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Irrigation,Drainage andRiver Engineering30W Pemberton BSc, FICEHead of Irrigation and Drainage DepartmentSir Murdoch MacDonald and Partners
C E Rickard BSc, CEng, MICE, MIWEMHead of River Engineering DepartmentSir Murdoch MacDonald and Partners
Contents
Part A: Irrigation and Drainage
30.1 Irrigation – fundamental concepts 30/330.1.1 Introduction 30/330.1.2 Soil moisture 30/330.1.3 Crop water requirements 30/330.1.4 Irrigation efficiency 30/430.1.5 Effective rainfall 30/530.1.6 Salinity and leaching requirement 30/5
30.2 Irrigation methods 30/630.2.1 Introduction 30/630.2.2 Surface irrigation 30/630.2.3 Sprinkler irrigation 30/730.2.4 Trickle irrigation 30/930.2.5 Sub-irrigation 30/930.2.6 Irrigation canal design 30/9
30.3 Drainage of agricultural land 30/930.3.1 Introduction 30/930.3.2 Sub-surface drainage of irrigated land 30/1030.3.3 Drainable surplus 30/1030.3.4 Drainage of lands subject to excess
rainfall 30/1030.3.5 Drain spacing 30/1130.3.6 Drain flow 30/1130.3.7 Drainage layouts 30/1130.3.8 Drainage of heavy soils 30/1230.3.9 Bedding systems 30/1230.3.10 Surface drainage for irrigated land 30/12
Part B: Land Drainage and River Engineering
30.4 Land drainage and flood alleviation 30/1230.4.1 Objectives of land drainage 30/12
30.4.2 Rivers as natural drains 30/1230.4.3 Economic issues 30/13
30.5 Hydrology 30/1330.5.1 Introduction 30/1330.5.2 Measurement 30/1330.5.3 Statistics 30/1330.5.4 Flood flow calculation methods 30/1330.5.5 Hydrographs 30/1430.5.6 Curve number method 30/1430.5.7 The Flood studies report 30/14
30.6 Channel regime 30/1430.6.1 Regime flow 30/1430.6.2 Regime formulae 30/1430.6.3 Practical applications 30/15
30.7 Sediment transport 30/1530.7.1 Basic concepts 30/1530.7.2 Sediment transport estimates 30/1530.7.3 Sediment transport equations 30/1530.7.4 Stable channel design 30/16
30.8 Channel design 30/1630.8.1 Channel flow formulae 30/1630.8.2 Channel stability 30/1630.8.3 Other considerations 30/17
30.9 Channel improvements 30/1730.9.1 Channel clearance 30/1730.9.2 Realignment 30/1730.9.3 Revetments and lining 30/17
30.10 Embankments 30/1930.10.1 Introduction 30/1930.10.2 Design 30/20
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30.10.3 Stability 30/2030.10.4 Construction 30/2030.10.5 Rood walls 30/21
30.11 Detention basins, washlands and catchwaterdrains 30/21
30.11.1 Detention basins 30/2130.11.2 Washlands 30/2130.11.3 Catchwater drains 30/21
30.12 Structures 30/2230.12.1 Introduction 30/2230.12.2 Retaining walls 30/2230.12.3 Bridges 30/2230.12.4 Weirs 30/22
30.12.5 Gated control structures 30/2330.12.6 Tidal outfalls 30/24
30.13 Pumping 30/2430.13.1 Single or multiple pumps 30/2430.13.2 Motive power 30/2430.13.3 Pumps 30/2430.13.4 Control 30/2530.13.5 Pump station building 30/2530.13.6 Other types of pumping installation 30/26
References 30/26
PART A: IRRIGATION AND DRAINAGE
30.1 Irrigation - fundamental concepts
30.1.1 Introduction
Irrigation is desirable where natural rainfall does not meet theplant water requirements for all or part of the year. Irrigation isessential for agriculture in the desert but even in areas such asnorthern Europe it can improve the yield of crops normallygrown under rainfall conditions only.
30.1.2 Soil moisture
The soil can be considered a moisture reservoir. Soils can beclassified under the International Soil Science Association(ISSA) system as follows:
Fraction Particle size(mm)
Coarse sand 2-0.2Fine sand 0.2-0.02Silt 0.02-0.002Clay < 0.002
Water is held by the soil in the soil pores. The amount of waterheld can be defined as follows:
(1) Saturation: the state of complete soil wetness when nofurther water may be added to the soil.
(2) Field capacity (FC): the condition reached after water hasdrained from the soil by gravity.
(3) Permanent wilting point (PWP): the condition reached afterplants have extracted all the moisture they can from the soil.
(4) Available water: defined as (FC-PWP), the amount ofwater held by the soil that plants can use.
Plants respond to how tightly the water is held by the soil whichis defined as soil moisture tension. Generally, it is assumed thatthe soil moisture tension at field capacity is 0.3 bar pressure. Soilmoisture tension at PWP is assumed to be 15 bar.
Typical moisture contents for various soils are shown inTable 30.1.
Table 30.1
Moisture content (percentage by weight)
Soil typePermanent Available
Field capacity wilting point moisture
Coarse sand 8 4 4Fine sand 15 8 7Silt 28 18 10Clay 45 30 15
With knowledge of the crop rooting depth, the available soilmoisture and the crop water requirements, it is possible to selecta suitable irrigation interval (time between irrigations). Not allwater in the root zone is readily available to the crop. It isnormal to allow the crop to deplete only 50% of the availablemoisture before irrigating again. More detailed guidelines aregiven by the Food and Agricultural Organization.1
30.1.3 Crop water requirements
Crop water requirements are defined as the depth of waterrequired to meet the water loss through evapotranspirationCETcrop) of a crop. The effect of climate on crop water require-ments is given by the reference crop evapotranspiration CET0)which is defined as the rate of evapotranspiration from anextensive surface of green grass of uniform height (8 to 15cm):
ET^-k^ET, (30.1)
where kc is the crop coefficient which varies with crop, growthstage, growing period and prevailing weather conditions
The most reliable method of estimating £T0 is generally con-sidered to be the PENMAN method. This method is bestdescribed by Doorenbos and Pruitt1 which also gives details ofcrop coefficients for a wide range of crops. Values of £Tcrop arenormally calculated for 10-day periods. A typical crop coeffi-cient curve is shown in Figure 30.1.
A simpler method was proposed by Blaney and Criddle2 in
Figure 30.1 Example of crop coefficient curve.(After J. Doorenbos and W. O. Pruitt (1977) Crop waterrequirements. Food and Agriculture Organization Irrigation andDrainage Paper No. 24.)
InitialCrop
development Mid-season Late
Plan
ting
date
Appr
ox. 1
0%gr
ound
cov
er Mat
urity
Harv
est
70-8
0%gr
ound
cove
r
Crop
coe
fficie
nt K
30.1.4 Irrigation efficiency
It is necessary to account for losses of water incurred duringconveyance and application to the field. Efficiencies can be
30.1.4.2 Field canal efficiency (Ef)
Ef is dependent on type of field channel used and area served.
Blocks larger than 20 ha Unlined canals 0.90Lined or piped 0.95
Blocks up to 20 ha Unlined canals 0.80Lined or piped 0.90
30.1.4.3 Distribution efficiency (Ed)
Distribution efficiency (Ed) is dependent on area served, the levelof water management, and canal seepage which is the maincomponent. Canal seepage can be calculated separately fromseepage rates given in Table 30.11 (page 30/9).
Typical overall values for EA are given below:
Irrigationmethod
Sprinkler
TrickleBasin
FurrowBorder
Application practices
— Daytime application,moderately strongwind
— Night application
— Poorly levelled andshaped
- Well levelled andshaped
— Poorly graded and sized— Well graded and sized
£a, % waterapplication efficiency
Soil textureheavy light
60 6070 7080 80
60 45
75 6055 4065 50
Period
Days
15/5-31/5 171/6- 3/6 34/6-30/6 271/7- 8/7 89/7-31/7 231/8-17/8 17
18/8-31/8 141/9-16/9 16
Meanmonthlytemp.(0Q
11.619.7
19.3
21.3
8.7
% annualdaytime(P)
5.951.11
10.022.898.315.554.574.53
Cropcoefficient(V
0.350.350.961.051.141.141.020.75
Waterrequire-ments(mm)
27.86.6
164.151.2
159.9112.683.040.8
646.0
Note: Southern latitudes apply 6-month difference as shown.
which the monthly crop water requirements £Tcrop (in milli-metres) are found by multiplying the mean monthly tempera-ture Tm (0C) by the monthly percentage of annual daytime hoursp and a monthly crop coefficient k\
ETcrop = (OA6Tm + Z)kp
Table 30.2 shows the monthly percentage of p for differentlatitudes.
A sample calculation of water requirements for maize plantedmid May near Saskatoon (latitude 520N) is shown in Table 30.3.
A simple calculation of gross irrigation requirements (/gross)can be made as follows:
i^-(fT^,-W EI (30.2)where £Tcrop is the crop water requirements, /?e is the effectiverainfall and Ea is the field application efficiency
Table 30.3
divided into three parts: (1) field application; (2) field canal; and(3) distribution efficiency.
30.1.4.1 Field application efficiency (Ea)
Ea is dependent on soil type and type of irrigation system used.Typical values are given in Table 30.4.
Table 30.4
Table 30.2 Monthly percentage of annual daytime hours (p) for different latitudes
Latitude
40°42°44°46°48°
50°52°54°56°58°60°
NorthSouth
Jan.JuL
6.766.636.496.346.17
5.985.775.555.305.014.67
Feb.Aug.
6.726.656.586.506.41
6.306.196.085.955.815.65
Mar.Sep.
8.338.318.308.298.27
8.248.218.188.158.128.08
Apr.Oct.
8.959.009.069.129.18
9.249.299.369.459.559.65
MayNov.
10.0210.1410.2610.3910.53
10.6810.8511.0311.2211.4611.74
Jun.Dec.
10.0810.2210.3810.5410.71
10.9111.1311.3811.6712.0012.39
JuLJan.
10.2210.3510.4910.6410.80
10.9911.2011.4311.6911.9812.31
Aug.Feb.
9.549.629.709.799.89
10.0010.1210.2610.4010.5510.70
Sep.Mar.
8.298.408.418.428.44
8.468.498.518.538.558.57
Oct.Apr.
7.757.697.637.577.51
7.457.397.307.217.106.98
Nov.May
6.726.626.496.366.23
6.105.935.745.545.044.31
Dec.Jun.
7.526.376.216.045.86
5.655.435.184.894.564.22
Continuous supply with nosubstantial change in flow 0.90
Rotational supply in projectsof 3000 to 7000 ha androtation areas of 70 to300 ha, with effectivemanagement 0.80
Rotational supply in largeschemes (> 10 000 ha) andsmall schemes (< 1000 ha)with respective problematiccommunication and lesseffective management:
based on predeterminedschedule 0.70based on advance request 0.65
The total irrigation requirements at the head of the system (7sys)can be calculated from:
/s^ = ̂ ff (30.3)
30.1.5 Effective rainfall (Re)
All rainfall is not effective in providing water for crop use. Thecalculation of effective rainfall is discussed in detail by Dastane.3
A simple method has been developed by the US Soil Conserva-tion Service which relates effective rainfall with £rcrop and meanmonthly rainfall (see Table 30.5).
For example, with a monthly rainfall of 50 mm, an ETCTOp of100mm and an effective soil storage of 100mm, the correctionfactor is 1.02 and the effective rainfall is 1.02 x 35 = 36 mm.
30.1.6 Salinity and leaching requirement
All irrigation water contains some dissolved salts. If no effort ismade to move salts through and beyond the root zone, the soil
salinity level will increase to make it unfit for plant growth. Theprocess of dissolving and transporting soluble salts downwardsto below the root zone is known as leaching.
The maximum leaching requirements can be calculated from:
Leaching requirement (LR) for surface or sprinkler irrigation
,»- EC9LR~5ECe-ECw
For drip and high frequency sprinklers (almost daily)
ro- EC»2Max£Ce
£CW = electrical conductivity of irrigation water, mmho/cm
ECe = electrical conductivity of the soil saturation extract fora given crop to the tolerable degree of yield reduction(see Table 30.6)
Max ECC = maximum tolerable electrical conductivity of thesoil saturation extract for a given crop (see Table30.7)
Alkalinity and toxicity may also affect soil permeability andcrop growth. For further details, Ayes and Westcott4 can beconsulted.
Salinity hazard has been classified by the US Department ofAgriculture (USDA) as shown in Table 30.6.
Table 30.6
Salinity of water(mmho/cm)
<0.25
0.25 to 0.75
0.75 to 2.25
>2.25
Salinity hazard
Low
Medium
High
Very high
Suitable for most crops andsoilsSuitable for moderately salttolerant cropsNot suitable for lowpermeability soilsGenerally not suitable forirrigation
Table 30.5 Average monthly effective rainfall as related to average monthly E7"crop and mean monthly rainfall
Monthlymeanrainfall(mm) 12.5 25 37.5 50 62.5 75 87.5 100 112.5 125 137.5 150 162.5 175 187.5 200
Average 25 8 16 24monthly 50 8 17 25 32 39 46ETCTOP 75 9 18 27 34 41 48 56 62 69(mm) 100 9 19 28 35 43 52 59 66 73 80 87 94 100
125 10 20 30 37 46 54 62 70 76 85 92 98 107 116 120150 10 21 31 39 49 57 66 74 81 89 97 104 112 119 127 133175 11 23 32 42 52 61 69 78 86 95 103 111 118 126 134 141200 11 24 33 44 54 64 73 82 91 100 109 117 125 134 142 150225 12 25 35 47 57 68 78 87 96 106 115 124 132 141 150 159250 13 25 38 50 61 72 84 92 102 112 121 132 140 150 158 167
Where net depth of water that can be stored in the soil at time of irrigation is greater or smaller than 75 mm, the correctionfactor to be used is:
Effectivestorage 20 25 37.5 50 62.5 75 100 125 150 175 200
Correctionfactor 0.73 0.77 0.86 0.93 0.97 1.00 1.02 1.04 1.06 1.07 1.08
In many instances, the usual inefficiencies of water applica-tion satisfy the leading requirements, but it is sometimes neces-sary to allow additional irrigation water for leaching. Theleaching efficiency varies with soil type and may vary from 100%for sandy soils to perhaps as low as 30% for swelling heavy claysoils.
30.2 Irrigation methods
30.2.1 Introduction
The choice of method of irrigation is dependent on technicalfeasibility and economics. Normal methods fall into four maincategories: (1) surface; (2) sprinkler; (3) trickle; and (4) sub-irrigation.
30.2.2 Surface irrigation
Surface irrigation is still the most common method of irrigationemployed and is suitable for the irrigation of most soils with aninfiltration rate of less than 150 mm/h and for lands with a flattopography with an overall slope of less than 3%, althoughthese limitations are exceeded in some situations.
There are four main types of surface irrigation: (1) basin; (2)border strip; (3) furrow; and (4) corrugation irrigation.
30.2.2.1 Basin irrigation
Water is applied from a small canal by gravity to fill a level basinsurrounded by earth bunds. In practice, these basins are oftensmall but the most efficient irrigation is obtained by using largebasins, at least a hectare in area and accurately levelled. Watershould be applied to those basins at a rate of at least 2 to 4 timesthe infiltration rate of the soil.
Basin irrigation is most suitable for very flat and level landand soils with low infiltration rates. When adopted for uneventopography the basin size must be kept small in order to limitthe quantity of land levelling required. Land levelling rates inexcess of 1000m3/ha should be avoided. The cultivation ofpaddy rice is normally done using basin irrigation. Basinirrigation is illustrated in Figure 30.2.
30.2.2.2 Border strip irrigation
The land is divided into strips separated by earth bunds whichrun generally down the slope, and water is applied at the head ofthe strip and allowed to flow down the slope infiltrating the soilas it flows across it (see Figure 30.3). The strip is graded at aneven slope along its length in the direction of flow and levelacross the strip. Efficient irrigation is obtained by choosing thestrip width, length and discharge to meet the soil infiltration rate
Table 30.7 Crop salt tolerance for selected crops
Figure 30.3 Surface irrigation methods
and land slope conditions to give as constant a depth of water aspossible infiltrated over the length of the strip. Typical borderstrip designs from the USDA Yearbook are given in Table 30.8.Border strips are suitable for land with a more pronouncedexisting slope, thus reducing the amount of land levellingnecessary. (For more information on design of sprinklers, seework by Barrs.6)
30.2.2.3 Furrow irrigation
Furrow irrigation is used for the irrigation of row crops or cropsgrown on beds between furrows. Furrow irrigation usuallyimplies sloping land although horizontal furrows can be usedfor row crops within level basins. Water is applied to the upperend of each furrow and flows down the furrow with waterinfiltrating into the beds between the furrows on which the cropis grown.
Furrow spacings are a function of crop and type of tillagemachinery used. Typically furrows are spaced 0.75 to 1.05mapart. Table 30.9 gives recommended maximum furrow lengthsin metres for various soil types, furrow slopes and average depthof water applied over the whole field.
Furrow slopes should be checked for erodability. The maxi-mum non-erosive flow in furrows (Qn) can be estimated from:
Qm = 0.60/5 (30.3)
Surface drainFurrowirrigation
Siphons
BorderStrip
Irrigation
Field channel
Figure 30.2 Basin irrigation
Field channel Basin
Siphon pipe
Drain
Bund
Crop
BarleyWheatTypical vegetables
(beans, carrots,lettuce, onions)
Forage, grassesFruit treesDate palm
Yield potential100%(ECj (ECJ
8.0 5.36.0 4.0
1.0 0.74.6 3.11.7 1.14.0 2.7
90%(ECj
10.07.4
1.75.92.36.8
(ECJ
6.74.9
1.13.91.64.5
75%(ECj
13.09.5
2.87.93.3
10.9
(ECJ
8.76.4
1.95.32.27.3
50%(ECj
18.013.0
4.611.14.8
17.9
(ECJ
12.08.7
3.17.43.2
12.0
Max EC,
2820
8188
32
Table 30.8 Typical border strip designs
Depth Strip StripSlope applied width length Flow
Soil type ((%) (mm) (m) (m) (1/s)
0.25 50 15 150 240100 15 250 210150 15 400 180
1.00 50 12 100 80Coarse 100 12 150 70
150 12 250 70
2.00 50 10 60 35100 10 100 30150 10 200 30
0.25 50 15 250 210100 15 400 180150 15 400 100
1.00 50 12 150 70Medium 100 12 300 70
150 12 400 70
2.00 50 10 100 30100 10 200 30150 10 300 30
0.25 50 15 400 120100 15 400 70150 15 400 40
1.00 50 12 400 70Fine 100 12 400 35
150 12 400 20
2.00 50 10 320 30100 10 400 30150 10 400 20
where Qm is in litres per second and S, the furrow slope, is in percent. Generally, cross-slopes in furrow irrigation should be lessthan the major ground slope down the furrows to limit thefurrow flows breaking out.
30.2.2.4 Corrugation irrigation
Corrugation irrigation is a variant of furrow irrigation in whichthe furrows are very small. It is suitable for medium soils only
and is used for close-growing crops such as wheat. The corruga-tions are some 10cm deep and spaced 40 to 75cm apart.Because the corrugation flows are small, slopes up to 8% havebeen used. This method of irrigation is not widely used outsidethe US.
For more details of surface irrigation methods, Booher5 canbe consulted.
30.2.3 Sprinkler irrigation
30.2.3.1 Types of sprinkler
The application of water by overhead sprinklers takes manyforms which include the following.
(1) Permanent and solid set - a network of pipes and sprinklerswhich covers the whole area to be irrigated. No movementof equipment within a season is necessary. This is the mostexpensive form of sprinkler irrigation.
(2) Lateral move sprinklers - sprinklers on a lateral line that ismoved by hand after each irrigation application to the nextarea to be irrigated. This is the most widely used system.
(3) Traveller systems - these are motorized methods of movingsprinklers and include:
(a) sideroll - lateral pipe and sprinklers on wheels, pushed byhand or small motor from one position to next irrigationposition;
(b) mobile rain gun - single gun winched across field whilstirrigating and fed from a hose reel;
(c) centre pivot - overhead lateral with sprinklers whichrotates about centre whilst irrigating;
(d) linear move - similar to centre pivot but moves laterallyacross the field.
The most common system used in developing countries, wherelabour is inexpensive, is the lateral move sprinkler system. Indeveloped countries where labour is expensive, various forms oftravellers are more common. Sideroll is suitable for low crops asthe lateral is not normally more than 1.8 m above the ground.Rain guns with their high water-pressure requirements and,hence, high energy costs are best used for supplementaryirrigation. Centre pivot and linear move are becoming the mostpopular traveller systems in arid areas. Permanent and solid setsare very expensive and hence used only on high-value crops.
30.2.3.2 Sprinkler design
Individual sprinklers provide a cone of precipitation so that
Table 30.9 Maximum recommended furrow lengths (m). (After Booher (1974) Surface irrigation. Food and Agriculture Organization Landand Water Development Series No. 3.)
Soil type Fine Medium Coarse
Furrow average depth of water applied (cm)slope (%) 7.5 15 22.5 30 5 10 15 20 5 7.5 10 12.5
0.05 300 400 400 400 120 270 400 400 60 90 150 1900.1 340 440 470 500 180 340 440 470 90 120 190 2200.2 370 470 530 620 220 370 470 530 120 190 250 3000.3 400 500 620 800 280 400 500 600 150 220 280 4000.5 400 500 560 750 280 370 470 530 120 190 250 3001.0 280 400 500 600 250 300 370 470 90 150 220 2501.5 250 340 430 500 220 280 340 400 80 120 190 2202.0 220 270 340 400 180 250 300 340 60 90 150 190
Distance (m)Figure 30.4 Sprinkler application patterns
overlap of sprinkler patterns is necessary to give a reasonableuniform application as shown in Figure 30.4.
The discharge of a sprinkler Q in cubic metres per second:
Q = CFJ2gH (30.4)
where: C= the contraction coefficient varying between 0.79 and
0.98, F= the cross-sectional area of the nozzle in square metres,g = 9.81 m/s2 and H= the height of hydraulic head behind thenozzle in metres
To give a particular precipitation rate over a field there is arange of solutions of sprinkler spacing, nozzle diameter andpressure as shown in Table 30.10.
Sprinkler 1pattern
Sprinkler 2pattern
Sprinkler 3Sprinkler 2
Combinedpattern
Sprinkler 3pattern
Sprinkler 4pattern
,Sprinkler 5pattern
Sprinkler 4
Table 30.10 Spacing and precipitation rates of single-nozzle sprinklers. (After Baars (1973) Design of sprinkler installations. Department ofIrrigation, Civil Engineering, Agricultural University, Wageningen)
Details of sprinklerNozzle Pressuresize(mm) (atm)
4.0 3.03.54.0
5.0 3.03.54.0
6.0 3.54.04.5
7.0 3.54.04.5
8.0 3.54.04.5
9.0 3.54.04.5
10.0 3.54.04.5
11.0 3.54.04.5
12.0 3.54.04.5
Discharge
(m3/h)
.02
.11
.191.63.76.88
2.562.742.903.483.733.964.444.745.045.676.066.427.127.608.068.639.239.79
10.1810.8811.55
Diametercoverage(m)
303132333435363637404142434344444546464748484950495052
Square and rectangular spacing of sprinklers (m)12x12 12x18 18x18 18x24 24x24 24x30 30x30 30x36
Precipitation rate (mm/h)
7.1 4.77.7 5.18.3 5.5
7.5 5.08.1 5.48.7 5.8
7.9 5.98.5 6.390 6.7
8.1 6.08.6 6.59.2 6.9
10.3 7.711.0 8.211.7 8.8
9.8 7.910.5 8.411.1 8.912.4 9.913.2 10.6140 11.2
12.0 9.612.8 10.313.6 10.914.1 11.315.1 12.116.0 12.8
Note: Exceeding the line is not recommended for ideal irrigation.
Appl
icat
ion
(mm
)
The variation of head between sprinklers is normally limitedto ± 0.2/f where H is the design head at the sprinkler, takinginto account any difference in ground level at sprinklers. It isthis criterion which limits the lateral pipe diameter and length.
Movable laterals are normally made of aluminium or galva-nized steel, the aluminium being lighter and hence easier tohandle, and the galvanized steel being cheaper and more easilyrepaired.
The supply pipeline can be designed using the normal pipefriction formulae described in Chapter 5.
The calculation of head loss in sprinkler lines havingsprinklers at constant spacing can be calculated using theChristiansen formula:
*•-*£ (30.5)
where hz = head loss in the sprinkler line in metres, h = head lossin 100 m line in metres, through which a quantity of water flowswhich corresponds to the total discharge of all sprinkl6rs on theline, n = number of sprinklers on the sprinkler line, a = spacingof the sprinklers and/= factor which varies with the number ofsprinklers, n, as follows:
n f n f n f n f2 0.625 8 0.398 14 0.370 20 0.3593 0.518 9 0.391 15 0.367 25 0.3544 0.469 10 0.385 16 0.365 30 0.3505 0.440 11 0.380 17 0.363 40 0.3456 0.421 12 0.376 18 0.3617 0.408 13 0.373 19 0.360
30.2.4 Trickle irrigation
The basis of trickle irrigation is to provide irrigation water toindividual plants. A plastic pipe is run along the ground at thebase of a row of plants and water is carried to each plantthrough orifices in the pipe or using an emitter. Trickle irriga-tion is more accurately described as localized irrigation as itincludes a wide range of emitters such as micro-sprinklers andbubblers.
Trickle irrigation is most suitable for row crops and trees andis generally able to use more saline water supplies than surfaceirrigation or sprinkler irrigation. The design of localized irriga-tion systems is described by Vermeirei and Jobling.7
30.2.5 Sub-irrigation
Sub-irrigation is only suitable for specialized soil conditions.High horizontal permeability and low vertical permeability are
Table 30.11 Seepage rates from canals. (After Etcheverry (1915)Irrigation practice and engineering. McGraw-Hill)
Seepage lossesType of soil (m3/s per million m2)
Impervious clay loam 0.8-1.2Medium clay loam 1.2-1.7Clay loam or silty soil 1.7-2.7Gravelly clay loam or sandy clay or
gravel cemented with clay 2.7-3.5Sandy loam 3.5-5.2Sandy soil 5.2-6.4Sandy soil with gravel 6.4-8.6Pervious gravelly soil 8.6-10.4Gravel with some earth 10.4-20.8
required, or a barrier layer beneath the root zone. Water ispassed to the crop from open feeder ditches via buried perfor-ated pipes. Control of the water level in the ditches determinesthe quantity of water available to the crops. A combined systemof irrigation and drainage is common with the ditches and pipesdoubling for both irrigation and drainage.
30.2.6 Irrigation canal design
The basic and most common method of designing a rigidboundary channel is the Manning equation.
The design method and values of Manning's n are described insection 30.8. Earth canals which transport significant quantitiesof sediment can be designed, using a regime method or one ofthe sediment transport formulae described in sections 30.6 and30.7.
30.2.6.1 Freeboard
Freeboard is defined as the distance between the design waterlevel and the canal bank top level.
Minimum freeboard above design water level for earth canalscan be defined by:
/2> = 0.2 + 0.235el/3 (30.6)
with a minimum value of 0.3 m
where Fb is the freeboard in metres and Q is the design dischargein cubic metres per second
30.2.6.2 Canal seepage
The quantity of water that will seep from the canal is normallymeasured in cubic metres per second per million square metresof wetted perimeter. Seepage rates for various materials inwhich the canals are constructed are given in Table 30.11.8
30.3 Drainage of agricultural land
30.3.1 Introduction
Agricultural drainage is necessary to remove excess water fromthe soil to improve the agricultural potential.
The benefits of drainage may include:
(1) Seed germination - excess moisture associated with lowtemperatures impairs germination. Waterlogging may causeseeds to rot and not germinate.
(2) Crop growth - most crops require air in the root zone togrow.
(3) Control of water table - high water tables will limit depth ofroot zone.
(4) Disease - waterlogged crops are more susceptible to disease.(5) Yield gain - generally higher crop yields are experienced
from drained land.(6) Poaching - wet soil that carries stock experiences surface
damage by grazing animals.(7) Cultivation - improved drainage will allow easier access for
cultivation machinery.(8) Salinity - control of salinity in crop root zone.
Drainage systems can be defined as subsurface and surface.Surface drains are designed to remove excess runoff from theland which would otherwise cause localized flooding. Subsur-face drainage is designed to remove excess water from the soilmass. It is discussed in the following sections.
30.3.2 Sub-surface drainage of irrigated land
Sub-surface drainage for irrigated lands in arid areas is nor-mally associated with the control of the water table depth. Mostcrops grow best with the water table below their root depthalthough crops may not be affected by a higher water table for ashort period. Rice is an exception since it grows well in totallywaterlogged conditions.
Recommended minimum water table depths are shown inTable 30.12. The necessary drainage is frequently achieved byproviding perforated drainage pipes below ground at regularintervals. It is necessary to install the drains below the desireddesign water table depth.
Table 30.12 Minimum water table depths
Water table depth below ground level (m)Fine textured
Crop (permeable soil) Light textured soil
Field crops 1.2 1.0Vegetables 1.1 1.0Tree crops 1.6 1.2
The shallowest drain depth for water table control is:
//+0.5/z+ 0.Im
where H= design water table depth given above and H = rise inwater table resulting from the maximum individual rechargefrom a water application.
30.3.3 Drainable surplus
The quantity of water to be removed by a subsurface drainagesystem can be estimated from a water balance:
Qs = Rf+Sc +S-Dn (30.7)
where Q5 = water to be removed by drainage, Rf= recharge tothe water table from rainfall or irrigation, Sc = seepage fromcanals or rivers, S{ = groundwater flow into the area andDn = groundwater flow out of the area.
Recharge (7?f) to the water table will vary with soil type,irrigation method and efficiency of water management.
Food and Agriculture Organization Paper No. 389 Drainagedesign factors gives the estimated recharge for various con-ditions as shown in Table 30.13. Seepage from canals can beestimated using Table 30.11.
Groundwater inflow and outflow can be calculated from dataon groundwater slope, flow cross-section and soil permeabilityusing Darcy's law, which states that:
K=^L (30.8)
where V= flow velocity in metres per day, K= hydraulic con-ductivity of the soil in metres per day, and h/L = hydraulicgradient.
And Q= VA
where Q = flow in cubic metres per day and A = area of flow insquare metres
30.3.4 Drainage of lands subject to excess rainfall
The drainage of irrigated land in arid areas is described above.However, many areas require drainage due to an excess ofrainfall. The drain discharge due to rainfall rises to a peakfollowing a rainstorm and then recedes.
For the design of a buried pipe-drainage system, the dischargeis often based directly on rainfall data. For instance, in the UKfield drainage design is based on 5-day rainfall divided by 5 togive the daily drainage rate with return periods as shown inTable 30.14. Typical drainage rates in northwest Europe wouldbe of the order of 7 to 10 mm/day. Drainage systems incorporat-ing mole drainage are normally based on a 1-day rainfall value,because of the shorter response.
The design depth to water table is often taken at 0.5m forshallow rooted crops and 0.75 to 1 m for deep-rooted and high-value crops. Drains in the UK, in practice, are usually laid at
Table 30.14
Design rainfallCrop exceedance
Specialist high value crops 1 yr in 25Horticultural 1 yr in 10Roots 1 yr in 5Intensive grass, cereals 1 yr in 2Grassland 1 yr in 1
Less than For soils having a low infiltration1.5 mm/day rate.
1.5 to 3.9 mm/day For most soils, with the higher ratefor more permeable soils and wherecropping intensity is high.
3.0 to 4.5 mm/day For extreme conditions of climate,crop and salinity management, andunder poor irrigation practices.
More than For special conditions, e.g. rice4.5 mm/day irrigation on lighter textured soils.
Approximate design drainage rates are likely to be in thefollowing ranges:
Irrigation method
Sprinkler
TrickleBasin
Furrow, border
Application practices
Daytimeapplication,moderately strongwind
Night application
Poorly levelled andshaped
Well levelled andshaped
Poorly graded andsized
Well graded andsized
Average recharge aspercentage ofirrigation waterdelivered to the fieldSoil textureheavy light
30 3025 2515 15
30 40
20 30
30 40
25 35
Table 30.13 Estimated recharge to watertable as related toirrigation method and soil type
depths ranging from 0.75 m in low permeability soils to 1.25 to1.5 m in permeable soils. A more detailed discussion of drainagedischarge design is given by Smedema and Rycroft.10
30.3.5 Drain spacing
The required spacing of drains can be calculated using theHooghoudt equation:
T2_^dh.4KJ^q~ q (30.9)
where Ka = hydraulic conductivity above the drain in metres perday, Kb = hydraulic conductivity below the drain in metres perday, h = height of water table above the drain level midwaybetween the drains in metres, q = drain discharge in metres perday and d= equivalent depth - function of depth to imperme-able barrier (D) and drain spacing (L) (see Table 30.15)
Table 30.15 Equivalent depths (d) for 80mm corrugated PVCpipe drains
L(m) 5 10 15 20 25 30 35 40 45D(m)
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.250.5 0.43 0.46 0.47 0.48 0.48 0.49 0.49 0.49 0.490.75 0.53 0.62 0.66 0.68 0.69 0.70 0.71 0.71 0.721.00 0.59 0.74 0.81 0.85 0.88 0.90 0.91 0.92 0.931.25 0.62 0.83 0.93 1.00 1.04 1.07 1.09 1.11 1.121.50 0.63 0.89 1.03 1.12 1.18 1.22 1.26 1.28 1.301.75 0.64 0.94 1.11 1.22 1.30 1.36 1.40 1.44 1.472.0 0.64 0.97 1.17 1.31 1.41 1.48 1.54 1.58 1.62
The Hooghoudt equation allows two layers of soil with differinghydraulic conductivity (A8, Ab) (see Figure 30.5). Values ofhydraulic conductivity can be measured in the field using theauger hole method. Alternatively, the designer can use valuesmeasured on similar soils elsewhere. The single auger holemethod requires a hole some 80mm in diameter to be boredbelow the water table. The water in the hole is then pumped orbaled out and the rate at which it refills is measured. From thesemeasurements the value of hydraulic conductivity can be calcu-
lated. For more details, see van Beers's work.11 Typical values ofhydraulic conductivity are given in Table 30.15. A more detailedexplanation of the calculation of drain spacings is given in ILRIBulletin no. 8.12
30.3.6 Drain flow
Drain pipe sizes can be calculated using the Darcy-Weisbachequation for smooth pipes and Chezy-Manning for corrugatedpipes. For which have a constant discharge along their length:
Q = 89^2-711 °-57 smooth pipesQ = 38^2-67/ °50 corrugated pipes
where Q = discharge in pipe, in cubic metres per second, cp — pipeinternal diameter, in metres and /=hydraulic gradient, in metresper metreIt is common to 'over design' the pipe to allow for some siltationwith the drain capacity normally increased by some 30%. It isnormal to assume that the hydraulic gradient line coincides withthe pipe soffit, i.e. the pipe flows full.
If the drains are installed in hydraulically unstable soils theywill require to be surrounded by a gravel envelope. Generally,soils with a high clay content will be stable and will not requirean envelope. Granular envelopes are normally 50 to 100mmthick. The gradation of the filter should be designed using theUS Bureau of Reclamation method.13
30.3.7 Drainage layouts
Typical layouts of a buried drainage system, regular and irregu-lar are shown in Figure 30.6.
Open Main Drain
Open or buriedpipe collectordrain
Buried pipe field drains
Open or buriedpipe collectordrain
(b) IrregularFigure 30.6 Typical layouts of buried drainage systems
(a) RegularOpen Main Drain
Figure 30.5 The Hooghoudt equation (definitions)
Table 30.16 Hydraulic conductivity (m/day)
K(m/day)
Coarse gravelly sand 10-50Medium sand 1-5Sandy loam/fine sand 1-3Loam/clay loam/clay, well
structured 0.5-2.0Very fine sandy loam 0.2-0.5Clay loam/clay, poorly
structured 0.02-0.2Dense clay, not cracked and
no bio-pores < 0.002
Collector drains can be open ditches or buried pipes. Buriedpipe collectors are to be preferred where sufficient ground slopeis available. Pipe drain slopes should not be less than 0.0005whilst open collector slopes can be as low as 0.0001. To allowdrains to be cleaned, they should not exceed 30Om in lengthwithout a manhole or outfall into an open channel.
30.3.8 Drainage of heavy soils
For soils with very low permeability it becomes uneconomic toinstall drainage systems with buried field drains at spacings ofbetween 1 and 5 m as are indicated by the use of the Hooghoudtequation. A common solution is the use of mole drainage.Moles are installed by using a mole plough that draws a 75-mmdiameter bullet through the soil at a depth between 400 and600 mm. The mole forms a tunnel in the soil and some fissuringin the upper soil area. Mole drains are normally spaced at 1 to3 m and drawn across the line of the collector drains which havepermeable fill in the pipe drain trench above the drain (seeFigure 30.7).
Collector drains are normally spaced at 20 to 60 m.Moling is best suited to clay soils with a minimum clay
content of 30% and the moles have a relatively long life in stablecalcareous clays. However, remoling will be necessary on aver-age every 5 yr or so.
Efforts have been made to increase the life of mole drains byfilling the tunnels with gravel. However, this is very expensiveand for normal field cropping is not economic.
30.3.10 Surface drainage for irrigated land
Surface drainage is often provided to irrigated land to collectexcess irrigation supplies and runoff from rainfall. For surfaceirrigation typical surface drain capacities can be based on 24 h, 1in 5yr rainfall with 24 to 48 h storage on the field. For ricedrainage, the drain capacity should be sufficient to allow thedrawdown of water in the paddies where this is part of thecultivation pattern.
Typical values of surface drainage capacity are in the range of2 to 41/s per hectare.
PART B: LAND DRAINAGE AND RIVERENGINEERING
30.4 Land drainage and flood alleviation
30.4.1 Objectives of land drainage
The drainage of agricultural lands has already been discussed inthe first part of this chapter. To the river engineer the term 'landdrainage' has a broader interpretation, encompassing both theremoval of excess water and the prevention of flooding of theurban as well as the rural environment.
In general terms, the problem of ineffective land drainageoccurs when inflow into the system exceeds outflow, so thatthere is a build-up of water over a period of time. This mayoccur rapidly over a few hours in response to heavy rainfall, or itmay be a gradual rise in water table during wet periods.Flooding occurs when a channel has inadequate capacity toconvey the amounts of water flowing into it, or when flooddefence works fail. Thus, the solutions to land drainage prob-lems invariably involve either control of inflow into the systemor works to improve the capability of the drainage channels tocarry flows through the system. The basic objective is to reducethe frequency and/or the intensity of inundation to acceptablelevels, appropriate for the situation.
30.4.2 Rivers as natural drains
Rivers are the Earth's natural drainage channels, conveyingsurface flow from the land to the sea or to inland lakes andmarshes. Some rivers are essentially ephemeral (wadis) and flowfor only very brief periods, often with very high discharges andconsequently devastating erosive power. Others are seasonal,being dry for part of the year, but flowing steadily during thewetter months. Others still are perennial, generally flowingthroughout the year but with varying intensity. Most rivers inEurope fall into this latter category.
No two rivers are the same, but rivers exhibit similaritieswhich, to a certain extent, can be defined mathematically, thusenabling engineers to assess the problems with which they arefaced. Perhaps the most fundamental property of a river is itsflow or discharge. However, as has been indicated above, this isnot a fixed property - the flow varies both spatially and withtime. There are ways in which the flow in a river can becontrolled or reduced, but often the engineer is faced with theproblem of designing a structure or a scheme which is capable ofwithstanding the flow which passes through a specific point orreach of the river. It is therefore necessary to estimate the riverflow for which the scheme or structure must be designed and thisinvolves an exercise in statistics which is described later in thischapter.
Figure 30.7 Mole drainage
30.3.9 Bedding systems
Bedding is a common method for the drainage of flat heavy landsubject to excess rainfall. Wide beds are most suitable formechanized agriculture and are up to 30 m in width. Drainage ismainly by surface runoff with some interflow in the topsoilregion as shown in Figure 30.8. The shallow drains are normallysome 0.5m in depth. The raised crowned beds are normallybuilt up over time by ploughing in such a way to turn the soiltowards the centre of the bed.
Figure 30.8 Wide beddingIMPERMEABLESOlL DRAIN
Overland- flow '
Interflow TOPSOIL
Rainfall
Soil fissuring Mole Plough
Direction of ploughingPipe drain
Mole channel
Permeable fill
30.4.3 Economic issues
Since funds are limited and there is always competition fromother potential schemes, it is necessary to undertake some formof economic evaluation of proposed drainage improvementworks. Such an evaluation requires the estimation of the benefitswhich might accrue from the scheme and the costs of itsimplementation.
For an urban flood relief scheme, some of the benefits areobvious and can be evaluated in a straightforward manner.Elimination of the physical damage caused by flooding is onesuch benefit, which can be assessed by counting the cost ofreplacement or repair of goods and property so damaged. Inaddition, there are less tangible costs of flooding which must beevaluated, such as loss of production due to flooding of indus-trial properties and disruption to traffic resulting from floodedroads. These too must be estimated. Finally it is now commonpractice to evaluate the intangible factors such as the distresscaused to the public by flooding, particularly to those people ina high-risk area. From a knowledge of the frequency of floodingthe present value of all the 'damage' likely to occur during thelifetime of the proposed works can be estimated. The benefits soderived should then be compared with the estimated costs of theworks so that competing schemes can be compared on a similarbasis or to determine the most economic level of protectionwhich could be provided.
For agricultural lands it is possible to estimate the increasedvalue of production generated by improved drainage, althoughthis can involve some fairly subjective assessments. In general,the agricultural benefit will accrue as a result of either a loweredwater table and/or a reduced risk of periodic flooding, bothenabling a wider range of crops to be grown and/or better yieldsto be achieved as well as extending the period for whichagricultural operations are possible and improving 'traffickabi-lity' of the land. Thus, an estimate of the increased value ofannual production is made possible by the drainage works andthis figure is capitalized over the life of the scheme to determinethe benefits. As with the urban scheme the benefits are thencompared with costs as a means of evaluating schemes.
30.5 Hydrology
30.5.1 Introduction
The design of river engineering and land drainage works isbased on hydrological criteria, predominantly estimates ofchannel flow and its variation with time.
The ideal basis for the calculation of design parameters is along period of recorded data which can then be analysed usingstatistical methods. Such data are often not available, but arecord from a neighbouring catchment may be, and this can becorrected for use in the area concerned. Even short periods ofdata are useful, but if no records exist or their reliability isdoubtful, empirical techniques of parameter estimation can beemployed.
30.5.2 Measurement
The measurement of channel flow (discharge) is most commonlyundertaken by velocity-area methods or at flow-measuringstructures. Flows are measured over a range of stages (waterlevels) so that a stage-discharge relationship can be developed.Velocity-area methods depend upon the use of a current meterto record velocities and a knowledge of the cross-sectional areato which the velocity measurements can be applied, the productof these two variables being discharge. Flow-measuring struc-tures are operated on the principle that there is a unique
relationship between level upstream of a structure and dis-charge. Flumes and weirs are commonly used on small riverswhereas velocity-area methods are usually applied to medium-and large-sized channels.
In recent years, permanent flow measurement installationsusing electromagnetic or ultrasonic gauging techniques havebeen developed as an alternative to weirs and flumes. Essen-tially, these methods measure velocity at a defined section.
Where records of channel flow are not available, rainfallrecords may be used to estimate likely flows. Within the UKrainfall records are maintained by the Meteorological Office,which operates over 5500 gauges, and flow data are archived bythe Institute of Hydrology.
30.5.3 StatisticsMethods of statistics14 are frequently used in hydrology toestimate the return periods of natural events. A flood flow is saidto have a return period of, for example, 50 yr if, on average overa long period of time, that flow is equalled or exceeded once in a50-yr period. Frequency analyses are required so that standardsof protection can be met, risk assessments made and economicanalyses undertaken.
The Fisher-Tippett type 1 extremal distribution (commonlyknown as the Gumbel distribution) is often used to analyseannual maxima series of discharge and rainfall. For a series ofdata values, QM, the magnitude of the event of return periodTyr, QT, is given by:
CT = <2A + d(0.78>> -0.45) (30.10)
I<2M n /^" 2\whereC?A = —, , = ̂ _ (__- Q^ ,
y—— loge f — loge ( 1 — — J J and n = number of years of data
The probability, PN, of an event of return period T yr beingequalled or exceeded during a period of TV yr, is given by:
/>N=1-[1-(1/D]N (30.11)
Typical acceptable frequencies of flooding within the UK areonce a year for grassland, once in 10 yr for arable land and oncein 100 yr for urban areas. Where the risk to life is high, as incoastal situations, standards of protection even higher thanthose for urban areas may be considered.
30.5.4 Flood flow calculation methods
If statistical methods cannot be used, the design flow may beestimated from empirical equations relating rainfall and/orcatchment characteristics to runoff. Early formulae to evaluatethe 'maximum flood', £max, were of the form:
Qa** = CA" (30.12)
where C = coefficient depending on the type of climate andcatchment, A = catchment area and « = an index, usuallybetween 0.5 and 1.2
The rational formula is representative of the many formulaedeveloped to relate rainfall to peak runoff and is of the form:
Q = CiA (30.13)
where g = peak discharge, C= runoff coefficient, depending onthe characteristics of the catchment, /= rainfall intensity andA = catchment area
This approach is based on the assumption that maximum flowoccurs as the result of the maximum rainfall intensity to beexpected within the 'time of concentration' of the catchment.'Time of concentration' is the time taken for rainfall falling onthe most remote part of the catchment to reach the part of thedrainage network under consideration. Nowadays the rationalformula is generally only used for urban drainage design, forwhich the assumptions are valid.
Rainfall formulae have also been developed relating theintensity, duration and frequency of events. The Bilham for-mula15 related these for the UK by the following equation:
r = 25.4[(1.25777V)0282-0.1] (30.14)
where r = total rainfall in millimetres, T= duration of storm inhours and TV= probable number of occurrences in 10 yr
30.5.5 Hydrographs
A more accurate approach to flood peak and volume prediction,which has been widely accepted and developed since its incep-tion by Sherman, is the unit hydrograph concept. A hydrographis a plot of discharge versus time (Figure 30.13(b)) and, as such,gives the engineer much more information than just a peak flowestimate. The unit hydrograph of a catchment is defined as thehydrograph of direct runoff resulting from a unit of effectiverainfall generated uniformly over the catchment at a uniformrate during a specified period of time.
Where runoff and rainfall records are available the catch-ment's unit hydrograph can be derived from these data and usedto produce hydrographs of runoff for any given rainfall profile.Since such data are often not available, synthetic unit hydro-graph techniques have been developed which relate catchmentcharacteristics to the parameters of the unit hydrograph. TheUS Soil Conservation Service (USSCS)16 and the EnvironmentalResearch Council17 have presented such techniques.
30.5.6 Curve number method
The curve number method developed by the USSCS uses thefollowing methodology:
(1) Rainfall is converted to discharge using a curve numbergraph based on catchment characteristics.
(2) Discharge is developed into a basin hydrograph using theUSSCS unit hydrograph.
(3) The design drainage rate is taken from the peak of thehydrograph.
This method is described in detail in the USDA Nationalengineering handbook, section 4.
30.5.7 The Flood studies reportIn the UK, the Flood studies report provides a comprehensiveguide to the estimation of maximum floods, return periods andflood volumes for any site. The methods described apply to bothgauged and ungauged catchments.
Of the two methods developed for ungauged catchments - themean annual flood plus growth curve and the synthetic unit
hydrograph plus design storm - the latter is given more weight-ing in the report. However, for preliminary estimates of peakdischarge the simplicity of the former technique is very appeal-ing. Use of the Flood studies report is described in detail bySutcliffe.18
30.6 Channel regime
30.6.1 Regime flow
A channel is said to be in regime when, over a hydrologicalcycle, the channel shows no appreciable change in its width,depth or gradient. Regime theory postulates that for a stablechannel there is a relationship between the channel parametersof width, depth, gradient and flow. Thus if any one of these fourparameters is artificially (or naturally) changed, the channel willadjust itself so that regime conditions are re-established.
Also fundamental to river regime is sediment transport, whichis discussed in more detail later. Sediment is material which ispicked up, transported and deposited by the river. It can varyfrom very fine clay particles, often referred to as wash load, tolarge cobbles and boulders which can be moved by a river inflood. A regime channel is generally transporting sedimentwhich is similar in size to the material which forms the bed andbanks of the channel.
Many regime theories have been developed from a study ofirrigation canals and the application of regime theory to naturalrivers raises the problem of what flow should be taken assignificant in determining the dimensions of the stable channel.The low flows which go on for most of the time cause little or nochange in the channel section, while the maximum flood flowswhich occur for a few hours at intervals of several years causerapid but temporary changes which the river will subsequentlytend to restore. The 'bank-full' flow is commonly used in theUK, where it has been quantified as the flow which is exceededfor 0.6% of the time (i.e. about 2 days per year on average).
30.6.2 Regime formulae
Of the many regime formulae postulated, that of Lacey isprobably one of the most useful. The Lacey formulae can beexpressed as:
«_WEQl/6
Ws = 4.83eQ]/2
_2A6V2
dm j-
where Q = design flow in cubic metres per second, 5=channelslope, ^5 = water surface width in metres, dm = mean depth =A/Wt in metres, A = cross sectional area of flow in squaremetres, V= flow velocity = QjA in metres per second, E= shapefactor = P/ W^ P=channel wetted perimeter in metres, ande = width factor and B=bed width (generally taken as IV5).
The values of K and x vary with the size of sediment present inthe channel bed, as shown in Table 30.17. The shape factor Etakes account of the differences between wide shallow channelsand narrow channels. For the former, the value of E approachesunity.
The width factor e varies with soil type, in response toerodibility. Its value may range between 0.7 for stiff soils (clays)to 1.0 for erodible soils (sands and silts).
The silt factor / depends on the size of sediment which willform the channel bed in the long term. Indicative values of/are
Table 30.17
Sediment median K xgrain size, D50(mm)
Z)50 < 0.2 0.000206 $0.2 < D50 < 0.6 0.000274 i0.6 < Z)50 < 2.0 0.000303 fZ)50>2.0 0.000188 £
0.4 to 0.6 for clays and 1.0 for sands. Actual values adoptedshould be based on local experience of stable channels.
In the context of rivers in the UK Nixon19 carried out aninteresting study of channel characteristics in 1959. He exa-mined twenty-nine British rivers and attempted to establishregime formulae which were independent of bed materialproperties. His formulae are thus a simplification of reality,since they imply that the channel cross-section is independent ofthe material from which it is formed. Nevertheless, they can beused as a guide to channel dimensions for British rivers,although it should be noted that they are not applicable tochannels with gravel beds. The Nixon formulae (converted tometric units) are:
^s = 3.02 b05
^ = 0.552-'F=0.6ieb°17andA = LMQM*
where W^ A and Fare defined as above, <4 = mean depth = A/Ws in metres and Qb = bank-full flow in cubic metres per second
30.6.3 Practical applications
Regime equations can be used as a guide in the design of newchannels or of remodelling works to existing channels. In theabsence of any other information the characteristics of a repre-sentative reach of existing channel will give a good indication ofwhat is appropriate for new works in terms of channel widthand depth, bed slope and bend radius. It is worth rememberingthat, if it is necessary to improve the conveyance of a riverchannel (as part of a flood improvement scheme) it is better toincrease the depth of flow and/or the slope rather than thechannel width, because natural adjustments of the former tendto occur at a much slower rate.
A channel may be capable of carrying the required flow withsmaller dimensions and a steeper gradient than the regimevalues but the higher velocities generated will increase the widthand depth by scour and will reduce the gradient by deposition ofthe eroded material at the lower end of the reach. Similarly,many channels have been made with excess width to carry themaximum flood flows, but sediment deposition has subse-quently resulted in a narrower meandering deep channel withinthe main channel. The most common error arising from lack ofknowledge of the regime theory is when a river is straightenedby cutting across meanders without considering whether theresultant shortening of the course and increase of slope willproduce scouring velocities.
30.7 Sediment transport
30.7.1 Basic concepts
Sediment load is generally divided into two categories depend-
ing on the mechanism of its transport. The 'wash load' com-prises relatively fine material and the rate of wash load transportis mainly determined by its rate of supply from the drainagebasin rather than the transport capacity of the stream. Thismaterial settles out rather slowly and can be maintained insuspension in large quantities by relatively slow-flowing water.Particles of 0.06 mm or finer are often considered as the washload fraction. More accurately, wash load can be defined as thatfraction of sediment which is finer than the Z)10 size of material(Z)10 = SiZe for which 10% is finer) found on the channel bed.
In contrast, the 'bed material load' transported is almostentirely a function of the transporting capacity of the flow. Theuse of the term 'bed material' indicates that this is what thesediment load mainly comprises. It should not be confused with'bed load' which has been used in the past to describe the largerparticles transported near the bed of the channel.
Sediment loads are normally expressed as parts per million(p.p.m.) by weight (i.e. 1 g of sediment in 1 g of water= 1 p.p.m.).Transport rates are generally expressed in tonnes per day.
30.7.2 Sediment transport estimates
Sediment transport estimates are most commonly required sothat the rate of scour or deposition in a channel can bepredicted. In northern Europe problems of sediment transportare generally modest and concentrations of less than1000 p.p.m. are common. However, in tropical or arid zones,sediment loads in rivers in flood can exceed 20 000 p.p.m., andmuch higher loads have been recorded in extreme cases. Clearly,if such highly charged water is diverted from a river into, forexample, a slow-flowing irrigation canal, there will inevitably beextensive deposition in the canal.
Sediment transport estimates may also be required in thedesign of river regrading works where it is necessary to checkwhether the changes imposed (such as steepening the rivergradient) are likely to lead to excessive erosion.
Of course, wherever possible, attempts should be made tomeasure sediment transport rates in situ. This is a notoriouslydifficult operation and even the most carefully controlled sam-pling can yield widely differing results. This is not only becausethe sampling technique is prone to error, but also becauseextreme sediment loads occur in relatively short-lived floodswhich are unpredictable.
30.7.3 Sediment transport equations
There are many sediment transport equations, none of whichcan claim a high degree of predictive accuracy. An estimatewhich is in the range 0.5 to 2 times the actual value is all that canbe reliably expected. The recent Ackers-White20 equations areamongst the most accurate although these are rather cumber-some. The more simple Engelund and Hansen21 equation yieldssimilar levels of accuracy.
The Engelund and Hansen equation for bed material load canbe expressed as:
1600OyFWS1-5
(S-D2D50
where A'= sediment concentration in parts per million, 5 = sedi-ment specific gravity (normally 2.65), V— average flow velocityin metres per second, d= average channel depth in metres,S= channel slope and Z)50 = median sediment size of bedmaterial in metres
Thus, a channel flowing at 1.0 m/s at a depth of 1 m with a slope
of 0.5 m/km and a median sediment size of 1 mm wouldtransport:
16 000 x 2.65 x 1.0 x 1.0^(0.0005)'5 , _ .1.6? x 0.001 =174p.p.m.
30.7.4 Stable channel design
Recent research by White, Paris and Bettess20 has resulted in thepublication of a set of Tables for the design of stable alluvialchannels. These tables have been derived from the results ofextensive flume experiments, and list values of sediment size,sediment concentration, channel flow, flow velocity, channelslope, depth of flow, channel width and friction factor. Given asediment size and any two of the other parameters, it is possibleto estimate values for all the other variables.
30.8 Channel design
30.8.1 Channel flow formulae
The most commonly used and universally accepted channel flowequation is that of Manning, which can be expressed in terms offlow as:
ARW12
n
where Q = channel flow in cubic metres per second, A = channelcross-sectional area below water level in square metres, R = hyd-raulic radius, A/P in metres, P=wetted perimeter in metres,S—channel slope and n — Manning's roughness coefficient
The selection of an appropriate value for Manning's roughnesscoefficient, which in normal engineering practice lies in the range0.010 to 0.150, is a matter of judgement and experience. Ven TeChow22 gives comprehensive guidelines, with values for allcommon situations supported by photographs of typical chan-nels. Table 30.18 gives some illustrative values.
In selecting an appropriate value of Manning's n the effect offuture changes in the nature of the channel must be considered.Perhaps the most significant factor is vegetation which, if left
Table 30.18 Values of Manning's n*
unchecked, can reduce the capacity of a channel to a fraction ofits design capacity. Thus, whereas it is feasible to construct anearth channel which will have an n value of 0.025, experience hasshown that even modest vegetative growth will increase this to0.035. The problem is worse in tropical zones where plantgrowth is prolific, and irrigation canals can require clearanceevery few months. It is therefore recommended that, for suchchannels, a minimum value of n = 0.030 is adopted, with highervalues if it is known that maintenance will be infrequent.
30.8.2 Channel stability
Manning's equation gives us a simple tool for determining thechannel size, but gives us no information as to the long-termstability of the channel. It is most important, therefore, that acheck is made on likelihood of sedimentation or scour occur-ring. Sedimentation has already been discussed. To assess thescouring potential of a stream it is common to determine thetractive force, defined as:
-C0 = CyRS (30.15)
where T0 = unit tractive force in newtons per square metre,Y = water specific weight (9810 newtons per cubic metre),S= water surface slope, R = hydraulic radius in metres andC=coefficient depending on the shape of the channel and thepart of the channel considered
Unless the channel is particularly narrow, values of C of 1.0 forthe bed and 0.76 for the banks are usually assumed.
For non-cohesive soils there are recommended values oftractive force on the channel bed for a range of soil types,recommended limiting velocities of flow are also given. Guide-lines are given in Table 30.19.
Table 30.19 Suggested limiting tractive force (TO) and flow velocity(V) values
Material
Fine sands andnon-colloidal silts
Firm loamStiff clay and
colloidal siltsFine gravelGraded colloidal
silts to gravelCoarse gravelCobbles and
shingles
Clear water
V \(m/s) (N/m2)
0.55 1.90.75 3.6
1.15 12.50.75 3.6
1.20 20.61.20 14.4
1.50 43.6
Water transportingcolloidal siltsV T0(m/s) (N/m2)
0.85 4.01.00 7.2
1.50 22.01.50 15.3
1.65 38.31.50 32.0
1.65 52.7
Source: Etcheverry.(1915) Irrigation practice and engineering. McGraw-Hill.
The figures in Table 30.18 can be used as a guide to determin-ing limiting slopes for channels in terms of the movement of bedmaterial. For the channel sides, even though the tractive force islower, the banks may be less stable because of the effect ofgravity. In practice it is found that, for fine non-cohesivematerial, small amounts of sediment in the water tend to cementthe particles together and the use of tractive force theory isconservative. However, side slope stability should be consideredfor coarse non-cohesive materials (medium-sized gravels andabove).
Surf ace I channel
Concrete lined channel (smooth finish)Brick-lined channelMortared rubble masonry
Earth channel: clean, uniformEarth channel: very overgrown with
weeds, etc.
Minor stream: clean, straightMinor stream: sluggish, weedy with deep
pools
Major stream: regular section
Floodplain
Normal range of n(design value)
0.012-0.017 (0.015)0.012-0.018 (0.015)0.017-0.030
0.020-0.030 (0.025)
0.050-0.120
0.025-0.033 (0.030)
0.050-0.080
0.025-0.060
0.025-0.15Of
Notes: *Assumes channel flowing at or near full stage, lower flows may result inhigher n values because of the relative significance of obstructions.
fOn floodplains the value of n depends very much on the type ofvegetation, its height and the season. A typical value for short grass mightbe 0.030, whereas dense brush in summer might be 0.100.
30.8.3 Other considerations
The channel shape, as well as its size, is also important. Forirrigation canals and drainage channels bed width to depthratios (B :d) of between 3 and 4 are often used. Channelsdesigned using the Lacey regime equations tend to have largerB\d ratios. Such channels have lower tractive force values butthis principle cannot be extended too far since drains with bedswhich are too wide will tend to form sub-channels at lower flowsleading to a higher local tractive force and, hence, erosion.
Channel side slopes depend mainly on the nature of theground in which they are cut. Slopes of 1:1.5 (vertical: horizon-tal) and 1:2 are quite commonly adopted, with flatter slopes of1:3 or even 1:5 if the bank material is highly erodible.
Other considerations may dictate the choice of side slope suchas use of the slopes for grazing (where flow is intermittent) andecological factors (e.g. desire to re-establish reed growth, henceshallow slopes).
For bends in channels a rough guide to the appropriate bendradius is I Q x W^ (W^ = water surface width), but this takes noaccount of the erodibility of the bank material. Lacey proposeddesign radii of 12SV (? (Q = design flow in cubic metres persecond) which is recommended where space is available forchannels in fine alluvial soils. If possible, the designer shouldmeasure the radii of other stable channels in the area to give aguide to acceptable values. For lined channels (concrete, brick,masonry, etc.) a minimum radius of 3 Ws is recommended, with5 W^ being used where possible.
30.9 Channel improvements
30.9.1 Channel clearance
In northern Europe the clearance of trees, brushwood andweeds offers greater improvement in reduced flood levels inrelation to cost than any other form of channel improvement.Such works should be executed with a sympathetic approachsince our rivers are frequently areas of great natural beauty andare well used by anglers and for other recreational interests (seeFigure 30.9). Some very valuable advice in this respect can befound in a handbook published by the Royal Society for theProtection of Birds and the Royal Society for Nature Conserva-tion.27 The aim should be to retain an appearance which is asnatural as possible without prejudicing the aim of improvedchannel conveyance.
As an alternative to channel clearance, consideration should
be given to forming a bypass channel which would leave asensitive reach of channel untouched. The bypass would be setat a level where it only operated in flood conditions, thuspreserving the main stream for all normal flows and allowinguse of the land taken up by the bypass for grazing.
30.9.2 Realignment
Natural rivers and streams often follow a meandering course,and lower flood levels in a particular area can be obtained bystraightening the course downstream by diversions acrossmeanders or by a completely new channel. Realignment willeliminate sharp bends where erosion takes place and willproduce a more stable course, but in rivers of high amenityvalue long straight reaches will lead to complaints that the riverhas been converted to a 'canal' and in such cases a course withsweeping curves is preferable.
With any realignment exercise it is essential to check that theincreased slope which will result will not lead to instability. Inextreme cases the shortening of a river reach can result inupstream progressing degradation which could underminebridge foundations or cause the collapse of river frontages.Where realignment results in a gradient steeper than the stableregime gradient, the provision of weirs may be necessary to limiterosion. In small channels, or in short lengths of larger channels,bed scour may be prevented by the use of some form of flexiblebed protection such as dumped stone or gabion mattress.
30.9.3 Revetments and lining
30.93.1 Introduction
A revetment is any means of protecting a channel bank fromerosion or undermining. Revetments are frequently required onriver bends, in the vicinity of structures (where flow may bemore turbulent), where the natural bank material is unstable,and where the wash from boats causes progressive collapse ofbanks. Revetments may be rigid (e.g. sheet piling) or flexible(e.g. dumped stone).
Channel lining is used where both bed and bank scour are tobe prevented or where it is necessary to streamline the channel.Linings, too, can be rigid (e.g. reinforced concrete) or flexible(e.g. gabion mattress). Flexible linings are often preferablebecause they will accept some settlement or damage whilstretaining their appearance and integrity.
Typical details of rigid and flexible revetment systems areillustrated in Figures 30.10 and 30.11. The most importantconsiderations in revetment design are:
(1) Adequate scour protection at the toe to prevent undermin-ing.
(2) Flexibility if settlement is likely.(3) Adequate weephole provision if the revetment is imperme-
able and rapid drawdown of river level is possible.(4) Cost (taking into account locally available materials, es-
pecially in developing countries, and availability of inexpen-sive labour).
(5) Environmental acceptability.
30.9.3.2 Traditional methods
One of the earliest methods of bed and bank protection was theuse of brushwood faggots or fascines and these are still used incertain conditions. For revetment purposes the brushwood ismade up into tight bundles laid side by side end-on to thechannel, with successive layers stepped back to conform withthe bank slope. The bundles are held down by stakes but, after a
Bank sown with grass and wild flowersand set back to retain tree
Figure 30.9 Conservation in river engineeringCourtesy. Nature Conservancy Council (1983) Nature Conservationand River Engineering
Flood leve
Bank set back to improveaccess and increase marginal]habitat
Sympathetic treatmentPond excavatedto create fillfor bank
Unsympathetic treatmentBank established with rye grass Tree lost
Flood level
Mortared Pitching
Figure 30.10 Typical rigid revetments
few floods or tides the whole mass is impregnated with silt whichholds and preserves the brushwood. Brushwood is also used inlonger lengths to build up large mattresses which are floated intoposition and sunk by loading with stone to protect the river bedor the lower bank slopes.
Stone is also much used for revetment work. This may be inthe form of dumped stone (usually machine placed), dry stonepitching (hand placed), pitching grouted with bitumen, ormortared stone pitching, the latter being rigid. The following
Reinforced Concrete Wall
formula may be used to estimate the minimum stone sizerequired for a given flow velocity:
0.01 IV6S(s -I)3sin3(p- a) (30.16)
where W= critical weight of stone in kilograms (two-thirds ofstones to be heavier), V= flow velocity in metres per second,s=stone specific gravity (assume 2.5 if no other information),
Maximum water level
Originalbed
Maximum water level
Concrete Lining
Mortaredstone pitch in
Bed levelConcretebacking
Concretetoe
Maximum water level
Bed level
Can be formed from a fabricmattress filled with cementgrout (placed under water)
Bed level
Flexibleprotection
AnchoredSheet PiIe Wall
Steel sheetpiling
Anchorblock
Tie rod
Concretecope
Maximumwaterlevel
Precast Wall withSheet Pi Ie Toe
Steel sheetpiling
Massconcrete
Backfill
Precast concretewall unit
INTERLOCKINGCONCRETE GEOTEXTILEBLOCK SYSTEM
Figure 30.11 Typical flexible revetments
p = a stability factor, 70° for random stone (riprap) and a = slopeof the bank (i.e. angle with horizontal, in degrees)
Note: Impinging velocity may be assumed to be 1.25 x averagevelocity on the outside of bends. Thickness of stoneshould be at least 1.5D where D is the effective diameter ofthe normal size of rock specified. Bank slopes should notexceed 1:1.5 (vertical to horizontal). A filter of gradedgravel or geotextile should be provided under the stone toprevent the leaching of fines from the bank.
30.9.3.3 Gabions
Gabions and gabion mattresses have been used for many yearsfor revetment and lining work. The gabions are crates formedfrom wire or plastic mesh and filled with stone. Common sizesare 2 x 1 x 1 and 2 x 1 x 0.5 m. The gabion mattress is similarbut comes in units of 6 x 2 m with a range of thickness (up to500 mm). The gabion crates/mattress are subdivided by diaph-ragms and are packed with stones of a size generally just largerthan the mesh size. Good-quality control during filling isessential to achieve the desired effect. The end-product is aflexible permeable structure of considerable erosion resistance.It is normal to use a filter beneath the gabions to prevent thewashing out of fines. Table 30.20 gives indicative mattressthicknesses for a range of conditions. The mattress can be usedalone as a flexible lining, or in conjunction with gabion boxes asillustrated in Figure 30.11.
30.93.4 Concrete, geotextile and other methods
In recent years the use of concrete block systems and geotextile
Table 30.20 Gabion mattress thickness
Clays, heavy cohesive soils:maximum water velocity(m/s) 2 3 4.5minimum mattress thickness(mm) 170 230 300
Silts, fine sands:maximum water velocity(m/s) 2 3 N/Aminimum mattress thickness(mm) 230 300 N/A
Shingle with gravel:maximum water velocity(m/s) 3.5 5 6minimum mattress thickness(mm) 170 230 300
revetments has become popular and there are many proprietarysystems on the market. Most of these systems are flexible,permeable and allow the growth of grass and waterside plantsthrough them. Most of the concrete block systems have mecha-nical interlocks which prevent the lifting of individual units,some are connected together by polypropylene strands or gluedto geotextile mats so that they can be placed in large units. Theincorporation of a geotextile fabric filter under the armouring iscommon practice. This prevents fines being washed through thearmouring in the same way that a graded gravel filter does understone protection.
Various forms of geotextile reinforcement are also available.These can be placed on the soil surface and sown with anappropriate grass seed to give a natural looking erosion-proofsurface (in limited erosion conditions). They can even be pro-vided with grass already established. The use of jute fibres forthis purpose is also coming into vogue because of its environ-mental acceptability (it biodegrades within 1 or 2 yr leaving anestablished grass cover).
Steel sheet piling is commonly used in the neighbourhood ofweirs, locks or sluices, where there is wave action or heavyturbulence. In fine bed material such as silts, corrugated asbes-tos cement sheets have been used as an inexpensive form of sheetpiling. This material is only suitable to support up to 1 m faceswhere damage by boats, etc. is not expected.
There are many other proprietary revetment systems, any oneof which may be appropriate in certain circumstances. Theseinclude:
(1) Grout-filled mattress (rigid, permeable, can be placed underwater).
(2) Fabric with pockets which can be filled with soil (allowsrapid planting of appropriate waterside plants).
(3) Plastic webbing spanning between vertical supports.
30.10 Embankments
30.10.1 Introduction
Embankments are provided along river channels to preventflooding. They are normally set back from the river so thatduring floods they provide the necessary increase in the water-way section by providing both extra width and extra depthwithout overflow. In urban areas the land required to set backthe embankments may not be available. In fact, there may notbe space for the wide-based embankment at all and a flood wall,
Filtercloth
0.20 m thickgravel backing
DUMPEDSTONE (RIP-RAP)
Heavy-duty filter cloth(as alternative to gravel filter)
0.30 m thick hand-placed.stone pitching
Dumped stone
GABION REVETMENTd - Maximum anticipated
scour depth
Scouredbed
River bedGabion mattress
Box gabion
DRYSTONEPITCHING
Precast Blocksfabricatedinto Mats
Proprietrygeotextilemat
Sown witfgrass seed
Anchoringpegs
or a 'half-bank' supported by a wall may have to be used (seeFigure 30.12).
Most flood embankments in the UK and northern Europe areof moderate dimensions, not exceeding 4 m in height and manynot exceeding 2 m. They are constructed of the best materialavailable, preferably containing some clay to make the bankswatertight. Pure clays are not ideal because they tend to crackon drying. Low flood embankments are often constructed frommaterial dredged from the river channel, frequently sands andgravels with varying silt content. This achieves two objectivessimultaneously by enlarging the channel and forming floodbanks. Such banks may be designed to overtop every 5 or 10 yror so when providing protection to agricultural land. Providedthat the land-side slopes are relatively flat and a good soil andturf cover is provided, overtopping of long reaches of suchembankment will cause little damage.
Clay cores are not normally provided in embankmentsalthough these can be specified where residential properties areprotected and even small amounts of seepage would therefore beunacceptable. Steel sheet piling driven down the middle of anembankment has also been used to cure a local seepage prob-lem.
30.10.2 Design
Bank top levels should provide a freeboard above the designflood level to allow for settlement and damage by cattle orpedestrians. The freeboard also allows for any inaccuracies inthe estimation of flood level. A minimum freeboard of 0.5 m forembankments is normal, with the lower figure of 0.3 m adoptedfor walls.
The bank top width is often about 2 m but may be increasedto permit the passage of a tractor along the top with variousmaintenance equipment.
Side slopes on the river face should not be steeper than 1:2(vertical: horizontal) and on the landward face may be some-what flatter. Where space permits and fill is available the bankmay be constructed with very flat slopes on the landward side sothat the area may be mown as part of the adjoining field. Arablecultivation may also be permitted on such slopes provided thatthe crest of the embankment is clearly demarcated to preventgradual lowering by farming operations. The planting of treesor shrubs on the embankment should be discouraged. The idealsurface treatment is good turf grazed by sheep or cut mechani-cally several times per year.
Embankment slopes of 1:2.5 can be mown by tractor, butflatter slopes are preferred. Slopes of 1:3 or flatter may be
grazed by cattle, but cows tend to cause more damage thansheep so careful management will be necessary.
The hydraulic gradient through the embankment is the slopeof a line from the high-water mark on the river side to thelandward toe of the embankment and this should not exceed1:4. In important cases a 'flow-net' through the embankmentand its foundation should be calculated to reveal any risk of'piping' due to excessive rates of seepage.
30.10.3 Stability
The subsoil adjoining many rivers in their lower reaches consistsof recent deposits of alluvium and is often waterlogged. Suchmaterial may be unable to support the increase of superimposedload due to the construction of the flood bank. Unless the softmaterial is a thin layer, the solution is to construct an embank-ment of reduced height with a flood wall on top of it. Alterna-tively, the use of geotextile layers under the embankment may beconsidered. Appropriately designed, these effectively reinforcethe subsoil and reduce settlement.
The most serious condition for bank instability arises whenthe flood level drops rapidly after prolonged retention at a highlevel. The increased pore water pressure in the embankment hasinsufficient time to dissipate, shear strengths are reduced and aclassic slip failure may result.
In practice, with embankments of no more than 4 m high,stability is rarely a problem given a good fill material adequatelycompacted. However, if problem soils are encountered, the useof geogrid reinforcement or alternative forms of constuctionshould be considered.
30.10.4 Construction
The area of the base of the embankment should be stripped ofturf and topsoil to a depth of 0.25 m. This may be stockpiled forlater use on the surfacing of the bank. Where the subsoil isweak, or the embankment high, compaction of the subgradebefore placing fill should be considered. This will reduce settle-ments experienced subsequently. The fill should then be depo-sited and rolled down in layers, generally by bulldozer or tractorshovel.
Finally, the embankment will be cased with topsoil and sownwith grass seed. The height of the newly completed bank shouldmake allowance for settlement which is likely during the firstyear at the rate of about one-tenth of the height of the bank,depending on the type of material and the degree of compaction.
Figure 30.12 (a) Flood embankment (b) Half-bank with wall (c) Flood wall (d) Flood wall on embankment
Maximum water level
Berm RiverHydraulicgradient
Maximum water level
Maximum water level
Sheet pilecutoff
30.10.5 Flood walls
Where space or foundation conditions do not permit theconstruction of embankments, flood walls may be used. To meetamenity requirements, the wall may have to be cased in brick, ornatural or artificial stone, or a half bank may be formed behindit which will provide support, seal any leakage and overcomeobjections to an exposed concrete face.
Where it is necessary to allow access through a flood wall inresidential or industrial areas and steps over the wall areimpracticable, flood gates may be incorporated. These are side-hung hinged steel gates with a rubber seal and stout lockingdevice. Normally left open, these gates are closed by theresidents when flood levels get dangerously high. Similar floodgates may be provided on vehicular access ways in which casethey may be bottom hinged or housed in a slot below road leveland raised when necessary.
30.11 Detention basins, washlands andcatchwater drains
30.11.1 Detention basins
The peak discharge in a river or stream can be reduced bystoring some of the flow in a detention basin temporarily. Theflow into the basin should be controlled by banks and weirs sothat flows up to the bank-full capacity can pass down the streamleaving the basin empty. In flood the excess flow can then bespilled into the basin over a weir or through a sluice so that flowdown the channel is still restricted to a safe value (Figure 30.13).When the peak of the flood has passed the basin can be emptiedthrough an escape sluice back into the river. The escape sluicemay be manual or automatic and should be designed to allowrapid emptying of the basin so that the storage is available in theevent of a second flood in quick succession.
Residual flow
Figure 30.14 Combined channel improvement andstorage - economic assessment
30.11.2 Washlands
The use of washlands is common in the Fens of the southeastUK and elsewhere. Flood embankments are constructed wellback from the river enclosing an area which may be more than1 km wide and 30 km long. This area acts both as a detentionbasin and as a flood channel. Large quantities of water can betemporarily stored and because of the large cross-sectional areaof the waterway at flood level, velocities and surface gradientsare very small. The area of land involved is considerable and itsuse is usually restricted to summer grazing or the production ofhay or lucerne. If the washland is only required for majorfloods, lesser floods may be excluded by a lower bank alongsidethe river and the washland may be cultivated. For small schemesuncontrolled flow on to the washlands may be acceptable butlarge schemes rely on sluices to regulate the flow into and out ofthe washlands, thus optimizing their use.
30.11.3 Catchwater drains
In many cases the water from areas of high ground runs downinto low-lying areas to create or accentuate drainage problems.The upland area may be more extensive than the lowland andalso produces a greater runoff per unit area. The upland flowmay be diverted away from the lowland area by a catchwaterdrain. In the case of lowland pumped drainage schemes thediversion of the upland water will reduce both the capital cost ofthe pumping plant and its subsequent running cost.
The catchwater drain is located on the edge of the upland andis designed to intercept the streams running down into thelowland area. Control structures are necessary to allow someflow to follow the original course in dry periods to avoid ashortage of water in the lowland area. In flood periods the whole
Provided that an adequate area of suitable land exists in theriver valley, the maximum flow passing downstream may bereduced to any desired extent above the normal flow, but thevolume of storage required increases in greater proportion thanthe reduction of residual flow, and economy may require acompromise between the provision of storage and channelimprovements downstream (Figure 30.14).
In order to spill a substantial part of the flow into the basinwhen the safe residual flow is reached without a further increasein the flow passing downstream, a long side weir may be usedseparating the basin from the normal channel, or an automaticsluice of adequate capacity designed to maintain a constantwater level on the main channel side may be used. The long sideweir may take the form of a low embankment suitably protectedagainst erosion by stone, gabion, concrete block or geotextilereinforcement.
Figure 30.13 Flood detention basin, (a) layout, (b) estimation ofstorage capacity
Time T
Safe carrying capacity of channel
High gro
Sluice
Low bank
F low
Q
Cost
Chea
pest
sche
me
flow may be diverted down the catchwater to a suitable outletaway from the lowland area.
30.12 Structures
30.12.1 Introduction
The design of river structures must involve an understanding ofthe fundamentals of channel hydraulics, sediment transport andthe associated problems of scour and deposition. It must beappreciated that any structure in a river is likely to interfere withthe natural regime of the channel, the consequences of whichcan be far-reaching unless the process is fully understood and iscatered for in the design.
Some of the structures commonly encountered in river engi-neering are described below.
30.12.2 Retaining walls
The riverside retaining wall is used to support the river bank ora road or building adjacent to it and is used where a slopingrevetment is inappropriate. The wall should be designed for therapid drawdown condition where the water table behind thewall remains high when the river level drops quickly afterprolonged flooding. Weepholes and gravel drains behind thewall may be provided to relieve the hydraulic pressure, but thesemust not be assumed to eliminate the pressure differential, onlyto reduce it. It is normal to provide a cutoff (see Figure 30.12(d))on the base of the wall. This member serves three functions:
(1) It provides a key to improve the factor of safety againstsliding.
(2) It reduces seepage under the wall when river levels are high.(3) It provides stability in the event of bed erosion in front of
the wall.
Various alternative walls have been described in section 30.9.3.Steel sheet piling is very common in river work, particularly forwall heights in excess of 3 m. Sheet piling has the considerableadvantages of speed of erection and no need for dewatering.
30.12.3 Bridges
30.12.3.1 General
Bridges spanning rivers affect channel regime inasmuch as thepresence of abutments and piers changes the flow patterncausing local acceleration of flow. The simplest way to avoidproblems is for the bridge to span the entire river channel,although this is often uneconomic or impractical for large rivers.Even if the bridge abutments are outside the main river channelit will be necessary to check that the approaches do not undulyrestrict floodplain flow.
As a guideline, a bridge should be designed to have an openarea of not less than 80% of the channel cross-sectional flowarea for bank-full conditions, although smaller values may beappropriate where the natural river is very slow-flowing.
30.12.3.2 Scour at bridges
Bridge piers and abutments must be designed such that they arenot undermined by any scouring of the river bed or banks.Scour may be caused locally by the presence of the piers, or itmay be a feature of the natural channel, occurring duringfloods.
The following equations, put forward by Holmes24 for rivers
in New Zealand, may be used to give indicative scour depths.The local scour (d&) due to the piers must be added to the generalbed scour (D5).
^ = 0.8 VT^
where ^8 = local scour depth in metres, V= flow velocity inmetres per second and b = projected pier width in metres
D5 = (y VK)U A]W
where Ds = depth of scour measured from flood water level inmetres, y = rise in water level to flood level upstream of thebridge measured from normal water level in metres, W= water-way width at bridge, no deduction for piers, in metres, A = wa-terway area at bridge, no deduction for piers, in square metres,V= mean flow velocity upstream, no allowance for scour, inmetres and K=^(W/4.$3Q112), maximum value K= 1, Q = dis-charge in cubic metres per second
30.12.3.3 Afflux at bridges
Unless a bridge is designed with very streamlined approach andexit conditions, head losses are likely to be significant duringflood flows. The head loss at a bridge may be estimated from thefollowing equation:
AL-^OI-P?). (30.17)
where /zL = head loss (drop in water level through bridge) inmetres, K1 = flow velocity upstream in metres per second,K2 = flow velocity through bridge in metres per second, g = acce-leration due to gravity (9.8!metres per squared second) andC= coefficient depending on the degree of streamlining (1.2 fornormal bridges)
30.12.4 Weirs
Weirs are designed for one or more of the following roles:
(1) Measurement of flow (as part of a hydrological network).(2) Control of channel depth for navigation.(3) Reducing the slope of steep, erosive channels.(4) Creation of ponds to improve river fisheries.(5) Control of rivel level to allow perennial irrigation abstrac-
tions.
The design of weirs is discussed in Chapters 5 and 22.For flow measurement the Crump weir is often adopted. This
is a robust structure offering minimal obstruction to the passageof sediment and debris. It is modular up to a drowning ratio of80% and can be used for measurement in any state of drowningif a crest tapping is provided (i.e. if both upstream and crestwater levels are measured). For modular flow the equation is:
Q= 1.961Vh15 (30.18)
where Q = discharge in cubic metres per second, W= weir widthin metres and h = upstream head over the weir in metres
Where it is necessary to record both high and low flowsaccurately it is normal to provide a compound weir. A typicalstructure might comprise a Crump weir for high flows with aparallel Vee-notch weir for low flows, the crest of the Crumpweir being above that of the Vee-notch.
For controlling navigation depths a wide range of weir typeshave been used. The weir will, of course, incorporate a lock topermit the passage of vessels and often sluice gates are providedto pass flood flows (these effectively reduce the range of waterlevels between normal and flood flow conditions and they maybe essential to avoid flooding upstream). The typical weircomprises a crest and a stilling basin, the design of which isdiscussed in Chapter 22. Provision must be made to reduceseepage under the weir and to resist uplift on the downstreamapron under all flow conditions.
Wherever the head loss across the weir is more than about1 m, or where the foundations comprise permeable soils, a checkon underseepage, uplift and exit gradient should be made. All ofthese can be determined by plotting a flow net using an electricalanalogue device or an appropriate computer program.
For larger weirs the use of a physical model is recommendedto test the design before the prototype is built. This is particu-larly important where entry and exit flow conditions are notstraightforward or where heavy sediment loads are expected.
For rivers which are used by migratory fish, weirs should beprovided with a fish pass. The most common type is a series ofpools connected by small weirs or submerged orifices. The poolsshould not be less than 3 m long and 2.5 m wide with not lessthan 1 m depth below the connecting openings. The difference inlevel between successive pools should not exceed 500 mm.
30.12.5 Gated control structures
Gated control structures are required to maintain a design waterlevel or range of water levels throughout the range of flowconditions experienced. For an unregulated weir the water levelupstream rises with increasing flow, but the introduction of awater control gate can enable a relatively constant upstreamwater level to be maintained. In effect, the gates (which have asill level lower than the weir crest) allow the waterway area to beincreased, thus permitting the flow to increase without raisingupstream water level.
The most common form of water control gate is the verticallift sluice gate, but radial gates and hinged tilting gates are alsoused extensively. Figure 30.15 illustrates the three types dia-grammatically.
30.12.5.1 Vertical lift gates
For small sizes the vertical lift gate may be a standard penstockgate fabricated from cast iron or steel sliding in cast iron or steelframes and normally operated by screwed spindles and simplereduction gearing.
Larger vertical lift gates are fabricated from welded steelplates and sections having wheels and guide rollers running onwheel tracks built into recesses in the piers and abutments. Suchgates are usually suspended from the gearing by plate linkchains or steel wire ropes passing over sprockets or groovedwinding drums mounted on the overhead steel superstructure.
To decrease the loading on the gearing and so increase themanual speed of operation, the gates are often counterweighted,the counterweights being connected directly to the chains orropes. Modern vertical gates have wheels, bushed with self-lubricating bearing material, bearing on stainless steel axlesfixed to the gate structure and having a fairly low coefficient offriction.
Opening and closing of vertical lift gates may be by manualmeans or electric power. When electric power is used theopening and closing of the gates may be made automatic,controlled by the upstream water level usually by incrementalmovements of the gate as a result of variations in water level ofabout 100 mm. Operation of the gate under automatic control ispurposely made slow with time delays between each incremental
Figure 30.15 (a) Vertical sluice gate(b) Float operated radial gate(c) Tilting gate
movement in order to prevent 'hunting' which can result if theincremental movement causes large changes in upstream waterlevel.
30.12.5.2 Radial gates
Radial gates are constructed so that the resultant reaction of thewater loading passes through the centre of rotation of the gateand thus there is no component of water load to be handled bythe gate-lifting mechanism. The water loading is transmitted tothe pivot bearings by gate arms and thence to the concrete work.Unlike vertical lift gates, there is no need for groove recesses inthe piers and abutments and the side members are requiredsimply to provide seal bearing surfaces. With automatic-typeradial gates the weight of the gate structure is balanced bycounterweights on extensions to the arms which support thegate water load.
The lower edge of the gate closes on to a sill and is sealed by astrip of rubber or similar material. The ends of the gate haverubber or leather seals sliding on the metal plates set in theabutments. Radial gates are normally operated by electricmotors but there is a form of radial gate operated by floats. Thefloat operated radial gate (see Figure 30.15(b)) is a special typeof radial gate designed to maintain a constant upstream waterlevel automatically. It can be very useful in remote locationswhere the power supply is unreliable or non-existent.
The floats are located in chambers in the abutments or piers.Water from the river upstream passes over an adjustable weirand into the float chamber, from which it escapes at a constantrate through a valve. If the upstream water level rises, waterflows into the float chamber at a greater rate than it escapes,causing the float to rise in response to the increased level. Thefloat acts on the gate arm causing the gate to open, which in turnincreases the flow through the structure and lowers theupstream water level. Conversely, if the level falls, flow over the
Outlet
WeirchamberControl-water
control weir does not balance the escape of water from the floatchamber, and the gate closes until normal level is restored.Again, the design must be such as to avoid hunting but inpractice it has been found possible to control the level of a largeriver within 12 mm of a set level. In major floods the gate risescompletely out of the water and there is a possibility that thedownstream water level will take control and prevent the gatefrom closing when it should. This situation can be avoided byproper design of the control arrangements.
30.12.5.3 Tilting gates
Radial and vertical gates have the disadvantage that flow takesplace under them and floating debris of all kinds collects againstthem and has to be removed manually. This is avoided by tiltinggates which are hinged at the bottom edge and allow the waterto pass over them. The gates are lowered by links or chains andmay be operated manually or automatically by electricity. Thegate when fully open lies flat in a shallow pit formed in thefoundation to give maximum discharge, or in some cases stillforms a low weir in the lowered position. In rivers carryingcoarse sediment there may be difficulty in lowering the gate tobed level, but in practice the water weiring over the gate usuallykeeps the area immediately downstream clear. This type ofsluice has the advantage that any failure of power supply orother operating failure will not allow water levels to fall belowthe gate level. As an adjustable weir it is particularly suitable forsmall installations with manual operation.
30.12.6 Tidal outfalls
Tidal outfalls are required where drainage channels dischargethrough a sea wall or tidal embankment. Their function is toallow discharge at low tide but to prevent the tidal water fromflowing back into the drainage system during the high tide.Essentially they consist of a culvert through the tidal embank-ment with a tidal flap or door, which may be at the outer end ofthe culvert or in a chamber in the embankment (Figure 30.16).Where beach levels near the sea wall are relatively high theculvert may be extended for some distance out on the foreshoreand a door on the outer end could be subject to severe waveaction. At the wall itself the door can be sheltered by wing wallsand breakwaters, and still greater protection is obtained by theuse of a chamber in the embankment.
The door is usually circular or square, of cast iron, steel, orplastic, hung from the top by double hinges which allow thedoor to seat freely and to accommodate small obstructions suchas weed or sticks on the seat. There should be sufficient spacebetween the bottom of the door and the apron on to which thewater falls to avoid debris from being trapped behind the doorand preventing closure. There should also be adequate clearancebetween the sides of the door and the wing walls or sides of thechamber. When the door is in a chamber it must be mounted onthe upstream wall and the chamber itself must be built up abovehigh tide level. It is not advisable for the chamber to be sitedinland of the tidal embankment as the intervening culvert will beunder pressure at high tides. If the door is some distance out on
the foreshore there is the possibility of tidal water breaking intothe culvert behind the door, and it is therefore advisable toprovide a sluice or penstock capable of shutting off the culvert atthe tidal embankment or at the inland end.
For small drainage channels discharging into a river a similararrangement is used. The culvert through the river bank is fittedwith a flap gate, which is a small version of the tidal door,generally circular and of diameter 300 mm to 1.2 m. Larger flapgates may be rectangular and can be counterweighted to mini-mize the head loss required to open the gate. Counterweightedgates would not be used in tidal situations where waves couldcause cyclical movements leading to damage.
30.13 Pumping
30.13.1 Single or multiple pumps
Pump capacities for land drainage installations are commonly inthe range 500 to 20001/s. It is usual to provide some standbycapacity in pumping stations but, in the case of pumps whichoperate infrequently, a single pump may be appropriate forsmall stations. For larger stations where two or more pumps arerequired it may be appropriate to omit a standby unit, but wherethe design capacity is regularly achieved, one standby pumpshould be provided. Pumps of variable capacity allow flexibilityin running but add to capital costs and complicate automaticrunning.
30.13.2 Motive power
Most modern pumping installations are powered by electricity.This provides the considerable advantages of automatic opera-tion and reduced maintenance. In some rural areas failures ofsupply are not uncommon but these are not often long enoughto cause difficulties.
Where electrical power is not readily available, particularly indeveloping countries, direct diesel-driven pumps are normallyused. For long-life applications the diesel is of the straightvertical cylinder, slow speed, nonautomotive type and generallyrequires continuous attendance.
30.13.3 Pumps
Land drainage pumps are required to produce large outputs atlow heads, generally between 3 and 7 m but occasionally up to10m. This flow-head characteristic falls into the axial andmixed flow bowl range. For the lower heads the axial-flow pumpis used but typically the mixed-flow bowl pump of the verticalspindle configuration is used. A typical pump station layout isshown in Figure 30.17.
With axial- and mixed-flow bowl pumps, small head varia-tions have a relatively large effect on the quantity of waterpumped. Consequently, careful selection of pump and primemover is required to cope with all demands. Also, because of thehead-flow characteristics, the discharge pipe usually has asubmerged termination to provide syphonic recovery. To reduce
Stoplogs forwate r re tent ionand ma in t enance
Emergencypenstock
Tidal f l a p door
C u t - o f f piles
Figure 30.16 Tidal outfall
Figure 30.17 Typical mixed-flow bowl pump installation
the risk of reverse flow a syphon breaker is installed. Thiscomprises a paddle-operated butterfly valve which is held shutby forward flow and opened to atmosphere during reverse flow,thus breaking the syphon.
30.13.4 Control
Unmanned electrically powered pumps are generally stoppedand started automatically by preset level sensors located in thesuction sump. The level-sensory equipment normally comprisesmercury float switches but there is a move to use more sophisti-cated sensory equipment such as ultrasonics because of their lowmaintenance, ease with which preset levels can be changed, and
because they are more compatible with remote instrumentationand automation.
Fully automatic stations are now common in the developedcountries. Details of water levels and pump operation can beconveyed automatically by a telemetry system to a centralmonitoring station. The same system can carry warning alarmsto signal pump failure or other maloperation.
30.13.5 Pump station building
Vertical spindle axial- and mixed-flow bowl pumps are sus-pended in sumps close to the back wall which is curved to reducethe risk of swirl and vortices occurring. To maximize on sump
Water level
Syphon breaker
Control room JPump and motor/
High Water Level
Low Water Level
Figure 30.18 Typical electro-submersible pump installation
Maximum water level
Removablebar screen
Stop log grooves
Control panel Removable handrailControl electrodes
Electro-submersible pump
Ground level
Stop loggrooves
Maximum water level
Flap Valve with chainfor manual operationfor gravity outlet
Automatic screen rake
Stop logs
configuration with minimum civil substructure, particularly fornon-standard applications, sump model tests are often carriedout prior to design.
The main walls of the sump may be of reinforced concrete orsteel sheet piling. Incoming channels are normally screened withsloping steel bars to prevent weeds and large items of debrisentering the sump. Screens require raking either manually orautomatically.
Buildings are generally required to give protection against theelements for plant and operating staff, to avoid damage due tovandalism, and to improve the appearance of the station. Forremote locations and in developing countries little or no super-structure is provided and equipment is suitably rated for theoutside locations.
30.13.6 Other types of pumping installation
For rivers carrying a high sediment load conventional channeloff-takes can become quickly blocked, so requiring continualdredging. One method which has been employed to overcomethis is the use of floating pontoons to carry the pumps, or tocarry the suction pipes from land-based pumps. Such installa-tions are also useful where the water level varies to the extentthat the water's edge recedes leaving a conventional station dry.
The most recent development towards changing the pumptype is the electro-submersible pumpset. This is being favouredbecause it can be located below ground level requiring little orno ground equipment or superstructure. A typical arrangementis shown in Figure 30.18.
References
1 Doorenbos, J. and Pruitt, W. O. (1977) Crop water requirements.Food and Agriculture Organization Irrigation and Drainage PaperNo. 24.
2 Blaney, H. F. and Criddle, W. D. (1950) Determining waterrequirements in irrigated areas from climatological and irrigationdata. US Department of Agriculture - SCS TP 96.
3 Dastane, N. G. (1974) Effective rainfall in irrigated agriculture.Food and Agriculture Organization Irrigation and Drainage PaperNo. 25.
4 Ayes, R. S. and Wescot, D. W. (1976) Water quality foragriculture. Food and Agriculture Organization Irrigation andDrainage Paper No. 29.
5 Booher, L. J. (1974) Surface irrigation. Food and AgricultureOrganization Land and Water Development Series No. 3.
6 Baars, C. (1973) Design of sprinkler installations. Department ofIrrigation, Civil Engineering, Agricultural University,Wageningen.
7 Vermeirei, L. and Jobling, G. A. (1980) Localized irrigation, Foodand Agriculture Organization Irrigation and Drainage Paper No.36.
8 Etcheverry, B. A. (1915) Irrigation practice and engineering.McGraw-Hill.
9 Food and Agriculture Organization (1980) Drainage designfactors. Food and Agriculture Organization Irrigation andDrainage Paper No. 38.
10 Smedema, L. K. and Rycroft, D. W. (1983) Land drainage.Batsford Academic.
11 Van Beers, W. J. F. (1963) The Auger hole method, ILRI,Wageningen.
12 Van Beers, W. J. F. (1979) Some nomographs for the calculation ofdrain spacings, ILRI, Bulletin No. 8, Wageningen.
13 United States Bureau of Reclamation (1978) Drainage manual. USGovernment Printing Office.
14 Ven Te Chow and Yevjevich, V. M. (1964) Statistical andprobability applied hydrology. McGraw-Hill.
15 Bilham, E. G. (1962) The classification of heavy falls of rain inshort periods. HMSO.
16 US Department of Agriculture (1968) A method of estimatingvolume and rate of runoff in small watersheds. US Department ofAgriculture, Soil Conservation Service.
17 National Environment Research Council (1975) Flood studiesreport. NERC, London.
18 Sutcliffe, J. V. (1978) Methods of flood estimation - a guide to theFlood studies report. IOH.
19 Nixon, M. (1959) 'A study of the bank-full discharges of rivers inEngland and Wales'. Proc. Instn. Civ. Engrs, 12, p. 157.
20 White, W. R., Paris, E. and Bettess, R. (1981) Tables for thedesign of stable alluvial channels. Hydraulics Research Station.
21 Vanoni, V. A. (ed) (1975) Sedimentation engineering. AmericanSociety of Civil Engineers, New York.
22 Ven Te Chow (1959) Open channel hydraulics. McGraw-Hill.23 Royal Society for the Protection of Birds and the Royal Society
for Nature Conservation (1984) Rivers and wildlife handbook.24 Holmes, P. S. (1974) 'Analysis and prediction of scour at railway
bridges in New Zealand.' New Zealand Engineering, (Nov) p. 313.
Suggested further reading
Withers, B. and Vipond, S. (1983) Irrigation: design and practice.United Nations (Economic Commission for Asia and the Far East)(1953) River training and bank protection.Brandon, T. W. (ed.) (1987) River engineering, Part 1: 'Designprinciples'. Institution of Water Engineers and Scientists, London.
