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है”ह”ह

IS 12731 (1989): Hydraulic design of impact type energydissipators - Recommendations [WRD 9: Dams and Spillways]

Indian Standard

’ IS 12131 : 1989

HYDRAULIC DESIGN OF IMPACT TYPE ENERGY DISSIPATORS - RECOMMENDATIONS :

UDC 627-838 :624-04

@J BIS 1991

BUREAU OF INDIAN STANDARDS MANAK BHAVAN, 9 BAHADLJR SHAH ZAFAR MARG

NEW DELHI 110002

May 1991 Price Group 6

Spillways Including Energy Dissipators Sectional Committee, RVD 10 : . 1

FOREWORD

This Indian Standard was adopted by the Bureau of Indian draft finalized by the Spillways Including Energy Dissipators by the River Valley Projects Division Council.

Standards on 23 June 1989, after the Sectional Committee had been approved

Energy dissipators are used to dissipate excess kinetic energy possessed by flowing water. This energy or velocity head is acquired by the water where the velocity is high, such as in a chute or drop and energy dissipators are incorporated into the design of these structures. An effective energy dissipator must be able to retard the flow of fast moving water without damage to the structure or to the channel below the structure.

Impact type energy dissipators direct the water into an obstruction that diverts the flow in all directions and in this manner dissipates the energy in the flow. into a pool of

In some structures the flow plunges wate t where the energy is diffused. BatHed outlets, baffled aprons, check-drops and

vertical stilling wells are examples of impact type energy dissipators ( se.~ Fig. 1 ).

The impact type energy dissipator is considered to be more efficient than the conventional hydraulic jump type. Generally, the use of an impact type energy dissipator results in a smaller and more economical structure,

Two most widely used impact type energy dissipators are baffled apron drops and baffled outlets which are covered in this standard. The baffled apron drops are used in canals or waste way channels to provide dissipation of energy at drops in grade. It can also be used for small spillways where foundation conditions are not favourable for providing a conventional energy dissipator. Baffled outlets are suited to pipe outlets ( flowing full or part ) or open channel outlets letting down discharge into a canal or a small water course.

For the purpose of deciding whether a particular requirement of this standard is complied with, the final value, observed or calculated, expressing the result of test or analysis, shall be rounded off in accordance with IS 2 : 1960 ‘Rules for rounding off numerical values ( revised )‘. The number of significant places retained in the rounded off value should be the same as that of the specified value in this standard.

IS 12731: 1989

Indian Standard

HYDRAULIC DESIGN OF,IMPACTTYPE ENERGY DISSIPATORS - RECOMMENDATIONS

1 SCOPE 1.1 This standard covers rccommcndations on hydrau-

lic design of balncd apron drop and baffled outlet type

cncrgy dissipators.

2 TERMINOLOGY 2.0 For the purpose of this standard, the following

definitions shall apply. Examples of various impact

type energy dissipators arc given in Fig. 1.

2.1 Baffle A cross wall, set of vanes or blocks or similar dcvicc

placed across the llow to cvcnly spread the Ilow in a11

directions, cffcct more uniform distribution ofvclocity

and dissipate cncrgy.

2.2 Depth of Entrance Flow a) For the bafllcd apron drops, il is the actual

depth of flow at the cntrancc section.

b) For the baffled outlets. the depth of cntrancc

flow shall bc taken as the square root of the

cross-sectional area of the cntrancc flow

irrespcctivc of the actual shape of the arca.

2.3 End Sill A vertical, srcppcd, sloped or dcntatcd wall con-

structcd at the downs&cam end of the stilling basin.

2.4 End Walls ( for Baffled Outlets ) Walls of the stilling baiin on both the sides, flared out

at 45” to the direction of the flow.

2.5 Entrance Section Top horizontal portion at the beginning of the bal’flcd

apron drop.

2.6 Entrance Sill A sill placed at the end of the cntrancc section to the

chute tocnsurc subcritical Ilow in the cntrancc section.

2.7 Froude Number Froudc number of the cntrancc Ilow Ibr the baflcd

outlet.

2.8 Riprap Protection to the cmbankmcnt material against erosion

due to wave action, velocity of flow, rain wash, wind

xlion, clc, provldcti by plxing i\ prolcclion loycr 01 rock fr:p~cnls.

2.9 Side Walls ( for BaiIled Apron Drops ) Walls on both the sides of the baffled apron drop to

contain the Ilow.

2.10 Stilling Basin A short length of paved portion at the exit course of an

outlet structure or below a spillway, chute, or drop, in

which major part of the energy of flowing water is dissipated and water is discharged into the down-

stream channel in such a manner as to prevent damage

to the structure or dangerous scour of bed or banks of

channel.

2.11 Wing Walls ( for Baffkd Apron Drops ) Walls constructed at the foot of the drop, in continu- ation of the side walls and normal to the direction of the tlow. to dccrcasc percolation and to retain the back fill along the slope.

3 SYMBOLS

3.1 The symbols used in the standard are given below:

A = c, = d = d, = d, = d, = I?= = ES” =

e =

e, = F, =

f = g =

II =

=

II* =

Cross-scctlonaiaroa of the entrance flow

Depth of cut-off at the end of the chute

Pipe diameter

Depth of Flow at the entrance

Critical depth = ( q2/g )‘lJ Depth of llow in the upstream channel

Energy of the flow over the entrance sill

Energy of the flow in the upstream

channel

Hcightofthecnd wallat thccndsillof the

stilling basin

Height of the end sill

Froudc number of ihe entrance liow as

defined in 5.7

Dimension of the fillets of the baffle

Accclcration due to gravity ( usually

g = 9.81 m/s2)

Height of the end walls in the portion of length I, of the stilling basin ( for baffled outlet )

&ightof the bafflc block ( for the baffled

apron drop )

lfcighl of the sitlc walls ( for the bnfllcd

apron drop ) mcasurcd normal to the

chute slope

-

IS 12731:

t1, =

hn = hs = i =

L = I1 =

1, =

P =

'. v, = v, = V" = w=

= l

x =

1989

Height of the side walls mcasurcd vcrti- tally Net head to bc dissipated Height of the cntrancc sill Total number of layers of filter below riprap .

Total length of stilling basin Length of the stilling basin up to the baffle Length of the stilling basin downstream of the baffle up to the end sill Projection at the top of the divide walls inside the basin, over the portion of length 1, Discharge Unit discharge Dimension of the inner web of the baffle Thickness of web of the baffle Thickness of flange of the baffle Velocity of flow cntcring the basin . Velocity of flow over the end sill Bottom velocity of flow downstream of the stilling basin Critical vclocily Mean velocity Velocity of fIow in the upstream channel Width of the chute ( for bafllcd apron drop) . Width of the stilling basin ( for bafllcd

0u1lc1 )

Height of thccnd sill over the bed of the discharge channel downsticam of the still- ing basin

4 HYDRAULIC DESIGN OF BAFFLED APRON

DROPS TYPE ENERGY DISSII’A’I’ORS

4.1 Basically, the baffled apron or chute consists of a sloping apron, usually on a 2 : 1 or flatter slope, with multiple rows of baffle blocks equally spaced along the

chute as shown in Fig. 2. The flow passes over, around

and bctwccn the baffle blocks and appears to slow down succcssivcly at each bafllc block. The ability

or the bafllcd apron drop to accommodate a widely lluctuating tail water clcvation makes it cspccially suitable as an cncrgy dissipator at the end of a canal or waste way. The length of the baffled apron dots not affect the cfficicncy of the structure. It is cffcctivc in dissipating excess cncrgy for drops ol’ any magniludc

but it nlaj~ bccQmc uneconomical for lnrgc Ilows with

g-cot depths, due to the wide section and numerous

blocks rccluircd. Whcrc ‘an cxccss of, trash, trees or

weeds accompany the Ilow. t&y may bccomc lodged

in the bafllc blocks restricting the flow. Removal 01

this material is somctimcs difficult.

4.2 Gcncralizcd design procedure is suggested in this standard for dctcrmining the principal dimensions of the baffled apron drops for a unit discharge (discharge per unit width of the chute) not exceeding 30 m3/s per mstrc and the approach velocity less than the critical velocity based on the unit design discharge. In case whcrc approach velocity exceeds critical velocity in- corporation of entrance sill to make it subcritical shall bc done as given in 4.3.2.

4.3 Design Criteria

The principal fcaturcs of hydraulic design of baffled apron drop consist of dctcrmining the size and dimcn- sions in rcspcct of:

‘a) width of the chute and entrance section, b) dimension and spacing of baffle blocks, and c) heights of side walls and wing walls.

4.3.1 The width W of the chute and the width of the cntrancc’ section shall be the same and detcrmincd such that the unit discharge does not exceed 30 m3/s per mctre width. The depth of the entrance flow d, shall bc such that the avcragc entrance velocity of flow is slower than the critical velocity V : c

whcrc V = (gq)1/3 c

The lcngtb of the cntrancc section shall be atlcast equal to 2 d,.

4.3.2 If for a given clcvation of the inlet floor, the approach velocity is grcatcr than V,, subcritical flow ( and hcncc the entrance velocity slower than V,) can bc cnsurcd in the entrance section by providing an entrance sill of height tS at the end of the entrance section. The rcquircd hclghl of the sill above the inlet floor shall bc worked out from the energy balance bctwccn the inlet and the upstream channel :

Thus ES” = E= + hi + hs or hS = EsU - ESc - hl

whcrc

the cncrgy of the flow in the upstream ctianncl and the cncrgy of the Ilow over the sill, rcspcctivcly.

v2 v2 hi = 0.5 -A-. - -!!-

2g 28

2

and thus

h, =du -d/1.5

Altcrnativcly, IaS can bc worked out considering it as a weir tocnsuresubcritical flow in thcapproach channel.

4.3.3 The dimensions and spacing of the baffle blocks along the chute as well as height of the side

walls shall bc dctcrmincd as follows:

a)

b)

cl

d)

6

f)

6)

h)

j)

k)

Set the longitudinal slope of the chute floor and side walls al 2 : 1 or llaucr.

The height of baffle 11 shall bc 0.8 d,, whcrc

d, is critical depth.

Set the first row of baffle blocks which is in the form of a scrrarcd horizontal broad crcstcd weir, immcdiatcly starling from the sill, as shown in Fig. 2. The configuration ol’ the short

and long sections is important and the long section, cvcn though notof full width, shall be placed next to the side walls.

Place the second row of the baffle blocks at a distance 4 If measured along the slope from the sill.

Place the subscqucnt rows of the baffle blocks at 2 t/ interval, but not grcatcr than 2 mctrcs.

Dcterminc the dimensions of the baffle blocks, the spacing bctwecn the baffle blocks and spacing bctwccn the two rows of the baffle blocks as well as the height of the side walls and wing walls, as shown in Fig. 2.

Alternate rows of bafllc blocks should bc slag-

gercd so that each block is downstream from a space in the adjacent row.

Adjust the width of thcchutc W or the width

and spacing of the baffles so that convcnicnt

baffle block widths can bc used.

A minimum of lhrcc rows of baffle blocks ( in addition to Ihe cnlrancc weir ) shall bc used. The baffled apron shall bc cxtcndcd so ihat the lop of atlcast one row of baffle blocks will bc bclow.thc bottom grade of the outlet chan- ncl as shown in Fig. 2.

In addition to the dissipation of the cncrgy of th$ flow, adcquatc protcclion in the form of gravel or riprap of suitable size shall bc provided on each side of the structure from

the top of the slope to the downsucam wing-

wall cxtcnding lavxally a dismcc cqual LO

the wall height. RockKill al the bottom OC the

apron may not lx ncccssary.

ml

4

IS 12731 : 1989

The stability of the structure shall be checked against uplift ( assuming a sudden cessation of the flow ) and sliding ( assuming removal of the earth material downstream by erosion ). The provision and details ofcut-offs at the upstream and downstream ends shall accord- ingly be delermined. .

The preliminary design worked out as above shall beconfirmed by hydraulic model studies for satisfactory performance.

4.4 Sample Computation .

A sample computation given in Annex A may prove helpful in designing a baffled apron drop.

5 HYDRAULIC DESIGN OF BAFFLED OUTLETS

TYPE ENERGY DISSIPATORS

5.1 The stilling basin for the baffled outlet is contained in a small box like structure, which requires no specific Lail water for successful performance. Generalized design procedure is suggested in lhis standard for

’ dctcrmining Lhe basin size and critical dimensions for a range of cntrancc velocity up to 15 m/s and Froude number from 1 to 9.

5.2 Energy ‘dissipation in the stilling basin is initiated by flow striking the vertical hanging baffle and being iurncd upstream by the horizontal portion of the baffle and by Ihc floor which leads to formation of vertical cddics and turbulence. The structur6,>herefoce, re- quircs no specific tail water for energy dissipeon.

5.3 In addition to the dissipation of the energy; the stilling basin, the bed and banks of the canal or the waler cobrsc have to be protcaed against erosion, by providing riprap.

5.4 This type of stiliing basin is subjected to large dynamic forces and turbulcncc which must be consid- crcd in the structural design. The structure shall be

made sulficicntly stable to resist sliding against the impact load on the baffle. The entire structure shall resist sevcrc vibrations inherent in this type of device, and the individual structural members shall be suflicicndy strong to withstand the large dynamic loads.

5.5 When a structure is required to be designed for vclocilics or Froude numbers exceeding those spccificd in 5.1. or lo wilhstand unusual hydraulic conditions, model studies for the spccific’case under consideration shall be conducted.

5.6 Design Criteria

The principal fcaturc of hydraulic design of impact lypc cncrgy dissipator consist of dctcrmining the

3

IS 12731 : 1989

following pilJlITlClCfS : a) Thcorctical maximum cntrancc velocity and

Froudc number of the cnlrancc flow;

b) The apron clcvation,

c) Si/& of the stilling basin and appurtenant struc-

tures; and

d) Riprap and filter downstream of the basin.

5.7 Entrance Velocity and Froude Number

b)

cl

d)

c)

Dctcrminc the net maximum head hn to bc

dissipated, as the diffcrcncc bctwccn the

maximum upstream water lcvcl and ccntrc

lint of the pipe al the stilling basin or the

invert ofthc open channel at the stilling basin

( as the cast may bc ), accounting for the fric-

tional and other losses in the system.

Entrance velocity may bc compuicd by the

formula v, = m

Cross-sectional arca of”the flow at the cn-

lrancc may bc computed from the lotal dis-

charge Q and the cnuancc velocity V,, from

the formula

A + 1

Depth of cntrancc flow may be calculated

from the formula

d, =K

Froudc number of the cntrancc flow may bc

dctcrmincd from the formula

Fl = vJm-- 5.8 Apron Elevation Unlike the aprons for the hydraulic jump type stilling

basins, thcrc is no special rcquircmcntol the apron

clcvation in rcliltiOn lo lhc lail waler clcvalion, for Ihc

impact lypc cncrgy dissipal()r. Tail waler as high as

(e,+ 7‘/2) (Fig. 3) will improve lhc pcrli)rmancc

by reducing exit vclocitics, providing smoother waler

surface and reducing tcndcncy towards erosion.

Howcvcr, cxccssivc tail waler lcvcl causing some llow

to pass over the bal’lk should bc avoidctl, as fnr as

possible.

5.9 Size of the Stilling bin and Appurtenant

Structures

59.1 Wid~hofhasin (W) shall bcdctcrmincd with lhc

help of Fig. 4A, ulilising the values of d, and I;, as cal-

culalcd from 5.7.

59.2 The other rclcvam dimensions of the stilling

basin and appurtcnanl structures LO the basin wldLh 1Y

shall bc rclatcrl in accordance with Fig. 3. The

dimensions I, and lP arc the suggc~~~l minimum

lhickncss for the hangmg bafllc and arc not rc’latctt IO

the hydraulic pcrformancc of the structure.

5.9.3 The notches shown in Fig. 3 are provided to aid in cleaning out the basin after prolonged non-use of the

structure. If cleaning action is not considered neces-

sary, the size of the notches may be reduced or

climinatcd all togcthcr.

5.9.4 The invert of the entrance pipe shall be kept at

the clcvation as shown in Fig. 3, in line with the

bottom of the baffle, regardless of the size of pipe. If

the cnlrancc pipe slopes downward, the outlet end of

the pipe shall bc turned horizontal, or the invert shall

bc filled to form a horiyantal surface for atleast one

pipe diamctcr upslrcam from the portal. For slopes 150

or greater, the horizontal length of pipe shall be atleast

thrice the diamctcr.

5.9.5 If the pipe flows partially full, it shall be vented

at the upstream end. The diameter of the vent shall be atlcasl one-sixth the diameter of the pipe.

5.9.6 If the flow cntcrs the basin through a rectangular open channel, its invert shall be decided in the same

way as that for a pipe. Its width shall bc less than the

basin width. The channel walls shall bc as high as the

basin width and the invert shall be horizontal for a

minimum of three channel widths upstream from the basin.

5.10 Riprap and Filter Downstream of the Basin

To prcvcnt damage of the bed and banks of the canal

or water course by the erosive action of the flow

passing over the end sill of the stilling basin, riprap is

usually placed on the bed and banks of the downstream

channel. Scvcral factors affect the stone siy& required

to resist the forces which tend LO movcriprap. In terms

of llow Icaving a stilling basin, these factors are vcloc-

ity, flow direction, turbulcnccand waves. The purpose

of this section is to give the design cnginccr a

guidclinc to dctcrminc the size of riprap to bc used

downstream of the stilling basin and to dctcrminc the

type of liltcr or bedding material placed below the

riprap.

5.10.2 A tentative curve giving the minimum stone diamctcr as a function of bottom velocity Vb is shown

in Fig. 5. The bottom velocity, Vb at which the flow

strikes the riprap may bc best asccrtaincd from model

studies. Howcvcr, for the purpose of preliminary

design, the bottom vclocily may bc calculated as given

in 5.10.X

5.10.3 For the compuicd values of entrance velocity

V, and Froudc number I;,, calculate the velocity over

the end sill V2, usmg Fig. 4B. If the bed of the dis-

charge channel downstream of the basin is at the same

clcvation as the top ol the end sill, bollom velocity V,

4

l--

I

will be the same as the velocity over the cn(t sill Vz.

If, however, the bcd of the discharge channel is at adistance X below the top of the cn(i sill as shown inFig. 3, an approxirnalc cslimalc of lhc bouom velocityVb can bc obtained by multiplying the mean velocityin the downstream (Vm = q / diffcrcncc bctwccn theTWL and bed of the discharge channel) by aturbulence correction factor appropriate to the flowconditions. In thepresent case, this factor may bc takenas 1.4.

5.10.4 The required stone (iiamcxcr may bc [ictcrmincdfrom Fig. 5. More than 60 pcrccnt of the riprapmixture shall consist of stones which have length,width and thickness dimensions as nearly alike aspracticable, and bc of the size indicated by the curvein Fig. 5, or larger and shall not bc flat slabs.

5.10.5 Besides the size and weight of the individualstones, other factors that alfcct satislautory perform-ance of a protective riprap arc the type of filter rnatcrialplaced beneath the riprap, the thickness of each layerand its possible pcrm~ability to water and sand. Atypical example of filter construction is shown inFig. 5 which may bc used as a guideline.

5.10.6 To prevent a filter Iaycr from lifting by waterentering thcchanncl through bcd or banks, the pcrnlca--bility to water of the construction as~ whole and of

each separate layer shall bc greater than that of the

underlying material. To maintain a sufficient per-meability to water, the following condition shall bcsatisfic@.

~15 (i th layer) d,, first Iaycrdl~ ( i_ 1 Ill Iaycr) =

d15 subgmdc=5t040

5.10.7 Depending upon the shape and gradation of thegrains, roughly the following ratios be used :

Lfalcrial Ratio

To

Homogeneous round grains (gravel) 5:10

Homogeneous angular grains 6:20(broken gravel, rubble)

Well gra(icd grains 12:40

prevent the filter from clogging it is advisable thatd~ of a layer bc larger than 0.75 mm.

~-5.10.8 A riprap protection may fail bccausc waves orground water flowing into the charmcl remove materialfrom beneath the riprap. To prevent the loss of finematerial from an undcrl ying fiItcr material or sub-gradc, the following rcquircmcnts with regard toimperviousness to sand shall bc met:

IS 12731:1989

dl~ first layer= = 5

~, subgrade

d~Ofirst layer= =5t060

d~Osubgrade

5.10.9 The mtio in the equation given in 5.10.8depends on the shape and gradation of-the grains asgiven below :

lvfaterial Ratio

Homogeneous round grains (gravel) 5 : IO

Homogeneous angular grains 10: 30( broken gravel, rubble)

Well graded grains 12: 60

5.10.10 If it is uneconomical to compose a specialmixture and locally available material is to be used forthe filter, the sieve curves for the subgrade and filtermaterial Iaycrs should run about parallel forthe smalldiamclcr grains.

5.1.0.11 The following thickness for the filter layersshall bc regarded as a minimum for a filter construc-tion mitdc in dry condition.

ikfaterial Ratio

Sand, fine gravel 0.05 m to 0.10 m

Gravel 0.10 m to 0.20 m

Stones 1.5 to 2 times thelargest stone dia.

With filter constructed under water, these ~icknesse~have to bc increased considerably.

S.1O.12 In case, the concrete blocks or slabs are to beused in place of riprap, the openings between themshall not be greater than 0.5 times d8~ of the under-lying material,

5.10.13 At structure-to-filter and filter-to-unpro-tcctcd channel joints, the thickness of the filter shall beincreased as shown in F@ 5.

5.10.14 The protective riprap shall be laid at least onebasin wide downstream of the stilling basin.

5.10.15 Sample ComputationA sample calculation given in Annex B may provehelpful in designing a baffled outlet.

5

IS 12731 : 1989

ANNEXA

(Ckzuse4.4)

DESIGN OF RAFFLED APRON DROP

(Sample Calculations)

A-l DATA

Q = 200 m3/s ( design discharge )

W = 15 m ( chute width )

d, = 4.2 m (entrance flow depth)

Slope of the chute = 2.5 : 1

Elevation of the entrance floor ’ = cl. 51.00 m

Elevation of the downstream = cl. 36.OOm channel

A-2 COMPUTATIONS

200 Unit discharge q = 15 = 13.33 m3/s/m

(Icss than

Critical vclocily V, = w 30 mYs/m)

= VT07 m,s

Actual cntrancc velocity 4 13.33 v, =d=-pj-

1 *

which is less than critical velocity

= 3.17 m/s

Hcncc the cntrancc arrangcmcnt is pcrmissiblc and cntrancc sill is not ncccssary. Length of the cnuancc section =2d, = 8.2 ( Min )

Dimensions of the Baf!le Blocks

First Row

H=0.8dc =0.8x

3 13.33 x 13.33 = 0.8 x \I = 2.lm

9.81

Wjdth of the individual block of = I .5/l scrratcd weir = 3.15m

Longer section of the scrratcd = 1.25 II weir = 2.625 m

Shorter section of the scrratcd = 5/8 It weir = 1.312 m

Portion adjacent to side walls

Adopt

Width and spacing of the full width block

Part width blocks near the side wall

Longer section

Shorter section

&d%equent Rows

Distance from the first row

Baffle block width = 4m

Baffle block height = H = 0.8 dc = 2,lm

= 4m Spacing between the blocks

Spacing between the subsequent rows of the baffle blocks

I-leight of the side walls H,

llw = 3H =3x2.1 =6.3m

Wing Walls

II, = YV - Hw ( fi)

cos 0 S

= 0.5 tt = 1.05 m

= 4m

= 1.5m

= 2.6Om

= 1.3m

= 4H =4 x2.1 = 8.4 m

=2H = 4.2 m

where s is the slope of the chute expressed as s : 1 (horizontal : vertical)

Here s = 2.5, HW = 6.3

II, = 6.3 JizF

2.5 = 6.785 m say 6.8

Length of the wing walls = 1.5 x H, = 10.177 say 10.20 m

Cut off at the end of the chute, C,

For flow depth exceeding 2 m, adopt C, = 1.0 m Fig. 6 shows the general arrangement of the baffled apron drop designed above.

6

IS 12731 : 1989

ANNEX B

(Clause 510.15)

DESIGN OF BAFFLED OUTLET

(Sample Calculations)

B-l DATA

Maximum upstream water lcvcl = cl. 97.60 m

Maximum tail water lcvcl

Design discharge ’

Pipe diameter

Centre lint elevation of pipe

Average ground lcvcl

Head loss in the pipe system

= cl. 86.40 m

= 8 m’/s

= 1.20 m

= cl. 86.00 m

= cl. 85.40 m

=0.6Om

B-2 COMPUTATION

NCL head hn = (97.60-86.00) - 0.60 = ll.Om

Entrance velocity V, 42 g x 11.00 = 14.7 m/s

Cross sectional area = A =Q/V, = 8 of flow at cntrancc 14.7

= 0.544 mz

Depth of cntrancc Bow

,=d, =fl= /=4 = 0.737m

Froudc number of the entrance flow

Since the entrance velocity and Froudc number arc within the range spccificd in 5.1, an impact type cncrgy dissipator will bc

suilablc in this cast.

W USC Fig. 4A with F, = 5.47 to find 7

I = 7.5

Hcncc, W = 7.5 x. 0.737 = 5.52 m; adopt W = 5.50 m

Calculate other relevant dimensions of the still- ing basin from Fig. 3.

Since the roof of the pipe is higher than the tail water elevation, the pipe will flow partially. Hcncc the pipe should have a vent diameter of

atlcast y = 200 mm

Apron lcvcl of the stilling basin

=TwL- e, + 7‘ ( 2 )

Since 5.50 e, X-z--=

6 6 0.917m say0.90m

and

7’~ JK =3~z2&j3m~y2.Jom 8 8 -

Apron lcvcl = 86.40 - (

O-90 + T )

= el. 84.50 m Top of end sill = cl. 84.50 + e, = e1.84.50 + 0.90 = cl .85.40 m.

v, Enter Fig. 4B with F, = 5.47 to get - = 0.15

vi

Thcrcforc, velocity of flow over end sill V, = 0.15 x vr

V, = 2.2 m/s = V,

Enter Fig. 5 with Vb = 2.2 m/s

Stone diamctcr of the riprap required = 350 mm

The riprap pro&lion should bc extended at least one basin width, that is, 5.5 m downstream of the stilling basin.

IS 12731:1989

GATEI 1

*

Uls w L.-

7’(

--— --— — D/s w.

------ -

——.~------ .-~...,.-..’,..a. -.

-;. .-.

ID

1A Typical Check Drop for a Canal

---1 U/S W L (d 15&2.L!n)

16 Baftled Outlet for a Canal

- [,20..BAFFLE BLOCKS 1300~225mm AT

-------- ---1 bx

STEPS 1065.x 152.5mm “ ‘–- u . .

*lC Baffled Chute with Stilling Basin ( for a Lower Jhelum Outfall Structure, India )

u/S WL (cl IS.f.gm) PERFORATEDBAFFLE

\el. 28.5S.0/S WJ-—— —-

Perforated Bathed Stilling Basin for a Canal Fall Stilling Basin ( Cambay Brartch of Mahi Canal, India )

BAFFLE BLOCKS— :

J

.

m p-—.

In

I

J

PLAN

IE Baflled Apron Drops for Canals and Small Spillways

T

‘SLEEVE VALVE

IF Vertical Sleeve Valve Stilling Well

FIG. 1 VARIOUSIMPACTTYPEENERGYDISSIPATORS

8

IS 12731 : 1989

ENTRA

SIDE

Bh A

Y

‘7 ENTRANCE SECTION

-

*- Min.2d,

1 -%CTfON ‘V

A Y I III

uu

llll rm

I-JU

CID r-m

Dll

nn nn l-m

I 111

SlOE WALL I

e--J

I’ lt WING WALL

--I

FIG. 2 BAFFLED APRON DROP ( GENERAL ARRANGEMENT )

9

IS 12731 : 1989

I- STILLING BASINS 4

PLAN

I ‘ EN0 SILL

SECTION -X X

r END WALL

I X __ t Ii = 3/L w L = b&W 4 = t/2 W 12 = 5/e w

--L5O c tl/2W

et = l/6 w

P = I/zW

T = 3/6W

tb. tpgi50-200mm

f .‘75 -150mm

SECTION-Y Y

DOWNSTREAM ELEVATION

FIG. 3 BAFFLED OUTLET - TYPICAL DETAILS

10

IS 12731 : 1989

2 3 L

F, - FROUDE NUMBER

4A

6

0 1 2 3 L 5 6 7 8 3

F,= FROUDE NUMBER

48

FIG. 4 BAFFLED OUTLKT - DHGN CURVES

11

IS 12731 : 1989

1000

500

100

t 50

:

: w I 9 10 0

z 5

2 u-l

1

0.2 0.1 0.5 1.0

FINE GRAVEL /\ *\

EXAMPLE OF FILTER CONSTRUCTDN

rRIpRAp

PROTECTEO-+----UNPROTECTED

INCREASE IN FILTER THICKNESS AT THE JOINTS OF PROTECTED AND UNPROTECTED PORTIONS

BOTTOM VELOCITY, m/s -

FIG. 5 BAFFLED OUTLET ( DETAILS OF RIPRAP PROTECTION:)

u SEC A-A

WINGWALLS’------ -----z \R,PRAp ON 1 Ct’lO SIDE SLOPE

SEC B-B

UlJl

1.5

t 1

l-m

PART PLAN

FIG. 6 GENERAL ARRANGEMENTS OF BAFFLED APRON DROPS

12

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