The Objectives of storm water drainage

•To prevent erosion in hillside areas (paved roads and terracing are needed)

•To prevent land-slides

•To improve the hygienic conditions with regard to the conveyance of wastewater

•To limit inconvenience to people and traffic

•To limit damage to unpaved roads

•Prevent damage to housing, in case the elevation of ground floor is below street level.

•Collection for reuse purposes, Agriculture use, domestic use and recharge the aquifer

Storm water management : Collection System Design principles

Basic Definitions

• Storm water: Precipitation or rainfall that does not infiltrate into the ground or evaporate into the air.

• Runoff: Storm water, and associated substances, discharged into streams, lakes, sewers or storm drains.

• Watershed: Land area from which water drains toward a common surface water body in a natural basin. 

Components of Storm water drainage system

The main components of the storm water drainage system are:- Pipes- Channels- Culverts- Inlets- Pumping station- Manholes- Gutters

1. Road Drainage : a. Roof type roads b. Channel type roads

Comparison criteria between the methods1. Efficiency2. Operation and maintenance3. Public safety4. Traffic requirements5. Required space6. Cost7. Reliability

Methods of Storm Water collection

2. Open channel drainage 3. Sewer Drainage Circular sewers Elliptical sewersBox culverts

4. Individual property collection Roof collection:a. Roofs of the buildingsb. Green house roofs (agriculture)

Box culvert

Open channel

Circular

Example 1

Two types of concrete storm water drains are compared:

• Pipe, diameter 2.0m, running full

• Open channel, rectangular profile, bottom width 2.0m and water depth 1.0 m

•The drains are laid at gradient of 1.0%, manning coefficient = 0.013

Determine the velocity of flow and discharge rate for the circular drain

Determine the velocity of flow and discharge rate for the rectangular open culvert

W 0.70 1.00 1.00 0.60

H 0.31 0.335 0.365 0.38

A 0.217

0.335 0.365 0.228

R 0.217

0.335 0.365 0.38

V 0.571

0.762 0.8075 0.830

Q 0.124

0.255 0.295 0.189

0.30 0.32 0.35 0.38 0.38

hydraulic calculation of road drainage.

Channel- type roads

Road width= 6 m

Width of street gutter= 0.6 m

Super elevation= 0.08 m or 3%

Kerb height= 0.30 m

Road gradient 1%

Friction factor= 50 (1/n Manning equation)

Roof- type roads

hydraulic calculation of road drainage.

0.30 0.30 0.265 0.23 m

W 0.60 1.20 1.20 Section width

H 0.30 0.2825 0.2475 m

A 0.18 0.339 0.297 m2

R 0.20 0.2825 0.2475 m

V 0.54 0.6807 0.623 m/s

Q 0.097

0.231 0.185 m3/s

Road width= 6 m

Width of street gutter= 0.6 m

Super elevation= 0.07 m or 3%

Kerb height= 0.30 m

Road gradient 1%

Friction factor= 50 (1/n Manning equation)

Channel type Roof type

Information needed for the design of storm water drainage system

1. Metrological and hydrological data

• Rainfall intensity

• Storm duration and occurrence

2. Topographical data

• Boundaries of the catchments areas

• Point of collection

3. Classification of catchments areas

• Industrial, domestic, …..

• Build up areas (run-off coefficient)

4. Soil investigations

• Permeability (run-off coefficient)

Methods of Run-off Computation

Rational method

Q = C i A

Where ;

Q = is the run-off in m3/sec

C = is the Run-off coefficient

i = is the average rainfall intensity in mm/hr ,

A = is the drainage area in m2

Runoff Coefficient (C)

Development Coefficient

Pavement, Road/Parking

0.9

Commercial / Public lots

0.7

Residential Communities

0.6

Parks / Unimproved

Areas

0.3

Irrigation Areas 0.2

Natural Zones 0.05

The runoff coefficient depends on:• The slope of the area• Type of roofs (flat or sloping roofs)• Type of soil, absorption capacity of the soil• Intensity of rain fall, duration of rain fall, previous rain fall.

Only a part of the precipitation upon a catchments area will appear in the form of direct runoff.

Composite runoff coefficient:When a drainage area consists of different surface types (or land use), a composite runoff coefficient is used by applying the weighted average method.

Example 2:

A catchments area has a total area of 0.2 Km2. The land use of this area is

distributed as follows:

Area Code Area (m2) Land Use Runoff- coefficient (C)

A1 3000 Buildings 0.70-0.95

A2 5000 Paved driveways and walks 0.75-0.85

A3 2000 Portland cement streets 0.80-0.95

A4 190,000 Soil covered with grass 0.13-0.17

Find the composite runoff coefficient for this catchment area.

Solution

totalA

CACACACA

comC 4*43*32*21*1

Take the lower value for the range of the C:

16.0200000

13.0*1900008.0*200075.0*50007.0*3000 comC

Take the higher value for the range of the C:

21.0200000

17.0*19000095.0*200085.0*500095.0*3000 comC

(For conservative design use the higher value of Ccom .)

Drainage area

The drainage area is determined according to the topography. The boundaries of each drainage area (catchment's area) are called watershed lines.

Precipitation and evapotranspiration

Rainfall can occur in several ways from very short rains with high intensity (tropical storms) to rains even during several days with low intensity (drizzle)

In hydrologic studies the following aspects are important:

• Annual rainfall and distribution over the year

• Short term intensity

• Arial rainfall

• Quality of rainfall

Measurement of rainfall: Rain gauges: The ordinary rain gauge for manual observation is normally standardized within a country.

Analysis of rainfall data

Estimating areal rainfall from point rainfall:

• Arithmetic mean

• Thiessen method: depends on the area

• Isoyetal method: depends on the area

19.2

14.6

26.9

45.0

50.029.8

6.5

15.4

17.5

19.5

28.2

10

10

20

20

30

30

40

40

Effective Rainfall

Assessments of effective rainfall provide an indication of how much of the rainfall over an aquifer outcrop actually contributes to the recharge of groundwater .

The effective rainfall from year 1982 till year 2004 is calculated based on the FAO general formula for effective rainfall (Pe.) :

Pe. = 0.8 * P - 25 for average rainfall (P) > 75 mm/month

Pe. = 0.6 * P - 10 for average rainfall (P) < 75 mm/month

I=aTb

Where; I is the rainfall intensity (mm/min),

T is the duration time (min),

and a, b are constants and related to the number of return years.

This equation is fit for Gaza Strip rainfall condition

Intensity return period

Design frequency of rainfalls

• sewers in residential areas: T= 1 to 2 years

• sewers in business areas: T= 2 to 5 years

• flooding caused by rivers: T= 10, 25, 50, 100, 500 years

Return Period: 2 years – a: 4.06 – b:-0.636

Duration 5

min

15

min

30

min 1 h 2 h 3 h 6 h 12 h 18 h 24 h

Pj= p24h X

0.875

Rainfall

(mm) 7.3 10.9 14 18 23.2 26.9 34.6 44.5 51.6 57.3 50

Return Period: 5 years – a: 6.18 – b: 0.649

Duration 5

min

15

min

30

min 1 h 2 h 3 h 6 h 12 h 18 h 24 h

Pj= p24h X

0.875

Rainfall

(mm) 10.9 16 20.4 26 33.2 38.2 48.8 62.2 71.7 79.4 69

Return Period: 10 years – a: 7.95 – b: 0.660

Duration 5

min

15

min

30

min 1 h 2 h 3 h 6 h 12 h 18 h 24 h

Pj= p24h X

0.875

Rainfall

(mm) 13.7 20 25.3 32 40.5 46.5 58.8 74.4 85.5 94.2 82

Design Periods of storm water facilities

• Drains: 30-100 years

• Sanitary sewers:

concrete, asbestos cement pipes: 10-60 years

glazed stone ware pipes: 40-100 years

Plastic (PVC, PE): 20-30 years

• Pumping Stations:

buildings, concrete works: 20-80 years

equipment (pumps, drives, etc.,) 10-20 years

Time of Concentration (Tc)

The time of concentration is the time associated with the travel of run-off from an outer point, which best represents, the shape of the contributing areas.

The Kirpich formula will be suitable to be used in determining the concentration time for over land run-off flows:

Tc = (L) 1.15 / ( 52 (H) 0.38 )

Where; Tc is the Concentration time in minutes,

L is the Longest path of the drainage area in meter,

H is the Difference in elevation between the most remote point and the outlet in meters.

If the duration of the rainfall (tr) is equal to the time of concentration (tc), then the total run-off gradually increase to the peak discharge.

Q Q

tc=tr tc tr

Example 3

A4

A3

A2

A1

0.5 hr

0.5 hr

0.5 hr

0.5 hr

Triangular basin of 20 km2 surface area.

A1= 2 km2 Run-off coefficient= 0.8

A2= 4 km2 constant rainfall intensity= 0.1m/hr

A3= 6 km2 Time of concentration= 2 hours

A4= 8 km2

Time in hr.

A1 A2 A3 A4 Total

00.51.01.52.02.53.0

00.160.160.160.160.160.16

000.320.320.320.320.32

0000.480.480.480.48

00000.640.640.64

00.160.480.961.601.601.60

A2= 400 du

C2=0.7

T2= 5 min

A1= 300 du

C1= 0.3

T1= 15 min

Example 4Use the rational method to find the 10 –years design runoff for the are showing in the figure.

• Time of concentration: Tc = t1 + t2 = 15+5 = 20 min

• Runoff coefficient: C = {(3x0.3)+ (4x0.7)}/7 = 0.53

• Rainfall intensity: I = 32 mm/hr.

• Design peak runoff: CIA= 0.53 x 22 x 7= 82 m3/hr.

Duratio

n

5

min

15

min

30

min 1 h 2 h 3 h 6 h 12 h 18 h 24 h

Rainfal

l (mm)

13.

7 20

25.

3 32

40.

5

46.

5

58.

8 74.4 85.5 94.2