Storm Runoff Estimation

STORM RUNOFF ESTIMATION

Peak run-off rate depends on.

❖ Type of precipitation.

❖ Intensity and duration of rainfall.

❖ Rainfall distribution.

❖ Soil moisture.

❖ Direction of storm/duration/storm frequency.

❖ Climatic conditions.

• Shape, size, type of catchment basin.

• Impermeability factor.

 

(a) Rational Formula:

❖ Assuming that if rain falls on an impervious surface at a constant rate, the resultant runoff from the surface would be equal to the rainfall.

Runoff rate = Rate of rainfall.

Time of Concentration:

Period after which the entire catchment area starts contributing to the runoff in drains.

 

Critical Rainfall Duration:

Maximum runoff obtained from rain having duration equal to the time of the concentration

Based on the basic principles, the rational formula was evolved by Fruhling, Kuichling lioz bied,menuM (S).

Q_p → Peak rate of runoff in cumecs

K → Co-efficient of runoff

A → Catchment area in hectares

P_e → Critical rainfall intensity (cm/hr).

 

Co-efficient of Runoff (Impervious Factor)

❖ The co-efficient of runoff (k) is the impervious factor of runoff, representing the ratio of precipitation to runoff.

❖ The value of k increases as the impreviousness of the area increases.

k = Ratio of precipitation to runoff

❖ Value of k varies with the type of surface.

k=1 → Perfectly impervious surface.

[For parks, lawns, gardens, k = 0.05 -0.25

Wooden lands, k = 0.01 -0.20]

Average Impermeability Factor (I) (k):

A₁,A₂, A₃ → Area of different surfaces of catchment.

I₁, I₂, I₃→ Corresponding impermeability factors for different surfaces.

Intensity of Rainfall:

❖ It is expressed in cm/hr.

❖ Automatic rain gauges measures the rainfall intensity in cm/hr and is plotted on graph.

❖ Readymade curves, standard depth-duration curves of a particular rain gauge station, representing different frequencies are provided by Meteorological Department.

❖ The value of intensity obtained is the rainfall at the rainfall gauge station and is called as point rainfall intensity.

❖Intensity at any point on catchment area = Point rainfall intensity × Areal Distribution Factor (dispersion factor)

 

Time of Concentration:

i) The Inlet Time (or) Overland Flow Time (or) Time of Equilibrium

The time taken by water to flow overland from the critical point upto the point where it enters the drain mouth.

T_i = Inlet time in hours.

L = Length of overland flow in kilometers.

H = Total fall of level (m) from critical point to mouth of drain.

 

ii) Channel Flow Time/Gutter Flow Time

Time of flow from drain channel mouth to considered point.

Time of concentration,

❖ Intensity of rainfall during T can be easily obtained from standard intensity duration curves.

❖ In the absence of standard intensity - duration curves, following formulae is used.

 

a) Localities with frequent rainfall (5 year frequency)

b) Rains with frequency of 10 years

c) Rains with frequency of 1 year

d) Kuichling's formula

(b) Empirical formula for computing the peak discharge for larger catchment areas

 

(i) Burkli - Ziegler Formula:

 

ii) Dicken's Formula: (North Indian Catchments):

Qp = CM^3/4

M → Catchment area in sq.km.

C→ Constant depends on runoff factors (11.5 generally) (Increases for hilly terrains).

 

iii) Ryve's Formula: (South Indian Catchments):

C_i = 6.8 (Less for flat catchment and more value for hilly areas).

 

iv) Inglis Formula: (Fan-Shaped Catchments-Bombay): super di

v) Nawab Jung Bahadur Formula: (Hyderabad Deccan Catchments): 

C₂ varies between 48 to 60

M'→ Catchment area in acres.

vi) Dredge (or) Burge's Formula:

L→ Length of drainage basin in Kilometers.

 

 

PROBLEMS

1. The surface on which the rain fall occurs in a district is given below.

The total area of the district is 36 hectares and the maximum rain intensity is 5 cm/hr.

Determine

i) Co-efficient of runoff.

ii) Total runoff for district.

Solution: Runoff ratio for entire area.

Rational Formula:

2. In the same problem, if the time of concentration for the area is 30 minutes, find the maximum runoff (if maximum rain intensity is not given). Also use formula, R=900/(t+60).

Solution: Time of concentration is given as 30 min.

Rain intensity = mm/hr (using given formula)

Rational formula for peak discharge,

Maximum runoff = Q_p = 0.4875 cumecs.

 

3. The surface water from airport road is drained to the longitudinal side drain from across one half of a bituminous pavement suface of total width 7.0 m, shoulder and adjoining land of width 8.0 m on one side of the drain. On the other side of the drain, water flows across from reserve land with average turf and 2% cross slope towards the side drain, the width of this strip of land being 25 m. The inlet time is 10 min. The runoff coefficients of the pavement, shoulder and reserve land (turf) are 0.8, 0.25 and 0.35 respectively. The length of the land parallel to road from where water is expected to flow to side drain is 400 m.

Estimate the quantity of runoff flowing in the drain assuming 10 year frequency. The side drain will pass through clayey soil with allowable velocity of flow as 1.33 m/s.

4. In problem 1.1, if the density of population is 250 per hectare and the rate of water supplied per day is 225 Iped. Calculate the quantity of:

a) Sewage flow for which the sewers of a separate system, should be designed?

b) Storm water for which the sewers of a partially separate system are designed?

Solution:

i) Sewage Flow - Separate system.

Population Density = 250 persons per hectare.

Total population = 250/hectare × 36 hectare

= 9000.

Water supply = 225 lpcd

Rate of water supply = Per capita water demand x Population

= 225 lpcd × 9000

= 202500 l/d

= 2025 m3/d= 2025 24×60×60 m3/s

= 0.0234 m3/s.

Assuming the sewage discharge is 0.8 times the water supplied.

Rate of sewage produced (Average Flow) = 0.8 x rate of water supplied

= 0.8 x 0.0234 m3/s.

= 0.0187 m3/s.

Now assuming the peak flow as three time the average flow

Peak rate of sewage flow= 3 x Average flow

= 3 x 0.0187 = 0.056 m3/s.

= 0.056 cumecs.

 

ii) Partially Separate System

In case of partially separate system, the storm water from roofs and paved yards of houses will be allowed to enter the sewers.

5. A population of 30,000 is residing in a town having an area of 60 hectares. If the average co-efficient to runoff for this area is 0.60 and the time of concentration of the design rain is 30 min. Calculate the sewage discharge for the sewers in a combined sewerage system. Assume suitable data.

Solution:

Assuming average per capita water demand as 120 litres/day/person and that 80% of the water supply will be reaching the sewers as sanitary sewage.

= 80% of water supplied x Population

Quantity of sanitary sewage produced per day [80x100]/120pcd x30,000

=2880x10³l/d

Average flow = 2880 m³/d

Quantity of sewage produced per second =2880/24 x 60 x 60 = 0.033m3/2

Assuming the maximum sewage discharge as 3 time of average sewage discharge

Maximum Sewage Discharge = 3 x Average Flow

= 3 x 0.033 m³/s.

= 0.1 m³/s.