Quantity of storm water
When rain falls over the ground surface, a part of it percolates into the ground, a part is evaporated in the atmosphere and the remaining part overflows as storm water. This quantity of storm water is very large as compared with sanitary sewage.
Factors affecting storm water:-
The following are factors which affect the quantity of storm water:
1. Rainfall intensity and duration.
2. Area of the catchment.
3. Slope and shape of the catchment area.
4. Nature of the soil and the degree of porosity.
5. Initial state of the catchment.
If rainfall intensity and duration is more, large will be the quantity of storm water available. If the rainfall takes place very slowly even though it continues for the whole day, the quantity of storm water available will be less.
Harder surface yield more runoff than soft, rough surfaces. Greater the catchment area greater will be the amount of storm water. Fan shaped and steep areas contribute more quantity of storm water. In addition to the above it also depends on the temperature, humidity, wind etc.
Estimate of quantity of storm water:-
Generally there are two methods by which the quantity of storm water is calculated:
1. Rational method
2. Empirical formulae method
In both the above methods, the quantity of storm water is a function of the area, the intensity of rainfall and the co-efficient of runoff.
Runoff from an area can be determined by the Rational Method. The method gives a reasonable estimate up to a maximum area of 50 ha (0.5 Km2).
Assumptions and Limitations
Use of the rational method includes the following assumptions and limitations:
1. Precipitation is uniform over the entire basin.
2. Precipitation does not vary with time or space.
3. Storm duration is equal to the time of concentration.
4. A design storm of a specified frequency produces a design flood of the same frequency.
5. The basin area increases roughly in proportion to increases in length.
6. The time of concentration is relatively short and independent of storm intensity. The runoff coefficient does not vary with storm intensity or antecedent soil moisture.
7. Runoff is dominated by overland
8. Basin storage effects are negligible.
The minimum duration to be used for computation of rainfall intensity is 10 minutes. If the time of concentration computed for the drainage area is less than 10 minutes, then 10 minutes should be adopted for rainfall intensity computations.
This method is mostly used in determining the quantity of storm water. The storm water quantity is determined by the rational formula:
Q = cla/360
Effects of Flow Variation on Velocity in a Sewer
Due to variation in discharge, the depth of flow varies, and hence the hydraulic mean depth
(r) varies. Due to the change in the hydraulic mean depth, the flow velocity (which depends
directly on r2/3) gets affected from time to time. It is necessary to check the sewer for maintaining a minimum velocity of about 0.45 m/s at the time of minimum flow
(assumed to be 1/3rd of average flow). The designer should also ensure that a velocity of 0.9 m/s is developed atleast at the time of maximum flow and preferably during the average flow periods also. Moreover, care should be taken to see that at the time of maximum flow, the velocity generated does not exceed the scouring value.