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.
Rational method:-
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.
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