Question

I used my catchement area to calculate my runoff using the national method.

how will i go on calculating the volume needed for storage?

which equations will i use ?

i want to have a storage tank that is pumped to the wetland. how will i distribute the volume and calculate the sizing for each.

Can you explain with an example if you can?

Answer 1

While calculating runoff from the rational method, due care should be taken while calculating intensity as it is a variale quantity.

It is prescribed to study minimum of 25 years rainfall data.

There are mainly two methods used first is the method of calculating intensity using IDF Curve ( intensity-duration-frequency) and the second is approximate method.

An IDF Curve gives the rainfall intensity for required frequency and duration. The intensity varies with frequency and duration of rainfall.

Frequency is generally given by return period, it is the average time in which a particular intensity of storm will be equal or greater than specified condition.

For a given duration of rainfall, intensity increases with increase of return period, simmilary intensity of storm decreases with increase in storm distribution.

Suitable probability distribution is fitted to the selected duration data series, commonly used probability distribution is Gumbels extreme value distribution.

Precipitation depth for a given return period is calculated using frequency factor:

X​​T = X + K​​​​​​T​​​​​S

Where,

X is mean of the rainfall data

K​​​​​​T is Frequency factor

S is standard deviation

For calculating volume using approximate method rainfall data of more than 25 years are calculated and design is done according to the average maximum hourly rainfall occured in the region.

Discharge can be calculated using rational method on hourly basis and depending upon the average duration of rainfall in region daily discharge can be calculated.

STORAGE CAPACITY:

To arrive at storage capacity of tanks average monthly data should be calculated and as per table-3 in the picture one day rainfall data should be arrived for storage capacity.

Table provides the thumb rule for storage as per empirical study given by green green building ratings - IGBC.

suppose you have a monthly average of 300mm so as per table one day storage required is 7.5% of the monthly rainfall and it is 22.5 mm, and if the average runoff coefficient is 0.8 and area of catchment is 200m2 then as per rational formula

Q = C*I*A

Where

C= coefficient of runoff = 0.8

I = intensity for storage = 22.5mm

A = Area for storage = 200m2

Then the derived discharge Q = 0.8*0.0225*200 = 3.6 m​​​​3

So the storage tank of minimum 3.6m3 is required to store the water which can vary depending upon the usage.

Number of storage tanks depends on the network characteristics , catchment area, Topography, location of wetland and number of dicharge or end points of networks.

I hope I am able to solve your queries.