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Urban Drainage

Expected Learning Outcomes

To gain familiarity with approaches to design and simulation of
piped drainage systems.
To demonstrate appreciation of the approaches to urban drainage
in the context of flood risk.
2.0 Urban drainage case

The plan of a piped storm water drainage system is given below.
1.0 1.1
Pipe Length
Impervious area
The contents for the table above, together with the values of:
CV for all areas the gradient of all pipes time of entry to be assumed are available via the DLE (everyone gets different data).
First go to the spreadsheet ‘CW numbers’ to find your personal

Then go to the document ‘CW data’, locate your personal
number in the left hand column and read off your data. Note – you
should not need to print the files ‘CW numbers’ or ‘CW data’.
The system will consist of concrete pipes, available in diameters 150,
225, 300, 375, 450, 525 mm etc (increasing by 75 mm). Assume
roughness ks is 0.6 mm. Use the rainfall formula: i = 750/(t+10),
i (intensity) in mm/hr, t (duration) in minutes.

Determine hydraulic properties using the appropriate HR Wallingford table (which you already have) or directly using the Colebrook-White equation.(36%)
Select a suitable size for each pipe using the Modified Rational Method
for design. Show the process of your calculations (assumed diameters
that were too small or too large) as well as the final design.
(6/25 = 24%)
Determine an appropriate alternative value for the rainfall intensity
needed in the final design of pipe 1.2 using the Wallingford Procedure
method for determining rainfall (as opposed to the rainfall formula
above that you used in your design). Location: Plymouth.
(3/25 = 12%)

In ‘simulation’ rather than ‘design’ mode, create, for the system as
designed, the time-area diagram for the outflow from pipe 1.2. Then
consider the rainfall pattern below where i = the rainfall intensity used in the final Modified Rational Method design of pipe 1.2. (Time = 0 is
the beginning of the rain.)
Time (minutes) Rain intensity
0 – 1 0.5  i
1 – 2 1.2  i
2 – 3 1.6  i
3 – 4 1.3  i
4 – 5 0.4  i

The Questions

Determine the resulting hydrograph for outflow from pipe 1.2.
(7/25 = 28%)
Explain the difference between your value of the peak flow-rate
determined in 2. and the capacity determined for pipe 1.2 in 1.
(2/25 = 8%)
3 . (28%)
Say Q1.2 is the calculated flow for pipe 1.2 as designed in 1. above.
Suppose it is a requirement of the Environment Agency that the flow to the receiving water does not exceed 0.25

 Q1.2. Determine the capacity of storage necessary to contain flows in excess of this limit,
using the Modified Rational Method, considering a range of storm

Give dimensions of a suitable tank that would provide this
storage, with a brief explanation.
(4/25 = 16%)
Explain the aim of a requirement like this. Apart from providing a
storage tank, discuss how else it might be possible to satisfy this
(3/25 = 12%).

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