# Water Management

Assignment Requirements

Urban drainage exercise

1.0    Expected Learning Outcomes

• To gain familiarity with approaches to design and simulation of piped drainage systems.

2.0    Scenario

The plan of a piped urban drainage system is given below

 Pipe Length (m) Impervious area (ha) 1.0 2.0 1.1 3.0 1.2

The contents for the table above, together with the values of:

CV for all areas

time of entry to be assumed

are available via the DLE.

Here is the data:

Urban Drainage Exercise – DATA

L = length           Ai = impervious area           S0 = gradient (1 in …)

te = time of entry (minutes)

 Pipe:   Number 1.0 L (m) 1.0 Ai (ha) 2.0 L (m) 2.0 Ai (ha) 1.1 L (m) 1.1 Ai (ha) 3.0 L (m) 3.0 Ai (ha) 1.2 L (m) 1.2 Ai (ha) CV S0 te

 30 95 0.47 200 0.49 120 0.55 70 0.54 110 0.46 0.95 190

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 chart (which you already have).

3.0    The exercise

• Select a suitable size for each pipe using the Modified Rational Method for design. (35%)

 Pipe number Length (m) Gradient (1 in…) Impervious area (ha) Sum impervious area (ha) Assumed diameter (mm) Pipe-full velocity (m/s) Pipe capacity (l/s) Time of flow (min) Time of concentration (min) Rain intensity (mm/hr) Calculated flow-rate (l/s) Comments

1. 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 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

Determine the hydrograph for outflow from pipe 1.2.  Comment on how your pipe 1.2, designed using the Modified Rational Method, would ‘perform’ when exposed to this rainfall, and discuss any reasons for the peak flow-rate determined in 2. being higher or lower than the capacity determined in 1. (35%)

1. 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 durations. Give the possible dimensions of a suitable tank that would provide this storage. Explain, with some background, why such limits on outflow may be necessary. Apart from providing a storage tank, briefly explain how else it might be possible to satisfy this requirement? (30%)

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