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Why is the dimension of the detention drain bigger than drain?


Purpose of drain;

Drain is built to compare between the flow from the catchment (CiA) and the drain flow (manning equation), and to determine whether the drain provided is sufficient to cater the flow from the catchment.


Purpose of Pond/Detention Drain;

The detention drain is used to retain the water before flowing out to the stream. The sole purpose is to compare between pre development and post development. As per requirement, the discharge of an area after development has to be the same or less than the pre development discharge (Qpost <=Qpre). Hence the mitigation such as detention drain is implemented in order to control the outflows.


Given the example in the project file, the input of drain between pre development and post development are as follows;


Pre Development Drain (Project): 

Development Area

6 ha

Catchment Coefficient

0.40 (Forest cover)

Peak Flow

1.661 m3/s

Drain Dimension

1200 x 450 mm


Post Development Drain (Project):

Development Area

6 ha

Catchment Coefficient

0.95 (Commercial and Business Centre)

Peak Flow

3.94 m3/s

Drain Dimension

1200 x 900 mm


The Post Development Drain dimension is bigger compared with the Pre Development Drain dimension. This happens due to different area coefficients of the development and the dimension calculated is WITHOUT CONTROL. The dimension is iterated based on the peak discharge and flow discharge, whether the dimension given is sufficient to cater the water coming from the catchment, regardless of Pre Development discharge.


Then, both the information of Pre-development and Post-development (coefficient, tc, etc.) are assigned in a detention drain design. 


The result of the detention drain dimension is as follows:


The dimension required with outlet control; ~6500 x 5000 mm



The dimension of the detention drain is way too big compared to the post development drain dimension. And what is the reason? You may refer to the image below to understand the reason behind it.


Detention drain dimension given is a few orders of magnitude larger than comparing it with the Post Development drain. Why is this so?


The explanation can be partly found from the above diagram, the detention drain has the purpose of retaining the water and controlling the flow so that the flow is less than Pre Development discharge (<1.661 m3/s).  Where the water is retained, the flow in the detention drain is expected to be slower( even a lot smaller than the pre development flow 1.66m3/s), which means that a bigger storage size is needed to cater for all of the saturated water. Essentially the detention drain is no longer a “drain” ( a conveyance), rather, it is a “storage”.


One might also point out that  drain size is determined using Manning Equation, which is a static equation, where as a “storage” like detention drain or pond involves level routing equation that is based on a completely different methodology. Hence, this also contributes to part of the huge size of detention drain. But to what extent the contribution is due to different analysis methodology is unknown until we carry out a full scale dynamic routing for drain and detention drain.