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Detention Drain

 

Detention drain is an option for the stormwater management facilities; it is usually used for an area which does not have much space for an OSD or Pond. The detention drain is usually larger than drain that only serves the conveyance purpose, but often it is still more economical, both in terms of space or construction cost consideration because now you don’t need to do a dedicated structure for water storage purposes. This is the reason for its popularity.  

To do detention drain in MiTS MSMA module:

  1. The Detention Drain module in Pond Sizing tab only can be applied to a single network drain for the dimension design.
  2. If your detention drain network consisting of multiple branches, then you need to approximate the drain network into a rectangle/ irregular pond with the same capacity
  3. Synced end invert level of drain into detention drain is possible towards a single network

Project file (this project file is created in MiTS 2.7.13.0)

Single Network 

Case:

User needs to design a stormwater management to an area with a size of 30 m x 30 m x 1.5 m RC detention storage tank. The calculation is manually calculated as per spreadsheet.

The summary of the design output for the storage tank is as follows:

PSD = 177.7 (l/s) (Based on nearest impervious percentage; 75%)

SSR = 1307.4 m3 (Based on nearest impervious percentage; 75%)

Volume Provided = 1350 m3

Sizing Dimension = 30m x 30m x 1.5m

However, due to past development, the space required is not enough. Therefore, a detention drain might be an option.

Following is the modelling on detention drain and the results obtained are to be compared with the original proposal (30m x 30m x 1.5m RC detention storage tank).  

As a summary, users can opt to another detention type (ie detention drain), as the result is almost the same.

Variable Detention Storage Tank Detention Drain
PSD (l/s) 177.7 176.38
SSR (m3) 1307.4 1389.27
Volume Provided (m3) 30m x 30m x 1.5m= 1350 3m x 160m x 3m= 1440

 

Syncing the drain invert level (IL) into the detention drain:

 

  1. Start TDL and IL:

    Refer the Start GL and the Start IL from the sump 1 in the drain module as their first TDL and IL.


TDL 27 m, IL 24 m, depth 3 m


First TDL and IL

2. End TDL and IL:

Refer to the end invert level of the final drain. 


End invert level 23.2 m 

Since the depth is fixed to 3 m, therefore the end TDL is 26.2 m. Thus, insert 26.2 m in the End TDL box. 

 

If the depth of the drain is not fixed, then the TDL is based on the ground level. The calculation of the greyed box is based on the following equations:

Dstart = TDL/GLstart – ILstart

Dend = TDL/GLend – ILend

ILend = ILstart – length/gradient

 

3. Proceed to the next dimensions which are the width, length and gradient. Insert the values according to the drain design. 

 

Multiple Networks

 

MiTS detention drain in OSD is applicable for a single network as the example above. 

However, if you have multiple branches of drain networks, you have to approximate it with proper detention storage facilities with proper parameters. The following constraints must be satisfied: 

  1. The capacity is the same for the detention facility in Pond Sizing tab and the actual drain on the key plan layout
  2. The depth of the Pond, or the depth of the end IL ( for detention drain) is the same as the final drain depth. This is important to ensure that the orifice design is correct with regards to head water level parameter.

It is thus best to design it in a rectangular dimension or in irregular shape with the same capacity. As both rectangular or irregular shape or detention drain can do in the approximation, but the detention drain has too many parameters to tune around and complicate things unnecessarily. With Depth fixed, the rectangular pond, on the other hand, just has Width and Length to tune in order to match the capacity. The extra degree of freedom is only 1 whereas detention drains the extra degree of freedom is considerably more. 


Example of multiple networks

For example, refer to the drainage module and the dimensions used in the box drain are 3 m width, 3 m depth with 160 m total length of both networks. The total drain volume is thus 1440 m3:


Box drain dimension

 

Pond Sizing consideration

 

Rectangle Pond ( recommended):

The above configuration can actually be approximated if you are using a rectangular type, with only three inputs needed which are the length, width and the depth. Note that the Depth is 3m, the same with the final depth of the detention drain. Also note that the capacity for this rectangle pond is the same as the total capacity of the detention drain network. That’s all that matters

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Provided storage 1440 m3