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Sub-basin modeling question 1

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Scott Horowitz

Scott Horowitz

Civil/Environmental
Joined
Jun 16, 2017
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13
Location
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I apologize for any confusion this post may cause as a result of the questions that I am asking or due to the information that I am presenting, but nonetheless, I appreciate any guidance that can be offered.

Two existing drainage basins combine storm water runoff with the hydrologic properties below:

Basin A - Existing
1. 54 acres
2. 70-minute Tc
3. 87.0 CN value

Basing B - Existing
1. 164 acres
2. 110-minhute TC
3. 92.1 CN value

Combined 100-Year storm water runoff Q - 760 CFS.

A proposed residential development (Basin C) will be located within Basin (B). Storm water runoff from Basin A, Basin B and Basin C will combine at the existing discharge point. The post-development hydrologic properties are below:

Basin A - Proposed
1. 30 acres
2. 37 minutes Tc
3. 91.1 CN value

Basin B - Proposed
1. 156 acres
2. 75 minutes Tc (different path than existing; the existing path is now one of the shorter duration pathways in the basin due to the development)
3. 86.6 CN value

Basin C - Proposed
1. 75 acres
2. 53 minutes Tc
3. 92.3 CN value

Combined 100-year storm water runoff Q (undetained from the development)for Basin A, Basin B and Basin C - 1,225 CFS
Combined storm water runoff Q (100-year)for Basin A and Basin B - 830 CFS

How are scenarios such as the above usually handled? The combined storm water flowrate from Basin A and Basin B for the post-development conditions (830 CFS) is greater than the combined storm water flowrate from Basin A and Basin B for the pre-development conditions (760 CFS), essentially removing any ability for the proposed residential subdivision discharging storm water runoff. Would either the Tc and/or CN value for Basin A be left those for the pre-existing conditions?
 
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If your goal is to mitigate the developed peak flow, this is commonly accomplished with a detention pond.

Peter Smart
HydroCAD Software
 
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Understood, and that is what the intent is. The question that I have is specific to the existing drainage basins in the post-development condition. In this case, Basin A and Basin B will be reduced in area due to the proposed residential development. The development's location within the basins remove the grassland and thus increase the CN values for Basin A and Basin B. Additionally, the time of concentration (Tc) paths are adjusted since the existing paths are no the longest-duration pathways in the post-development condition; there are different pathways longer in duration, but not as long as the original Tc values. Thus, the existing basins, in the post-development condition, are smaller in area and Tc durations with higher CN values.

When I enter these values into the model for the post-development condition for Basin A and Basin B, the combined storm water runoff flow rates are greater than the flow rates for the pre-development condition, before adding the proposed residential development into the model as its own basin (Basin C). I need to calculate the allowable discharge from Basin C to meet the pre-development flow rates, but if the flow rates for Basin A and Basin B are greater in the post-development condition (830 CFS for 100-year storm) than the pre-development condition (760 CFS for the 100-year storm), there won't be any allowable discharge from Basin C. What would be a good recommendation in this case? Would I adjust the Tc or CN values back to the original values for the post-development conditions?

Thank you very much.
 
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So the detention pond would be a regional detention pond?
 
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Whatever you call it, if you're increasing the flow from basins A and B you will presumably have to treat that flow.

Peter Smart
HydroCAD Software
 
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total area increased and basin c got smaller? should probably check your numbers
 
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First, look at ways to reduce the CN for A+B in your watershed. if not possible, with your total post-development increases (+ 43 acres), you need to look at providing some detention in A+B. You may need to provide series of detentions to reduce flow down to pre-development condition.
 
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