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How to model depressional area in HEC-HMS

How to model depressional area in HEC-HMS

(OP)
For a project I'm working on, a subbasin 1A flows to a depression which then overflows to an existing culvert. I am unsure what is the correct way to model this. I tried both these ways:

Method 1. I modeled the depressional area as a reservoir with a spillway outlet. I had to add an arbitrary orifice outlet at the bottom of the depression with diameter 0.001 to effectively produce 0 CSS discharge under the overflow elevation.

Method 2. I divided the depressional storage by the drainage area and included it as an initial abstraction over the entire subbasin 1A under the Simple Surface input. I made a separate reservoir with the spillway set at the depression overflow elevation to model discharge flows.

However, I get different discharges for both methods. Method 1 results in an increased discharge over method 2.

RE: How to model depressional area in HEC-HMS

Method 2 seems kind of double dipping the impact and that's why you got a lower discharge. I would go with method 1,and also make sure since you are modeling the depression as reservoir, when you calculate your basin hydrological parameters, don't include any factors from the reservoir.

RE: How to model depressional area in HEC-HMS

hi,
if you know the size of the depression or volume then use storage discharge curve in reservoir..do not apply any arbitrary outlet or orifice..follow below steps
1- calculate what is the peak discharge from 1A including depression storage area...but donot use reservoir in this run
2- once know the peak discharge...now apply reservoir with storage discharge curve.
3- in storage discharge curve if apply maximum discharge at full volume for example: say your full volume of depression is 10000m3 and peak discharge is 3m3/s.

Volume Discharge
0 0
500 0
5000 0
8000 0
10000 3
11000 3

by this the pond will act as natural pond and you will get exact outflow discharge from the pond.

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