HEC-HMS Add Base Flow to Wetland pond

[swazimatt]

thread789-447696

We have a wetland pond modelled to detain flow from the piped network for the 10year event and overland flows for the 100 year event. The catchment for the overland flow is different (smaller) to the 10 year pipe catchment. My stormwater engineer has informed me the level in the pond is lower for the 100 than the 10, which is obviously wrong. They have basically just run the 100yr with the 100yr catchment and ignored that the pipe network will still be directing flow to the pond. This is fine for the catchments that overlap, but we are missing a big portion of catchment that would still contribute flow through pipes at max flow (the balance runs overland to a different location)

the pipe network has been modelled on different software so there is no extensive pipe network in hec. Is there a way we can add the 10year pipe flow to the pond either as a constant flow or as a catchment with different rainfall?
I understand the pond hydrograph will be wrong, but we only need to check the max pond levels

Please bear in mind that i have only used HEC for 5 minutes so may not have described the HEC part 100% correctly but have a case of a junior engineer using hec telling me the software doesn't lie - which is true, but rubbish in rubbish out

We tried setting the pond height at the 10 year and adding the 100yr overland storm flow to it, but the 10yr levels drain before the 100yr storm peak arrives. This would also be a doubling of the piped flow for the overlap portion of 100 and pipe catchments but if it was still contained within the pond it would be a good conservative result
The next thing i was thinking of doing was to adjust the pipe only catchment area. eg if i have 1000m2 catchment and 100yr peak rainfall is 100mm/hr and 10yr is 80mm/hr could i just adjust the catchment to 800m2 and run it with the 100yr rainfall (probably not this easy)?

I think the simplest is to calculate the 10yr flow from the 10yr only catchment and add that to the pond as a constant base flow

 

[psmart]

There are several ways to add the extra flow.

1) You can add a constant flow to any pond by setting the "Base Flow" on the Advanced tab.

2) -or- You can use a link to define a constant inflow, making it more visible in the model. A link also allows you to set a different base flow for each event.

The problem with options 1 and 2 is they assume that the extra flow is constant, while it probably varies over time, like the other hydrographs in your model. To accommodate the varying flow:

3) If you have a predefined hydrograph (from HEC or otherwise) you can enter it using a link. Again, a link will let you define a separate hydrograph for each event.

4) -or- You could model the missing flow in HydroCAD using additional subcatchment(s).

The choice is yours. Personally, my preference would be option 4,3,2,1 in that order.



Peter Smart
HydroCAD Software

http://www.hydrocad.net