Channel flow within a subbasin

[DPAJR]

Neither HEC-HMS nor Hydrocad allow the runoff from a subbasin to be routed through a reach that is part of that basin.

I have a situation where a small channel runs completely through a subbasin and the reality is that the runoff feeds directly into the channel every foot of the way. Since there are several major drainages upstream I really need to know whether the runoff from this final subbasin is going to cause the channel to overflow it's banks or possibly the adjacent highway which is the last terrain feature downstream. At the end of this reach a culvert runs under the highway and into a very large lake.

The only thing I can come up with so far is to model the reach as a whole series of small reaches and the hillsides as a series of narrow strips. That would be a lot of work and would probably raise a few eyebrows.

Any ideas on how to model this?

Thanks,

 

[SMIAH]

Can't this be done using Kinematic Wave in HEC HMS (?)

 

[sam74]

First I would model the culvert at the end of the reach as a pond using the software you mention above and see if there is significant back up caused by this structure. That could certainly cause the channel to overflow. The channel flow will likely have very little flow time versus the rest of the basin.


If there is no significant back up then you could perform a quick check of a typical section of channel and slope with Mannings equation to see if there is enough capacity to carry the flow you calculated already or model the channel in Hec Ras. Just assume a worst case scenario of the total basin flow as a check. If a certain area floods you could always break the area upstream of this point up into a separate drainage area to lower the flow to a more realistic number.

 

[gbam]

Two approaches come to mind. First, as SMIAH questioned is to use the Kinematic Wave Hydrologic method. The second is to discretized your flow rate entering the channel. If you have a minimum Tc using RATIONAL Method this can be done rather easily. The second approach would be used only if the reviewing agency does not allow the Kinematic Wave Approach.

 

[psmart]

If I understand correctly, the issue is how to handle the continuous runoff contribution along the channel COMBINED with a separate incoming hydrograph. As such this is not really an issue of what routing procedure to use, but how to integrate the runoff and routing effects along the length of the channel.

As you suggest, the common solution is to use one or more reach segments to route the flow through the subcatchment, depending on how accurately you are trying to predict the flow depth.

For further details about the options for modeling a channel in HydroCAD please see

http://www.hydrocad.net/reach.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net