Question regarding Catchment Time of Concentration

[salhu]

I want to challenge a statement made elsewhere as follows;

Statement: In general the SWMHYMO model shows that the run off from Highway row discharges downstream before the peak flow from the external areas reaches the highway culverts.

Challenge: How this statement can be made, when most Tc values are less than the rainfall duration i.e.12 Hours? Therefore at the time of Time of concentration, for most of the catchments, rainfall would still be going on, and all the areas, including those from the pavement would be still contributing to the peak flows.

Do you see that the challenge could be wrong in any ways?

Thanks in advance....KH

 

[cvg]

I think your assumptions are incorrect. I would tend to agree with the assumptions made by others. Unless the offsite drainage area is quite small and close to the highway - the pavement drainage peak discharge will occur before the peaks from the offsite area can reach the culverts. Pavement drainage would rarely coincide with offsite drainage.

 

[salhu]

Actually catchment areas upstream of the culverts vary from 1 hectare to about 1000 hectares. (culverts are the outlets of these catchment areas) And pavement areas are most downstream parts of these catchment areas, before outletting to the culverts.

All catchment areas have Time of concentration less than 12 hours, that is why i thought that i could challenge the statement. Do you still think that my challenge is not justified?

 

[cvg]

I wouldn't challenge it.

If you think about the fact that the maximum precipitation will occur at the same time on the roadway as it does on the entire 1,000 Hectare watershed, the runoff from that precip will need to travel a kilometer or more until it reaches the culvert. However, the flow from the roadway may only need to travel a few hundred meters. It will arrive first and not combine with the peak from the offsite watershed.

 

[bltseattle]

It is a requirement of the unit hydrograph methods that the Tc be less than the storm duration, so that alone is not a basis to challenge anything. If the Tc were longer than the storm duration, then a more sophisticated hydrologic routing should be used....

 

[salhu]

thank you cvg and bltseattle. I am satisfied with your responses.

In case Tc is longer than the storm duration, what do you recommend as a next step in modeling? should the modeling be repeated for longer storm duration or do something else?

 

[cvg]

that would indicate either a very long drainage area, or a very short storm duration. You might want to split up into smaller subareas and route the flow

 

[bltseattle]

Out here in Western Washington HSPF hydrology model is the standard for large undeveloped to urbanizing basins, SWMM is more the standard hydrology model for large urban basins.

You could use an event model for small to medium size basins (say up to 100 acres or so) but should route the flow, as cvg said, so that the attentuation in open channels and travel time lags can be assessed. I usually start with the WaterWorks software package (because it is easy to use and not too buggy) although I'm sure that different design programs could be used. Waterworks has several reach routing algorithms such as Muskingum, kinematic, etc.

 

[cvg]

HEC-1 is the standard in Arizona and can be used to model small or large, developed or undeveloped basins. It can implement various types of runoff, infiltration and routing analysis.