Time of concentration issues

[gadwall]

I am working on a residential development approximately 35-acres in size. The property is extremly flat( less than 0.2% grade from fron to back.) There was a drainage way on the property that has been relocated. I requested a copy of the drainage calcs for the relocation project and the Town Engineer said there were no calcs. (I thought this was strange). Anyway, I performed my own analysis of the overall basin using HydroCAD. I looked at the 10-yr and 100-yr storm events.

When I submitted my report to the Town, the Town Engineer said that it was to information to digest and that he really only needed a report on my development. I disagree. The Development will be directly influenced by the flow in the drainage way. The WSE must be known to apply tailwater conditions to the model.

The drainage way enters my site by a double 6x6 box culvert. The original channel was very iregular in section. It was approximately 3 feet wide at the bottom and about 4 feet deep. The relocated channel is 6 feet wide at the bottom, 4 feet deep, with 3:1 side slopes. Historically there have been no reports of flooding in this area. The HydroCAD model showed the average depth of flow in the channel to be 2.72 feet and 3.78 feet for the 10-yr and 100-yr storms, respectively. I do not believe this to be representative of the situation.

So, here is my major issue. The time of concetration for the development will be on the average of 5-10 minutes. The off site areas have times of concentration in the 30-90 minute range. When the calculations are performed, there seems to be a reduction of runoff. This scenario may be true for a storm of short duration. The runoff from the site will reach the drainage way well ahead of the off site runoff. What happens when a longer duration storm occurs? How do I model this scenario? The Town engineer is trying to push on-site detention, but I am not sure that it is really needed. Any suggestions?

Oh yeah, the model seems to be plagued with [80] Warning messages. I have tried changing the finer routing control to no avail. I think that because the area is so flat that the tailwater is causing a reverse flow condition. How do I model this?

 

[psmart]

I agree that the timing of your site vs. the upstream contribution may be significant. Modeling all the contributing areas is a good way to go. You may have a situation where detention is counterproductive, and letting your peak go before the upstream contribution could produce a smaller combined flow.

You didn't say what runoff method you're using. For the SCS TRS-20 procedure, the standard 24-hour rainfall distributions include all events up to 24-hours, so the single analysis will also consider the effects of shorter storms.

You DEFINITELY need to address the warning messages. In general they indicate that the modeling assumptions have been exceeded and that the results are not valid. Click each message for details - and resolve the problem. Do NOT rely on any results until you have resolved or fully understand the warnings.

Warning 80 indicates a potential reverse flow. This can occur when water levels in a "downstream" node rise faster than an upstream point. If you still have questions about a specific model you can send the HydroCAD .HCP file to support@hydrocad.net

If you feel your results are inconsistent with historical flooding reports, you should examine all the input parameters to see if any adjustments are appropriate. You may be able to calibrate the model to provide a better match to the observations. However, unless you have accurate rainfall measurements to correlate with these observations, this anecdotal information should be used be caution.

 

[gadwall]

Mr. Smart,

Thanks for your prompt reply. I am using the SCS TR-20 method from your program w/ the Sim-Route option. I am going to send you a copy of the HydroCAD file. I still am having truoble resolving th warning 80 messages.

Thansk again.