debgallery
Civil/Environmental
- Aug 20, 2007
- 25
A little bit of background...
I'm doing a peer review of a Stormwater Masterplan by another consultant. The watershed has ten sub-basins, six of which are included in FEMA's FIS for the region, meaning there are published peak flows for the 100-year flow (I'm focusing on the 100-year peak flow for simplicities sake in this post). These are based on regression equations developed by FEMA a long time ago. The consultant used PCSWMM to generate their own hydrographs for the basins, calibrating where gage data was available. The basins range in size from 600-acres to 18,000-acres and are between 20% and 95% developed. They're located in the mountains of Colorado with generally steep slopes.
When comparing the results of the peak flow analysis from PCSWMM to FEMA, five of them are surprisingly close but the peak flow in one basin doubled with the PCSWMM model (400-cfs to 800-cfs). This basin is 2,500-acres, is roughly 20% developed and is ungaged. I decided to run it through the USGS NSS software and got a peak 100-year flow of 137-cfs (73% above 7500 elevation).
Now my questions....
Is PCSWMM the right model to use for this scenario (strictly hydrologically speaking)? My understanding of EPA SWMM is that it is for primarily urban watersheds. The majority of these sub-basins are less than 40% developed.
Is USGS the right model to use? The documentation on the software says it is appropriate for natural streams, which they further define as being less than 10% developed. If the basins are more than 10% developed does it automatically eliminate NSS as an option?
Is there another method that might be more appropriate?
I'm thinking of maybe generating a hydrograph in NSS (I think you can do this) and then using it in the PCSWMM model for the sub-basins where it is appropriate.
The masterplan is recommending upgrading a lot of infrastructure based on these flows and I want to make sure they are balancing "being conservative" with reality.
Really looking forward to hearing your opinions.
I'm doing a peer review of a Stormwater Masterplan by another consultant. The watershed has ten sub-basins, six of which are included in FEMA's FIS for the region, meaning there are published peak flows for the 100-year flow (I'm focusing on the 100-year peak flow for simplicities sake in this post). These are based on regression equations developed by FEMA a long time ago. The consultant used PCSWMM to generate their own hydrographs for the basins, calibrating where gage data was available. The basins range in size from 600-acres to 18,000-acres and are between 20% and 95% developed. They're located in the mountains of Colorado with generally steep slopes.
When comparing the results of the peak flow analysis from PCSWMM to FEMA, five of them are surprisingly close but the peak flow in one basin doubled with the PCSWMM model (400-cfs to 800-cfs). This basin is 2,500-acres, is roughly 20% developed and is ungaged. I decided to run it through the USGS NSS software and got a peak 100-year flow of 137-cfs (73% above 7500 elevation).
Now my questions....
Is PCSWMM the right model to use for this scenario (strictly hydrologically speaking)? My understanding of EPA SWMM is that it is for primarily urban watersheds. The majority of these sub-basins are less than 40% developed.
Is USGS the right model to use? The documentation on the software says it is appropriate for natural streams, which they further define as being less than 10% developed. If the basins are more than 10% developed does it automatically eliminate NSS as an option?
Is there another method that might be more appropriate?
I'm thinking of maybe generating a hydrograph in NSS (I think you can do this) and then using it in the PCSWMM model for the sub-basins where it is appropriate.
The masterplan is recommending upgrading a lot of infrastructure based on these flows and I want to make sure they are balancing "being conservative" with reality.
Really looking forward to hearing your opinions.