Modeling sub basins
Modeling sub basins
(OP)
I have a project where there is a large bowl shaped area in the upper reaches. There are several dry gullies that would become intermittent streams in the event of a large storm. They all lead to the same culvert. Is it appropriate to model the entire area as a single catchment/sub basin or should it be divided into separate sub basins for each potential stream bed?
Thanks
Thanks





RE: Modeling sub basins
"There are several dry gullies that would become intermittent streams in the event of a large storm. They all lead to the same culvert. "
Meaning they would fill with runoff and not be conveyed to the culverts? or is there storage that you're trying to account for?
RE: Modeling sub basins
RE: Modeling sub basins
It can be a tedious task to calculate flows with areas like this. If you're producing flows to size culverts, be conservative (within reason) in your estimates. There's a number of fators that can quickly change within a watershed (ie tree removals) which could impact the flows later on.
If the requirement is to size the roadway culvert for a major system flow it may be beneficial to be as accurate as you can with the flows.
Hope this helps.
RE: Modeling sub basins
RE: Modeling sub basins
RE: Modeling sub basins
It's actually not just one culvert it's a series of five existing culverts. The area is now up to 589 acres due to escalating requirements from the reviewing agency. The soil types vary from Group D at the upper elevations to Group A at the middle and lower elevations.
FEMA requires the use of software off of their approved list. I am using HEC-HMS since TR-20 seemed to be user unfriendly and TR55 only allowed 10 nodes.
They require the use of a 100 year storm from the NOAA Atlas when actually the rain gage data for the last 51 and a half years shows that there has not even been a storm equivalent to NOAA's ten year storm.
The whole thing has been frustrating. I hope to avoid FEMA in the future.
dpa
RE: Modeling sub basins
If your five culverts are each under the same road, each conveying a different basin under the road, I say stick with Rational and call it a slightly longer day.
If your culverts are in series along the same drainage path / creek / etc, then you're probably best to use HMS and do a more detailed model.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Modeling sub basins
RE: Modeling sub basins
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Modeling sub basins
This is a series of five culverts in a large ravine some concrete, some corrugated. There are about nine soil types. Several are HSG "A" but some are "D". Vegetation also varies throughout with grass in the upper reaches and desert shrubs, and grass in the lower reaches. Usage varies too with houses in the lower areas and almost no houses in the upper areas. I ended up with ten subbasins. Now all I have to do is get HEC-HMS to stop changing back to metric.
dpajr
RE: Modeling sub basins
See Chapter 4 of NRCS TR-55.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com