Modeling sub basins 1

[DPAJR]

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

 

[Ryb01]

Just wondering if you could provide some clarification..

"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?

 

[DPAJR]

No there is no storage. They all converge at various points and eventually the last one leads to a culvert that runs under a two lane road. It's like several Y-shapes with their bottom legs coming together at a common point.

 

[Ryb01]

Caculating flows would all depend on the level of accuracy you're looking to achieve. More drainage areas refines the accuracy and increases the amount of work required. Also, the more you break the overall area down into smaller components, the larger your flows will be.

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.

 

[DPAJR]

Thanks. I am going with more subbasins like you say.

 

[francesca]

For culvert design, I'd calculate the longest time of concentration and go with the Rational Method for a storm of that intensity/total acreage. If you break it up into sub-basins, you're dealing with different times of concentrations, and combining hydrographs. It seems overly complicated for a culvert.

 

[DPAJR]

Thanks Francesca,

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

 

[beej67]

If you're sizing one culvert I'm with Francesca, use rational method and call it a day.

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

 

[cvg]

while it may seem that more is better, that is not always the case. There is a strong argument for keeping it simple. However, 589 acres is possibly too large for the rational method which was never intended for large watersheds.

 

[beej67]

Also be aware that some states like you to add a correction factor for larger storms to the rational method equation, that varies by storm frequency. Check your state drainage design manual if there is one.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[DPAJR]

God tips everyone,

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

 

[beej67]

If you're only trying to get a design flow to size a single culvert, then you might be able to get away with doing the NRCS graphical peak discharge method by hand to determine a peak flow, instead of fooling with routing it. Once you have your curve number it's a pretty simple calculation, and it's certainly a defensible design approach.

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