manning's equations for ephemeral streams

[tramphardrocker]

I'm using a cabin building site adjacent to and above a dry stream to begin learning methods for calculating flood event flow volumes and rates to assess risk to that site. It is in the Bradshaw Mounains above Prescott, AZ. It is in a pine forested, 5 sq. mi. watershed with dendritic subcatchments feeding the main downslope drainage.

Is it legit to use Manning's equations for stream channels to estimate flow volumes and rates without doing a full bore hydrology study.

I have surveyed 3 cross sections 100 feet apart to get area, wetted perimeter and slope to use in the calcs.

Eventually, I hope to get a handle on the processes and software required to do this type of work. I see that it will be a daunting task. The watershed I'm studying has all the elements for training purposes.

 

[cvg]

estimate your watershed area and use the regional regression equations for arizona, quite easy. I believe you can find them in the ADOT hydrology manual. chapter 10

http://www.azdot.gov/Highways/Roadw...ADOTHighwayDrainageDesignManual_Hydrology.pdf

unless you have a true high water mark, not sure what good your method would be to calculate peak flood flow rate and how would you estimate the flood frequency?

 

[beej67]

You're getting your terms mixed up, rocker.

Hydraulics is the math of how water flows, and how deep it will be, given a certain flow rate.
Hydrology is how you figure out what the flow rate is in the creek.

For your task, you can't use Manning's equation (hydraulics) without first knowing the flow rate (hydrology). Manning's equation will indeed be appropriate, but only after you've first determined the flow rate somehow. For any hydrologic method, you'll need to delineate the boundary of the watershed and characterize that watershed in some fashion. The simple way for a basin of your size is what cvg recommends - USGS regression equations based on nearby stream data.

Dig out the regression equations for your region, and see what the inputs are. The form of those equations vary a little from place to place. It might just be based on watershed area and some sort of location factor. It might also be based on impervious coverage, or something similar. At that point, it's just a mapping exercise to get the right numbers to plug and chug to get your flow. Then you use that flow with some kind of Manning's analysis.

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

http://www.campbellcivil.com

 

[LincolnPE]

Use HEC-RAS for channel flood analysis to take into account backwater effects. Look for a nearby downstream control boundary condition, like a weir, culvert or bridge, or other constriction.

Put your surveyed sections into HEC-RAS and start investigating all the channel parameters, experiment with different flowrates while doing so like the others have suggested.

 

[tramphardrocker]

I hand cranked the regression equations using the field data I gathered and came up with the same number a flood dept. guy got with his ArcView program. So, I guess I'm on the right track. Thanks for input.

 

[beej67]

Well, all you've really proven with that exercise is that his ArcView program is using the regression equations.

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

http://www.campbellcivil.com