Peak flows with HEC-RAS 1

[WaterGeo]

New to HEC-RAS but need to find peak flows just downstream of a small earthen dam, where the downstream reach passes under two roads.

I have HEC-GeoRAS and HEC-RAS.

Any tips or links to papers that can walk me thru this?

TANKS!!

Chris Erichsen
GeoIntel GIS
WWW.GEOINTEL.COM

 

[RWF7437]

You can't do this with HEC-RAS. HEC-RAS is primarily an hydraulics program, NOT an hydrology program. In other words, you need to estimate, or know , the flows in order to calculate water surface elevations and other things of interest BEFORE you can model your system in HEC-RAS.

For flood studies submitted to FEMA these flows are often calculated using regional regression equations. But, any method may be used depending on the purpose of your study and the accuracy needed. An accuracy of +/- 100% is not uncommon since natural flows are only probabilities.

good luck

 

[RWF7437]

http://www.wsi.nrcs.usda.gov/products/w2q/H&H/Tools_Models/Ras.html

 

[cvg]

try HEC-HMS or HEC-1

 

[WaterGeo]

I need to calculate the peak flow upon dam breach, not watershed runoff. Is that still something other than HEC-RAS?

Chris Erichsen
GeoIntel GIS
WWW.GEOINTEL.COM

 

[RWF7437]

There is a dam break analysis function in HEC-RAS 4.0. I've never use it but I believe it is based on the older SMPBreak developed by the Dept. of the Interior.

By all means, get the documentation, if you don't already have it and read it. See the link provided in my previous post.

good luck

 

[cvg]

try the FEMA website below

http://www.fema.gov/plan/prevent/damfailure/publications.shtm

 

[Drew08]

If all you need is the peak discharge and not the full breach hydrograph, use the peak breach discharge eq. developed by Fread, whose method is now used in BOSS DAMBRK (originally NWS DAMBRK), Eq 6-7 in:

http://www.civilsoftware.co.kr/Pds/dambrk_user mannual.pdf

Or the NRCS TR-60 peak breach discharge method, which will likely be more conservative:

http://directives.sc.egov.usda.gov/TR_210_60_Second_Edition.pdf

 

[cvg]

you will also need to determine if you are investigating a sunny day failure (piping) or operational failure (overtopping during IDF).

 

[WaterGeo]

Thanks all.

I have a small earthen dam, where 1/4 mile downstream is a road culvert then an interstate culvert.

I need to calculate peak flow at both culverts upon dam breach (will do sunny day, 10/25/100 yr storms)

So I shouldn't create a drainage profile in HEC-RASGeo for this?

Thanks!!

Chris Erichsen
GeoIntel GIS
WWW.GEOINTEL.COM

 

[Rye1]

Hey Geo,
I would not approach your problem with HEC-RAS and HEC-geoRAS. Kind of like shooting a rabit with a cannon. Do you have to do a "breach" analysis? Where I work, you have to show that the spillway and barrel/riser system will pass the correct flows. Depending upon the downstream impacts of a failure, you may have to model a 1/2 PMP, or 1/3 PMP or just the 100 year event. For small earthen dams, I like to delineate by hand, I would go with ArcHydro for large basins. I take my hand delineations and build polygons in GIS. That give me subbasin size. Next is CN and Tc estimation. I like to do this in HydroCadd. HydroCadd is a nice little program with a small learning curve. It does a few things very well and is not as complicated to use as HEC-1 or HEC-HMS.

If you are required to conduct a breach analysis. I would probably go with HEC-RAS. You are going to get into some unsteady modeling which is another animal altogether.

Robert Billings, PE, PH
New River Engineering, PLLC

http://www.newrivereng.com

 

[WaterGeo]

That you Robert, very helpful.

I do use ArcHydro, so are you saying I should delineate the drainage area using ArcHydro. I don't have HydroCad, so is there another tool or paper to use for calculating the CN for 1/2 PMP?

Thanks!

 

[RWF7437]

"I need to calculate the peak flow upon dam breach, not watershed runoff. Is that still something other than HEC-RAS?"

AND

"I need to calculate peak flow at both culverts upon dam breach (will do sunny day, 10/25/100 yr storms)"

AND
"I do use ArcHydro, so are you saying I should delineate the drainage area using ArcHydro. I don't have HydroCad, so is there another tool or paper to use for calculating the CN for 1/2 PMP?"

OK, I'm confused. If you not are calculating watershed runoff why would you need to delineate the watershed? or calculate 10/25/or 100 year storms? , or know the Curve Number (CN)? Why would you use 1/2 PMP, is there some regulation requiring you to do so ?

The PMP is usually used to design the emergency spillway. HEC-RAS will model two types of breaches of earthen dams: overtopping or piping. Either type of breach can also be modeled for the almost instantaneous failure of concrete dams.

If you are working toward meeting some regulation, that regulation should describe, clearly, the methods of analysis and design which are acceptable.

 

[cvg]

http://www.nws.noaa.gov/oh/hrl/hsmb/hydraulics/dam_break.html

read Fread's paper

 

[WaterGeo]

Its a permit, and my obj is to get the peak flows at two road crossings downstream of an earthen dam. Is this a watershed runoff model (i.e. HEC-HMS)? If so, how do you account for the stored water volume?

(thanks cvg for the read - good info)

 

[RWF7437]

A permit?
From who (plural)?
To do what?
Under what conditions?
What if it is impossible to meet those conditions ?

Any sizable dam, storing any significant volume of water, located within a quarter mile of a highway culvert, is likely to exceed the culvert capacity and overtop the roadway. It is similarly likely that this flood wave can move downstrem for a considerable distance with little or no attenuation and also overtop the Interstate highway. What will you do then ?

 

[Rye1]

"Its a permit, and my obj is to get the peak flows at two road crossings downstream of an earthen dam. Is this a watershed runoff model (i.e. HEC-HMS)? If so, how do you account for the stored water volume?"

You need to put together a watershed runoff model. I see to concerns with your explanation of the project. First, you need to look at your state's regulations and talk with the regulators. You need to know if your dam is classified as low, intermediate, or high hazard. This classification should point you toward the required precipitation frequency event.

Once you have the regulations and you know what you have to work with, you can start on the watershed runoff model.

I would strongly suggest, and I don't know your background, so I apologize if I step out of bounds, but if you've never put together a runoff model, maybe you should contact an engineer in your state that has dam hydrology experience. There is a lot of liability tied up in these types of projects even if you are only doing calculations. If the dam were to ever breach, and someone was hunt, the attorneys would cast a wide net and sue ever engineer that just looked at the dam on a Sunday evening stroll.

 

[WaterGeo]

Thanks Robert, While I am not new to water quality and runoff models, I was unsure which was more suitable for dam break analysis. I was offered to do this by a client, but since it is permit related, and I do not maintain insurance for these sorts of projects, I am passing on it.

Oh, to sum up the thread - the answer is: HEC-HMS, with ArcMap pre-processing using ArcHydro

 

[Rye1]

Kewl. You have to know when to holdum, when to walk away, and when to run...

I've done all three including running.

HEC-HMS after ArcHydro is becoming an industry standand related to FEMA floodplain mapping. Unlike the Hec-geoRAS preprosser for HEC-RAS, the Hec-geoHMS hasn't ever really gained a foothold.