When to Add in Flow Changes

[themoy08]

I was wondering what the general consensus is of when to add in a flow change to a HEC-RAS model, specifically when dealing with USACE/FEMA 100-year evaluations.

Obviously if there is a confluence with a major tributary that warrants a flow change, but when doing models for FIS Reports how is it determined when they enter in a flow change. Is there a rule of thumb of say after you add so much drainage area you should make a flow increase, or something like that? In the FIS reports a lot of flow changes happen a road crossings, dams, etc.

Basically we are evaluating a site that is in an approximate FEMA zone, but has a detailed study area just downstream of the site. In adhering to FEMA is it more appropriate to use their reported flow from the detail area (that is 1.5 miles downstream) through my entire reach, or develop flows at the upstream end of the site and make flow change at some point along the stream reach. Which brings me back to the original question of when to make that flow change? Thanks for any input.

[cameron]

The simple, quick, and cheap answer would be to just use the discharge from FEMA as it is approved and you don’t have to do anything.

How much does the drainage area change from the FEMA discharge to your study area? If it is small then the FEMA discharge probably won’t change much. If you have a tributary coming into your project area and would change the discharge significantly, then I would think about doing an analysis to come up with better discharge.

A rule of thumb I have used in the past is having flow changes when the drainage area changes by 10%, at confluences, FEMA discharge locations, detention basins/dams, and sometimes major roads.

Since your study is in an Zone A, another option you may also look at just doing regression analysis (StreamStats) as it is simple and FEMA approves that. It would need to be checked against the FEMA discharge downstream though as you wouldn’t want it to be higher than downstream.

[themoy08]

Awesome thanks for the tips. Yeah the one thing that makes this a little weird is that the detailed section uses some regional analysis USACE did in the 70’s and it produces flows way higher than what StreamStats does (USGS SIR 2008-5102 [PA]). The FIS also says they used SIR 2008-5102 in the approximate areas. I’m just not sure how they meshed the two sections together seeing as the flows are drastically different. My initial plan for it all was to use the StreamStats at the upstream end then bump up the flow towards the downstream end and use the wsel from the detailed FEMA section as my DS end conditions. Those end conditions are based on flows that are a lot higher than StreamStats though.

I was thinking of just taking the conservative approach and using the detailed area flow through my reach, and will probably just stick with it hearing your thoughts. Maybe I’ll do an alternative model run with StreamStats and normal depth end condition as a point of comparison.

Thank again.

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