HEC RAS W/ LATERAL FLOWS

[sam74]

I've posted this on the Boss hec ras forum as well but have had no responses.

I'll try to keep this short. Initially we have modeled (steady state) what appears to be an inadequate culvert system on a blue line stream. It is at an older goverment facility. Culverts procede downstream with two 36", two 30", two 30", and two 36" RCPs among others above and below.

The initial model worked out okay and showed flooding on most storm events at most all culverts from top to bottom. It was a simple model assuming all flooding went over the road deck and back into the stream. The bad part is that over 30 years of observation shoots holes in our model.

An expert reviewed our model and input and saw no issues. Stream monitoring devices and rain gauges are set to be placed to aid in future model adjustment. However, the expert has suggested we look at lateral flows into a nearby parallel street to see if this would reduce flooding at the road decks. We have three such situations occuring with two 30", two 30", and two 36" RCPs. We have sectioned off approximate lateral weir dimensions and developed a rating curve for each. This was used as output from the system for these three occurences slightly upstream of the road decks. We also played with road deck min/max WSE settings to prevent runoff across them.

I finally have everything adjusted so that the program will run and pass all storm events lateral flow optimizations. The first culvert looks good upstream and has a WSE close to what was calculated in the original model. The area downstream of this culvert hits the channel and goes crazy. 1 and 2 year events look reasonable (previously the only flows not crossing the road deck). Then every higher storm event's flow and WSE gets progressively lower. My input in the flow table gets higher but the program begins to ignore the table and ramps down the flow of the 25, 50, 100, and 500 year events to values of less than 1 cfs. The remainder of the model is a mess after this.

Ever heard of the program ignoring the flow table? Any suggestions ou there?

 

[gbam]

Are you sure the program ignores the table or just loses the flow out the lateral structures?

Sounds like the model ran fine without lateral structures. I would begin with their input data. Try to balance your flows inorder to account for the discharge and where it leaves the main channel.

Also, try running a single larger storm event, say 100-year, to get the bugs worked out of the model.

Hope this helps!

 

[sam74]

I'm not sure where the flow is going but it is not out the lateral structure. For instance my flow input for the upstream section is 72.77, 86.03, 117.81, 135.43, 160.73, 190.45, 201.94, and 243.61 cfs for storm events in increasing order. When looking at the lateral structures profile output table my Qs upstream of the structure are 72.77, 86.03, 84.12, 67.71, 46.52, 22.95, 10.68, and 0.08 cfs in the same order as the storm events as listed above. Doesn't make much sense does it? I think I'm going to eliminate the rating curves and try an actual weir.

 

[gbam]

The rating curves should work. Confirm your station for the lateral structure. Does your any of your rating curves for the split fit these discharge values?

Are you changing flow rates at any xsec? or is it uniform through the entire reach?

Also you can round your flow rates to the nearest cfs. It wont make any impact.

 

[sam74]

I'm pretty sure I have double checked everything. Rating curves contain flow ranges and depths that should be more than sufficient. I also have tried several different varitations with similar results.

My flow values change at numerous xsecs due to additional inflow as well as upstream flow changes due to lateral structures. But generally only one flow change input between culvets.

Another employ here suggested that maybe the s/w didn't agree to well with the majority of the channel flow being diverted through the lateral structure and the big loss of energy gradient in the main channel?

Anyway I've put that particular scenario aside for the moment and I am looking at modeling the same system with multiple reaches. Where previously I had a lateral structure I will continue a reach with a road deck and no culvert at the end to model the flow into the side street. Then I will add a lateral culvert from this reach to a downstream reach to simulate the culverts in the main channel. Hopefully this will work better.

Yeah, I've got a bad habit with decimals.

 

[sam74]

That last idea didn't work well at all. I thought my original problem might be a bug so I emailed the files to the HEC RAS team. I actually got a reply the same evening. It took us a while to get on the same page but I think it is a bug. It appears after several manipulations and scenarios that HEC RAS has trouble with new steady flow input downstream of an optimized lateral structure. I originally thought it might have something to do with the fact that I had the three lateral structures so close but if I delete the steady flow inputs in the channels downstream of them and allow only the flow calculated by the optimization process to pass through I get pretty good looking numbers (constants). The channel continues on to a bridge crossing and on the other side of this (which is about 300 ft downstream of the last lateral structure) I had another steady flow input. Looking at the profile table on this section it did the same thing as it did in the midst of the lateral weirs. For instance my steady flow input for 1,2,5,10,25,50,100, and 500 yr events was 154, 164, 166, 171, 186, 200, 199, and 213. The flows listed at this same cross section in the profile table were 154, 160, 120, 80, 20, 0, 0, 0. I couldn't get the HEC RAS guy to admit to anything but he said they were working on this in their next release. He offered the advice that I break the area with multiple lateral weirs into three separate reaches and optimize them by hand. I did and this worked. Hope this helps someone

 

[gbam]

Sounds like you were able to get it worked out. I have not run into this problem yet. But I will keep an eye open for it. GL!