Boundary conditions

[Anonymous]

I am currently performing HnH for a small bridge in AL. The bridge is less than 200′ downstream of a small dammed lake. Due to the lake’s ability to lag the storm I have opted to use the SCS methodology for hydrology and therefore are running an unsteady model. The model runs great and yields reasonable results. However, this project sits at the upstream edge of what could be called a swamp and historically the existing road floods often with smaller storms. Therefore I assume it to be tailwater controlled. The model does not yield this high water elevation using normal depth and I have no stage/flow data to use.

The question: How would you recommend modeling this and getting this elevated outlet WS elevation?

[Marulke]

How far downstream of your reach of interest does your model extend?

I would place the downstream boundary so far down that you capture any geometry that may control the water level at the bridge. I would also check that reasonable changes to the downstream boundary condition (slope, if you use normal flow) does not effect the slope.

You may also consider if inflow from tributaries downstream of the bridge may affect the tailwater level, for example other inflow to the swamp.

When running a flood hydrograph (un-steady) you should also consider the flood duration and total flood volume. If the swamp has a large storage capacity an will dampen the flood, a flood with longer duration (larger total volume) but lower peak may give the highest water levels. Try a steady run with the Qmax from your unsteady and compare levels.

[Anonymous]

Im having the same problem and i cant find a good idea to solve it. I think the unsteady flow lacks of a known water surface elevation as a downstream control.

My case is a tributary wich delivers its flow to a big river. The only information I own about the river is the 100 year flood water surface elevation, then I wanted to find a Known WS condition to enter the elevation.

The only solution I can find is modeling a 1D downstream my small river “tributary” and connect it to the big one. The main problem about it is that I dont have the flow data of the big river and data about its slope or section.

If someone solves this issue or have a better idea please dont hesitate to help us.

Thank you

[Author]

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