Flow splitting in HEC-RAS

[Trackfiend]

I am trying to figure out a way to split the flow in a HEC-RAS model. I have a main channel that has a distributary that splits off near the end of the main channel. Here are the knowns:

Main Channel Distributary
L = 16000 ft L = 1400 ft
A = 1675 ft2 A = 686 ft2
Q = 14000 cfs Q = ?
R = 7.9 R = 3.4

What I am attempting is to find what percentage of flow diverts from the main channel and into the distributary since HEC is requesting a flow at the beginning of the distributary and just beyond the distributary in the main channel.

I also had to run HEC-RAS with bogus flows at each of these junctions in order to obtain my wetted perimeter and flow area. For simplicity, I made sure the entire area and perimeter were used.

Any tips?

 

[Trackfiend]

One formula that I've come up with is this:

Q1 = Q2*(SQRT(L2/L1))*(A1/A2)*(R1/R2)^(2/3)

Am I wondering down the right track here?

 

[Trackfiend]

I've also noticed that HEC-RAS has a "flow optimization" tool that allows the user to guess at the flows and then HEC will give what should be the flows at the split in each channel.

Should I be matching up the energy grade elevation and water surface elevation at the junction for each of the (3) cross sections (upstream of split, distributary, and downstream of split) within a certain tolerance?

 

[gbam]

I would suggest using the optimization and provide an initial guess. Be careful for convergence errors/warnings. I have had to adjust my guess a couple of time before the model converged. Make sure you look at the output and the allowable iterations if you run into convergence issues.