I have a combined 1D/2D model consisting of a reservoir (modeled as level pool) connected to a 1D reach via an inline structure (the dam) with the 1D reach connecting to a 2D area further downstream. The dam is 63 ft high and stores 1400 ac-ft. I used the Breach Parameter calculator (Froehlich 1995) in the Breach Data editor to set the breach parameters. The model runs fine with no errors but the breach flow calculated by the model is only ~3,500 cfs. When calculating the breach flow by hand using the Froehlich 1995 formula, the breach flow is > 45,000cfs. I can’t seem to figure out why the model is so off.
Any suggestions as to why the model is off or what I can do to trouble shoot this issue would be greatly appreciated.
Things I have tried:
1. Deleted the storage area and added it back in
2. Checked profile plot to make sure the model is breaching
3. Checked the model breach dimensions and parameters to make sure the dam is breaching how the froehlich equations predict – all checks out
4. Checked the volume accumulation at a profile line cut within the 2D area downstream of the dam. It shows ~ 80 ac-ft at the end of the simulation and based on the 1D profile plot, the reservoir has drawn down about 55 feet corresponding to a volume loss of ~1200 ac-ft
5. Changed time and step and cross section spacing – no change to result