Drawdown in area of abrupt changes

[dwolford]

I have a HECRas model with a 400ft-wide floodplain and closely spaced cross-sections in a localized area where modifications are proposed. The right overbank includes an area raised by fill and with buildings which force 60% of the overbank flows into the (efficient bedrock) channel. An upstream cross-section includes the high (right) overbank and buildings just described, and a downstream cross-section (10' downstream) has a lower right overbank with no obstructions. A 0.5' drawdown occurs at the upstream cross-section though the froude # there is LT 0.5. Apparently, flows forced out of the right overbank by terrain and blockages force more water in the channel where flow is more efficient thereby causing an increased total velocity which results in the drawdown. Since flow here is subcritical, no drawdown should occur (in the real world), but this may be a problem with model instability when very close cross-sections are used in a wide floodplain. Is there a way to remove the drawdown in this situation?

 

[gbam]

dwolford-have you visited the specific energy diagram for sub critical regime, recently. If memory serves me correctly, one gets a reduced WSE for a contraction in sub critical regime and a bump in a supercritical regime. I vaguely remember that the two results were counter intuitive. School was a long time ago. Anyone else recall this?

 

[francesca]

Yes, more velocity head means lower water surface elevation, very easily seen at a bridge.