Thursday, September 17, 2009

Sediment Mass Plot/Invert Change Issues

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.

A very common "head-scratcher" for sediment modeling is the situation where your mass plot is showing a lot of accumulation of sediment at a given cross section (mass plot curve rises with time, indicating deposition), but the channel invert shows no change over the same time period. Usually this is due to the placement of the moveable bed limits.

Keep in mind the spatial plot only shows the minimum elevation node for each cross section. As a result, a depositional zone may not be captured correctly in that plot, if the mobile bed stations are missplaced. This example shows the mobile bed limits placed low in the channel. As the bed aggrades, the low point moves out to the mobile bed limit and its elevation remains constant throughout the simulation. The Sediment Spatial plot will indicate the cross section is stable, when in reality it is depositing a lot of sediment. A check of the mass plot will help to figure this out. One solution would be to move the bed limits out onto the banks, so that the minimum cross section point moves up with the deposition . Also, you could move the bank stations inside of the moveable bed limits, if that is acceptable to your conveyance distribution (remember, the minimum bed elevation is reported inside the bank stations). Or, you could leave it as is, and just recognize that the channel invert plot is misleading.

Tuesday, September 15, 2009

Initial Reservoir Elevation-Pilot Flow

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.

A common question when modeling reservoirs with cross sections (dynamic routing) is how do I get my reservoir elevation to start where I want it to? Using Pilot Flow at your inline structure is a very easy way to guarantee your water surface elevation starts exactly where you want it to. First, you want to make sure that the pilot flow is equal to your initial conditions flow into the reservoir. Now, if you only did this, the reservoir would never fill up, as it would be passing everything that came into the reservoir, right out of it (no storage). To get your reservoir where it should be, go to the unsteady flow editor, and select Options...Internal RS Initial Stages. Pick the cross section just upstream of the inline structure and assign it the starting water surface elevation you desire. Now keep in mind this will only set the initial water surface elevation in the reservoir. For the remainder of the simulation, the reservoir level will move based on inflow, outflow, and storage (as it should). However, pilot flow is constant throughout the simulation. It does not change with reservoir level. Therefore only use this technique if the pilot flow is small compared to the flood discharges you are looking at. A great application is a dam breach flood where the discharge from the dam breach will be orders of magnitude greater than any pilot flow you use.