Tuesday, September 15, 2009

Initial Reservoir Elevation-Pilot Flow

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © 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.


  1. One thing to note with the pilot flow is that it is a minimum flow the model will use.
    If you're passing more than this flow over the spillway, it will not take additional water to account for the pilot flow (a good thing). This could become a problem if you're modeling multiple events in a row as the reservoir will not go down to normal pool level after an event because of the pilot flow.

  2. We are modeling a bridge that crosses the upstream tail of a pond that has a downstream dam. Is there anything amiss with modelling the dam purely with cross-sections in (rather than an in-line structure)? It doesn't have any outlet structures (gates, pipes), just a primary spillway (a large concrete weir) and an earthen emergency overflow spillway.
    We are using static (FEMA effective) flows and starting at normal depth downstream of where the two spillway channels converge.

  3. I have the filling curve of the reservoir which developed at constant discharge rate. In my case study I have to do sunny day failure is there any possibility that I can maintain the reservoir water elevation at specific level and model start reducing it with time during dam breaching?

    1. Yes, you just have to balance inflow to the reservoir with outflow. Whether you do that with pilot flow, or with gates, as long as what is coming in is what is going out, your reservoir will stay level (until you breach it of course!).


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