Friday, March 10, 2017

Rating Curves for Dams

Written by Jesse Rufener, P.E., CFM | GPD Group
Copyright © The RAS Solution 2017. All rights reserved.

Version 5 of HEC-RAS allows the use of rating curves for inline structures (and lateral structures).  Rating curves can be added through the Outlet RC feature in the inline structure editor.  Inline structure rating curves can be useful for evaluating the impacts of structures, such as labyrinth dams, where it may be difficult to correctly capture the geometry and/or flow properties within the inline structure editor.

Click the Outlet RC button and in the following menu you can add your rating curve based on upstream flow or water surface elevation. 

Please note that the rating curve must account for any influence of downstream tailwater as HEC-RAS does not with the rating curve option. 

When using the inline structure rating curve, the top of the dam must be above the highest elevation on the rating curve if you only want the rating curve to account for flow over the outlet feature.  You can check the Stage and Flow hydrograph to see how the rating curve is contributing to flow past the inline structure.  In the image below, the total flow is a combination of the rating curve and flow over the structure as I didn’t have the top of dam elevation set high enough in the first iteration.

Note that the peak flow is 34,471 cfs with a peak HW stage of 927.86.  When I raised the top of the dam to be above the top elevation of the rating curve, the peak flow is 33,967 cfs with a peak HW stage of 930.47 as shown in the Stage and Flow hydrograph below.  The peak flow rates are within 1.5%, but the peak HW elevations differ by almost 3’.

The Outlet RC feature is also available for lateral structures.