Calibrating Watersheds in Maryland and Adventures in HEC RAS
Katie Scott, PE
Coastal Resources, Inc.
Annapolis, MD
Author: Scott, Katherine A., P.E.
In Maryland, the Application of Hydrologic Methods document developed by the Maryland Hydrology Panel (appointed by MD SHA and MDE) lays out a process to calibrate watersheds to existing USGS river gages. Regression equations have been developed based on the gages and the calculated peak discharge is considered calibrated when it falls within one standard deviation of the regression equations peak discharge. To simplify the process of calibration the GIS-Hydro program was developed jointly by MD State Highway Administration (MD SHA) and University of Maryland (UMD).
There have been instances where using GIS-Hydro was not effective in calibrating watersheds, specifically smaller watersheds and those on the Eastern Shore. CRI utilized a different approach and method to calibrate smaller watersheds. In a study of six watersheds, a correlation between the size of the watershed and the runoff flow was developed. Another instance where it was difficult to calibrate a watershed to the regression equations was on the Eastern Shore. In this case, local river gages were compared to calibrate the watershed for a stream restoration project.
The presentation will show how watersheds were calibrated. This process may be replicated in other areas of Maryland. The importance of calibrating watersheds will be discussed. Calibration of watersheds can prevent erosion of downstream stream channels and can provide confidence in the design of structures and delineation of floodplains.
In addition some tips and tricks on using HEC RAS will be addressed such as: importing DEMs using a built in process, burning out stream channels using channel modification, using courant number, calculate maps to show change in shear from existing to proposed, elevate floodplains based on jurisdiction requests, using HMS to enter hydrographs into HEC RAS, working with dam breaches, and fixing unstable models by opening 2d connections. (I am thinking this part may be a separate talk or even a workshop.)
About Katie Scott
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