Abstract

Automating Manning's "n Coefficient Value Assignments for Hydraulic Modeling
Track: Water Resources
Authors: Kyle Gallagher

As part of a flood inundation modeling effort for the USACE Tulsa District, an automated approach was developed using Python to assign Manning's "n coefficient values to approximately 9,000 hydraulic cross section overbanks, covering over 2,200 river miles within the Arkansas and Red River basins. Manning's "n coefficient values account for surface roughness in open flow channels and are used to calibrate one-dimensional hydraulic models. The approach substituted for the Extract N Values tool in HEC-GeoRAS due to having extremely wide cross sections that produced numerous elevation data points, thus allowing for a less intensive HEC-RAS calibration. Numerous raster calculations were undertaken to determine how much of a cross section's area fell in each land use category, with a decreasing influence the further away from the channel. ModelBuilder was used for testing and the process was converted to a Python script in order to leverage user-input parameters and batch processing.