Upper Midwest Environmental Sciences Center

Application of Wind Fetch and Wave Models for Habitat Rehabilitation and Enhancement Projects - 2012 Update

By Jason Rohweder, James T. Rogala, Barry L. Johnson, Dennis Anderson, Steve Clark, Ferris Chamberlin, David Potter, and Kip Runyon

Abstract

Models based upon coastal engineering equations have been developed to quantify wind fetch length and several physical wave characteristics including significant height, length, peak period, maximum orbital velocity, and shear stress. These models, developed using Environmental Systems Research Institute's ArcGIS 10.x Geographic Information System platform, were used to quantify differences in proposed island construction designs for the Harper's Slough Habitat Rehabilitation and Enhancement Project in the U.S. Army Corps of Engineers St. Paul District. Weighted wind fetch was calculated using land cover data supplied by the Upper Mississippi River Restoration - Environmental Management Program's (UMRR-EMP) Long Term Resource Monitoring (LTRM) for each island design scenario. Figures and graphs were created to depict the results of this analysis. The difference in weighted wind fetch from existing conditions to each potential future island design was calculated. A simplistic method for calculating sediment suspension probability was also applied. This analysis involved determining the percentage of days that maximum orbital wave velocity calculated over the growing seasons of 2008-2012 exceeded a threshold value taken from the literature where fine unconsolidated sediments may become suspended. This analysis also evaluated the difference in sediment suspension probability from existing conditions to the potential island designs. Bathymetric data used in the analysis were collected from the UMRR-EMP LTRMP and wind direction and magnitude data were collected from the National Oceanic and Atmospheric Administration, National Climatic Data Center.

Introduction

The U.S. Army Corps of Engineers (USACE) tasked the Upper Midwest Environmental Sciences Center (UMESC) of the U.S. Geological Survey (USGS) with upgrading geospatial models developed to quantify wind fetch length and calculate several physical wave characteristics that can be altered by Habitat Rehabilitation and Enhancement Projects (HREP). The models originally developed in ESRI ArcGIS software version 9.3 were upgraded to be able to operate using the most current version (10.x). Within the wind fetch model a feature was added to allow the automated calculation of weighted wind fetch results. Features were also added to the wave model to allow the automated calculation of sediment suspension probability and also a revised water density parameter which accepts a raster surface as input. A new model was also developed to allow the user to delineate the area of potential effects based on the magnitude of difference between alternative project design results.

Using the upgraded models, UMESC was then asked to perform specific analyses to model weighted wind fetch and also calculate the probability that fine unconsolidated particles would be suspended due to wind-generated waves for Harper's Slough HREP within the St. Paul District of the USACE. Wave output data were created with algorithms that used wind fetch, wind direction, wind speed, and water depth as primary input parameters. The results of these analyses depict how wind fetch and fine unconsolidated particle suspension are affected by alternative HREP management scenarios, allowing managers to quantify gains or losses between these proposed management scenarios.

Wind Fetch Model

Wind fetch is defined as the unobstructed distance that wind can travel over water in a constant direction. Fetch is an important characteristic of open water because longer fetch can result in larger wind-generated waves. The larger waves, in turn, can increase shoreline erosion and sediment resuspension. Wind fetches in this model were calculated using scripts designed by David Finlayson, U. S. Geological Survey, Pacific Science Center, while he was a Ph.D. student at the University of Washington (Finlayson 2005). This method calculates effective fetch using the recommended procedure of the Shore Protection Manual (USACE 1984).

The wind fetch scripts that the model operates from were developed by Finlayson using the Python scripting language and were originally designed to run on the ArcGIS 9.0 (Environmental Systems Research Institute ([ESRI] Redlands, California) Geographic Information System (GIS) platform. However, in 2008, these scripts were updated in order to operate using the most current ArcGIS revision at that time, 9.2 and now were updated again in order to operate using the most current ArcGIS version, 10.x. The model was also modified to more efficiently meet the needs of USACE planning personnel. These modifications give the model the ability to calculate wind fetch for multiple wind directions based upon a text file listing individual compass directions and also to calculate a weighted wind fetch output if individual wind direction weightings are supplied.

Wave Model

A model was constructed within ArcGIS 10.x to create several useful wave outputs. Significant wave height, wave length, spectral peak wave period, shear stress, and maximum orbital wave velocity can all be calculated using this model according to equations taken from the USACE Shore Protection Manual (USACE 1984) and the USACE Coastal Engineering Manual (USACE 2002). This model uses wind fetch, wind direction, wind speed, and bathymetric data as inputs to calculate these wave outputs. The model was updated to allow the user to automatically calculate the probability that aquatic areas within the area of interest will have maximum orbital wave velocities sufficient to suspend sediments based upon the wind direction and speed data used and a user-defined maxiumum orbital wave velocity threshold.

Delineate Area of Potential Effects Model

A new model was developed to allow the user the ability to identify the areas where the differences between the values from two raster datasets are above a certain specified threshold (magnitude). So, for instance, if the user wanted to know where there is at least a 600 meter change in weighted wind fetch when comparing existing conditions to another scenario the model will generate a shapefile identifying those areas. Required inputs to the model include two rasters to compare against one another, a number identifying a desired magnitude of difference, and a location to save the output shapefile.

07/08/14 Update

Updates were made 07/08/14 regarding the way a 1-hour average wind speed is calculated using the wave model. This is described on page 20 of the revised Contract Report below using the following text "The wind averaging interval in seconds (t; 120 seconds for the 2-minute interval used here) is used to compute a 1-hour average wind speed. This is done by calculating the ratio of wind speed of any duration to the 1-hour wind speed using the equations below (USACE 2002)."  An input line was added to the tool's dialog window for the user to enter the wind averaging interval. The default is 120 seconds.

Download Wind Fetch and Wave Models

To use the wind fetch and wave models, there are some preliminary steps that need to be followed for them to function correctly on the computer. First are a few software requirements that need to be met:

1. ArcGIS 10.0 or more recent
2. A Spatial Analyst License
3. Python 2.4 or more recent (Automatically installed with ArcGIS)
   

Pywin32 (Python for Windows extension)
Pywin32 allows Python to communicate with COM servers such as ArcGIS, Microsoft Excel, Microsoft Word, etc. Python scripting in ArcGIS cannot work without this extension. This extension can be downloaded at: http://sourceforge.net/projects/pywin32/files/pywin32/

References Cited

Finlayson, D. 2005, david.p.finlayson - Puget Sound Fetch. School of Oceanography, University of Washington, Seattle, WA. Accessed October 16, 2007, from http://david.p.finlayson.googlepages.com/pugetsoundfetch

U.S. Army Corps of Engineers (USACE), 1977, Shore Protection Manual, Third Edition. Coastal Engineering Research Center, Fort Belvoir, Virginia.

USACE, 1984. Shore Protection Manual, Coastal Engineering Research Center, Fort Belvoir, Virginia.

USACE, 2002, Coastal Engineering Manual, Engineer Manual 1110-2-1100, U.S. Army Corps of Engineers, Washington, D.C. (in 6 volumes).

Impact of UMESC Science

This project was completed in 2013.

Please direct problems downloading files to Jason Rohweder.