Puget Sound Fetch

Introduction

The unobstructed open water distance fronting a beach or fetch is one of the three variables on which the flux of wave energy onto a shoreline depends (the other two being wind velocity and duration). In the absence of long-term wave or wind records, it may be the only one of the three variables that can be measured. In closed water bodies such as Puget Sound, the direction of net-shore drift (sediment transport) is largely determined by maximum fetch ( Schwartz, Wallace et al., 1989). As a result, fetch is a common proxy for wave energy and finds its way into a wide variety of shoreline indices in both scientific and management applications.

The Shore Protection Manual (CERC 1984) details how to calculate fetch manually for a single beach. The technique requires measuring 9 radials centered about the direction from which the wind blows. The mean length of the 9 radials is the fetch. This process must be repeated for each location and for each direction under consideration. Obviously, the task is laborious if fetch must be determined for more than one or two beaches.

I was faced with the need to calculate fetch for every shoreline in Puget Sound as part of my research. I chose to solve the problem once and for all ( for Puget Sound) by calculating the fetch for every water pixel in a 90-meter DEM of Puget Sound. To save others the trouble, I have provided the resulting rasters below.

References Cited

Download the fetch length calculation script

You can download the fetch calculation script for ArcGIS 9.0 by visiting my script page. This script integrates with the new geoprocessing framework so that you can use the fetch script in a wizard, as part of a larger model, or in command line mode.

Download the Fetch Rasters

There are 18 rasters in the table below: one for each point of the compass, one raster that represents the arithmetic mean of the 16 directional fetch rasters---the mean fetch (Figure 1), and finally, the 90-meter DEM used to identify landmasses.

Each fetch grid was calculated according to the Shore Protection Manual method (i.e., averaging the 9 radial distances at 3 degree increments with the 5th radial centered on the target compass point). In order to make it easier to align high-resolution DEMs with the fetch grids, the fetch is calculated across the downwind shoreline and over downwind landmasses. The algorithm I used involved rotating the land raster 9 times, hence there is some error in the fetch measurement on upwind shorelines (from re-sampling the shoreline 9 times). This should not be much of a problem in practical use because these are fetches that are < 180 meters (i.e., hardly any waves).

In some cases fetches are shown to be NEGATIVE. Negative fetches are Minimum distances (but could be 100's or 1000's of kilometers longer). They result on water bodies open to the ocean or the edge of my land DEM. For example, almost all fetches in the Strait of Juan de Fuca are minimums since I didn't measure fetches out past the Pacific Ocean. Fetches that are POSITIVE are Absolute distances. These fetches are closed by upwind topography. The mean fetch grid was calculated from the absolute values of the 16 directional fetch grids, but the negative signs were put back in if any of the original fetches was a minimum measure.

The value of the raster is the fetch distances in meters.

The data is in ESRI ASCII Grid format. This format is an ASCII text file with a simple 7 line header followed by the grid array (in row-major order). Cell resolution is 90 meters, projection is UTM Zone 10 North.

Raster Description	File Names (Size)
Fetch, Wind Blowing from 000.0 (N)	psfetch0000.zip (10 Mb)
Fetch, Wind Blowing from 022.5 (NNE)	psfetch0225.zip (12 Mb)
Fetch, Wind Blowing from 045.0 (NE)	psfetch0450.zip (11 Mb)
Fetch, Wind Blowing from 067.5 (ENE)	psfetch0675.zip (12 Mb)
Fetch, Wind Blowing from 090.0 (E)	psfetch0900.zip (11 Mb)
Fetch, Wind Blowing from 112.5 (ESE)	psfetch1125.zip (12 Mb)
Fetch, Wind Blowing from 135.0 (SE)	psfetch1350.zip (11 Mb)
Fetch, Wind Blowing from 157.5 (SSE)	psfetch1575.zip (11 Mb)
Fetch, Wind Blowing from 180.0 (S)	psfetch1800.zip (10 Mb)
Fetch, Wind Blowing from 202.5 (SSW)	psfetch2025.zip (11 Mb)
Fetch, Wind Blowing from 225.0 (SW)	psfetch2250.zip (12 Mb)
Fetch, Wind Blowing from 247.5 (WSW)	psfetch2475.zip (12 Mb)
Fetch, Wind Blowing from 270.0 (W)	psfetch2700.zip (11Mb)
Fetch, Wind Blowing from 292.5 (WNW)	psfetch2925.zip (12 Mb)
Fetch, Wind Blowing from 315.0 (NW)	psfetch3150.zip (12 Mb)
Fetch, Wind Blowing from 337.5 (NNW)	psfetch3375.zip (12 Mb)
Mean Fetch Distance	meanfetch.zip (17 Mb)
Land DEM from which fetches were calculated	psland_90m.zip (7 Mb)