Appendix A: Wave Exposure Calculation

Wave processes are the dominant controlling process for shore morphology and sediment redistribution and are probably the dominant control of biota that use the shore zone. Consequently, it is important to have an index of wave exposure for each shoreunit.

Ideally wave exposure estimates would be based on the consideration of wave energy units; however, the calculation of wave energy for a particular shore unit would involve the use of complex wave climate models that use wave fetch characteristics, historical wind climate measurements from shore stations with over-the-water corrections for the area under consideration, wave generation routines, and wave refraction and shoaling routines. The application of these routines would be required for each shore unit.

This method for estimating wave exposure is based on some standard engineering practices for estimating wave heights for a particular wind speed and direction. It was developed for the British Columbia Physical Shore-zone Classification (Howes et al. 1994). The method involves the consideration of the wave fetch window; that is, the open-water area offshore from the shoreunit over which waves can be generated by winds - the larger the fetch window, the greater the wave exposure.

Estimation of wave exposure involves the consideration of two fetch indices: effective fetch and maximum fetch. The wave exposure estimates provided by this technique represent a first approximation of wave exposure. Important controls such as the associated local wind climate and wave refraction are ignored for the sake of simplifying the estimate. As such, the Wave Exposure estimates provide a first-order estimate of wave energy expended within the shoreunit.

Intertidal biotic assemblages have been used as an index of wave exposure categories and have been found to agree reasonably well with the Wave Exposure estimates defined from the "fetch model". Analysis of detailed ground survey stations of intertidal biota showed about 75% agreement with the fetch model and about 85% agreement with video-imagery revisions of the "fetch model". In other words, the Wave Exposure categories defined as part of the fetch model agree with community assemblages observed in the field.

Effective Fetch

Effective fetch calculation involves the measurement of the fetch distance along several directions from a given point from the shore and is a standard engineering measurement for shore protection studies (CERC 1977).

Wave fetch measurement values and exposure calculations

Unit	Shore Normal Azimuth (o)	Shore Normal Fetch (km)	Left 45o (km)	Right 45o (km)	Max. Fetch Azimuth (o)	Max. Fetch (km)	Effective Fetch (km)	Exposure Category
A	135	15.7	8.2	19.5	131	29	14.6	SP
B	066	19.0	13.0	11.0	073	40	14.9	SP
C	035	13.5	11.5	50	082	75	23.6	SE

To simplify the large number of measurements required for a mapping area (for example there are over 1,800 shore units mapped in the Southern Strait of Georgia), a "modified effective fetch" measurement was developed. The "modified effective fetch" technique involves the measurement of three fetch distances: the shore-normal or perpendicular to the general trend of the shore unit, 45^o to the left of the shore-normal and 45^o to the right of the shore normal.

These three measurements are used to compute a modified effective fetch for the shoreunit based on the fetch equations:

Maximum Fetch

The wave climate of a particular point cannot be characterized by effective fetch alone because waves may be generated in an area remote from the shore unit and propagate into the area of the shore unit. These waves are commonly refered to as swell. A good BC example is that of the Juan de Fuca Strait where waves are generated locally (indexed by effective fetch) and are relatively small, but large swell, generated in the open Pacific, can penetrate into the Strait. The maximum fetch of a shoreunit is intended to provide an index of the swell waves and, to a lesser extent, refraction effects.

The Maximum Fetch is the maximum fetch distance in kilometres that can be measured from a centre point of the estuary.

Wave Climate Fields for Calculation of Estuary Wave Exposure

There are several wave climate data fields that need to be calculated to determine the wave exposure of the estuary. These are defined below:

Maximum Fetch Direction - the azimuth (in degrees from true north) of the direction of the maximum fetch.

Maximum Fetch Distance - the distance in kilometres of the maximum fetch as measured along the Maximum Fetch Direction.

Shore Normal Direction - the azimuth (in degrees from true north) of the normal to the general orientation of the shore unit. That is, if the general trend of the shoreline is from northwest to southeast with open water to the east), then the Shore Normal Direction would be 45^o.

Fetch Distances - the distances in kilometres as measured along a line 45^o to the left of the Shore Normal, along the Shore Normal and along a line 45^o to the right of the Shore Normal.

Modified Effective Fetch - the distance in kilometres as calculated from the Fetch Distance Measurements.

Exposure Category - the exposure category provides a summary indicator of wave exposure for the unit.

The following classes of wave exposure have been utilized and are derived from knowledge of Maximum Fetch and Modified Effective Fetch. These are accompanied with the appropriate code for entry in the wave exposure attribute of the project and overview database, and definition.

VP	Very Protected :	Maximum wave fetch less than one kilometre; usually the location of all-weather anchorages, marinas and harbours.
P	Protected:	Maximum wave fetch less than 10 km; usually areas of provisional anchorages and low wave exposure except in extreme winds.
SP	Semi-protected:	Maximum wave fetch distances in the range of 10 to 50 km. Waves are low most of the time except during high winds.
SE	Semi-exposed :	Maximum wave fetch distances between 50 and 500 km. Swells, generated in areas distant from the shore unit create relatively high wave conditions. During storms, extremely large waves create high wave exposures
E	Exposed:	Maximum wave fetch distances greater than 500 km. High ambient wave conditions usually prevail within this exposure category, which is typical of open-Pacific type conditions.

Effective and maximum fetch wave exposure Matrix

Modified effective fetch (km)

This procedure, although preliminary and subject to refinement, offers an objective, repeatable basis for estimating wave exposure. Wave exposure, in turn, is of critical importance in determining species distribution, sediment mobility and a variety of other shore processes.