III. B. 2. Wave Characteristics |
Waves are characterized by length, period, and height, and are the physical representation of energy moving through water. The short-period waves generated by local winds and vessel wakes are superimposed on the water elevation that varies with tide, season, and longer-term influences. In addition to winds and vessels, waves may be generated by geologic sources (i.e., large-scale bluff collapse, seismic forces). Though the magnitude of the latter can be theoretically calculated based on energy considerations, the occurrence is not yet predictable and is beyond the scope of this study. The wave energy is translated across the water and is ultimately expended on the shoreline, working to erode, transport, and deposit beach sediment (U.S. Army Corps of Engineers 2002; Terich 1987). Compared with other locations in the U.S., Puget Sound is considered to be a moderate wave-energy environment, even in the most exposed locations (Macdonald and Witek 1994).
Wind Waves
Wind waves are short-period waves that are created by winds blowing over a distance of open water, or fetch. The wave conditions in Puget Sound are normally quite mild (less than 3 ft wave height), but waves of considerable height (greater than 6 ft) have been reported during storms. Wind blowing over the water surface imparts energy to the water, which is expressed in surface current and in the development of surface waves. The main factors that affect the generation of such waves are fetch – the distance over which the wind works on the water; duration – the length of time the wind blows over the water; and wind speed. In open water, with known fetch and time, and for a given wind speed, wave height and period can be calculated. The basic definitions applied to surface waves are shown in Figure III-8.
Figure III-8. Surface wave definitions. L is wave length, H is wave height, and d is the still-water depth. Wave steepness is defined as H/L. Other definitions and relationships can be found in USACE (2002)
Natural events, such as storms, wave heights, and wind velocity and duration, are categorized by their statistically determined return interval. The “100-year storm,” for instance, is defined as the storm that has a 1/100 (1%) chance of occurring in a given year, and the 50-year storm has a 1/50 (2%) chance of occurrence in a year. Because the events are governed by independent random processes, it is possible to have more than one, 100-year event in a given year or in successive years. The specialized field of extreme value statistics (or extremal analysis) is used to arrive at these return-interval estimates. By assuming that the distribution of storm intensity follows certain statistical rules, one can arrive at return estimates by extrapolating from shorter measurement records. It is typical that the 100-year wave height can be estimated by extrapolation from a 20-year record of extreme wave data. Coastal structures are usually designed to withstand conditions with given return intervals. The design condition is selected based on analysis of the risk of encountering and surviving the extreme event during the life of the structure. The intended length of service, cost of replacement, and consequences of failure are all factors that should be considered when selecting the return interval to be used for design.
Knowledge of the wave conditions at a coastal site is necessary to predict sediment transport rates, design methods to protect or restore the beach, or design infrastructure, such as marinas and port facilities. Design wave criteria are usually based on years of wave data that allow calculation of a given return-interval condition. Coastal structures may be designed to resist the 50- or 100-year wave condition. The annual sediment transport rate along a stretch of beach may be determined by applying an appropriate numerical model for an entire year of wave action. The wave data necessary to complete these engineering or management activities may be obtained from direct measurement of the waves or, as is more often the case, by calculation of wave conditions based on measured wind, fetch, and duration. A large proportion of the annual transport may occur during a single storm. Because storm frequency, duration, and strength, as well as more moderate weather conditions vary from year to year, it is necessary to consider long-time series data to decide what constitutes “normal conditions.”
Winds have been measured in many more locations and for longer periods of time than have waves, so the known relationship between wind speed, duration, and fetch are used to “hindcast” waves. Many methods of wave prediction are available in the oceanographic literature, from simple empirical equations (U.S. Army Corps of Engineers 1984) to elaborate numerical models (Meteorological Service of Canda 2000). Under relatively uniform and steady wind conditions in the open ocean, waves can be determined with fairly high accuracy. Calculation of waves in Puget Sound and around land masses such as Bainbridge Island requires special consideration of factors such as over-water wind speed, air-sea temperature differences, and steering by land forms. In addition, as waves enter shallow water or encounter currents, they change height and direction (U.S. Army Corps of Engineers 2002). Because sediment transport is highly dependent on the angle the wave makes with the shoreline, these shoaling effects should be considered. Such wave transformations are treated in more detail below.
There are no permanent meteorological stations on Bainbridge Island. The West Point Lighthouse on the north side of Elliot Bay may be used to obtain regional wind information. Both wind speed and direction data from 1984 to present are available from this station. These data should be carefully evaluated for application to specific sites around Bainbridge Island, because local features may substantially change wind conditions and their related waves and currents. Washington State Ferries (WSF) has also begun collecting wind data aboard a limited number of ferries transiting Puget Sound routes, including those between Bainbridge Island and Seattle. Depending on location and numbers of observations, some of these data may be useful for wave estimates.
The ShoreZone Inventory (Washington State Department of Natural Resources 2001) classifies the shorelines of Bainbridge Island by Wave Exposure Class (see Appendix A Wave Exposure Map). The fetch distance limits the maximum wave heights around the Island. This means that under even very strong winds blowing for a long time, the wave height will reach only a limited maximum because the maximum height is ultimately governed by the distance over which the wind blows and not by the wind speed or duration (U.S. Army Corps of Engineers 2002). The ShoreZone Inventory has assigned three exposure classes to the waters around Bainbridge Island based on fetch distance and the potential wave heights that may be generated: ‘semi-protected’ with a fetch distance of 6 to 30 miles (10 to 50 km); ‘protected’ with a fetch of between 0.6 to 6 miles (1 to 10 km); and ‘very protected’ with less than 0.6 miles (1 km) of fetch. Based on the USACE (2002), these distances correspond to the following significant wave heights: 0.6 mi fetch – 0.8 ft wave height; 6 mi fetch – 2.6 ft wave height; 30 mi fetch – 6.0 ft wave height. The “significant wave height” is a statistical way of representing the sea state and corresponds to the average of the highest 1/3 of the waves present. The maximum single wave may be nearly twice as high as the significant height (Goda 1985). The east side of Bainbridge Island and Restoration Point is predominantly semi-protected. The west side of Bainbridge Island and around Eagle Harbor are predominantly protected. Finally, the bays and inner harbors of Bainbridge Island (such as the Point Monroe Lagoon, Port Madison Bay, Manzanita Bay, Fletcher Bay, inner Blakely Harbor, inner Eagle Harbor, and inner Murden Cove) are very protected.
The Coastal Zone Atlas provides estimates of deep-water wave heights (Washington State Department of Ecology 1980). Generally, deep-water wave heights around all of the shoreline areas of Bainbridge Island are estimated to range from 0.5 to 2 feet, with the exception of Restoration Point on the southeast tip of Bainbridge Island, where deep-water wave heights are estimated to be 2 to 4 feet, based on the long fetch toward the south. Again, these estimates are based on the significant wave height.
The eastern shore of Bainbridge Island is exposed to both southerly and northerly winds (and waves) from Puget Sound. The southern and western shorelines face smaller bodies of water, and the potential for large storm waves is somewhat limited because of the reduced fetch. The maximum fetch in Puget Sound can reach 35 miles. Around Bainbridge Island, the typical fetch distance is between 4.7 and 7.9 miles (7.6 and 12.7 km) (Schwartz et al. 1989). Around Bainbridge Island, waves come from mostly the southwest, and the wave height can range from 2 to 5 feet high (Canning and Shipman 1995a). The maximum significant wave height occurs during winter storms and can reach heights of 5 to 6 feet (Washington State Department of Ecology 1979a; Washington State Department of Ecology 1980). These estimates vary somewhat based on the assumptions of the authors but are generally consistent. They should not be used for engineering purposes, because interaction with the local sea bottom changes wave characteristics through the process of shoaling, refraction, or diffraction.
Few actual measurements of waves have been made in Puget Sound. Wave buoy data were collected at a location two miles southwest of West Point from September 1993 to December 1994 (Shepsis et al. 1995). Based on this record, the significant wave height was reported to be 3.3 ft (wave period of 5.1 sec). Additionally, Shepsis et al. (1995) reports that wave heights from 1.0 to 1.3 ft were observed 40% of the time, wave heights from 1.3 to 2.25 ft were observed 25% of the time, wave heights from 2.25 to 3.2 ft were observed 15% of the time, and waves greater than 3.2 ft were observed 10% of the time; the remaining times were reported calm (Williams et al. 2001).
Vessel-Generated Waves
Vessels operating in Puget Sound generate wake waves that have characteristics that depend on the size, speed, hull shape, draft of the vessel, and water depth in which the vessel is operating. The waves generated by an individual vessel are of short duration relative to the amount of storm-generated (or wind-generated) waves; however, depending on the number of vessels and their characteristics, the wake waves may cause a beach to establish a new equilibrium. This new equilibrium may result in changes to the beach slope or size and gradation of beach material.
Recent studies have shown that the passenger-only fast ferries operating through Rich Passage at full operational speeds (i.e., 34 knots) can produce nearshore wave heights of 2.1 feet (and wave periods of about 8.4 sec), and other vessels may produce waves up to 2.2 feet (and wave periods of about 4.5 sec) (Anchor Environmental 2000). Both of these measurements were made with wave gauges deployed at about –4 ft MLLW for 1- to 2-months duration.
Tsunamis
The extension of the surface expression of the Seattle Fault passes through Bainbridge Island just south of Blakely Harbor. The east-west linear feature can be seen on the Topographic and Bathymetric Relief Map (Appendix A) and is named the Toe Jam Hill Fault. The U.S. Geological Survey (USGS) predicts that a zone surrounding this fault line will be a zone of probable ground rupture in the event of a major earthquake (Nelson et al. 2002). The expected ground motion would be uplifted on the south side of the fault and subsidence north of the fault. Amount of motion depends on the magnitude of the earthquake (Nelson et al. 2002). The NOAA Pacific Marine Environmental Laboratory (PMEL) has developed numerical models of probable water-level change and tsunami inundation associated with the potential displacement (Koshimura and Mofjeld 2001). Earthquakes and the consequent tsunami are considered inevitable in the long run, but the timing is presently beyond the ability of prediction. Coastal ecosystems would be impacted by the ground motion, attendant slope failure, and tsunami.
Wave Transformation
As waves move toward shore, the bottom eventually affects them. This occurs at a water depth that is about half of the wave’s length (i.e., the distance between two wave crests, Figure III-8). Wind waves in Puget Sound seldom exceed lengths of 80 feet, so would feel bottom at a maximum of about 40 feet. The encounter with the bottom slows the wave travel and reduces the wavelength. Since the wave period does not change, the wave becomes steeper and eventually breaks. The characteristics of the breaker; e.g., plunging, surging, or spilling, depends on bottom slope and, for the same offshore wave condition, the breaker type can vary with tidal elevation. For waves that approach the shore with their crests at an angle to the local depth contour, the part of the wave in shallow water travels more slowly than that part in deeper water and the wave crest will bend toward the shore. This process, known as refraction, tends to align the wave crest with the shoreline and decrease the angle the breakers make with the beach. Though the process of refraction can be performed graphically, in all but the simplest cases, numerical models are used to predict the transformation of waves from deep to shallow water. Figure III-9 shows the path that wave rays, drawn perpendicular to the wave crest, would follow as waves approach a hypothetical shoreline. The wave energy expended at the shoreline depends on the breaker height squared, so accurate prediction is important to estimate sediment transport.
When waves break, part of their energy is lost in turbulence, while the other part is transferred to beach sediment. After breaking, water surges up the beach, exerting strong forces on sediment. A return flow along the bottom, the undertow, balances this shoreward movement of water. Areas on the shore where wave rays converge receive more wave energy and will have higher breaker heights than areas where waves diverge. As a result of this convergence and divergence, wave energy is concentrated at the headlands and diminished in bays.
Figure III-9. Wave Refraction (Source: Macdonald et al. 1994).
The above explanation relates to refraction caused by interaction with the bottom. Refraction also takes place when waves encounter currents that slow one part of the wave more than another. Tidal currents may, therefore, influence the wave direction. This aspect of wave propagation has not been applied to studies around Bainbridge Island.
Another type of wave transformation is called diffraction. Diffraction occurs when a barrier, such as a small island, a breakwater, or a jetty, interrupts a train of waves. The energy is transferred along a wave crest, and this creates waves in the sheltered area. The combined effects of both diffraction and refraction are important in modifying wave energy and direction. These are taken into account in recent numerical calculation methods (Kirby et al. 2002).
These processes are important because they change the breaking wave height and the angle that the waves make relative to the shoreline and, therefore, affect the direction and rate of wave-generated sediment transport.
Wave-Generated Currents
The currents in the Bainbridge Island nearshore are generated by tide, local wave breaking, and wind. The tides are the most persistent and predictable source of current, but the wind waves are also important, because breaking waves suspend sediment, which is then transported by even minor current flow. Tidal currents tend to act along the length of a shoreline and vary in magnitude with distance from the deepest part of a passage to the shore. Waves also generate longshore currents. Waves that break at an angle to the shoreline impart momentum in the direction of wave breaking and generate a current in the surf zone. Even when waves approach with their crests parallel to the shore, they transport water, which builds up against the shoreline. The excess may then move longshore until it finds an outlet where it can move offshore in the form of a rip current. On the open sandy coast, rip currents are often observed at the breaks in sand bars. Rip currents can also be observed adjacent to natural features or structures, such as boat ramps or groins that extend perpendicular to the shoreline. Rip currents are narrow coastal jets that transport water and suspended sediment away from the beach. Undertow is a less rapid though sometimes persistent seaward current along the bottom that transports water to the offshore. Figure III-10 is a schematic that shows several types of currents that originate from breaking waves (see also Komar 1998a).
Figure III-10. Current Systems
| <<iii.b.1. tides |