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Chapter 5 PDF (2.8 Chapter Coastal Processes of the Elwha River Delta By Jonathan A. Warrick, Andrew W. 5 Stevens, Ian M. Miller, and Guy Gelfenbaum Abstract To understand the effects of increased the west. This establishes surfzone sediment supply from dam removal on currents and littoral sediment transport marine habitats around the Elwha River that are eastward along much of the delta, a basic understanding of the delta. Offshore of the surfzone the region’s coastal processes is necessary. currents are largely influenced by tidal This chapter provides a summary of circulation and the physical constraint the physical setting of the coast near to flow provided by the delta’s headland. the Elwha River delta, for the purpose During both ebbing and flooding tides, of synthesizing the processes that move the flow separates from the coast at and disperse sediment discharged by the tip of the delta’s headland, and this the river. One fundamental property produces eddies on the downstream side of this coastal setting is the difference of the headland. Immediately offshore between currents in the surfzone with of the Elwha River mouth, this creates a those in the coastal waters offshore of situation in which the coastal currents the surfzone. Surfzone currents are are directed toward the east much more largely dictated by the direction and frequently than toward the west. This size of waves, and the waves that attack suggests that Elwha River sediment will the Elwha River delta predominantly be more likely to move toward the east come from Pacific Ocean swell from in the coastal system. Chapter 15 112 Coastal Habitats of the Elwha River, Washington—Biological and Physical Patterns and Processes Prior to Dam Removal Introduction (Warrick and others, 2008). No fine- grained sediment has been deposited on Dam removal on the Elwha River the submarine delta, although deposits will result in fundamental increases in of fine-grained sediment are expected sediment delivery to the Strait of Juan de following dam removal (Warrick and Fuca (Strait). To understand and predict others, 2008). the effects of this increased sediment Another element of the Elwha supply on marine habitats around the River delta setting is the fundamental delta, a basic understanding of coastal difference between processes and processes is necessary. This chapter currents within the surfzone and synthesizes information about the within the coastal zone offshore of the physical setting of the Elwha River delta surfzone (fig. 5.2). The fundamental to provide a general understanding of difference between these zones is the the coastal processes that will determine relative importance of waves in dictating future changes to the coastal ecosystems currents. Near the beach, breaking in and around the Elwha River delta waves provide a dominant influence on after dam removal. currents (Komar, 1998). Waves in the surfzone are not only responsible for the back-and-forth motions of water as each wave passes, but also can drive currents Coastal Setting along the beach when waves approach at an oblique angle (fig. 5.2). These The Elwha River discharges into wave-generated currents are the primary the Strait of Juan de Fuca, a waterway drivers of alongshore beach sediment between the Salish Sea and the Pacific movement in littoral cells (Komar, Ocean (fig. 5.1). The Elwha River delta 1998). is sheltered from much, but not all, of Offshore from the surfzone, waves the waves and swells from the Pacific still impart substantial back-and-forth Ocean, although it is exposed to the movements in the water-column and abundant and energetic exchange of along the seafloor, but the currents water between the Salish Sea and the primarily are driven by tidal, wind- Pacific Ocean. This setting—sheltered generated, or other coastal currents. from the ocean and yet exposed to This may result in a situation, such as strong tidal and estuarine circulation—is shown in figure 5.2, in which currents a defining feature of the Elwha River in the surfzone and the coastal zone are delta. in opposing directions. This situation Another defining feature is the would cause differences in the transport broad and shallow submarine delta that directions of water and sediment in extends several kilometers offshore these zones. Although shear will not from the present shoreline (fig. 5.1B). always be present between the surf- and The origins of this submarine delta coastal zones, transport within these are discussed in Warrick and others zones will be produced by different, (2011, chapter 3, this report). This and largely independent, processes. feature strongly influences the waves For the Elwha River delta, the surfzone and coastal currents that act upon this extends only several to tens of meters section of coast. Acoustic, video, and off of the shoreline depending on wave physical sampling techniques have height and tidal stage. More information shown that the submarine delta seafloor about these processes is available in the is composed of coarse sand to boulders reviews of Nittrouer and Wright (1994) and Komar (1998). Coastal Processes of the ElwhaCoastal River Setting Delta 113 A. B. 125° 123° 138 0 49° Submarine Delta 136 -30 Stra 134 -60 it of Salish Sea Jua E n deFuca 132 W -90 48° Elwha fig. B River 130 -120 Northing, in kilometers PACIFIC 128 Elwha River Ediz Hook -150 Elevation, in meters OCEAN Port Angeles -180 290 295 300 305 310 315 Easting, in kilometers 47° EXPLANATION Shaded relief from Puget Sound LIDAR Consortium Sites of current measurement Base modified from U.S. Geological Survey W West 1:100,000-scale digital data E East Figure 5.1. Elwha River and Strait of Juan de Fuca, near Port Angeles, Washington. (A) The Strait of Juan de Fuca is the primary waterway between the Salish Sea and the Pacific Ocean. (B) The local topography and bathymetry of the Elwha River delta. wave crests Coastal Current oblique waves Surfzone Current breaking waves zone of shear Figure 5.2. Schematic diagram showing alongshore currents of nearshore waters. Surfzone currents (green arrow) are caused by breaking waves when they have oblique approach angles. The coastal currents (blue arrow) offshore of the surfzone are influenced by tidal, wind-driven, or other regional currents and may or may not be in the same direction as the surfzone currents. Chapter 5 wa11-0558_fig 05-2 114 Coastal Habitats of the Elwha River, Washington—Biological and Physical Patterns and Processes Prior to Dam Removal Waves Strait of Juan de Fuca, the Elwha River delta “fetch” (the distance over which The Strait of Juan de Fuca has an wind can blow to generate waves) is intermediate wave climate between about 70 km. One effect of this 70-km the energetic outer coast exposed to fetch is that waves generated within the Pacific Ocean and the more placid the Strait will be limited to periods conditions of the Salish Sea (Shipman, of less than 8 seconds (Komar, 1998; 2008), although few measurements U.S. Army Corps of Engineers, 2002). of the wave conditions within the Waves with longer periods come Strait of Juan de Fuca are available to from Pacific Ocean swell, where the corroborate this understanding. To better fetch is much longer. Our records of characterize waves, the U.S. Geological waves at the Elwha River delta show Survey (USGS) installed acoustic combined influences from low period Doppler current profilers (ADCP) along (4–8 seconds), locally generated wind the seafloor at two sites at about 10 m waves and longer period (greater than water depth, and sampled waves hourly 8 seconds) ocean swell (fig. 5.3B). The between January 26 and May 2, 2006 most common condition measured (fig. 5.1B). Although limited in their consisted of wave periods in excess of duration, these measurements provide 8 seconds (fig. 5.4B), which indicates important information about the general that Pacific Ocean swell dominate wave wave conditions at the delta. conditions more frequently than wind Waves heights at the Elwha waves. River delta regularly exceeded 1 m and Because the waves at the Elwha occasionally exceeded 2 m (fig. 5.3A). River delta originate from the northwest In contrast, waves of the outer coast of direction, these waves break at oblique Washington exceeded 10 m during this angles along much of the shoreline, and same winter-to-spring season (Ruggiero these obliquely breaking waves drive and others, 2010). The wave heights surfzone currents and littoral sediment at the Elwha River delta rose and fell transport. Observations of wave rapidly in response to locally generated breaking angles have been obtained wind waves and swell from in the from airborne light detection and Pacific Ocean fig.( 5.3). Wave heights in ranging, or “lidar,” of the ocean water excess of 1 m generally lasted for only surface, historical aerial photography of a day or less (fig. 5.3), and wave heights the beach, and numerical modeling of were most commonly less than 0.5 m wave propagation in the Strait (fig. 5.6). (fig. 5.4A). Combined, these observations show The narrow shape of the Strait that the northwest wave approach of Juan de Fuca causes waves to be imparts normally aligned waves on the focused into a narrow northwestern western side of the delta and oblique directional band. For example, more wave breaking angles on the eastern than 90 percent of the wave energy side (fig. 5.6). Thus, transport of littoral that approached the Elwha River delta sediment should be negligible west of during our observations came from the the river mouth and eastward on the northwest (fig. 5.5). Warrick and others eastern side of the river mouth. This (2009a) provide further evidence that conceptual model for littoral sediment northwest wave directions dominate all transport is consistent with recent seasons of the year.
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