SHOREZONE LITERATURE REVIEW - DISRUPTION OF PHYSICAL PROCESSES|
Disruption of Physical Processes
Attenuation of wave energy, and sediment recruitment and transport are the primary physical processes that are altered by shoreline structures. When shallow littoral waters become partially or completely isolated from circulation with the main body of the lake, localized thermal, chemical, and physical regimes become established. These alterations can be beneficial to some species, but detrimental to others. When separated from the buffering effects of the main water body, littoral waters exhibit generally warmer and much greater diel temperature fluctuations. Larval forms of some fishes (e.g., catostomids and cyprinids) may benefit from the lack of wave energy and elevated temperatures, as could Eurasian milfoil (Myriophyllum spicatum L.). Erosion of an undeveloped shoreline by wave action results in a continuous input of sediment that is episodically supplemented by large inputs from slope failures. Sediment added to the system by erosion or slope failure is transported along shore by wave energy in the direction of prevailing winds (Lawrence and Davidson-Arnott 1997). Irregular shoreline orientation creates distinct areas of deposition and erosion (Nordstrom 1989). Shoreline areas lacking in sediment supply are prone to increased erosion of existing beach substrate, and the reduction of sediment sources in one area results in erosion in other areas (Lawrence and Davidson-Arnott 1997). Shoreline structures could potentially interrupt the process of sediment transport by preventing the input of sediment from the shore, disrupting wave energy, or blocking the movement of sediment along the shoreline.
Macdonald et al. (1994) concluded that shoreline armoring could deprive beaches of sediment recruitment from the uplands, resulting in scour of the existing substrate and ultimately reducing the beach to substrate particles too large to be mobilized by wave action. Erosion in front of bulkheads is exacerbated by reflected wave energy (Macdonald et al. 1994). However, Kraus and McDougal (1996), in an updated literature review of the effects of seawalls on beaches, concluded that reflected wave energy is "probably not a significant contributor to beach profile change or to scour in front of seawalls," but the scour is more likely a result of along-shore processes, or a combination of long-shore and cross-shore processes. A gradient in sediment flux is necessary for scour or a reduction in beach profile to occur, and gradients typically occur along shore as a result of insufficient sediment supply (Kraus and McDougal 1996). Thus, the deprivation of sediment supply caused by bulkheads could ultimately result in lowering beach profiles in front of bulkheads and increased erosion at neighboring properties, including properties without bulkheads. Reflected wave energy at seawalls could contribute to the suspension of particles, thus facilitating their transport (Kraus and McDougal 1996).
These processes would be expected to occur in Lakes Washington and Sammamish at a smaller scale due to the relatively reduced energy of the lacustrine systems. The implications for ESA-listed salmonids are uncertain. Changes in sediment composition could affect prey availability, habitat availability, and spawning. Juvenile fall chinook in the Columbia and Snake Rivers showed a preference for shallow water over sand substrates (Key et al. 1994a and 1994b; Garland and Tiffan 1999), and preliminary data indicates a similar preference by chinook fry in Lake Washington (Tabor, pers. comm., 9 June 2000). A reduction in fine sediments (from an interruption of supply) and shallow water due to bulkhead construction could reduce the availability of shallow sandy habitat.
Lorang et al. (1993) investigated shoreline changes that resulted from lake-level regulation in Flathead Lake, Montana, and found that docks and seawalls "intercept the natural supply and migration of plunge zone gravels, resulting in accelerated, localized backshore erosion on the downdrift side and heavy aggregation of migrating gravels on the updrift side." An effect of the regulation of the level of Flathead Lake was that the extended high lake level (similar to Lake Washington - early spring filling, extended into late fall) prevented the summer establishment of riparian vegetation (Lorang et al. 1993). Historically, peak water levels in Flathead Lake increased steeply in May with spring runoff, peaked briefly in June, and declined steeply in July and August; beaches formed at high lake level were colonized by riparian vegetation over the summer (Lorang et al. 1993). These findings could have implications for the establishment of riparian and emergent vegetation in Lake Washington. Additionally, if gravels are captured around shorezone structures in Lake Washington, these accumulations could further enhance the attraction of smallmouth bass, and potentially increase the availability of desirable spawning substrates associated with these structures (see observations by Malcom, pers. comms., 1999).
Bonham (1983) proposed emergent vegetation as effective wave-energy attenuation and scour prevention, and as an alternative to armored shorelines. Bonham (1983) tested the ability of four species of emergent vegetation to attenuate wave energy in large British canals and rivers, and found that a 2-meter-wide bed of any of the four species on a 1v to 4h slope, was capable of dissipating approximately two-thirds of boat wake energy and inhibiting wave break. Rolletschek and Huehl (1997) described the impacts of reed-protecting structures on shorelines; however, only the abstract was available. The structures, apparently designed to protect shoreline marshes from wave action, enhanced the accumulation of organic sediments within the marshes, modifying sediment chemical properties (Rolletschek and Huehl 1997).