VI. A. 3. Impacts - Physical Processes and Biological Considerations |
Shoreline stabilization structures have a variety of physical and biological impacts to the nearshore environment that often depend on the location (both along the beach profile and shoreline), design, type of material used, and size of the structure. These structures may cause profound impacts to nearshore geomorphology, hydrology, and wave energy, some of the most important factors controlling the development and distribution of nearshore habitats (Conceptual Model, Figure II-3).
Possibly the most significant effect of stabilization structures is a direct impact to regional geomorphology via the impoundment of potential natural sediment sources (Macdonald et al. 1994). These structures can induce three main types of sediment loss (Macdonald et al. 1994):
Erosion of fine-grained sediment from the active beach, causing it to become narrow and coarser
Erosion or impoundment of sediment, stored in backshore areas, which is usually added to the longshore transport system during severe storm events.
Impoundment of sediment from adjacent upland sources (e.g., feeder bluffs), that previously reached the beach, but is now trapped behind the structure beyond the reach of waves.
This impoundment of natural sediment sources can influence erosion processes that alter the structure and function of native habitats (and properties) at areas both near and distant from the site of impact. Shoreline structures designed to affect shoreline sediment transport (e.g., groins) will cause similar beach erosion and accretion impacts in adjacent areas (Pilkey and Wright III 1988).
Placement of structures below the ordinary high water mark may exert their most chronic impacts on nearshore hydrological processes, which include altered wave energy and current patterns, obstruction of littoral drift and longshore sediment transport, and altered fluctuations of temperature, salinity, and water levels (Williams and Thom 2001). Hardened shorelines with vertical or recurved slopes also alter hydrology by deflecting wave energy downward, causing scouring of the bottom sediment at the toe and periphery (Macdonald et al. 1994) (Figure VI-8) and may alter groundwater dynamics relative to the upland. According to this representation, wave reflection forces increase as armoring methods intensify. This wave reflection ultimately results in elevation loss and habitat change (e.g., loss of eelgrass). Studies conducted by the WSDOT in Rich Passage along the south shore of Bainbridge Island revealed that armored shorelines reflected larger breaking waves, causing increased scour in the upper intertidal zone (Anchor Environmental 2001).
Placement of hard structures also radically alters the distribution and extent of existing habitats, resulting in a large-scale replacement of soft beach substrates with hard, rocky shore habitats that support different animal communities (Williams and Thom 2001). One of the more widely recognized biological impacts is the permanent loss of fish (e.g., surf smelt, Pacific sand lance, and rock sole) spawning and shellfish habitat on upper intertidal beaches. Exacerbating these direct impacts is the indirect loss of additional spawning habitat from downdrift beach coarsening and erosion, and the loss of shading riparian vegetation (Macdonald et al. 1994; Thom et al. 1994b; Macdonald 1995; Antrim and Thom 1995; Penttila 1996; Allee 1982; Macdonald 1995; Antrim et al. 1995).
Figure VI-8. Relative beach impacts versus shore protection method (from Macdonald et al. 1994).
Direct physical disturbance associated with construction of all shoreline stabilization structures temporarily causes several types of direct impacts, which vary with the size and extent of the structure and the time needed to build it (Williams and Thom 2001). In the short term, heavy equipment associated with construction causes local noise (e.g., pile driving), which can disrupt nesting waterfowl and alter animal behavior and distributions. Air and water pollution from machinery and watercraft exhaust emissions may also cause local impacts (Mulvihill et al. 1980; Kahler et al. 2000). Other construction impacts include temporary bottom disturbance, which increases sediment suspension, erosion, sediment compaction, and turbidity. Other obvious and immediate impacts associated with construction include burial or excavation of both subtidal and intertidal habitats and fauna, trampling, and direct mortality from heavy equipment operation (e.g., dredging or barge groundings) (Armstrong et al. 1991).
Water quality may degrade in areas of extensive shoreline modifications. Residential and commercial development and impervious surfaces in upland habitats and watersheds can increase stormwater runoff, sediment erosion, and loading of nutrients and toxic pollutants (Williams and Thom 2001). Shoreline development can increase local nutrient loading to the point of eutrophication, with removal of vegetative buffers exacerbating these problems (Short and Burdick 1996).
Ambient light levels in nearshore habitats are increased when structures replace riparian vegetation, which provides shade to the upper intertidal zone. Shade reduces temperature and desiccation stress to insects, marine invertebrates, and fish eggs laid by intertidal spawning fish species (Penttila 1996; Penttila 2000). Likewise, the increase in artificial lighting that often accompanies anthropogenic shoreline alterations can modify salmon behavior and predator avoidance (Simenstad et al. 1999; Azuma and Iwata 1994). Conversely, overwater shading by anthropogenic shoreline alterations may also unnaturally reduce local light levels, reducing primary productivity rates and eliminating critical shallow-water vegetated habitats.
Shoreline stabilization methods may affect the recognized functions of estuarine and nearshore marine habitats for juvenile salmon by altering substrate, hydrologic, and water property conditions that affect prey production (Williams and Thom 2001). Shoreline modifications usually involve riparian vegetation removal, which displaces trees and shrubs that normally overhang onto beaches. More current research is clarifying the important role of leaf litter and insect fall from this riparian vegetation in nearshore detritus production and salmon food webs (Simenstad and Cordell 2000; Levings and Jamieson 2001) (unpublished data, KCDNR 2002). Structures may fragment the nearshore landscape, thereby altering natural patterns of habitat use and movement by fish, as well as by animals that use upland habitats (e.g., birds and mammals) (Castelle et al. 1994; Desbonnet et al. 1994). Shoreline structures that intrude into the intertidal zone also affect patterns of detritus and large woody debris recruitment (Hugh Shipman, WDOE, personal communication, 2002). Though not well studied in marine nearshore habitats, large woody debris provides added structural complexity that provides shelter and refuge for a variety of species in freshwater systems (Knutson and Naef 1997; Kahler et al. 2000).
As shown above, shoreline stabilization has substantial effects on physical processes that reduce the number and diversity of habitats, as well as the intertidal habitat area (Douglass and Pickel 1999). These modifications have substantial effects on nearshore processes and the ecology of many species, including spawning habitat for forage fish such as surf smelt, sand lance, and herring, as well as prey production and refuge areas for salmonids (Macdonald et al. 1994; Allee 1982). Thom et al. (1994b) summarized the potential effects of shoreline armoring to selected nearshore resource species in Puget Sound based upon knowledge of critical links between physical effects, habitats, and biological resources (Table VI-1).
The seawall constructed at Lincoln Park in West Seattle provides the best-documented example from Puget Sound of the direct (e.g., alteration of upper beach substrata) and indirect impacts (e.g., lowering of beach and coarsening of substrata) of a hard shoreline structure on nearshore habitats (Figure VI-9). The lesson learned at Lincoln Park was that the seawall, which was originally located above the influence of the tide, had major effects on seaward habitat conditions well into the subtidal zone. The effects were evident and extensive for decades after placement of the seawall, and only re-nourishment of the beach with sand and gravel could begin to restore some of the original (pre-seawall) habitats and functions. This beach continues to need periodic renourishment to maintain some historic habitat elements. However, the process of renourishment has its own associated impacts on plant and animal communities that recolonize over an extended period of time.
Table VI-1. Summary of Armoring Effects to Resource Species in Puget Sound (from Thom et al. 1994b) .
| Armoring Effects | |||||||
| Resource Species | Armoring-related Habitat Shift | Loss of Spawning Habitat | Loss of Shoreline Riparian Vegetation | Loss of Wetland Vegetation | Loss of Large Organic Debris | Changes in Food Resources | Loss of Migratory Corridors |
| Surf Smelt | l | l | l | Å | |||
| Pacific Sand Lance | l | l | l | Å | |||
| Rock Sole | l | l | l | Å | |||
| Juvenile Salmonids | l | l | l | l | l | l | |
| Pacific Herring | Å | Å | |||||
| Hardshell Clams | l | Å | l | ||||
| Geoduck | m | ||||||
| Oysters | m | m | m | ||||
| Dungeness Crab | Å | Å | Å | ||||
| Sea Cucumber | m | m | |||||
| Sea Urchins | m | m | |||||
l: Well-documented evidence of negative effects
Å: High potential for negative effects, but not documented
m: some potential for long-term effects, but not documented
Figure VI-9. Changes in the beach at Lincoln Park following seawall construction in the mid 1930s (from Thom et al. 1994b)
| << vi.a.2. regional focus - bainbridge island |