Enhancing armored shorelines in order to restore some natural conditions is relatively new from both design and scientific perspectives, and several conclusions from our study will help guide future efforts:
(1) Nursery area for fish: Nearshore fish used shallow-water enhancements along armored shorelines. More fish (juvenile salmon and larval fish, depending on the year) were at enhanced shorelines, often with higher feeding activity.
(2) Foraging opportunities: Invertebrates that are prey for juvenile salmon and other fish colonized the low gradient, finer-grained intertidal habitats that were incorporated into the armored shorelines. Most of these aquatic invertebrates increased in taxa richness and numbers after enhancements were in place.
(3) Riparian value: Certain types of terrestrial insects increased in abundance and taxa richness where patches of shoreline vegetation were planted. Vegetation in urban parks requires routine maintenance, so progression to a more natural riparian zone will be somewhat limited in this setting.
(4) Connectivity: Linkages between aquatic and terrestrial zones are broken on heavily armored shorelines. When artificial barriers are removed and aquatic habitats merge with terrestrial habitats, this provides the opportunity for biological and physical processes to reconnect across the ecotone.
(5) Physical resilience: In heavily urbanized settings, habitat enhancements have limited ability to restore larger-scale processes such as sediment supply, and this may lead to the need for maintenance. Human use of urban beaches can create some surface sediment loss and structural impact such that occasional maintenance and nourishment may be necessary.
It is encouraging that enhanced shorelines can provide benefits similar to those at more fully restored shorelines, and shoreline enhancement should be a management goal in locations where larger scale restoration is not possible due to high levels of urban development. In restoration planning, scientific research can be useful (1) prior to restoration in helping to define project goals, (2) during project design by incorporating data to optimize the likelihood of desirable ecological responses, and (3) after completion of restoration to document performance, to identify problems, and to provide critical information for adaptive management. The Seattle seawall is scheduled for replacement starting as soon as 2013, and using the data collected on design elements at OSP should prove useful for planning other shoreline enhancements along this highly urbanized segment of Elliott Bay. The habitat bench, pocket beach, and planted shoreline vegetation have all had beneficial effects, and these features along with other studied features such as textured and sloped seawalls should be incorporated into project designs. Each of these design elements result in different ecological responses, and using a combination of shoreline enhancements would produce a more diverse community than that along the uniform seawall, riprap, and pier structures that currently exist along the urban Seattle shoreline.