Benches, beaches, and bumps: how habitat monitoring and experimental science can inform urban seawall design

Published source details Cordell J.R., Toft J.D., Munsch S. & Goff M. (2017) Benches, beaches, and bumps: how habitat monitoring and experimental science can inform urban seawall design. Pages 421-438 in: D.M. Bilkovic, M.M. Mitchell, P.M.K. La & J.D. Toft (eds.) Living Shorelines: The Science And Management Of Nature-Based Coastal Protection. CRC Press, Boca Raton, Florida.
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This study is summarised as evidence for the following.

	Action	Category
	Create large ridges or ledges (>50 mm) on intertidal artificial structures Action Link
	*Create grooves and* small protrusions, ridges or ledges (1–50 mm) on intertidal artificial structures** Action Link

Create large ridges or ledges (>50 mm) on intertidal artificial structures

A replicated, randomized, controlled study in 2008–2011 on three intertidal seawalls in Puget Sound estuary, USA (Cordell et al. 2017) reported that large ledges created on seawall panels, along with grooves and small protrusions, supported higher macroalgae, microalgae and invertebrate species diversity and live cover, with more rockweed Fucus distichus and mussels Mytilus spp., than seawall surfaces without added habitats. After 42 months, the macroalgae, microalgae and invertebrate species diversity was higher on seawall panels with ledges, grooves and protrusions than on seawall surfaces without (data reported as Evenness index, not statistically tested). Total live cover was 83–84% on panels with ledges, grooves and protrusions and 74% on surfaces without (data not statistically tested). Rockweed and mussel abundances were statistically similar on panels with long ledges (rockweed: 5% cover; mussels: 6%) and short ledges (rockweed: 13%; mussels: 12%), and higher on both than on seawall surfaces without (both 1%). Abundances of six other species groups were not statistically tested (see paper for results). It is not clear whether these effects were the direct result of creating ledges, grooves or protrusions. Large ledges were created on concrete seawall panels (height: 2.3 m; width: 1.5 m; thickness: ~150 mm) using a formliner. Each panel had three long (length: ~1.5 m; width/height: ~0.5 m) or six short (length: ~0.7 m; width: ~0.2 m; height: ~0.5 m) evenly-spaced horizontal ledges. Panels were either smooth or had grooves and small protrusions on their surfaces. One panel of each ledge-surface combination was randomly arranged spanning high–lowshore on each of three vertical concrete seawalls in January 2008. Seawall surfaces were intertidal areas of seawall cleared of organisms (dimensions/spacing not reported). Macroalgae, microalgae and invertebrates were counted on panels (excluding downward-facing surfaces) and seawall surfaces during low tide after 42 months.

(Summarised by: Ally Evans)
Create grooves and small protrusions, ridges or ledges (1–50 mm) on intertidal artificial structures

A replicated, randomized, controlled study in 2008–2011 on three intertidal seawalls in Puget Sound estuary, USA (Cordell et al. 2017) reported that seawall panels with grooves and small protrusions, along with large ledges, supported higher macroalgae, microalgae and invertebrate species diversity and live cover, with more rockweed Fucus distichus and mussels Mytilus spp., than seawall surfaces without added habitats, but that flat panels (i.e. without large ledges) with grooves and protrusions did not. After 42 months, ledged seawall panels with grooves and small protrusions supported higher macroalgae, microalgae and invertebrate species diversity (data reported as Evenness index, not statistically tested), rockweed abundance (5–13% cover) and mussel abundance (6–12%) than seawall surfaces without added habitats (rockweed/mussels: both 1%), but flat panels with grooves and potrusions did not (rockweed: 0%; mussels: 6%). Total live cover was 83–84% on ledged panels with grooves and protrusions, 81% on flat panels with grooves and protrusions, and 74% on seawall surfaces (data not statistically tested). Abundances of six other species groups were not statistically tested (see paper for results). It is not clear whether these effects were the direct result of creating grooves and protrusions or ledges. Concrete seawall panels (height: 2.3 m; width: 1.5 m; thickness: ~150 mm) were moulded with and without groove habitats and small protrusions (dimensions not reported) using a cobble-effect formliner. Panels had three long or six short horizontal large ledges, or were flat. One panel of each surface-ledge combination was randomly arranged, spanning high–lowshore on each of three vertical concrete seawalls in January 2008. Seawall surfaces were intertidal areas of seawall cleared of organisms (dimensions/spacing not reported). Macroalgae, microalgae and invertebrates were counted on panels (excluding downward-facing surfaces) and seawall surfaces during low tide after 42 months.

(Summarised by: Ally Evans)