Study

Between a rock and a hard place: environmental and engineering considerations when designing coastal defence structures

  • Published source details Firth L.B., Thompson R.C., Bohn K., Abbiati M., Airoldi L., Bouma T.J., Bozzeda F., Ceccherelli V.U., Colangelo M.A., Evans A.J., Ferrario F., Hanley M.E., Hinz H., Hoggart S.P.G., Jackson J.E., Moore P., Morgan E.H., Perkol-Finkel S., Skov M.W., Strain E.M., van Belzen J. & Hawkins S.J. (2014) Between a rock and a hard place: environmental and engineering considerations when designing coastal defence structures. Coastal Engineering, 87, 122-135.

Actions

This study is summarised as evidence for the following.

Action Category

Create small adjoining cavities or ‘swimthrough’ habitats (≤100 mm) on intertidal artificial structures

Action Link
Biodiversity of Marine Artificial Structures

Create hole habitats (>50 mm) on intertidal artificial structures

Action Link
Biodiversity of Marine Artificial Structures

Create groove habitats (1–50 mm) on intertidal artificial structures

Action Link
Biodiversity of Marine Artificial Structures

Create groove habitats (1–50 mm) on intertidal artificial structures

Action Link
Biodiversity of Marine Artificial Structures

Create pit habitats (1–50 mm) on intertidal artificial structures

Action Link
Biodiversity of Marine Artificial Structures

Create 'rock pools' on intertidal artificial structures

Action Link
Biodiversity of Marine Artificial Structures

Create pit habitats (1–50 mm) on intertidal artificial structures

Action Link
Biodiversity of Marine Artificial Structures

Create 'rock pools' on intertidal artificial structures

Action Link
Biodiversity of Marine Artificial Structures

Create pit habitats (1–50 mm) on intertidal artificial structures

Action Link
Biodiversity of Marine Artificial Structures

Create 'rock pools' on intertidal artificial structures

Action Link
Biodiversity of Marine Artificial Structures
  1. Create small adjoining cavities or ‘swimthrough’ habitats (≤100 mm) on intertidal artificial structures

    A replicated study in 2011–2014 on 30 rock gabions on open coastline in the Irish Sea, UK and in the Eastern Scheldt estuary, Netherlands (Firth et al. 2014) found that gabions with small swimthrough habitats created amongst rocks of mixed sizes (small and large) supported similar macroalgae and invertebrate species richness and abundance to those amongst rocks of regular sizes (small or large), but that abundance was higher on gabions with regular small rocks than regular large ones. In the UK, after 12 months, 12 mobile and non-mobile invertebrate species were recorded in and on gabions with swimthroughs. Species richness and abundance was similar in and on gabions with swimthroughs amongst mixed-sized rocks (9 species/gabion, 252 individuals/gabion) and regularly-sized ones (8 species/gabion, 191–366 individuals/gabion). Abundance was higher in and on gabions with small regular rocks (366 individuals/gabion) than large regular ones (191/gabion). In the Netherlands, after 16 months, 14 macroalgae, mobile and non-mobile invertebrate species were recorded and overall species richness was similar on all gabion designs (data not reported). Small swimthrough habitats were created amongst rocks in gabion baskets (500 × 500 × 300 mm; 76 mm mesh size). Swimthroughs were either amongst rocks of mixed sizes (small: 60–100 mm and large: 180 mm), or amongst regularly-sized small or large rocks. Five of each design were placed at midshore on a boulder beach in the UK in April 2011 and at lower-midshore on a sandy beach in the Netherlands in September 2012. UK gabions were dismantled and invertebrates counted after 12 months. In the Netherlands, macroalgae and invertebrates on external horizontal gabion surfaces were counted after 16 months.

    (Summarised by: Ally Evans)

  2. Create hole habitats (>50 mm) on intertidal artificial structures

    A replicated, controlled study in 2010–2011 on an intertidal seawall in the Teign estuary, UK (Firth et al. 2014) found that hole habitats created on the seawall, along with rock pools, supported higher macroalgae and invertebrate species richness than seawall surfaces without holes or pools. After 19 months, macroalgae and invertebrate species richness was higher in holes (3 species/hole) than on seawall surfaces without (1/surface). Barnacles (Cirripedia) were recorded only in holes. It is not clear whether these effects were the direct result of creating holes or rock pools. Hole habitats were created in May 2010 by replacing seawall blocks with water-retaining troughs during construction of a vertical sandstone seawall. Fifteen cube-shaped holes (150 × 150 × 150 mm) were created at highshore. Water pooled in the base of holes (depth/volume not reported). Holes were compared with 15 mortar seawall surfaces (150 × 150 mm). Macroalgae and invertebrates were counted in holes and on seawall surfaces during low tide after 19 months. Three holes and seven surfaces had been buried by sediment and no longer provided habitat.

    (Summarised by: Ally Evans)

  3. Create groove habitats (1–50 mm) on intertidal artificial structures

    A replicated, controlled study in 2010–2011 on an intertidal seawall in the Teign estuary, UK (Firth et al. 2014a) found that groove habitats created on the seawall supported similar macroalgae and invertebrate species richness to seawall surfaces without grooves. After 19 months, macroalgae and invertebrate species richness was similar in grooves (1 species/array) and on surfaces without grooves (1/surface). Groove habitats were created in May 2010 by scraping a trowel across wet mortar between blocks during construction of a vertical sandstone seawall. Arrays of 5–10 grooves (length: 150 mm; width/depth: 1–5 mm) were irregularly-spaced on 150 × 150 mm seawall surfaces. There were 15 surfaces with grooves and 15 without at highshore. Macroalgae and invertebrates were counted in grooves and on surfaces without during low tide after 19 months. Two arrays of grooves and seven surfaces without had been buried by sediment and no longer provided habitat.

    (Summarised by: Ally Evans)

  4. Create groove habitats (1–50 mm) on intertidal artificial structures

    A replicated, controlled study in 2012─2013 on an intertidal groyne on open coastline in the Irish Sea, UK (Firth et al. 2014b) reported that groove habitats created on a concrete block placed in the groyne supported similar macroalgae and invertebrate species richness to groyne surfaces without grooves. Data were not statistically tested. After 13 months, a total of four species were recorded in grooves and on groyne surfaces without grooves. Groove habitats were created on two vertical sides of a concrete block (1.5 × 1.5 × 1 m) using a mould. Ten horizontal grooves (length: 1 m; width/depth: 50 mm) were cast 50 mm apart on each side. The block was placed at midshore in a boulder groyne during construction in February 2012. Surfaces with grooves were compared with vertical surfaces of adjacent groyne boulders (dimensions/material not reported). Macroalgae and invertebrates were counted in grooves and on groyne surfaces without during low tide after 13 months.

    (Summarised by: Ally Evans)

  5. Create pit habitats (1–50 mm) on intertidal artificial structures

    A replicated, controlled study in 2011–2013 on an intertidal breakwater on open coastline in the English Channel, UK (Firth et al. 2014a) found that creating pit habitats on the breakwater increased the macroalgae and invertebrate species richness on breakwater surfaces. After 24 months, macroalgae and invertebrate species richness was similar on surfaces with large (10 species/surface) and small (9/surface) pits, and higher on both than on surfaces without pits (3/surface). Six invertebrate species groups recorded on surfaces with pits were absent from those without. Pit habitats were created in August 2011 by drilling into the vertical sides of concrete breakwater blocks. Arrays of 100 large (diameter: 22 mm) and small (14 mm) round pits (depth: 25 mm) were evenly-spaced on 1 × 1 m breakwater surfaces. There was one surface with each of large, small and no pits on each of eight blocks at mid-lowshore. Pits were angled to retain water. Macroalgae and invertebrates were counted on surfaces with and without pits during low tide after 24 months.

    (Summarised by: Ally Evans)

  6. Create 'rock pools' on intertidal artificial structures

    A replicated, controlled study in 2010–2011 on an intertidal seawall in the Teign estuary, UK (Firth et al. 2014a) found that rock pools created on the seawall, along with holes, supported higher macroalgae and invertebrate species richness than seawall surfaces without pools or holes. After 19 months, macroalgae and invertebrate species richness was higher in pools (3 species/pool) than on seawall surfaces without (1/surface). Barnacles (Cirripedia) were recorded only in pools. It is not clear whether these effects were the direct result of creating rock pools or holes. Rock pools were created in May 2010 by replacing seawall blocks with water-retaining troughs during construction of a vertical sandstone seawall. Fifteen square pools (150 × 150 mm; depth/volume not reported) were created at highshore. Pools were shaded. Pools were compared with 15 mortar seawall surfaces (150 × 150 mm). Macroalgae and invertebrates were counted in pools and on seawall surfaces during low tide after 19 months. Three pools and seven surfaces had been buried by sediment and no longer provided habitat.

    (Summarised by: Ally Evans)

  7. Create pit habitats (1–50 mm) on intertidal artificial structures

    A replicated, controlled study in 2010–2011 on an intertidal seawall in the Teign estuary, UK (Firth et al. 2014b) found that pit habitats created on the seawall supported similar macroalgae and invertebrate species richness to seawall surfaces without pits. After 19 months, macroalgae and invertebrate species richness was similar in pits (2 species/array) and on surfaces without pits (1/surface). Pit habitats were created in May 2010 by pushing a stick into wet mortar between blocks during construction of a vertical sandstone seawall. Arrays of four round pits (diameter: 25 mm; depth: 25 mm) were evenly-spaced on 150 × 150 mm seawall surfaces. There were 15 surfaces with pits and 15 without at highshore. Pits were angled to retain water. Macroalgae and invertebrates were counted in pits and on surfaces without pits during low tide after 19 months. One array of pits and seven surfaces without had been buried by sediment and no longer provided habitat.

    (Summarised by: Ally Evans)

  8. Create 'rock pools' on intertidal artificial structures

    A replicated, controlled study in 2012–2013 on an intertidal groyne on open coastline in the Irish Sea, UK (Firth et al. 2014b) reported that rock pools created on a concrete block placed in the groyne supported similar macroalgae and invertebrate species richness to groyne surfaces without pools. Data were not statistically tested. After 13 months, a total of five species were recorded in large deep pools, four in small deep pools, three in each of large and small shallow pools, and four on groyne surfaces without pools. Rock pools were created in the top surface of a concrete block (1.5 × 1.5 × 1 m) using a mould. Cylindrical pools were either large (diameter: 250 mm) or small (150 mm), and either deep (depth: 200 mm) or shallow (100 mm). There were three of each size-depth combination. The block was placed at midshore in a boulder groyne during construction in February 2012. Pools were compared with horizontal surfaces of adjacent groyne boulders (dimensions/material not reported). Macroalgae and invertebrates were counted in pools and on groyne surfaces during low tide over 13 months.

    (Summarised by: Ally Evans)

  9. Create pit habitats (1–50 mm) on intertidal artificial structures

    A replicated, controlled study in 2012–2013 on an intertidal groyne on open coastline in the Irish Sea, UK (Firth et al. 2014c) reported that pit habitats created on a concrete block placed in the groyne supported similar macroalgae and invertebrate species richness to groyne surfaces without pits. Data were not statistically tested. After 13 months, a total of three species were recorded in deep pits, two in shallow pits, and four on groyne surfaces without pits. Pit habitats were created on two vertical sides of a concrete block (1.5 × 1.5 × 1 m) using a mould. Two arrays of each of 16 deep (50 mm) and 16 shallow (20 mm) round pits (diameter: 20 mm) were evenly-spaced in 250 × 250 mm areas on each side. The block was placed at midshore in a boulder groyne during construction in February 2012. Surfaces with pits were compared with vertical surfaces of adjacent groyne boulders (dimensions/material not reported). Macroalgae and invertebrates were counted in pits and on groyne surfaces without during low tide after 13 months.

    (Summarised by: Ally Evans)

  10. Create 'rock pools' on intertidal artificial structures

    A replicated, paired sites, controlled study in 2012–2013 on an intertidal breakwater on open coastline in the Irish Sea, UK (Firth et al. 2014c) found that rock pools created on the breakwater supported higher macroalgae and invertebrate species richness than breakwater surfaces without pools. After nine months, macroalgae and invertebrate species richness was higher in pools (4 species/pool) than on breakwater surfaces without (2/surface). Two species groups (1 macroalgae, 1 mobile invertebrate) recorded in pools were absent from breakwater surfaces. Rock pools were created on a boulder breakwater in June 2012 by pouring concrete into existing core holes in breakwater boulders. Nine cylindrical pools were created (depth: 100 mm; diameter/shore level not reported). Pools were compared with horizontal surfaces on breakwater boulders adjacent to each pool with surface areas matching inside pool surfaces (dimensions/material not reported). Macroalgae and invertebrates were counted in pools and on breakwater surfaces during low tide after nine months. Four pools leaked water and did not provide rock pool habitat.

    (Summarised by: Ally Evans)

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