Considerations for Designing Resilient Living Shorelines Molly M - - PowerPoint PPT Presentation

Considerations for Designing Resilient Living Shorelines

Molly M Mitchell Don Donna M Marie B Bilkovic

Living S Sho horeline nes T Tech T h Trans nsfer W Worksho hop, p, R RAE Beaufo fort, N NC Oct 2019 2019

Sea level “report cards” for the US

https://www.vims.edu/research/products/slrc/compare/index.php

Marshes are dynamic  resilient

To keep pace with sea level: a) Marshes migrate b) Marshes accrete Controlled by: Plant production Sediment availability Sediment respiration Δ Temperature Δ CO2 Δ Water Levels Controlled by:

SLR Land elevation

Δ Water Levels

Mitchell & Bilkovic. 2019.

Modified by human activities & decisions

Photo by Karen Duhring Photo by Skip Stiles

https://appliedecologistsblog.com/2019/05/01/can-living-shorelines-survive-the-rising-seas/#comments

Photo by Karen Duhring Photo by Skip Stiles

https://appliedecologistsblog.com/2019/05/01/can-living-shorelines-survive-the-rising-seas/#comments

Migration Prospects in Chesapeake Bay

Low – marsh projected to decline overtime due to typically

high elevations in the surrounding lands

Medium – opportunities for marsh migration are limited by

development

Best – Best likelihood for marsh migration

Conservation priorities for promoting marsh migration “DNH_marsh_conservation_2050” shapefile. Developed by Center for Coastal Resources Management, Virginia Institute of Marine Science. Nov 2018. Contact M. Mitchell (molly@vims.edu) or J. Herman (herman@vims.edu).

Migration potential in living shoreline design

 Focus planting in the upper elevations of the tidal range

 Dubois, K. 2017. Overcoming barriers of Living Shoreline Use and Success

 Preserve riparian land where elevations are suitable for marsh migration  Don’t mow! What to plant for best migration potential?

MSL 2016 MSL 2050 2 ft

Accretion potential, also affect by siting

• Sediment supply coming from
• Plants (organic)
• Watershed
• Adjacent lands (via runoff or tidal

waters)

• Marsh front edge erosion
• Current CB management goals

are to restrict sediment in waters

Fagherazzi et al. 2013. Oceanography, 26(3): 70-77.

Today 2050-2060

Enhancing accretion in living shoreline design

Marsh plant stem density, stem height, and aboveground biomass influence the marsh’s ability to trap sediments and attenuate wave energy

200 400 600 800

Spartina alterniflora Spartina patens

Marsh plant density (stems/m2)

Natural Marsh Marsh-sill

 Marsh-sill low marsh stem counts lower than natural fringing marshes; high marsh similar  No evident trajectory across age of marsh sampled (1-11 yrs)

Bilkovic and Mitchell, 2017

Enhancing accretion in living shoreline design

Living sills add a dynamic component to wave energy attenuation

Growth on structures reduces wave transmission as oysters grow

 Hall et al. 2017 Growing Living Shorelines and Ecological Services via Coastal Bioengineering. In Living Shorelines: The Science and Management of Nature-based Coastal Protection, CRC Press

Sills increase in height, width and density over time

When natural dynamic components are compromised…

Thin layer deposition artificially maintains elevations in the tidal frame

Considered a beneficial use of dredge material and might help marshes stay within the tidal frame Might improve grass density leading to increased accretion Need more evidence of long-term usefulness

Dynamic designs are the path to resilience

 Site

te

• allow for landward marsh migration with rising sea levels
• don’t mow the marsh as it migrates

Enhan ance

• use sills (rock or oyster) *with windows* to encourage sediment accretion
• encourage tall, dense plant growth
• encourage mussel/oyster growth on marsh and sills

Main intain in

• when marshes can’t migrate, and natural sediment supply is low, add thin layers
• f sediment to maintain elevations
• remove invasive species

Commu mmunicate – property owners need to know that living shorelines

aren’t static

Thank you!

molly@vims.edu