Presentation

Ecosystem Tradeoffs
Associated with Tidal Marsh
Sills
Karen Duhring
Marine Scientist
Virginia Institute of Marine Science
International Submerged Lands Management Conference
October 29, 2007

Presentation Outline
Typical erosion protection projects in
Virginia’s portion of Chesapeake Bay
Preferred approaches from an ecosystem
perspective
Marsh sill case studies
Summary of ecosystem tradeoffs
associated with marsh sills

Typical Erosion Protection Projects
on Virginia’s s Tidal Shorelines
Revetments
Bulkheads
Bank Grading & Structures

Northumberland Co.
County
Lancaster Co.
Permit site
Potomac
River
Chesapeake
Bay
Tidal Shoreline Permit
Sites
Lower Northern Neck,
VA
1999-2007
Widespread
distribution
regardless of energy
Middlesex
Co.
Rappahannock River
Cumulative impacts
throughout
watersheds

Cumulative Impact of
Shoreline Hardening
Cumulative length new
structures permitted
each year
18.2 miles / yr average
Cumulative length
permitted 1993-2006
255.5 miles
Direct loss & isolation
of tidal wetlands
Source:
VIMS Tidal Shoreline Permit Database

Order of Preferred Approaches
for Shoreline Protection
1. No Action
Leave shoreline in
natural condition or
restore habitats
Reduce risk through land
use changes
2. Non-Structural
Maximize vegetation
cover

Order of Preferred Approaches
for Shoreline Protection
1. No Action
Leave shoreline in
natural condition or
restore habitats
Reduce risk through land
use changes
2. Non-Structural
Maximize vegetation
cover
3.“Hybrid”
Use structures to support
natural erosion buffers
(e.g. tidal marshes,
beaches)
4. Revetments
5. Bulkheads

Preferred Approaches
that may affect public submerged lands
Tidal marsh enhancement or creation
Beach nourishment in sand-limited system
Bank grading with cut and fill to create or enhance
tidal marsh
Offshore breakwater systems for high-energy beach
shorelines
Tidal marsh sills for low to medium energy shorelines

Gapped sills with backfill from
graded bank
Marsh Sill
with Planted Marsh
Low profile revetment
backfilled with sand to
create or enhance tidal
marsh
Imported sediment from
upland source or suitable
bank grading material
Offensive approach
structures built in sand transport
region to address impinging waves
before they reach upland areas

Typical Sill Cross-Section
Planted tidal marsh
Height near mean high
water elevation
Average 20-30 feet
Source: Jefferson Patterson Park project, Maryland

Marsh Toe Revetment
Natural Marsh Edge Stabilization
Low profile
revetment placed
along the eroding
edge of existing
tidal marsh

VIMS Study
36 marsh structures in 2004-2005
2005
A Survey of the Effectiveness of
Existing Marsh Toe Protection
Structures in Virginia
July 2006
K.A. Duhring, T.A. Barnard &
C.S. Hardaway, Jr.

Marsh Sill at VIMS
Marsh Sill at VIMS
since 1983
Salt Bushes
Saltmarsh Cordgrass
Stone Sill

Before
After 1 year
High, vertical, sandy bank
Undercutting erosion at bank toe
Narrow, patchy fringe marsh + 5 ft wide
Tidal creek with regular boat wakes
Graded bank now stable
Suitable material used for backfill
Planted tidal marsh + 25 ft wide
Gapped sills (3)

Combination Approach
Graded Bank with cut & fill + Planted Tidal Marsh + Gapped Sills
Tidal opening
Saltmeadow hay
Spartina patens
Smooth
cordgrass
Spartina
alterniflora
Approx. mean high water
+ 25 feet

Before
After 1 year
Regular high
tides
Remnant tidal marsh at high
energy setting
Extensive beds of submerged
aquatic vegetation (SAV)
Existing revetment stabilizing
bank
Desire to protect 1 oak tree from
saltwater intrusion
“Perched” marsh not
accessible to marine organisms

Tidal Openings
Straight
Offset
gapped
Offset end at
upland revetment

Before
After 10 years
Non-vegetated sand/mud,
narrow intertidal zone
Open water (before sill)
Sediment movement
Nutrient cycling by infaunal
community
Eroding upland bank (not
visible)
Stone sill & wide vegetated intertidal area
Less open water area
Sediment trapping
Nutrient cycling by marsh plants and infaunal
community
Stable upland bank with dense vegetation (not
visible)

Aerial habitat
Polychaete worms, crustaceans, molluscs
Bivalves – clams, oysters, mussels
Essential prey for blue crab, benthic
feeding fish (e.g. spot, croaker)
Habitat Diversity
Comparison
spiders, insects, wading birds, waterfowl,
mammals
Marsh surface & belowground
microbial fungi and bacteria, algae,
meiofauna (nematodes, copepods, rotifers,
protozoa)
Foraging invertebrates (polychaetes,
gastropod mollusks, fiddler crab, blue crab,
amphipods)
Filter feeders (ribbed mussel, oyster)
Aquatic pools and channels
Small & juvenile fish, shrimp, blue crab

Habitat in Riprap
Reefs
Lower diversity &
abundance than marshes
& oyster reefs
May support similar or
higher nekton abundance
than bare sediment

Potentially Negative Effects of
Marsh Sills
Covering shallow water benthic infauna
Hydrodynamic changes - altering tidal
exchange, wave height and wave direction
Construction access & maintenance
impacts
Altering sediment transport
Altering habitat use at marsh edge

Potentially Positive Effects
Wave attenuation including boat wakes
Sediment stabilization
Vegetation stabilization
Allows for efficient nutrient cycling, complex habitat
structure
Tidal marsh creation and restoration
Protection and restoration of salt marsh habitats may be essential
to the maintenance of high benthic production and consumer
biomass in estuarine ecosystems (Seitz et al 2006)
Habitat diversity & complexity
For terrestrial, wetland and aquatic organisms

Current Guidelines for Marsh Sills
Use only if action is required and nonstructural
methods are not sufficient
Minimize channelward encroachment
Make structure as porous & low as possible
Minimize / restore construction access
impacts
Periodic maintenance required
Pruning overhanging branches, removing excess
tidal debris, replacing scattered stone, additional
planting

Main Conclusions about
Tradeoffs
Marsh sills are better choice for erosion protection
in low & medium energy settings than bulkheads
Not as appropriate at high energy settings or where
significant aquatic resources would be impacted
(e.g. SAV, unique shellfish areas)
Tidal marsh creation & restoration at the land-water
interface is beneficial for public trust resources
More research is needed & underway to compare
actual productivity before and after sill construction,
evaluate effects of tidal gaps

Any Questions About Marsh Sills