Download Puget Sound

Puget Sound Coastal
Geomorphology
Background materials for NearPRISM
discussion of Puget Sound shoreline
geomorphology
September 11, 2000
Hugh Shipman
WA Dept of Ecology
Questions
• How is nearshore biology impacted by changes in
sediment supply associated with a large landslide or
with shoreline armoring?
• How do geomorphological processes affect
landscape-scale ecological processes (e.g. migratory
pathways, eelgrass patchiness)?
• Are there predictable relationships between
geomorphological process, beach substrate, and
biological community?
Puget Sound
• 2000 miles of shoreline
• Narrow, steep nearshore zone
• Significant lateral heterogeneity
– Geology and geomorphology
– Wave exposure and tidal conditions
– Hydrology (rivers, streams, groundwater)
• Nearshore ecology linked to shoreline
geomorphological process
Nearshore
• Def: Top of bluff to limit of photic zone
• Distinct geomorphological regimes - river
deltas, barrier beaches, rocky shores, and
open bluff-backed coast
• Wave action determined by local wind field
and shoreline shape/bathymetry
• Dominance of longshore sediment movement
• Mixed gravel and sand beaches
• Complex beach hydrology (e.g. seeps)
Puget Sound
Shoreline
Sediment transport
• Sources
– Bluff erosion vs. Fluvial
– Highly variable, often coarse sediment
• Longshore Transport
– Low transport rates, particle size dependence
– Major factor in geomorphological variability
• Sediment sinks
– Spits, lagoons and flats, deep water
• Littoral cells
Keuler, 1988
Shoreline modification issues
•
•
•
•
Armoring
Nourishment
Docks and piers
Riparian changes
• Shoreline Guidelines
• Endangered Species Act
Shoreline Armoring
• Loss of littoral sediment
• Wave reflection/scour
• Hydrological impacts
• Loss of riparian vegetation
• Passive erosion
• Cumulative impact
Modified
shoreline
[DNR, 1998]
Ecology = f ( Physical Processes)
• Topography
– elevation, slope
• Sediment structure
– substrate, particle size
• Sediment dynamics
– turnover, disturbance
• Hydrology
– estuaries, seepage
• Geochemistry
Beach Models
• Longshore sediment movement
– Long-term changes to the nearshore tied to
changes in longshore sediment supply
• Beach sediment dynamics
– Short-term changes in beach state related to
waves and tides
Sediment Budget model
• Address beach processes over 100s-1000s meters
and over years to centuries (Large Scale Coastal
Behavior)
• Important for evaluating questions relevant to coastal
managers
• Useful for addressing landscape-scale ecological
questions (e.g. habitat distribution)
• Model “currency” is sediment - possibly reflecting
behavior of different particle sizes
• Model cells might be segments of shoreline
UPLAND
Shoreline armoring,
hydrologic modifications
Bluff erosion
Streams
Dunes and
backshore
Rivers
NEARSHORE
Longshore
Transport
Upper Beach
Longshore
Transport
Lower Beach
Groins, jetties
Tide Flats
Small estuaries
Deep Water
OFFSHORE
Beach Dynamics model
• Address changes in beach sediment at a scale of
centimeters to meters, and over time periods of
hours, storms, or seasons.
• Important to understanding relationship between
sediment, biological, and chemical processes.
• Links to other PRISM modules (oceans, nutrients)
• Model “currency” is chemical constituent (e.g.
carbon)
• Model cell is a unit area (1-10 meters square?)
Information Sources
• Limited literature on estuarine and mixed
gravel/sand beaches
• Little observational data on Puget Sound
• Limited knowledge of geomorphologicalbiological interactions outside of large river
deltas