coastal processes
TRANSCRIPT
Coastal Systems
Coastal Settings Open coasts can be divided into two general categories based on the
dominant processes acting on the coast over long periods of time (1000’s of years or more)
1. Erosional Shorelines dominated by processes that form erosional features along
the shore Typically high-relief rocky coasts in tectonically active areas (e.g., the
Pacific coast of North America) Typically high-relief rocky coasts
2. Depositional Shorelines dominated by processes that form deposits along the shore Include a wide spectrum of environments
• Deltas, barrier islands, reef coasts, glaciated coasts, etc. More typical of passive, or trailing-edge, continental margins (e.g., the
Atlantic coast of North America)
Features of erosional coasts
11.4-Thurman and Trujillo, 2004
Features of erosional coasts
Cabo Rojo, PRCabo Rojo, PR
Sea arch
Wave-cut cliff
Features of depositional coasts
11.7-Thurman and Trujillo, 2004
Features of depositional coasts
21, 25-RPI
Barrier islands along the South Carolina coast
A wave moving onto the shore
20.5-Tarbuck & Lutgens, 2005
Wave erosion
Wave erosion
Breaking waves exert a great force
Wave erosion is caused by
• Wave impact and pressure
• Abrasion by rock fragments
Wave erosion
20.6, 20.7-Tarbuck & Lutgens, 2005
Wave refraction Wave refraction
• Bending of a wave• Causes waves to arrive nearly parallel to the
shore• Consequences of wave refraction
• Wave energy is concentrated against the sides and ends of headlands
• Wave energy is spread out in bays and wave attack is weakened
• Over time, wave erosion straightens an irregular shoreline
Wave energy focused on headland
Wave energy dispersed over bay
9.19b-Thurman and Trujillo, 2004
Wave refraction
Wave Reflection and Refraction
Wave Motion/Refraction
10.20b-Garrison, 2005
Wave refraction,Maili Point, Oahu
Movement parallel to the (↔) shoreline• Caused by wave refraction• Along most shorelines, waves strike the
shore at an angle• Waves that reach the shoreline at an angle
cause the sediment to move along a beach in a zigzag pattern called beach drift
Sediment movement on the shore
Sediment movement on the shore
Movement parallel to the (↔) shoreline• Oblique waves also produce longshore
currents• Currents in the surf zone
• Flow parallel to the coast
• Easily moves fine suspended sand and rolls larger sand and gravel along the bottom
Movement of sand by longshore current
20.10-Tarbuck & Lutgens, 2005
Sediment movement along a beach
11.19-Segar, 2007
Beach Drift and Longshore Currents
Movement perpendicular (↕) to the shoreline
• Caused by breaking waves• Swash (↑)
• Backwash (↓)
Sediment movement on the shore
Swash →← Backwash
Domes beach, Rincón, PR
T11.2-Thurman and Trujillo, 2004
11.2a-Thurman and Trujillo, 2004
11.2a-Thurman and Trujillo, 2004
Seasonal variation in
beach profile
11.19-Segar, 2007
Barrier Systems
Sandy Point, Cape Romain, South Carolina
38-RPI
Barrier Systems
Thin strips of land built a few to several tens of meters above sea level
Form due to the combined action of wind, waves, and longshore currents
Act as a barrier – protect mainland coast from open-ocean wave energy, storm surge, etc.
Protected environments form behind barriers – bays, lagoons, marshes, tidal creeks
Barrier Systems
Most common along passive margins Economically important Vulnerable to storms, rising sea level,
erosion One of the major forces impacting
barriers today, as well as in the past, is rising sea level
11.9c-Thurman and Trujillo, 2004
Heavily developed barrier islands
Tom’s River, New JerseyBarrier islands north of Charleston, SC
21-RPI
Chesapeake Bay – a
drowned river valley
Pamlico Sound – a bar-built
estuary
12.5-Garrison, 2005
Coastal Erosion andBeach Loss
Coastal erosion is a natural process that redistributes sediments along coasts
Coastal erosion becomes a problem when coastal development impedes or ignores the natural movement of sediment along the shore
This problem is exacerbated by continued sea level rise
Coastal erosion is a natural processNorth Island, SC
32-RPI
Figure 20.16
Wave erosion caused by strong storms, Long Island, New York
Coastal development makes erosion a problem
Sea-level rise makes the problem worse
Studies show a 150x Studies show a 150x erosion multiplier for erosion multiplier for sea level rise on sandy sea level rise on sandy shorelines. Hence, for shorelines. Hence, for a mean 0.24 m rise by a mean 0.24 m rise by 2050, beaches will 2050, beaches will recede 36 m (118 ft). recede 36 m (118 ft). ((Leatherman et al., Leatherman et al., 2000)2000)
Stabilizing the shore
Shoreline erosion is influenced by several local factors including
• Proximity to sediment-laden rivers• Degree of tectonic activity• Topography and composition of the land• Prevailing wind and weather patterns• Configuration of the coastline and nearshore
areas
Stabilizing the shore
Three basic responses to erosion problems1. Building structures (Hard stabilization)
2. Beach nourishment
3. Relocation
Stabilizing the shore
Hard stabilization perpendicular to the shore
• Jetties• Usually built in pairs to develop and maintain
harbors
• Extend into the ocean at the entrances to rivers and harbors
Jetties are built to prevent deposition
Figure 20.17
Stabilizing the shore
Hard stabilization perpendicular to the shore
• Groins• Built to maintain or widen beaches
• Constructed at a right angle to the beach to trap sand
• Because of increased erosion on the downdrift side of the groin, additional groins may be built resulting in a groin field
11.20-Thurman and Trujillo, 2004
Interference of sand
movement
Hard stabilization like the groin shown here interferes with the movement of sand along the beach, causing deposition of sand upstream of the groin and erosion immediately downstream, modifying the shape of the beach.
Figure 20.18
Groins along the
New Jersey Shore at Cape May
Longshore transport
Jetties and Groins
Jetties are always in pairs Groins can be singular or many (groin field) Both trap sand upstream and cause erosion downstream
11.22-Thurman and Trujillo, 2004
Stabilizing the shore
Hard stabilization parallel to the shore• Breakwater
• Barrier built offshore and parallel to the coast
• Protects boats from the force of large breaking waves
Groins and Breakwaters
11.7-Jones and Jones, 2003
Figure 20.19
Coastal Stabilization Structures
Stabilizing the shore
Hard stabilization parallel to the shore• Seawall
• Barrier parallel to shore and close to the beach to protect property
• Stops waves from reaching the beach areas behind the wall
• Often the building of structures is not an effective means of protection
Seawalls and beaches
Seawalls are built to reduce erosion on beaches
Seawalls can destroy recreational beaches
Seawalls are costly and eventually fail
11.25-Thurman and Trujillo, 2004
Seawalls are designed to protect beachfront property from erosion
13.21-Segar, 2007
Figure 20.20
Seawall in Seabright, New Jersey
Where’s the beach?
Wine Island?
10.26-Merritts et al., 1998
Seawalls can’t compete with sea-level rise
Responses to Coastal Erosion and Beach Loss
Hard stabilization “Armoring” the shoreline
Responses to Coastal Erosion and Beach Loss
Hard stabilization “Armoring” the shoreline
Responses to Coastal Erosion and Beach Loss
USGS
Illegal extraction of beach sand is one of the major causes of coastal
erosion in Puerto Rico
Morelock et al.
Illegal extraction of beach sand is one of the major causes of coastal
erosion in Puerto Rico
Stabilizing the shore
Alternatives to Hard Stabilization• Beach nourishment
• The addition of large quantities of sand to the beach system
• Only an economically viable long-range solution in a few areas
• Abandonment and relocation of buildings away from the beach
Responses to Coastal Erosion and Beach Loss
Beach nourishment
Wrightsville Beach, NC
10.24-Merritts et al., 1998
Figure 20.21
Beach nourishment,Miami Beach
USGS
Locations of three submerged sand deposit areas on the insular shelf of
Puerto Rico
Aerial photograph of the Escollo de Arenas, a subtidal sand and gravel deposit that extends some 6 kilometers north-northwestward of Punta Arenas on the northwest coast of Vieques Island, east of Puerto Rico.
Responses to Coastal Erosion and Beach Loss
Caveats of beach nourishment Not permanent solution Expensive Need appropriate sand source
Responses to Coastal Erosion and Beach Loss
Still, with appropriate financial and sand resources, nourishment can be a viable solution in some settings
Responses to Coastal Erosion and Beach Loss
Abandonment and relocation
Alternatives to hard stabilization
Restrict the building of structures too close to the shore
Eliminate programs that encourage construction in unsafe locations
Relocate structures as erosion threatens them
Figure 10C
Relocation of the Cape Hatteras lighthouse, North Carolina
Coastal Hazards Increasing coastal development and rising sea level puts
more people and property at risk
Combined wind and storm surge damage
Iniki flooding reached>800 ft inland>25 ft above sea level
New Directions?
Avoid development of eroding lands Discourage additional development in erosion hazard zones Enforce “setbacks” Acquire high-value coastal lands Construction guidelines for hazard areas Nourish eroding shorelines
Natural resources are those that are derived from the Earth and that exist independent of human activity
- Cutter and Renwick, 2004
New Directions?
Without a sound understanding and appreciation of the dynamics of the coastal zone, and thoughtful environmental management of the shore
We may lose one of our most precious natural resources