Coastal processes

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


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


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 (↓)


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


Three basic responses to erosion problems1. Building structures (Hard stabilization)

2. Beach nourishment

3. Relocation


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


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


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



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


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


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


Hard stabilization “Armoring” the shoreline


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


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


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.


Caveats of beach nourishment Not permanent solution Expensive Need appropriate sand source


Still, with appropriate financial and sand resources, nourishment can be a viable solution in some settings


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

Education

Coastal processes