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Chapter 12 - Coastlines


Coastlines are a dynamic interface between land and sea. Coastlines preserve evidence of many process from the past, going back hundreds, thousands, even millions of years. Coastlines are shaped by an ongoing series of processes involving daily wind and wave action, tides, occasional storms and superstorms, earthquakes, and massive tsunamis. Coastlines reflect process of their origin including erosion of bedrock features, and are influenced by regional geology, geography, and climate.

Understanding coastline dynamics is important considering that about 75% of the worlds megacities are on coastlines. According to the United Nations. presently about 40% of the world’s population lives within 100 kilometers of the coast, with hundreds of millions living in low-lying coastal areas (below about 10 meters elevation).

Wave erosion is persistent and intense
, especially when storm waves combine with high tides. As a result, coastal landforms are generally delicate, and short-lived features. The sediment supply to coasts are offset by erosion rates along shorelines. Sediment supply is influenced by climate factors and geography, and can vary significantly from place to place, season to season, and by isolated events, such as changes caused by a massive superstorm (Figure 12-2).

Click on thumbnail images for a larger view.

New York City
Fig. 12-1. New York, the largest coastal city in North America. More than 12 million people in the US live in regions within 3 meters above current sea level.


Classifications of Coastlines and Shoreline Features

Three different classification schemes of coastlines include:
a. Primary or Secondary Coastlines
b. Active or Passive Margins
c. Emergent or Submergent Coasts

Note below that characteristics of each classification scheme overlap and complement each other.

hurricane katrinaFig. 12-2. Hurricane Katrina, North America's most expensive disaster, wiped out an estimated 328 square miles of coastal land along the Gulf of Mexico.


Primary and Secondary Coastlines

Primary: Young coasts formed by terrestrial influences, not significantly altered by marine processes.

Coasts that have been significantly changed by marine processes after sea level has stabilized.

Jade Beach
Fig. 12-3. Jade Beach, CA


Primary Coasts - 5 Types

Ria Coasts: Drowned river valleys caused by a rise in sea level.
Examples: Chesapeake Bay (Figure 12-4).

Glacial Coasts: Coastlines influenced by recent glacial activity such as glacial cut “U shaped” valleys called “fjords.” Examples: Norway, British Columbia, Alaska, Hudson Valley, New England region, Long Island (Figure 12-5).

Deltaic Coasts:
Coastlines associated with active river and delta systems.
Examples: Mississippi and Nile Rivers (Figure 12-6).

Volcanic Coasts
: Coastlines associated with recent or active volcanoes (mostly basaltic or andesitic volcanoes).
Examples: Hawaii, Aleutian Islands, Japan, Philippines, Indonesia (Figure 12-7).

Fault/tectonic Coasts:
Coastlines associated with major active fault systems along continental margins
Example: San Andreas fault going off shore at San Francisco (Figure 12-8).

Chesapeake and Delaware Bays and the Delmarva Peninsula
Fig. 12-4. Ria Coast: Chesapeake and Delaware Bays (estuaries), and the Delmarva Peninsula. Sea-level rise has back filled river valleys draining into the Atlantic Ocean. Ridges on land became peninsulas.

Kenai_fjords National Park, Alaska
Fig. 12-5. Glacial Coast: Kenai Fjords NP, Alaska

Nile River Delta from space
Fig. 12-6. Deltaic Coast: Nile River Delta

New land forming on Hawaii
Fig. 12-7. Volcanic Coast: Hawaii Volcanoes NP

Thornton Beach where the San Andreas Fault runs out to sea.
Fig. 12-8. Tectonic Coast:
Thornton SP, San Francisco


Secondary Coasts

Secondary coasts are coastlines that have been significantly changed by marine processes after sea level has "stabilized" allowing "erosional" and/or "depositional" processes to dominate shaping of the landscape. However, to explain this better, we need to examine the other classifications of coastlines first.

Both primary and secondary coasts
are influenced by whether they are "active" or "passive" continental margins (the second method of "coast classification").

Both primary and secondary coasts
are influenced by whether they are emergent or submergent coastlines (the third method of "coast classification" - discussed below). Passive margins tend to be submergent due to the ongoing rise in sea level (Figure 12-10). In contrast, active margins can be both "emergent or submergent" depending on local tectonic forces, such as caused by faulting.
Active versus passive continental margins Sea level trends
Fig. 12-9. "Active" versus "passive" margins. Fig. 12-10. Changing sea level trends.

Coastlines on "Active" vs. "Passive" Continental Margins

In North America, the Pacific Coast is an "active continental margin" whereas the Atlantic Coast is a "passive continental margin" (Figures 12-11 and 12-12)

An active continental margin is a coastal region that is characterized by mountain-building activity including earthquakes, volcanic activity, and tectonic motion resulting from movement of tectonic plates. Active margins typically have a narrower and steeper continental shelf and slope. They can also be subsiding or uplifting. Active continental margins are also associated with subduction zones, often include a deep offshore trench. The Pacific Coast is an active margin that is characterized by narrow beach, steep cliffs, rugged coastlines with headlands and sea stacks (see features discussed below).

Passive continental margins occur where the transition between oceanic and continental crust which is not an active plate boundary. Passive margins are characterized by wide beaches, barrier islands, broad coastal plains. Offshore passive margins typically have a wider and flatter continental shelf and slope. They are usually subsiding. Examples of passive margins are the Atlantic and Gulf coastal regions which represent setting where thick accumulations of sedimentary materials have buried ancient rifted continental boundaries formed by the opening of the Atlantic Ocean basin.

North Carolina Outer Banks satellite view
Fig. 12-11. Passive margin: North Carolina's Outer Banks region showing coastal plain, rivers, tidal estuaries, lagoon, barrier islands, and shallow Atlantic continental shelf.

Satellite view of San Francisco and Monterey Bay region
Fig. 12-12. Active margin: San Francisco Bay and Monterey Bay region has actively rising coastal range mountains and sinking coastal basins.


Erosional coastal landforms or features (on Secondary Coastlines)

Emergent coastlines typically have sea cliffs carved by wave and current action along the shoreline. The geometry of a coastline is largely a reflection of how some rocks along a coastline are more resistant to erosion.

Sea Cliffs
and Wave-Cut Platforms
* Sea cliffs form where persistent wave erosion carves into elevated coastlines.
* Waves erode the base of cliff, causing it to subside or fail (Figure 12-13).
* Waves carve a flat surface where they scour the seabed leading up to the beach creating a wave-cut platform.

* When sea level locally falls falls (such as from uplift of a regional earthquake) wave action scours out a new wave-cut platform, leaving remnants of the old seabed surfaces exposed as expose wave-cut bench (Figure 12-14). A wave-cut bench is a flat bench-like platform of rock typically preserved in the upper surf zone associated with an actively eroding sea cliff on an emergent coastline (Figures 12-14 and 12-17, also see marine terraces below).

are rocky shorelines that have resisted wave erosion more than surrounding areas, forming points or small peninsulas that jut seaward. Small sandy beaches typically occur in bays between headlands (Figure 12-15).

Sea stacks
are large rocky outcrops that have resisted wave erosion and stand offshore as the beach and sea cliff continues to erode landward
* Mound of rock and debris that eventually is taken to sea by waves.
* The product of a sea arch caving in (Figure 12-16).

A sea cave is an underground passage or enclosed overhang carved into a sea cliff carved by focused wave action (Figure 12-17).

A sea arch is a natural rock arch caved by wave action. Sea arches form where two caves join together or where a cave cuts through a narrow fin of rock (Figure 12-18).

Marine terraces are elevated step-like benches formed by the combined effects of long-term wave erosion during the rise and fall of sea level on an emergent coastline (Figures 12-19 to 11-22). Marine terraces are old wave-cut platforms and benches that have been elevated by the land rising relative to the ocean surface.

Sea cliff at Del Mar Dog Beach, CA
Fig. 12-13. Sea cliff rise above the wave-cut platform (with beach) at Del Mar Dog Beach, CA

Wave-cut platform on Drakes Beach, Point Reyes National Seashore, CA
Fig. 12-14. Wave-cut platform, wave-cut bench, and sea cliffs on Point Reyes National Seashore, CA

Headlands at Point ReyesFig. 12-15. Headlands and bays at Point Reyes National Seashore.

sea stacks at Olympia National Park
Fig. 12-16. Sea stacks along the coast at Olympic National Park, WA

Wilder Ranch State Park, wavecut bench and sea cave
Fig. 12-17. A sea cave and wave-cut benches at Wilder Ranch State Park, Santa Cruz, CA.

sea arch at Natural Bridges State Park, Santa Cruz
Fig. 12-18. A sea arch at Natural Bridges State Park, Santa Cruz, CA.


Elevated marine terraces on the California coastline (examples in northern and southern California are illustrated in Figures 12-19 and 12-20).

California preserves much evidence of geologic, geographic, and climatic changes caused by ice ages. During the last ice age, alpine glaciers and ice caps covered upland regions in the Sierra Nevada Range and Cascades volcanoes, but lower elevations were ice free (Figure 12-21). The formation of continental glaciers in North America and Europe caused sea level to fall almost 400 feet, causing the shoreline to migrate seaward as much as 10 to 70 miles westward of the current coastline in some locations. This rise and fall of sea level happened with each glaciation cycle (of which there were many through the ice ages of the Pleistocene Epoch). In places where the California coastline is slowly rising, each of the major glaciation cycles is preserved as a step-like bench, called a marine terrace. The formation of marine terraces is illustrated in Figure 12-22.

Davenport marine terraces
Fig. 12-19. Marine terraces at Davenport, California

California glaciation
Fig. 12-21. California at the peak of the last ice age. Glaciers covered the higher mountains, lakes filled inland valleys, and a coastal plain was extended offshore.

Formation of marine terraces
Fig. 12-22. Formation of marine terraces. This example shows the formation of two terraces. At least seven major terrace levels are preserved in some areas along the California coast.

Marine terraces on San Clement Island, California
Fig. 12-20. Step-like marine terraces on San Clement Island located offshore in southern California


Depositional coastal landforms or features:

Spits are ridges of sand projected from land into the bay (Figure 12-23).

A Bay-mouth bar is a sandbar that stretches across a bay, separating it from the ocean (Figure 12-24).

Barrier islands are ridges of sand islands that run parallel to the coast (Figure 12-25).

Spits: Rockaway Spit (NY) and Sandy Hook (NJ), NY)
Fig. 12-23. Spits: Rockaway Spit (on Long Island, NY) and Sandy Hook Spit (New Jersey project into outer New York Harbor and Raritan Bay.

Baymouth Bar at Bolinas Lagoon
Fig. 12-24. Bay-mouth bar: Bolinas lagoon has a baymouth bar composed of sand from the Point Reyes. Peninsula.

Barrier islands along Cape Cod's south shore, Massachusetts
Fig. 12-25. Nauset-Monomoy barrier islands along Cape Cod's south shore, Massachusetts.


Emergent and Submergent Coasts

Another important factor in understanding shorelines is tectonic activity and the rise and fall of sea level.

Submergent coastlines display characteristics caused when sea level rises or the land sinks down. Submergent coastlines:
* Contain estuaries and barrier bars, and barrier island systems.
* Ridges that separate valleys that propel into the sea.
Example: East Coast (see Figure 12-4).

Emergent coastlines
display characteristics caused when sea level drops or the land rises (from tectonic uplift).
* Wave cut platforms and elevated marine terraces.

Example: West Coast California (Figure 12-26).

In some regions around the world, tectonic forces are pushing rocks up along coastal regions, mostly in regions associated with active continental margins. There areas are called emergent coasts and display features including sea cliffs and marine terraces (see below). Where sea level is rising faster than land is rising, or where coastal areas are sinking, it is called a submergent coast. Submergent coasts are associated with passive continental margins with wide coastal plains and continental shelves. Estuaries are associated with submergent coastlines formed when sea level rises and floods existing river valleys. Active margins can have both emergent and submergent coastlines in close proximity to each other.

Torrey Pines State Preserve, CA
Fig. 12-26. San Diego's coastline displays characteristics of both emergent and submergent coastlines, having both seacliffs, headlands and marine terraces (emergent), and bays and estuaries filling flooded river valleys (submergent). View south along the Coast Highway at Torrey Pines Nature Preserve.


Common Shoreline Features of Beaches and Barrier Islands

Figure 12-27 illustrates common shoreline features associated with beaches and barrier islands.

A beach is an accumulation of mostly sand (and some gravel) along a shoreline where wave action winnows away finer sediment. Beaches occur in the intertidal zone (the zone between highest and lowest tides). Above the high tide line the upper supratidal part of the beach is mostly impacted by wind (forming dunes) and storm surges (Figure 12-28).

A barrier island is a long and typically narrow island, running parallel to the mainland, composed of sandy sediments, built up by the action of waves and currents (Figure 12-29). Barrier islands serve to protect the mainland coast from erosion by surf and tidal surges. Examples include the Outer Banks in North Carolina and Padre Island in Texas. Barrier islands are most common on submergent coastlines associated with low-relief regions such as is present along the Atlantic Coast and Gulf Coast of the eastern United States. They form where the sea floor remains shallow for a long distance offshore.

An estuary is the mouth of a river or stream where the tide-driven flow allows the mixing of freshwater and ocean saltwater (Figure 12-30). A lagoon is a saltwater-filled bay or estuary located between a barrier island and the mainland.

A tidal flat is a nearly flat coastal area (at or near sea level) that is alternately covered and exposed by the tides, and consisting of unconsolidated sediments.

Coastal Depositional Environments
Fig. 12-27. Coastal environments extend from offshore to inland estuaries and bays.

Coastal Dunes at Point Reyes Peninsula
Fig. 12-28. Beach and coastal dunes at Point Reyes National Seashore, California

Fire Island, A barier island on the south shore of Long Island, NY
Fig. 12-29. Fire Island, NY, is a barrier island on the south shore of Long Island, NY.

Elkhorn Slough
Fig. 12-30. Tidal marshes and tidal flats along an estuary, Elkhorn Slough, CA.


Coral Reefs, Keys, and Atolls

Biogenous carbonate sediments can accumulate faster than sea level is rising. Skeletal reefs (including coral reefs) thrive in the surf zone, and are able to weather wave action, although they can be heavily damaged by superstorm wave energy. The sediments generated by wave erosion and bioerosion (critters eating critters) contribute to the buildup of carbonate islands (keys) and atolls associated with fringing reefs forming around extinct and eroding volcanic islands (Figures 12-31 to 11-33). Keys and reefs of the world experience exposure and erosion during low sea levels during the ice ages.

Carbonate depostional environments
Fig. 12-31. Landforms associated with carbonate depositional environments.

Coral reefs and keys on an atoll in the Marshall Islands.
Fig. 12-32. Coral reefs and keys, Kwajalein Atoll, Marshall Islands, South Pacific Ocean

Mataiva Atoll
Fig. 12-33. Mataiva Atoll, Tuamotu Archipelago, South Pacific Ocean


Shoreline Erosion

Shoreline erosion depends on several factors:
1) Amount of sediment to buffer land: If the sand supplied to a beach is less than the amount removed by shoreline erosion processes, the beach will retreat landward.
2) Amount of tectonic activity: Uplift along the coastline allows erosion to provide sediments to a coastline. If the coast is not rising, then shoreline will retreat landward.
3) Topography: Coastal uplands provide more sediments to beaches than flat coastal plain regions.
4) Composition of land: Hard bedrock (such as granite) is harder to erode than softer unconsolidated deposits.
5) Waves and weather: The greater the waves and storm-generated currents, the more material can be eroded.
6) Coastline configuration: Coasts facing prevailing storm waves are eroded faster than isolated bays and down-wind protected shorelines (Figure 12-34).

Coast erosion of headlands and bays
Fig. 12-34. Wave refraction focuses wave energy on headlands and deposits sand in quieter bay settings.


Seasonal Erosional Changes to a Beach Profile

During the winter, storm-wave energy is most intense. Waves wash up on the beach and erode sand, and transport it offshore to where wave-driven currents aren't so strong and the sand accumulates on offshore bars. Heavier materials (gravel and boulders) are concentrated on the beach (Figure 12-35).

During the summer, lower wave energy prevails, and the sand gradually migrates back onshore, gradually expanding the beach seaward (Figure 12-36).

ano nuevo
Fig. 12-35. Cove Beach at Año Nuevo State Park (CA) in winter.

ano nuevo sandy beach
Fig. 12-36. Cove Beach at Año Nuevo State Park (CA) in summer.


Longshore Currents and Longshore Drift

A longshore current is a current that flows parallel to the shore within the zone of breaking waves. Longshore currents develop when waves approach a beach at an angle (Figure 12-37). Longshore currents cause sediment transport called longshore drift. Longshore drift is the movement of sediments along a coast by waves that approach at an angle to the shore but then the swash recedes directly away from it. The water in a longshore current flows up onto the beach, and then back into the ocean in a “sheet-like” formation. As this sheet of water moves on and off the beach, it can transport beach sediment back out to sea. Objects floating in the longshore current move in a zigzag pattern up and down the beach as it moves down current.

Longshore current and longshore drift
Fig. 12-37. Longshore currents and longshore drift are caused by waves approaching the beach at an oblique angle.


Rip Currents and Rip Tides

A rip current (or just “rip”) is a current that flows away from the coast. Rip currents form when wave break strongly in one direction, but weakly in another. In the surf zone, breaking waves produce currents that flow both along the shore and out to sea. Rip currents typical form on beaches with a sand bar and channel system in the nearshore area. A rip current forms as a narrow fast-moving current of water moving in an offshore direction. Obstructions in the water can also deflect current offshore. Rip current vary in size and speed (up to 6 miles an hour, or faster than an olympic swimmer). Rip currents move offshore and dissipate beyond the breaker zone. If caught in a rip current, swim parallel to shore to leave the current before heading for shore.

A rip current is different than a rip tide, which is current associated with the swift movement of tidal water through inlets and the mouths of estuaries, embayments, and harbors caused by the rise and fall of tides.

Rip Currents
Fig. 12-38. Rip currents are wave-generated currents that move in an offshore direction.

Rip Currents
Fig. 12-39. Rip current can vary with size and intensity depending of waves and shore geometry.


Tidal currents (including rip tides) are strong erosional forces where they are restricted at the mouths of inlets and straights between bodies of water. One example include the narrow straights of the Verrazano Narrows (between Staten Island and Brooklyn on Long Island, NY) (Figures 12-40 and 12-41). Another example is the Golden Gate Narrows between San Francisco and Marin County in northern California (Figure 12-42). In both cases, the seabed has been scoured deeply by the daily tidal flows. Tidal flows redistribute sediments building submerged tidal deltas at opposite ends of the channel that need to be dredged frequently to mitigate hazards to shipping.

New York City waves, winds, currents, and longshore and tidal currentss
Fig. 12-40. Dominant winds, waves, and currents of the New York City region.
Bathymetry of the New York City region
Fig. 12-41. Bathymetry shows tidal current scour in the Verrazano Narrows.
Bathymetry of San Franciso bay
Fig. 12-42. Bathymetry shows tidal current scour in near the Golden Gate Bridge.

Coastal Littoral Cells

A coastal cell is a relatively self-contained "compartment" within which sediments circulate. A coastal cell contains a complete cycle of sedimentation including sources, transport paths, and sinks. In the San Diego area, the Oceanside Coastal Cell extends from Dana Point to La Jolla Canyon; some of the sand is lost to Carlsbad Canyon as well (Figure 12-43). Streams and cliff erosion provide sediments to the shore zone. The arrow on the map indicates the predominant longshore current direction (and the direction of the migration of beach sand along the coast). Most of the sand moves down the coast and eventually drains down La Jolla Canyon and is deposited as turbidity flow deposits on the La Jolla Canyon deep-sea fan in the San Diego Trough (Figure 12-44).

Oceanside Littoral Cell Compartment
Fig. 12-43. Map of the Oceanside littoral cell and Carlsbad and La Jolla Canyons offshore.

La Jolla Canyon
Fig. 12-44. Sediments move from shore down La Jolla Canyon to the San Diego Trough.

San Diego's Coastal Erosion Problems Related To the Oceanside Coast Cell

The dominant swell direction in northern San Diego County is from the northwest. This creates longshore currents that move sediments (longshore drift) from north to south along area beaches. The sand on northern San Diego County beaches are mostly derived from sediments derived from coastal erosion in the shallow nearshore, beach, and sea cliffs along the coast between Dana Point and Oceanside (much of it from along the undeveloped coast within Camp Pendleton north of Oceanside). In addition, large quantities of sandy sediments are contributed to beaches from streams (small rivers) that, during episodic floods, dump large amounts of fresh sediment into the nearshore environment, contributing about half of the sand supply to area beaches over time. The amount of sand from river sources is highly variable with the seasonal weather, year to year.

Large waves (swell) especially during high tides in stormy conditions can erode, transport, and deposit large quantities of sediments.


Shoreline Erosion Problems

Shoreline changes quickly with natural forces; they are not a stable landforms. Coastlines, especially on the East Coast and Gulf regions, are constantly changing, especially from the impacts of superstorms. These coastal regions are underlain by unconsolidated sediments that are easily eroded by strong currents. They remain relatively stable, as long as there is a new supply of sediment to replace materials eroded by longshore currents, tides, and storm waves. Figure 12-45 illustrates how much shorelines can change. In less than two centuries, Fire Island's eastern spit has grown about 5 miles longer. The sediments creating this new land came at the expense of coastal lands father east on Fire Island, making the island increasing narrower. Barrier Islands are prone to be breached by storm erosion, creating new inlets, and filling in others.

Many attempts have been made, often at great expense, to try to prevent the effects of erosion and deposition along coastlines. Common construction efforts include jetties, groins, and seawalls to protect harbors, infrastructure, and communities.

Fire Island-Robert Moses State Park
Fig. 12-45. Fire Island (on Long Island, NY) has steadily grown about 5 miles longer since 1825 by longshore drift (see Figure 12-40). Fire Island Inlet at the west end of Fire Island is scoured by rip tides, adding sediments to the tidal delta in Great South Bay.


Jetties and Groins

Jetties are built at entrances to rivers and harbors. Their purpose is to protect properties from storm and wave damage, and to keep sand out of channels (so that there is no beach). Jetties require high maintenance costs to manage because they impede longshore drift (which is continues relentlessly). Most the costs are for dredging sand from one side, and moving it down current to replenish sand to community beaches. Loss of the sand supply makes down current areas susceptible to beach loss and coastal erosion (a major problem for Southern California's coastal communities, Figures 12-46 and 12-47).

Dana Point Harbor Jetty
Fig. 12-46. Dana Point Harbor and Jetty
(Orange County, CA)

Oceanside harbor
Fig. 12-47. Oceanside Harbor and Jetty (Northern San Diego County, CA)


Groins are built as barriers perpendicular to the beach in an attempt to stabilize shorelines. Their purpose is to trap sand migrating along the shore by longshore drift (Figure 12-48). Figure 12-9A is an aerial view of a wash-over fan created by a breach in Sandy Hook Spit (on the New Jersey side of New York City's Outer Harbor (see Figure 12-40).The inlet formed when coastal storm waves and currents cut an inlet across the spit. Note the sand trapped on the left side of the groins (longshore drift is moving left to right). Figure 12-49B shows an accretionary prism of sand building up at the end of Sandy Hook Spit. Figure 12-50 shows the growth of Rockaway Spit on the north east side of New York's Outer Harbor. It has grown nearly 2 miles since the end of the Civil War (1866). The area has been heavily modified by construction of groins and a jetty to keep the inlet to Jamaica Bay accessible to boat navigation.

Lonshore Drift
Fig. 12-48. Groins are designed to trap migrating sand by impeding the flow of longshore drift.

Sandy Hook groins, washover fan, and accretionary prism.
Fig. 12-49 (A&B). Groins, a washover fan, and accretionary prism on Sandy Hook, New Jersey.

Sandy Hook groins
Fig. 12-50. Map showing the growth of Rockaway Spit impacted by construction of groins and a jetty.


Other structures used to protect properties from the destruction by the sea

* Breakwaters are structures used to protect boats from large waves (jetties and groins are forms of "breakwaters").
* Seawalls are walls built to protect land structures from large waves and coastal erosion (Figure 12-52 show an example of some seawalls used to stop cliff erosion).
* Rip Rap are piles of large boulders put on the beach or shoreline. They are cheap but take up beach space and are not as permanent as a seawall, and are unsightly and dangerous. However, they do create habitat for sea life that needs a hard substrate to live (Figure 12-52).
* Beach nourishment adds large amounts of sand to the beach to keep water away from land structures. Sand is dredged form harbor areas or mined from sand bars offshore and pumped onshore in slurries. The process is quite expensive.

Seawalls in Encinitas California
Fig. 12-51. Seawalls built in Encinitas, CA are an attempt to stop sea cliff erosion.

Rip-rap was used in the construction of the breakwater (Oceanside Harbor).
Fig. 12-52. Rip-rap was used in the construction of the breakwater (Oceanside Harbor).


The "Dam Problem" - Dams have been constructed on most of the small rivers and streams throughout upland regions of San Diego County. The intentions of dam construction were to store water (reservoirs) and to reduce flood damage in low-lying communities. The problem is that dams have largely shut off the supply of sand from rivers and streams to the shore. One of the largest dams is for Lake Hodges on the San Dieguito River near Escondido, California (Figure 12-53). Construction of highway and railroad bridges, dikes, and causeways also restrict the flow of sediment-bearing water, preventing the migration of sediment to the coast. As a result, less sand is finding its way to the shore, resulting in narrower beaches. Without the protection of well-developed beaches, erosion of the sea cliffs are progressively endangering homes and infrastructure along the coast.

Lake Hodges Dam
Fig. 12-53. Lake Hodges Dam.

Dam construction: The easy way to kill a coastal community.

Dams on rivers trap sediments that would otherwise find their way to ocean beaches. A classic example was the construction of a dam on Matilija Creek in Ventura County. The dam is currently being demolished in order to return the sediment flow to sensitive habitats along the river downstream, but also to return a sediment supply to the Ventura County coastline (Figure 12-54). Many other dams constructed in the 19th and 20th century are being removed for the same reasons.

Ventura River Delta
Fig. 12-54. The Ventura River (left) supplies massive amounts of sediments to the coast during infrequent floods, then persistent coastal erosion processes take over the action. Construction of the Matilija shut of much of the sediment supply to the coast. The dam is now being removed.

Coastal Dynamics—The Unending Saga

Sea-level rise due to global warming is a highly political topic of our times. The world's Scientific Community has been studying the changes happening around the world for many decades. Real-time observations show that the atmospheric temperatures are steadily rising along with the concentrations of greenhouse gases in the atmosphere. What is perhaps most alarming is that the rate of sea-level rise is accelerating as the ice caps melt and the oceans expand from increased warmth (there are many NASA and NOAA websites on these matters). However, the threat doesn't seem to fit within the interest span of politicians and corporations keen on making high profits from the extraction of coal, oil, and gas. sea-level rise will likely continue unabated until either we either consume all the economically available carbon-based resources, or human populations collaborate to change the fate we, collectively, are facing.

Chapter 12 quiz questions
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