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Coastal Erosion Assessment for Maine FIRMs and Map Modernization Plan
Beach and bluff erosion processes along the Maine coast
An Eroding Coast
With simple sea-level rise, the coastal floodplain also rises. For example, in southern Maine, the salt marsh environment behind barrier beaches is generally an A-Zone (100-yr flood hazard area). The B-Zone (500-yr flood hazard area) is commonly about 6 inches higher than the A-Zone. So, the sea-level rise that has occurred in southern Maine since 1912 would have changed a 1912 back barrier marsh B-Zone to a A-Zone. Even in the absence of coastal erosion, a rising sea will gradually change the elevation of the floodplains and necessitate remapping of flood hazard areas.
Causes of Shoreline Change
Natural processes and human activities cause shoreline change. In addition to the gradual rise of the sea, other natural processes affect the position of the shoreline. The most important of the natural processes are waves, tides, and storm surge.
On a daily basis, waves and tides redistribute sediment along the shoreline. Waves that reach a shoreline at an angle produce a current that carries sediment along the shoreline. This sediment transport undercuts coastal bluffs and moves beach sand parallel to shore. Many shoreline areas have a predominant wave attack and thus a dominant sediment transport direction (e.g. Higgins Beach, Scarborough and Great Hill, Kennebunk). In an area with a natural balance of sediment supply and removal, the shoreline position remains stable. Along most Maine shorelines, however, the sediment budget is not balanced and there are areas of erosion and areas of accretion caused by waves.
Tides are also an important natural force that causes shoreline change. Tides flow in and out of estuaries and transport sediment along the margins of channels leading to and from the ocean. Tidal currents predominantly carry sediment from the ocean shoreline into the estuary in most Maine locations (e.g. the Kennebunk, Scarborough, and Webhannet Rivers). In a few places sediment is carried to the sea from an estuary. The Saco and Kennebec Rivers deliver sand to the coast. In either case, tidal currents continually alter shorelines and coastal floodplains in and around estuaries.
Storm surge is the natural elevation of the sea due to the influence of wind and atmospheric pressure near the coast. Coastal flooding results from storm surge and surge statistics define the 100-year floodplain. Storm surge is also important in changing the shape of the shoreline. An elevated sea combined with storm waves results in erosion of frontal dunes and coastal bluffs. Single storm events can cause tens of feet of dunes to erode (e.g. February 1978 Blizzard and October 1991 "Perfect" Storm). In general, such dune erosion is seasonal and most loss from storms is replaced in a year. A storm with a 10- to 100-year recurrence interval may permanently change the location and elevation of the frontal dune and permanently shift the beach profile inland. After significant storms, coastal floodplain boundaries along Maine beaches are likely to have moved inland permanently. Prioritization for remapping flood hazard areas along beaches should consider the age of the map relative to the timing of significant storms.
Human activity is a dominant force affecting the shoreline position and rates of shoreline change. The two primary actions that affect erosion and accretion are the engineering of seawalls and jetties. Seawalls are prevalent along about half of the beach shorelines. A significant, but yet undetermined, percentage of bluff shoreline is also armored to prevent shoreline change. In both coastal settings, the stabilized shore prevents the high tide line from moving inland. In fixing the shoreline, the natural release of sediment from the bluff or dune is prevented and the local sediment budget is permanently altered with consequences for adjacent shoreline stability.
Seawalls also alter the rate of sand transport along the beach. By causing increased wave reflection, the amount of sand resuspended in the water in front of a seawall can increase and lead to more sand being carried in an alongshore direction, away from the beach profile. The increase in alongshore-sand transport can be 10 times that which occurs in a natural beach and dune system. Seawalls can significantly alter the local sand budget of beaches. An effect of seawalls can be to alter the orientation of the shoreline and thus the way waves break and run up the beach and cause coastal flooding.
The third way that seawalls affect coastal flood hazards is by creating a beach profile that is out of equilibrium. On beaches where seawalls have regularly reflected waves for several decades or more, the natural inland migration of the beach has not taken place. Instead, the beach seaward of the wall has lowered while the dune behind the wall has remained static. Compared to a natural beach profile in an adjacent area, the amount of disequilibrium can be on the order of 50 feet. The artificially lowered beach profile influences the height of the 100-year wave envelope and wave runup elevation. These factors, in turn, affect the inland extent of flooding and the need to reevaluate flood hazard areas.
Dredging. Dredging for navigation also influences shoreline positions. Unstable sandy channels adjacent to beaches have had regular and recurring maintenance dredging for many decades. The cumulative amount of sediment redistribution due to dredging is as significant as or more significant than natural processes in most local sediment budgets.
Cumulative Human Action
The combined influence of jetty engineering, seawalls, and dredging has accelerated shoreline change and the inland positions of floodplains in Maine in the last century. In the last 40 years, up to 2 million cubic yards of sand have been moved in the Wells Embayment (Ogunquit to Kennebunkport) under the influence of human activity and dramatic shoreline changes resulted. In the last century, about 4 million cubic yards of beach sand moved north in Saco Bay from Saco to Scarborough. This movement was away from a jetty at the Saco River and toward the jetty at the Scarborough River. The rate of sand movement north in the bay is about 3 times the natural rate that existed prior to jetty and seawall construction in Saco. Over 30 houses in Saco have been destroyed by the combination of shoreline change and coastal flooding that is attributed primarily to human activity.
Last updated on February 8, 2006.
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