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Coastal Geologic Processes
The interplay of winds, waves, tides, and currents continually reshapes coastal environments. With time, beaches and dunes shift location through erosion and accretion of sand grains.
Winds. The interaction of winds from variable directions contributes to the formation of sand dunes. In Maine, the strongest winds usually blow from the northeast and occur during storms. Along the southern Maine coast sustained winds of 130-145 km/hr (80 to 90 mph) are expected twice a century (ANSI A58.1, 1982). Another source, the U.S. Coast Pilot, suggests that sustained winds in excess of 185 km/hr (115 mph) may occur about every 50 years and may be accompanied by gusts 30% higher (240 km/hr [150 mph]). Prevailing non-storm winds blow most commonly from westerly directions with a northerly component in winter and a southerly component in summer. Coastal development, which blocks the wind, can interfere with the natural processes that form and maintain the dunes.
Waves. Large swells accompany storms and often continue long after the storm has passed. Surf from breaking swells can erode sand rapidly from the frontal dune at high tide and shift the beach profile to a more gradual slope with a larger offshore sand bar. The offshore sand bar helps to break waves further offshore and reduce dune erosion. The natural shift of sand between the dunes and offshore bars can be reduced by seawalls and shoreline riprap.
Tides. Tides are important agents of sediment redistribution over the beach. They sort sediments and may cause currents along the shoreline. Tides also determine the level at which storm waves attack the beach and frontal dunes. If a storm passes while the tide level is low, most of the wave energy is expended eroding lower portions of the beach and the dunes are preserved. However, many storms last longer than one tidal cycle and at high tide the frontal dune is eroded by waves.
Tides are variable along the Maine coast. Along the beaches from Kittery to Portland the tidal range is about 2.7 m (9 ft), while at Eastport, in far eastern Maine, the range is 5.6 m (18 ft). "Spring" tides increase high water levels by 0.3 to 0.6 m (1 to 2 ft) above mean high water depending on position along the coast. These tides occur semimonthly when the moon is either new or full and are increased even further when the moon is in perigee, nearest the earth.
Additional astronomical movements in the solar system enhance tides every few years and result in exceptionally high (and low) "proxigean" tides. These tides may bring water levels 0.6 to 0.9 m (2 to 3 ft) above mean high water in the Portland region and 1.2 m (4 ft) above the mean in Eastport.
Currents. Currents along the shore can be caused by winds, waves, or tides. Commonly, currents close to the beach are weak and variable. However, during extreme weather conditions, currents may move sand one way along a beach or offshore. A net movement of sand along a beach can cause erosion at one end and growth at the other. Erosional zones may have narrow frontal dunes while accretional zones may have multiple low dune ridges.
During storms, the combination of winds, tides, waves, and currents can move considerable volumes of dune and beach sand in short periods of time. Between storms the smaller waves tend to reshape the beach and move sand to the upper "dry" beach where, with the help of winds, the frontal dune can be rebuilt. The natural cycle of erosion and restoration results in many visual changes to the beach and dunes.
References
American National Standards Institute, Inc., 1982, Minimum Design Loads for Buildings and Other Structures, ANSI A58.1, New York.
National Ocean Service, United States Coast Pilot, No. 1, Eastport to Cape Cod, 1988, 24th Edition.
Last updated on October 6, 2005