Preliminary examination of a report on shoreline changes from 1949 to 1989 suggests that 62% of the sandy shoreline studied on Maui is eroding at an average rate of 1.25 ft/yr (HWANG AND FLETCHER, 1992), and as much as 30% of Maui's shoreline has experienced beach loss or significant narrowing (MAKAI OCEAN ENGINEERING, INC. AND SEA ENGINEERING, INC., 1991). Based on field and photographic observations, nearly all of this beach degradation is in front of or adjacent to shoreline armoring such as seawalls and revetments.
Typically, these armoring structures are erected when coastal erosion threatens beachfront development. Armoring the shoreline usually halts coastal erosion and protects property and structures, but on shorelines undergoing long-term retreat, it often leads to beach loss (FLETCHER, ET AL., 1997). The impact that armoring has on the adjoining beach creates a conflict between the rights of coastal property owners to protect their land and the rights of the public to utilize the beach resource.
Coastal Erosion
Sea-level rise, wave and current impacts, and sediment deficiencies drive coastal erosion (Figure 4). Sea-level rise, currently averaging about 2.5 cm/decade on Maui, causes the littoral system to shift landward by eroding the upland area-usually a coastal dune or the coastal plain. This natural process, known as coastal erosion, has occurred for millennia as sea level has risen nearly 110 meters since the last ice age. The retreat of the shoreline-and associated loss of coastal lands-is the natural response of the beach to rising sea levels (TAIT AND GRIGGS, 1990) and has been the underlying premise of coastal engineering theory for over thirty years (e.g. "The Bruun Rule", BRUUN, 1962). The influx of sediment released to the active beach by erosion of the coastal upland helps maintain beach width.
Figure 4. Causes of coastal erosion. Sea-level rise, wave and current action, and sediment deficiencies drive coastal erosion.
Certain human activities create significant sediment deficiencies and aggravate coastal erosion. These include sand mining, dune alteration (e.g., dune grading and building on dunes), construction of shoreline structures such as seawalls, revetments, and groins, degradation of coral reefs, and construction of harbors and navigational channels.
Sand mining on the beach removes sediment from the beach system leading to beach narrowing and deflation. Up until the early 1970's, large volumes of sand were mined from beaches around Maui to provide cement aggregate for construction and lime for sugar cane processing.
Dune grading entails bulldozing the upper portion of the dune to flatten it, often in order to allow an unobstructed view of the ocean or as part of a building's construction. This practice sharply reduces the dune's natural capacity to buffer coastal erosion and other coastal hazards. Furthermore, if the dune is then covered with soil fill, subsequent coastal erosion will release silt and other fine-grained sediments to the ocean, which degrade water quality.
Harbors and navigational channels can interfere with sediment transport. Sand moved by nearshore waves and currents is deposited in these artificial depressions and is removed from the littoral system.
Beach Loss
Armoring shorelines undergoing long-term retreat with structures such as revetments and seawalls halts coastal erosion, but refocuses the erosion onto the beach in front of the structure (TAIT AND GRIGGS, 1990; FLETCHER ET AL., 1997). This causes beach narrowing, a decrease in the usable beach width, and beach loss, the volumetric loss of sand from the active beach (Figures 5 and 6). Coastal armoring often aggravates erosion along downdrift properties by decreasing the supply of sediment to downdrift areas.
Figure 5. Coastal erosion vs. beach loss. Coastal erosion does not normally threaten beaches, but armoring to protect coastal lands may lead to beach loss (after TAIT AND GRIGGS, 1990).
Figure 6. Beach width on an armored vs. a natural shoreline, Speckelsville, Maui.
The site-specific history of coastal processes for a particular beach segment must be assessed to help guide the most effective beach management practices. Certain management tools-beach nourishment and dune restoration, for example-can counteract coastal erosion and beach loss. Other management tools-such as requiring sufficient building setbacks and wiser construction codes-can delay or prevent the need to armor the shoreline to protect beachfront development. Hence, coastal erosion does not necessarily present a conflict between coastal property owners and the public. It can be mitigated through effective beach management strategies.
Reef degradation
Harbor and navigational channel construction compromise the reef's wave buffering capacity. If a portion of the reef is dredged during harbor and channel construction, larger waves can reach the shoreline and accelerate erosion.
Many other human activities degrade water quality and harm coral reef ecosystems. Since most carbonate sand ultimately derives from the coral reef ecosystem, poor water quality reduces the amount of sand produced by the reef and delivered to the beach. Impacts to water quality caused by human activities include: siltation, nutrient loading, and urban runoff. In addition, over-fishing can deplete the reef ecosystem of certain species of fish and upset the ecological balance necessary for healthy coral reef ecosystems, and introduced species have disrupted the preexisting food web, which also disrupts the reef ecosystem. Finally, anchoring on reefs causes physical damage to coral, as does standing on or touching these sensitive creatures.
In some cases, coastal erosion can have adverse effects on water quality and harm the reef. The erosion of dirt embankments or coastlines that have been artificially filled releases fine sediments to the nearshore waters. Furthermore, the significant increase in drainage outlets for recent developments and concrete channelization for flood protection have both had significant impacts on near-shore water quality and sediment loads.
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