Geotextile and Vegetation Terraces for Sconset Bluff Stabilization

摘要

Coastal bank terraces, constructed using natural geotextile materials and vegetation, provide a sacrificial buffer to protect landward properties from coastal storm damage and downslope erosion. Sconset Bluff is a glacial coastal bank on the east coast of Nantucket Island, located off the mainland of Massachusetts. This bluff, which ranges in height from 4.6 meters (15 feet) to more than 21.3 meters (70 feet) near the historic Sankaty Head Lighthouse, has exhibited historic shoreline retreat at an average rate of 0.9-1.2 meters (3-4 feet) per year over the past century. Coastal storms erode the toe of the bluff, causing bank failure and collapse. Where entities seek to halt or slow landward retreat, local, state and federal regulatory standards require the use of non-structural shoreline protection strategies, if feasible. Private landowners with houses landward of the top of Sconset Bluff formed the Siasconset Beach Preservation Fund (SBPF), a non-profit group, to collectively pursue a comprehensive approach to coastal erosion control. Coastal bank terraces are an important component of the shoreline protection utilized to protect Sconset Bluff. These terraces can slow shoreline retreat, but to be an effective long-term solution they must be supplemented with other measures, such as beach nourishment, which protect the toe of the bank against erosive undercutting. SBPF is currently engaged in an effort to permit a beach and dune nourishment project along three miles of the eroding shoreline; bank terraces have been and are currently used at Sconset in isolation, without accompanying forms of protection, while the lengthy permitting process proceeds. Even with shoreline nourishment, terraces remain critical for protecting landward resources since nourishment material alone will not remedy portions of the bank that are already over-steepened as a result of undercutting and subsequent collapse. Terraces are needed to prevent runoff- and gravity-induced downslope failure. Ultimately, when used in combination with nourishment, terraces will stabilize the bank slope and prevent additional bank face retreat, protecting landward houses, properties, and public utilities.