Risk assessment as tool for coastal erosion management

Abstract

The coastal zone of Cartagena city, Colombia provides illustration for applying risk assessments as a basis for guiding coastal management. Much of this coastal reach is heavily urbanized with a few areas of suburban to rural zones. In this study 32.6 km of shoreline under the city's jurisdiction were evaluated for coastal erosion management, calculating the Hazard and Vulnerability Indexes, which together constitute the Coastal Erosion Risk Index which provides a single numerical evaluation of risk. A range of variables were utilized in the calculations including forcing processes (wave height, storm surge, littoral exposure), coastal susceptibility (sandy and rocky shores: dune height, percent washovers, beach width, beach slope, cliff/platform, rock types, structure, weathering), as well as socio-economic, ecological and cultural factors (e.g., land use, percent urbanized, population density, infrastructure, cultural heritage, agency conservation/protection, ethnic communities, etc.). Coastal forcing and susceptibility are the basis for determining the Hazard, resulting mostly from induced susceptibility as a result of 60 years of over-reliance on shore-hardening structures, interrupting/reducing sediment supply, and over-development in terms of urbanization (loss of protective landforms, irreversible coastline modification, beach narrowing, and the growing coastal squeeze). From a management viewpoint, much of the Cartagena urban area has limited options, but the buffer zone between the shoreline and development must be increased (e.g., no new development; planned retreat), building codes updated (e.g., building floors at/below ground level should be flood-proofed, and have uses compatible with/recoverable from flooding/wave impact), and minimize additional shore-hardening (e.g., utilize beach nourishment). Rural and suburban areas in the region still have time to adopt more stringent management including development bans, disallowing replacement of buildings lost to erosion/flooding, large set-back requirements for new development, utilizing ecosystem management for natural protection, and removal of some groin systems. Virtually all management solutions will be costly, so plans are needed at the government level to develop financing systems.