Tsunamis as Long-Term Hazards to Coastal Groundwater Resources and Associated Water Supplies

Abstract

Tsunamis are potential hazards in most of the world, (Figure 1), but risks are higher in areas associated with close and direct proximity to high seismic activity areas in marine and coastal areas, such as Indonesia, Japan, Chile, Peru, and SIDS (small island developing states) (UNISDR, 2009) (Figure 2). They generally hit in relatively limited areas: the coastal zone up to few kilometres inland from the shore. Tsunamis are short-term events, but impacts on human infrastructure and environment can be severe and enduring. Especially coastal freshwater resources, most often the source of public and private water supplies, are vulnerable to the imprint of saltwater that comes along with the tsunami flooding event. Safe, adequate, accessible, and socially acceptable water supply is of essential importance, both as a priority in the immediate aftermath of a disaster like a tsunami, but also in the longer term to ensure proper human health and prosperous livelihoods. Time horizon and severity of tsunami impacts on water resources and water supply systems depend on: magnitude and extent of the event, type of land mass hit (island vs. large mainland, low topography vs. rising coasts) and its natural protection (e.g. mangroves), population density, type and extent of freshwater resources present, and dependence on them for water supply. Often, alternative water supply solutions can be provided in the interim of a tsunami, from external sources and with external support because of the often limited geographical extent of the impacted areas. However, this is not always possible e.g. islands, and other aspects are relevant: recovery and sustainability of fundamental and primary freshwater resources, human health, optimal and efficient use of resources at any time of the disaster risk management cycle (Figure 3). Here, a distinction between groundwater and surface water systems is relevant. Groundwater systems are generally much slower in recovering due to longer internal residence times. However, since groundwater sometimes is the only freshwater resource available in coastal areas and on small islands, it is critical to assess the vulnerability and potential impacts. Since significant coastal populations around the world depend on groundwater for their water supply, either from decentralized, often small schemes and private wells, primarily in rural and peri-urban areas, or from larger centralized schemes, primarily in urban areas,