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Title: SMALL DAMS, PRE-RESERVOIRS: PROACTIVE, SUSTAINABLE MEASURES FOR ADDRESSING WATER SHORTAGE, FLOOD RISK, WATER POLLUTION AND COASTAL EROSION

Author: A. D. Toumazis and K. Kyrou

Year: 2009

Publisher: European Water Resources Association (EWRA)

Description:

Areas facing drought/ water shortage problems have adopted policies of constructing dams in order to store the water for later use. River flow in such areas carries with it significant quantities of sediments (the result of soil erosion), tree branches (the result of the drought) and polluting substances(especially in the case of a fire). This multi-phase river flow is associated with various problems such as bridge openings being blocked causing floods upstream of the bridge, collection of sediments in the receiving dams, dam storage capacity reduction and natural sediment cycle arrest. As a result the river delta and the adjacent coastal areas are starved from sediment supply, the water stored in the dams is polluted with pollutants carried by the river flow. Measures have been taken and are being taken to address individual problems:

  • Dams are built to address the water shortage problem,
  • Bridges/ culverts/ water drainage systems are designed to accommodate the increasing flood events – it is not practical to design for obstructed openings,
  • Coastal defences are constructed to address the problem of coastal erosion.

In a holistic approach, the above problems are addressed simultaneously and proactive, sustainable measures developed in a recent research program are proposed. Small dams/ pre-reservoirs are constructed at the entrance of the river to the reservoir and upstream of potential flood areas. These are low elevation dams capable of storing the water carried by small rainfall events (return period of about one year). River water (together with sediments, any branches and pollution) is arrested and temporarily stored within the basin/ pond upstream of
the structure (which is normally empty). After the storm, water, practically free from sediments and pollution, is allowed to enter the main dam reservoir and is diverted for irrigation or domestic supply, or continues its flow to the sea at a controlled rate. This controlled rate of flow reduces the risk of flooding downstream. Sediments collected in the small dam basins are sorted and suitable material is transported to the coastal area, thus restoring the natural sediment cycle. Any pollution (result from fire or accidents) is treated before discharge to
the environment. The small dams, pre-reservoirs are considered as sustainable, pro-active measures that reduce the risk of flooding, address the risk of water pollution, contribute towards the alleviation of the drought problem and restore the sediment cycle and the supply of sediments to the coastal environment.