Use of wetlands in flood control

Dr Meththika Vithanage

‘Wetlands are areas of marsh, fern, peat land or water whether natural or temporary with water that is static, flowing, fresh, brackish or salt including areas of marine water, the depth of which at low tides does not exceed six meters’. This is the definition given by Ramsar Convention for Wetlands which was held in 1971 in Iran. Some wetlands are confined to freshwater and some are confined to saltwater.

Several wetlands are inundated by water for a few months of the year. A few wetlands are made by human beings. Therefore, wetlands are classified as freshwater wetlands, saltwater wetlands and anthropogenic wetlands although there are overlaps in this classification. Sri Lanka consists of several different types of wetlands.

Wetlands are typically associated with rivers and streams. Most of the streams are located in the Central, Western and Southern part of the country.

There are around 103 rivers in Sri Lanka; most of them are seasonal and others are perennial. Although Sri Lanka does not have natural lakes there are several flood plain lakes. These are called Villu and are mainly confined to the cut-off river bends. Freshwater marshes are another type of freshwater wetlands. They are associated with rivers; fill up with surface runoff, river floodwater and also groundwater seepage. Wathurana swamp forest and Muthurajawela are examples of freshwater marshes. These are characterized by partially decomposed organic material.

Deltas, estuaries, mangroves, lagoons and marine wetlands are considered as saltwater wetlands. Since the velocity of flowing water in rivers and streams is reduced when they come close to the sea, they often divide into branches resulting deltas. Finer sediments are deposited and form mudflats. Lagoons, estuaries and mangroves are brackish water bodies.

Tanks, reservoirs, saltpans and agricultural wetlands are classified under anthropogenic wetlands. All different types of wetlands are considered as factories since they provide several necessities for human beings.

Coastal erosion

Water, food, medicine, energy, flood control, retention of sediments, prevention of coastal erosion and purification of water by removing heavy metals, are few of the benefits that we get from wetlands. Wetlands are very sensitive environments and therefore they should not be merely abandoned but properly managed like other areas. In the past, wetlands were considered as wastelands and obstructed in many ways. Many were drained to be replaced by farmlands, settlements, infrastructure and waste dumping sites.

Urbanized areas

Recent flooding in the South Western part of the country has demonstrated the value of wetlands adjacent to streams and rivers in heavily urbanized areas. Such areas flood because of encroachment into the flood passage and the lack of sufficient storage to temporarily absorb floodwaters during storms.

Rapid urbanization and the associated land use changes in the country has encroached upon or destroyed important wetlands adjacent to rivers and streams. Properly maintained wetlands provide flood control benefits in many ways. First, they act as a hydrologic sponge to absorb excess floodwaters during major storm events which are subsequently released slowly in terms of both surface water and groundwater.

This essentially protects the downstream areas from excessive flooding by reducing the flood peaks and water levels. It has been estimated that only 15 percent of a watershed in wetlands can reduce flood peaks by as much as 60 percent.

Wetlands also help slow down the floodwaters reducing soil erosion along streams and rivers. Wetlands hold surface water, providing water storage and shoreline stabilization. Different wetlands have different storage capacities, depending upon their size, depth, frictional resistance, shoreline shape and position within a watershed.

Shoreline stabilization refers to the stabilization of soil and vegetation along bodies of water. It reduces sediments being carried into streams and lakes, which is harmful to both the land being degraded and the ecosystems in these waters.

They act as sediment traps and with the detention of floodwaters in the wetland over several days they act as a kidney to remove the pollutants often associated with floodwater during storms.

This will have significant benefit to the quality of water flowing into rivers and streams downstream.

The slowed water also builds up the soil forming higher, more insulated ground where terrestrial grasses and other vegetation will grow reducing the erosive power even more.

Environmental degradation

Coastal wetlands are extremely important for protection of inlands from severe storms or tsunamis. It has been suggested that for every mile of vegetative wetlands storm height can be reduced by one foot.

Policy and decision makers often lack awareness of the interconnection between the valuable ecosystems and people’s livelihoods or between environmental degradation and poverty. As a result, wetlands are inadequately managed leading to the destruction of its environmental services and products.

Wetlands are an important part of our ecosystem. Our economic well-being and quality of life largely depend on our natural resources and wetlands are the vital link between our land and water resources. We have already lost many of our wetlands.

We must take action to protect wetlands to ensure that the functions and related values they provide will be preserved for present and future generations.

The writer is a Research Fellow at the Institute of Fundamental Studies, Kandy