Towards more effective floodplain management

Effective floodplain management, like watershed management, is an iterative process of identifying and assessing alternative ways of reducing the impact of floods (particularly of catastrophic events) in flood-prone areas. Decision-making in floodplain management involves compromises between the costs and benefits of alternative actions. It also requires that upper catchment areas be considered part of the solution and not as the ‘source’ of the problem.

In the past, structural responses (e.g., dams, levees, dikes, etc.) were emphasised and, indeed, in the early- to mid-20th century engineers prevailed in debates over the best means to tame the awesome power of floodwaters. With ‘flood control’ as their explicit objective, engineers around the world spent decades (and billions of dollars) building dams, embankments and levees to prevent floodwaters from inundating floodplains. These structures were often combined with dredging to straighten and deepen stream channels. According to the World Commission on Dams (WCD 2000), some 13 per cent of all large dams, or over 3,000 worldwide, were built with a specific flood-mitigation function.

Most flood defences were built as individual local schemes, with little consideration of their impacts across the wider river catchment, their impacts on the aquatic and coastal environments or, indeed, even their broad economic impacts. The fact that embankments and other engineering structures were most effective only for small- to medium-sized flood events was often not recognised. Also, river, road and other embankments sometimes inhibited the discharge of rainwater from water-logged areas into the river system (particularly where the number of sluices in the embankments is insufficient) and accordingly increased the dimension of flooding.

The available water storage of a typical reservoir is generally much less than the volume of a major flood surge. Moreover, structural solutions often have spill-over effects, shifting problems from one location to the next. For example, emergency releases of water during periods of high rainfall can dramatically and dangerously increase water levels immediately downstream of dams.

It should be evident that individual flood alleviation schemes cannot be considered in isolation and that a solution in one part of a river basin may be detrimental for other areas further downstream. Recently, numerous restoration projects have been implemented to reverse the impacts of earlier engineering works such as the Rhine Action Plan on Flood Defence adopted in 1998 after major floods in 1993 and 1995 (Leentvaar 1999). Increasingly, management of flood risks is moving away from structural engineering solutions toward programmes that work with natural processes. The impetus for this shift came from a number of major destructive events over the last 50 years, including:

• 1953 coastal flooding in the Netherlands that led to the Delta works;

• 1988-9 floods in Bangladesh that led to the Flood Action Plan and the National Water Management Plan;

• the upper Mississippi floods of 1993;

• the Rhône floods of 1993;

• the Rhine floods of 1993 and 1995;

• the Yangtze floods of 1998 in China; and

• the Elbe floods in Europe in 2002 - which once again drew attention to the important role of non-structural catchment measures.

The new approach weighs alternative actions in floodplain management in the context of whether overall flood effects are positive or negative. Although attention is usually focused on the negative effects of floods, there are highly important positive effects that warrant recognition and consideration. Flooding in many low-lying areas in Asia is a vital element of the culture and economy of the people. Annual floods along many rivers carry fine sediments and nutrients that renew the fertility of the land and aquatic habitats, and the continuous flow of silt-bearing irrigation water helps control diseases in many areas. In a region where agriculture and fishing remain vitally important, the loss of these beneficial effects could potentially lead to unacceptable economic and social disruption. However, what is beneficial to some may inflict heavy economic costs upon others. The challenge is to balance costs and benefits.

New flood management approaches are steadily introducing or expanding the role of non-structural measures within integrated floodplain management programs. Key measures include the identification of natural storage areas, such as swamps and wetlands, where excess water can be directed and temporarily stored during periods of flooding. The World Commission on Dams (WCD 2000) categorises the components of an integrated approach to floodplain management according to those which reduce the scale of floods, those which isolate the threat of floods and those which increase people’s capacity to cope with floods (Table 2).

A similar approach is also evident in the Mekong River Commission’s (MRC 2001) promotion of ‘Integrated Floodplain Management’, which comprises a mix of four types of management measures. These reflect the flooding, flood risk and flood hazard characteristics of a particular floodplain, the specific social and economic needs of flood-prone communities, and the environmental and resource management policies for the floodplain.

Table 2: Complementary approaches of integrated flood management

Source: World Commission on Dams (2000)

The importance of regional co-operation has also been stressed by the South Asian Floods Project (SAF) of the International Centre for Integrated Mountain Development. The project facilitates information exchange in the Hindu Kush-Himalayan region (http://www.southasianfloods.org). It stresses that one of the most cost-effective means of reducing the impact of floods is the nonstructural approach of providing people with sufficient advance warning for them to escape from approaching disasters. It further points out the importance of timely and reliable information on weather and river flows, and the open exchange of information among countries. Priority needs to be given to the development of such information gathering and dissemination networks in countries where no such systems currently exist.

Finally, there is a clear need to develop improved capacities for river systems to respond to flooding in both rural and urban environments, and by balancing land use more carefully. Agricultural and forestry policies, practices and incentive schemes need to be redirected towards reduction of flood risks and the restoration of the roles of formerly undeveloped floodplains for storing water and reducing peak flows downstream. Indeed, flood storage could become a recognised land use in development plans, which should be encouraged and compensated through government incentives. For example, over 25,000 homes have been relocated from the Mississippi floodplain since 1993, and thousands of hectares of marginally productive low-lying areas have been reconverted from agriculture to natural areas (Galloway, Jr. 1999).

Urban flood in Bangkok, Thailand (courtesy of the Royal Irrigation Department of Thailand through Water Resources Section, UNESCAP)