Flood control and urban drainages management    

A flood is caused by excess water surpassing the limits of its confines. Floods can be caused by heavy rainfall and overflowing river banks, with enough force to sweep away massive objects such as cars, huts and trees.

Flood is a natural process in which the river / stream uses to spill to its plains. Hazards occur if people occupy these areas during floods. The flood impact has been mainly due to the lack of flood plain management. The flood hazards of some communities on the banks of the major rivers / streams had major social and economic impacts which can be seen after many years without a major flood event.

Types of floods

The main types of flood which may impact urban areas can be classified in the following two types:

Due to urbanization: these are floods related to the increase of the impermeable areas and man- made drainage such as conduits and channels. Usually the land use surface in small urban basins are made of roofs, streets and others impervious surfaces. Runoff flows through these surfaces to the storm sewers. It changes the hydrologic cycle, increasing the overland flow and decreasing the groundwater flow. Under these circumstances the peak discharge increases together with the flood frequency. In addition, the washed urban surfaces during rainy days increase the pollution load in urban environment and to downstream rivers.

The source control of urban drainage has been developed by detention and retention ponds, permeable surfaces, infiltration trenches and others source control measures. Usually this type of control does not exist and the impacts are transferred to downstream in the major drainage. The cost to control of this impact is transferred from the individual to the public, since the county has to invest in hydraulics works structures to reduce the downstream floods impacts.

Due to flood plain occupation: these are natural floods which mainly occur in medium and large sizes rivers. When no reliable urban plan and regulation exists, the population occupies the flood plain after a sequence of low flood years, because these areas have a flat topography and are near to valuable city land and have a low cost. However, when a larger flood occurs, flood damage increase and the municipality is requested to invest in flood protection in this area.

Flash floods

Flash floods are dangerously fast moving floods caused by a large amount of heavy rain in a localized area. This torrential downpour may transform a normally calm area into a powerful river of death in only a few short minutes. Many people are caught unprepared for the power and speed of a flash flood, which results in dangerous situations.

 

Flash floods are slightly different from normal floods. Normal floods involve water rising and overflowing from its normal path. However, flash floods appear quickly and move swiftly across land with little warning. Flash floods occur for a variety of reasons including concentrated rainfall during a slow moving thunderstorm, hurricanes, and tropical storms.

The water in a flash flood moves at such a high velocity that it can move boulders, uproot trees, demolish buildings, and destroy bridges. The huge amount of water in such a flood be anywhere from ten to twenty feet tall and usually carry a substantial amount of debris.

The most devastating flash floods are from failure of hydraulic structures. When such structures break, an enormous amount of water is suddenly unleashed, destroying everything in its path.

Effects of Floods

Floods make an enormous impact on the environment and society. Floods destroy drainage systems in cities, causing raw sewage to spill out into bodies of water. Also, in cases of severe floods, buildings can be significantly damaged and even destroyed. This can lead to catastrophic effects on the environment as many toxic materials such as paint, pesticide and gasoline can be released into the rivers, lakes, bays, and ocean, killing maritime life. 

Floods may also cause millions of Rupees worth of damage to a city, both evicting people from their homes and ruining businesses. Floods cause significant amounts of erosion to coasts, leading to more frequent flooding if not repaired.

However, floods do make a slight positive impact on the environment. Floods spread sediment containing beneficial nutrients to topsoil that might never arrive there otherwise.

Urban drainage and flood control management

Some of the flood control principles in urban drainage and flood control management are:

  • Flood control evaluation should be done in the whole basin and not only in specific flow sections; 
  • Urban drainage control scenarios should take into account future city developments; 
  • Flood control measures should not transfer the flood impact to downstream and upstream reaches, giving priority to source control measures; 
  • The impact caused by urban surface wash-off and others related to urban drainage water quality should be reduced; 
  • Management of the control starts with the implementation of Urban Drainage Master Plan in the municipality; 
  • Public participation and awareness among public in the urban drainage management should be increased; 
  • The development of the urban drainage should be based on the cost recovery investments.

The main causes that hinder in implementation of the above principles are the following:

  • Urban development in the developing countries cities occurs too fast and unpredictably e.g. Development of cities like Bangalore, Chennai, Mumbai, etc., 
  • Urbanization in outer peripheral urban areas usually is developed without taking into account the city regulations.
     
    This urbanization is as follow: 
  • Unregulated developments: In outer peripheral urban areas of big cities the real estate is low priced. As consequence private land owners develop urbanization without the infra-structure, selling it to the low- income population; 
  • Invasion of public areas (such as public green areas) usually the invasion of public areas which were planned in Urban Master Plan for future parks, public construction and even streets. Due low income conditions (the homeless) and slow decision-maker by public administration these developments are consolidated receiving water and electricity, most of illegal developments lack sewers, water and electricity.
  • Outer peripheral urban and risk areas (flood plains and hill side slope areas) are occupied by low income population without any infra-structure. Spontaneous housing development in risk areas in Humid Tropics cities are on land prone to flooding: Municipality and population usually do not have sufficient funds to supply the basics of water, sanitation and drainage needs; 



 
  • Lack of appropriate garbage collection and disposal decreases the water quality and the capacity of the urban drainage network due filling. 
  • The local bodies and the public do not have enough knowledge on how to deal with floods using the above principles.
  • The local bodies and the public do not have enough knowledge on how to deal with floods using the above principles. 
  • Lack of institutional organisation in urban drainage at a municipal level such as: regulation, capacity building and administration. In Asian cities there is a lack of comprehensive project organisation and clear allocation of responsibilities; adequate urban land-use planning and enforcement;
  • capability to cover all phases and aspects of technical and non-structural planning; 
  • In cases where separate system is applied solid waste is dumped into storm sewers. This is a situation which occurs also in some developed countries. 
  • When there are unrealistic regulations for urban occupation related to social and economic conditions the owner uses different procedures in order not to comply. 
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