A flood is caused by excess water surpassing the limits of its confines. A flood can happen during a myriad of events, including heavy rainfall and overflowing river banks. Floods present a significant danger with enough force to sweet away massive objects such as cars and trees.

Flood is a natural process in which the river / stream uses to spill to its plains. Hazards occur when the population occupies these areas during a sequence of small flood. 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 increases 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 are 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.
• 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.

Watershed management:

The watershed management approach is holistic and aimed at optimising the use of land and water resources, preventing soil erosion, improving water availability including through water harvesting and water saving schemes and by so doing avoiding or minimizing natural disasters, which could be caused by droughts or floods. The emphasis was on conservation of rainwater to the maximum extent possible.

The approach has two components, namely, (i) soil and water conservation works comprising of bunding, trenching, terracing, harvesting structures such as ponds, water channels stop- dams and (ii) promotion of sustainable production systems including through introduction of improved technologies and practices of conservation farming, forestry, aquaculture, animal husbandry production systems.

Specific activities under the above two major components included: (i) putting adequate vegetative cover on the soil during the rainy season; (ii) conserving as much rain water as possible at the place where it falls; (iii) draining out excess water with a safe velocity and directing it to storage ponds; (iv) avoiding gully formation and putting checks at suitable internals to control soil erosion, and to recharge ground water; (v) maximising productivity and increasing crop intensity; (vi) safe utilisation of marginal lands through alternative land use systems; (vii) ensuring sustainability of the ecosystems; (viii) and reducing risks of natural disasters.

Flood Control in developing countries

Flood control in most of the developing countries are not adequate to minimize the related impact. The main aspects of these policies are presented below:

Urban Drainage in developing countries is designed based on the concept of draining water from urban surfaces as quickly as possible through pipe and channel networks; but this increases the peak flow and the cost of the drainage system. There is no control of peak increase at a minor drainage level and most of these impacts will appear in the major drainage.

Flood Plain : A common scenario in an uncontrolled urbanization is that flood plain occupation by the population takes place, in a sequence of years with small flood levels. When higher flood levels return, damage increases and the public administrations have to invest in population relief. Structural solutions have higher costs and it is feasible only when damages costs are greater than their development or due to intangible social aspects. Non-structural measures have lower costs, but there are some difficulties in their political implementation.

Plan Development

Urban flow control is developed by sub-basins and regulated by modules which are defined by a political city division but have the technical restrictions related to basin flow. The main flood control policies may be summarized based on the drainage system as follow:

In the major drainage: hold urban space for detention or in same way create linear parks within the river boundaries for storage flood volumes, sediment and trash detention and water quality improvement. Since part of the upstream urbanization is not controlled due to lack of law enforcement, as described before, this policy is used to avoid urban drainage impact to be transferred to downstream reaches. Instead of having the garbage and sediments distributed in conduits or along the rivers and channels, it is retained in specific places for cleaning and reduce the maintenance costs. In some situations this is not the best solution, it has to be evaluated based on local conditions.

When, in the major drainage, the selected solution for flood control are storm sewers or increase channel capacity, the plan or design has to control their downstream impacts.

In the flood plain: use non-structural measures: flood areas zoning of low and high risk areas; constructions standards and overall regulation related of this control. Some of the common recommendations are the following: (i) public flood area which has its value increased due to urban development pressure has to be occupied with urban infrastructure such as parks, sports fields, playgrounds, etc. If these areas are not developed they could be invaded.

There are a myriad of ways to prevent floods. The creation of flood plains and winding streams are two of the best ways to hinder the accumulation of water by providing a route for the drainage of water. Also, the protection of wetlands helps to maintain a natural drainage system to provide a place for the excess water to gather. Such devices allow the water to evaporate before it can accumulate, creating flooding conditions.

Levees also impede the collecting of water. Levees are embankments composed of soil and earthen material that are used to prevent annual flooding in many areas. With a levee, a huge amount of rain water is required before flooding occurs. However, one drawback to their use is that if water is able to get past the levees, then the flooding is significantly worse and causes several times more damage.