Watershed Hydrology

Lesson 32 Flood Management

Flooding is caused by the inadequate capacity within the banks of the rivers to contain the high flows brought down from the upper catchment due to heavy rainfall. Areas having poor drainage characteristics get flooded by accumulation of water from heavy rainfall. Flooding is accentuated by erosion and silting of the river beds resulting in reduction of carrying capacity of river channel, earthquakes and landslides leading to changes in river courses, obstructions to flow, synchronization of floods in the main and tributary rivers and retardation due to tidal effects. Some parts of the country mainly coastal areas of Andhra Pradesh, Orissa, Tamil Nadu and West Bengal experiences cyclones which often are accompanied by heavy rainfall leading to flooding.

 

The damages caused by floods in terms of loss of life, property and economic loss due to disruption of economic activity are all too well-known.Floods can be mainly categorized in two types:

a)     Flash floods, occurs when heavy rainfall persists only for a short time period (usually only a few hours) yet can cause major damage and death due to their sudden arrival. It can also be caused by dam brusts or overflows.

b)    Riverine floods, occurs when water rises above its natural banks, often caused by prolonged and heavy precipitation. Riverine floods take days, weeks or months to rise it maximum and return to normal, much longer than it takes for a flash flood to.

 

32.1.1 When and where can flood occur?

a)     Floods can occur anytime of the year.

b)    In countries having monsoon season, like India, at the time of heavy rains flood often occur.

c)     In countries of high latitude floods occur primarily in the spring season due to snow melting and frequent storms.

d)    Flood occurs all over the world except Antarctica.

e)     Highland are also unlikely to be stuck by flooding.

f)      Floods usually affect floodplains, low lying flat areas near large water bodies.

 

32.1.2 Measurement of Flood Severity

There are six categories to measure the flood severity in a region as follows:

a)     Depth of flow

b)    Duration of flow

c)     Velocity of flow

d)    Rate of rise of water

e)     Frequency of flood i.e. how often the floods occur in an area, which greatly depends on the topography and climate.

f)      Seasonality i.e. time of year in which flood often occurs.

 

32.2 Structural Approach for Flood Management

The general approach to tackle the problem of floods in the past has been in the form of physical measures with a view to prevent the flood waters from reaching potential damage centres. The main thrust of the flood protection programme undertaken in India so far has been in the nature of taking structural measures like:

a)     Embankments, flood walls, sea walls.

b)    Dams and reservoir

c)     Natural detention basin

d)    Channel improvement

e)     Drainage improvement

f)      Diversion of flood waters

 

32.2.1 Embankments, Flood Walls, Sea Walls

The most common and generally economical form of protection to provide immediaterelief from inundation is construction of embankment. The embankment system along the river is planned to restrict the river in its existing course and they are designed to avoid over-flowing of banks by increasing the channel capacity to pass the probable floods. Generally, these are constructed with easily available earth in the nearby area.

 

Floodwall is a kind of retaining wall, constructed about parallel to the river course. The flood walls are constructed with RCC or masonary materials, depending on their availability in nearby area. Design of flood wall is done on the basis of water pressure likely to be developed on the wall. A key is provided at base for making the wall safe against sliding. For checking the overtopping of flood water, a sufficient free board is also provided to the flood wall. Sea wall is an example of flood wall. The sea walls are constructed in sea shore areas for protecting the sea bank from the soil erosion due to wave action.

 

32.2.2Dams and Reservoirs

Dams and reservoirs are constructed for storage of flood water to reduce the flood peaks. These are constructed at upstream end of the area, which is required to protect and from where the stored water can be safely discharged into the channel downstream side. The degree of flood reduction depends on the storage capacity of the reservoir. If reservoir capacity is more, then reduction in flood peak will be more and vice versa.

 

32.2.3Channel Improvement

Channel improvements involve providing of proper grade, deepening and widening of the channel section, to increase the flow carrying capacity. The main objective of this work is to decrease the stage and duration of the flood by increasing the flow velocity. The work of channel improvement is recommended for narrow and shallow depth channels having very small catchment.

 

32.2.4Drainage Improvement

Drainage improvements are used to carry out for providing relief against the recurring drainage congestion and water logging inside the urban as well as rural cropped areas.

 

32.3 Non-structural Measures for Flood Management

Structural measures can never completely eliminate the risk of flooding. Nevertheless, because of their physical presence, they have the potential to create a false sense of security, leading to inappropriate land use in the protected areas. Non-structural measures play an important role in reducing not only the catastrophic consequences of residual risks, but also adverse impacts on the environment. Non-structural flood management measures such as land use regulations; flood forecasting and warning; flood proofing; and disaster prevention, preparedness and response mechanisms; have limited environmental consequences and should be actively considered as viable options, both as independent or complementary measures.

32.3.1 Land use Regulations

Next to their important role in reducing the risk due to flooding land use regulations can contribute substantially to environmental preservation. For example certain land use regulations can lead to the reduction of surface runoff as well as to the conservation of ecosystems. Furthermore floodplain zoning can regulate the location of polluting industries and sewage treatment plants, thus preventing the dispersion of hazardous substantials due to flooding.

32.3.2 Flood Forecasting and Warning

Of all non-structural measures, flood forecasting and warning is the most widely accepted and has been used since the latter half of the 20th century. Flood forecasting allows concerned people and authorities to take preventive and emergency measures. Authorities can respond appropriately with dam operations, opening and closing the gates of various flood management structures, anticipatory releases to increase reservoir storage capacity, etc. Moreover hazardous material can be brought away from the affected area, thus preventing environmental damages.

 

32.3.3Flood Proofing

Flood proofing, a combination of long-term, non-structural and minor structural measures, as well as emergency actions, is important not only in reducing damage due to flooding, but also in preventing the negative impacts on the environment such as the spread of pollutants. It includes provision of quick drainage facilities such as the cleaning of primary and secondary drainage channels and clogged cross-drainage works before the onset of the flooding season. Moreover flood proofing measures include removing goods, equipment and harmful industrial, agricultural and domestic chemicals, beyond the area subject to flooding or out of contact with flood waters, by constructing high ground or small embankments.

 

32.3.4Emergency Preparedness, Response and Recovery

The awareness of the community at risk of flooding should be raised and maintained, with a clear understanding of their role in responding to emergency situations appropriately. This is critical in organizing coordinated evacuation from the affected area, maintaining healthy and hygienic conditions and preventing environmental pollution in the flooded areas. Flood-prone populations should be dissuaded from storing harmful chemicals during the flood season, made aware of likely pollutants in flood waters and advised of the ways to avoid their adverse impacts.

  

References

Subramanya K. (2008). Engineering Hydrology, Tata McGraw Hill, Third edition, 309-313.

 

Suggested Reading

Singh, G., Venkataraman C., Sastry G., and Joshi B. P. (1990). Manual of soil and water conservation practices, Oxford and IBH publishing Co. Pvt. Ltd.