Flood management

Prepared by Bibhabasu Mohanty

Dept. of Civil EngineeringSALITER, Ahmedabad

MODULE- IV

Content

Indian rivers and floods, Causes of flooding, Alleviation, Leeves and floodwalls, Flood ways, Channel improvement, Flood damage analysis

Introduction cause of misery for the country

since ages because, major habitation clusters

like towns and cities been located near rivers since the beginning of civilization

India National Commission on Irrigation and Drainage (INCID), ‘Flood’ as a relatively high flow or stage in a river, marked by higher than the usual, causing inundation of low land .

The flood problem in the country is mainly due to southwest monsoon during the months from June to October.

The average annual rainfall of India is about 1170 mm, of precipitation takes place in about 15 days and less than 100 hours altogether in a year.

The average annual flow of the rivers of India has been estimated to be about 1869 BCM.

The Brahmaputra and the Ganga rivers contribute the major part of these flows.

The rivers carry major portion of their flows during the southwest monsoon period when heavy and widespread rainfall occurs.

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.

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.

Indian rivers and floods

• The rivers in India can be broadly divided into the following four regions for a study of flood problems:

1. Brahmaputra River Region2. Ganga River Region3. Northwest River Region4. Central India and Deccan Region

Brahmaputra River Region Covers the States of Assam,

Arunachal Pradesh, Meghalaya, Mizoram, Northern parts of West Bengal, Manipur, Tripura and Nagaland.

Catchments of these rivers receive very heavy rainfall ranging from 110 cm to 635 cm a year which occurs mostly during the months of May/June to September.

Main problems are overspills, drainage congestion, bank erosion, landslides, change in river courses.

Ganga River Region Covers the States of Uttar Pradesh,

Bihar, South and central parts of West Bengal, parts of Haryana, Himachal Pradesh, Rajasthan, Madhya Pradesh and Delhi.

Normal annual rainfall of this region varies from about 60 cm to 190 cm of which more than 80% occurs during the southwest monsoons.

Damage is caused by the northern tributaries of Ganga by spilling over their banks and changing their courses, inundation and erosion problems are confined to a relatively few places.

Northwest River Region

Main rivers in this region are the Sutlej, Beas, Ravi, Chenab and Jhelum, tributaries of Indus, all flowing from the Himalayas.

Region covers the States of Jammu and Kashmir, Punjab, parts of Himachal Pradesh, Haryana and Rajasthan.

Compared to Ganga and Brahmaputra River regions, flood problem is relatively less in this region. Major problem is that of inadequate surface which causes inundation and water logging over vast areas.

Central India and Deccan RegionImportant rivers in this region are

Narmada, Tapti, Mahanadi, Godavari, Krishna and Cauvery.

Andhra Pradesh, Karnataka, Tamil Nadu, Kerala, Orissa, Maharashtra, Gujarat and parts of Madhya Pradesh.

Annual rainfall in Western Ghats around 500cm, remaining 75 to 125 cm.

Stable and sufficient capacity to carry flood water, except the delta region of Orissa.

Problems like congestion, bank erosion, landslides, change in river courses

Causes of flooding

Intensity of rainfall in catchment area

Topography of catchmentSedimentation of rivers and

reservoirsObstruction in river flowContraction of river sectionInadequate cross drainage works

Other causes

Heavy melting of snow and ice

Failure of dam

Because of earthquake

Alleviation (Flood mitigation)Flood control aims to attempt:

Modify the floodModify the susceptibility to flood

damageModify the loss burdenBearing the loss or living with floods

Flood control measures

Construction of protective worksReduction of flood flows by storageModifying the susceptibility to flood

damageSoil conservation in catchment area

Structural mitigation measuresStorage reservoir

detention reservoirretarding reservoir

Confining river flow by embankments:

Embankments constructed parallel to river bank to continue the river water within a specified section.

Surrounded area protected from flood.

Channel improvement works:Increase the discharge or velocity of

stream or to decrease the stage and duration of flood.

Increase in size of cross section by widening and excavating stream bed.

Increasing velocity by smoothing river bed.

Diversion works

Flood wall: Concrete walls constructed just on

the river bank.Act as retaining wall.

Flood ways:Low lying areas along river known

as floodways.

These are vast depression into which portion of flood water diverted from river through natural or artificial channel.

Floodways ordinarily used during major floods and land in floodway used for agricultural purpose.

Construction of cut-off:During heavy rainfall when large

flood discharge approaches the sharp bend of the

river, it overflows its bank and submerges the surrounding area.

so cut-off constructed to reduce travel time and water flow with high velocity along a straight path.

Runoff reduction by watershed management.

AforestationContour farmingCheck damsGullyingBank protectionDiversion drains

Non- structural mitigation measures

Flood plain zoningFlood forecastingFlood proofingMathematical modeling

LeveesAn earth dyke or embankment.Used for flood control.Constructed along the river.Material placed in layers and

compacted with least pervious material placed along the river side of levee.

Height and top width of levee depends on H.F.L of river.

Levee cross section must be adjusted to fit the site and available materials.

Minimum top width 3 m usually recommended.

River side protected by stone pitching and country side protected by turfing.

Generally, levees run along river bank with a margin of 2 m between toe of levee and top of river bank.

Sometimes, it may run some distance away from river bank.

NATURAL LEVEES FORMATION

Location of leveesLocated on both the banks such that

sufficient area of flow is provided between them.

If flood plain between levees is shallow and flat, channel width is large, therefore height of levee is low.

A deep and narrow flood plain will necessitate high levees.

When a city or agricultural districts is to protect a ring levee may be provided.

Flood wallsFlood wall is usually of masonry or

concrete.Usually constructed in which

enough land is not available for levees.

If land is costly or limited, it is more economical to construct flood wall.

Constructed on river bank.Proper foundation should be

provided and precaution should be taken against scouring.

Flood waysLow lying areas along the course of

the river are known as floodways.

Depression into which a portion of the flood water diverted from a river through natural or artificial channel for temporarily storing.

After flood recedes, the store water is permitted to flow back to river.

Two functions in flood control:

Create large shallow reservoirs which store a portion of flood water, decrease flow in main channel.

Provide additional outlet for water, increase velocity.

Construction is limited by topography of the valley.

Availability of low value land.

Floodways used during periods of major floods, land can be used for agricultural purpose.

Diversion channel carrying water may be natural or artificial.

Channel improvement

Channel improvement as flood control measures.

River can be made to carry higher discharges at lower levels by improving its hydraulic conditions.

Increase in size of cross section of channel

Realignment of channel along a shorter route

Increasing velocity of flow in channelIncreasing waterways at crossing

Flood damage analysis

Direct damage: damage to crops, houses, human lives, live stock, roadways, rail tracks etc.

Indirect damage: due to transport loss factories, shops and business in flood affected centre may be closed. Temporary unemployment.

Two types of losses due to flood disaster,

Tangible lossIntangible loss

Tangible loss: loss can be estimated in terms of money value.

Personal properties like building, materials etc.

Loss of cropsLoss due to disruption of businessLoss due to disruption of road and

railways

Intangible losses: can't be estimated in money values.

Loss of human life and cattleLoss of health due to diseaseLoss caused by social distress

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