Flood management

Flood Management

• Floods occur when discharge exceeds bankfull capacity.

• Water leaves channel to cover adjacent land – the flood plain.

• Human occupancy of this flood zone creates need for hazard response.

Recurrence intervals

• This refers to the frequency with which a particular flood height can be expected to return.

• Established from past records.

• Formula: • Recurrence interval = nº of peaks in list + 1

ranked position of discharge x

Human Impact on Run-off and flooding

• Most activity tends to increase flood risk, by reducing the interception store and thus increasing the amount of surface run-off:

• Deforestation • Urbanisation • Cultivation• Whereas afforestation reduces the flood

risk by encouraging infiltration.

Human response to Flooding:

1. Flood protection – decreases risk of bankfull capacity being exceeded

2. Flood abatement – reduces stormflow and reduces peak discharge levels

3. Behavioural responses – societies adopt different coping strategies

Human response to Flooding: Flood Protection

1. Modification to channel or banks:• Bank raising and dredging both increase bankfull

capacity• By increasing the hydraulic radius, channels also

become more efficient (velocity increases and so water levels drop)

• Widely used (e.g. Mississippi – 3000 kms of raised levées – up to 15 m high)

Human response to Flooding: Flood Protection

2. Artificial channel linings:• Concrete lined channels create smoother

wetted perimeter and so increase velocity• Thus water levels drop and flood risk is

reduced• Expensive, and high maintenance• E.g. Los Angeles

Human response to Flooding: Flood Protection

3. Dam construction:

• Multi-purpose, but key tool for flood protection

• Controlled release of water stored in reservoir through sluice gates can spread discharge over a longer period (reducing peak flows)

• Effectiveness depends on relative scale of reservoir’s catchment area to that of the whole drainage basin

• Geo-politics can cause problems –

e.g. India and Bangladesh (Ganges), Spain and Portugal (Tagus), Zimbabwe and Mozambique (Limpopo and Floods of 2000)

Human response to Flooding: Flood Protection

4. Flood relief channels:• effectively increases bankfull capacity and

diverts flow away from high impact zones• requires there to be space on floodplains to skirt

around high impact zones, so not always possible

• e.g. River Exe at Exeter, River Thames at Windsor

Human response to Flooding: Flood Protection

5. Spreading grounds:• Diverting flood water to low impact flood plain

zones, for storage• Reduces downstream peak flows• Low impact zones can be recreational land use• Flood water will evaporate or eventually infiltrate,

replenishing groundwater supplies• E.g. Los Angeles basin

Human response to Flooding: Flood Protection

6. Debris dams:• To trap sediment in upper catchments to prevent

downstream bed aggradation• Maintains higher bankfull capacities downstream• Periodic need for emptying, but can be used for

construction materials• Especially important in semi-arid, mountainous

catchments• E.g. Los Angeles Basin

Human response to Flooding: Flood Protection

7. Straightening of sinuous rivers:

• Increases gradient → increases flow rates

• Thus water levels drop and flood risk is reduced

• Also reduces deposition and averts bed aggradation

• Also keeps channels navigable

Human response to Flooding: Flood Abatement

• Tackles problem at source by reducing surface run-off. Achieved by:

• Afforestation or reforestation of upper catchment slopes (e.g. River Exe)

• Comprehensive protection of vegetation

• Terracing of farmland

• Contour ploughing

Human response to Flooding:

Behavioural responses

1. Accepting the loss – fatalism often only option in countries like Haiti or Bangladesh

2. Public relief funds – emergency response to hazard event requires funding, materials, technical support, rebuilding. Sources vary from UN agencies to governments and NGO’s

3. Flood insurance – a standard response in flood prone communities in the North

Human response to Flooding:

Behavioural responses

4. Monitoring and Prediction – data on rainfall and stream discharge can be used to produce accurate predictions of the timing of flood surges

• Can be used for communities to prepare for actual flood event or for authorities to organise evacuations

• Not always possible – flash floods have too short a lag time (e.g. Boscastle, Devon, U.K., 2004), lack of technical equipment / personnel (Haiti, 2004), or communication systems (Bangladesh, 2004)

Human response to Flooding:

Behavioural responses

5. Floodplain zoning• Planning authorities can prohibit certain land-

uses in the more flood prone floodplain zones6. Flood proofing• Individuals bear responsibility for reducing

likely flood damage to property• Techniques: water-proof garden walls, windows

and doors; sandbags; buildings on stilts; removal of damageable goods to higher levels.