Floods

WHAT IS A FLOOD?

Abnormal progressive rise in the water level of streams or rivers which may result in overflowing.

FLASH FLOOD

A sudden and extreme volume of water which flows rapidly and causes inundation and which, because of its nature, is difficult to forecast.

CAUSES OF FLOODS 

• Heavy Rainfall 

• Dam or levee failures

• Torrential rains from cyclones

• Tsunamis

• Storm Surges

• Burst water mains

TYPES OF FLOODS

• Flash Floods

• River Floods

• Coastal Floods 

LEVELS OF FLOODING

Inundations due to flooding vary in cause and severity can be categorized as follows:

MINOR FLOODING

• Due to the accumulation of excessive surface runoff.

• Flood waters consigned to the flood plain immediately along a river/channel or in random low lying and topographically depressed areas.

• Flooding is relatively shallow and there is no perceptive flow of water as when inundation is rapidly spreading to adjacent areas.

MAJOR FLOODING

• Due to overflowing of rivers and lakes, unexpected and serious breaks in dikes, levees and other protective structures or uncontrolled releases of dam water .

• Coverage of a wide continuous area and rapid spreading to adjacent areas of relatively lower elevation.

• Flooding is relatively deep in most parts of the flood-stricken areas. Currents of flowing flood water will be swift as the flood spreads to other areas.

LIKELY IMPACT

Flood waters can destroy infrastructure, particularly those at ground level, Crops and livestock can also be destroyed and considerable water damage to critical assets will occur in the path of flood waters. Many persons may be killed especially when flash floods occur but the injuries are few.  Food supply may become an issue.

FLOOD WATCH

Designation of the period during which flood monitoring forecasting and flood warnings are carried out.

The principal activities are:

• Flood monitoring. 

• Data collection and processing.

• Assessment of probability of flooding.

• Dissemination of information to public.
  

PHASES OF FLOOD WATCH

Alert: Period of intense data collection, monitoring and assessment, when the possibility of flooding is present, but its probability is relatively low.

Warning: Issued when indications show that water levels will exceed the alert level within 24 hours. The Flood Watch converts to flood warning and the appropriate Advisories are issued.

A Flood Watch is activated and terminated in accordance with the established conditions and criteria developed by the National Disaster Organisations.

EMERGENCY ACTION

Official announcements are issued before during and after the occurrence of floods.  These are intended to appraise the public in the affected area of the present and projected flood situation.

FLOOD ADVISORY

Content of such bulletins include:

• Present and expected situation of the meteorological cause of flooding.

• Present and forecasted hydrological situation.

• Areas likely to be affected.

• Time and severity of flooding

• Precautionary  measures recommended.

• Routine announcement of next bulletin.

• Safety precautions to be taken before the flood, when Advisories are issued and during and after the flood.

Before the Flood

• Know the flood warning system in your community and ensure that your family knows the warnings.

• Learn all you can about the flooding

• Monitor weather conditions

• Keep on hand material like lumber, plywood, nails, roped, wires, plastic sheeting, sandbags, etc.

• Keep a portable transistor radio with spare batteries and emergency equipment.

• Store all chemicals away from flood waters.

• Store livestock feed and supplies above expected water levels.

During the Warning

• Listen for emergency instructions

• Watch for rapidly rising water 

• Store drinking water in sealed plastic containers as water service may be interrupted.

• Move household items to higher levels 

• Get livestock to higher ground

• Evacuate if necessary when it is safe to do so, don't move quickly.

• Turn off electricity at the main switch before evacuating

During the Flood

• Avoid areas subject to flash flooding

• Don't attempt to cross rivers or flowing streams where water us above the knees.

• Beware of water-covered roads and bridges

After the Flood

• Re-enter building with caution. Use flashlights, not lanterns or torches as flammables may be inside.

• Be alert for fire hazards such as broken electrical wires.

• If the building has been under water, do not switch on the main, wait for professional assistance. Never touch electrical switches while wet or standing in water 

• Don't use appliances or equipment until they have been cleaned, dried and thoroughly checked for damage.

• Report utility lines (electricity, water, gas and telephone) to the appropriate authorities.

• Boil all water and don't eat left-over food until it is checked for contamination

• Keep away from disaster areas as your presence may hamper rescue efforts

MITIGATION MEASURES

Flood Mitigation Measures may be divided into three (3) main areas:

• Control over the river

• Control over the land

• Other measures

Control Over The River

Reliance is mainly on the physical alteration to the channel, flood plain or watershed to control the river. 
Measures include:

• Dams and reservoirs built on mainstreams or tributaries that store excessive water and releases it gradually after the threat has passed.
• Levees or floodwalls confine flood waters to a floodway, thereby reducing flood damage.
• Channel improvements which include:

1. Straightening to remove undesirable bendways

2. Deepening and widening to increase size of waterways 

3. Clearing to remove brush, trees and other obstructions

4. Lining with concrete to increase efficiency

• Watershed Treatment which renders the soil more absorbent of excessive rainfall until flood heights have receded.

Measures include:

1. Crop rotation.

2. Construction of terrace.

3. Contour strip cropping.

4. Selective planting and reforestation.

Control Over The Land

Measures are embodied in the following Land Use Policies:

• Designated floodways and encroachment lines are the lateral boundaries of the floodway where no construction or land filling should be permitted.

• Zoning is a legal tool used by governments to control development.

• Subdivision Regulations specify the manner in which land may be divided. Typical provisions show the extent of the flood plain on maps. Floodway limits or encroachment lines prohibit filing in channels and floodways that restrict flow and require that each lot contain a building site with an elevation above the flood level.

• Building Codes are standards for construction of buildings and other structures and if enforced can reduce damages to buildings in flood-prone areas.

Some requirements include:

1. Establishment of basement elevations and first flood elevations consistent with potential flood levels.

2. Structural strength to withstand water pressure or high velocity of flowing water

3. Prohibition of equipment that might be hazardous to life when submerged.

4. Prevention of flotation of buildings by requiring proper anchorage.

Other Measures

These include flood proofing , flood forecasting, warning and evacuation systems.

• Flood Proofing is a combination of structural changes and adjustment to properties which can be used in new or existing construction. Action include. seepage control, protective coverings, elevation or raising anchorage and under pinning.

• Flood Forecasting is reliable accurate and timely forecasting of floods coupled with timely evacuation to save lives and reduce property losses.

• Temporary Evacuation removes persons and property from the path of flood waters.

• Permanent Evacuation removes an affected population from areas subject to inundation. This involves the acquisition of lands and the removal of improvements. The acquired lands can be used for agriculture, parks or other purposes that would not interfere with flood flows or result in material damage.

• Flood Insurance assists by compensating for flood damage but insurance rates should realistically reflect the flood risk in order to avoid encouragement of improper development of flood plains.

Most flooded sections in the reason are monitored at the natural level

 

Tropical Systems

Landslides