Disaster Cases E Ofori FLOODS Causes Flood is

Disaster Cases E. Ofori

FLOODS- Causes • Flood is a state of high water level along a river channel or on the coast that leads to inundation of land, which is not usually submerged. • Floods may happen gradually and also may take hours or even happen suddenly without any warning due to breach in the embankment, spill over, heavy rains, etc.

Types • • • There are different types of floods namely: flash flood, riverine flood, urban flood, Dam failure etc.

Flash Floods • Flash floods can be defined as floods which occur within six hours of the beginning of heavy rainfall, and are usually associated with storms • Requires rapid localized warnings and immediate response to reduce damage.

Causes a. Heavy downpour of rainfall b. Heavy siltation of the river bed reduces the water carrying capacity of the rivers/stream. c. Blockage in the drains lead to flooding of the area. d. Landslides blocking the flow of the stream. e. Construction of dams and reservoirs

Hazards • Primary • Secondary

Hazards - primary • Floodwaters transport larger particles as suspended load including rocks and sediment, automobiles, houses and bridges. • Massive amounts of erosion can be accomplished by flood waters. Such erosion can destroy bridge structures, levees, and buildings causing their collapse. • Water entering human built structures cause water damage. Homes, ruining of furniture , damage to floors and walls , and anything that comes in contact with the water is likely to be damaged or lost.

Hazard - primary • Flooding of automobiles usually results in damage that cannot easily be repaired. • The high velocity of flood waters allows the water to carry more sediment as suspended load. When the flood waters retreat, velocity is generally much lower and sediment is deposited. After retreat of the floodwaters everything is usually covered with a thick layer of stream deposited mud, including the interior of buildings. •

Hazard-flooding • Flooding of farmland usually results in crop loss. • Livestock, pets, and other animals are often carried away and drown. • Humans that get caught in the high velocity flood waters are often drowned by the water. • Floodwaters can concentrate garbage, debris, and toxic pollutants that can cause the secondary effects of health hazards.

Hazard-secondary • Disruption of services a. Drinking water supplies may become polluted, especially if sewerage treatment plants are flooded. b. This may result in disease and other health effects, especially in under developed countries. c. Gas and electrical service may be disrupted. d. Transportation systems may be disrupted, resulting in shortages of food and clean-up supplies.

Hazard-Long-term effects • Location of river channels may change as the result of flooding, new channels develop, leaving the old channels dry. • Sediment deposited by flooding may destroy farm land (although silt deposited by floodwaters could also help to increase agricultural productivity).

Hazard-long-term effects • Jobs may be lost due to the disruption of services, destruction of business, etc. (although jobs may be gained in the construction industry to help rebuild or repair flood damage). • Insurance rates may increase. • Corruption may result from misuse of relief funds. • Destruction of wildlife habitat

Effects • The most important consequence of floods is the loss of life and property. • Structures like houses, bridges; roads etc. get damaged by the gushing water, landslides triggered on account of water getting saturated, boats and fishing nets get damaged. • There is huge loss to life and livestock caused by drowning. • Lack of proper drinking water facilities, contamination of water (well, ground water, piped water supply) leads to outbreak of , diarrhoea, viral infection, malaria and many other infectious diseases.

Effects • Flooding also leads to a large area of agricultural land getting inundated as a result there is a huge crop loss, resulting in shortage of food, and animal fodder. • Floods may also affect the soil characteristics. The land may be rendered infertile due to erosion of top layer or may turn saline if sea water floods the area.

Flood management • Flood forecasting and warning has been highly developed in the past two decades. With the advancement of technology such as satellite and remote-sensing equipments flood waves can be tracked as the water • local civil authorities to communicate advance warning for appropriate mitigation and preparedness measures.

Flood risk reduction • Mapping of the flood prone areas is a primary step involved in reducing the risk of the region. • Historical records give the indication of the flood inundation areas and the period of occurrence and the extent of the coverage.

Flood risk reduction • Warning can be issued looking into the earlier marked heights of the water levels in case of potential threat. • In the coastal areas the tide levels and the land characteristics will determine the submergence areas. • Flood hazard mapping will give the proper indication of water flow during floods.

Flood risk reduction • Land use control will reduce danger of life and property when waters inundate the floodplains and the coastal areas. • In areas where people already have built their settlements, measures should be taken to relocate to better sites so as to reduce vulnerability. • No major development should be permitted in the areas which are subjected to high flooding. • Important facilities like hospitals, schools should be built in safe areas. In urban areas, water holding areas can be created like ponds, lakes or low-lying areas.

Flood risk reduction • Construction of engineered structures in the flood plains and strengthening of structures to withstand flood forces and seepage. • The buildings should be constructed on an elevated area.

Flood control • Flood Control aims to reduce flood damage by: • decreasing the amount of runoff with the help of reforestation and protection of vegetation, • clearing of debris from streams and other water holding areas,

Flood Control • conservation of ponds and lakes • Flood Diversion include levees, embankments, dams and channel improvement. Dams can store water and can release water at a manageable rate.

Flood Control-Structural measures • Structural measures include a. storage reservoirs b. flood embankments c. drainage channels d. anti-erosion works e. channel improvement works f. detention basins g. Infiltration ponds

Flood Control-nonstructural measures • • • flood forecasting Flood plain zoning flood proofing Flood Disaster preparedness Education and awareness creation

Flood frequency • The recurrence interval of a peak flow is the number of years, on average, in which the specified flow is expected to be equaled or exceeded one time.

Recurrence interval • The recurrence interval of a peak flow is the number of years, on average, in which the specified flow is expected to be equaled or exceeded one time. • For example, a peak flow with a 100 -year recurrence interval will, on average, be equaled or exceeded once every 100 years • It has an exceedance probability of 0. 01 (a 1% of being exceeded in a given year)

Recurrence Interval • Recurrence intervals refer to the average number of occurrences over a long period of time • For example, a 100 -year flood is statistically expected to occur about 10 times in a 1, 000 -year period, rather than exactly once every 100 years. • It should be noted that the occurrence of a flood of a given recurrence interval in a given year does not affect the probability of such a flood occurring again the next year.

Flood frequency • Flood frequencies can be determined for any given stream if data is available for discharge of the stream over an extended period of time. • Such data allows statistical analysis to determine how often a given discharge or stage of a river is expected. • From this analysis a recurrence interval can be determined and a probability calculated for the likelihood of a given discharge in the stream for any year.

Weibull formula • Use data values of yearly discharges n. The yearly discharge values are first ranked. • Each discharge is associated with a rank, m, with m = 1 given to the maximum discharge over the years of record, m = 2 given to the second highest discharge, m = 3 given to the third highest discharge, etc.

Weibull formula • The smallest discharge will receive a rank equal to the number of years over which there is a record, n. • Thus, the discharge with the smallest value will have m = n. • The number of years of record, n, and the rank for each peak discharge are then used to calculate recurrence interval.

Flood frequency-Weibull equation • Probability, P = m/(n + 1) • A graph is then made plotting discharge for each year of the record versus probability • The graph usually plots recurrence interval on a logarithmic scale. • From the best-fit line, one can determine the discharge associated with the flood with a given probability or recurrence interval

Flood designations • • Recurrence interval or frequency probability Example 10 -year flood, 20 -year flood, 50 – year flood, 100 -year flood etc.

Probability of occurrence • Probability of occurrence of a discharge P, • P = m/(n+1) • The value, P, is called the annual exceedance probability. • For example, a discharge equal to that of a 10 -year flood would have an annual exceedance probability of 1/10 = 0. 1 or 10%. • This would say that in any given year, the probability that a flood with a discharge equal to or greater than that of a 10 year flood would be 0. 1 or 10%. • Similarly, the probability of a flood with discharge exceeding the 100 year flood in any given year would be 1/100 = 0. 01, or 1%.

Flood risk probability Probability 2200 2000 1800 1600 Discharge 1400 1200 1000 800 600 400 200 0 1 10 Probability (%) 100

Flood Hazard Mapping • Flood hazard mapping is used to determine the areas susceptible to flooding when discharge of a stream exceeds the bank-full stage. • Using historical data on river stages and discharge of previous floods, along with topographic data, maps can be constructed to show areas expected to be covered with floodwaters for various discharges or stages.

Flood hazard mapping

Human Influence

Engineering Approaches • Channel modifications – a. Channelization can enlarge cross-sectional area and thus create a situation where a higher stage is necessary before flooding. b. Enlarging the cross-sectional area, higher discharge can be held within the channel. c. Channelization also increases water velocity, and thus reduces drainage time.

Engineering Approaches • Dams - Dams can be used to hold water back so that discharge downstream can be regulated at a desired rate. • Retention ponds - Retention ponds serve a similar purpose to dams. Water can be trapped in a retention pond and then released at a controlled discharge to prevent flooding downstream.

Engineering Approaches • Levees, Dikes, and Floodwalls • These are structures built along side the channel to increase the stage at which the stream floods.

Floodways • Floodways areas that can be built to provide an outlet to a stream and allow it flood into an area that has been designated as a floodway • An area which provide an outlet for flood waters during periods of high discharge.

Regulatory Approaches to Reduce Vulnerability • With a better understanding of the behavior of streams, the probability of flooding, and areas likely to be flooded during high discharge, humans can undertake measures to reduce vulnerability to flooding.

Regulatory Approaches to Reduce Vulnerability • Among these non-structural measures are: a. Floodplain zoning - Laws can be passed that restrict construction and habitation of floodplains. b. Instead floodplains can be zoned for agricultural use, recreation, or other uses wherein lives and property are not endangered when (note that I did not use the word if) flood waters re-occupy the floodplain.

Regulatory Approaches to Reduce Vulnerability • Floodplain building codes - Structures that are allowed within the floodplain could be restricted to those that can withstand the high velocity of flood waters and are high enough off the ground to reduce risk of contact with water.

Regulatory Approaches to Reduce Vulnerability • Floodplain buyout programs - In areas that have been recently flooded, it may be more cost effective for the government, to buy the rights to the land rather than pay the cost of reconstruction and then have to pay again the next time the river floods.