FLOODS A river flood is a flow in excess of the channel capacity to accommodate the peak discharge, and occurs when the amount of water arriving on land exceeds the capacity of the land to discharge that water by infiltration, surface flow or drainage pipes. Flooding of river valleys and coastal areas is the most frequent of natural hazards and is one of the most significant in terms of death, injuries and long-term social and economic impacts. Flooding regularly claims over 20,000 lives a year and affects 75million people globally. Thus, strategies are put in place to minimize the impacts of these hazards.
The 3 broad groups are prediction, mitigation and response. However, the effectiveness of these strategies vary depending on the level of development of the country, the level of cooperation between the authorities, population and private organizations and the severity of the hazard. It is also important to note that effective hazard management cannot rely on just a single strategy and generally, a multi-pronged approach, which is a combination of strategies, is necessary to minimize the severe impacts of the hazard. Prediction 1. Prediction through the calculation of flood recurrence interval a.
North Dakota – Accuracy and reliability of data b. Flood recurrence interval is defined as the probability that a flood of particular magnitude will occur once or more in any given year. It can be used to examine the flood frequency (how often an area will experience flood) and flood magnitude (the size of a flood event). The records of a river’s discharge over the longest time available are ranked according to the discharge volume. The formula (n+1)/R is used to calculate the recurrence interval, where n is the number of discharge levels in the record and R as the rank of discharge.
The recurrence intervals then can be plotted against discharge to determine the statistical probability of flood events. c. Using the flood recurrence intervals, a hazard map can be produced, which will aid in mitigation and response strategies. d. However, the statistical prediction of flood recurrence intervals Is only an indication of probability based on past records, and it is known that rivers are constantly changing during to erosion and deposition, thus the statistical prediction will change. Also, the accuracy of the extrapolated values epends on how much data is available, and most data tend to be relatively recent and cover only a few decades. These factors lead to the inaccuracy of flood recurrence interval values. For example, North Dakota had two 250 year floods within 110 years. e. Thus, prediction through flood recurrence interval is only reliable if the flood records are longer and fuller, and if the records are constantly updated as events happen. 2. Forecasting f. Bangladash [Rmb: NWS] – Level of development g. Flood forecasting uses satellite readings and radar to predict the occurrence of floods.
This method of prediction is more accurate than using the recurrence interval method to predict the occurrence of floods. Forecasting can be split into short-term and long-term forecasting. Short-term forecasting is based on the atmospheric circulation pattern monitored through satellites. It has a higher accuracy of around 24h to 72h lead time, and is useful for emergency actions. Long-term forecasting is based on rainfall run-off modeling for different scenarios using historical flow data, and the likelihood of the river flooding is determined based on the current rainfall conditions and weather observations. . Flood forecasting is more common in DCs than LDCs, as DCs have the availability of financial resources and technical expertise. For example, in US, river gauges to track water levels, extensive radar networks are used to determine rainfall volume and location, and computer models are sued to predict how the water will flow downstream, just to predict the floods. i. However, as LDCs lack the financial and technical expertise, much of the satellite data have to be obtained from the National Weather Service (NWS).
For example, the forecast system in Bangladash (Climate Forecast Applications of Bangladesh) is funded by US to improve flood warnings. j. Thus, flood forecasting using satellite images have a very high accuracy, but is more common in DCs due to their financial and technical expertise. However, DCs can also gain international help to forecast floods too. Mitigation 1. Early warning system a. 2000 Britain flood & Bangladash [rmb: NCAR, ADPC] – Level of development b. Early warning systems allows people to be evacuated rom the hazard zones, and gives time for properties to be moved, and for temporary flood defences to be employed. c. For example, in Britain, forecasting and warning are important in flood management. In the great flood of 2000, there were 200 general warnings and media coverage was extremely effective in providing the public with flood warnings. These warning enabled the residents to clear ground floor and basements of at-risk properties and use sand bags to try to prevent water entering buildings. . However, early warning systems are generally more seen in DCs, as early warning systems required the improved communication technology for more efficient dissemination of warning to public, and LDCs generally lack the financial and technological resources required. e. For example, in LDCs, there are 15 flood-prone countries with no warning systems, and at least 40 more with inadequate systems. f. However, with the help of international support, LDCs too can have an early warning system.
For example, Flood forecasting in Bangladash has benefitted from technology from the United States in 2009, and scientists at the National Centre for Atmospheric Research (NCAR) have provided the technology to provide forecasts to Bangladash agencies in 2003. Also, the establishment of the Asia Disaster Preparedness Centre (ADPC) has allowed Bangladash to have a 10-day lead time in terms of flood warning. g. Thus, early warning systems are effective in minimizing the damage to properties and human lives by giving them time to evacuate and prepare temporary defences.
However, even though DCs have the financial and technical resources for more efficient dissemination of warnings, LDCs too can have an effective warning system if they have help from international support. h. Also, it is important to note that the effectiveness of this strategy depends on the efficiency of forecasting. Thus this mitigation approach must go hand in hand with forecasting in order to ensure the accuracy of the warning systems. 2. Floodplain zoning (soft-engineering method) i. New Zealand, Ciliwung River in Jarkata j.
Based on the hazard map produced using the recurrence interval data, the floodplain can be divided into areas of different degrees of flood risk. Once the flood-hazard areas have been established, local authorities may exercise controls, such as zoning areas to limit the use of flood-prone areas or relocating the developments. k. For example, in New Zealand, development is excluded in highest risk areas. Land-use which retains the floodplain as a natural floodwater storage areas and wetland, such as soccer pitches in urban areas, helps to reduce the flood risk downstream.
In the outer areas of the floodplain, which are at reduced risk of flooding, there would be regulations to minimize flooding such as flood-proofing of buildings. l. However, it is only appropriate for new developments, as it is not feasible for existing urban areas due to the need to relocate structures or resistance by people who lose the advantages of a floodplain location. Moreover, despite the fact that this management technique can be used in both DCs and LDCs and it is cheap and effective, it is less effective in LDCs due to the lax enforcements in LDCs.
With limited space in LDCs, there is a need to occupy every single space they have. Moreover, there will be squatters occupying such ‘danger zones’, and it is difficult to relocate the squatter settlements, such as those around Ciliwung River in Jakarta, which has around 350,000 inhabitants. m. Thus, floodplain zoning is only effective if it is applied to new developments, if government enforcements are strong, and if the people cooperating with the government. Also, the effectiveness of the floodplain zoning depends on the accuracy of the recurrence interval data.
Thus, floodplain zoning cannot do without prediction. 3. Building of physical barriers like Levees (hard-engineering method) n. 2011 Mississippi flood vs 1993 Mississippi-Missouri flood – Severity of hazard o. However, though floodplain zoning is the best solution environmentally, engineering structures are still need to protect existing development in highly urbanized areas. Structures may include physical barriers like levees. Levees artificially raise the height of the banks, so as to increase the channel depth of the river.
This allows for greater discharges to be accommodated. Levees can also be used to restrict and divert water to low-value land on the floodplain. p. For example, during the 2011 Mississippi flood, the US authorities deliberately flooded certain rural lands upstream of New Orleans to prevent large damages to the city. The Morganza spillway was built to allow its floodwaters to escape onto rural land with less people and property value, such as the town of Butte La Rose. 25,000 people from the rural towns were evacuated. Also, their homes were estroyed by the floodwaters, and many were advised to pack ‘for a long time’ as their homes were expected to be badly affected in the long-term. However, authorities argued that by doing so, more lives can be saved in the more densely populated urban areas. q. However, the effectiveness of these structures depends heavily on the severity of the hazard. If the discharge of the river exceeds of a certain level, the defences could be over-topped, leading to catastrophic effects. For example, in the 1993 Missisippi-Missouri floods, more than 70% of the levees suffered damage and were breached in the upper Mississippi-Missouri.
At least 50 people died, 72,000 homes flooded, 50,000 people evacuated and crop losses at $2. 6 billion. One of the causes of the failure of the levees was because the magnitude of the flood was a 1:500 year flood, and the levees couldn’t cope with the large discharge, and thus many overtopped. r. Thus, structural measures such as the building of levees are effective strategies of minimizing the flood, only if the magnitude of the flood is of a small one. Response 1. Evacuation a. 1993 Mississippi-Missouri Floods b.
Also with the hazard map produced through the recurrence interval data, evacuation routes can be planned out for the people so that they will know what to do and where to go when a flood event occurs. For example, with the help of a hazard map, 50,000 people were evacuated during the 1993 Mississippi-Missouri Floods. However, evacuation can only be successful if the hazard map is accurate, and if the people cooperate with the government, as many might not evacuate even in the presence of the flood event, especially in LDCs.
Thus the level of cooperation between the authorities and the people must be there, and there must be accurate prediction to produce an accurate hazard map so that evacuation routes can be accurately planned. In conclusion, the flood hazard can be managed if a multi-pronged approach is adopted, ie. Prediction, mitigation and response. Greater and continuing research needs to be undertaken to study flood recurrence intervals and to produce effective hazard maps. Planners then need to respond accordingly, either by restricting development on flood-prone areas or by ensuring that adequate measures are taken to mitigate the impacts of he hazard. However, the effectiveness of these strategies depend greatly on the level of development of the countries and level of cooperation of the authorities and the people. The DCs generally have a higher rate of success of managing the hazard due to the availability of financial and technical resources. However, when the event is unexpected and of high-magnitude, it may not be easy to minimize the impacts even if all the measures are in place. All in all, prediction needs to be supplemented by other strategies like mitigation measures to further minimize the effects of earthquakes.