Flood Prevention and Mitigation

Background

Flood disaster is one of the most serious natural disasters in the world. Floods are often distributed in densely populated areas with high agricultural reclamation, concentrated rivers and lakes and abundant rainfall, such as warm temperate zone and subtropical zone in the northern hemisphere. China and Bangladesh are the areas where floods occur most frequently in the world. Flood disasters are also serious in the United States, Japan, India and Europe.

Globally, floods cause enormous economic damage and loss of life every year, in the last century floods killed about 100 000 humans (Jonkman, 2005).

During flood events, local authorities, civilians and armies often work together to place tens of thousands of sandbags, attempting to prevent large breaches in flood defences. These events show that emergency measures, such as sandbags and big bags, are often used during threatening floods to reinforce flood defences or protect critical infrastructure from flooding. Despite these attempts, dike breaches often occur, causing large floods in the otherwise protected areas.

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Types of Flooding Problems

Urban Flooding

Flooding because of poor drainage of local stormwater runoff is often a greater problem.Extremely rapid urbanization associated with economic development and migration from rural areas has seen populations in some cities triple or even quadruple within a decade, making it difficult for city planners to provide adequate infrastructure.

Newcomers are often unfamiliar with past flooding history. Because they are unaware of the flood risk or simply have no choice, they have settled on flood-prone land. Cities have expanded into surrounding farmland where standards of flood protection are lower.

Drainage systems in many city centers have become antiquated, while the value of assets and property at risk has escalated. Zoning land and regulating land use are done poorly or not at all, often due to lack of awareness of flood risk and reluctance of local governments to inhibit development and forgo revenues.

Urban flooding in Australia

Flash Flooding

Flash flooding, typically occurring in remote or mountainous areas, causes 70% of loss of life due to flood disasters. Landslides, mud flows, and debris flows caused by the sudden onset of intense storms account for many of these casualties.

In China, under a national program over the past decade, areas at greatest risk from flash flooding have been identified and mapped, and warning systems combining technology and preparedness have been progressively implemented.

Flash flooding

Rural Flooding

Poor drainage of local stormwater runoff is a serious problem in rural as well as in urban areas, causing crop losses and property damage. The problems are often aggravated by capture of local stormwater runoff behind dikes. Dikes intercept local stormwater runoff that otherwise would drain to rivers or lakes. This often results in waterlogging behind dikes for long periods of time. Pumping is often used to relieve the waterlogging, but this remedy may be unavailable when river floodways are near capacity during floods.

The occupation and use of flood detention areas creates a dilemma for flood management when floods occur, increasing damages and requiring well-organized flood warning and evacuation procedures to avoid casualties in these areas.

Aggravated waterlogging also occurs behind embankments in polder areas that are developed, often informally, in floodplains and flood detention areas. Polder embankments obstruct natural drainage paths, which may lead to higher waterlogging levels or longer durations of waterlogging behind the embankments. At the same time, polders interfere with natural flood storage in floodplains and regulated flood storage in flood detention areas.

Rural Flooding

Coastal Flooding

Coastal flooding results from typhoons or other storms occurring in conjunction with high tides. On average, about four coastal floods occur annually in the PRC, mostly in July to September, and particularly in Fujian, Zhejiang, and Guangdong provinces.

Sea walls have been constructed in many coast al areas, but standards of construction vary and all are prone to damage. In 2004, Typhoon Yunna caused storm tides of 1-3 meters above normal high tide level along the Zhejiang coast, breaching sea walls in more than 1,200 locations; causing the evacuation of over 467,000 people; and damaging 200 docks, over 3,000 fishing boats, and 42,300 hectares of aquaculture development.

Losses caused by storm tides are expected to increase due to global warming and associated rising sea levels. As living standards improve, coastal regions will grow and higher-value assets will be exposed to coastal flood hazards.

Coastal Flooding

Preventive measures

Flood disasters seem to occur every year in human history. Although it is inevitable for natural disasters, we can effectively prevent them through some scientific knowledge, so as to reduce human, financial and material losses.

1. People in flood-prone areas should listen to flood forecasting more during the rainy season, and understand the possible height of the water surface and the possible impact areas.
2. If you listen to the flood warning, under the condition of ensuring your own safety, you can use sand bags to build a waterproof wall outside the threshold, and then use old carpets, old cotton wool, etc. to plug the gap between doors and windows.
3.  When it is known that the flood is approaching, if time is sufficient, it should be transferred to a safe area in accordance with the scheduled route.
4. If the flood comes, people who can not be transferred in time should quickly avoid the buildings, hillsides, flood shelters and other highlands nearby, or immediately climb up trees, high walls, roofs and other high places for shelter.
5. If surrounded by floods, do not panic. We should try to get in touch with the local government flood control department as soon as possible, report our position and danger, and actively seek rescue.
6. If you accidentally wash away or fall into the water, you must keep calm. Grasp as much as possible the floating boxes, boards, wardrobes and any other things that can float into the water. And learn to send out distress signals, such as shouting for help, shaking branches or clothing, etc. Never swim blindly, lest you run out of energy.
7. After the flood, the relevant departments should do a good job in hygiene and epidemic prevention to prevent the epidemic of malaria and cholera.

These are individual flood control measures. For city managers,there are also a lot of work to be done .

Maintenance of Flood Control Infrastructure

Systematic monitoring and maintenance programs are important in managing flood control infrastructure. Because of the extent of structural measures that have been implemented in the PRC, monitoring and maintenance are enormous tasks. There is a legacy of aging flood defense structures that require monitoring and maintenance:

· Dikes are often unreliable due to a range of factors such as geotechnical faults in embankments, unsound foundation conditions, erosion, and scour. Past monitoring and maintenance have been inadequate.

· The potential failure of a huge number of endangered reservoirs in the PRC threatens lives and property downstream. In addition, many endangered reservoirs do not store water up to the design level and cannot control floods.

· In some places, river engineering works have fallen into disrepair.

· Flood diversion channels are becoming blocked by sediment deposition resulting from low flow velocities during non-flood periods and requiring very expensive dredging and clearing

Watershed Conservation

The deposition of sediment transported in streams is an important modifier of flood hazard, generally with adverse effects.

Soil erosion and fluvial sediment transport are natural phenomena, and deposition on river banks and floodplains can enhance fertility.

The important role of soil conservation in integrated flood management needs greater recognition, particularly during the development of master plans at the river basin scale.

Resettlement

Exposure to flood hazard can be contained at or near current levels by:

1. regulating residential development to manage population increase in hazard areas,
2. Managing the types of enterprise permitted in hazard areas,
3. integrating planning of public infrastructure. Without such regulation, danger and damages from future floods will inevitably increase unless additional structural measures are implemented to modify the flood hazard.

Land use regulations:

• · type of buildings that are appropriate;
• · land use that is appropriate, which may exclude residential use or be limited to uses such as recreation and agriculture;
• · siting of public services such as schools, hospitals, emergency services, and flood refuges;
• · storage of hazardous chemicals and materials;
• · routing and/or siting of key infrastructure such as electricity substations, water supply, water treatment, and sewerage facilities.

Method for the reliability analysis of emergency measures

Flooding will occur if the flood defence system fails forming a breach in the flood defence. This will happen when the emergency measure fails and the flood defence fails or when the flood defence fails in spite of a correct functioning emergency measure (bottom branch in Figure 1). Thus, even when emergency measures are successfully applied, the flood defence can still fail; for example, when a dike covered with sandbags overflows.

The failure probability of a flood defence system while taking into account implementation of emergency measures is determined in four steps:

1. Determine the failure probability of the emergency measures [P(E)] (Reliability of emergency measures section);
2. Determine the failure of the flood defence system without emergency measures, which is equal to the failure probability of the flood defence system given failure of emergency measures [P(D|E)] (Failure of the flood defence given successful implementation of emergency measures section);
3. Determine the failure probability of the flood defence system given successful implementation of emergency measures [P(D|Ec)] (Failure of the flood defence given successful implementation of emergency measures section);
4. Determine the combined failure probability of the flood defence system taking into account implementation of emergency measures?by solving Eqn (2) (Combining the failure probability of the flood defence and the emergency measure section).

The method is based on the Dutch situation, where flood defences have been part of the countries' critical infrastructure for centuries. Different countries may have different protocols for emergency measures, leading to different reliabilities of the flood defence system. Even though these differences may lead to different reliabilities, the method developed in this paper can be applied in other flood-prone areas protected by flood defence systems, to determine the combined reliability of the system?

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