In the absence of human settlements, flooding is a natural process. However, when human settlements and flood-affected areas overlap, flooding becomes an imminent challenge. On the one hand, the development of flood plains (also known as flood plains) will affect the dynamic process of flooding; on the other hand, periodic floods will affect buildings and infrastructure.
Schematic diagram of floodplain before and after development. The right bank in the picture above is undeveloped; after the development of the right bank in the picture below, the hidden flood hazards of the original houses on the left bank have been increased. Image source: https://www.fema.gov/sites/default/files/documents/fema-480_floodplain-management-study-guide_local-officials.pdf
Schematic diagram of floodplain before and after development. The right bank in the picture above is undeveloped; after the development of the right bank in the picture below, the hidden flood hazards of the original houses on the left bank have been increased. Image source: https://www.fema.gov/sites/default/files/documents/fema-480_floodplain-management-study-guide_local-officials.pdf
Effective floodplain management requires identifying and understanding which areas are at risk of flooding, making plans and zoning them to prevent damage to personnel and infrastructure. Most communities in the United States rely on the National Flood Insurance Program (NFIP, National Flood Insurance Program) Flood Insurance Rate Map ( FIRM ) to determine the flood risk range in the community.
Although FIRM can be used as a baseline, it also has limitations. The Naturally Resilient Communities (a partner of the American Planning Association, the American Society of Civil Engineering and other institutions) has proposed more than 50 solutions and cases to improve the resilience of communities to floods. One of the solutions is to draw enhanced floodplain mapping ( enhanced floodplain mapping ).
Spillway is the area through which water flows within the floodplain area. The current federal standards that the United States complies with in drawing spillways allow new development projects to invade natural spillways, but should ensure that the resulting increase in flood height does not exceed one foot (about 0.3 meters). If this method is used to draw a floodway map, almost half of the natural floodway will be filled. Such encroachment not only leads to an increase in the height of the flood, but also speeds up the flow of the flood.
In order to ensure that the current residents of the community do not suffer more serious floods due to new development projects, the occupation of floodplains should be restricted to avoid aggravating flood risks. The entire range of the floodplain can be regarded as a spillway, or a part of it can be drawn as a spillway, and no occupancy of the natural spillway is allowed. This concept is called drawing a complete spillway.
Mapping the residual risk
FIRM does not consider floods that occur when river embankments, dams, and sea embankments are destroyed or submerged. Without this information, large-scale densely populated areas may suffer rapid and severe floods, and there is almost no warning for the destruction of the above-mentioned structures. It is difficult to imagine how much impact these flood infrastructures will have if they fail. Therefore, it is necessary to predict it, and map the area protected by dams and other structures to help emergency response and help community residents realize their risks.
Map future risks and impacts
As sea levels rise and precipitation events become more extreme, and land use changes over time, the flood map of the future will be very different from today. Buildings currently constructed have a reasonable life span of 50 to 100 years; development projects that are currently believed to be protected from flooding may be damaged in the near future. In order to take into account the impact of future floods, the effects of sea level rise, coastal and river erosion, and changes in precipitation patterns should be included in the flood mapping as much as possible.
The US National Oceanic and Atmospheric Administration has mapped out predicted sea level rise for the entire US coast , helping to visualize the possible future of sea level and storm events.
The national climate assessment made an overall prediction of changes in land use over time and changes in precipitation . Where possible, local granular information should be used to more accurately predict possible future situations.
Mapping the natural system of flood protection
Ensuring the integrity of natural habitats (including wetlands, beaches, dunes, mangroves, etc.), the benefits of which are usually not reflected in floodplain mapping. Similar to the risks caused by the loss of flood protection infrastructure, when analyzing risks, the intact natural habitat provides overlooked flood protection benefits. Therefore, clarifying the location of existing natural habitats and the locations of lost natural habitats is an important step to more effectively manage the impact of potential floods.
In addition to mapping existing natural infrastructure, efforts should also be made to consider how sea level rise and precipitation patterns and other long-term climate changes will affect the health and location of such infrastructure. Several modeling tools have been developed to simulate where the natural habitat may exist in the future as the sea level rises.
As a rough introduction in this article, drawing a map covering the entire floodplain should consider the potential risks brought by the existing flood control infrastructure, and strive to prepare for the impact of future precipitation, sea level rise and land use changes, and prevent problems. Otherwise, more effective and comprehensive flood management can be carried out.
Editor in charge: Feng JingProofreading: Shi Yun
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