Urban flood Hazard level: Very low ?

In the area you have selected (San Diego) urban flood hazard is classified as very low based on modeled flood information currently available to this tool. This means that there is a chance of less than 1% that potentially damaging and life-threatening river floods occur in the coming 10 years (return period of c. 1 in 1000 years). Therefore, based on this information, flood hazard does not need to be explicitly considered for your project. Although the hazard is considered to be very low or non-existent in the project location based on the information available in ThinkHazard!, additional information may show some level of hazard. If local or additional information sources suggest that there is flood hazard, follow the recommendations below and seek expert guidance on additional recommended actions. It is recommended that local flood regulations and conditions possibly leading to highly localized water nuisance problems are considered. In particular, it is recommended to check the condition of and possible flaws in local water management systems, e.g. poorly dimensioned or maintained sewerage or drainage channels. Always consider taking no-regret measures.

Climate change impacts: In western, central and eastern continental US, medium confidence in an increase in the number of extreme rainfall events. In northeast US, medium confidence in an increase in days of intense precipitation. In southeastern US, model projections are inconsistent in changes in rainfall. In Alaska, high confidence in an increase in days of intense precipitation and the number of extreme rainfall events. The present hazard level may increase in the future due to the effects of climate change. It would be prudent to design projects in this area to be robust to river flood hazard in the long-term.

Recommendations

• EXISTING INFORMATION: Obtain pre-existing flood hazard information. The high-level information available in this tool may indicate the presence of flood hazard in your project area. However, because flood hazard can change dramatically over short distances, the exact geographical location of your project should be checked against pre-existing flood hazard information. More information
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  o Consult local flood zoning information (if readily available). o Consult local open access flood hazard maps (if readily available).

  Acquisition of available flood hazard data should be undertaken for all sites and all hazard levels. Information about flood hazard can take multiple forms, including spatial maps of flood hazard, flood zoning information, reporting of previous events and local knowledge.

  Local flood zoning information, typically from government planning departments, can indicate the likely flood hazard for specific locations. Local scale flood hazard maps can estimate the likelihood of flooding at a given location. Flood hazard maps may be available from government agencies or through local consultants and may be available at a variety of scales and with variable data quality. Additionally, urban planning authorities may indicate that some zones have critical drainage problems. Overwhelmed sewer and drainage systems can significantly contribute to the flood hazard in an area, so information regarding drainage issues should also be sought where available.

  Most government-based flood hazard information will pertain to river floods, but some countries also produce surface water maps (flood hazard from intense rainfall). High-density developed areas are more prone to flooding from direct rainfall onto the land surface, since concrete surfaces force water to flow over the ground rather than infiltrate into it. Surface water flood maps should be acquired for such areas wherever possible.

  In addition to local/country level flood hazard maps, regional and global flood hazard maps have emerged in recent years (e.g. SSBN, Aqueduct). These indicate the spatial extent of flood hazard over large scales but have important limitations. Flood models used to generate hazard data are subject to significant uncertainty and their output should be used tentatively. Flood hazard maps produced over large scales highlight where there is predominant flood hazard. However, owing to their coarse resolution, national or regional food hazard maps should not be used to provide information at local (building) scales and certainly not to inform detailed planning and engineering design. For this, localized flood modeling is required, with better topography and river channel data.

  Observations of previous flood events can define flood hazard. Monitoring of flood events is becoming more routine, and archives of previous events now exist (Dartmouth Flood Observatory [http://floodobservatory.colorado.edu/], Disaster Charter [https://www.disasterscharter.org/web/guest/home], and Copernicus [http://www.copernicus.eu/]). The identification of previous events at a given location will clearly indicate the presence of a significant hazard. This can be used to refine the hazard levels provided by the ThinkHazard! tool. However, a location may have no events recorded in these archives; this does not mean there is no hazard – it simply means that an event has not yet been recorded officially.

  Documentation of previous events, through local news reports, can also provide a useful insight into flood risk. Organizations such as Reliefweb (reliefweb.int) and FloodList (floodlist.com) provide information about previous events and their impacts. The local government may hold written records or observations into previous flood events. Likewise, residents near the project location may have a good understanding of the local flood behavior, particularly if they have resided there for a significant period. These sources of local knowledge should be used to further inform the likely flood hazard level at a given location.

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• INTERDEPENDENCY: Consider vulnerability of other assets within the project's dependency network. If your project is interdependent with other projects, it is important to assess the vulnerability of the entire network if the service provided is critical. More information
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  In addition to the specific location of a project or development, it is also necessary to consider other locations that, should they be impacted by a flood, would adversely affect your project or development. The consideration of other locations applies particularly to crucial infrastructure that is required to remain operational during a flood event. It is also important to ensure that escape routes remain available at all times during flood events.

  o If your project provides a critical service and is reliant on external services (e.g. power supply, computer networking and communications), the vulnerability of those external services to flood should also be considered. Externally provided services may be based in a different physical location and therefore be subject to a different level of flood hazard. o Access to the project or development may become restricted during a flood event. Ensure that escape routes are available at all times.

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• LOCATION ASSESSMENT: The high-level information available in this tool may indicate the presence of urban flood hazard in your project area. Before committing significant resources to this issue, a study of the immediate environment can help you to assess whether more detailed assessment and/or intervention should be considered. More information
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  Flooding in urban areas is a very local phenomenon, so the hazard information provided by this tool should be considered the preliminary action in defining a flood hazard level. Flood hazard can vary dramatically over short distances, depending on local topography, drainage capacity and distance to waterways.

  A local assessment should be undertaken to identify any obvious sources of flooding (rivers/streams). A lack of rivers or streams nearby does not rule out the possibility of flooding resulting from direct rainfall onto the land surface. Identify whether any structural flood defenses are in place: particularly levees (large structures preventing rivers overtopping their banks) and storm drains (systems designed to remove large volumes of water from the land surface in an area of interest). Location assessment can provide a greater understanding of the likely flood hazard, through the interpretation of both natural and human aspects of the local environment.

  o Be aware that the hazard level provided by this tool is a first estimate of urban flood hazard. Further clarification of the local flood hazard will be required. o Assess the local topography (i.e. how flat or steep your area is). Flooding is most likely to occur in relatively flat areas beside rivers (‘floodplains’), in local low-points in the land surface (‘depressions’), or along valley bottoms. If your project is situated at the base of a hill or on flat terrain it is more likely to be at risk than if it is situated on the top or side of a hill. o Check for streams and rivers near to your project area. You can use tools such as published maps, Google Earth, or open source GIS software such as QGIS, to identify waterways. Do not dismiss small streams – these can swell rapidly and dramatically in flood conditions and cause serious local flooding. In some areas, rivers may not run all year around; such ‘ephemeral’ rivers are still capable of causing significant flooding at certain times (this type of river is most common in arid and semi-arid regions).
  o Flash-flooding may arise from proximity to small streams and/or from intense rainfall directly onto the land surface. If excess rainfall cannot infiltrate the land surface it will flow over it, generating flood hazard. This phenomenon is often prevalent in high-density developed areas, where impervious surfaces (e.g. concrete) force water to flow overland. o If your location of interest is in a high-density developed area (e.g. in a city), assess the existence and quality of local flood defense structures. The presence of a levee, how often it is maintained, and the size of the flood it defends against, will indicate the level of flood risk at a location of interest. Storm drains should be similarly investigated, especially ensuring there are no blockages in the drains. This information may be provided by local or national governments. o Identify debris from previous floods. The presence of large objects, such as boulders, near to even small waterways may indicate the presence of powerful flood waves. Such behavior is particularly prevalent in small, steep tropical catchments.

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• FLOOD HAZARD MANAGEMENT: Your project or development should consider flood management measures, such as catchment management and implementation of flood defenses. More information
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  o Identify whether flood management procedures are in place in the local region, and how your development will integrate within the existing flood management framework.

  Flood management includes all measures that may alleviate flood risk. This can range from management of upstream catchment areas through to the implementation of flood defenses. Flood management is likely more applicable to projects in low-density developed areas (e.g. rural areas or the outskirts of towns and cities), as flood management in more built-up urban areas is often under the control of local and national authorities. Management of upstream catchment areas is sometimes defined as being ‘Green’ protective measures, as they often involve the restoration of natural wetlands or the removal of impermeable surfaces. Some examples of Green protective measures include: -Measures to reduce runoff: replacing impermeable surfaces (e.g. concrete) with natural landscapes and afforestation are measures that will reduce the rate at which rainfall moves across the surface of a catchment and into a river. -Storage of runoff: wetlands and reservoirs can store excess water during extreme flows, reducing the magnitude of the flood event. Often the restoration of natural wetland areas will alleviate flood risk, whilst also providing additional benefits to local eco-systems. The implementation of flood defenses is sometimes referred to as ‘Grey’ protective measures, as they involve the implementation of built structures to protect areas or transport water away as quickly as possible. Examples of Grey measures include: - Capacity enhancement of rivers: bypass channels and channel deepening/widening are measures that can be used to increase the amount of water that can pass through a river channel. - River flood defenses: levees can be built to separate the hazard source (river) from the receptor (e.g. a building). - Flash-flood defenses: storm drains can be constructed to enable the rapid removal of excess surface water from an area of interest. Permeable concrete could be used as part of a sustainable urban drainage system (SUDS).

  The flood management options available will depend upon the size of the development, the resources available and the willingness/ability of other land users in the catchment area to participate in a flood management scheme. In the case of large developments, encompassing significant proportions of a river catchment, a number of flood management procedures will be available, including measures to reduce and store floodwaters. For small-scale developments, the implementation of upstream, flood alleviation measures may not be possible. Instead, the implementation of flood defenses, aiming to protect the structure or development from a floodplain, or the development of flood resilient structures, may represent the available options. Sustainable Urban Drainage Systems (SUDS) are designed to reduce the impact of new urban developments on surface water drainage. These systems take a holistic view of storm water management and include measures to increase infiltration (such as permeable concrete), detain and store flood water (such as surface wildlife ponds or subsurface storage tanks) and increase evapotranspiration (‘green’ roofs). Further information on SUDS can be found at http://www.susdrain.org/. Regardless of the measures implemented, it is important when implementing flood management measures that they are not treated in isolation; their impact on other areas may be significant.

  The World Meteorological Organization (WMO) identified that attempts to control flooding via traditional Grey methods in single locations or developments have had little success in reducing flood losses and damages. Instead, the WMO has outlined what it calls Integrated Flood Management. Integrated Flood Management moves away from measures of flood control, such as engineered flood defenses in single locations, and instead, promotes the management of the water cycle as a whole. This approach not only aims to alleviate flood risk in highly vulnerable areas but also aims to create opportunities from flooding by maximizing the net benefits from the use of floodplains. This may include restoring natural floodplain areas or replenishing agricultural land. Where possible, this integrated approach to flood management represents the ideal approach to alleviate flood risk. However, such approaches will likely require a framework for collaboration across many different sectors and organizations. The Associated Programme on Flood Management provides documentation, guidance and test cases outlining how to implement Integrated Flood Management strategies.

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• DO NOT INCREASE HAZARD: Built infrastructure may alter flood hazard. Constructing a significant piece of infrastructure can significantly alter the landscape and potentially influence how an area responds during a flood. Any alteration of the landscape should be undertaken with consideration as to how this will influence the local hydrology. More information
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  o Consider how your development may affect local drainage behavior.

  When building a new development, consider how it will impact local hydrology. Built infrastructure can significantly change how an area responds to rainfall and how water is transported, potentially increasing flood risk. For example, replacing large permeable areas with impermeable surfaces or structures will increase the amount of excess water that flows over the land surface rather than infiltrates into it. Sufficient drainage systems will need to be in place to transport the excess water that previously would have been absorbed by the landscape, away from the site. However, this will increase the speed at which rainfall enters local river networks, potentially magnifying flood risk in downstream areas.

  This section is less applicable to high-density developed areas, where an intense built environment will have already altered flood hazard (i.e. it is likely your development will not exacerbate this effect). If your project will contribute to urbanization of low-density developed areas, its effect on flood hazard may be more significant.

  Obstruction of waterways, for example with a poorly located bridge, can increase adjacent, as well as upstream, river flood risk. This will create the potential for flooding to occur more regularly and to a greater magnitude. Any development that encroaches upon the drainage capacity of a channel can also magnify flood risk by allowing waterborne debris to gather, further restricting flow.

  Buildings and developments also have the potential to exacerbate downstream river flood risk by reducing floodplain storage capacity. If a development removes space that previously stored excess water, the scale and/or speed of run-off will increase. These considerations also apply to flood defenses and changes in channel capacity. The implementation of river flood defenses will, by its nature, remove some amount of floodplain storage and exacerbate flood risk elsewhere. Similarly, measures to increase river channel capacity, with a view to making an area more drainage efficient, will likely increase the magnitude of flooding downstream.

  Developments can increase the likelihood of flash flooding in an area: impervious surfaces (e.g. concrete) force excess water to flow over land rather than infiltrate into it. Extreme rainfall is, therefore, more likely to translate into surface water flooding than if the land surface was kept permeable. The installation and maintenance of urban drainage systems (e.g., storm drains), can effectively remove water from the surface; however, by decreasing the time it takes for rain water to reach the river channel, such systems can lead to an increase in river flood risk downstream. Where drainage systems are poorly maintained, blockages can occur and lead to increased surface flooding. It is therefore essential that provision is made for the maintenance of any new drainage systems.

  Aside from direct human impacts, the impacts of a development on local flood behavior should also be considered from an environmental viewpoint. Floodplains are valuable ecological resources that will provide a habitat for a wide variety of plants and animals. All developments should be considered in terms of their potentially negative impact on local ecosystems, either through the removal of natural floodplain areas or through the amplification of flooding.

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