River flood Hazard level: Very low ?

In the area you have selected (Hong Kong, SAR) river 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. Surface flood hazard in urban and rural areas is not included in this hazard classification, and may also be possible in this location. Please see 'Urban Flood' for consideration of urban surface and river flooding.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: Medium confidence in an increase in heavy precipitation. 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

• LOCATION ASSESSMENT: The high-level information available in this tool may indicate the presence of river flood hazard in your project area. Before committing significant resources to this issue, a study of the surrounding landscape can help you assess whether more detailed assessment and/or intervention should be considered. More information
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  Since flooding is a very local phenomenon, the hazard information provided by this tool should be considered the preliminary action in defining river flood hazard level. Flood hazard can vary dramatically over short distances, depending on factors such as local topography and distance to waterways. To further define potential flood risk, a local assessment should be undertaken to identify whether any sources of flooding exist (rivers/streams). If they do, it would then be necessary to identify whether the location of interest is situated in an area that may become flooded, usually by identifying whether the area is located within a natural floodplain (see Guidelines for Environment Risk Assessment and Management). The objective of the location assessment is to gain a greater understanding of the likely flood hazard at your location, through the interpretation of the local landscape.

  • Be aware that the hazard level provided by the tool should be considered a first estimate of flood hazard. Further clarification of the local river flood hazard will be required.
  • Consider the position of your project in the landscape by assessing 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 on the top or side of a hill then you are unlikely to be directly at risk from river flooding; if your project is situated at the base of a hill or on flatter terrain then you are more likely to be at risk. Note that flood risk may not always originate from the closest point on the river; you may also be at risk from flood waters that overflow from the channel upstream before flowing downhill over the floodplain.
  • Check whether there are any streams or rivers in your project area. You can use tools such as published maps, Google Earth, or open source GIS software such as QGIS, to identify nearby waterways. Do not dismiss small streams as 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).
  • Flash-flooding is unlikely to be represented in this tool, be aware that any small streams may pose a risk that is unaccounted for.
  • If any such streams or rivers are present, and your project is not obviously protected by its position in the landscape, assume that the hazard level for your location is higher than ‘Very Low’.
  • Identify debris that may have been deposited during previous flooding. The presence of large objects, such as boulders, in the vicinity of even seemingly small waterways may indicate the presence of powerful flood waves. Such behaviour is particularly prevalent in small, steep tropical catchments.

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• EXISTING 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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  Acquisition of the available flood hazard data should be undertaken for all sites and all hazard levels. Information about flood hazard can take multiple forms, this includes 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 provide a good indication of the likely flood hazard for specific locations. Flood hazard maps provide an estimate of 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. Depending on the quality of the data, local scale flood hazard maps can provide a reliable understanding of the flood risk at a given location.

  In addition to local/country level flood hazard maps, large-scale flood hazard maps have emerged in recent years (see GAR15, Aqueduct [used in this tool], SSBN). These data are available for most locations and may provide further insight into the spatial extent of flood hazard over large scales. The limitations of these data, particularly those produced over large scales, cannot be overstated. Flood hazard data are typically derived from the output of computer models. These flood models themselves are subject to significant uncertainty and therefore their output should be used tentatively. Flood hazard maps produced over large scales may highlight where there is a predominant flood hazard and will indicate the need to do more localised flood modelling with better topography and river channel data in these areas for detailed planning and engineering design. 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 engineering design.

  Observations of previous flood events can also be used to define flood hazard. Monitoring of flood events is becoming more routine, with a number of centres building archives of previous events (see 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 occurring 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 ‘Think Hazard!’ tool. However, the non-existence of an event in these archives should not be used to infer the non-existence of a 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 previous events, and help to define flood risk at a more local scale. Organisations such as Reliefweb (reliefweb.int) also represent a useful resource, providing information about previous events and their consequences. In addition to flood hazard modelling, mapping and documentation, local knowledge can provide a useful source of additional information that can be used to identify flood hazard. Local government officials may provide written records, observations or insight into previous flood events that have occurred. Likewise, local residents may have a good understanding of the local flood behaviour, particularly if they have resided near the site location over a significant period of time. These sources of local knowledge should be used to further inform the likely flood hazard level at a given location.

  • Consult local flood zoning information (if readily available).
  • Consult local open access flood hazard maps (if readily available).

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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 during 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. Therefore transport links, not located in the vicinity of the project or development may need to be considered.

  • If your project provides a critical service and is reliant on external services (e.g. power supply, computer networking and communications) then the vulnerability of these 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.
  • 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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• FLOOD HAZARD MANAGEMENT: Your project or development should consider flood management measures, such as restoration of natural wetlands, removal of impermeable surfaces, or implementation of flood defences at the project site and in the upstream catchment. More information
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  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 defences. Management of upstream catchment areas are 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 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 defences are 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.

  • Flood defences: Flood defences can be implemented to separate the hazard source (river) from the receptor (e.g. a building).

  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 rainfall-runoff. For small-scale developments, the implementation of upstream, flood alleviation measures may not be possible. Instead the implementation of flood defences, aiming to protect the structure or development from a floodplain, or the development of flood resilient structures, may represent the available options. 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 (See WMO- Integrated Flood Management). Integrated Flood Management moves away from measures of flood control, such as engineered flood defences 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 maximising the net benefits from the use of floodplains. This may include resorting natural floodplain areas or replenishing agricultural land. Where possible, this integrated approach to flood management represents the most ideal approach to alleviate flood risk. However, such approaches will likely require a framework for collaboration across many different sectors and organisations. The Associated Programme on Flood Management (APFM) provides documentation, guidance and test cases outlining how to implement Integrated Flood Management strategies.

  • Identify whether flood management procedures are in place in the local region, and how your development will integrate within the existing flood management framework.

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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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  It is important when building a new development to consider how it will impact upon local hydrology. Built infrastructure can significantly change how an area responds to rainfall and how any area transports water, potentially increasing flood risk. For example, replacing large permeable areas with impermeable surfaces or structures will increase rainfall-runoff. 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.

  Obstruction of water ways, for example by the development of a poorly located bridge, can also increase adjacent flood risk, as well us upstream. 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 water borne debris to gather, further restricting flow.

  Buildings and developments also have the potential to exacerbate downstream flood risk by reducing floodplain storage capacity. If a development removes space that previously stored excess water, then the scale and/or speed of run-off will increase. These considerations also apply to flood defences and channel conveyance measures. The implementation of flood defences 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.

  Aside from direct human impacts, the impacts of a development on local flood behaviour 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.

  • Consider how your development may affect local drainage behaviour.

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