o Consider how your development may affect local drainage behavior.
o Consider how your development affects local drainage behavior and its implications for flood risk in downstream areas. For instance, increasing impermeable paved areas (preventing water passing through to the soil) through development can increase flood hazard by increasing the amount and speed at which water drains from your site.
o Check to see if other planned projects in your area or upstream may affect your project, for example by moving water downstream to your site more quickly thereby increasing flood hazard at your planned site.
o Consider getting guidance on how to integrate your development into local flood management practices (see the Associated Programme on Flood Management).
o Be aware of the potential impacts of your development on local ecosystems.
o Implement sustainable drainage techniques, ensuring limited impact on other areas (See Associated Programme on Flood Management and PPS 25).
o Some form of Flood Risk Assessment is recommended to gain a greater understanding of the local drainage behavior and inform sustainable drainage design.
o If applicable, consult other river users about your plans.
o Ensure that any implemented protection measures do not exacerbate flood risk in other areas.
o For larger projects, a detailed Flood Risk Assessment will be required.
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.