- Flooding has recently caused misery to thousands and will affect human settlements for the foreseeable future. Flood risks present significant constraints on existing and new developments. In extreme cases, planning blight can prevent much needed improvements to facilities in existing settlements leading to urban decay.
- New developments on infill sites or contiguous to existing settlements may be aborted causing increased pressure on the green belt. Planning gain contributions from developers which could finance much needed improvements to existing infrastructure may be lost to that community.
- Increasing pressure for development means that all types of marginal land, including those subject to flooding are being brought forward as potential development sites. Flood risks are a material factor in planning decisions and government policy is addressed in a series of guidance documents published over the last few years. Details of practice and implementation have varied across the nation, but are usually based on regional maps indicating perceived levels of risk published by the Environment Agency based on existing conditions at the time of compiling the maps. These are accompanied by guidance tables listing types of development appropriate for each risk level.
- No policy document can foresee the complexities of every development problem and it has been recognised recently that policy and its implementation requires a degree of flexibility so that appropriate and informed development decisions can be taken to achieve policy targets. In 2006, the Welsh Environment Minister issued a clarification letter which indicated that objections from regulators should be based on future flood risks at the site under consideration rather than simplistic interpretations of policy.
- Such changes help open the way to securing permission to develop in higher risk zones by providing appropriate mitigation measures. In our experience there are several key factors to consider at an early stage. Some are listed below.
- Many flood prone areas are seen as helping to mitigate flood risks by providing temporary storage of flood waters. In fact they often only have sufficient capacity to store a few minutes of peak river flow from a design flood event which may last for hours or days. The contribution of these areas is in providing conveyance for the flood waters to pass downstream, not storage. Developing such areas can be feasible provided the conveyance is replaced by other means such as river works or relief channels / culverts.
- Land raising is the gold standard of flood protection because it takes the protected area out of the flood plain. It is expensive. However, land raising need not extend across the whole of the development area. The protected lower hinterland can be considered for development provided the raised land between it and the source of flooding is erosion resistant and has a large aspect ratio (width / height) such that a breach failure is inconceivable. It is important not to rely on traditional flood defences, rather the design should aim to ‘fine tune’ the geometry of the river channel and valley bottom so that flood flows can pass safely without affecting the residential areas or impacting on other reaches of the watercourse.
- Simplistic interpretations of Flood Risk concentrate on depth and velocity of flood waters. For a realistic interpretation it is important to include quantitative assessments of post development betterment or detriment arising from changes in Rate of Rise, Speed of Inundation and Flood Escape Routes. The tables shows a simple scoring system.
|Rate of Rise & Speed of Inundation||0.5||Hydrographs show similar time/flood profile for pre & post with around thirty minutes delay until inundation.|
|Flood Hazard Rating||-0.5||Pre = (1.1+0.5)*1.4=2.24. Post = (1.1+0.5)*1.8=2.88.|
|Flood Escape Routes||0.5||Post development will provide a nearby (<10m) dry escape route where they are absent pre-development.|
|Total (Post Development)||0.5||Slight Betterment Indicated.|
The Flood Hazard Rating is a standard DEFRA/EA derived ratio that combines flood depth with flood velocity to establish an indication of risk. Our assessments extended this concept to acknowledge benefits of other key factors during a time of flood, and also to demonstrate that slight changes in depth did not always change the real-world impact. This approach was very successful in communicating actual risks to stakeholders, which included non-professionals, and was acknowledged by the EA as a useful contribution to Flood Risk Assessment.
The design sheet below is an example of Flood Resistant Design for the proposed Merthyr Vale Renewal area.