What is a Preliminary Flood Risk Assessment?
A Preliminary Flood Risk Assessment will include analysis of all potential sources of flooding at a site and their potential impact, taking into account a number of factors including, any proposed development and the use of the site. Often requested by the Local Planning Authority, a Preliminary Flood Risk Assessment will act as a scoping assessment to determine whether the risk of flooding at the site is acceptable or whether a more detailed assessment, including modelling and surveying site levels, is necessary.Contact Us
What's Included in a Preliminary Flood Risk Assessment?
Context (the first stage in the process) – The first stage of the Preliminary Flood Risk Assessment includes a detailed description of the site, existing surface cover, proximity to nearby water bodies and the flood zone that the site lies within. The proposed development will also be detailed, including the proposed footprint of any buildings, any external landscaping, and the proposed site levels. The most important part of this stage is determining the site's existing and proposed vulnerability classifications. These are central to the Preliminary Flood Risk Assessment as they will determine the scope of the assessment and help to identify the level of risk at the site.
Flood Sources – After the context and scope of the Preliminary Flood Risk Assessment has been determined, all the potential sources of flooding at the site are evaluated on a site-specific basis using information from broader Strategic Flood Risk Assessments, if available, and any other relevant databases. These sources are addressed in more detail in the section below.
Conclusions (most important stage) – The final and most important stage of the Preliminary Flood Risk Assessment is to conclude the likely risk of flooding at the site from each of the previously considered sources. This will, in turn, determine whether further assessment is needed, in the form of a Detailed Flood Risk Assessment, or if the flood risk is considered to be acceptable. It may also allow for flood mitigation measures, that would reduce the risk of flooding, to be recommended at this stage without the need for further assessment.
Why Do I Need a Preliminary Flood Risk Assessment?
Flooding can be caused by a number of factors including, but not limited to, the proximity of surface water bodies, the underlying hydrogeology at a site and the surface cover of the site and its surroundings. A Preliminary Flood Risk Assessment will evaluate all of these factors to determine the potential for flooding at a given site.
Fluvial (rivers & lakes) – Rivers and lakes are one the most important potential sources of flooding and exceptionally large areas can be affected during flood events. It is, therefore, important to carry out a Preliminary Flood Risk Assessment to determine the risk to individual sites based on the presence of any flood defences, the assumed flood risk zone that the site lies within and the estimated maximum depth of a flood event.
Pluvial (surface water) – Geology, surface cover and periods of heavy rainfall can lead to significant surface water flooding which can be exacerbated by poor drainage. These factors all need to be assessed in detail as part of a Preliminary Flood Risk Assessment in order to ascertain the potential for surface water flooding.
Groundwater – The underlying geology and hydrogeology at the site, along with the depth of the water table, are important factors in determining the risk to a site from groundwater flooding. Groundwater flooding occurs when the water table rises and reaches the surface, and this can have a lasting affect as it takes a long time for the water table to decline.
Canals & Sewers – It is rare for canals to flood as they are regularly monitored and can be managed in times of heavy rainfall. It is possible, however, for canals to flood as a result of river flooding in areas where canals meet rivers and in built up areas, such as inner-city regions, the flood risk can be very damaging. Similarly, when flooding occurs from other sources, usually from rivers, it can lead to flooding of sewers which can impact densely populated residential areas.
Effect on the wider area – As well as assessing the potential impact of flooding on the site, it is also important to determine the risk the development itself poses to surrounding sites. Development often leads to an increase in impermeable surfaces or the introduction of impermeable surfaces where none were previously present as buildings are built and areas of hardstanding are placed. This potential impact needs to be considered in a Preliminary Flood Risk Assessment and it will be necessary to show that the development will not increase the risk of flooding in the surround area.
How Do I Get a Preliminary Flood Risk Assessment?
We can carry out Preliminary Flood Risk Assessments for a range of development projects including new builds and change of use on rural and urban sites.
1. Contact us and let us know your requirements
2. Following initial consultation we can provide an outline proposal detailing how we can help and the costs involved
3. Our environmental consultants will then carry out a Preliminary Flood Risk Assessment including a screening of flood sources and determination of the flood risk at a site
4. We can turnaround a Preliminary Flood Risk Assessment within days of your initial enquiry, allowing you to stick to schedules and minimise any disruption to your project