Homes England Planning Application: Topic to consider – Flooding

We all know that Chalgrove has a problem with flooding. The village is in a low-lying area with a watercourse running through its centre. The airfield sits slightly higher that the village and the lie of the land means that some of the water run-off from the airfield arrives in the village. Development of the airfield will undoubtedly hugely increase the amount of water runoff.

Paragraph 163 of the National Planning Policy Framework (NPPF) includes:

When determining any planning applications, local planning authorities should ensure that flood risk is not increased elsewhere.”

The planning application appears to recognise that development of the airfield will increase the flood risk to the existing village however it is not enough to simply deal with the risk of flooding to Chalgrove; the site itself has to be safe and areas downstream have to be protected as well. The planning application actually does not do any of this.

The Flood Risk Assessment (FRA) that accompanies the planning application for the airfield says that the risk of groundwater flooding on the airfield site is medium and that they have mitigation planned that will deal with this. The drainage solution proposed is the widespread use of sustainable drainage solutions (SuDS).

SuDS have two purposes: 1) trap surface runoff from impervious surfaces and release it more slowly (at most at the Greenfield runoff rate); and 2) remove pollutants to ensure it is clean when released (again as it would be when Greenfield).

The FRA does not present a true picture of flood risk and the methods suggested in the application to deal with flood risk will be largely ineffective and likely to cause harm to the surrounding local watercourses.

The points that have been found are:

The FRA reports that the risk of flooding from ground water is medium, however, the British Geological Survey (BGS) report states that there is a high risk of flooding from groundwater on this site.

The BGS data and mapping shows that groundwater is very close to the surface (within 0.5m) across 83% of the site.  (This is supported with local knowledge reporting how the water sits on the surface of the airfield, particularly across the southwest corner near the bypass). The planning application does not reflect this fact!

The applicant’s plan is to use SuDS to deal with surface water drainage, which should be kept separate from groundwater. Their scheme, however, proposes to flush polluted water directly in to a groundwater wetland.

Chalgrove Brook and Haseley Brook are of high importance for water quality and it is extremely important that polluted water is not discharged into these local watercourses.

Other types of SuDS proposed in areas of high ground water are to be separated from groundwater by a membrane. This is known to be a common failure point. It is inevitable that the proposed swales will leak polluted water to the groundwater. Tanks used to store floodwater are not going to have enough storage for the purpose and there is a risk of these being unweighted and becoming unstable due to the level of groundwater being underestimated.

The SuDS should be designed to discharge water, at most, at the rate of Greenfield run off rates. (most of the airfield is Greenfield). The FRA is based on a custom value of median annual maximum flood, rather than the standard value (with no evidence to say why). Our calculation, using the standard method, shows that runoff is only 44% of that given by the applicants in their FRA. This means that with slower runoff, more storage of floodwater is required. In this case 227% more. Also further runoff storage is required for drainage from landscaped areas. The FRA has ignored this source of runoff.

The FRA has made calculations for the amount of pollutants in the runoff but it only accounts for runoff from roofs. It should be noted that runoff from cul-de-sac type roads are substantially more heavily polluted and the pollution load is higher still from retail area, main roads and commercial areas.

We have some basic issues with the Flood Risk Assessment and our conclusions include:

a) The SuDS types proposed for the site do not achieve the water quality standards necessary to avoid discharging polluted water to our local watercourses.

b) The site presents an increased flood risk to Chalgrove and Stadhampton; it relies heavily on permeable paving which is prone to block and which needs regular repair and maintenance. The flood risk is likely to increase in the future as the climate changes and residents add paving to their gardens and extensions to their houses.

c) The microdrainage modelling that supports the FRA uses some non-standard parameter values including the rainfall and the percentage runoff from impervious surfaces.

d) The plans as submitted would cause flooding to the new properties and to Chalgrove and Stadhampton; they would pollute local watercourses; and they would pollute groundwater.