The fight continues. The Inspector has asked for the Hydraulic Modelling of the entire drainage system to be approved by the Local Planning Authority.

It is the view of ERVPS that the drainage system being proposed for Watermill Bridge is inadequate and at some point will be overwhelmed by runoff from the developed site. At the appeal we pointed out the flaws in the hydraulic modelling used by the Developer’s consultants to access the proposed drainage system and the adequacy of the proposed attenuation basins.

Best practice in parts of Britain, i.e. East Anglia and the North West, dictate that an attenuation basin should drain within one to two days of a storm event, so that the full attenuation volume is available for the next storm event. Our view is that the proposed attenuation basins will not drain in two days after every storm event because of the topographic inadequacies of the Watermill Bridge site.

At the appeal hearing the Developer’s expert hydrologist told the Inspector that their hydraulic modelling conforms to British Standards and that it was standard industry practice to assume that the attenuation basin would be empty at the start of any storm event and anyway it is too difficult to predict the probability and size of a second storm event, so they don’t consider more than one storm event.

To quote the Inspector “At the Inquiry, the appellant explained that the modelling of the attenuation basins assessed rainfall events ranging from 15 minutes to 24 hours. The basins have been designed such that they would be 50% empty after 24 hours and fully empty 48 hours after a 1 in 100-year rainfall event. ERVPS criticised some of the assumptions contained within the modelling, such as the assumption that the basins would be empty at the start of the modelled event. However, I accept the appellant’s evidence that the modelling has used an approach which is standard for such assessments within the UK and consistent with national policy”.

Because the whole Common Farm site is low lying relative to the River Enborne and the Developer is relying on gravity to take the runoff away from the houses and roads the proposed attenuation basins have to be as low as possible in the landscape. This means that the outlets from the attenuation basins must be within the flood range of the river. In fact, the outlet from the proposed attenuation basin-1 is very low in the flood range of the river, which means that relatively standard floods will overwhelm the outlet valve at the base of the attenuation basin.

When the flood height of the river is equal to the height of the water stored in the attenuation basin the outlet valve will not function. The water level in the attenuation basin will follow the receding river flood height and if the river does not recede to low flow levels within two days there is no chance the attenuation basin will be empty within one to two days. We can see no evidence that the impact of flood height on the efficiency of the outlet valve has been taken into account in the modelling provided by the developer and his consultants.

The Inspector has asked for the Hydraulic Modelling of the entire drainage system to be approved by the Local Planning Authority. I quote from his decision document. “No development shall commence until a detailed surface water drainage scheme for the site, based on the principles within the Flood Risk Assessment (Ref: HLEF76836 Version 2 dated 20 October 2021 by RPS group) has been submitted to and approved in writing by the Local Planning Authority.

The scheme shall include:  C) detailed hydraulic calculations for all rainfall events, including those listed below. The hydraulic calculations shall consider the connectivity of the entire drainage system, including the discharge location. The results should include design and simulation criteria, network design and results tables, manholes schedule, tables and summary of critical results by maximum levels during the 1 in 1, 1 in 30 and 1 in 100 (plus an allowance for climate change) rainfall events. The drainage features shall have the same datum as the submitted drainage layout.

In our view, the entire drainage system must include the outlet valve and the outflow pipe from the valve to the river. Any hydraulic calculations that do not consider the impact, over a realistic time period, of high river levels on the efficiency of the outlet valve are worthless. It is the responsibility of Basingstoke and Deane Borough Council Planning Authority to ensure that the Developer can demonstrate that the attenuation basins will drain completely within two days even following a 1 in a 100-year storm (plus 40% for climate change). If this cannot be demonstrated using a realistic simulation then written permission should not be given for development to begin. Surely, it is not national policy to approve a drainage system based on an unrealistic modelling exercise.

All documents submitted by the developer and their consultants should be available for public scrutiny.

We ask you to write to Basingstoke and Deane Borough Council Planning to remind them that it is their responsibility to ensure that the Sustainable Drainage System is in fact sustainable using a realistic model of the topography, flood regime and its impact on the functioning of the proposed infrastructure. Please quote the following references:

Planning Application – 21/03394/OUT

Appeal Decision – APP/H1705/W/23/3326191