Flooding And Griffin Lane Development – Letter Received from Solihull MBC Planning Dept
I am aware that you have been waiting on a response to the questions you asked following the flooding event at Griffin Lane. I apologise for the delay in my reply, which in part has been due to collating information from colleagues in the Drainage team at SMBC together with the Environment Agency for reason that there has been a recent change in responsibility for management of certain watercourse transferring from the Environment Agency to the Local Authority. Responsibility to investigate flooding from an Ordinary Watercourse and surface water now rests with the Local Lead Flood Authority. However, in the case of Griffin Lane, the Environment Agency has been explicit that they will continue to liaise with the local authority drainage team for the future development of this site to ensure that any development does not increase flood risk elsewhere.
As you will recall, you asked that the following questions be responded to:
- Have SMBC contacted the environment agency to discuss recent events, do they accept that the level of flooding was ‘normal’
- Where will the water that sat on the griffin lane field (8 feet deep in places) go when the building is complete? one small balancing pond will not take this
- Given that residents have been assured that the existing balancing pond was functioning correctly isn’t the future design using exactly the same methodology that has been proven not to work? why should we expect it to work in future?
My colleague, from the SMBC drainage team responds as follows:
1. The officers of the Environment Agency have been aware of the historic flooding in 2007 and are aware of the recent flooding event. The flood level observed in November was again exceptional and is understood to be due to a blockage of the screen that protects from debris entering the culvert that passes under the canal. The screen and the existing balancing pond is the maintenance responsibility of Dickens Heath Consortium led by Redrow Homes. Council officers will be carrying out regular inspections when there is the threat of heavy rainfall and alert Redrow Homes to any particular issues. We will in the meantime work with Redrow Homes to ensure whatever precautionary measures are made available, including maintenance of the drainage outfalls from the balancing pond and locally sited sand bag provision.
2. The developer proposes a redesign of the outfall structure and screen to maintain the passage of flow and hence reduce the risk of blockage. The developer also proposes that the dwellings will be elevated above the normal flood level to safeguard against flooding. The normal flood volume will then lie within the established flood plain outside the perimeter of the housing. As the redesign of the outfall structure and screen will ease the passage of flows, the developer anticipates that the heavy rainfall runoff will continuously drain to the watercourse and discharge to the culvert. The capacity of the existing balancing pond and the new pond to be located within the Public Open Space is undergoing scrutiny at present as part of the technical approval process.
3.Based on observations during the flood by Council officers, it appears that the cause of the flood was indeed the blocked outfall structure at entrance to the culvert. Due to this, the balancing pond was overwhelmed causing a “cork in a bottle effect”. The established principles of balancing pond design employed in this case was that there is an outfall to a culvert and the balancing pond can “balance” the excess volume draining to the culvert.
To supplement the above, the Environment Agency have provided the following comments:
The EA were consulted on the original application and reviewed the FRA, ref 20133/PH/08-12/3096. We responded to the consultation and objected to the development in our letter dated 27 September 2012.
We stated the FRA, submitted by M-EC, failed to take the impacts of flooding from the combined effects of local ordinary watercourses and the Dickens Heath Surface Water system which discharges to the attenuation pond to the south of the site or the culvert that runs beneath the canal into account.
The modelling suggested the previous flood incidents were caused by the existing screen arrangement and we requested evidence of how this issue will be addressed and who would be responsible for future maintenance.
It was also suggested that the watercourse running along the southern boundary of the site would be modified and its banks relaxed to allow more storage in the channel rather than flooding onto site. We requested drawings that would show this would be feasible.
In response to our objection we received an amended FRA responding to the points raised in our letter.
The developer stated that following the installation of the new secondary channel it would direct flows to the canal culvert. The remaining channel would be utilised to a lesser extent although additional capacity has been made available to aid the flow and movement of water. A revised floodplain was submitted taking the new channel into account and how the site is now protected.
In view of this submission, the developer had demonstrated that the proposed development can proceed safely. We therefore removed our objection and requested a safeguarding condition to ensure the scheme accords with the NPPF.
These conditions require the developer to:
· Limiting the surface water run-off generated by the site during a range of critical storms so that it will not exceed the run-off from the undeveloped site and not increase the risk of flooding off-site.
· Provide compensatory flood storage alongside the existing watercourse that runs along the southern boundary of the site.
· Install a new screen on the combined watercourses, as per the recent planning permission.
· Set the finished floor levels no lower than 600mm above the highest predicted 100yr + climate change level in the local area.
· Ensure a responsible authority will assume responsibility for the future maintenance of the screen in perpetuity.
· Ensure overland flooding from upstream surcharged sewers will be allowed to safely flow through the site and join the watercourse. (the model shows a water depth of 155mm)
As such, we wouldn’t require a further FRA to be produced. However, we will require sufficient details to be submitted to the LPA to satisfy the conditions attached to the planning permission.
Lastly, I would wish to add that I spoke with My Colleague shortly before the Christmas break. He explained that the existing culvert and watercourse will both be widened and works to enable this will be done through either S38 of the Highways Act 1980 or through the Drainage Act.