Nigel Woonton
Area Flood Defence Manager
Environment Agency
Bromholm Lane
BRAMPTON
Huntingdon PE28 4NE

Dear Sir,

1.1 Subject: Bin Brook Flooding

Thank you for your letter of 7 December. Whilst we accept your observations on the extreme nature of the storm, we are nevertheless very concerned that such a flooding could occur again in the foreseeable future under significantly lower rainfall and a shorter return period flood. As you are probably aware, once the Gough Way culvert is flowing full, the flooding damage will occur.

Following upon your letter and pending the studies by the EA, we have carried out our own preliminary study of the problem and possible solutions utilising water engineers living on the Estate with experience of flood studies in UK. We would like to draw your attention to some of our observations in advance of discussions with yourselves. These are as follows:

You mention in your letter about the possibility of flood boards across external doorways. We consider that this will not prove to be an adequate remedy, as in the case of several houses in Gough Way water entry was first through the floor including cases where there were concrete floors. At some houses where there are suspended wooden floors and no water entered through the external doors due to successful "sandbagging", water once again came in from underneath through the floorboards.

There are two alternative processes of flooding from Bin Brook in Gough Way, as evidenced in October. Firstly there is the flooding of houses on the south side of Gough Way (odd nos 37-47) from the relief channel. Secondly there is the flooding of the majority of the affected houses when the culvert is full and is first evidenced by the discharge through two inspection manholes in the street. Both floodings occur at approximately the same time, caused by the Gough Way culvert reaching capacity.

Based on previous studies, the overall capacity (bank full-start of flooding) of Bin Brook at Gough Way is 4 m3/s. This includes the relief channel to make up for the deficiencies with the original culvert. It is our understanding that the overall capacity of Bin Brook has not been increased from its original natural state, although in the last fifty years there has been the development of Hardwick and Coton into more substantial villages. In addition there has been the M11 which appears to have no balancing ponds over the 2km length within the Bin Brook catchment.

Our flood estimates show that the 1 in 10 year return period flood of the 13.4 km2 catchment up to the entry point to the Gough Way culvert is 4.2 -5.5 m3/s depending on method of analysis adopted. These estimations exclude the runoff from the additional 1.5-2.0 km2 of catchment downstream of the culvert/start of relief channel, which flows directly into the relief channel from the adjacent fields (as occurred in October) and which will further reduce the capacity at the upstream end of the channel. In other words it appears that there will be overtopping leading to flooding at approximately the 10 year return period flood. In this context we understand that the EA's target criterion for flood defences in urban areas is the 1 in 100 year event.

The above return period presupposes that the entrance to the Gough Way culvert remains clear during the flood. This is generally not the case. Whilst the authorities do carry out regular checks and clearance, the major debris causing blockage of the trash screen of course occurs rapidly during floods and it is local residents (from their own interests) who have been continually going out during periods of high flows to clear the debris; however this obviously cannot be done as frequently as required, particularly at night. In fact it was local residents who kept the trash screen clear in the flood build-up on the night of 21 October, but this could not be continued when the flood became too high. Early next morning when the flood had died away, large debris including a large block of polystyrene were found against the trash screen.

Based on observations of the flood, in particular the flow characteristics in the defined pathway (defined by fences or walls) from the start of the culvert to Gough Way (between houses 35 and 37), which carried nearly all the spillage, a crude estimate of the peak excess flow (above 4 m3/s) has been 1.5-2.0 m3/s. This would indicate, based on the duration of the excess flood, a surplus volume probably of the order of 20 000 - 30 000 m3 and certainly less than 40 000 m3. The peak discharge and excess volume of water implies considerable attenuation in the catchment and this is borne out by witnesses of the flooding upstream and in Coton. We would be very interested to know your assessment of this flood.

We understand that the October flood could approximate to a 1 in 100 year return period event. If this is the case, the volume of flood flow to be dealt with (above 4 m3/s) appears to be relatively low, when compared to the damage which was caused to property in the Gough Way Estate, along Barton Road and further downstream along Bin Brook.

In this context it is understood that a typical solution adopted by the EA in such situations is the construction of a flood storage reservoir with regulation to restrict the downstream flow to bank full conditions. Although the terrain is relatively flat, a particular feature of Bin Brook is that it is incised in excess of 2m below general ground level and it is thought that an offstream flood storage reservoir taking the flow in excess of 4 m3/s could be possible either on the west side of the M11 or between Gough Way estate and the M11. We understand that the EA encourage the development of such reservoirs into permanent wildlife lakes and this certainly could be an added attraction, particularly on the east side of the M11.

1.2 Summary

Leading from the above observations, we do feel that there is a considerable risk of flooding of a significant number of properties in the Gough Way Estate and also in parts of Barton Road at flood events of about 10 years return period. This could be more frequent if there is any blockage of the trash screen at the culvert entrance or other impedance in either the Bin Brook channel downstream or the relief channel. At the same time, we also feel that at least in engineering terms, there may be a relatively straightforward solution.

You have received estimates of the cost of repairing the flood damage from some of the residents in this area. Our inquiries show that these figures are incomplete as many residents are still awaiting better estimates from the loss adjusters and builders or may have under-estimated the cost. You may wish to obtain revised figures, but we should be surprised if the total insurance claim for Gough Way Estate residents will be less than £1 million.

You will appreciate that the residents of the Gough Way Estate, and not only those affected by the recent flooding, are very keen to discuss with yourselves possible solutions and when they are likely to be implemented. We do consider that a resolution of the problem is urgent and look forward to meeting with you in February. It is our intention that this meeting should be open to all residents of this Estate plus those affected in Barton road. The Residents Association has set up a "Bin Brook Action Group" which will follow up any action required after that meeting.

We would welcome a response to this letter provided it does not delay the meeting with you.

Yours faithfully,

BRIAN HERBERT Chairman

cc Anne Campbell
Cllr Chris Lakin
Cllr Nicola Harrison
Cllr Malcolm Schofield