Nigel Woonton
Area Flood Defence Manager
Environment Agency
Bromholm Lane
Brampton
HUNTINGDON PE 28 4NE

Dear Nigel,

Subject: Bin Brook Flooding

We welcomed hearing your and Peter's viewpoints on 13 February on our flooding problems. I said that I would provide you with some of the information on the flooding, which I had researched, and I apologise for not responding sooner but there was further data on levels which I wanted to establish. As you aware, I have for obvious reasons taken a particular interest in the behaviour of Bin Brook and possible solutions, particularly as the problem is so site specific. Herewith are some of my own independent analyses and assessments, which I hope you may find helpful.

(i) Flood Estimates

I enclose in Appendix A some of the hydrological work which I had arranged to be undertaken informally to get "a better hold of the problem". The flood characteristics up to the start of the Gough Way culvert (13.5 km2) have been estimated by the standard unit hydrograph approach and the FEH statistical method. I have also pro-rata'd the flood estimates in the 1978 Mott MacDonald report (up to the Cam confluence) to allow for the smaller catchment at the Gough Way culvert. In the case of the unit hydrograph method, the volume of water in excess of the capacity of the culvert plus relief channel (4m3/s) has also been estimated. These results are summarised in the table below.

Return Period (years) 5 10 25 50 100

1978 Study Peak(m3/s) - 3.7 - 5.6 6.5

FEH Statistical Peak (m3/s) 3.4 4.2 5.2 6.0 6.9

FEH Unit Hydrograph (m3/s) - 5.2 - 8.3 9.8

Peak Excess volume (m3x 1000) - 22 - 129 190

I understand that the difference between the 1978 estimates and the recent FEH statistical method is the fact that the latter tends to be more severe, although dependent on the catchment contributors selected. Nevertheless, although one can argue about the choice of input parameters, it is apparent that the present arrangement at Gough Way is only capable of discharging the 1 in 10 year return period event. This of course excludes possible blockage of the trash screen, impedance to the flow in the first 200m downstream of the culvert and also the additional contributing catchment discharging direct into the relief channel from adjacent fields (and of course climate change).

Post the 1978 flood event and the subsequent relief channel in 1982, there were near flooding events in 1984 and 1993 and hence the 1 in 10 year return period appears to be backed up by fact.

(ii) Flow Characteristics on 21 October

The excess flood (above 4 m3/s) flows down two reasonably well defined channels in terms of pathways with vertical (fence) sides at Gough Way. These are shown in Figure 1 and also in the enclosed photographs. For want of other information such as river gauging, it is considered that a reasonably good estimate of the flood can be obtained by assuming uniform flow and utilising Manning's formula for the flow in these pathways. The paths/channels are:

Path A: between the start of the culvert and Gough Way. It was a width of 3.5m and a gradient of 0.0176.

Path B: between Gough Way and towards the end of the culvert. It has a width of 3.0m and a gradient of 0.0036. It has dense vegetation on the right side as shown in the photos.

Due to its smaller size and lower gradient, Path B act as a constriction to the outflow of the excess flood entering Gough Way, resulting in a greater spread of the flooding up the road as shown in Figure 1. A series of levels of the peak flood , as recorded along Gough Way, have been surveyed, and these showed good agreement, indicating a depth of flow at the start of the pathway of 1.0m.

As indicated in the accompanying "Note on Flood Estimation", it has been possible to estimate the peak "excess" flow at 3.9 m3/s. It is considered that this is accurate to plus or minus 10%. The total peak flood would on this basis be 7.9m3/s, which compares well with the FEH statistical method for an event with a return period of above 100 years, which surely is not unreasonable based on the rainfall probability.

In addition it has been possible to generate an approximate hydrograph knowing the overall duration and rough levels at other times during the flood. As indicated in the Note, the excess volume of water has been estimated at 45 000 m3 and once again this may be only accurate to plus or minus10%. In terms of the flood characteristics, it can be seen that once the Gough Way culvert is overtopped, the problem is considerably exasperated caused the constriction of Path B.

One small point: I note from Figure 1, which was taken from the 1978 report, that it was originally planned to divert Bin Brook further to the west than has actually occurred. This would most probably have prevented flooding, assuming an adequate size relief channel.

(iii) Flood Attenuation

In view of the considerable difference between the statistical and unit hydrograph results, I asked one of our hydrologists to "run" the 21 October storm by the unit hydrograph method based on 100 mm rainfall in 18.5 hours. This is enclosed in Appendix A, giving a peak of 10.7 m3/s and an excess flood volume of 250 000 m3.

The peak flood of 7.9 m3/s and excess volume of 45 000 m3 (both estimates to 10% accuracy) estimated in (ii) clearly indicate the very large amount of attenuation upstream. This is borne out by the reported local flooding in Coton and the extent of surface flooding/sheet flow either side of the A603, as reported by several people. Almost certainly there was also flooding on the west side of the M11, as this tends to act as a constriction. The degree of attenuation was also evident by the high flows, albeit below 4 m3/s, in Bin Brook for most of the following day, although the rain had stopped the previous evening.

My own impression of Bin Brook between the M11 and Gough Way is that, at flows above bank full (say 3-4 m3/s), the flow will tend to spread out as sheet flow, as occurred on 21 October. Two conclusions can possibly be reached from this: firstly mathematical modelling will not accurately reflect severe flood conditions and, secondly, a relatively small amount of additional storage could resolve the flooding problem.

(iv) Land Ownership between Gough Way and M11

I understand that a letter has been recently sent to your Mr Nigel Fawthrop from the Cambridge Preservation Society concerning the Coton Countryside Park. In addition to the farmland on the west side of the M11, the CPS also own land on the east side which straddles Bin Brook. The ownership of the other land between the M11 and Gough Way is either Cambridge University or colleges. As indicated at our meeting, there are good contacts within the Gough Way Estate if a preferred location for a balancing pond is on this land.

However some of the land owned by the University close to the Barton road has recently been taken up by one of the colleges for sports fields and there may be future similar developments planned. This is another reason why we are keen to have the possibility of a balancing/off stream pond studied as soon as possible.

I would be pleased to come and discuss all these aspects with yourself or Peter, if you so wish.

Yours sincerely

Chris Wagner