Re Application 18/00832/OUM 52 homes between 4 and 5 Soham Road Fordham
As a result of the explosion of development applications in Fordham, I have become concerned about infrastructure capacity. I have tried directly and through our MP to find the capacity of the sewage system. Each application has been met with the bland assurance that there is sufficient capacity in the system. The drainage report that accompanies this latest application is therefore of interest to me. A potential problem has been identified and:
“The following alternative options were tested to mitigate the impact of flood detriment resulting from the proposed development, but were found to be ineffective or unfeasible:
1) Increasing the pump rate at FORCSP from 10l/s to 13l/s, resulted in detriment
at DG5 locations downstream of the pumping station.
2) Increasing the storage capacity at the pumping station was not a viable solution
as a large storage volume was required to reduce the flood detriment.”
The UK average for waste water is 150 litres per day per person, or 0.00174 litres per second. Current housing in Fordham amounts to about 1200 homes, and, with the UK average of 2.3 persons per house, equates to 4.792 litres per second of sewage.
Including this application, consent has been given or is being considered for another 355 houses and a 75 bed care home. I calculate this provides an additional load on the system of 1.56 litres per second, making the total average flow of 6.36 litres per second for the village.
Also to be taken into account is the likely load of surface water. In Fordham there are culverts by the Ironbridge that discharge water from Mill Lane and Hillside Meadow, and by the bridge on Carter Street near the Victoria Hall. The report states the “Mitigation solutions are designed to prevent detriment to the existing sewerage network performance during a 1 in 30 year critical duration storm event” This implies that my suspicion that most surface water from roads drains into the sewage system is correct. Furthermore, AW expects consumers to prove that their roof rainwater is NOT discharged into the sewage system in order to reduce the estimate of their sewage output and gain a discount.
I estimate that the built up area of Fordham is about 1km2. A very conservative estimate is that only 2. 5% of this area is roof and road draining into the wastewater system. A 24 hr period with 20mm of rain (less than an inch, and a typical amount in wet months of the year) would therefore produce a flow of 10 litres per second.
Including my rainfall estimate makes a total average flow on a wet day of 16.36 litres per second, well above the capacity of 10 litres per second.
It would appear that a considerable proportion of the system contributes to storage to even out the flow but my calculations are of average daily flow and it is obvious that there are peak times for both domestic wastewater and rainfall contribution.
I believe that the sewage system in Fordham is incapable of taking developments of this scale without high risk of raw sewage overflows, and that this development ought to be considered in the light of existing planning consent for developments elsewhere in Fordham, which were not taken into account in the Drainage Report.