Planning Application MW.0027/22: Oxford Flood Alleviation Scheme
Thank you for consulting on the above application by the Environment Agency.
I believe that the plans do not represent efficient use of public money and that improvements can be made with reductions in costs.
I have been working with Hinksey and Osney Environment Group (HOEG) for over two years and have, together with Kevin Larkin, Architect, designed a pumped option that can function as a backup with additional capacity that is cost effective, reliable, and has flexibility in its location. Furthermore, the pumped scheme is unobtrusive and can be scaled to move quantities of water equal to or greater than OFAS at time-critical periods during a flood. It offers a degree of control of water level not available to a gravity scheme.
It is apparent that there are man-made obstacles that impede flow of floodwater between Botley Road and the East side of the railway at the A423 Southern Bypass and that two of these remain as hindrances in the OFAS application plans. These are:
- Culvert under the Old Abingdon Road/new road bridge(s) at the Abingdon Road T-Junction, and:
- The railway permanent way.
The wide 2nd stage channel does not add protection to properties above that offered by the 1st stage channel and additional mitigating measures that have low environmental impact. Clearance and maintenance of existing waterways, along with the 1st stage channel, will optimise flow across the wider Meadow flood plain and focus the main discontinuities in head drop at OAR and the railway, which in themselves determine the effectiveness of OFAS.
(It is important to emphasise that at a flow rate in excess of, say, 30 cubic meters/sec during a flood there is no opportunity for significant temporary storage of water. A 2.5 sq. km area, roughly equal to that of the floodable Meadow plain, would fill to 1m depth in 24 hours. Peak Hinksey Meadow flow rate is estimated to be 65 cubic meters/sec in a flood. The 2nd stage channel, with a nominal volume of 300,000 cubic meters, would fill less than 90 minutes under peak flow conditions.)
OFAS does not address the 100-year flood event and passage of water across the OAR and railway in a robust fashion. Water levels of up to 56.28m AOD at Hinksey Stream Railway Br D/S (ES Appendix Q Table 4) threaten both Abingdon Road highway (A4144), and properties in New Hinksey and Weirs Lane estate.
It is clear that supplementary measures for letting down more water from above OAR to East of the railway are required, beyond anything that the 2nd stage channel and OFAS, as currently proposed, can achieve.
Network Rail have stated in correspondence with HOEG that they will ‘do what is needed’ to ensure that their obligations for drainage across the permanent way under the 1843 Oxford Railway Act are observed. This is an instance where development over past decades has brought this particular aspect of flood relief into sharp focus over a relatively short period of time, since the millennium during episodes of flooding. In numerous other places across the country progressive works of culvert and bridging under railways (and roads) has kept flood levels equalised on either side.
It is now time to add extra railway drainage capacity upstream of the soon-to-be-replaced A423 bypass bridge, that will be accommodated in the expanded East culvert on its route to the Thames and Sandford Weir. The tracks have already been raised along this section a few years ago, and together with planned upgrading of Oxford Station an opportunity now arises to improve both flood protection of the permanent way and concomitantly West Oxford by constructing a new culvert.
If a new railway culvert is factored into a revised OFAS, then the need for the unsightly, destructive and costly 2nd stage channel is completely removed, with Scenario A2 taking its place.
The Argument for a Pumped Scheme
The topography and near-surface stratigraphy of the Hinksey flood plain is ideally suited to trenching and emplacement of buried pipes. A pair of 2m diameter pipes can carry up to 44 cubic meters per second of water, powered by electric axial-flow pumps in a pumping station at the head end. This is more than the flow rate of OFAS at full capacity.
A pumped system has a number of advantages over gravity flow, which may be summarised as follows:
- Flexibility in terms of maximum flow rate, and location of the pumping station.
- Components are industry standard, with pumps powered by extremely reliable electric motors.
- High degree of control of flow rate, with commencement of pumping at an early stage of flooding thereby reducing peak water levels. It may be operated remotely.
- A pumped scheme is additive, and contributes more than 40 cubic meters/sec over and above gravity flow off the Meadow.
- Pipes can be incorporated into culverts and channels.
- Construction requires considerably less removal of spoil off-site than OFAS, and once completed has much smaller visibility. It has a low environmental impact.
Water, and other fluids, are pumped through pipes in thousands of locations across the world. It is the most efficient way to convey water in large volume across large distances. Drainage of land areas below sea level depend on pumping massive volumes of water with high reliability.
A pumped layout for West Oxford flood alleviation is both very short, with a small head of pressure, and demands a relatively low amount of power. The favourable ground conditions and even topography of Hinksey Meadow make a pumped scheme less intrusive and more effective than OFAS, either as the primary means of flood alleviation, or as a backup system.
The pipes can negotiate all obstacles between Botley Road and the East side of the railway.
However, a new culvert under the railway will be needed at the Southern end.
Designed as a backup system, placement of a pumping station below OAR would offer a highly effective method of complementing Scenario A2. This would draw water off the floodplain and convey it at high volume under the railway to a discharge point that could be below Sandford Weir, giving more control of water levels and reducing the effects of pooling and back-flow during heavy flooding.
Summary
The Applicant is proposing an expensive and over-engineered flood alleviation scheme that will take several years to complete.
OFAS is dependent for drainage at the southern end upon replacement of the impaired A423 Southern Bypass railway overbridge, which will require collaboration with Network Rail and OCC Highways. This is a complex operation that will in itself involve several years' work.
Together with its other shortcomings, OFAS is also critically flawed in not incorporating adequate drainage across the railway, which forms part of Network Rail’s obligations under the 1843 Oxford Railway Act. By adding a new culvert under the railway upstream of the A423 overbridge, the permanent way would become essentially transparent to flood water flow and the head drop across the railway would reduce to almost zero. The railway will also benefit from better flood protection, at a time when upgrades are due to take place for Oxford, including full electrification.
Once this accompanying work is included into a revised scheme, hydrological modelling of flood water flow will show a marked reduction in peak water levels, and thereby eliminate any need for a 2nd stage channel.
Inclusion of a pumped system, which has flexibility of scale and location, will minimise environmental damage to an historic, recreationally important and environmentally sensitive part of Oxford and the wider county.
Flood alleviation for West Oxford would then be secured for the next century against anticipated weather events, while also embodying a significant margin of safety against climate change.
By an hydraulics engineer