West Flood Solutions Study By OXFORD FLOOD ALLIANCE

Produced for discussion by all the Authorities involved in flood-risk management in the City of Oxford. August 2008

Introduction

The Oxford Flood Alliance is a core group of active residents distilled from all those who suffer flood problems or threat in Oxford. Area groups have united to create a six-person committee that has the mandate of the people whose area they reside in, and endorsement of the others. This committee has links with all sub groups and issues up-dates to an informal, unlimited membership by web-site and E-mail, and the group encourages input by any concerned resident. The group is supported with technical and survey advice by the author of this report. The Author has a previous career in flood measurement and civil engineering, and a thirty-year interest in the navigation and natural character of the .

The Problem

The three major problems in the city are:- Water in the meadows north and south of the , high levels in the / Coldharbour/S. area, and ground water levels owing to insufficient movement through the South Oxford area. All flood problems being increased by continuing man-made additions in the flood plain, and are therefore becoming greater in severity.

What happens

As levels in the Thames rise the lock-keepers open gates to reduce ‘headwater’. When a gate is opened the keeper phones the info to the keeper downstream. Weirs fall like dominos along the river. Smaller rivers discharge into the Thames to increase peak levels. The affect seems to be a hastening of that peak level on its way downstream. Instead of making efforts to hold back water where possible, land drainage priorities and ingrained habits cause the rapid movement of water to the Oxford City area. This seems to be manageable until water does reach Oxford where man-made obstacles in the flood plain begin the accumulation of standing water in the low-lying parts of the city and its environs. Water from the largely un-controlled R.Cherwell adds to the problem. Water overtops the Thames and fills the meadows north and south of the Botley Road. It is constrained by man- made development in all areas of the city and also by vast areas of land-fill in places like Redbridge so that levels are artificially increased. (Brian Wilson’s submission to the Public Inquiry on the proposed Bodliean book store at Osney gives more background and history on Oxford’s flooding; including the strong warnings given in the 70’s about over-development.) Obviously in a moderate flood there is no damage and the water usually recedes quickly. But large floods, as have occurred four times in the last ten years, do not get through Oxford and begin a steady rise, which begins to inundate property. Recently, houses that had never flooded before were overwhelmed and future forecasts show a worsening scenario. 3000 homes in the City could be affected.

Solutions (letters to suit identification on accompanying maps)

A) Thames west bank (north of Botley Road): The banks of the main channel, especially between Tumbling Bay and Medley have deteriorated and no longer keep enough water in the main channel. At one area we have measured a drop in bank levels of at least 18 inches caused by the trampling by cattle. It is important that the levels are restored to match the headwalls of the weir at Tumbling Bay. The extra water passing across the fields here plays a major role in inundating the Botley Road area. Without these breaches it is possible that the water may not come over the road near P.C.World and into Duke and Earl Street. The history of flooding in that area seems to bear this out and it is known that the reduction in bank height is a relatively recent occurrence.

B) Osney main channel: Repairing the banks will put more water in the main channel past Osney Island. This is not a problem with the rapid speed it travels but there are two works that would complete this part of the solution. 1) An impermeable embankment along the East Street towpath of about 18 inches. (This bank has reduced in level since first built.) This would put it nearer to the higher bank levels on the city side of the river and allow a big increase in capacity. 2) The Osney Mill, when in operation, had a large adjustable weir gate that could be opened during flood conditions. It is currently blocked by a concrete wall. From rough measurements we estimate this structure could take at least ten times the water it now passes. These works would allow the North Street sluice to be closed longer, thus relieving pressure in the Osney Stream to the west of Osney Island.

C) Castle Mill Stream: This has a large capacity, but is restricted by the weir at the Castle. It limits the flow but there are a number of smaller weirs and gates in the area that could be operated to get more flow. These may not be familiar to the flood risk manager. It was witnessed in June 08, that when the allotments and Middle Fisher Row flooded there was a fall of at least four feet at the Castle Mill weir.

D) Botley Road/Lamarsh Rd: Water comes over the crest and flows down Earl Street and into Duke Street. The flow is sufficient to overwhelm emergency pumps. Local residents tell that the water used to pass down the Lamarsh Road to the meadows, but many alterations to the Botley Road levels during redevelopment have changed its path. The County surely have a moral obligation to correct this situation: minor surface works would re-direct water to Lamarsh Road as it did in the past. A culvert could be placed in Lamarsh Road to take water to the flood meadows to the south. It could at this time be co-ordinated with the (Kingerlee) redevelopment site, so that some of the costs would be covered under planning gain.

E) Oriel Mews: A notorious development where fundamental warnings on flood risk were ignored by the Council. The ground was raised so much that Earl Street and Duke Street now become a lake during flooding. It is possible to dig a culvert with non-return valve thru the new estate to empty to the meadow. This will work with the slightest head of water and our survey of field and street levels shows that any required gradient is attainable (ground levels available on request).

F) Stream – inlet from the Thames: A fixed concrete weir 33’ wide 3’ deep. This allows a continuous flow into the stream, for self-maintenance and natural benefits. Before the Osney Channel was dug, Seacourt Stream used to be a main navigation channel bypassing Oxford, but it has diminished thro time and is only a few feet wide in places. The last alterations to the inlet weir reduced the summer flow and the stream has been shrinking so it carries less water. We think that resetting the weir structure could allow more water in summer, while allowing the reduction of its flow during a flood. This would lessen inundation at the Bullstake area of the city.

G) Seacourt Stream/ Hinksey Drain: The EA. are embarked on the clearing of obstructions at Kennington and up to .

H) Osney Island: The damage is not done by overtopping water but by it coming through the ground at South and Bridge Street. It is felt that the current Thames Water pumps are unreliable and that simply increasing their capacity and reliability will stop the flooding to property.

I) Godstow- causeway: This has slumped over time and now allows vast extra flow into the Binsey/ Botley area. Raising the road would give better control of water and also prevent the isolation of the village.

J) The : Witnesses state that after flood peaks, control weirs are left open far longer than necessary; the Wolfson reach becomes depleted while excess water is allowed to add to the rising levels in the Abingdon road area. The University needs to be more aware of the need for better water control.

K) Willow Walk: This causeway forms a dam which raises water levels and concentrates water from the meadows into Hinksey and Bullstake Streams. A number of culverts could be run through the causeway to balance the levels, or at the very least the blocked outlet at the eastern end of the causeway should be re-connected to the stream that runs at the south side of the Ferry Hinksey estate and enters the Bullstake behind The Oxford Mail works.

Solutions - ownership Action

A) Thames, West bank repairs EA., Charlie Gee(tenant/owner) B) Osney Mill EA., City Council + land-owner B) East Street – bank repairs City Council C) Castle Mill Stream – small weirs Navigation/ EA flood risk dept. D) Botley Road surface reshaping/Lamarsh culvert ThamesWater,City,County Council E) Oriel Mews culvert (non-return) ThamesWater,County Council F) Seacourt Stream inlet EA. G) Seacourt/Hinksey Stream – clear obstructions EA./ Network Rail H) South Street, Osney – better permanent pump Thames Water I) Godstow–Wytham causeway County, EA. J) River Cherwell University, EA. K) Willow Walk City Council

Effects upstream/downstream

There are parts of the catchment that could store more water without harm; and other parts that will receive less. This has been discussed in the document and requires further work. It is a complex subject but readers should note that many of these solutions are about returning conditions to what they once were when maintenance was more rigorous and flooding less damaging.

Key solutions

• Osney Mill - rebuilding control gates. • Thames, west bank – repairs, including East Street improvement. • Botley Road surface alteration/ Lamarsh drainage culvert. • Osney Island – more efficient pump system.

These four works, we feel will make the most difference to controlling floodwater in Oxford; we would welcome more discussion on these with the relevant authorities.

Conclusions

Structures and bank and road levels have changed over time; usually in a way that adds to flooding problems. Our review is based on intimate local knowledge and aimed at stimulating debate on a wide range of relatively inexpensive solutions. We don’t mind who does the work but there is an appropriate ‘owner’ of each remedy. If the work load can be spread then hopefully there is more chance of earlier resolution. The foregoing are just some of the ideas that have come out of much research. They have the potential to greatly reduce the scale of damage in Oxford, and we hope they will be considered seriously by the local authorities. The measures are thought to be good short-term solutions with reasonable cost implications. A more in-depth analysis can be provided for each of the solutions suggested.

Hamblett 13/8/08 - West Oxford Flood Solutions5.doc