Dear Mr Post,

RE- Freedom of Information request - Dredging abandoned, saving Marlow sacrificing Bisham

I refer to your request dated 14^th August 2009 regarding dredging of the Thames near Marlow and the proposed Marlow Flood Alleviation Scheme.

Dredging

With regard to dredging, on the River Thames this can be carried out for two main reasons: Navigation and Flood Risk Management (FRM). Navigation dredging is undertaken to meet our statutory duty to maintain navigational depths in the main river channels, in the lock cuts, and in areas close to moorings. This work consists mainly of removal of shoals, based upon regular monitoring and survey work that tells us how sediment has moved. The need for shoal removal does not prove that the Thames is silting up generally and our surveys show that this is not the case.

There are two separate methods of dredging for FRM purposes, the first of which is maintenance dredging which typically consists of improving conveyance through the removal of large volumes of silt and only has a negligible effect on reducing flood risk during small flood events.

The second method is re-profiling, in which the riverbed cross-section would be significantly changed by removing part of the hard bed material. This was carried out following the 1947 flood, lowering the bed of the river by about 30cm, and as you correctly pointed out this was completed in the mid 1990s.

After completion of this activity a study was commissioned to look at the future justification of re-profiling works for FRM purposes. Further deepening would require bank stabilisation works, and would involve much greater costs in disposing of the dredged material than previously, and have major environmental impacts. Hence the study concluded that further re-profiling of the Thames could not be justified on economic or environmental grounds.

A subsequent study was commissioned in 2008 to better understand the benefits and impacts of dredging and to ensure that decisions on potential for dredging in the future are based upon recent data.

Halcrow consultants have analysed riverbed level surveys undertaken between 1998 and 2008, from Hurley to Teddington. The results indicate a dynamic riverbed profile, with net change to volumes of material in the general order of plus/minus 10,000-20,000 cubic metres each year.

The changes in volume of material are clearly related to river flows in the period between surveys. The study results support the general rule that deposition of material and increase to riverbed levels occur over periods of lower river flows (e.g. 2004 to 2007), whilst erosion and lower bed levels can be observed over periods of higher flow (e.g. 2003, 2007 to 2008).

The report recommends a further monitoring programme to improve our detailed knowledge of long term trends in riverbed profiles.

I attach the summary for the above report for you information.

Marlow Flood Alleviation Scheme

With regard to the proposed Marlow Flood Alleviation Scheme, the purpose of the scheme is to protect nearly 300 properties at risk from flooding.  Houses in other areas such as Bisham will not be protected by this scheme and will still be at risk of flooding.

The scheme comprises a combination of flood walls and flood bunds along with a groundwater pumping scheme. Modelling carried out by us shows that the building of bunds and walls will have no detrimental effects at other locations.  In addition, we are proposing to provide compensating flood storage areas upstream of Marlow and Bisham to compensate for the loss of floodplain as a result of the construction of the works.

Please refer to the attached flood map indicating the areas our modelling has shown will be protected and those that will remain at risk during a 1 in 100 year event (1% chance of flooding in any given year).

Further details of the scheme are included in the recent Newsletter which is attached along with two drawings showing the general layout of the proposed works.

If you require further details about this scheme please contact Doug Hill, Asset Systems Management Team Leader on 01276 454513 or by [email address].

You should note that this information is based on that currently available as of the date of this letter. You may feel it is appropriate to contact our office at regular intervals, to check whether any amendments/ improvements have been made. Should you re-contact us after a period of time, please quote the above reference in order to help us deal with your query.

This information is provided subject to the enclosed notice which you should read.

Yours sincerely

Marie Edwards

External Relations Officer

Tel: 01276 454427

Email: [email address]

Enc

Notice

Newsletter

Dredging Summary

Maps x 3

2

Environment Agency

Frimley Office, Swift House, Frimley Business Park, Camberley, Surrey, GU16 7SQ

Tel: 01276 454450 Fax: 01276 454301