Proposed new flood defences

As part of our active interest in the development, improvement and protection of Cockermouth we have prepared the following letter requesting additional information from the Environment Agency concerning the proposed flood defences that would run from Victoria Bridge all the way round to the end of Waterloo Street.

Details of the proposals can be found on the EA website at http://environment-agency.gov.uk/homeandleisure/floods/131663.aspx and a map indicating the extent and type of defences is located at http://environment-agency.gov.uk/static/documents/Map_of_our_proposals.pdf

 

 

Dear Mr Bacon,

Proposed Flood Defences for Cockermouth

Members of our Committee attended the exhibition of the Environment Agency proposals for enhanced flood protection on Wednesday 7th September. A number of questions arose for which we did not obtain answers on the day. I would therefore be very grateful if you could provide us with responses to the questions identified below. Please note that I am copying this letter to Mike Apperley for the information of the Cockermouth Flood Action Group.

  1. What effect will there be on Bitterbeck under flood conditions from an increased head of water along the Cocker, and what will be the consequences?
  2. What consideration has been given to strengthening of the existing house walls on Hatters Court since in the plans they are to act as 2.5-3m defence walls? [These are rubble walls and we believe that they cannot retain anything like that head of water. They could collapse causing a very powerful destructive surge to be directed through town. Note that these are listed buildings and any works will be subject to listed building restraints].
  3. What consideration has been given to the potential opening up of new flood-water pathways from the increased hydrostatic pressure arising from the higher head of water under flood conditions?
  4. Specifically, has consideration been given to the effect of the increased head of water on the known flood-water pathway through the basement of HSBC Bank?
  5. With increased head and the restriction of flow at Cocker Bridge, will this not increase the likelihood of both damage to the listed structure and the direction of water through the open bridge parapet onto Main Street? [Specific concerns are for localised increased head of water directly against the upstream parapet as a result of the inevitable pressure wave that will build at that point].
  6. What protection against destructive scour is planned to the Cocker Bridge foundations and what structural surveys have been carried out to ascertain the suitability of the structural integrity of Cocker Bridge when exposed to the increased head and flow of water? [The likelihood of greater scour occurring with the proposed works should be incorporated in to the viability and consequence of the scheme. Any Bridge collapse at this point would lead to severe flood ingress to the Town Centre].
  7. In order to assess and understand the various flood risks, could you set out what the various probability statistics actually mean in terms of height of water above a known datum (ideally some known landmark within the Town Centre).
  8. Have conscious decisions been made to create sacrificial areas of town in order to protect other areas, and, if so, what are these areas and what is the basis of these decisions? [Specifically we are concerned that Market Place has been designated ‘sacrificial’]
  9. What arrangements are being put in place to ensure that the residents of Cockermouth, in particular those most affected, fully understand the physical and visual intrusiveness of the various proposals on normal life, in particular the raising of the height of the protective walls? [We believe that the rivers in Cockermouth are an important part of the ‘added value’ of living here and creating a ‘Fort Knox’ will significantly undermine this. Clearly a balance has to be struck between flood risk and ‘ambiance’, but it is crucial that we all understand the negatives of the proposals as well as the positives so that informed decision
    s can be made]. In particular, would you consider undertaking a ‘house to house’ survey of the most affected residents such as on Ruby Banks, the results of which may be used to support the case for proceeding (or otherwise)?
  10. What consideration has been given to schemes to delay flood-water reaching Cockermouth as cost effective alternatives to raising flood defences within the town itself? [Examples might be increased forestation and/or sacrificial areas upstream of Cockermouth]

Many thanks in anticipation of your response,

Yours Sincerely,

Phil Campbell

(Chairman)

 

One thought on “Proposed new flood defences

  1. Following our letter to the EA the below text is a response from Adrian Bacon received on 29 September.

    Thank you for your letter date 18th September 2011, please find answers to your questions in order below:

    1. The proposed flood defence improvement scheme will not increase the frequency of flooding from Bitter Beck. The flooded area associated with Bitter Beck has been modelled with and without the defences in place and it makes no difference to the flood extents. We will also place flap valves on any drainage outfall on Bitter Beck which pose a flood risk through backing up.

    2. The proposals for the flood defence improvement scheme involves raising flood walls to a level that will provide protection only up to a level that has a 1% chance of being exceeded in any year. In higher order floods the defences will be overtopped and thereafter any increase in hydrostatic loading due to the new defences will be limited. We have already carried out a visual structural inspection of the properties and from this a programme of waterproofing and strengthening works will be incorporated into the scheme.

    3 During the design of flood defences we routinely make an assessment of all the potential flood pathway which increase the risk of flooding to the area we area trying to protect. These can include; seepage, drainage networks and overland flows, where necessary we incorporate measures to control these within our designs.
    4 The scheme will not increase the frequency at which the water enters into the basement of HSBC Bank. We will consider whether the raising of flood defences will mean that more water passes into the basement of the Bank and if necessary we will incorporate appropriate control measures into our design.

    5. The situation here is similar to that described in our response to question 2. As we are only raising flood defences to a 1 in 100 year level (1% AEP), our modelling shows that at this level the parapet walls will not be acting as flood defences. The proposed scheme will not increase the loading on Cocker Bridge beyond that which it was able to withstand in the November 2009 event.

    6. During the design of the new defences we will assess the potential impact of increased flows through Cocker Bridge, including additional scour problems. We will ensure our works do not exacerbate the current level of risk of scour at the structure and if necessary scour protection measures will be carried out in consultation with Cumbria Highways.

    7. The soffit level of the arch at Cocker Bridge is at a level of 45.73OmAOD. The road level at the bridge is around 46.25mAOD with the minimum level of the wall parapet is 47.35mAOD.The design levels including 300mm freeboard for each event at the upstream face of Cocker Bridge are as follows (annual exceedance probability AEP);

    • 1 in 25 Year Event (AEP 4%) 45 .494mAOD
    • 1 in 50 Year Event (AEP 2%) 45.886mAOD
    • 1 in 75 Year Event (AEP 1.3%) 46.131mAOD
    • 1 in 100 Year Event (AEP 1%) 46.3O4mAOD
    • 1 in 200 Year Event (AEP 0.5%) 47.138mAOD

    8. We have not taken any decisions to create “sacrificial areas” of the town. Market Place has not been designated as “sacrificial”. However, it is not our intention to defend the Riverside Car Park downstream of South Street Foot Bridge.

    9. We have had the first of a series of public drop in sessions to obtain the views of Cockermouth residents on design alternatives to inform the developing design. We want to produce proposals that are acceptable from aesthetic, performance, environmental, technical and affordability perspectives. We are endeavouring to strike the balance that you describe and will be meeting again with residents of Rubby Banks Road. Once we have consulted widely we will finalise our proposals and make a planning application. Everyone will have the opportunity to comment on the planning application as part of the consultation process, before the town’s elected representatives on the planning committee meet to either approve or reject the application. All the defences work together to reduce the flood risk to Cockermouth and with this in mind we are trying to utilise our available budget to deliver a scheme that everybody is happy with.

    10. As part of the appraisal process for the scheme we looked at a number of options to store water upstream of Cockermouth for both the Derwent and Cocker Rivers. This included utilising the existing upstream reservoirs and creating a new storage area. On the Cocker the volume of water that would need to be stored for a 1 in 100 year event with no changes to the current linear defences would be 560,000m3, which is equivalent to 145 football pitches flooded to an average depth of 2m. The Derwent flood storage options required even more significant areas and offered no benefits upstream of Cocker Bridge on the Cocker. There are limited sites available for a storage area, and during a flood event once the area has been filled it offers no further flood defence. Furthermore, the protection against flooding is dependent on where rain falls to ensure that the storage area does actually operate. If the rain falls downstream of the storage area it will offer no flood protection and the only viable location for storage is 4km upstream. The storage option was over 3 times the costs of the linear defences.

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