From: Sandra Sheldrick Sent: 19 September 2011 13:57 To: Subject: Flooding From: Yusuf Kaya Sent: 10 September 2011 16:43 To: grant.whyte Cc: Alex Mitchell; bruce.weir Subject: YK T/C WITH A & B FLOOD OFFICER AND EMAIL SEPT 11 Grant, Thank you for discussing the Gasworks flooding issues with me on Thursday. The main points of our discussion are summarised below. 1) Kaya Consulting were asked to review existing documentation for the Gasworks site with particular reference to flooding. We are independent consultants and only express our professional opinion on the subject and of course we are happy to provide the necessary supporting information to back up our views expressed in various correspondence. 2) After reviewing the relevant documentation, we have highlighted a few issues associated with flooding risk of the site. Of these, the differences in the predicted peak water levels between the Carl Bro (CB) model and Dougall Baillie (DB) model and the potential effect of overland flow paths in case of culvert blockage were the two main issues which concerned us the most. We have also highlighted other issues which would result as a consequence of the above two main issues. 3) As the differences in water levels between the CB and DB model results were significant (being of the order of 0.65m), we have carried out our own investigation and extended our existing mathematical model of the upper parts of the Milton Burn (which we have set up recently for a different project) downstream to cover the site and areas further downstream. By doing so we were able to identify the source of the differences between the CB and DB models. It appears that the main reason for the differences in water levels between the CB and DB models come from the head loss at the three sharp bends in the watercourse just downstream of the site. If a reasonable head loss were included in the model (based on experience), our model predicted water levels are similar to CB model results. Therefore, we believe that the DB model may not be properly taking the effects of the three bends into account (hence producing lower water levels). 4) You have indicated that you have made your decision based on CB model results showing the predicted water levels with and without the proposed flood alleviation scheme involving a 1400mm bypass culvert starting from the supermarket car park and including a retaining wall to protect the gasworks site from flooding. You indicated that because there are no differences in the predicted peak water levels between existing and defended cases as per the CB report, you were happy to have no objection on flooding grounds. You also indicated that as this is a 'decision in principle', additional work by the developer will need to be carried out at the detailed application stage to show there is no flood risk to site and others. I have indicated my concern that there is a possibility that more detailed assessment at the detailed design stage may show larger parts of the site at risk of flooding and this could result in a smaller developable area than that proposed at present. 5) You have also indicated that you require the developer to implement an acceptable maintenance regime to reduce the risk of blockage of the Argyll Street culvert. You indicated that under the new legislation, the council have responsibilities in maintaining watercourses and this will reduce the risk of large debris arriving at Argyll Street culvert from upstream. My view is that regardless of the risk of blockage, existing overland flow paths should be retained so that flooding risk to others are not increased. 6) You indicated that the flood alleviation scheme being constructed by the council at present is exactly the same as considered by CB in their 2006 report with the exception of the retaining wall to protect the Gasworks site being taken out. After phone call note: Following our discussion, I checked the model results tables in the CB report and they indicate that there are no differences in peak water levels between the existing and defended (Gasworks defended) cases as assessed by CB in 2006. However, the model clearly indicates that the site floods and by protecting it flood waters that would be stored within the site will be passed downstream, increasing flooding risk to others (although this may or may not show in the model results depending on how it was modelled). SPP in paragraph 203 specifically covers such situations and indicates that "Piecemeal reduction of the flood plain should be avoided because of the cumulative effects of reducing storage capacity". In this respect, raising the flood plain on the grounds of it does not have a significant impact on flooding risk to others contravenes the SPP. However, SPP also refers to land raising, providing a number of strict criteria are met. SEPA only allow land raising in brownfield sites providing appropriate compensatory storage could be provided. Our main concern is exactly for this reason that if it was found during the detailed application stage that larger parts of the site would flood, this would require larger volumes of compensatory storage to be provided on-site and at this stage it is not known if the proposed development would be able to provide such required storage at the right level. Our view is that this should be determined now, because a decision is going to be made by the council by comparing this site with others. Otherwise, you will be comparing a site with known uncertainty with a site with its actual flooding risk already determined in detail. As it is clear that there are differences between the two model results, and you have not favoured the standard precautionary approach (i.e. accepting higher water levels until proven otherwise, as advocated in SPP), we believe that this should be assessed now and best estimate be determined (so that a like with like comparison can be made with other sites). This remains one of our main concerns. Based on the above, our views expressed in our previous correspondence remain. I trust items 1 to 6 above summarise the main points of our discussion. Please feel free to comment, in particular, please confirm item 6. Also, please feel free to comment on my after phone call note above. Kind regards, Yusuf This message may contain privileged or confidential information which is intended for the addressee only. If you have received this message in error, please destroy all copies in your possession or control and notify the originator immediately. Recipients may not forward, disclose or copy this message to any third party without the proper consent of Kaya Consulting Limited. Kaya Consulting Limited does not accept liability for direct, indirect or consequential damages arising from alteration of the contents of this message by others.