Engineering review on the final closure of Saemangeum Dike

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HR Wallingford has commenced an engineering review on the Final Closure of Saemangeum Dike. This draft final report represents the completion of the second phase of studies by HR Wallingford and is issued for review by KARICO and discussion with their representatives during meetings planned in Ansan during the second week of October 2005. The report contains some detailed matters which deserve consideration but the following overall conclusions are worthy of particular note: 1. Much of the work has been carried out by KARICO and RRI is of excellent quality and only deserves some small comments. However, there are a small number of issues that do require serious attention. 2. Scour either side of the existing bed protection will remain a problem and will become worse as velocities increase during the final phase of closure. We have considered the processes taking place and recommended that the bed protection be extended by a further 50 meters either side of the dike centre-line. 3. When estimating stable stone weights, the increases from estimated mind gap velocities to peak velocity, for example at the progressing ends of the closure bunds, has not been taken into account. We have applied appropriate speed up factors varying between 5% to 14% to allow for this, but the presence of flow asymmetry means that these increases may be exceeded. We have also allowed for high turbulence, which may be particularly evident in the vortex sheets emanating from the ends of the dikes. 4. We make recommendations for increases to the the stone weights and/or proportions of gabions to take account of these larger velocities. These changes are significant, requiring more heavy stone (up to 6t in weight) and higher proportions of gabions. In some cases modifications to the existing sill and bed protection will be necessary. Making appropriate modifications will require serious attention by KARICO in the following respects: i. To ensure that appropriate stability criteria have been adopted for all materials to be used. RRI have carried out very useful physical modelling, but not all material weights and combinations of gabions for bed protection, sill and closure bund were covered by this work. We have attempted to fill the gaps in understanding by the use of published stability formulae, but further physical modelling to confirm our results would be advisable. ii. To ensure that the financial and physical resources necessary to support these design and construction changes are put in place. 5. We have no particular recommendation to make on the issue of whether the March-April or April-May closure period is to be preferred. On the grounds of stability and wave overtopping, the later period is marginally more favorable, but this difference is not sufficient to require the use of the later period if the earlier period is preferred for construction or other reasons. 6. To the extent that information has been provided to us, procedures for construction appear to be satisfactory. 7. The problem of water leakage through the (extended) bed protection layer after final closure has been completed is significant. A strategy involving carefully timed pumping of gravel and sand into closure bund and bed protection layer is recommended.