Great Brak Island - measures for flood defence

[摘要] ENGLISH ABSTRACT: This thesis firstly presents the analysis of large flood events, causing low-lying property in and around the Great Brak estuary to be inundated. The nature of several South African estuaries renders adjacent properties vulnerable to river and ocean related flooding. An important objective of the study is to develop a feasible form of flood defence for the Island, a residential development situated in the lower estuary basin, 250 m from the estuary mouth. The Island regularly experiences inundation due to high water levels in the estuary which have partially driven the need for the artificial manipulation of the estuary mouth berm using millions of litres of potable water.The lowest property on the Island is situated at +2.2 m MSL whereas the highest flood level ever recorded in the estuary was +2.9 m MSL. This extreme water level was achieved by an equivalent 1 in 10-year flood, flowing into the estuary and coinciding with a closed estuary mouth. Research into climate change predictions for regional overland precipitation trends and for sea level rise have been included in the study. The sea level is predicted to rise by about 1 m in the year 2100 whereas the frequency of extreme floods is set to increase for the catchment area in the next century. Large storm events may cause direct wave attack on the shorefront properties of the Island due to the rising sea levels and the increase in storminess. Much higher extreme water levels are therefore expected in the estuary and a form of flood defence is therefore sought; also, to reduce the current need for 'wasteful water releases to artificially induce open mouth conditions.Various possible measures for flood defence have been identified for application either directly around the Island, in the surf-zone to dissipate wave energy or upstream of the estuary, at the Wolwedans Dam to attenuate large floods from rainfall in the catchment. A Multi-Criteria Analysis approach has been followed to objectively identify the preferred flood defence measure. The evaluation criteria were based on the hydraulic -, environmental-, and economic performance of the proposed flood defence measure. The Multi-Criteria Analysis identified a combined solution of an Armoured Dike- and Rock Revetment structure directly around the Island, to be the preferred solution.The proposed solution was conceptually designed for various lifetimes and extreme flood conditions to find the least expensive option. An order of magnitude cost estimate for the construction of the proposed solutions was derived, which formed the basis of comparison to the costs foreseen if no flood defence measure is implemented (the Do-Nothing alternative). The study found that the proposed solution becomes economically feasible if designed for a lifetime of 33 year and more. The most attractive solution was found to be the combination of a +3.0 m MSL crest level Armoured Dike and a +5.0 m MSL crest level Rock Revetment, which is estimated to cost R95 million to construct.

[发布机构] Stellenbosch University