Evaluation of Flow Reduction due to Hydraulic Barrier Engineering Structure: Case of Urban Area Flood, Contamination and Pollution Risk Assessment

Abstract

In this study a vertical barrier forming the exclusion system in relation to partials extending into an impermeable stratum was analysed using a 3-D numerical modelling used to quantify the effect of a hydraulic barrier on flow which allows taking the anisotropy and heterogeneity of the site in a complex hydrogeological context and hydraulic barrier into account. A simulation in this study shows that for 0 % cut off the % reduction in flow is 0 and for 100 % cut off the % reduction in flow is 96, 94 and 92 % at 5, 10 and 15 days respectively due to leakage through the sheet piles, even with 100 % of the aquifer cutoff, the % of groundwater inflow impounded never reaches the 100 %. Also the change in trend where the % reduction in flow increases significantly with % cut off occurs at around 60 % cut off. That is, the reduction in flow through the aquifer only becomes significant after 60 % cut off by the sheet piles. The sensitivity analysis allows determining the factors of influence. A sensitivity analysis indicates the relationship appears relatively less sensitivity to varying the hydraulic conductivity, but very sensitive to the % cut-off. Therefore the effect of the sheet piles start to be significant after cut off exceeds 80 % and that the total profile length matters, i.e. the 60 % cut off must be applied to the whole width of the aquifer and not a portion of the aquifer, i.e. the minimum required to adequately reduce flow under the levee is 80 % cut off. From this study it can be derived that less water flows to a levee structure surrounded by sheet piles, depending on the depth of the sheet piles in proportion to the depth of the water bearing layer. The relationship is independent for the hydraulic conductivity but dependent on the ratio between the installation depth of the sheet piles beneath the Piezometric level and the depth of the bottom of the water bearing layer beneath the same Piezometric level. This study demonstrated the major influence of the technical design of the barrier on the simulated flow disturbances. The current approach can be applied elsewhere in related field for variety of application including formulating a resource management strategy, contamination containment and settlement risk. Overall, the capacity of decision makers to understand flow systems, how they function and respond to the placement of hydraulic barriers in the area they manage will form the basis for the operational management of the resources and infrastructure.