AN EMPIRICAL STUDY OF FLOOD WAVE IMPACT PRESSURESTO DETERMINE THE EFFECTIVENESS OF NEW SEAWALL DESIGNSUSING A DAM-BREAK APPROACH

Abstract

Coastal flooding and erosion, a major consequence of coastal natural events can result in physical devastation, threats to human health and safety, detrimental effects on ecosystems, and severe economic losses to individuals and to society. These potentially devastating consequences are therefore justifying efforts to reduce both their occurrence and severity. Seawalls of varying slopes with wave energy dissipaters to create various degrees of roughness on their surfaces have been proposed as a potential evolution in the design of coastal defences. This present study therefore aimed at investigating the energy dissipating ability, in terms of impact pressures, of newly designed seawalls which incorporate unique energy dissipaters, with the ultimate goal of predicting the effectiveness of these new designs. To achieve these aims, a novel technique for generating floodwater waves has been developed and applied to new seawall models. A Low Cost Wave Tank (LCWT) with water release gate mechanism (dam-break method) was primarily designed and constructed for this purpose. Apart from the smooth surface wall model, geo-grid materials of varying textures and grit sizes have been used to model different degrees of surface roughness with each model subjected to varying wave heights and wall angles. The experimental tool and technique had been found to be effective and relatively economical while the gate release system represented a good approximation of instantaneous dam-break problem. The innovative imaging system (IS) and the sensor signal capture (SSC) techniques used for estimating flow velocity were found to be in close agreement with the commonly used PIV method, thus, could be a useful laboratory scheme for analysing hydrodynamics model studies. The study has also found the location of maximum impact pressures for the vertically inclined smooth surface wall model, to be varied from that of sloping forms which is in agreement with the propositions of most previous researchers. The maximum impact pressures have been found to be about 1.4 to 40 times the hydrostatic pressure which is within the range previously suggested by other researchers. Also, angle 75o appeared to be the best sloping position for the wall models investigated and in general, IMACTS wall seemed the most superior surface in terms of energy dissipation in vertical form. This implies that the harder the surface of the defence wall or/and the higher the degree of surface roughness the greater the energy of the floodwater waves that would be dissipated. Again, the predictive model equations proposed in this study are useful for the purposes of assessment of the suitability of the seawalls and the mitigation against flood hazards.