دانلود رایگان مقاله شیب بستر و وابستگی اقلیمی موج متوسط حمل و نقل شن و ماسه

Abstract

We analyse measured 46 year changes in nearshore bathymetry to quantify rates of net shore-normal sand transport (qx) in 10–20 m depths at northern Gold Coast, Australia. These are significant both in understanding local disequilibrium shoreface evolution and as prototype data that may be used to assess theoretical predictive methods. This analysis is feasible because the migration history of a nearby river mouth has formed a disequilibrium shoreface lobe that is evolving towards an equilibrium profile shape, which is identified in adjacent non-lobe profiles. The data include various profile transect surveys from 1966 to 2012, sediment characteristics and locally recorded wave conditions. A generally applicable form of qx that is a function of both a depth-dependent horizontal bed transport forcing component and bed slope qx = qx(h,S) is developed and quantified in terms of long term average behavior for this site. Dependence of the forcing component on wave climate is shown from higher than average qx derived from the lobe response during a recent five year period with abnormally high waves. Our qx(h,S) quantifies the balance between generally shoreward wave boundary layer sand transport and the opposing effect of the seabed slope, as incorporated in the predictive methods.

7. Discussion

7.1. Depth, slope wave dependence of qx A database of long term surveys documenting progressive evolution of disequilibrium lobe profiles at depths of 10–20 m has been analysed to provide average rates of shoreward sand transport (qx) under the wave conditions prevailing at the northern Gold Coast. The analysis These factors have been quantified for this particular location in terms of both long term (41–46 years) shoreface response and a shorter recent five year period during which there was an unusually high occurrence of storm waves of Hs N 4–6 m compared with the long term average. The lobe response during the period of relatively high waves shows that qx increases with increased wave energy through the depth-dependent forcing component qx(h,0). The analysis shows that the shoreface equilibrium profile has a best fit value A ≈ 0.23 (m1/3) for the shape of form h = Axn with n = 2/3. However, it indicates the possibility that n ≈ 0.746 for which the equivalent A = 0.131 (m0.235). Observed qx(h,0) decrease with increasing water depth approximately as qx(h,0) = Ce−0.42h where, with qx is expressed as bulk m3 /m/year, C varies with wave conditions prevailing during the period considered. C ≈ 5000 m3 /m/year for the long term average 1966–2007. However, C is higher at 6,500 m3 /m/year for the extended period 1966–2012 and ≈16000 m3 /m/year for the 5 years 2007–2012 due to the unusually severe wave conditions in 2009.