The cross-shore distribution of grain size in the longshore transport zone

[摘要] ENGLISH ABSTRACT: Calculation of longshore sediment transport rates is a typical part of coastal engineering work.One of the important inputs to such calculations is the sediment grain size. A single,representative grain size is typically required. The inter-tidal beach is the most convenient andcommon area from which grain size data can be obtained. Yet only a fraction of the longshoretransport occurs at the beach, with the bulk of the transport occurring in the surf zone, wheresampling is difficult. Sediment transport calculations can be improved if the representativegrains size is also characteristic of this area. A better understanding how the grain size in thelongshore transport zone compares to the beach grain size is required.A review of relevant literature indicates that limited attention has been given to quantifyingthe grain size in the longshore transport zone. No previous investigations were found thattried to link the longshore transport zone grain size to that found on the beach. Acomprehensive analysis of beach and longshore transport zone grain sizes was thereforeundertaken and is described in this thesis.Beach grain sizes were compared to those in the longshore transport zone for three differentlocations around the world: Published grains size information, together with detailed waveand profile data, was obtained from the US Army Field Research Facility at Duck; a seconddata set was obtained from measurements done at Bogenfels in Namibia; a third dataset wascompiled from sampling undertaken by the author in Table Bay, South Africa. A total of 189samples were collected at four sampling lines in Table Bay between September 2005 andSeptember 2006. Samples were collected across the entire profile from the primary dune to awater depth of 10 m. Samples were collected by grab in the offshore, and by swimming anddiving in the surf zone. The location of the four Table Bay sampling lines was chosen so as toobtain data from beaches with different wave and grain size characteristics. Together with theDuck and Bogenfels data, data from six different beaches was therefore available for study.A settling tube was used to determine the grain sizes. Verification of the settling tube analysesagainst conventional sieving indicated a good comparison. However, the settling tube provedunsuitable for processing of samples with coarse to very coarse material, for which sievingwas conducted instead.The grain size at the mid-tide level has been used to characterise the beach. The limits of thelongshore transport zone were defined by calculating the cross-shore distribution of longshoretransport with the Unibest model. Simpler methods, such as the depth of closure, eitheroverestimated or underestimated it, depending on which wave condition was used in the depthof closure formula.It was found that the beaches with steeper mid-tide beach slopes, such as Bogenfels andnorthern Table Bay, had coarser median grain sizes than more gently sloping beaches such asfound in the south of Table Bay. On energetic beaches, the mid-tide beach grain sizes weresignificantly coarser than those in the surf zone, by more than twice. At less exposedlocations, such as Duck and the central Table Bay beaches, this difference was less. Atsheltered locations, such as the southern sampling lines in Table Bay, the mid-tide beach grainsizes are virtually the same as those found in the surf zone. The surf similarity parameter wasused to compare the characteristics of the different sites. This parameter was defined using theaverage wave height seaward of breaking, and the mid-tide beach slope. The ratio between thelongshore transport zone grain size and the mid-tide beach grain size was found to be similarto the inverse of the surf similarity parameter for the six beaches that were studied.These findings have led to an improved understanding of the grain size in the longshoretransport zone and allow a better characterisation of the representative grain size to use forsediment transport calculations.

[发布机构] Stellenbosch University