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

Soltau, Christoph (2009-12)

Thesis (MScEng (Civil Engineering))--University of Stellenbosch, 2009.


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 and common area from which grain size data can be obtained. Yet only a fraction of the longshore transport occurs at the beach, with the bulk of the transport occurring in the surf zone, where sampling is difficult. Sediment transport calculations can be improved if the representative grains size is also characteristic of this area. A better understanding how the grain size in the longshore transport zone compares to the beach grain size is required. A review of relevant literature indicates that limited attention has been given to quantifying the grain size in the longshore transport zone. No previous investigations were found that tried to link the longshore transport zone grain size to that found on the beach. A comprehensive analysis of beach and longshore transport zone grain sizes was therefore undertaken and is described in this thesis. Beach grain sizes were compared to those in the longshore transport zone for three different locations around the world: Published grains size information, together with detailed wave and profile data, was obtained from the US Army Field Research Facility at Duck; a second data set was obtained from measurements done at Bogenfels in Namibia; a third dataset was compiled from sampling undertaken by the author in Table Bay, South Africa. A total of 189 samples were collected at four sampling lines in Table Bay between September 2005 and September 2006. Samples were collected across the entire profile from the primary dune to a water depth of 10 m. Samples were collected by grab in the offshore, and by swimming and diving in the surf zone. The location of the four Table Bay sampling lines was chosen so as to obtain data from beaches with different wave and grain size characteristics. Together with the Duck 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 analyses against conventional sieving indicated a good comparison. However, the settling tube proved unsuitable for processing of samples with coarse to very coarse material, for which sieving was conducted instead. The grain size at the mid-tide level has been used to characterise the beach. The limits of the longshore transport zone were defined by calculating the cross-shore distribution of longshore transport with the Unibest model. Simpler methods, such as the depth of closure, either overestimated or underestimated it, depending on which wave condition was used in the depth of closure formula. It was found that the beaches with steeper mid-tide beach slopes, such as Bogenfels and northern Table Bay, had coarser median grain sizes than more gently sloping beaches such as found in the south of Table Bay. On energetic beaches, the mid-tide beach grain sizes were significantly coarser than those in the surf zone, by more than twice. At less exposed locations, such as Duck and the central Table Bay beaches, this difference was less. At sheltered locations, such as the southern sampling lines in Table Bay, the mid-tide beach grain sizes are virtually the same as those found in the surf zone. The surf similarity parameter was used to compare the characteristics of the different sites. This parameter was defined using the average wave height seaward of breaking, and the mid-tide beach slope. The ratio between the longshore transport zone grain size and the mid-tide beach grain size was found to be similar to 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 longshore transport zone and allow a better characterisation of the representative grain size to use for sediment transport calculations.

AFRIKAANSE OPSOMMING: In kusingenieurswese word langstrandse sedimentvervoer (langsvervoer) gereeld bereken. Belangrike invoer vir sulke berekeninge is die verteenwordigende korrelgrootte van die sand. Die sandkorrelgrootte van die benatte strand is die maklikste om te bepaal, al vind net ‘n klein gedeelte van die langsvervoer op die benatte strand plaas. Die grootste gedeelte van die langsvervoer kom in die brandersone voor, waar dit moeilik is om monsters te neem. Sedimentvervoerberekeninge kan verbeter word as die korrelgrootte ook hierdie sone verteenwoordig. Dit is dus van belang om te verstaan hoe die korrelgrootte van die benatte strand met die gemiddelde korrelgrootte oor die algehele langsvervoersone vergelyk. ‘n Literatuursoektog dui aan dat min aandag al gegee is aan hoe die korrelgrootte op die benatte strand met dié in die langsvervoersone vergelyk. ‘n Ontleding van sandkorrelgroottes op die benatte strand en in die langsvervoersone is dus onderneem en word in die tesis uiteengesit. Strandkorrelgroottes word met dié in die langsvervoersone vergelyk vir drie verskillende gebiede in die wereld : (1) Gepubliseerde data oor korrelgroottes, sowel as strandhellings en golwe, is van die Field Research Facility by Duck in die VSA verkry ; (2) ‘n tweede stel data is van opmetings by Bogenfels in Namibie verkry; (3) die derde stel data is saamgestel deur die skrywer tydens opmetings in Tafelbaai, Suid-Afrika. ‘n Totaal van 189 monsters is tussen September 2005 en September 2006 by vier verskillende opmetingslyne in Tafelbaai geneem. Monsters is geneem van die duin tot in 10 m waterdiepte. In dieper water is grypmonsters geneem, terwyl dié in the brandersone verkry is deur te duik. Die posisie van die vier Tafelbaaise opmetingslyne is gekies om strande met verskillende golftoestande en korrelgroottes te dek. Tesame met die data van Duck en Bogenfels is dus ses datastelle vir ontleding beskikbaar. ‘n Valbuis is gebruik om die korrelgroottes te bepaal. Die valbuismetode het goed met gewone sifresultate vergelyk, behalwe vir monsters met baie growwe korrels. Hiervoor is sifwerk gebruik. Die korrelgrootte van die benatte strand is gebruik om die strand te karakteriseer. Die grense van die langsvervoersone is bepaal deur die dwarsstrandse verdeling van die langsvervoer met die Unibest-model te bereken. Eenvoudiger metodes, soos die berekening van die sluitingsdiepte, het minder betroubare resultate gelewer. Daar is gevind dat steiler benatte strande, soos Bogenfels en Noord-Tafelbaai se strande, groter gemiddelde korrelgroottes as platter benatte strande, soos dié in suidelike Tafelbaai, het. Op blootgestelde strande is die benatte strand se korrelgrootte tot meer as twee keer groter as dié in die brandersone. By gedeeltelik beskutte strande, soos Duck en Sentraal- Tafelbaai se strande, was die verskil minder. Op beskutte strande, soos die Suid-Tafelbaai se strande, is gevind dat die benatte strand se korrelgrootte amper dieselfde as dié in die brandersone is. Die brandergelyksoortigheidsfaktor is gebruik om die verskillende strande te vergelyk. Die golfhoogtes net buite die branderlyn en die benatte strandhelling is gebruik om die veranderlike te bereken. Daar is gevind dat by die ses strande wat ondersoek is, die verhouding van die korrelgrootte in die langsvervoersone en dié van die benatte strand ongeveer gelyk aan die omgekeerde brandergelyksoortigheidsfaktor is. Die bevindings het tot ‘n beter begrip van die wisseling van die korrelgroottes in die langsvervoersone gelei. Gevolglik kan ‘n akkurater verteenwoordigende sandkorrelgrootte bepaal word.

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