Sand mass density and borrow material compatibility for beach nourishment

Ocean & Shoreline Management 13 (1990) 89-98

Sand Mass Density and Borrow Material Compatibility for Beach Nourishment G. G. Garland Department of Geographical and Environmental Sciences, University of Natal, King George V Avenue, Durban 4001, Republic of South Africa (Received 13 February 1989; accepted 12 May 1989)

A BS TRA CT Design of successful and cost-effective beach nourishment schemes requires a sound understanding of the hydraulic character of both borrow material and native beach sand. Techniques for assessing borrow material compatibility exist, but have proven inadequate in some situations. Most methods use a particle size parameter to represent sand transportability, although hydraulic theory and the nature of post-erosional lag deposits on Durban beaches suggest that mass density may also be a factor of importance. Results of a flume test on natural and artificially constituted samples of Durban beach sand showed that transport of sand particles was unrelated to median particle size, but inversely correlated with sample mass density. The study concludes that where beaches consist of mineralogically heterogeneous material, a density factor should be included in any attempt to assess sediment transport rates and borrow material compatibility.

INTRODUCTION As a m a n a g e m e n t technique, beach nourishment has certain engineering and environmental advantages over other m e t h o d s of erosion control. In particular, the need for p e r m a n e n t sediment retention structures on beaches is r e d u c e d to a minimum ~ and risk of total or catastrophic failure is slight. 89 Ocean & Shoreline Management 0951-8312/90/$03.50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland

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However, beach nourishment is costly, and it is important that any nourishment scheme is carefully designed and implemented. Dean 2 has noted that cost-efficiency of nourishment schemes is in large measure dependent on the ability to predict the littoral performance of available fill or borrow material, in order to compare it with native beach sand. To this end, a number of methods for fill performance prediction have been developed. The three most popular of these are Krumbein and James' o v e r f i l l ratio, 3 refined by Dean, 2 which determines the volume of borrow material needed to retain permanently 1 m 3 of beach sand; the r e l a t i v e retreat rate 4 which is the ratio between rates of landward retreat of nourished and unnourished beaches; and the r e - n o u r i s h m e n t f a c t o r 5 which indicates the frequency with which re-nourishment must take place. Campbell et al. 6 found that neither the relative retreat rate nor the overfill ratio could successfully predict nourishment requirements for beaches in Durban; and in evaluating a beach nourishment scheme on the Gold Coast of Australia, Chapman ~ claimed that all such critical ratio concepts were simplistic and unsoundly based. Central to these approaches is the assumption that sand particle transportability in the littoral system can be adequately represented by a particle size parameter (such as the median particle size (Ds0)), or a parameter representing particle size distribution (like the Phi sorting coefficient). 4 Indeed, with respect to sand in the active zone of a beach area, James 5 observed that 'The average residence time for grains depends on their size, so that system behaviour is dependent on the textural properties of the constituent materials.' And it was claimed by Chapman ~ that ' . . . coarser non-cohesive sediment is less likely to be moved by any given wave than fine sediment...' Although these statements might hold true for monomineralic sands of uniform density, they may not be correct for material of heterogeneous mineral composition, especially when varying combinations of heavy and light minerals are involved. The literature on sediment transport mechanics makes it quite clear that when materials of different densities are concerned, a density parameter must be included in any quantitative estimate of sediment transport rate (e.g. Ref. 7). From observations on Durban beaches after periods of severe erosion, it appeared that post-erosional lag deposits consisted mainly of accumulations of heavy minerals (Fig. 1), much of the lighter fraction having been removed by wave and current action. This led to the decision to carry out laboratory tests based on the hypothesis that, as far as Durban beach sands are concerned, the mass density of sediment is a better predictor of sand transportability than Dso.

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Fig. 1. Heavymineral concentration after an erosive event. C H A R A C T E R I S T I C S OF D U R B A N B E A C H SAND Sand on Durban beaches is typically straw-coloured, with a median diameter ranging between 200 ,um and 300 ~m. 6 Composition is variable: quartz is normally the dominant mineral, although feldspar can form a major constituent. The presence of significant quantities of ilmenite, magnetite and zircon was recorded by King & Maud, s and Cooper & Mason 9 found ilmenite, magnetite and garnet to be the major constituents. Blackshaw ~'' measured heavy mineral contents between 0-26 and 10.5% by mass.

LABORATORY PROCEDURE Komar and Miller ~ showed that parameters which determine particle transport and the threshold of sediment motion under wave action are similar to those defined by Shields (1936--in Ref. 11) for unidirectional current, This proposition was later confirmed by Swart & Fleming s " theoretical analysis. In the present study, rates of sediment transport were tested in a small, open-ended flume, attached to a vibration unit in order to minimize surface armouring. Three 10 g specimens of each sample were subjected to unidirectional current of 5-0 m min -~ for i min. All sediment emerging from the end of the flume

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was collected, dried and weighed, and calculated as a percentage of the original sample. Two series of tests were carried out. The first was to establish the relationship between heavy mineral content, and mass density in Durban beach sands, and to assess the effects of mass density on particle transport under conditions of controlled particle size. To this end, nine samples were artificially constituted from two natural samples of Durban sand, one with 4% and the other consisting of 97% minerals with densities in excess of 2-86g ml -t. In the first sample, all heavy minerals were removed by suspension in bromoform; the second sample consisted of the natural 4% heavy mineral sample; and the others were varying combinations of the two natural samples, designed to produce a sample set in which heavy mineral content increased by intervals of approximately 10%. Although it had been intended to maintain a uniform value for Dso in all samples, this proved impossible since the heavy mineral component was in all cases very much finer than the rest of the material, and as it increased, the value for Dso became progressively smaller. Relationships between heavy mineral content, mass density and particle size are shown in Fig. 2. For the second series of tests, 26 samples were collected between the high and low tide marks at various points along the beach between the northern harbour pier and Snake Park (Fig. 3). Since the sand pumping scheme commenced operation in 1982,13 the material on Durban beaches consists of an admixture of borrow material and native sand; these samples should be representative of most sand presently in the system. Values of mass density for the sample set varied from 2.34 to 4.17 g ml -~, and Dso ranged from 108 m to 377 m (Fig. 4). Shell fragments were removed by passing the material through a 2 mm sieve, and values for Dso were determined by sieving; mass density was ascertained by displacement of water; heavy mineral content was determined by separation in bromoform with a specific gravity of 2.86.

RESULTS AND DISCUSSION For the artificially constituted samples, the percentage of material transported to the end of the flume varied between 1.2 and 9%. Figure 5 shows a clear inverse relationship between the amount transported and mass density, and a similar link between transport and heavy mineral content. Conversely, D~o exhibited a strong positive relation-

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ship with the amount transported--that is, greater quantities of material were moved from the samples with larger particle sizes. Similar trends were observed in the natural samples, where the amount transported was highly correlated with the log of mass density (r =0-832; p = 0-001); while there was a weak, positive relationship (r=0.237, not significant) between amount transported and Dso (Fig. 6). Inverse relationships between mass density and particle transport conform to established theory relating to the mechanics of sediment transport, where particles of low volume and surface area but high mass are more resistant to entrainment and transport than whose with low mass but high volume and surface area. However, the positive relationship between Ds0 and transport is contrary to hydraulic theory, in which larger particles are considered to be less erodible than fine material. For this study, the answer to this conundrum lies in the relationship between particle size and mineralogy. The fine fraction of Durban sands is dominated by heavy minerals such as magnetite (with an average density in the order of 5.3 g ml-1), and ilmenite (whose density is between 4.5 and 5gml-l). Coarser material consists mainly of quartz with some feldspars, with densities ranging from 2-5 to 2.65 g ml-L The small, dense particles of heavy

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Fig. 6. Relationships between sand transport under test conditions and (a) mass density; (b) median particle size, for the natural samples. CONCLUSIONS The results show that mass density has an important, and possibly dominant, influence on the hydrodynamic performance of Durban beach sands. Median particle size inadequately represents sediment transportability; and in beach materials with a large proportion of fine-grained heavy mineral particles, the assumption that high values of Dso lead to greater stability may be invalid. Further, since the reverse could be true, this might lead to serious errors in computation of overfill ratios, relative retreat rates, re-nourishment factors and the like. Where nourishment schemes are planned for beaches of mineralogically heterogeneous sediments, sand mass density must be taken into account during prediction of borrow material performance. REFERENCES 1. Chapman, D. M., Beach nourishment as a management technique. In Proceedings of the 17th Coastal Engineering Conference, (Vol II). American Society of Civil Engineers, New York, 1980, pp. 1636-48.

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2. Dean, R. G., Compatibility of borrow material for beach fills. In Proceedings of the 14th Coastal Engineering Conference, (Vol H). American Society for Civil Engineers, New York, 1974, pp. 1319-49. 3. Krumbein, W. C. & James, W. R., A lognormal size distribution model for estimating stability of beach fill material. Technical Memorandum no. 16, US Army Coastal Engineering Research Center, Fort Belvoir, Virginia, 1965. 4. James, W. R., Beachfill stability and borrow material texture. In Proceedings of the 14th Coastal Engineering Conference, (Vol II). American Society for Civil Engineers, New York, 1974, pp. 1334-44. 5. James, W. R., Techniques in evaluating suitability of borrow material for beach nourishment. Technical Memorandum no. 60, US Army Coastal Engineering Research Center, Fort Belvoir, Virginia, 1975. 6. Campbell, N. P., Macleod, D. C. & Swart, D. H., Bypassing and beach nourishment scheme at Durban. In Proceedings of the 26th International Navigation Congress, Permanent International Association on Navigational Conferences (PIANC), Brussels, 1983, pp. 1-12. 7. Yalin, M. S., Mechanics of Sediment Transport. Pergamon Press, Oxford, 1972. 8. King, L. C. & Maud, R. M., The Geology of Durban and Enoirons. Department of Mines, Geological Survey of South Africa, Pretoria, 1964. 9. Cooper, J. A. G. & Mason, T. R., Barrier washover fans in the Beachwood mangrove area, Durban, South Africa: Cause, morphology and environmental effects. J. Shoreline Mgt, 2 (1986) 285-303. 10. Blackshaw, J., An analysis of fluvial and beach sediment in the lower Mgeni river and immediate beach vicinity. Honours dissertation, Dept. Geographical and Environmental Sciences, University of Natal, Durban, 1985 (unpublished). 11. Komar, P. D. & Miller, M. C., Sediment threshold under oscillatory waves. In Proceedings of the 14th Coastal Engineering Conference, (Vol I). American Society of Civil Engineers, New York, 1974, pp. 756-75. 12. Swart, D. H. & Fleming, C. A., Longshore water and sediment movement, In Proceedings of the 17th Coastal Engineering Conference, (Vol. H). American Society of Civil Engineers, New York, 1980, pp. 1275-1294. 13. Garland, G. & Kruger, D., Durban's beach reclaimed. The Geographical Magazine, 82 (11) (1985) 608-11.