Dunes Growth Estimation for Coastal Protection Muhammad Zikra Department of Ocean Engineering, Faculty of Marine Technology, ITS, Kampus ITS Keputih Sukolilo, Surabaya 60111 Abstract: This paper describes the investigation of dunes growth estimation caused by Aeolian sand transport based on field survey and empirical model formulation. The empirical model is based on considerations of the turbulent kinetic energy relationship as proposed by Coastal Engineering Manual, 2000. The empirical method is then tested against field data collected on the Espiguette spit, France. The result indicated that the winds blowing from the dunes or back beach towards to the shoreline are not efficient in transporting sand. This is because of sheltering provided by the dune itself or there is a limited supply of sand landward of the dunes. Keywords: wind, dunes growth, sand transport INTRODUCTION The transport of sand by wind (Aeolian sand transport) is an important component part in the dunes growth (Carter, 1988; Pye and Tsoar, 1990). These sand dunes are important for coastal protection to provide protection from flooding due to high-water levels, shoreline erosion and wave overtopping due to storm. These dunes often result from the natural accumulation of windblown sand originating on the beach face. However, they may also be man-made. Dunes can be made artificially by: (a) beach nourishment, (b) grading existing sand available on the dry beach, or (c) removing sand from below the high-water line during low tide and using it to construct a protective dune (beach scraping). In addition, sand transport under wind is known a continual and natural process that is often bringing significant change on the beach area. Wind transport can cause the removal of sand or its redistribution within the littoral zone and gives influence on dunes formation and evolution. Onshore winds carry sand from the beach and deposit it in backshore marshes, in developed backshore areas, or in natural or man-made dunes, while the offshore winds carry sand from the beach into the sea or lake. In Rhone Delta area, the wind-blown sands are responsible for big input and output of sand on accretion beach called the Espiguette spit at Rhone Delta, Mediterranean Sea (Sabatier, 2001). This condition must be controlled to protect the stability of dunes as coastal protection structure for the area behind it. For that reason, it is important to be able to quantitatively predict how much sand will be transported by wind at a given coastal site, which direction the sand will be transported and where it will be deposited. In order to understand the process, the procedures for calculating wind-blown sand transport in the Espiguette spit will be derived in this paper. The use of empirical theoretical equation of Aeolian sediment transport is one way to quantify and predict the sand movement under wind forcing. 181
METHODOLOGY The research was performed using data collected at sandy spit on the eastern part of the Rhone Delta, France (at Espiguette spit), which shows accretion and erosion since several decades (Figure 1). Based on field measurement the characteristic of the beach is known as sandy beach with nominal diameter, D 50 = 0.20 mm and the astronomic tide amplitude is of +/- 0.30 m. Wind data were obtained from Coungeron Station, France for interval period from 1961-1995 (Zikra et.al 2007). This wind data were measured by anemometer located at 10 m above the ground. Percentages of average wind speed and wind direction is given in table 1. Because there is a limitation on data especially moisture content of sand (no available data on daily precipitation data and monthly evaporation data records) then the sand transport will be calculated in dry condition and wet condition. Table 1. Wind Climate data from 1961-1995 Figure 1. The coastline at Rhone Delta, France (Adopted from Sabatier and Provansal, 2000) Direction F0= F0= F0= F0= F0= F0= F0= F0= F0= F0= F0= F0= Total F0>34 Degree 0-1 24 5-7 8-1 11-13 14-16 17-19 20-22 23-24 25-27 28-30 30-33 (%) 0 0.229 2.397 2.258 1.093 0.361 0.173 0.036 0.016 0.001 0.000 0.000 0.001 0.000 6.556 20 0.212 1.936 1.363 0.387 0.054 0.022 0.008 0.000 0.000 0.000 0.000 0.000 0.000 3.981 40 0.221 1.912 0.812 0.185 0.077 0.013 0.001 0.001 0.000 0.000 0.000 0.000 0.000 3.222 60 0.237 2.120 0.818 0.293 0.105 0.030 0.003 0.003 0.000 0.000 0.000 0.000 0.000 3.609 80 0.271 3.067 1.569 0.869 0.408 0.132 0.027 0.012 0.000 0.001 0.000 0.000 0.000 6.357 100 0.334 3.026 1.758 1.182 0.481 0.253 0.078 0.018 0.004 0.004 0.000 0.000 0.000 7.139 120 0.371 3.084 2.559 1.957 1.043 0.454 0.098 0.042 0.011 0.009 0.001 0.000 0.001 9.632 140 0.088 0.599 0.427 0.187 0.059 0.075 0.004 0.004 0.001 0.000 0.000 0.000 0.000 1.445 160 0.185 1.066 0.625 0.245 0.075 0.013 0.003 0.001 0.000 0.000 0.000 0.000 0.000 2.215 180 0.132 0.901 0.403 0.120 0.028 0.005 0.001 0.000 0.000 0.000 0.000 0.000 0.000 1.591 200 0.186 0.936 0.328 0.080 0.028 0.007 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.564 220 0.255 1.517 0.550 0.133 0.038 0.005 0.004 0.000 0.000 0.000 0.000 0.000 0.000 2.502 182 Neptunus, Vol. 14, No. 2, Januari 2008: 181-188
Direction F0= F0= F0= F0= F0= F0= F0= F0= F0= F0= F0= F0= Total F0>34 Degree 0-1 24 5-7 8-1 11-13 14-16 17-19 20-22 23-24 25-27 28-30 30-33 (%) 240 0.186 1.610 0.992 0.333 0.066 0.022 0.004 0.000 0.000 0.000 0.000 0.001 0.000 3.214 260 0.139 1.163 1.370 0.721 0.209 0.053 0.018 0.004 0.001 0.001 0.000 0.000 0.000 3.679 280 0.177 1.792 2.079 1.618 0.576 0.201 0.023 0.009 0.001 0.000 0.000 0.000 0.000 6.475 300 0.201 2.263 3.420 2.735 1.045 0.367 0.057 0.011 0.001 0.001 0.001 0.003 0.000 10.105 320 0.262 3.026 4.361 3.703 1.913 0.882 0.202 0.084 0.019 0.011 0.000 0.000 0.000 14.462 240 0.252 2.967 3.633 2.828 1.486 0.797 0.197 0.065 0.011 0.004 0.004 0.000 0.000 12.243 Total (%) 3.936 35.381 29.324 18.671 8.053 3.504 0.764 0.270 0.051 0.032 0.007 0.005 0.001 100.0 RESULT AND DISCUSSION Aeolian sediment transport Many models have been proposed to predict Aeolian sediment transport rates (Bagnold, 1941; Kawamura, 1951; Lettau and Lettau, 1977; White, 1979). In this paper, the equation as proposed by Coastal Engineering Manual (CEM), 2002 is used to calculate Aeolian sediment transport. This wind blown sediment transport equation is based on considerations of the turbulent kinetic energy relationship which given by the formula 3 u*. q = K (1) g D where q represents sand transport rate in gm/cm-s, u * shear velocity, g acceleration of gravity, D mean sand grain diameter and K dimensional Aeolian sand transport coefficient. Value of K is a function of sand grain diameter which can be expressed by K = e -9.63 + 4.91 D (2) where D is in millimeter and K in grams per centimeter per second Equations (1) can be used to estimate sand transport rates for given wind speeds and mean sand-grain diameters. It is based on the data which include transport data for mean sand grain diameters up to 1.0 mm; consequently, the equations should not be used to estimate transport on beaches with mean grain diameters greater than about 1.0 mm. This equation can be recast into a dimensionless form, which allows it to be used with any consistent set of units. The revised equation is given by q v a ρ a 3 ' u = K * (3) g.d in which ν a represents the kinematics viscosity of the air and ρ a the mass density of the air. The dimensionless coefficient K' is given by K = e -1.00+4.91 D (4) in which D represents the median grain diameter in millimeters. Equation (3) reduces to original equation (1) when ν a = 0.147 cm 2 /sec and ρ a = 0.00122 gm/cm 3 are substituted into it. The results of the Aeolian sediment transport analysis are tabulated in tables below. Table 2 presents the summary of the analysis results obtained by not considering moisture conditions (precipitation or evaporation) and Table 3 shows analysis of sand transport with assumption that Dunes Growth Estimation for Coastal Protection.. 183
the precipitation is higher than evaporation (wet condition). The result indicated that the total transport is reduced from 44.61 m 3 /m-yr in dry condition become 14.54 m 3 /m-yr in wet condition. This result indicated that moisture content conditions are very important in Aeolian transport calculations. Table 2. Summary of wind blown sediment transport for dry condition Wind Direction Direction in which sand is transported (m3/m) North Total S = 12.77 North East Total SW = 0.82 East Total W = 3.75 South East Total NW = 5.54 South Total N = 0.33 South West Total NE = 0.33 West Total E = 4.08 North West Total SE = 17.01 TOTAL 44.61 Table 3 Summary of wind blown sediment transport for wet condition Wind Direction Direction in which sand is transported (m3/m) North Total S = 5.31 North East Total SW = 0.00 East Total W = 0.78 South East Total NW = 1.56 South Total N = 0.00 South West Total NE = 0.00 West Total E = 0.78 North West Total SE = 6.10 TOTAL 14.54 (a) 184 Neptunus, Vol. 14, No. 2, Januari 2008: 181-188
(b) Figure 2. (a) Wind-blown sand transport rose at dry condition, (b) Wind-blown sand transport rose at wet condition The results of the potential sand transport by wind at Rhone Delta beach (at Espiguette spit) are also shown in the form of a transport rose as shown on Figure 2. The transport rose shows the direction from which the sand is transported with the most being transported to the South and South East (seaward direction) Dune growth prediction The efficiency of sand transport on dunes growth is depending on the proportional to the square of the cosine of the angle between the wind direction and a vector perpendicular to the shoreline (CEM, 2002). Sand transport rates in the offshore direction, perpendicular to the general orientation of the dune toe, are given by equation q = q cosα cos 2 β, 180 o < β < 360 o where β is the angle the wind makes with the shoreline and α is the angle the wind makes with a vector perpendicular to the shoreline. Therefore, α = β 90 o and Therefore, cos (α) = cos (β - 90 o ) = - sin (β) q = - q sinβ cos 2 β, 180 o < β < 360 o The sin β term in the equation corrects the transport from the wind direction to a direction perpendicular to the shore, while the cos 2 β term is the efficiency term introduced to consider the sheltering effects of the dune. The result of dune growth estimation is presented in Tables 4. The table shows that winds blowing seaward from the dunes are not very efficient in transporting sand from the dunes back onto the beach (dune growth = 2.45 m 3 /m/year). This condition prevails at Espiguette spit, where much of the area landward of the beach is wetlands. In this case, we need stabilization of dune to control the landward movement of wind-blown sand into developed areas. Stabilization can be achieved using vegetation or sand fencing. The best type of stabilizing methods varies with geographical area, location on the dune, exposure of the site, whether the water body is salt or fresh water, etc. Dunes Growth Estimation for Coastal Protection.. 185
In present conditions at the Espiguette spit, the Ganivelles stabilisation method (Figure 3) can be improved and combined using vegetations method as sand trapping to stabilize the dunes and coastlines. In addition, when designing a dune system as coastal protection structure, the dunes should be set back ( ± 70m) from the shoreline (Figure 4) so that there is sufficient dry beach to provide a source of sand. Table 4. Estimated annual dune growth at rhone delta (Espiguette spit) Wind Direction Annual Transport Wind Angle Cos (a) Efficiency Dune growth (m3/m) β sin β cos2 β (m3/m/year) North 12.77 318-0.6691 0.552-4.72 North East 0.82 3 0.0523 1.000 0.04 East 3.75 48 0.7431 1.000 2.79 South East 5.54 93 0.9986 1.000 5.54 South 0.33 138 0.6691 1.000 0.22 South West 0.33 183-0.0523 0.997-0.02 West 4.08 228-0.7431 0.447-1.35 North West 17.01 273-0.9986 0.003-0.05 TOTAL 2.45 Figure 3. Dune conditions at Espiguette spit 186 Neptunus, Vol. 14, No. 2, Januari 2008: 181-188
CONCLUSION Figure 4. Reconstructing the dune using ganivelles and vegetation The result indicated that strong seasonal winds are responsible for big inputs and outputs of sand on the Espigette spit, Southern France which known as Tramontane and Mistral. The winds blowing from the dunes or back beach towards the shoreline which this winds blowing are not efficient in transporting sand. This is because of sheltering provided by the dune itself or there is a limited supply of sand landward of the dunes. In addition, the predictions of aeolian sediment transport could be significantly improved by implementing the models with environmental conditions such as moisture content, grain size distribution, micro-topography, salt cementation, shells remains, on the shear velocity, the threshold velocity and the roughness length. REFERENCES Bagnold, R.A. (1941), The physics of blown sand and desert dunes, 265 p. Bagnold, R.A. (1973), The nature of saltation and bed-load transport in water, Proc.R. Soc. London, Ser. A332, 473-504. Carter, R.W.G. (1988), Coastal Environments, Academic Press, 617 p. CEM. (2002), Coastal Engineering Manual, Department of Army Corps of Engineering, Washington, DC. Kawamura, R. (1951), The study of sand movement by wind, HEL 2-8, University of California Hydraulics Engineering Laboratory, Berkeley. Lettau, K. and Lettau, H. (1977), Experimental and micro-meteorological field studies of dune migration. In : Exploring the World s Driest Climate, Ed. Lettau and Lettau, University of Wisconsin Press, Madison. Pye, K. and Tsoar, H. (1990), Aeolian sand and sand dunes, London, 395 pp. Sabatier, F. (2001), Fonctionnement et dynamiques du littoral du delta du Rhône, Thesis, University of Aix-Marseille III, 272 p. Sabatier, F., and Provansal, M. (2000), Sandbar morphology of the Espiguette spit, Mediterranean Sea, France, Marine Sandwave Dynamics, Lille, France, pp.179-187. Dunes Growth Estimation for Coastal Protection.. 187
White, B.R. (1979), Soil transport by winds on Mars, Journal of GeophysicalResearch, 84, 4643-4651. Zikra, M. Bari, M.A. Min, G and Luong, C. (2007), Morphology changes in the Camargue, France, Group work Report, UNESCO-IHE Institute for Water Education, Delft, 169 p. 188 Neptunus, Vol. 14, No. 2, Januari 2008: 181-188