Artificial headlands for coastal restoration

Transcription

1 Artificial headlands for coastal restoration J. S. Mani Professor, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai 636, India Abstract Construction of a satellite harbour 15 km north of Chennai harbour has resulted in erosion on the down drift coast as this stretch of coast experiences depleted sediment supply during the southwest monsoon. Due to the geometry of the coast, the coast receives a marginal quantum of sediment during the northeast monsoon. To protect the coastline, new concept involving construction of artificial headlands is suggested instead of adopting conventional coastal protective structures, such as groynes, seawalls etc. This paper discusses the numerical model studies carried out to design the configuration of the artificial headlands to suit the prevailing wave and sediment transport characteristics. The studies suggest an optimum length of projection of 14 m for the headlands and an optimum spacing of m between the headlands to derive good results. Further the paper discusses the comparison of the results with a pair of groynes. Keywords: headlands, coastal restoration, erosion, accretion, beach 1 Introduction A satellite harbour was constructed during , 15 km north of Chennai harbour for handling coal and other products. Figure 1 shows the location of the harbour and the geometry of the adjoining coast. The region of interest experiences wave approach from northeast during October till February and from southeast during March till September. As the harbour is projecting into the sea for an effective length of about 1.5 km., the sediment transport induced by these waves is intercepted by the harbour, thereby affecting the equilibrium of the neighbouring coast. Though the coast on the south of the harbour experiences accretion, the north coast suffers damage due to erosion. Figure shows the coast on the north of the satellite harbour and the shoreline variations observed over the last few years. In order to stabilize the coastline, the dredged material & L. Garcia Andion (Editors) 4 WIT Press, ISBN

2 Coastal Environment V, incorporating Oil Spill Studies from the harbour basin (.96 m.cu.m) was deposited adjacent to the north breakwater expecting that this the dredge fill would help in maintaining the equilibrium of the north coast. As the erosion continued, alternate means of protecting the coast was thought necessary. Pulicat Lake Satellite harbour Ennore Creek Fisheries harbour India Chennai Harbour Figure 1: Coastal features in the study area. Distance from the baseline (m) 3 Dredge fill Shoreline Shoreline- Original coast Shoreline-1 6 Eroded coast Distance along shore (m) Figure : Observed coastline variations north of satellite harbour. & L. Garcia Andion (Editors) 4 WIT Press, ISBN

3 Coastal Environment V, incorporating Oil Spill Studies 3 This paper discusses on constructing artificial headlands as a method of maintaining the coastal equilibrium as against the conventional approaches involving construction of either rubble mound seawalls or groynes. Environmental characteristics The east coast of India experiences northeast monsoon during October-February and southwest monsoon during March-September. The wave height distribution observed at 17 m water depth off Chennai coast (fig.3) suggests that during northeast monsoon maximum wave height and wave period are of the order of.7m and 8.5s respectively. The corresponding values for southwest monsoon are.5m and 8s respectively. The rate of sediment transport along the coast Chandramohan et al. [1] indicate that the littoral transport is towards the north from March to September and towards the south from October to February. During March-September, the monthly transport rate varies between.5 and 1.5 x 1 5 cu.m. and during October to February, the rate varies from.5 to.5 x 1 5 cu.m. Northerly and southerly components of annual sediment transport along Chennai coast are estimated to be the order of.89 x 1 6 and.6 x 1 6 cu.m, respectively. This results in net northerly drift of.3 x 1 6 cu.m / annum Hs [m] Hm [m] Wave dir [rad] Wave freq [rad/s] VARIABLE MONTH Figure 3: Wave characteristics off Chennai coast. 3 Numerical model studies Figure 4 shows the shape of the artificial headlands obtained by stacking rubble of specific size in the required alignment. With the wave characteristics shown in figure 3 and adopting the following dimensions for artificial headland, numerical exercise was carried out to predict a) wave characteristics Ebersole [] b) wave height distribution within the surf zone Fredsoe and Deigaard [3] c) sediment transport in the surf zone Horikawa [4] and d) change in beachfront Horikawa [4]. & L. Garcia Andion (Editors) 4 WIT Press, ISBN

4 4 Coastal Environment V, incorporating Oil Spill Studies Length of the beach pocket: 7m. Length of each headland: m Number of headlands: Length of projection (Li) : 1,15 and m. Clear distance between the headlands: 3 m Wave approach SEA SIDE θ q sw Li Accretion ----le----- Artificial Headland Erosion la Coastline m. 5m. 7m LAND SIDE Figure 4: Artificial headlands and associated shoreline change. 3.1 Basic equations adopted for the studies Wave refraction and diffraction The nearshore wave characteristics were predicted based on the following equations Ebersole [] with the assumptions that the sea bed slope is small, waves are linear, harmonic and irrotational.. s cc cc 1 H H 1 H g H g (1) = k H x y cc g x x y y ( H cc g s cos θ ) + ( H cc g x y s sin θ ) = () ( s sin θ ) ( s cos θ ) = x y (3) with s : gradient of the wave phase function, k : wave number, H : wave height, C : celerity, Cg : group velocity and θ : wave angle Wave height distribution in the surf zone Wave height distribution between the wave breaking point and the coast was determined based on the following expression Fredsoe and Deigaard [3] H / D = exp(. 11 y / D b ) (4) with H : Wave height at given depth, D : Depth, y : Distance inshore of the breaking point, D b : breaking depth. & L. Garcia Andion (Editors) 4 WIT Press, ISBN

5 Coastal Environment V, incorporating Oil Spill Studies Sediment transport Computation of the sediment transport due to waves was based on the following expressions Horikawa [4] A w q wx = Q ub cosθ (5) w qwy = Qw ub sinθ (6) Qw = A w ( τ τcr ) / ρg (7) = B w / (1 λ ) s sgd ) f / (8) w ( o v w with q wx, q wy : sediment transport rate in x and y directions, θ : wave angle with respect to y axis, τ : available shear stress, u b : amplitude of orbital velocity near the sea bed, w o : fall velocity of the sediment, λ v void ratio, f w : wave friction factor, B w : a constant, d: sediment diameter, s: relative buoyant density of the sediment (ρ s -ρ)/ρ Shoreline advance and recession The expression given by Horikawa [4] for predicting the shoreline change is as follows y y + t / D )( q q ) (9) t = t 1 ( s x i i + 1 with Y t : shoreline position at any time t, Y t-1 : shoreline position at any time t-1, t : time step, q i : rate of long shore sediment transport at cells (i), q i+1 : rate of long shore sediment transport at cell (i+1), D s : Depth x : cell width. Results on the shoreline advance and recession with length of projections of 1 m, 15 m, and m are shown in figures 5 to 8 area and volume of accretion and erosion are given in the respective figures. 8 8 Area of advancement : 4.5 sq.m Volume of advancement : cu.m Advancement of shoreline Distance along coast (m ) Figure 5: Shoreline advance with Li=1 m. & L. Garcia Andion (Editors) 4 WIT Press, ISBN

6 6 Coastal Environment V, incorporating Oil Spill Studies 1 1 Area of advancement : sq. m 75 Volume of advancement : cu. m Advancement of shoreline Figure 6: Shoreline advance with Li=15 m. 1 1 Area of advancement : sq.m 8 Volume of advancement : cu.m Advancement of shoreline Figure 7: Shoreline advance with Li= m. 8 8 Area of erosion : 4.7 sq.m Volume of erosion : 37.5 cu.m Shoreline recession Figure 8: Shoreline recession with Li=1,15 & m. 4 Optimum configuration of artificial headlands Optimum configuration of artificial headlands was derived based on the quantum of sediment retained and bypassed by the headlands. To determine the bypassing & L. Garcia Andion (Editors) 4 WIT Press, ISBN

7 Coastal Environment V, incorporating Oil Spill Studies 7 capacity, it is assumed that the long shore sediment while being transported along the curved face of the sea wall is partly directed towards the offshore and the rest bypassed (fig.4). The quantum of sediment moved (a) offshore (b) bypassed were determined based on the following equations. θ min offshore ( sw ) θ max θ min qalongshore ( qsw * cosθ ) θ max q = q * sin θ dθ (1) = dθ (11) where q sw : Sediment transport along the headland. θ : the angle which varies from θmax to θmin as the beach advances in front of the headland. Variation in volume of sediment retained and bypassed for projection lengths Li of 1m.,15m.,and m. (fig. 9) suggests an optimum length of projection of 14m. for the artificial headlands. The length of beach experiencing accretion (la)and erosion (le) (fig. 4) for above projection lengths given in Table 1 suggests that for the optimum length of projection of 14m., clear distance of about m. between the headlands should be adequate for coastal restoration. Volume of sediment (cu.m.) Length of projection in m.(li) retained bypassed Figure 9: Optimum length for the artificial headland. Table 1: Lengths of accretion and erosion. Length of projection La Le 1m. 145m. 35m. 15m. 165m. 35m. m. 17m. 35m. 5 Comparison with groyne pocket Results of the numerical exercise conducted for the groynes with the same spacing of 3 m adopted for artificial headland are shown in figures 1 and 11. & L. Garcia Andion (Editors) 4 WIT Press, ISBN

8 8 Coastal Environment V, incorporating Oil Spill Studies Comparison between the groyne and the artificial headland (Table ) suggests that the volume of the beach that would develop with an optimum length of projection of 14m. would be about 37% more than the groyne pocket and there is no remarkable difference in the pattern of erosion. Average width of beach that would develop with artificial headlands is of the order of 1m over a sea front of 1 m (fig.6) whereas with groyne the average width of beach is less than 5m. over the sea front of 1 m (fig.1). 1 1 Area of advancement : sq.m 8 Volume of advancement : cu.m Advancement of shoreline Figure 1: Beach development with groyne. 8 8 Area of erosion : 4.7 sq.m Volume of erosion : 37.5 cu.m Shoreline recession Figure 11: Beach erosion with groyne. Type Table : Area and volume of beach development. Length Area of Volume of Area of (m.) accretion accretion erosion (sq.m.) (cu.m) (sq.m.) Headland Headland Headland Groyne 1,15 & Volume of erosion (cu.m.) & L. Garcia Andion (Editors) 4 WIT Press, ISBN

9 6 Advantages of artificial headlands 1. Advantage with the artificial headland is that with the reversal of the waves during the subsequent monsoon, the beach developed adjacent to the headland is first eroded exposing the rubble sea wall to experience angular wave attack and the beach behind it is guarded for the remaining period of the monsoon.. Offshore transport of beach sand is reduced as the beach is protected by the rubble sea wall. 3. Conventionally, short groynes (of length less than 15 m) are not preferred as they are not effective in restoration of the coast whereas artificial headlands with short length of projection (14 m in the present case) would be sufficient for coastal restoration. 7 Conclusions 1. Artificial headlands with length of projection of 14m. would be adequate to hold about 7 cu.m. of sand as against 53 cu.m with groyne of almost same length whereas the volume of beach sand eroded is of the same order.. Average width of beach that would build with artificial headlands is of the order of 1m over a sea front of 1m. whereas with groyne the average width of beach is less than 5m. over the sea front of 1m. References Coastal Environment V, incorporating Oil Spill Studies 9 [1] Chandramohan, P. Nayak, B.U. & Raju, V.S., Longshore-transport model for south Indian and Sri Lankan coasts. ASCE Journal of Waterway, Port, Coastal and Ocean Engineering 116, 48-44,199. [] Ebersole, B.A., Refraction diffraction model for linear water waves, ASCE Jl. Of Waterway, Port, Coastal and Ocean Engineering, 111. No 6. Pp ,1985. [3] Fredsoe, J. & Deigaard, R., Mechanics of Coastal Sediment Transport, World Scientific publications, Singapore, 199. [4] Horikawa, K., Nearshore Dynamics and Coastal Processes Theory, Measurement and Predictive Model. University of Tokyo Press,1988. & L. Garcia Andion (Editors) 4 WIT Press, ISBN