Artificial headlands for coastal restoration
J. S. Mani Professor, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai 600036, 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 200 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 1998-99, 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 2 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
Coastal Environment V, incorporating Oil Spill Studies, C. A. Brebbia, J. M. Saval Perez & L. Garcia Andion (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-710-8 222 Coastal Environment V, incorporating Oil Spill Studies from the harbour basin (0.96 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
Chennai India Harbour
Figure 1: Coastal features in the study area.
3000 Shoreline-1999 Dredge fill Shoreline-2000 2800 Original coast Shoreline-2001 2600 Eroded coast
2400
2200
Distance from the baseline (m) from baseline the Distance 2000 0 500 1000 1500 2000 2500 3000 3500 4000
Distance along shore (m)
Figure 2: Observed coastline variations north of satellite harbour.
Coastal Environment V, incorporating Oil Spill Studies, C. A. Brebbia, J. M. Saval Perez & L. Garcia Andion (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-710-8 Coastal Environment V, incorporating Oil Spill Studies 223
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.
2 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 2.7m and 8.5s respectively. The corresponding values for southwest monsoon are 2.5m 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 0.5 and 1.5 x 105 cu.m. and during October to February, the rate varies from 0.5 to 2.5 x 105 cu.m. Northerly and southerly components of annual sediment transport along Chennai coast are estimated to be the order of 0.89 x 106 and 0.60 x 10 6cu.m, respectively. This results in net northerly drift of 0.3 x 106 cu.m / annum.
Hs [m]
4.0 Hm [m]
Wave dir [rad]