Physical Model Studies on Wave Propagation Along Approach Channel and Its Effects on Harbour Tranquility

Abstract

Development of harbour basin along open coasts need to be connected to the sea with sufficient depth by dredging approach channels of required base width, side slopes and length . The side slopes of such channel also need to be maintained for no sliding condition under water. The side slopes of the channel mainly depend on the seabed material and its stable slope in the submerged conditions. The length of the channel depends on the sea bed bathymetry of the site proposed for development. While finalizing the channel alignment and its geometry, an in-depth understanding of the factors responsible for stability and the effects on the wave propagating along the approach channel and its effects on harbor wave tranquility are very important. 3-D shallow basin hydraulic physical models studies provides a very useful support for the decision to be taken in this regard under complex hydraulic conditions. The wave attenuation along port approach channel is responsible for the dispersion of the wave energy along the side slopes of the channel to outer regions. Due to this phenomenon total amount of wave energy entering into harbour basin will be reduced and this results in achieving the overall wave tranquility inside the harbor basin. This natural phenomenon of wave attenuation can be effectively utilized for optimization of the harbour layout resulting in reducing the lengths of breakwaters, reduction in the lengths of approach channel by orienting it perpendicular to the sea bed contours wherever possible. This effectively reduces the overall cost of investment of the project and in some case reduces the recurring cost of maintenance dredging as well. Hydraulic physical model studies are conducted to investigate the wave propagation along port approach channel by varying the side slopes of the channel, which effects of wave attenuation. The effects on wave propagation for regular and random waves are studied. The comparison of studies with regular and random wave generations are made. It is observed that the wave attenuation effectively results in 87%, 90%, 92% and 93% for channel side slopes of vertical, 1:5, 1:10, 1:20 slopes respectively. The wave energy dispersion increases with the increase in the channel dimensions. The available model studies data at CWPRS for the stage wise development of port over a period of about fifty years was collected .This data has revealed the effects of wave attenuation for different lengths and depths of channel. It is observed that with the increase in channel dimensions there is increase in wave attenuation effects. Based on these studies the suggestions made for the port development is successfully working in field. This can be effectively used by port planners for arriving at good conceptual layouts. Apart from wave heights the direction of the wave is another very important factor at the berthing locations inside the harbour basins for effective loading and unloading operations. Broad side wave incidence on the berthed vessels reduces the efficiency of operation and in some extreme cases damages the vessels also. The cost of the mooring devices mainly depends on the ship motions at berth, this in turn depends on wave incidence angle on the vessels. Thus wave directional spread studies for finalizing the berth alignment is an essential part of studies for port development. Studies are conducted to observe the directional spread of waves generated by 2-D wave generator in the model along the approach channel and adjacent shallow regions. The directional spread for waves approaching from different directions making different incident angle with the sea bed contour are observed by sketching the wave crests in the model. The use of 2-D long crested wave generators in 3-D shallow basin model for wave tranquility studies is widely adopted till date. This is mainly due to its amicability and ease with which it can be used and its low cost effective as compared to 3-D paddle type wave generators. Model studies conducted by various experts have revealed that use of such wave generators are fully justified from the point of port planning Hughes (1993). The 2-D wave generated will propagate over the complex bathymetric conditions in the model and gradually develops into 3-D wave state and resembles the prototype. The factors effecting the change of wave from 2-D to 3-D status viz., wave approach angle with the sea bed contours, presence of port structures like breakwater, approach channel, berthing structures etc., Based on these studies some useful conclusions were drawn for a model engineer in deciding the size of the model for port development studies. The wave tranquility studies conducted on the model for development of berths at different locations in the harbour basin, the effects of wave height for waves approaching from different directions, effects of wave incident angle at berthing location, the usefulness of wave incidence angle on decision making about the berthing alignment are discussed based on model results. The effects of providing sloped surface on the rear side of a berthing face, maintaining spending beaches within the harbour basin effectively facilitating wave run up and wave energy absorption. From the findings of the model studies it is suggested to utilize the natural wave attenuation phenomenon in aligning the port approach channel. This effectively increases the wave tranquility in the harbour basin. Based on wave tranquility studies for different berths in the harbour the advantages of providing sloping face behind berthing structure and effect of maintaining spending beaches within harbour basin in maintaining good wave tranquility are highlighted. The results of directional propagation studies conducted are helpful for a model engineer to select the boundary of physical model to be simulated depending on the sea bed contour and wave incidence angle. The studies conducted will be very useful for port engineers in general planning of the port layouts.