Modelling nourishments through the Durban piers: Research of sediment transport and surf conditions

Modelling nourishments through the Durban piers: Research of sediment transport and surf conditions

Bronsvoort, K.
Radermacher, M.
Van der Spek, B.J.
Spruit, R.
Van Tongeren, B.

Vella, G.

Civil Engineering and Geosciences

Hydraulic Engineering

2011-11-08

The Durban bight has a very dynamic and variable coastline which is constantly under attack by the Indian Ocean. To protect the Durban harbour from this energetic ocean, already in 1855 breakwaters have been constructed to protect the harbour and prevent sedimentation inside. The construction of these breakwaters and eventually extending these breakwaters towards the sea has influenced the sediment flow in front of the Durban beaches enormously. The natural sediment flow along the coast from South to North has been interrupted and this has changed the surroundings. The beaches of Durban are constantly eroding and nourishments are necessary every year to restore the beaches and preserve the safety of the citizens of Durban. The Durban Harbour has agreed to dredge all the available sand that is trapped by the breakwaters and deliver the sand back into the beach system. The Municipality of Durban has responded to this development by constructing a sand distribution system at the beaches, to nourish the sand in an optimal way. New nourishment pipelines have been constructed through three piers at the central part of the Durban coast to supply the sand directly in the surf zone. Although the rough sea is responsible for the erosion at the beaches, the sea also provides nice surf conditions. If the bathymetry has the right shape, perfect surf waves can appear. A combination of nourishments done by the Durban Harbour and the construction of the new nourishment pipelines can lead to the necessary change in bathymetry, resulting in the perfect surf waves. To investigate the consequences of the nourishments on the surf conditions and the movement of the sediment, a Delft3D computer model has been set up. Different nourishment scenarios have been investigated, as well as the available recorded data regarding the tide, the wave and the wind conditions. The total input data have been reduced in size towards representative parameters and converted to corresponding Delft3D input files. After running the constructed model with different scenarios for a simulation period of 15 days, the results have been checked on two different aspects. First of all the effect of the nourishments through the piers on the bathymetry is analysed. An overview of the most important model outcomes is bundled in the additional document ‘Modelling nourishments through the Durban piers, Figures’. The nourishments tend to move onshore and therefore into the coastal cells between the piers. Offshore transport doesn’t seem to be an issue for the dominant wave conditions, but during north-easterly and north-north-easterly waves it is advised not to carry out nourishments. The just nourished sediment might be transported offshore in that situation. The added sediment is partly used to fill up scour holes just north of each pier, but it also leads to the creation of a shallow area north of the tip of each pier. When every year a volume of 250,000 m3 of sand is supplied to the Durban bight, the coastal cells remain more or less in equilibrium, given that 40% of the total amount is nourished through the piers. The second part concerns the effects on the surf conditions. When analysing the physical wave parameters, no major changes can be noticed. However, the model is only run for 15 days, during which one nourishment is simulated. The results do show some subtle trends, which could lead to a bigger influence when successive nourishments are carried out. Waves start to break at the newly created shallow areas during low tide. This has a positive effect on the surf conditions, since the waves will break in a more favourable way. In case of multiple nourishments, the shallow areas will probably become shallower, so then the positive effect can be noticed during high tide as well. Furthermore, the orientation of the coastline between the piers will obtain a slight obliqueness with respect to the dominant wave direction. This causes the waves not to break at once along the whole wave crest, but more gradually from one side to the other, so that surfers can ride a wave uninterruptedly for a longer period of time. Finally a number of recommendations is given, which result from the whole process that the project has gone through. A few adjustments to the model might lead to a major improvement of the performance. Two of the most important recommendations concern a more detailed way of modelling the piers in the model and the inclusion of wind.

South Africa
surf reef
swell
artificial beach nourishment

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TU Delft, Section Hydraulic Engineering

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