This can be accomplished by constructing two jetties at either side of the mouth (see Fig. In order to protect existing infrastructure and property at the eroding side of the mouth and accommodate shipping (yachts fishery vessels), these entrances may be fixed at a certain position. Natural river mouths and coastal inlets fronted by barrier spits typically have entrances that migrate over time (see Sect. Compare the discussion in The Netherlands of the possible impact of an extension of the Maasvlakte (Port of Rotterdam) on the processes in the Wadden Sea.įunction 2: Stabilizing a natural river mouth or coastal inlet Figure 10.11: Stabilised river mouth. These aspects must be taken into account in the decision making process. These impacts may be felt at rather large distances from the structures due to the low settling rates. Long structures extending into open sea have an impact not only on the coastal morphology but may also affect the transport patterns of silt and e.g. The Sandbar Breakwater concept consists of making the inevitable growing sandbar updrift the basis for the port protection, while restoring the natural sand balance by repetitive nourishments downdrift, at the Sand Engine (Van der Spek et al., 2020). Along the Gulf of Guinea coast the longshore sediment transport is unidirectional and obstructions such as a breakwater, result into strong sedimentation updrift and continuous coastal erosion on the downdrift side. Image courtesy of CDR International B.V.įigure 10.10 shows a nature-based port solution on the West African coast, which has been named the Sandbar Breakwater. Figure 10.10: Westward looking aerial photo of the Sandbar Breakwater including the sand en- gine at Lekki, Nigeria. With a well-chosen layout of the breakwater, the currents can be guided such that the nuisance for shipping operations will be acceptable and scour is reduced as much as possible. The contraction may further lead to local scour (possibly undermining the stability of the breakwaters). Contraction of the currents (high velocities) just near the entrance to the port hampers the safe entering and departing of the port by vessels. This sedimentation/erosion pattern is avoided with the implementation of an adequate sand bypass system.Īlso, the further the breakwaters extend seaward, the more the (tidal) currents are affected (see Sect. On the downdrift side, erosion and coastline retreat will occur (see also Sect. Since wave-induced longshore sediment transports are interrupted, sedimentation and coastline accretion on the updrift side is expected. The impact of long structures on the morphodynamics of the adjacent coasts can be very large. The optimum length of the breakwaters follows from a balance between the construction costs of the breakwater and the maintenance costs of the entrance channel. This is because large sedimentation may occur in the unprotected channel and dredging operations are less efficient due to wave action. Here, the maintenance of an artificial channel is more expensive than for the situation in between the breakwaters. In many cases, the approach channel extends seaward of the end of the breakwaters. The breakwaters of the Dutch port of IJmuiden extend about 2.5 km into the sea, therewith (partly) preventing sedimentation in the approach channel. Generally, dredging is required to maintain an approach channel at the required depth for shipping. Material transported along the coast will accumulate against the jetty on the updrift side. This jetty or breakwater should extend at least through the breaker zone, even during storms, and even after the coast has moved forward as a result of accretion. When a longshore transport threatens to cause sedimentation in an entrance channel to a harbour or a river, this process can be interrupted by constructing a jetty slightly ‘updrift’ from the harbour or river (where the sand is coming from). \)įunction 1: Blocking the longshore transport of sand, which would otherwise settle into a dredged approach channel
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |