Shoreline changes and longshore transport at the Port of Ngqura

[摘要] ENGLISH ABSTRACT: The fixed sand bypass system at the Port of Ngqura was designed to match the natural longshore transportrate. Several factors, such as natural pebble presence and pump downtime, have resulted in thebypass system not matching the proposed transport rate. This shortfall has transpired to pressure fromparties with environmental interests arguing that, by missing the target, erosion would result along thenorthern beaches. Concern was also expressed by Transnet National Port Authority, the operators ofthe system, that the target was not a fair representation of the available sand for the bypass systemto pump. An investigation into the longshore transport rate and the Port of Ngqura's effects on thesurrounding shoreline thus became necessary.A review of existing literature revealed that the shoreline surrounding the port consists predominantlyof sandy beaches with underlying pebbles. The wave climate can be described by a dominant southwesterlyswell and a less frequent, but not insignificant, easterly swell. It was established that transportrates passing the Port Elizabeth Harbour are approximately 180 000 m³/year to 230 000 m³/year. Progressingnorthward, the stabilised shoreline resulted in limited sand supply from the northern beachesof PE and erosion has been witnessed at New Brighton Pier, amounting to approximately 160 000m³/year. At the Port of Ngqura it was concluded that the net transport rate can be estimated to 160000 m³/year to 200 000 m³/year in the northward direction, the latter being the chosen target rate.Studies of historical images and beach survey data determined that erosion in close proximity to the portwas limited, likely due to the proximity to the discharge point and a high level of pebbles. Progressingnorthward, erosion became more visible with the decrease of pebbles acting as a beach stabiliser. Onthe southern beaches, accretion was evident against the breakwater and extended to approximately 1.8km south of the port. The shoreline northward of the Swartkops River exhibited evidence of steadyerosion, believed to be as a result of the sand supply deficiency caused by the PE Harbour and stabilisedshoreline north of PE Harbour.Studies of the accretion concluded an approximate transport rate of 180 000 m³/year past the Portof Ngqura. Transport rates were also calculated along the southern shore as calibration data for theMIKE Littoral Process FM model.Longshore transport model simulations concluded an average annual net transport rate of 225 000m³/year in a northward direction. Considering all the results it was concluded that a transport rateof 200 000 m³/year accurately represented the transport rate at the Port of Ngqura. Studies into thevariability determined that, in order to match the natural transport rate, the average pumped rateshould fall between 180 000 m³/year and 220 000 m³/year after five years, and after 10 years, between190 000 m³/year and 210 000 m³/year.Furthermore, the cross-shore transport distribution was studied. The original and accreted profileswere modelled and the results compared. For the original profile, describing the shoreline prior to construction,it was determined that 92% of the transport occurred within the reach of the bypass pumps.It was then established that for the accreted profile, this amounted to only 74%. An investigationinto pump priority was conducted and described the shift in importance to seaward pumps due to theaccretion.Finally, recommendations were provided in accordance with the shoreline monitoring. Further topicsof study were also included.

[发布机构] Stellenbosch University