Andre Theron

Evaluation of 10 cross-shore sediment transport/ morphological models

Abstract Cross-shore sediment transport models are used to model beach profile changes in order to determine, for example, coastal set-back lines, behaviour of beach fill and beach profile variations adjacent to coastal structures. A study was undertaken to evaluate ten of the most well-known mathematical cross-shore transport models with regard to different model requirements. The characteristics of these time-dependent models were investigated and the pros and cons of each are listed. The ranges of the data used to verify and calibrate these models are noted. It is concluded that the models can be classified generally into three groups with regard to their theoretical basis (re. mainly sediment transport) and the extent to which they were verified (re. mainly morphodynamics). These groups are termed the “best”, “acceptable” and “less suitable” groups. However, it is very important to consider the specific purpose of a model application. In some instances one model may perform better while for a different purpose another model may be better. Data are generally lacking for accretionary events and for erosion cases where the significant wave heights exceed 2.5 m. Aspects presently usually not included in these models are also listed. Without direct comparative prototype tests the final conclusion as to which are the better models in practice cannot be given. Furthermore models may be best applicable under different specific conditions. Models are also constantly being improved and thus a comparative evaluation of the models can only be completely accurate for a relatively short time.

Review of the field-data base for longshore sediment transport

A literature search was undertaken to collect field data on longshore sediment transport. This yielded a large number of data sets (273 points for bulk transport rates) from a variety of sites around the world. Data are especially lacking for transport rates exceeding 0.2 × 106 m3/year, significant wave heights higher than 1.8 m, sediment grain sizes coarser than 0.6 mm and beach slopes steeper than 0.06 (= 114). A point rating system was devised whereby the quality of the data could be assessed. The recording method and the accuracy thereof as well as the representativeness of the data were taken into account. It was found that the evaluation was done reasonably objectively and consistently. The data were divided into three categories. The highest score achieved in the evaluation was only 71% thus reflecting the difficulty of measuring longshore transport accurately. It is recommended that longshore transport formulae be calibrated against the data in the higher category (60% and better) and then be tested against all the other data. This will ensure that the formulae will be tested in as many different conditions and sites as possible without the lower quality data contributing to the calibration constants.

ACCURACY AND APPLICABILITY OF THE SPM LONGSHORE TRANSPORT FORMULA

A special reflecting wall 12 m long and 2.1 m high was built off the beach at Reggio Calabria, and 30 wave gauges were assembled before the wall and were connected to an electronic station on land. It was possible to observe the reflection of wind waves generated by a very stable wind over a fetch of 10 Km. The experiment aimed to verify the general closed solution for the wave group mechanics (Boccotti, 1988, 1989), for the special case of the wave reflection.Significant features on Wadden Sea wave climate are evaluated in respect of the state of the art. Main emphasis was laid on an analysis of the governing boundary conditions of local wave climate in island sheltered Wadden Sea areas with extensions being sufficient for local wind wave growth. Explanatory for significant wave heights a reliable parametrization of local wave climate has been evaluated by using generally available data of water level and wind measurements.

IMPROVEMENT OF THE MOST ACCURATE LONGSHORE TRANSPORT FORMULA

A study of alternatives including a shoreline evolution numerical modelization has been carried out in order to both diagnose the erosion problem at the beaches located between Cambrils Harbour and Pixerota delta (Tarragona, Spain) and select nourishment alternatives.

A Simple Normalized Difference Approach to Burnt Area Mapping Using Multi-Polarisation C-Band SAR

In fire-prone ecosystems, periodic fires are vital for ecosystem functioning. Fire managers seek to promote the optimal fire regime by managing fire season and frequency requiring detailed information on the extent and date of previous burns. This paper investigates a Normalised Difference α-Angle (NDαI) approach to burn-scar mapping using C-band data. Polarimetric decompositions are used to derive α-angles from pre-burn and post-burn scenes and NDαI is calculated to identify decreases in vegetation between the scenes. The technique was tested in an area affected by a wildfire in January 2016 in the Western Cape, South Africa. The quad-pol H-A-α decomposition was applied to RADARSAT-2 data and the dual-pol H-α decomposition was applied to Sentinel-1A data. The NDαI results were compared to a burn scar extracted from Sentinel-2A data. High overall accuracies of 97.4% (Kappa = 0.72) and 94.8% (Kappa = 0.57) were obtained for RADARSAT-2 and Sentinel-1A, respectively. However, large omission errors were found and correlated strongly with areas of high local incidence angle for both datasets. The combined use of data from different orbits will likely reduce these errors. Furthermore, commission errors were observed, most notably on Sentinel-1A results. These errors may be due to the inability of the dual-pol H-α decomposition to effectively distinguish between scattering mechanisms. Despite these errors, the results revealed that burnt areas could be extracted and were in good agreement with the results from Sentinel-2A. Therefore, the approach can be considered in areas where persistent cloud cover or smoke prevents the extraction of burnt area information using conventional multispectral approaches.

Detection of sinkhole precursors through SAR interferometry: First results from South Africa

Sinkholes are an unpredictable geohazard that endangers life and property in dolomitic terrains. They are a significant threat in Gauteng, South Africas most populated and urbanized province. Small scale surface subsidence is frequently present prior to the collapse of a sinkhole. Therefore, the presence of precursory surface deformation can be exploited to develop early warning indicators. Spaceborne SAR interferometry is able to monitor small-scale surface deformation over large areas and was exploited to detect and measure precursors to sinkhole development. The first results of DInSAR-based monitoring of areas associated with sinkhole development using TerraSAR-X is presented here. Subsidence activity was detected on three successive interferograms over a 55 day period. The subsidence basin was approximately 100 m in diameter with a maximum vertical displacement of 66.7 mm. Field surveys revealed tension cracks along the edges of the subsidence basin. Dramatically, four months after the subsidence event, a high pressure water supply pipeline burst meters downslope of the basin. The first results indicate that high resolution, X-band interferometry is able to monitor dolomite-induced instability in an urban environment.