Berm height at temporarily open/closed estuaries in South Africa: analysis and predictive methods

[摘要] ENGLISH ABSTRACT: This study investigates the berm crest elevation at South African Temporarily Open/Closed Estuaries (TOCE), as well as the processes involved in berm growth, and the drivers that contribute to variation in berm height among estuaries. The relationship between wave runup elevation and maximum berm height at estuaries is evaluated. Additionally, the study presents suitable methods for the prediction of berm height at South African TOCEs, given the limited data availability.TOCEs along the wave dominated coastline of South Africa are subject to frequent inlet closure. During inlet closure, the presence of the wave built sand barrier (berm) restricts tidal influx and temporarily prevents catchment runoff from reaching the sea. The elevation of the inlet berm dictates the peak flood level in the estuary. A comprehension of estuary mouth behaviour, specifically the berm building processes present after estuary closure, is of paramount importance for the efficient management of these systems. This includes knowledge and quantification of the berm building processes, potential berm height and berm height variability.The recorded berm crest elevations of twenty prominent TOCEs along the South African coastline are presented. Several years of berm/mouth survey data and estuary water levels have been analysed for the selected locations, resulting in an extensive record of historical berm crest elevations. This provides improved estimates of the probable berm height at these estuaries, especially compared to previous estimates typically based on limited survey data.The primary drivers responsible for high berms and variation in berm height among estuaries were identified, viz. median sediment grain size, beach face slope, nearshore wave height and nearshore Iribarren number. The relationship between the berm height at the selected estuaries and the relevant coastal parameters were assessed. The beach face slope and the nearshore Iribarren number have a significant influence on the maximum berm height, and adequately describe the variation in berm height among estuaries. A multi-criteria analysis – the Berm Crest Elevation Criteria – and corresponding linear regression model is developed to investigate the relative importance of the dominant coastal parameters on maximum berm height. Additionally, the Berm Crest Elevation criteria provides an accurate first estimate of the maximum berm crest elevation at other, less studied TOCEs, based on only a few coastal input parameters.The vertical extent of wave runup is assessed to determine the potential limit of berm accretion. Existing runup parameterisations are implemented to simulate several years of wave runup elevation at the selected estuaries, based on recorded sea levels and offshore wave data. The predicted wave runup elevation provides an accurate estimate of the long-term variation of estuarine berm height. The Stockdon et al. (2006) wave runup parameterisation provides superior performance across the entire range of estuaries.The occurrence probability of the simulated wave runup elevation records were assessed to further elucidate the probability of wave runup associated with maximum berm height at estuaries. The findings indicate that the maximum berm height can be predicted by the 5% exceedance probability of wave runup. A theoretical threshold of runup exceedance probability and associated berm response is presented. Additionally, a design scenario of wave runup is proposed to estimate the vertical extent of sediment deposition caused by wave runup. The design scenario is based on the 2% exceedance probability significant wave height, 50% exceedance probability peak wave period and Mean High Water Spring (MHWS) tidal elevation.Lastly, a berm growth model is presented to predict berm height/growth on a short-term time scale. The model provides an incremental prediction of the morphodynamic response of estuarine berms subjected to wave runup and overwash.

[发布机构] Stellenbosch University