Beach recovery and studies in accretive sediment transport

Abstract

Erosion has been a focal point in sediment transport research while sediment accretion has been neglected until recent times. This thesis addresses some components of accretive sediment transport through modelling and new experimental measurements.The existing Grab and Dump model (Nielsen, 1988), a simple sediment transport model over rippled beds, is generalised to reflect a wider range of flow conditions, including acceleration skewed flows, boundary layer streaming, sheet flows, and cases with superimposed current. The revised model is compared to another semi-empirical sediment transport model and is found to have similar predictive abilities for small scale laboratory data. The results show that there are limitations to the model and more work is required to accommodate flows with no acceleration skewness and under the sheet flow regime.The local approximation method for non-linear irregular waves (Nielsen, 1989) is updated for converting surface elevation data into velocity, as many sediment transport models require velocity time series as input while surface elevation data are more readily measured and available. Data from three experiments are used to investigate this method, which was previously used to convert pressure measurements to surface elevation. The results are compared to the spectral transfer method, and found to perform similarly in most locations apart from near the breakpoint, where it performs better due to an additional multiplier which accounts for the increasingly negative timemean velocities closer to shore.New beach profile experiments were undertaken with an emphasis on the beach profile at equilibrium under random waves. The beach profiles were subjected to waves of various heights, with each case run until the profile appeared to be at equilibrium, for both accretive and erosive conditions. It was observed that the profile under accretive conditions behaves in a cyclic manner, where the outer bar is destroyed and recreated from the inner bar moving offshore, a process which also has a much longer time scale compared to the profile evolution under erosive conditions. The experimental profile measurements are used to evaluate an equilibrium type total sediment transport model utilising the relationship between the dimensionless fall velocity parameter and cross-shore bulk sediment transport proposed by Baldock et al. (2011). The model is applied to both laboratory and field measurements, and compared to an existing equilibrium shoreline model. The model is found to perform well under laboratory conditions where the profiles reach equilibrium but faces limitations with field measurements, which is an area for future work.ii Declaration by author Contributions by others to the thesisNo contribution by others. Statement of parts of the thesis submitted to qualify for the award of another degreeNone.iv

Entrainment and Transport of Well‐Sorted and Mixed Sediment Under Wave Motion

“…The results were consistent with the threshold value of 𝐴𝐴 𝐴𝐴𝑚𝑚𝑚𝑚𝑚𝑚𝑑𝑑50𝜔𝜔∕𝑤𝑤𝑠𝑠 = 0.038 (Dohmen-Janssen et al, 2002). Lower net transport rate of WS-M under waves with high velocity skewness is also consistent with the previous measurement of coarse (d 50 = 0.465 mm) and fine (d 50 = 0.125 mm) sand under velocity skewed flows (analyzed by Shimamoto (2016)). Interestingly, for the BM consisting of only 30% coarse fraction, the trend of the net transport rate as a function of <U 3 > is similar to that of WS-C.…”

Laboratory beach profile dynamics and responses to changing water levels with and without nourishment

“…The Gourlay number or dimensionless fall velocity, Ω, (Gourlay, 1968 and has been repeatedly demonstrated to be a useful parameter for predicting beach states and profile response (e.g. Nayak, 1970;Hattori and Kawamatta, 1980;Wright and Short, 1985;Shimamoto, 2016, Baldock et al, 2017 which has the form:…”

“…Some consider equilibrium to be achieved in just a couple of hours (Nayak, 1970;Srisuwan et al, 2014), while others reported several (Dalrymple and Thompson, 1974;Shimamoto, 2016),…”

“…Therefore, wave conditions for the present experiments were mostly chosen through experience gained from previous flume experiments at similar scales and with the same or similar sediment, where distinct barred or bermed profiles were observed to develop under known conditions (e.g. Shimamoto, 2016;Baldock et al, 2017).…”

Coarse Sand Transport Processes in the Ripple Vortex Regime Under Asymmetric Nearshore Waves

“…In this present study, only the coarse sand case is studied (coarse sand corresponding to a median grain diameter greater than 0.400 mm). Shimamoto (2016) shows the influence of sand grain size on transport behavior in the reference frame of different rippled beds and various hydrodynamic conditions, mostly with Figures 3–1 (page 49 from Shimanoto, 2016). This study shows that offshore transport occurs for high velocity skewness in the fine sand case, whereas offshore transport occurs for low asymmetry in the coarse sand case.…”