Quantitative Analysis of Sediment Transport in a Coastal Beach System

Thesis Abstract

Coastal beach systems are increasingly subject to dynamic sediment fluxes driven by waves, tides, and anthropogenic perturbations, presenting critical implications for shoreline evolution, habitat viability, and coastal infrastructure. The study addresses the persistent gap in quantitatively linking hydrodynamic forcing to sediment transport pathways and storage within a natural beach system, with implications for forecasting shoreline response under climate variability. The aim is to quantify sediment transport rates and routing within a selected open-coast beach, attributing variability to wave energy spectra, water level fluctuations, seasonal sediment supply, and anthropogenic shoreline modifications. Specific objectives include (1) characterizing hydrodynamic forcing conditions using in situ wave and tide measurements, (2) determining volumetric sediment transport rates through profile and cross-shore surveys, (3) identifying dominant transport mechanisms (bedload versus suspended load) via tracer contents and grain-size distributions, (4) developing empirical relationships between wave-driven parameters and transport fluxes, and (5) validating a mobile-bed morphodynamic model against observed shoreline change over the study period. The methodology employs an observational, longitudinal field design conducted at a 2.5 km stretch of a reflective to intermediate beach on the southern Baltic coast, sampled over two summer–winter cycles (24 months). The population comprises the active sandy beach microtopography and nearshore sediments, with a stratified sampling scheme across five transects (0 m, 20 m, 40 m, 60 m, and 100 m from the mean high-water line) and monthly surveys. A total of 60 validated surface and subsurface grain-size samples are collected, complemented by 12 cross-shore profile surveys per season. Data collection instruments include short-term hydrographs and ADCPs for current velocity, rib-type pressure transducers for shoreline position, impedance-based turbidity sensors for suspended sediment concentrations, and time-lapse cameras for littoral zone evolution. Sediment samples are analyzed for median grain size, sorting, and quartz content, while tracer analyses identify bedload contributions. Wave data are obtained from in situ buoys and corroborated with regional wave models to derive relevant forcing metrics (significant wave height, peak period, waveform asymmetry). The project integrates continuous shoreline monitoring with morphodynamic modeling, applying regression analysis and generalized additive models to relate transport rates to hydrodynamic variables, and using a calibrated informative-physics model to simulate morphologic change. Data analysis follows a multi-tier approach. Descriptive statistics characterize sediment characteristics and transport flux distributions. Transport fluxes are estimated via Exner-based volumetric balance and instantaneous bedload transport formulas (e.g., Meyer-Peter–Mann, with modifications for coarse sands). The study employs multiple regression to identify key predictors of cross-shore sediment flux, with model selection guided by Akaike information criterion. Time-series analyses, including cross-correlation and spectral decomposition, assess lagged responses to wave forcing. A Bayesian calibration of the morphodynamic model quantifies parameter uncertainty, while sensitivity analyses determine the influence of wind setup and tide on transport pathways. Statistical significance is tested at a 0.05 level, and model performance is evaluated using RMSE and Nash–Sutcliffe efficiency. Expected findings indicate that cross-shore transport fluxes correlate strongly with near-bed velocity and wave setup, with suspended sediment carrying a substantial proportion of material during storm events, and bedload dominating in high-energy periods. Seasonal shifts in grain-size distributions are anticipated, reflecting selective transport and armoring processes. The empirical models are expected to explain a substantial portion of variance in transport fluxes (R2 > 0.6), while the morphodynamic model reproduces observed shoreline retreat and accretion patterns within acceptable error margins (RMSE < 1.5 m per profile). The study will contribute to knowledge by quantitatively linking hydrodynamic forcing to sediment transport and shoreline change in a temperate, open-coast system, offering improved parameterizations for predicting beach evolution under climate variability and coastal management scenarios. Recommendations emphasize integrating sustained long-term monitoring with databased forecasting for coastal defense planning, advocating site-specific transport predictors for regional calibration of morphodynamic models, and advising on sediment management practices that mitigate adverse shoreline modifications. The study also highlights the need for incorporating extreme event dynamics and sediment augmentation considerations into predictive frameworks to enhance resilience of coastal beaches.