Analysis of data from 280 rivers discharging to the ocean indicates that sediment loads/yields are a log-linear function of basin area and maximum elevation of the river basin. Other factors controlling sediment discharge (e.g., climate, runoff) appear to have secondary importance. A notable exception is the influence of human activity, climate, and geology on the rivers draining southern Asia and Oceania. Sediment fluxes from small mountainous rivers, many of which discharge directly onto active margins (e.g., western South and North America and most high-standing oceanic islands), have been greatly underestimated in previous global sediment budgets, perhaps by as much as a factor of three. In contrast, sediment fluxes to the ocean from large rivers (nearly all of which discharge onto passive margins or marginal seas) have been overestimated, as some of the sediment load is subaerially sequestered in subsiding deltas. Before the proliferation of dam construction in the latter half of this century, rivers...

Geomorphic/Tectonic Control of Sediment Discharge to the Ocean: The Importance of Small Mountainous Rivers

Foreword 1. Introduction 2. Runoff, erosion and delivery to the coastal ocean 3. Temporal variations 4. Human impacts Appendices. Global River Database: Appendix A: North and Central America Appendix B: South America Appendix C: Europe Appendix D: Africa Appendix E: Eurasia Appendix F: Asia Appendix G: Oceania References Index.

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River Discharge to the Coastal Ocean: A Global Synthesis

This paper describes the development of a numerical model for studying the evolution of a wave train, shoaling and breaking in the surf zone. The model solves the Reynolds equations for the mean (ensemble average) flow field and the k–e equations for the turbulent kinetic energy, k, and the turbulence dissipation rate, e. A nonlinear Reynolds stress model (Shih, Zhu & Lumley 1996) is employed to relate the Reynolds stresses and the strain rates of the mean flow. To track free-surface movements, the volume of fluid (VOF) method is employed. To ensure the accuracy of each component of the numerical model, several steps have been taken to verify numerical solutions with either analytical solutions or experimental data. For non-breaking waves, very accurate results are obtained for a solitary wave propagating over a long distance in a constant depth. Good agreement between numerical results and experimental data has also been observed for shoaling and breaking cnoidal waves on a sloping beach in terms of free-surface profiles, mean velocities, and turbulent kinetic energy. Based on the numerical results, turbulence transport mechanisms under breaking waves are discussed.

A numerical study of breaking waves in the surf zone

An assessment is made of contemporary effective sea-level rise (ESLR) for a sample of 40 deltas distributed worldwide. For any delta, ESLR is a net rate, defined by the combination of eustatic sea-level rise, the natural gross rate of fluvial sediment deposition and subsidence, and accelerated subsidence due to groundwater and hydrocarbon extraction. ESLR is estimated under present conditions using a digital data set of delta boundaries and a simple model of delta dynamics. The deltas in this study represent all major climate zones, levels of population density, and degrees of economic development. Collectively, the sampled deltas serve as the endpoint for river basins draining 30% of the Earth's landmass, and 42% of global terrestrial runoff. Nearly 300 million people inhabit these deltas. For the contemporary baseline, ESLR estimates range from 0.5 to 12.5 mm yr −1 . Decreased accretion of fluvial sediment resulting from upstream siltation of artificial impoundments and consumptive losses of runoff from irrigation are the primary determinants of ESLR in nearly 70% of the deltas. Approximately 20% of the deltas show accelerated subsidence, while only 12% show eustatic sea-level rise as the predominant effect. Extrapolating contemporary rates of ESLR through 2050 reveals that 8.7 million people and 28,000 km 2 of deltaic area in the sample set of deltas could suffer from enhanced inundation and increased coastal erosion. The population and area inundated rise significantly when considering increased flood risk due to storm surge. This study finds that direct anthropogenic effects determine ESLR in the majority of deltas studied, with a relatively less important role for eustatic sea-level rise. Serious challenges to human occupancy of deltaic regions worldwide are thus conveyed by factors which to date have been studied less comprehensively than the climate change–sea-level rise question. © 2005 Elsevier B.V. All rights reserved.

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Effective sea-level rise and deltas: Causes of change and human dimension implications

Approximate layer-averaged equations describing the mechanics of turbid underflows are derived. Closure of the equations describing the balance of fluid mass, sediment mass, and mean flow momentum provides for the delineation of a three-equation model. A description of sediment exchange with the bed allows for the possibility of a self-accelerating turbidity current in which sediment entrainment from the bed is linked to flow velocity. A consideration of the balance of the mean energy of the turbulence yields a constraint on physically realistic solutions to the three-equation model. It is shown that the self-acceleration predicted by the three-equation model is so strong that the energy constraint fails to be satisfied. In particular, the turbulent energy consumed in entraining new bed sediment exceeds the supply of energy to the turbulence, so that the turbulence, and thus the turbidity current, must die. The problem is rectified by the formulation of a four-equation model, in which an explicit accounting is made of the mean energy of the turbulence. Sediment entrainment from the bed is linked to the level of turbulence in the four-equation model. Self-acceleration is again predicted, although it is somewhat subdued compared with that predicted by the three-equation model. The predictions of both models are summarized over a wide range of conditions.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112086001404

Self-accelerating turbidity currents

In terms of the gross first-order effects of plate tectonics, there appear to be three major classes of coasts and several subclasses, depending upon their position relative to the moving plates of the tectosphere: (1) collision coasts, that is, those on the collision edge of continents and island arcs; (2) trailing-edge coasts, that is, those on the trailing edge or noncollision side of a continent; and, (3) marginal sea coasts protected by island arcs. The trailing-edge coasts range in form from the tectonically new coasts facing beginning separation centers to the morphologically active coasts bordering the debris plains formed from the erosion products of the continents. The good coherence between certain morphologic and tectonic features of coasts was used as a guide in formulating a purely morphologic classification with tectonic implications. The morphologic classification is defined simply in terms of the width of the continental shelf and the relief of the adjacent land forms: (1) mountainous coa...

On the Tectonic and Morphologic Classification of Coasts

x8090 Submarine canyons serve as active conduits joining the shallow waters of the shelf to the deeper waters offshore. Canyon currents are generated by many forces, including those related to wind, surface waves, internal waves, tides, and suspended sediment. Studies of canyon currents indicate that submarine canyons can generally be divided into deep- and shallow-water regimes that are dominated by specific driving forces. The deep-water regime is generally exposed to energy from tides, intern al waves, and spin-off eddies from large-scale current systems, whereas the shallow-water areas are dominated by currents related to surface waves and wind. Strong down-canyon currents appear to be caused by a unique combination of air-sea-Iand interactions consisting of 1. a pile-up of water along the shoreline caused by strong onshore winds; 2. down-canyon pulses of water associated with groups of high incident waves; 3. excitation of standing edge waves that produce longer-period up-and down-canyon oscillations; and finally, 4. the formation of discrete pulses of down-canyon motion, which become more intense and lead to sustained down-canyon currents, as the weight of the sediment suspended by the currents overcomes the density stratification of the deeper water. Simultaneous measurements of currents and pressure in Scripps Submarine Canyon, and of winds, waves, and pressure over the adjacent shelf have been made for several years, with the strongest down-canyon current measured, 1.9 m sec-1 at a depth of 44 m, being recorded during the

Currents in Submarine Canyons: An Air-Sea-Land Interaction

Two important mixing mechanisms are operative within the surf zone, each having distinctive length and time scales determined by the intensity of the waves and the dimensions of the surf zone The first is associated with the breaking wave and its bore, which produce rapid mixing in an on-offshore direction This mixing, when normalized and averaged over the surf zone width Xb, gives coefficients of eddy diffusivity of the order of HbXb/T where Hb and T are the breaker height and the period of the waves The second process is advective and is associated with the longshore and rip current systems in the nearshore circulation cell For constant longshore discharge of water between cells Qi, this process gives a concentration Nn in the nth cell down-current from a continuously injected source of dye of

Nn=N0(Qi/Qm)n

where N0 is the concentration leaving the injection cell, and Qm is the maximum longshore discharge within a cell As an approximation, the concentration decreases exponentially with distance y from the injection point when n is replaced by y/Y, where Y is the spacing between rip currents This relation gives an apparent longshore eddy mixing coefficient of the order of Y〈υi〉 where 〈υi〉 is the longshore current velocity Along ocean beaches HbXb/T and Y〈υi〉 are about 10 m2 sec−1 and 100 m2 sec−1 respectively

C. E. Nordstrom

Papers

On the Tectonic and Morphologic Classification of Coasts

Currents in Submarine Canyons: An Air-Sea-Land Interaction

Mixing in the surf zone