R. L. Brouwer

Surfzone Monitoring Using Rotary Wing Unmanned Aerial Vehicles

This study investigates the potential of rotary wing unmanned aerial vehicles (UAVs) to monitor the surfzone. This paper shows that these UAVs are extremely flexible surveying platforms that can gather nearcontinuous moderate spatial resolution and high temporal resolution imagery from a fixed position high above a study site. The rotary wing UAVs used in this study can fly for ;12 min with a mean loiter radius of 1–3.5m and a mean loiter error of 0.75–4.5 m. These numbers depend on the environmental conditions, flying style, battery type, and vehicle type. The images obtained from the UAVs, and in combination with surveyed ground control points (GCPs), can be georectified to a pixel resolution between 0.01 and 1m, and a reprojection error—that is, the difference between the surveyed GPS location of a GCP and the location of the GCP obtained from the georectified image—of O(1 m). The flexibility of rotary wing UAVs provides moderate spatial resolution and high temporal resolution imagery, which are highly suitable to quickly obtain surfzone and beach characteristics in response to storms or for day-to-day beach safety information, as well as scientific pursuits of surfzone kinematics on different spatial and temporal scales, and dispersion and advection estimates of pollutants.

Modelling the influence of spatially varying hydrodynamics on the cross-sectional stability of double inlet systems, doi: 10.1007/s10236-013-0657-6

The cross-sectional stability of double inlet systems is investigated using an exploratory model that combines Escoffier’s stability concept for the evolution of the inlet’s cross-sectional area with a two-dimensional, depth-averaged (2DH) hydrodynamic model for tidal flow. The model geometry consists of four rectangular compartments, each with a uniform depth, associated with the ocean, tidal inlets and basin. The water motion, forced by an incoming Kelvin wave at the ocean’s open boundary and satisfying the linear shallow water equations on the f -plane with linearised bottom friction, is in each compartment written as a superposition of eigenmodes, i.e. Kelvin and Poincaré waves. A collocation method is employed to satisfy boundary and matching conditions. The analysis of resulting equilibrium configurations is done using flow diagrams.Model results show that internally generated spatial variations in the water motion are essential for the existence of stable equilibria with two inlets open. In the hydrodynamic model used in the paper, both radiation damping into the ocean and basin depth effects result in these necessary spatial variations. Coriolis effects trigger an asymmetry in the stable equilibrium cross-sectional areas of the inlets. Furthermore, square basin geometries generally correspond to significantly larger equilibrium values of the inlet cross-sections. These model outcomes result from a competition between a destabilising (caused by inlet bottom friction) and a stabilising mechanism (caused by spatially varying local pressure gradients over the inlets).

Influence of basin geometry on equilibrium and stability of double inlet systems

This study investigates the influence of basin geometry on the cross-sectional stability of double inlet systems. The inlet is in equilibrium when the amplitude of the inlet velocities equals the equilibrium velocity (~1 m s-1). This equilibrium is stable when after a perturbation the cross-sections of both inlets return to their original equilibrium value. The necessary amplitudes of the inlet velocities are obtained using an idealized 2DH hydrodynamic that calculates tidal elevation and flow in a geometry consisting of several adjacent rectangular compartments. Model results suggest that regardless of the inclusion or exclusion of bottom friction in the basin, stable equilibrium states exist. Qualitatively, the influence of basin geometry does not change the presence of stable equilibrium. Quantitatively, however, taking a basin surface area of 1200 km2, equilibrium values can differ up to a factor 2 depending on the geometry of the basin.

Time Evolution of Estuarine Turbidity Maxima in Well-Mixed, Tidally Dominated Estuaries: The Role of Availability- and Erosion-Limited Conditions

Using an idealized width-averaged process-based model, the role of a mud pool on the bed and time-varying river discharge on the trapping of fine sediment is systematically investigated. For this purpose, a dynamically and physically motivated description of erodibility is presented, which relates the amount of sediment on the bed to the suspended sediment concentration (SSC). We can distinguish between two states: in the availability-limited state, the SSC is limited by the amount of erodible sediment at the bed. Over time, under constant forcing conditions, the estuary evolves to morphodynamic equilibrium. In the erosion-limited state, there is an abundant amount of sediment at the bed so that sediment pickup occurs at the maximum possible rate. The SSC is then limited by the local hydrodynamic conditions. In this state, the estuary keeps importing sediment, forming an erodible bottom pool that grows in time. These two states can be used to explain the response of an estuary to changing river discharge. Under availability-limited conditions, periods of high river discharge push estuarine turbidity maxima (ETMs) downstream, while drier periods allow ETMs to move upstream. However, when the estuary is in an erosion-limited state during low river discharge, a bottom pool is formed. When the discharge then increases, it takes time to deplete this pool, so that an ETM located over a bottom pool moves with a significant time lag relative to changes in the river discharge. Good qualitative agreement is found between model results and observations in the Scheldt Estuary of surface SSC using a representative year of discharge conditions.

Tidal Inlet Hydrodynamics; Excluding Depth Variations with Tidal Stage

Introduction The dynamics of the flow in the inlet are described by the equation for uniform unsteady open channel flow. Variations in depth with tidal stage are neglected. The dynamic equation is complemented with a continuity condition that assumes a pumping mode for the back-barrier lagoon, i.e., the water level in the back-barrier lagoon fluctuates uniformly. Although these are simplifications, the advantage is that they allow relatively simple analytical solutions that are helpful in identifying mechanisms responsible for phenomena such as resonance, tidal choking and generation of (odd) overtides. As examples, analytical solutions by Keulegan (1951, 1967) and Mehta and Ozsoy (1978) are presented. Results of the analytical solutions are applied to a representative inlet and compared with numerical results. Inlet Schematization The tidal inlet system is schematized to an inlet and a back-barrier lagoon (Fig. 6.1). The inlet connects the back-barrier lagoon and the ocean. Its geometry is simplified to a prismatic channel with diverging sections at both ends. The backbarrier lagoon is schematized to a basin with uniform depth. Referring to Chapter 2, in the real world inlets have varying widths and depths and back-barrier lagoons are characterized by tidal flats and marsh areas. Therefore, the schematization presented in Fig. 6.1 is only a rough representation of an actual inlet. Governing Equations and Boundary Condition Dimensional Equations In deriving the governing equations, the major assumptions are 1) one-dimensional unsteady uniform flow in the inlet, 2) a uniformly fluctuating water level in the basin (pumping or Helmholz mode) and 3) negligible variations in cross-sectional area of the inlet and basin surface area with tidal stage. With these assumptions, the equation for the flow in the inlet is (Appendix 6.A): In this equation u is the cross-sectionally averaged velocity, positive in the flood direction, L is length of the prismatic part of the inlet, g is gravity acceleration, t is time, F = f /8 where f is the Darcy–Weisbach friction factor, R is hydraulic radius, m is entrance/exit loss coefficient, η 0 is the ocean tide and η b is the basin tide.

Engineering of Tidal Inlets

Introduction Back-barrier lagoons are home to recreational marinas and fishing ports. With a few exceptions, most vessels using these facilities are relatively small, with lengths in the 5–30 m range and a maximum draft of 5 m. To access the lagoon, vessels need to navigate the ebb delta channel and the inlet. This requires that both channel and inlet are relatively stable, have sufficient depth and an alignment relative to the wave direction that allows safe access and passage. Not many natural inlets satisfy these requirements and measures are needed to remedy the shortcomings. A distinction is made between soft and hard measures. Soft measures include the opening of a new inlet, inlet relocation, dredging and artificial sand bypassing. Hard measures are jetty construction and weir-jetty systems. In addition to providing boating access, inlets play a role in maintaining the water quality of the back-barrier lagoon; they serve as conduits for the exchange of lagoon and ocean water. Artificial Opening of a New Inlet The objectives of the artificial opening of a new inlet are to provide passage for vessels to the back-barrier lagoon and/or to improve water quality. With regards to the passage of vessels, design requirements include sufficient channel depth, width, alignment and stability. Improving water quality requires sufficient exchange, implying a large enough tidal prism. Examples of inlets that were artificially opened, but with different objectives, are Bakers Haulover Inlet (FL), Faro-Olhao Inlet (Portugal) and Packery Channel (TX). Bakers Haulover Inlet (Fig. 14.1a) was opened in 1925 for the prime purpose of improving water quality in the northern part of Biscayne Bay (Dombrowsky and Mehta, 1993). Faro-Olhao Inlet (Fig. 14.1b) was opened in 1929 to improve navigational access to the city of Faro (Pacheco et al., 2011). Packery Channel (Fig. 14.1c) was opened in 2006 with the objectives to facilitate recreational fishing and boating and to improve the exchange between the Gulf of Mexico and Corpus Christi Bay (Williams et al., 2007).

THE CONTRIBUTION OF ENTRANCE/EXIT LOSSES TO THE EQUILIBRIUM AND STABILITY OF DOUBLE INLET SYSTEMS

This study investigates the contribution of entrance/exit losses to the equilibrium and stability of a double inlet system. Previous studies concluded that for such a system no stable equilibriums can be found. An important assumption in these studies is that entrance/exit losses can be neglected as they are small compared to friction losses. However, analytical model results and numerical experiments carried out in this study show that stable equilibriums are possible when entrance/exit losses are not small compared to friction losses and a small amplitude difference between the inlets is allowed. Furthermore, parameter sensitivity analysis shows that the inlet length, entrance/exit losses, bottom friction losses and the hydraulic radius affect the range of amplitude differences for which stable equilibriums exist.