Modeling shallow-water hydrodynamics : rotations, rips, and rivers

by Long, Joseph W.

Hydrodynamic models are used as a diagnostic tool to understand the temporal variability of shallow-water processes that are difficult to completely resolve with traditional field measurements. For all simulations, modeled quantities are qualitatively or quantitatively compared with available measurements to gain confidence in conclusions derived from the modeled results. In this work we consider both vorticity motions and rip currents, which arise from alongshore inhomogeneities in the wave momentum flux but occur at much different time scales (O(min) vs. O(hours-weeks)). They each have an effect on sediment transport processes and dispersion of sediments or pollutants in the surf zone, which makes understanding their structure and persistence essential.

The vorticity motions of interest here are associated with spatial and temporal wave height variations caused by wave grouping and can exist with either normally or obliquely incident wave conditions. We find that these flows persist for O(1000s) but their lifespan is controlled by the sequence of wave forcing rather than bottom friction as previously hypothesized. These motions can also be observed in combination with either stable or unstable alongshore currents. Our results suggest that, at times, these alongshore propagating wave group forced vortices are misinterpreted as instabilities of the alongshore current.

Alternately, the rip currents considered in this research are controlled by strong wave height gradients in the surf zone generated by the refraction of incident waves over variable offshore depth contours. Thus, this type of circulation is governed by timescales associated with changing offshore wave conditions (O(hours - days)). We consider a four-week time period when variable offshore wave spectra were observed during a large-scale field experiment. The model and data are in good agreement for all wave conditions during the month and estimated model errors are similar to those found previously for alongshore uniform beaches. Through comparisons with remote sensing observations, the model proves it is capable of predicting rip currents when they are observed. Analysis suggests that the direction of the offshore wave spectra will dictate when and where rip currents will appear. We also find that for bi-modal offshore spectra, the relative amount of energy in each spectral mode is a better predictor of rip current development than the peak spectral characteristics.

Finally, some preliminary work to estimate water depths from the combination of hydrodynamic models and available data is also presented. We focus this work in a river meander for our initial tests. A simple analytical model shows skill in predicting the water depth at only one of the two river meanders considered. This discrepancy appears to be related to river curvature and as curvature weakens, the model accuracy decreases. This is hypothesized to be the result of dispersive mixing which is not accounted for in this simple model but confirmation is still required. At the same time, we perform simulations within a river meander to determine the efficacy of using coastal hydrodynamic models in riverine environments where the principles governing the flow are the same. Our initial tests of the Regional Ocean Modeling System (ROMS) suggests that it is able to reproduce the flow through a river meander which opens the door to developing one model that can simulate conditions from upland rivers out to the continental shelf.