Ernest R. Smith

Temporal and spatial variations of surf-zone currents and suspended sediment concentration

Temporal and spatial variations of surf-zone currents and suspended sediment concentrations were investigated at the U.S. Army Engineer Research and Development Centers Large-scale Sediment Transport Facility (LSTF). A longshore-uniform fine-sand beach, 35 m alongshore, 20 m cross-shore, and 25 cm thick was placed in the facility for these experiments. Two unidirectional, long-crested irregular wave conditions were examined, one resulted in predominantly spilling breakers and one in plunging breakers. Waves and currents, and sediment concentrations were measured at 20 and 16 Hz, respectively, at various longshore and cross-shore locations and throughout the water column. Both currents and sediment concentrations exhibit great temporal and spatial variations in the surf zone. The variation patterns, however, of the longshore current, cross-shore current, and sediment concentration are substantially different. Caution should be exercised when averaging these parameters over time and space. For the two wave cases examined, the temporal variations of longshore current, including those at principal incident-wave frequencies, were relatively small across most of the surf zone. Over 70% of the variations are within approximately F60% of the mean value. The wave motion, with a strong peak at principal incident-wave frequencies, dominated the temporal variations of cross-shore current. Temporal variations of suspended sediment concentration under the irregular waves were episodic, characterized by occasional large values induced by suspension events or due to horizontal advection. The variance of the concentration at the peak incident-wave frequency was not significant except very near the bed. Time-averaged longshore-current profiles over the predominantly rippled sand bed were logarithmic in shape below the wave trough. Depth-averaged longshore current (excluding the portion of water column above wave trough) matched well with the current measured at an elevation of 1/3 of the water depth from the bed. Time-averaged cross-shore current profiles were characterized by an onshore mass flux near the surface, and a balancing offshore flow below the wave-trough level (undertow). Sediment concentration decreased very rapidly upward through the water column across most of the surf zone except at the plunging breaker line where relatively homogeneous concentration was measured throughout much of the water column above 4 cm from the bed. Depth-averaged sediment concentration over the range from 1 cm above the bed to the bottom of wave trough roughly equaled the concentration measured at an elevation from the bed equal to 20% of the still- water depth.

Dependence of Total Longshore Sediment Transport Rates on Incident Wave Parameters and Breaker Type

Abstract Experiments were conducted in the Large-scale Sediment Transport Facility (LSTF) at the U.S. Army Engineer Research and Development Center to investigate the importance of wave height, period, and breaker type (spilling and plunging breakers) on total rate of longshore sediment transport (LST) and the cross-shore distribution of LST. Estimates computed by the CERC formula and Kamphius were compared to the accurately measured total LST rates. Several K-values were used with the CERC formula, including the recommended value of 0.39 and calculated values by Kamphuis and Readshaw, Ozhan, Bailard, and Del Valle et al. The recommended K-value and most of the calculated K-values overpredicted the measured total LST rates, but methods that included parameters to indicate breaker type gave good estimates. The Kamphuis and Readshaw equation, in which K is a function of surf similarity parameter, gave consistent estimates with measurements. The Kamphuis equation, which includes wave period and beach slope that in turn influences wave breaking, also compared well with the measurements. Additionally, the CERC formula has been used successfully if K is calibrated, and the formula gave excellent results if K was calibrated with measured data and applied to similar breaker types. The findings indicate that total LST rate is strongly influenced by breaker type. The cross-shore distribution of LST indicated three distinct zones of transport: the incipient breaker zone, the inner surf zone, and the swash zone. Transport in the incipient breaker zone was influenced by breaker type. Transport in the inner surf zone indicated that wave height was the dominating factor and independent of wave period. Swash zone transport, which accounted for a significant percentage of the total transport, showed a dependence on wave height, period, and beach slope.

Assessing the Resilience of Coastal Systems: A Probabilistic Approach

ABSTRACT Schultz, M.T. and Smith, E.R., 2016. Assessing the resilience of coastal systems: A probabilistic approach. Resilience is the ability to anticipate, prepare for, and adapt to changing conditions and withstand, respond to, and recover rapidly from disruptions. Methods and tools to quantify resilience are needed to provide actionable intelligence to plan, design, construct, and manage coastal systems. This paper describes how a probabilistic measure of resilience can be assessed for a coastal community using a Bayesian network. The measure of resilience is the joint probability of meeting two management objectives, one with respect to the level of system performance and the other with respect to the length of time required to restore system performance. This paper describes a pilot study to demonstrate the approach in Jamaica Bay, New York, a dense, urban, residential community located on the southern coast of Long Island. Results of the pilot study illustrate how practical information can be developed to support decisions about managing coastal systems. The pilot study provides insights into data and information requirements; the advantages, challenges, and limitations of the approach; and the feasibility of implementing this approach for operations. This approach to resilience assessment is well suited for coastal planning contexts because it explicitly incorporates information about uncertainty in the severity of coastal storm events, as well as uncertainty in how the system will perform when exposed to storm loads. The method challenges the community to establish explicit objectives for coastal resilience, identifies what data are needed to monitor progress toward objectives, and provides a platform from which to explore how those objectives might be achieved in practice.

Hydrodynamic Conditions Associated with an Onshore Migrating and Stable Sandbar

ABSTRACT Cheng, J.; Wang, P., and Smith, E.R., 2016. Hydrodynamic conditions associated with an onshore migrating and stable sandbar. In this study, large-scale three-dimensional laboratory data were analyzed to identify the hydrodynamic conditions associated with the onshore migration of a sandbar and the subsequent equilibrium state of a stable bar. The initial sandbar was constructed offshore and out of equilibrium, with a symmetrical shape. The bar became asymmetrical as it migrated onshore. As the rate of onshore migration slowed, the bar was restored to a symmetrical shape toward an equilibrium state. Wave and near-bottom velocity across the surf zone were measured during the onshore sandbar migration. The near-bottom velocity skewness analyzed wave by wave indicates that before the sandbar reached equilibrium, the velocity was skewed offshore in the nearshore region and was skewed onshore seaward of the bar. However, the velocity skewness pattern reversed when the beach profile reached equilibrium and the sandbar became stable. The location of maximum undertow velocity moved from nearshore to the bar crest as the sandbar evolved toward equilibrium. Furthermore, the peak onshore-directed acceleration was greater than the peak offshore-directed acceleration throughout the surf zone during the periods of both onshore migrating and stable sandbar. The maximum difference between the onshore- and the offshore-directed acceleration occurred at the seaward side of the bar crest. The analyses of the hydrodynamic conditions associated with sandbar movement in a controlled laboratory experiment provide insights on the mechanisms of sandbar migration.

NAVIGATION IMPROVEMENTS, MOUTH OF THE COLORADO RIVER, TEXAS

This paper describes an ongoing analysis of coastal and inlet processes at the mouth of the Colorado River Navigation Channel, Texas. A weir jetty system with impoundment basin down drift of the weir was constructed at the mouth of the Colorado River in 1985 to reduce the expected rate of dredging of the shallow-draft entrance navigation channel. The required rate of dredging has been about double the design estimate, however, due in part to diversion of the Colorado River away from its entrance to the Gulf of Mexico. A study is underway to better understand the processes involved, design a more efficient entrance, and provide the reduced dredging interval and volume originally desired, while placing the inlet processes within a regional sediment transport framework.

LONGSHORE SAND TRANSPORT CALCULATED BY TIME-DEPENDENT SHEAR STRESS

Based on longshore sand transport experiments performed in a large basin, measured sand transport rates obtained for spilling and plunging waves are compared to the Bodge and Dean (1987) and Watanabe (1992) distributed load formulas, representative of typical formulas applied in engineering practice. Neither formula estimates the measurements satisfactorily. The Bodge and Dean formula is sensitive to changes in energy flux and does not include threshold shear. The Watanabe formula includes critical shear, but transport estimates are made from time-averaged values of bottom shear stress, which did not exceed critical shear stress at most cross-shore locations. A new transport formula is introduced based on time-dependent shear stress calculated from the total velocity that includes the wave orbital velocity. The new formula gives reasonable estimates for both spilling and plunging breaker types. A conclusion is that it is essential to represent the time-dependent, or fluctuating, component of fluid motion in predictive equations of the longshore sand transport rate.

Longshore Sediment Transport as a Function of Energy Dissipation

Abstract : Experiments to measure waves, currents, and sediment transport rate for two breaker types, plunging and spilling, were conducted in a large-scale three-dimensional physical model. It was found that there was a large difference in cross-shore distribution and total sediment transport rate between the two breaker types. Total transport rates compared to existing predictive equations. The equations generally did not predict the data well. With the exception of the Kamphuis (1991) equation, which included a dependence on wave period, the predictive equations did not differentiate between breaker types.