: This Coastal and Hydraulics Engineering Technical Note (CHETN) describes a methodology for representing nonerodible substrates in a two-dimensional (2-D) coastal morphology change models, with emphasis on coastal inlets. The calculation procedure is described, followed by examples showing the functioning of the method.

Numerical modeling of coastal circulation encompassing the nearshore requires forcing by tide, surface gravity waves, and possibly other factors. In the nearshore, the wave-induced longshore current and setup are dominant hydrodynamic processes, and lateral boundary conditions representing tide and oceanic forcing typically do not include surface-wave contributions. Without proper boundary conditions, significant gradients in current and water level can occur that contaminate the solution in the internal domain. A standard strategy is to place the boundaries far from the site of interest, but this strategy greatly increases computational demands, and it may not be appropriate for long-term simulations. This paper describes a wave-adjusted boundary condition that accounts for wave-induced water level and current acting in combination with tidal forcing. The wave-adjusted boundary condition is demonstrated for an idealized case of a parallel-contour beach and for an engineering application at Ocean City, MD.

BACKGROUND: Morphology change models apply sediment transport formulas to calculate transport rates from which depth change is computed. Predictive sediment transport formulas are based on the presumption that there is sediment available corresponding to the transport capacity of the hydrodynamic forcing. At many coastal inlets, however, the sea bottom may be covered by hard or nonerodible material, which may or may not be covered by a layer of sediment. Hard-bottom location and composition are determined by geotechnical surveys such as coring, beach profiling and hydrographic surveying, diver inspection, and aerial photographs. Various forms of hard bottom are commonly encountered, imposing challenges for morphological modeling. Hard bottom may consist of material such as limestone, coral reef, sedimentary rock, submerged coastal structures such as jetty weirs, or even artificial material such as concrete. Figure 1 shows an example of a beach with rock outcrops extending from the shore into the inner surf zone. Sebastian Inlet, FL, shown in Figure 2, was blasted out of limestone, yielding an inlet that passes through an almost completely hard substrate. Exposed hard bottom is observed as darker bands under the water. If hard bottom is exposed, the actual transport rate will be less than the potential (predicted by a transport relationship). With less mobile sediment leaving the exposed area, transport rates in surrounding regions will be influenced by the presence of neighboring hard bottom even though these areas may have sediment available for transport. In this way, hard-bottom areas will promote erosion or reduced accumulation in surrounding areas with erodible bottom because the availability of sand is reduced. Exposed hard bottom, thus, constitutes a constraint on the sediment transport and associated bottom change to be calculated. A methodology was developed for representing hard bottom in 2-D morphology change models. The method applies a recalculation technique for transport rates at locations that are nonerodible and nearby areas, subject to certain principles discussed in this CHETN. BASIC PRINCIPLES: The hard-bottom routine is a 2-D extension of the one-dimensional (1-D) methodology applied in the shoreline change model GENESIS (Hanson 1989) to represent the control of seawalls on longshore sediment transport and shoreline evolution (Hanson and Kraus 1985, 1986). The methodology was also applied by Kraus and Larson (1998) and Larson and Kraus (2000) to simulate cross-shore sediment transport and beach profile evolution in the presence of hard bottom in the SBEACH model (Larson and Kraus 1998).

: The two-dimensional (2-D) circulation model M2D, developed under the Coastal Inlets Research Program conducted at the U.S. Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory, has been designed for local applications, primarily at inlets, the nearshore, and bays. M2D is computationally efficient, easy to set up, and has features required for many coastal engineering applications including robust flooding and drying, wind-speed dependent (time-varying) wind-drag coefficient, variably- spaced boftom-fiction coefficient, time- and space-varying wave-stress forcing, efficient grid storage in memory, two hot-start options, and the convenience, through control statements, of independently turning on or off the advective terms and mixing terms. If wave information is available, such as through coupling with the STeady state spectral WAVE model STwAVE, M2D will calculate wave friction and wave mixing. M2D can be coupled to regional circulation models through boundary conditions providing flexibility for large-scale applications and connectivity between models. A graphical interface for M2D has been implemented within the Surface-Water Modeling System (SMS) Versions 8.1 and higher. Features of the M2D interface are grid development, control file specification, model runs, post-processing of results, and visualization. M2D can be driven by larger-domain circulation models, such as ADCIRC, through boundary specification capabilities contained within the SMS. The Steering Module in SMS provides an automated means of coupling of M2D with STWAVE, which is convenient for projects that require wave-stress forcing for M2D as well as wave friction and mixing owing to breaking waves. The Steering Module allows the user to choose from seven possible coupling combinations, providing flexibility in conducting simulations of wave-driven currents and wave-current interaction.

During the past 5 years, the U.S. Army Corps of Engineers' Coastal Inlets Research Program has been taking a systematic regional modeling approach to calculating combined scales of nearshore circulation and providing established models, called "Community Models," for new projects that can involve modeling by different organizations. Here, we describe a regional circulation model for the coast of Long Island, NY. Domain coverage includes the New York Bight and Long Island Sound. The two-dimensional finite-element circulation model ADCIRC was applied because of its stability and flexibility in element sizes. Placement of tidal boundaries far from the area of interest allows free propagation of the tides throughout the domain, preserving the natural amplitude and phase variation. Since development of the original Long Island regional model, it has been applied to numerous studies around the north and south shores of Long Island. The community model benefited from bathymetry updates and increases in resolution for each project. Specific study applications are: flood shoal mining at Shinnecock Inlet; erosion control at Jones Inlet; storm surge calculations from Fire Island to Montauk Point; proposed relocation of Fire Island Inlet; and stability of two small inlets on the north shore of Long Island. Because each project could obtain, adapt, and apply the existing regional model and rigorous forcing conditions, considerable time and effort were saved.

Analysis of five high-resolution bathymetric data sets collected at Shinnecock Inlet, NY indicates the evolution of ebb shoal morphology between 1994 and 2000 was primarily controlled by migration of the main navigation channel. Increased wave activity during the 1997 El Nino accelerated the rate at which the channel was deflected toward the west. These bathymetric data are applied in this study for assessment of morphology change calculation conducted within the Inlet Modeling System developed by the U.S. Army Corps of Engineers Coastal Inlets Research Program. Circulation, sediment transport, and morphology change were calculated by the two-dimensional finite-difference model M2D, which was coupled with STWave for computation of wave-driven currents. A simulation was conducted for August to November 1997 in which waves from NDBC Station 44025 were input as forcing for STWave. Tidal forcing for M2D was prescribed with water levels extracted from a regional ADCIRC model. Major observed changes in inlet morphology were reproduced by the modeling system. These changes are: scour and westward migration of the navigation channel, accretion along the eastern flank of the ebb shoal, and accretion of the seaward extent of the ebb shoal.

Bypassing at inlets can occur across the ebb shoal, through tidal exchange, and by episodic collapse of shoals. To examine ebb-shoal and tidal exchange bypassing in a systematic way, we investigated sediment pathways at an idealized inlet with a coupled tide, wave, and sediment transport-morphology change numerical modeling system. The idealized inlet, ebb shoal, and channel were devised to test the coupled modeling system and isolate sediment transport pathways driven by wave and tidal forcing. The inlet, channel, ebb shoal, and bay dimensions approximate those of Shinnecock Inlet, New York. Five simulations consisting of tide forcing, wave forcing (fair-weather and storm), and combined tide and wave forcing were conducted. Patterns of calculated morphology change followed those found in nature. Simulations with waves impounded sand against the updrift jetty and eroded the bottom in the nearshore area on the downdrift side of the inlet. Wave breaking on the ebb shoal primarily moved material updrift, but also flattened the shoal by eroding the top and depositing material around its perimeter. For the forcing conditions examined, waves were the dominant transport mechanism. Tidal currents modified the morphology change primarily at the inlet entrance and on the updrift side of the ebb shoal by opposing the current during the flood tide.

Abstract : The interaction of waves and currents at an inlet entrance can be significant. Traditionally, numerical modelers have separated the processes of tidal circulation and wave transformation, but the surf zone and inlet are areas where the interactions are strong and should be numerically simulated to capture the resulting hydrodynamics. This paper describes performance of coupled wave and circulation models for both an idealized inlet setting and an application for Grays Harbor, Washington, concentrating on the influence of waves on currents. A comparison of tidal current simulations to tidal-plus-wave-induced current simulations shows that the interactions create gyres, longshore currents, rip currents, and "shadow zones" of relatively weak currents. It is concluded that accurate simulation of the hydrodynamics at coastal inlets requires coupling of wave and circulation models.

This paper describes the wave measurement and numerical modeling components of a regional monitoring and modeling system established for the south shore of Long Island, New York. The monitoring portion was begun in April 1998 and has produced a wealth of data on waves, currents, water level, and soundings at inlets. A directional spectral wave model incorporating nested grids with fine resolution at inlets provides an efficient and accurate means of calculating nearshore waves. Validation of the modeling system is presented, together with discussion of the managerial functions of the data and model.

This paper introduces a closed-form analytical solution of the one-dimensional (1D), depth-averaged linearized momentum and continuity equations that incorporates linear bottom friction and the non-linear wind stress. The solution describes wind-forced motion in a 1D basin with horizontal bottom as governed by water depth, basin length, bottom friction coefficient, wind speed, and fundamental frequency of an oscillatory wind. The solution displays in compact form general behavior and dependencies of the physical processes, including generation of wind-induced harmonies of the forcing motion, damping, and resonance. The solution can serve as a benchmark test for numerical models of the shallow-water equations, as well as provide estimates of wind-induced motion in enclosed water bodies.

PURPOSE: The Coastal and Hydraulics Engineering Technical Note (CHETN) described herein provides guidance on coupling the regional circulation model, Advanced Circulation (ADCIRC), with the local circulation model M2D. Coupling of models is an efficient and accurate means of calculating water level and current on a local domain, such as at and around an inlet. This CHETN also serves as a tutorial for coupling the two models within the Surface-Water Modeling System (SMS). BACKGROUND: Circulation modeling at tidal inlets requires accurate boundary conditions at the seaward edge of the domain. Regional models developed by the Coastal Inlets Research Program (CIRP) provide information that can be applied as boundary conditions for local (project scale) models. Examples of regional models available from the CIRP are Long Island, NY, southwest Washington, northern California, east coast of Florida, and the Maryland and Delaware coast. Water levels from solutions of these regional models can be applied as boundary conditions for local projects. Large-scale physical oceanographic processes such as the tide and storms can be represented in regional models and the information cascaded locally by furnishing boundary (driving) conditions to meet project analysis requirements. Also, multiple projects with great detail can be included within a computationally efficient regional domain (Militello, Kraus, and Brown 2000). Benefits include automatic calculation of mutual interactions among projects, such as multiple inlets sharing the same bay or lagoonal system, and the availability of baseline information from which new projects can be added. In addition, duplication of effort can be avoided, increasing efficiency and cost-effectiveness. Engineering applications require numerous simulations with modified grids to determine the consequences of project alternatives. An efficient means of conducting multiple simulations is to apply a local model, which eliminates the computational overhead of a large domain. By coupling regional and local models, the local model can propagate hydrodynamic conditions on the boundaries calculated by the regional model into its smaller domain. Coupling of the models is accomplished by passing time series of water level obtained from the regional model solution to the local model as boundary conditions. Because the boundary conditions for the local model were calculated on a regional domain, they contain spatial variations that preserve such features as tidal phase, surge distribution, and wind setup or setdown. Other boundary conditions or input, such as river discharge or wind, can be specified for the local model as required by the project. The coupled modeling approach is advantageous because the local model can apply ADCIRC- calculated water levels from previous studies as boundary conditions if they are available for the time period of interest. Alternatively, ADCIRC can be run for the time intervals required for the study, and then the local model can be applied. In both situations, modifications to the local grid can be easily made for project alternatives without the necessity of multiple simulations with the large

: Review of the literature of barrier island and barrier spit breaching reveals there is a paucity of information on the physical processes beyond qualitative reporting of case studies, despite the significant potential environmental and societal consequences that unintended breaches can bring. A breach susceptibility index is introduced to classify breaching potential by storm surge or equivalent inundation mechanism. A recent breach of the barrier spit enclosing Stone Lagoon, located on the Pacific Ocean coast, Humboldt County, northern California, provides background for the discussion. The spit is believed to have breached seaward between March 13 and 15, 2002. Aerial photography was flown four times at approximately weekly intervals from March 18, documenting breach closure, movement of the ephemeral ebb shoal, and initial recovery of the barrier spit.

A new interface component, called the Steering Module, has been developed within the Surfacewater Modeling System (SMS) that automatically controls coupling between circulation and surface-wave models. The Steering Module accepts user-specified information that describes which variables are passed between models and the frequency of passage. One-way coupling provides currents and/or water level to the wave model to calculate modification of the waves owing to the presence of a current and deviation of water level from a vertical datum, or provides wave-stress fields to the circulation model to calculate wave-driven currents. Two-way coupling provides both of these transfers. Because models can be defined over different domains and grids (or meshes), the Steering Module maps the information from one model onto the grid or mesh of the other. Input to the Steering Module is the time interval between passage of between models, and specification of variables passed between models. This paper describes coupling between the time-stepping finite-element circulation model ADCIRC and the steady finite-difference wave model STWAVE, and provides an example application for Grays Harbor, Washington. Remarkable differences in predicted current around the inlet entrance are found with and without the coupling.

: Shinnecock Inlet, New York, is a dual-jettied inlet located on the south shore of Long Island connecting Shinnecock Bay to the Atlantic Ocean. The down-drift beach, west of the inlet, experiences chronic erosion, and cost-effective and innovative measures for beach nourishment are being examined by the U.S. Army Engineer District, New York. The feasibility of mining of the flood and ebb shoals to serve as sources of material was examined in this report. Emphasis is on the concept of 'flood-shoal engineering' within an integrated inlet and beach system. Fifteen action alternatives were developed that involved dredging, modification of the jetties, and combined dredging and structural changes. The alternatives were evaluated by their potential changes to navigation conditions, availability of material for placement on the beach, changes to inlet and channel currents that would modify scour and deposition patterns, and changes in current strength near the beach that would modify erosion. The area of compatible material, established for the flood shoal from analysis of core samples, was the targeted mining area for the study and contains approximately 1.8 x 10(exp 6) cu yd of beach-compatible sand. Exploratory alternatives were also evaluated that involved dredging in other locations. Evaluation of alternatives was conducted through circulation, wave, and morphology modeling. A calibrated circulation model was applied to simulate each alternative and compare current strength and patterns to those for the existing condition. Wave modeling was conducted for one alternative that consisted of mining the attachment bar to determine changes in the wave patterns near the shore. Morphology modeling was conducted to calculate the long-term recovery rates of the system to mining of the flood shoal.

: This Coastal and Hydraulic Engineering Technical Note (CHETN) describes the methodology and input requirements for the leaky-barrier boundary feature in the Advanced Circulation (ADCIRC) coastal hydrodynamics code versions 40.02 and higher. This feature calculates flow over and through structures such as levees and jetties.

The sea breeze introduces harmonics of its diurnal cycle into water bodies, which can be notable for strong sea breeze and microtidal and nontidal conditions. The harmonics can be of the same magnitude as those produced by bottom friction and dominate those produced by nonlinear terms in the equations of motion. A closed-form analytic solution of the linearized depth-averaged equations of motion including friction is discussed for the situation of a sea breeze blowing on an idealized one-dimensional basin of constant depth. The solution reveals the generation of odd harmonics introduced by the quadratic wind stress and role of (linearized) friction. Sea-breeze forcing on the idealized basin is numerically modeled, and agreement with the analytic solution is found. The numerical model is then run with quadratic bottom friction and nonlinear terms to compare relative contributions to the generation of harmonics. Harmonics of the water motion are distinguished as forced, or arising from the wind forcing, and as response, or arising from the interactions within the water. The hydrodynamics of Baffin Bay, Texas, are modeled and spatial variation and relative strength of the harmonics investigated. Baffin Bay is a large shallow embayment with a weak connection to the Gulf of Mexico that experiences a strong southeast wind and sea breeze during the summer. The wind induces even and odd forced harmonics through the combined quasi-steady southeast wind and sea breeze. At Baffin Bay, ratios of the semidiurnal to diurnal amplitudes of water level and current speed are found to be comparable to M-4/M-2 ratios for U.S. Atlantic coast embayments.