Nonlinear Wave Evolution in Interaction With Currents and Viscoleastic Muds

A numerical model is extended to investigate the nonlinear dynamics of surface wave propagation over mud in the presence of currents. A phase-resolving frequency-domain model for wave-current interaction is improved to account for wave modulations due to viscoelastic mud of arbitrary thickness. The model compares well with published laboratory data and performs slightly better than the model with viscous mud-induced wave damping mechanism. Monochromatic and random wave simulations are conducted to examine the combined effect of currents, mud-induced wave dissipation and modulation, and nonlinear wave-wave interactions on surface wave spectra. Results indicate that current effects on wave damping over viscoelastic mud is not as straightforward as that over viscous mud. For example, while opposing currents consistently increase damping of random waves over viscous mud, they can decrease damping over viscoelastic mud due to high variations in frequency-dependent damping stemming from mud’s elasticity. It is shown that a model that assumes the mud layer to be thin for simplification can overestimate wave damping over thick mud layers

Modeling the Impacts of Sea Level Rise on Storm Surge Inundation in Flood-Prone Urban Areas of Hampton Roads, Virginia

Hampton Roads is a populated area in the United States Mid-Atlantic region that is highly affected by sea level rise (SLR). The transportation infrastructure in the region is increasingly disrupted by storm surge and even minor flooding events. The purpose of this study is to improve our understanding of SLR impacts on storm surge flooding in the region. We develop a hydrodynamic model to study the vulnerability of several critical flood-prone neighborhoods to storm surge flooding under several SLR projections. The hydrodynamic model is validated for tide prediction, and its performance in storm surge simulation is validated with the water level data from Hurricane Irene (2011). The developed model is then applied to three urban flooding hotspots located in Norfolk, Chesapeake, and the Isle of Wight. The extent, intensity, and duration of storm surge inundation under different SLR scenarios are estimated. Furthermore, the difference between the extent of flooding as predicted by the hydrodynamic model and the “bathtub” approach is highlighted

Characterizing Seagrass Effects on Hydrodynamics of Waves and Currents Through Field Measurements and Computational Modelling

Low-lying coastal and estuarine areas are among the most populated regions globally, have high economic significance, and are increasingly threatened by climate change, sea level rise, nuisance flooding, and extreme storms. Nature-based coastal protections are sustainable and sea-level resilient alternatives compared to traditional solutions such as dikes and seawalls. Submerged aquatic vegetation (SAV) or seagrasses can provide coastal flood and erosion protection by attenuating storm wave and current energy and stabilizing seabed sediments. However, more research is needed to understand the interactions between flow, SAVs, and sediments. These dynamic interactions affect flow at different scales and seagrass productivity. In this study, we present field measurements of current and wave evolution over a seagrass meadow in South Bay, Virginia. The high vertical resolution measurements show how currents change from above-canopy to in-canopy waters. Wave measurements indicate the dissipation and frequency modulation over the canopy. The results are compared with hydrodynamic simulations using a two-way coupled flow-vegetation interaction model that simulates nonlinear current and wave evolution as well as dynamics of highly flexible vegetation

Nonlinear Interactions between Longs Waves in a Two-Layer Fluid

The nonlinear interactions between long surface waves and interfacial waves in a two-layer fluid are studied theoretically. The fluid is density-stratified and the thicknesses of the top and bottom layers are both assumed to be shallow relative to the length of a typical surface wave and interfacial wave, respectively. A set of Boussinesq-type equations are derived for potential flow in this system. The equations are then analyzed for the dynamics of the nonlinear resonant interactions between a monochromatic surface wave and two oblique interfacial waves. The analysis uses a second order perturbation approach. Consequently, a set of coupled transient evolution equations of wave amplitudes is derived. Moreover, the effect of weak viscosity of the lower layer is incorporated in the problem and the influences of important parameters on surface and interfacial wave evolution (namely the directional angle of interfacial waves, density ratio of the layers, thickness of the fluid layers, surface wave frequency, surface wave amplitude, and lower layer viscosity) are investigated. The results of the parametric study are discussed and are generally in qualitative agreement with previous studies. In shallow water, a triad formed of surface waves (or interfacial waves) can be considered in near-resonant interaction. In contrast to the previous studies which limited the study to a triad (one surface wave and two interfacial waves or one interfacial and two surface waves), the problem is generalized by considering the nonlinear interactions between a triad of surface waves and three oblique pairs of interfacial waves. In this system, each surface wave is in near-resonance interaction with other surface waves and in exact resonance with a pair of oblique interfacial waves. Similarly, each interfacial wave is in near-resonance interaction with other interfacial waves which are propagating in the same direction. Inclusion of all the interactions considerably changes the pattern of evolution of waves and highlights the necessity of accounting for several wave harmonics. Effects of density ratio, depth ratio, and surface wave frequency on the evolution of waves are discussed. Finally, a formulation is derived for spatial evolution of one surface wave spectrum in nonlinear interaction with two oblique interfacial wave spectra. The two-layer Boussinesq-type equations are treated in frequency domain to study the nonlinear interactions of time-harmonic waves. Based on weakly two-dimensional propagation of each wave train, a parabolic approximation is applied to derive the formulation

Editorial: Coastal Flooding: Modeling, Monitoring, and Protection Systems

Coastal flooding has received significant attention in recent years due to future sea-level rise (SLR) projections and intensification of precipitation, which will exacerbate frequent flooding, coastal erosion, and eventually create permanently inundated low-elevation land. Coastal governments will be forced to implement measures to manage risk on the population and infrastructure and build protection systems to mitigate or adapt to the negative impacts of flooding. Research in this area is required to establish holistic frameworks for timely and accurate flooding forecast and design of protection systems

Dynamic Modeling of Inland Flooding and Storm Surge on Coastal Cities Under Climate Change Scenarios: Transportation Infrastructure Impacts in Norfolk, Virginia USA as a Case Study

Low-lying coastal cities across the world are vulnerable to the combined impact of rainfall and storm tide. However, existing approaches lack the ability to model the combined effect of these flood mechanisms, especially under climate change and sea level rise (SLR). Thus, to increase flood resilience of coastal cities, modeling techniques to improve the understanding and prediction of the combined effect of these flood hazards are critical. To address this need, this study presents a modeling system for assessing the combined flood impact on coastal cities under selected future climate scenarios that leverages ocean modeling with land surface modeling capable of resolving urban drainage infrastructure within the city. The modeling approach is demonstrated in quantifying the impact of possible future climate scenarios on transportation infrastructure within Norfolk, Virginia, USA. A series of combined storm events are modeled for current (2020) and projected future (2070) climate scenarios. The results show that pluvial flooding causes a larger interruption to the transportation network compared to tidal flooding under current climate conditions. By 2070, however, tidal flooding will be the dominant flooding mechanism with even nuisance flooding expected to happen daily due to SLR. In 2070, nuisance flooding is expected to cause a 4.6% total link close time (TLC), which is more than two times that of a 50-year storm surge (1.8% TLC) in 2020. The coupled flood model was compared with a widely used but physically simplistic bathtub method to assess the difference resulting from the more complex modeling presented in this study. The results show that the bathtub method overestimated the flooded area near the shoreline by 9.5% and 3.1% for a 10-year storm surge event in 2020 and 2070, respectively, but underestimated the flooded area in the inland region by 9.0% and 4.0% for the same events. The findings demonstrate the benefit of sophisticated modeling methods compared to more simplistic bathtub approaches, in climate adaptive planning and policy in coastal communities

Defining boat wake impacts on shoreline stability toward management and policy solutions

Coastal economies are often supported by activities that rely on commercial or recreational vessels to move people or goods, such as shipping, transportation, cruising, and fishing. Unintentionally, frequent or intense vessel traffic can contribute to erosion of coastlines; this can be particularly evident in sheltered systems where shoreline erosion should be minimal in the absence of boat waves. We reviewed the state of the science of known effects of boat waves on shoreline stability, examined data on erosion, turbidity, and shoreline armoring patterns for evidence of a response to boat waves in Chesapeake Bay, and reviewed existing management and policy actions in Chesapeake Bay and nearby states to make recommendations for actions to minimize boat wake impacts. In the literature, as well as in our analyses, boat wake energy may be linked to elevated turbidity and shoreline erosion, particularly in narrow waterways. In Chesapeake Bay, three lines of evidence suggest boat waves are contributing to shoreline erosion and poor water clarity in some Bay creeks and tributaries: 1) nearshore turbidity was elevated in many waterways during periods of expected high boating activity, 2) armoring was placed along about a quarter of the low energy shorelines of three examined tidal creeks that are exposed to relatively high boating pressure, and 3) 15% of the shorelines we examined throughout the Bay (9000 km) are low energy shorelines that are experiencing high erosion (≥0.3 m/yr) that cannot be attributed to wind wave energy. Still, there remain significant data gaps that preclude the determination of the overall contribution of boat waves to shoreline erosion throughout the Bay, notably, shoreline erosion data in low energy waterways, recreational boating traffic patterns, and nearshore bathymetry. Interim protective measures can be (and have been) applied in high risk waterways, such as small, low energy waterways that have high recreational boating activity, to help reduce shoreline erosion. Policy options used in Bay states and elsewhere include setbacks from the shore, wake restrictions, and speed restrictions; other more restrictive policies may include prohibition on boats of a certain size or limiting the number of passages. Finally, a systems-approach to boat wake impact management using uniform boat wake policies is likely to be the most effective for consistent shoreline protection