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1. Small-Scale Experiments’ Ability To Augment Large Lab Testing For Designing Nature-Based And Hybrid Solutions For Coastal Flood Hazard Mitigation

   https://doi.org/10.59490/coastlab.2024.697  

   MCMANN, DAN; KECK, JORDAN; TOMICZEK, TORI; LOMONACO, PEDRO; COX, DAN; LIBBY, MARGARET (April 2024, CoastLab 2024: Physical Modelling in Coastal Engineering and Science)

   Mangroves and other natural coastal defenses have the potential to augment or replace traditional engineered coastal structures in preventing adverse events such as wave overtopping. Natural, or “green” systems may reduce maintenance costs, reduce sediment erosion, and increase biodiversity compared to traditional “gray” infrastructure built from stone and concrete. To effectively inform the design of hybrid green-gray infrastructure, experimental results must be reliable, but testing at 1:1 scale is time-consuming, expensive, and available at only a few facilities worldwide. This study addresses a knowledge gap in defining the nature of the interactions between green and gray coastal defenses with a focus on overtopping and scaling experimental results. This study will compare data from mangrove-related experiments conducted at scales including 1:2 and 1:8 as part of a collaborative effort between Oregon State University (OSU) and the United States Naval Academy (USNA). The study aims to analyze this data and contribute to the joint compilation of a methodology for designing prototype-scale tests from small-scale experiments to identify the relative importance of friction and scaling effects between prototype and small-scale experiments. Testing conducted at USNA as part of this study included a 1:8 scale, 0.61m-wide (2ft.) flume that replicates the conditions of 1:2 scale experiments at Oregon State University. The experimental setup includes a model Rhizophora mangrove forest placed in front of a seawall, behind which overtopping is measured as volume per unit length either computed from overtopped water weight or directly measured by overtopped volume. Mangroves are modeled as central trunks with stilt roots, as this study focuses on the effects of the root structures on overtopping. Waves generated for the 1:8 experiments include regular waves with heights between 5cm and 10cm and periods between 1 and 2 seconds, scaled according to Froude similitude. Implications of scaled-up measurements of overtopping are also discussed. 

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2. Physical model comparison of gray and green mitigation alternatives for flooding and wave force reduction in an idealized urban coastal environment

   https://doi.org/10.1016/j.coastaleng.2023.104339  

   Van Dang, Hai; Park, Hyoungsu; Shin, Sungwon; Tomiczek, Tori; Cox, Daniel T.; Lee, Eunju; Lee, Dayeon; Lomonaco, Pedro (September 2023, Coastal Engineering)

   A 1:16 scaled physical model was constructed to investigate the effectiveness of a seawall, a submerged breakwater, and mangrove forests to mitigate overland flooding and forces on structures in an idealized urban coastal environment. The experiment was performed using tsunami-like waves at different water levels, wave amplitudes, and time scales to simulate long-wave dynamics. The baseline condition (no mitigation), seawall, submerged breakwater, and mangrove forest were tested individually, and the seawall and submerged breakwater were also tested in combination. Wave gauges, acoustic Doppler velocimeters, loadcells, and pressure gauges were used to measure wave elevations, velocities, forces, and pressures on coastal structures, respectively. The performance of these hard structures and mangroves was compared through their effects on wave elevation, particle velocity, and force reduction. Experimental results showed that each protecting structure reduced the horizontal wave forces and inland flow hydrodynamics in the low-water-level case, with a similar performance by the individual seawall, submerged breakwater, and four rows of mangroves. The combined configuration, when the seawall and submerged breakwater were installed simultaneously, caused the most significant maximum force percent reduction by approximately 50%, while mangrove forests arranged in eight rows resulted in a force reduction of 46% in the first building array. However, in the high-water-level cases, the impulsive force measured with the presence of the submerged breakwater was larger than in the baseline case; thus, the submerged breakwater may amplify the impulsive force on the vertical building rows for certain incident wave conditions. Generally, the combined hard structures induced the lowest force reduction factor measured in almost every building row compared to the seawall, submerged breakwater, and mangroves considered separately for all wave conditions and water levels. That means this multi-tiered configuration showed better performance than individual alternatives in reducing horizontal forces inland than the individual alternatives considered separately. 

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3. The sum of the parts: Green, gray, and green-gray infrastructure to mitigate wave overtopping

   https://doi.org/10.1016/j.coastaleng.2024.104615  

   Libby, Margaret; Tomiczek, Tori; Cox, Daniel; Lomónaco, Pedro (December 2024, Coastal Engineering)

   Hybrid approaches to shoreline protection, where natural (“green”) features are combined with hardened (“gray”) infrastructure, are increasingly used to protect coastlines from erosion and flood-based hazards. Our understanding of hybrid systems is limited, and it is unknown whether the components of these systems interact in any meaningful sense to provide flood reduction benefits that are greater or less than “the sum of the parts.” In this study, a large-scale physical model was used to investigate the overtopping of a vertical wall protected by a hybrid system where an idealized Rhizophora mangrove forest of moderate cross-shore width fronted a rubble-mound revetment. Configurations included the wall alone, the wall with a low- or intermediate-density mangrove forest without the revetment, the wall with the revetment, and the wall with an intermediate- or high-density mangrove forest and the revetment. The study isolated the reduction in overtopping of the wall by the revetment component, the mangrove forest component, and the interaction between the components of the hybrid system. The total reduction by the hybrid system was estimated within 5% accuracy as the sum of the reduction by each component minus the product of the component reductions. Comparison of the proportional reduction in overtopping by the mangrove forest on the wall alone and the wall with the revetment indicated that the mangrove forest reduced the overtopping of the revetment by approximately the same proportion that the forest reduced the overtopping of the wall. Therefore, (1) total overtopping reduction by the hybrid system was modeled as the reduction expected from the green and gray components in series. Additional analysis showed that (2) for the same wave conditions, a mangrove forest of moderate cross-shore width can have equal or greater protective benefits than a coastal revetment, (3) there is an exponential relationship between the discharge rate and the forest density, and (4) the mangrove forest, the revetment, and the hybrid system all provided greater reduction in overtopping as wave steepness increased. The tests in this study were conducted without wave breaking, with constant freeboard and water depth, with a specific revetment geometry, and without a mangrove canopy. Therefore, these results should be interpreted with caution if used for engineering design. 

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4. Quantifying Overtopping Performance Of Green-Gray Hybrid Infrastructure

   https://doi.org/10.59490/coastlab.2024.734  

   LIBBY, MARGARET; TOMICZEK, TORI; T_COX, DANIEL; LOMONACO, PEDRO (April 2024, CoastLab 2024: Physical Modelling in Coastal Engineering and Science)

   Wave overtopping of shoreline infrastructure can lead to significant flooding and consequent loss of life, impairment of transportation systems, and ecological damage. Coastal defenses against overtopping traditionally include hard structures, such as seawalls and revetments, and design guidelines for these structures, e.g., the EurOtop manual (Van der Meer et al., 2018), have been developed from empirical studies of overtopping. Recently, natural and nature-based features (NNBF) including mangroves, wetlands, reefs, and other systems have gained attention as alternatives to conventional engineered coastal protection systems. Field observations have identified the potential of emergent vegetation, particularly mangrove forests, to mitigate damage during extreme coastal flood events (Alongi, 2008; Tomiczek et al., 2020). However, there is a lack of research on engineering NNBF systems to achieve specific design requirements for overtopping protection. Hybrid or multi-tiered approaches to shoreline protection have also been proposed, where natural (“green”) features are combined with hardened (“gray”) infrastructure to protect coastlines and near-coast assets from erosion and/or flood-based hazards. For overtopping mitigation, hybrid designs can add the performance provided by emergent vegetation to the services of a revetment or a wall. It is unknown whether the green and gray features in a hybrid system perform independently and can be considered as separate design elements, or if the inclusion of one feature affects the performance of the other such that the hybrid system must be considered as a single, complex design element. This study constructed a large-scale physical model to investigate the overtopping performance of a hybrid system with an idealized Rhizophora mangrove forest seaward of a revetment abutting a vertical wall compared to that performance of the wall fronted by the revetment only, the wall fronted by vegetation only, and the wall alone. 

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5. Regional Data-Driven Modeling of Levee Failure Due to Overtopping

   Azhar, M; Vahedifard, F; AghaKouchak, A (May 2024, Geotechnical Frontiers 2025)

   There are growing concerns regarding the increase in flood risk due to climate change and land use/land cover changes. In light of these changes, levees play an increasingly critical role in safeguarding communities, infrastructure, and the natural environment. However, the average age of levees in the United States is 60 years, with the majority operating under marginal conditions. The most common failure mode of earthen levees is breach due to overtopping. Existing methodologies evaluate the site-specific probability of levee overtopping and do not provide a holistic view of flood risk across a wider area. Here we present a regional-scale overtopping model for levees using a data-driven overtopping model that uses five variables: (1) levee construction classification, (2) overtopping depth, (3) overtopping duration, (4) erosion resistance classification, and (5) duration of levee loading before overtopping. The overtopping model is applied to levee systems in Wilton, California. The probability of breach due to overtopping is presented for three distinct scenarios (overtopping duration) during a 50-year flood event. The results show the probability of breach for the Wilton Levee ranges from 0.32 to 0.91 for overtopping durations of 6 hours and between 6 to 24 hours. For durations exceeding 24 hours, the probability of breach increases to a range of 0.73 to 0.98. The proposed framework offers a viable tool for performing regional-scale levee risk assessment, offering broader implications for enhancing preparedness, response, and recovery strategies in the face of escalating flood risks. 

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