Dynamic revetments, a type of nature-based feature used to mitigate erosion, are engineered cobble berms placed on the upper beach to protect the backshore, and are inspired by the natural protective capabilities of cobble beaches. Dynamic revetments are one of the few nature-based erosion control mechanisms that could have the capacity to withstand energetic Pacific Northwest waves, and have proven successful in several cases on the coast. However, there is still relatively little known about the best way to design dynamic revetments, how often they need to be maintained, and the processes by which they reshape, dissipate wave energy, and promote the accretion of sand. We are using a combination of field and lab studies to investigate the performance of dynamic revetments and understand their behavior. We aim to use our results to inform engineering design practices.
Field studies:
Our fieldwork is focused on three beaches in the Pacific Northwest: Arch Cape, OR and Falcon Cove, OR, which are both naturally-occurring composite beaches, and Westport, WA, which has a recently constructed dynamic revetment. We have installed cameras at the three sites to monitor the runup of waves onto the cobble berms. We also have been collecting topography data throughout the active winter season at all three sites since winter 2022 to capture the changes in the cobble. In the 2023-2024 winter, we carried out an cobble tracking campaign at all 3 sites using Radio Frequency Identification (RFID) tags embedded in the cobble. The RFID tags enable the movement of the 300 tagged cobbles to be tracked using an antenna. We visited the field sites throughout the winter, including before and after storm and high water level events, to capture the movement of cobbles throughout the winter. We hope to use these results to inform the predicted maintenance for dynamic revetments.
Part of our field work also involves characterizing the grain size distribution of existing cobble berms, to better understand why those sites might suffer less erosion than nearby pure sand beaches. Better understanding the morphological characteristics of natural cobble berms will help us understand their dynamics, and will contribute to basic guidelines for engineered dynamic revetments. Cobbles are measured with photogrammetric techniques that rely on spectral properties of the photograph to generate grain size distributions, allowing the convenient generation of a much larger data set than would manual techniques.
We also study how aeolian processes impact dynamic revetments. During low tide, the sandy beach in front of the dynamic revetment becomes exposed and strong winds can move sand onto the revetment. In the field, we have measured wind velocities and sediment transport on the dynamic revetment in Westport, WA. We will expand this work by using an aeolian sediment transport model to calculate how much sand gets moved onto the dynamic revetment in a year.
Lab study:
Based on our experience in the field, we are also designing a lab experiment to study the impacts of dynamic revetment design (including total cobble volume, initial slope, and cobble specifications) on dynamic revetment performance. The lab study will be conducted at the US Army Corps Engineering Research Facility (EDRC) in Vicksburg, Mississippi in Fall 2024.
