Geotechnical evaluation of overwash chenier beach deposits

Rockefeller Wildlife Refuge in southwestern Louisiana continues to retreat along a shelly chenier shoreline. Shell hash dominates the beach, and this fabric behaves differently from typical quartz sands during loading and shearing. Segmented breakwaters were installed along parts of the refuge beginning in 2018 to reduce wave energy and land loss. The hydrodynamic benefits of breakwaters are documented, yet the longer-term effects on sediment fabric and geotechnical strength behind them are less well understood. To examine these effects, two adjacent cross-shore transects were evaluated one of which remained exposed to waves without any shoreline protection (Natural) and one protected by a breakwater (Hybrid). The analysis combined PANDA DCPT with laboratory measurements of D50 and Cu, minimum and maximum void ratio, direct shear tests, and dilatancy. Comparisons were made at two scales. First, a sample-to-sample assessment was conducted on thirty shell-hash specimens with a wide range of D50 and Cu values. Peak friction angle (ϕ′) varies nonlinearly with median grain size. Values rise quickly as D50 approaches ~1.0 mm (coincident with minimum void ratio) and then decline more gradually for coarser material. Higher Cu (poorer sorting) aligns with lower ϕ′ and reduced dilatancy. The maximum dilation angle ψmax follows the same pattern and peaks near D50 around 1.0 mm. Second, a site-by-site comparison reveals a systematic fabric shift behind the breakwater. Hybrid transect particle-size distributions show smaller D50 than Natural transect counterparts; Cu frequently exceeds 5, indicating poorer sorting. PANDA DCPT profiles suggest a thinner, weaker shell layer in the protected transect (~0.2 to 0.8 m versus ~0.4 to 1.0 m along Natural). These patterns imply a management trade-off. Breakwaters reduce wave energy and can limit storm-induced shoreline retreat in the short-term. The protected transect tends to retain finer and poorly sorted material with lower shear strength. Because the threshold for motion decreases with particle size, waves that reach the lee during storm-energy conditions can more readily remobilize the protected area; over longer timescales, sea-level rise may further increase the frequency of such exposure. Thus, shoreline protection alters fabric and strength, supporting short-term stability but potentially reducing longer-term geotechnical resilience. Design and modeling should incorporate the coupled roles of D50, Cu, and void ratio when estimating ϕ′ and dilatancy for this environment.