Effectiveness of Staggered Pile Arrangements for Managing River Mouth Closures on Gravel Beaches

Several small- and medium-sized rivers connecting to the Shichiri-Mihama coast are closed due to debris. The progression of river mouth closures increases the risk of flooding in the watershed, so countermeasures are necessary. In this study, piles were installed in a staggered pattern to prevent river mouth closures. The hydraulic model experiment was conducted to assess how countermeasure (staggered piles) installation affects topographic change under various wave conditions. The installation of the staggered piles was found to be effective in moving the location of the berm forward more than 10 cm offshore and reducing gravel accumulation by up to 57%. The effect of the staggered piles is particularly significant in wave conditions with small wave gradients. The pile row spacing should be short to maintain the complexity of the staggered arrangement. The effects of changes in the flow velocity field and differences in the bottom sediment should also be considered for practical use. Eventually, the installation of the staggered piles will be expected to prevent the long-term development of berms, thus preventing estuary closure.