The Dynamics of Cobble in and Near the Surf Zone

Abstract

Abstract : This research was driven by the need of the Navy to improve mine countermeasures in support of joint littoral warfare, for which a key paradigm is to locate a clear or low mine- and obstacle-density path. To accomplish this goal, it is imperative that one be able to predict the behavior of mines in shallow water under a variety of coastal-ocean conditions. A laboratory experimental program, coupled with theoretical modeling, was conducted at Arizona State University for the purpose of improving scientific understanding and predictive capabilities of the dynamics of the motion and scour/burial of cobbles/mines within very shallow water in and near the surf zone. Results of these studies are summarized in this report. To model the oscillatory water motion in different regions of the surf zone, three experimental installations were constructed and used: (1) dam break tank for flow in a swash region, (2) standing wave tank for oscillatory flow near the surf and (3) large wave tank to model flow in the entire surf zone, including wave breaking region. The data on the motion of cobbles/mines along solid bottom at different flow conditions were collected and the results were explained theoretically. In the case of a sandy bed, detailed data on the bottom morphodynamics (sand ripples and bars formation and evolution) were collected and explained theoretically. Different scenarios of cobble/mine behavior on a sandy bed were simulated experimentally and explained theoretically. These include: (1) slow, mostly onshore, drift of cobbles, (2) scour around cobbles, (3) periodic burial under drifting ripples, and (4) permanent burial under migrating sand bars.