Assessing the Impact of an Organic Restoration Structure on Boat Wake Energy

Erosion of unprotected levee banks decreases their structural integrity and increases the likelihood of failure. Several types of restoration structures for levee protection and stabilization have been used in the Sacramento–San Joaquin River Delta, California, to reduce erosion. The purpose of this paper is to describe the results of a field experiment designed to measure the effectiveness of organic restoration structures (brush bundles) in altering the hydrodynamic regime in the vicinity of levees, with specific focus on changes in boat wake energy. Two simple hypotheses were tested: 1) restoration structures reduce boat wake energy, and 2) energy reduction is dependent on water depth. Field work was conducted August 29-31, 2000 on Georgiana Slough, which is a tidally influenced (spring tidal range of 2 meters) distributary of the Sacramento River. Pressure sensors were deployed offshore and landward of the restoration structures. Data collection occurred with the bundles in place and with them removed. Boat wakes were generated during rising and falling tides to capture the effects of fluctuating water levels. Wakes were characterized by index wave height, period and energy. Comparing sample means of normalized energy with the bundles removed and with the bundles in place revealed a 60% reduction of energy by the bundles. It was also determined that energy reduction was tidally, or depth dependent. The reduction of energy by the structures indicates that they are an effective method to protect against boat-wake induced, levee erosion. ADDITIONALINDEXWORDS: Sacramento-San Joaquin River Delta, levee erosion Journal of Coastal Research SI 36 256-265 (ICS 2002 Proceedings) Northern Ireland ISSN 0749-0208 Boat Wake Energy 257 Journal of Coastal Research, Special Issue 36, 2002 Energy Dissipation Through Structures Previous work has examined energy dissipation through various coastal protective structures. WALTHER and LEE (1975) and MASSEL and BUTOWSKI (1980) examined the hydrodynamic effects of porous breakwaters. FONSECAand CAHALAN (1992) and KOBAYASHI et al. (1992) considered wave energy dissipation through sea grass and MASSEL et al. (1999) considered dissipation through mangrove forests. In addition, research has been conducted to quantify wave interaction, transformation and energy reduction over coral reefs (ROBERTS et al., 1975 and 1992; ROBERTS and SUHAYDA, 1983; LUGOFERNÁNDEZ et al., 1998 and 1998a). Several methods have been used to quantify energy reduction. ROBERTS et al. (1975), WALTHER and LEE (1975), and ROBERTS and SUHAY D A (1983) used spectral energy density and LUGO-FERNÁNDEZ et al. (1998; 1998a) and ROBERTS et al. (1992) calculated a wave transmission coefficient. FONSECAand CAHALAN (1992), ROBERTS et al. (1992), and LUGO-FERNÁNDEZ et al. (1998; 1998a) quantified energy reduction, Ered, using changes of wave height, and calculated a corresponding energy reduction: