Structure

Introduction

In coastal applications, it is important for hydrodynamic and sediment transport models to simulate wave transmission and flow penetration through rubble mound structures (d’Angremond et al. 1996; Garcia et al. 2004; Tsai et al. 2006). The present study applies the methodology simulating the permeability of rubble mound structures (Reed 2010) in the CMS to Dana Point Harbor on the southern California coast. The hydrodynamic calibration and sediment transport validation are conducted against the field measurements.

Study area and data collections

Dana Point Harbor is located in Orange County on the US Pacific coast, 40 miles southeast of Los Angeles, CA. The harbor is entirely manmade and is protected from ocean waves by a pair of riprapped breakwaters constructed in the late 1960s. The breakwaters, consist of a long shore-parallel West Breakwater of 5,500 ft and a shore-normal East Breakwater of 2,250 ft (Figure 1), were designed as permeable structures. As these structures can dissipate wave energy and reduce wave reflection, the current and sediment transport can pass through. As a result, fine sands are accumulated inside the West Breakwater and maintenance dredging is required periodically (County of Orange 2009).

Figure 1. Dana Point Harbor and the surrounding area. The red line denotes the CMS domain.

A field data collection program was designed for this study. A hydrographic multi-beam survey for the underwater portion along with the above-water mapping via the LiDAR scanning technology for the breakwaters was conducted in October 2009 (Fugro West 2010). Bathymetric data were also collected in the marina basins, harbor entrance and nearshore areas outside the harbor. Two ADCPs were deployed from November 2009 through January 2010 to collect current and water level inside (without wave data collection) and outside (with wave data collection) the West Breakwater (Figure 1). Because of the instrument failure, only initial six days of data were recovered from the outside ADCP. In the last 20 years, Orange County has conducted three maintenance dredges to remove fine sand material that moved through and deposited on the harbor side of the West Breakwater. The dredged volumes inside the West Breakwater are approximately 25,000 cy in 1990, 35,500 cy in 1999, and 54,000 cy in 2009 (County of Orange 1990; 1999; 2009).

Methodology

In the CMS, both the West and East Breakwaters were specified as permeable structures, through which wave transmission, flow and sediment seepage were implemented. Figure 2 shows the CMS rectangular grid domain that consists of 399 ´ 327 cells surrounding Dana Point Harbor. The water depth ranges from 0 m at Baby Beach to 9 m at the harbor entrance channel inside the marina and increases to more than 10 m outside the West Breakwater. The offshore area further deepens to a few hundred meters and is open to the Pacific Ocean. A variable-resolution grid system was created to discretize the entire harbor and the offshore region, which permits much finer grid resolution (5 m) in areas of high interest such as the harbor and the breakwaters. The model domain extends approximately 5 km alongshore and 4 km offshore, and the offshore boundary of the domain reaches to the 300‑m isobath.

Figure 2. The CMS model domain and configuration.