Coastal Engineering

Challenge. Quantify potential erosion and toe scour to aid in fast-track design of seawall proposed in response to erosion caused by Hurricane Sandy and the Thanksgiving Day storm

Solution. In fall 2012, Hurricane Sandy undermined sidewalks, an existing decorative wall along State Road A1A, and portions of the roadway. To support the installation of an emergency steel sheet pile seawall along this damaged section of roadway, INTERA quantified potential beach erosion and toe scour during storm events to aid in design of the proposed seawall. We also examined beach fill alternatives to quantify benefits and the reduction of scour at the wall. INTERA examined historical shoreline behavior; developed representative beach profiles; applied the United States Army Corps of Engineers (USACE) cross-shore erosion model, SBEACH, to evaluate storm-induced scour for 10-, 20-, 50-, and 100-year storms; and determined local scour at the seawall for selected wall design lives. Concurrently, INTERA examined minimum (defined by permitting agencies) and desired (defined by client) beach fill templates to quantify their effects on reducing scour at the seawall to support environmental permitting. Qualitative analyses included beneficial effects of beach fill on reducing wave damage potential to upland infrastructure and possible detrimental effects of no beach fill on fronting and adjacent beaches. We also delineated a potential upland source of beach fill material. The permitting agencies approved our analyses without revision.

Challenge. Estimate design scour at existing and proposed bridge piers under large, tidal-influenced flow conditions and deep water depths

Solution. INTERA performed hydraulic and scour analyses for the existing and proposed Tacoma Narrows Bridge piers. This site experiences large astronomical spring tides and tidal induced flow velocities. Additionally, the water depths range from 120 feet at the channel piers to greater than 250 feet in the center of the channel. Both existing and proposed piers are large, rectangular in shape, and skewed to the flow. These conditions, combined with the fine sand bed materials made for a challenging scour analysis. INTERA configured a 2-dimensional, depth averaged computer model (RMA-2 model) for the tidal waters encompassing the bridge site, and extending from Puget Sound to Olympia, Washington. The model calibration data included measured water elevation, velocity, and discharge data at and near the bridge site. The model simulated flow conditions for spring tides as well as for the 100-year, and 500-year return interval design conditions. To address the close proximity of the new and existing piers, INTERA designed, conducted, and analyzed physical model local scour tests.

Challenge. Design shoreline structures to meet the level of protection specified by the National Economic Development (NED) plan

Solution. Taking advantage of a large bedrock outcropping (Morgan Shoal) that lies just offshore, the City of Chicago and Chicago Department of Transportation considered an alternative shoreline protection scheme—a series of offshore breakwaters/islands to support ecological functions/habitat in their lee—in lieu of the NED plan’s rubblemound revetment along the 45th Street to 51st Street segment of this federal shore protection project. INTERA defined the appropriate wave climate near the project by employing the United States Army Corps of Engineers’ (USACE) Coastal Modeling System wave model (CMS-Wave) to transform waves from the offshore Wave Information Study station to the site and examined breakwater/island configurations and substrate stability in their lees via empirical formulas and desktop assessment tools. During the course of the design process, the public wanted to preserve the shoal and suggested a scaled back shoreline alternative. The reduced conceptual plan consisted of an onshore rubblemound revetment with an amphitheater area (comprising special placement of cut stone) and a small pebble beach, to mimic the existing pebble beach, in front of the amphitheater toe. INTERA developed typical sections of the rubblemound revetment including areas where the shoreline extended into the lake beyond the existing shoreline and the amphitheater area. We also participated in a meeting with the USACE Chicago District to ensure the design met federal and NED standards.

Challenge. Determine appropriate repairs or replacement of articulated concrete block (ACB) mattress installed to protect nearly 14 miles of roadway and embankment

Solution. County Road 707 along the Indian River experienced major damage during the 2004 hurricane season. Subsequent emergency repairs to the roadway and its embankment included installing ACB mattress. Storm events in 2005 caused considerable damage to portions of the repaired shoreline. INTERA assessed the condition of the existing ACB, modeled wave conditions at the site, and developed appropriate alternatives for repair or replacement of the ACB. Our engineering evaluation revealed that the damaged ACB mattress revetment could only withstand conditions corresponding to an approximately three-year return period event. Given these facts, INTERA developed three conceptual alternative designs. Alternatives considered removal of the existing revetment. Since this would have required significant costs and further disturbed this environmentally sensitive area, the desired alternative incorporated the existing protection and provided for offshore wave attenuation via mangrove plantings. This alternative had the advantage of a low cost, the environmental benefits associated with a mangrove forest, incorporating the existing revetment, and not interfering with any protection installed by homeowners because it sat below mean high water.