CARMEL RIVER LAGOON RESTORATION Scenic Road Protection Options
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CARMEL RIVER LAGOON RESTORATION Scenic Road Protection Options Prepared Under Contract to Whitson Engineers for: Monterey County Water Resources Agency 893 Blanco Circle, Salinas, CA 93901-4455 Prepared by: moffatt & nichol 2185 No California Blvd., Ste. 500 Walnut Creek, CA 94596 February 25, 2013 M&N Job No: 7871 Scenic Road Protection Options CONTENTS 1. INTRODUCTION ................................................................................................................. 1 2. Data Summary .................................................................................................................... 4 2.1 Topography ................................................................................................................ 4 2.2 Bathymetry ................................................................................................................. 4 2.3 Geotechnical Conditions ............................................................................................ 4 2.4 Additional Information ................................................................................................ 5 3. Coastal Processes .............................................................................................................. 6 3.1 Water Levels .............................................................................................................. 6 3.2 Wave Climate ............................................................................................................. 6 3.3 Sea Level Rise ........................................................................................................... 7 3.4 Littoral Transport ........................................................................................................ 8 3.5 Beach Morphology ..................................................................................................... 8 3.6 River Breaching .......................................................................................................... 9 4. Conceptual Protection Options .......................................................................................... 11 4.1 Alternative 1 - Revetment (Rip Rap) Located at the Toe of Slope ........................... 12 4.2 Alternative 2 - Seawall Located at Toe of Slope ...................................................... 12 4.3 Alternative 3 - Reinforced Earth Wall Located at Mid-Slope .................................... 13 4.4 Alternative 4 - Pile Wall Located at Top of Slope ..................................................... 14 5. REFERENCES .................................................................................................................. 16 APPENDICES A. County Aerial Photographs i Scenic Road Protection Options FIGURES 1. Project Site Overview 2. Depths (in fathoms, MLLW) near project site (NOAA Chart) 3. Nearshore Bathymetry (in ft, MLLW) 4. Wave Conditions (Annual Average) for Monterey Bay (~30 Miles Northwest of Project Site) 5. Wave Conditions (Annual Average) for Point Sur (~50 Miles South of Project Site) 6. Winter (left) and Summer (right) Wave Height for Monterey Bay 7. Winter (left) and Summer (right) Wave Peak Period for Monterey Bay 8. Observed Seasonality in Wave Conditions for Monterey Bay (Winter (left) and Summer (right)) 9. Wave Refraction Diagram for WNW, 11-second Swell (Howell, 1972) 10. Example of Nearshore Wave Model Domain (PMS Model) 11. Example of Nearshore PMS Wave Model Results 12. Alternative 1 – Riprap 13. 10+00 Cross Section Alternative 1 – Riprap 14. 14+00 Cross Section Alternative 1 – Riprap 15. 16+00 Cross Section Alternative 1 – Riprap, Alternative 2 – Seawall & Riprap 16. Alternative 2 – Seawall & Riprap 17. 10+00 Cross Section Alternative 2 – Seawall & Riprap 18. 14+00 Cross Section Alternative 2 – Seawall & Riprap 19. Alternative 3 – Reinforced Earth & Riprap 20. 10+00 Cross Section Alternative 3 – Soil Nail Wall & Riprap Grade Control 21. 14+00 Cross Section Alternative 3 – Soil Nail Wall 22. Alternative 4 – Pile Wall at Edge of Road 23. 14+00 Cross Section Alternative 4 – Pile Wall at Edge of Road ii Scenic Road Protection Options 1. INTRODUCTION This report presents an overview of the Scenic Road Protection Project, and provides a description of potential bluff protection options for times when Scenic Road is threatened by a northerly meandering Carmel River. The project description below describes the Carmel River history and management drivers. Subsequent sections include a summary of available topography and bathymetry, a description of the coastal processes influencing the beach dynamics along Scenic Road, and potential bluff protection options. The Carmel River drains an approximate 250 square mile watershed. Like many California streams entering the Pacific, there is a very productive estuary at the mouth, called the Carmel River Lagoon, which serves as rearing habitat for juvenile steelhead. The Carmel River Lagoon supports threatened species such as the South-Central California Coast (S-CCC) steelhead, California red-legged frog, Western snowy plover, and Smith’s blue butterfly. The Carmel River was also designated as critical habitat for S-CCC steelhead in September 2005 (County of Monterey et. al, 2012). The lagoon is not connected to the ocean during times of low or no river flow, when waves build a barrier beach across the mouth of the river. When the mouth of the river is closed, lagoon water levels rise between late fall and spring seasons and threaten private properties along the northern edges of the lagoon, as well as a parking lot and restroom facility operated by the State Parks. Therefore, since 1973 the County has mechanically breached the sandbar. Evidence shows that the barrier beach has been breached since at least the early 20th century (Final Study Plan, 2007). Historically, the breaches were created such that the river would flow directly west out to the Pacific Ocean; however, the river meanders north or south along the beach in response to beach build-up from wave action. Often numerous breaches are mechanically and naturally formed over the winter and early spring, depending on variable ocean and river conditions (James, 2005). Beginning in 2005, the sandbar has also been mechanically closed in the spring when the river flows have subsided in order to maintain adequate water levels for steelhead throughout the summer. A larger, deeper lagoon during the summer and fall increases the quality and quantity of fish and wildlife habitat until river flows resume during the winter (County of Monterey et. al, 2012). The rate of the lagoon drawdown after a breach and the post-breach lagoon water surface elevation are dependent on several factors, including breach location, channel length and width, tidal conditions, and the presence or absence of a rock sill along the outflow channel. Breaches which result in the lagoon reaching relatively low elevations reduce the available aquatic habitat area for fish and wildlife. Breaches directly west out to the ocean also have swift stream flows that push juvenile steelheads out to sea prematurely. When the river channel migrates along the northern or southern beach, a longer, meandering path is taken which extends the hydraulic gradient slowing the water exit. Local fisheries groups and agencies have preferred a northern meandering outlet channel alignment because, in the past, when the river channel migrated northwards, it reduced the rate and amount of drawdown (drop in lagoon water levels before and after a breach to the ocean) and subsequent loss of threatened juvenile steelhead that get flushed out to sea, as compared to when the channel flows along the southerly and westerly outlet channel (James, 2005). Scenic Road is located immediately north of the Carmel River mouth in Monterey County. It is a publicly maintained road that is threatened by erosion when the river takes a northerly route along the beach before discharging into the Pacific Ocean. The road runs around the northern headlands of Stewart’s Cove, southwest to intersect with Carmelo Street near the Carmel River State Beach parking lot and restrooms. Scenic Road provides recreational access to the State Beach, sole access to six private homes, and has a sanitary sewer pipe under the 1 Scenic Road Protection Options roadway. Monterey County (County) is investigating bluff stabilization methods to protect the road, sewer infrastructure, and homes. If a northerly river alignment on the beach were to persist over an extended period of time (several months), it is likely that the bluff fronting Scenic Road may also require protection from wave erosion. As a result, although river-induced scour is believed to be the primary mechanism that undermines the sand dune fronting the bluff (Thornton, 2005), the protection options developed for this study take both factors into consideration. Bedrock underlies the northern beach that prevents a deep exit channel from forming. A bedrock sill is also found on the south beach, and often mechanical breaches are directed towards this sill to prevent damage. In 1992, regulatory agencies informed the County that sandbar management did not qualify as an emergency due to the predictability of flooding at Carmel River Lagoon. In response, the County prepared an Interim Management Plan and Breaching Criteria and submitted it to the agencies. The County updated the agencies in the years following; however, a lack of supporting data and analysis was expressed by the agencies. In 2007 a Final Study