Cal Poly Humboldt Environmental Resources Engineering Senior Capstone.
This report provides an analysis of design alternatives and ultimately a preferred design to ensure the longevity of Highway 101 near Orick, CA which is under threat of sea level rise (SLR). Highway 101 is the main transportation corridor for communities stretching from Arcata to Crescent City, California. Caltrans provided an area of interest along Highway 101 just southwest of Orick, between mile markers 119.58 and 119.92. The project area of interest is illustrated below in Figure ES-1. This stretch of Highway 101 borders low lying pastures and Redwood Creek's historic estuary. The estuary has seen a significant reduction in size and function since the construction of the U.S. Army Corps. of Engineers levee in 1969.
Four alternative designs were developed to study options to protect this stretch of highway from a 100-year storm event in combination with 25-year sea level rise projections, approximately 3-feet. The designs considered are the installation of rock berm protection, restoring the historic Redwood Creek estuary, raising the roadway, and retreating the highway inland to higher elevation. These were compared using the Delphi method which analysed the alternatives based on a number of environmental, economic, and social criteria. The results of the Delphi matrix indicate that the estuary restoration alternative is selected for further analysis. After a discussion with the client, it was determined that a combination of the estuary restoration and raising the roadway alternatives would be preferred. An overview of this preferred design can be seen in Figure ES-1 above.
In order to protect the highway from SLR, the initial area of interest will be extended south to mile marker 119, about a half mile, and the highway will be raised 4-feet. The design for raising this section of highway will result in the widening of the existing road prism, the extension of the three existing culverts, and the replacement of a small bridge that will also be raised 4-feet to act as a wildlife crossing. The estuary restoration consists of a three phase process: During phase one the estuary will be excavated to match historical records and planting of native vegetation. During this phase a concrete box weir that is 6 ft wide, 4 ft deep, and 12 feet in length will be placed within the levee to allow flow into the estuary during higher river flows. Phase two will consist of surveying conducted by wildlife biologists, botanists, geologists, and hydrologists, and phase three will include any hydrological adjustments needed based upon the findings from phase two. Initial costs including the raising of Highway 101 and the restoration of Redwood Creek estuary were estimated at $19,400,000. Maintenance and operation costs for this design were estimated at $5,150,000 for the first five years and $136,000 annually after the first five years.
The proposed design was modelled using HEC RAS to model water flow through the concrete weir constructed connecting Redwood Creek and the estuary. The concrete weir can withstand a peak velocity of 25 ft/s without failure. To measure the viability of the concrete weir, the velocity of the flow through the concrete weir was measured for different SLR severities with the base case design storm of 50% of the 2-year flow, and the 100-year design flow.