Northern Hydrology and Engineering PO Box 2515, McKinleyville, CA 95519 Telephone: (707) 839-2195; email: [email protected] Engineering – Hydrology – Geomorphology – Water Resources TECHNICAL MEMORANDUM Date: 19 August 2010 To: Hank Seemann, Environmental Services Manager County of Humboldt Department of Public Works 1106 Second Street Eureka, CA 95501 From: Bonnie S. Pryor, M.S. Jeffrey K. Anderson, M.S., P.E. Re: Preliminary Design Review and Geomorphic Evaluation of Redwood Creek Flood Control Project, Orick, Humboldt County 1 Introduction The Redwood Creek Flood Control Project, located in the community of Orick in northern Humboldt County, is a levee system composed of two armored earthen embankments confining approximately 3.4 miles of lower Redwood Creek upstream of the mouth at the Pacific Ocean (Figure 1). The Flood Control Project was designed by the U.S. Army Corps of Engineers in the late 1950s and early 1960s (COE, 1966) and constructed by the Corps of Engineers between 1966 and 1968. As a pre-condition of the project, Humboldt County agreed to operate and maintain the levee system in accordance with the project’s Operation and Maintenance Manual (COE, 1969). The intended design of the Flood Control Project was a trapezoidal channel with fixed geometry and flood conveyance capacity; however, sediment deposition has caused a substantial reduction in flood capacity relative to design conditions (COE, 2008; NHE, 2009; NHE 2010). This technical memorandum is a preliminary evaluation of the geomorphic characteristics of the Redwood Creek Flood Control Project and the root causes of the impaired condition. This memorandum includes a review of the intended performance of the levee system as stated in the General Design Memorandum (COE, 1966), assessment of the geomorphic response within the Flood Control Project following channel excavation occurrences as documented by a time-series of topographic surveys, and assessment of long-term sediment transport patterns based on existing monitoring data. The stationing system (the numerical reference system used to designate locations along the channel) used throughout the memorandum is consistent with the original design document for the Flood Control Project, the General Design Memorandum (COE, 1966), and subsequent cross-section surveys. Elevations are referenced to the vertical datum NAVD88. Historical 1 planform maps and topographic surveys referenced to the NGVD29 datum were converted to the NAVD88 datum using the following equation: Elevation in NAVD88 = Elevation in NGVD29 + 3.33 feet. This conversion is identical to that used in COE (2008). Figure 1. Site map of project area. 2 2 Overview of the Flood Control Project Design and Current Flood Control Capacity 2.1 Flood Control Project Design The General Design Memorandum (GDM) (COE, 1966) provides the engineering design basis for the Redwood Creek Flood Control Project. The fundamental engineering components of the Flood Control Project consisted of enlarging the channel and constructing earthen levees on both sides of the channel (COE, 1966, page 4). Key design considerations included alignment, longitudinal profile, and cross-section. According to the GDM: “The channel alignment was selected to closely follow the existing alignment except near the mouth of the creek where the alignment has been changed to cut cross the [most-downstream meander] bend. The flow is directed toward the sand spit to accelerate its removal by erosion during rising flood stages. The invert grade closely follows the existing channel thalweg. Adopted channel cross- sections are the most economical which will meet the hydraulic and stability requirements, preserve the existing highway bridge and hold real estate requirements to a practical minimum.” (COE, 1966, page B-8) The adopted channel cross-sections were trapezoidal with a bottom width of 250 feet throughout the entire length and side slopes at one vertical on three horizontal (COE, 1966, page B-9). By setting the invert grade (the bottom of the flat, trapezoidal channel) at or near the existing channel thalweg (the line of maximum depth along the pre-levee channel), the project required excavation of approximately 463,000 cubic yards of sediment from channel bars to meet the design depth (COE, 1966, Table 1). The combination of building up levee embankments and lowering the channel profile was intended to provide a specific, fixed geometry for the design capacity of 77,000 cubic feet per second (cfs) (COE, 1966, page 6). The levee grades were designed to provide this capacity and “to provide at least three feet of freeboard above the maximum water surface resulting from design flood flow meeting the highest estimated tide level in the Pacific Ocean” (COE, 1966, page B-8). The GDM makes reference to the issue of sediment transport, noting that “The stream carries a substantial sediment load during floods” (COE, 1966, page B-7). This issue was well known at the time, as storms in the 1950’s and 1960’s produced widespread landslides, debris flows, bank erosion, and channel aggradation throughout the Redwood Creek basin, including within the Flood Control Project reach (Ricks, 1995). The downstream levee section was extended an additional 2,000 feet toward the ocean beyond the end point in the preliminary design to “provide for better flushing of silt during low flow” (COE, 1966, page 5) and to “cause the bulk of the sediment to deposit in the ocean where the prevailing littoral currents would progressively move it along the coast” (COE, 1966, page B-7). This extension was designed to “provide more comprehensive flood control for the valley, and the indicated high velocities will keep the lower reach of the channel clear of the bulk of debris and silt” (COE, 1966, page 8). However, no specific analysis was included in the GDM to demonstrate that the sediment supplied to the Flood Control Project could be conveyed to the ocean without depositing within the channel and causing a long-term reduction flood control capacity. Rather, the GDM provides more focus on potential erosion and channel bed lowering that could threaten the integrity of the levees. 3 Channel degradation was prevented by the construction of a control sill (concrete grade-control structure) at the downstream end of the Flood Control Project (COE, 1966, page 5). In addition to providing the design basis for construction of the levee system, the GDM specifies the level of operation and maintenance that will be required by the local agency (County of Humboldt) following construction. According to the GDM: “The Redwood Creek Flood Control Project has been designed to require only moderate operations by local interests. It is estimated that necessary operations, including inspection of flapgates, will average about $1,000 a year. “It is anticipated that yearly maintenance will be required on Redwood Creek. Minor displacement of riprap, minor restoration of levee slopes during early years of the project, small siltation problems in the lower reaches of the project, clearing of flapgates, maintenance and minor repair of access roads, and restoring effectiveness of the relief wells are anticipated. Average annual maintenance will cost an estimated $18,000” (COE, 1966, page 23). The GDM includes a budget breakdown for the annual maintenance cost estimate, which focuses on repairs of the embankments and riprap with minimal work within the channel. Neither the budget breakdown nor the narrative description of the annual maintenance requirements identifies removal or re-shaping of channel bed material as a requirement for maintaining the Flood Control Project. 2.2 Current Flood Control Capacity In December 2008, the Corps of Engineers issued a technical memorandum (COE, 2008) that analyzed the capacity of the Redwood Creek levee system, assessed the relative contribution of gravel and vegetation to capacity reduction, and estimated the quantity of gravel that would need to be removed to achieve certain thresholds. According to this report, capacity is highest at the upstream extent of the levee system and decreases progressively downstream. By dividing the levee system into three reaches, the Corps of Engineers determined that the upper reach (Reach 3) provides protection for a 250-year flood, the middle reach (Reach 2) provides protection for a 53-year flood, and the lower reach (Reach 1) provides protection for a 13-year flood. COE (2008) concluded that excess sediment is the chief reason for decreased channel capacity from the Flood Control Project design levels. NHE (2009) provided a technical review of COE (2008). NHE (2009) identified a number of interpolation cross-section errors in the lower section of the COE Final HEC-RAS Model. The interpolation errors resulted in reduced cross-section areas of the channel cross-sections that are not representative of the actual physical channel. NHE (2009) provided a revised HEC-RAS model (NHE 2007 Model) that corrected the interpolation errors in the COE Final HEC-RAS Model, and also developed another model (NHE 2008 Model) based on the County 2008 post- extraction channel surveys. The County 2008 channel surveys contain more cross sections than the 2007 surveys (30 vs. 20), which require less cross section interpolation in the HEC-RAS model. For this reason, NHE (2009) recommended that the NHE 2008 Model be used for making flood predictions. 4 NHE (2010) reassessed the flood capacity by reach using the NHE 2008 Model. Flow frequencies were estimated from COE (2008). Reach lengths were adjusted to delineate the target flow thresholds (e.g., the 500-yr and 100-yr flow capacities). The updated analysis demonstrates that Reach 3 provides protection for a 500-year flood, Reach 2 provides protection for a 100-year flood, and Reach 3 provides protection for a 33-year flood (Table 2). The new analysis demonstrates that only the approximate lower 2,250 feet of the Redwood Creek Project fails to provide three feet of levee freeboard at the 100-yr flood flow. Table 1.