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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 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 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 with fixed geometry and flood conveyance capacity; however, 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 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 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 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 ] bend. The flow is directed toward the sand spit to accelerate its removal by during rising flood stages. The invert grade closely follows the existing channel . 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 level in the Pacific Ocean” (COE, 1966, page B-8).

The GDM makes reference to the issue of sediment transport, noting that “The carries a substantial sediment load during ” (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, erosion, and channel 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 , 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. Summary of capacity by reaches for Redwood Creek Flood Control Project with three feet of levee freeboard (COE, 2008).

COE Reach Designation by Flood Frequency Flood Frequency Reach Station (ft) Flood Flow (cfs) (Year) (% Exceedance) Reach 3 11,414 to 17,956 64,150 250 0.4 Reach 2 6,014 to 11,414 54,550 53 1.9 Reach 1 333 to 6,014 43,100 13.2 7.56

Table 2. Summary of reassessment of capacity by reaches for Redwood Creek Flood Control Project with three feet of levee freeboard (NHE, 2010).

NHE Reach Designation by Flood Frequency Flood Frequency Reach Station (ft) Flood Flow (cfs) (Year) (% Exceedance) Reach 3 10,216 to 17,956 70,000 500 0.2 Reach 2 2,581 to 10,216 58,868 100 1 Reach 1 333 to 2,581 52,500 33 3

Although NHE (2010) analyses shows significantly more flood capacity than COE (2008), the overall trend described in the COE (2008) report, in which capacity is highest at the upstream extent of the levee system and decreases progressively downstream, is confirmed by the corrected model. These results suggest that the effects of the levee system’s sedimentation problems are progressively worse in the downstream direction.

As part the channel capacity analysis, the COE (2008) determined the location and volume of sediment needed to be removed from the channel in order for the Redwood Creek Project to convey the 100-yr flood with at least three feet of freeboard and the total volume of gravel that would need to be removed to restore the entire system to design conditions. The COE estimated that 178,000 cubic yards of sediment would need to be removed to convey the 100-yr flood and a total of 430,000 cubic yards of sediment would be required to restore design conditions (93% of the original excavation volume). Humboldt County has estimated that the hypothetical removal of 430,000 cubic yards of sediment to achieve design conditions would cost $4.4 million.

The volumes estimated by COE (2008) were based on the COE Final HEC-RAS Model, which as described above, contained cross section interpolation errors (NHE, 2009). Using the NHE 2008 Model, NHE (2010) recalculated the volume of sediment required to be removed to be 38,400 cubic yards from Reach 3 to convey the 100-yr flood with three feet minimum levee

5 freeboard. NHE (2010) did not recalculate the volume of sediment removal required to restore design conditions.

Neither study assessed the economic or operational feasibility of extracting gravel in the targeted areas which are within or adjacent to the Redwood Creek , and it remains uncertain whether regulatory approvals could be obtained for these extractions. In addition, there has not been an assessment of the potential for continued sediment deposition within the Flood Control Project that could lead to a long-term reduction in channel capacity, or the quantity and frequency of maintenance sediment removal that would be required following large-scale channel excavation.

3 General Patterns of Sediment Transport, Storage and Channel Morphology in Redwood Creek

The 2008 Channel Capacity Analysis (COE, 2008) and follow-up work (NHE, 2009) demonstrated that flood capacity had decreased substantially between construction of the Flood Control Project in 1968 and conditions in 2007 and 2008, due to sediment deposition. The following section provides a description of typical patterns of sediment transport and storage in the Redwood Creek basin to provide a context for the observed sedimentation trajectories observed following construction of the Flood Control Project.

Redwood Creek has one of the highest annual sediment yields in the conterminous for basins of similar size (278 square miles), with the exception of areas draining active volcanoes and glaciers (Nolan, Lisle, & Kelsey, 1987). The mainstem of Redwood Creek alternates between steeper, boulder dominated reaches confined by valley walls and wide lower gradient gravel-bed reaches. Pulses of sediment delivered to the channel during major storms can overwhelm the transport capacity of the channel, resulting in extensive channel aggradation (channel infilling). Long, intervening periods of moderate flows are necessary for channel configurations to return to pre-storm conditions (Nolan & Marron, 1995).

Nolan, Lisle, & Kelsey (1987) calculated effective (the flow that transports the most sediment over a period of record) from discharge and sediment data collected between 1971 and 1984 at the USGS Gaging Station 11482500 Redwood Creek at Orick. They determined the effective discharge is approximately 15,000 cfs (1.8 recurrence interval) with 50% of the sediment transported at flows below 11,000 cfs (0.36 year recurrence interval) and 90% of sediment transported at flows below 40,000 cfs (11.1 year recurrence interval) (Nolan, Lisle, & Kelsey, 1987). The recurrence interval of the effective discharge in Redwood Creek is similar to those found in other studies (Andrews, 1980). A distinct second peak occurs in the Redwood Creek effective discharge curve at 35,740 cfs. A second peak is relatively uncommon in effective discharge curves in other areas, but was also observed in two other North Coast in the same study. The occurrence of the second peak in Redwood Creek is attributed to high variability of flows and weak rocks that generate an almost unlimited supply of transportable (Nolan, Lisle, & Kelsey, 1987). The Redwood Creek effective discharge curve demonstrates that moderate flows are responsible for transporting the majority of the sediment, but that high flows also play a particularly important role in sediment conveyance in this system.

6 A channel is considered stable when the prevailing flow and sediment regimes do not lead to long-term aggradation or degradation and the average channel dimensions are maintained (Thomas, Copeland, & McComas, 2002). The cyclic pattern of aggradation during large storm events and incision and re-shaping of the channel bed during moderate storms appears to be the typical pattern of erosion and deposition in Redwood Creek (Nolan & Marron, 1995). Cyclic patterns of channel aggradation and degradation result in a range of channel geometries. Aggraded channels are typically wider, shallower and may have frequent mid-channel bars that split flow into multiple channels, while degraded channels tend to be narrower, deeper and have a tendency toward single-thread channels with alternating lateral bars and fewer mid-channel bars. Since Redwood Creek exhibits long-term aggradation-degradation cycles with variable channel geometry, the channel cannot be described as stable.

A related concept to the stable channel is the geomorphic concept of equilibrium. Equilibrium channels are those that exhibit a tendency to return to approximately their previous state following a channel disturbance if channel forming factors are not changed (Mackin, 1948). These channel forming factors include stream flow, sediment supply (quantity and caliber), composition of bed and bank materials, and valley gradient. These “channel forming factors” interact to create a unique channel form composed of a specific , channel geometry, and profile (Leopold, Wolman, & Miller, 1964). If the channel forming factors are altered, long term aggradation or degradation may occur until a new equilibrium is achieved. Channels are considered to be in disequilibrium if they are adjusting toward equilibrium, but there has not been sufficient time to reach equilibrium, while non-equilibrium channels have no tendency toward an equilibrium condition (Knighton, 1988).

Noland & Marron (1995) observed that episodic sediment deliveries in Redwood Creek produce the channel disturbance, and moderate flows over long periods of time reshape the channel to a pre-storm configuration. The tendency toward a pre-storm configuration implies a tendency toward an equilibrium condition, given sufficient time between episodic events. Aggradation in lower Redwood Creek has persisted for more than 30 years in Redwood Creek and recurrence intervals of storms that created the disturbance in the 1960’s and 1970’s have recurrence intervals of approximately 25 years. Therefore, the most likely state of the lower Redwood Creek channel at any given time immediately prior to and following construction of the Redwood Creek Flood Control Project is in disequilibrium.

Channel morphology is controlled by sediment supply, stream discharge, valley gradient, and the composition of bed and bank materials. Bars and pools are typical features in gravel-bed and occur regularly throughout the Redwood Creek basin. Flat-bed channel forms are not associated with gravel-bed streams with the exception of very short sections of channel located at cross-overs between meander bends (Leopold, Wolman, & Miller, 1964).

7 4 Preliminary Analysis of the Geomorphic Trends within the Redwood Creek Flood Control Project

The 2008 Channel Capacity Analysis (COE, 2008) identified a decrease in flood capacity between the construction of the Flood Control Project and 2007 due to an increase in sediment storage (sediment deposition). Sediment storage increases when sediment supply exceeds the transport capacity of the stream and decreases when sediment supply is less than the transport capacity of the stream. Local adjustments in gradient, channel geometry, grain size, and bank materials can increase or decrease the transport capacity of a given reach.

Channel surveys within the Flood Control Project demonstrate the spatial patterns and rates of channel infilling. Two intervals of project history are distinguished: the first period (1966 through 1987) extends from the time of construction of the Flood Control Project to a major aggregate mining operation in 1987 and 1988 that resulted in near re-excavation of the design channel ; the second period (1988 through 2009) extends from the major re-excavation event to the present. Modifications to parameters that affect transport capacity (channel gradient, channel geometry, grain size and bank materials) are provided where data exist. Sediment data collected at USGS Gaging Station 11482500 Redwood Creek at Orick provide an indication of long-term trends in sediment transport capacity between 1970 and 2009.

The volume of sediment supply to the Flood Control Project since construction is uncertain. A sediment wave was documented in the lower 26 kilometers (16 miles) of Redwood Creek above the Flood Control Project that was migrating at a rate of 800 to 1600 meters (one-half to one mile) annually (Madej & Ozaki, 1996). Although the movement of sediment into and through the Flood Control Project has not been specifically documented in previous research, it is reasonable to assume that sediment supply to the Flood Control Project has varied as a result of sediment wave propagation.

4.1 Data Sources and Methods The primary sources of data for the pre-project channel and the design channel are contour maps, centerline profiles, and descriptions of the channel geometry and gradient published in the GDM (COE, 1966). The primary sources for post-levee assessment are channel cross-section surveys completed in 1980, 1981, 1997, 2000, 2002, 2004-2005, and 2007-2009; and vertical or oblique aerial photography (1971, 1981, 1988, 1996, 2005-2009). Because channel width has been fixed since levee construction, average bed elevation is used as a reasonable estimate of sediment storage.

Changes in channel morphology between 1966 and 1997 are interpreted qualitatively from (1) aerial photos spanning this period, and (2) surveyed channel cross-sections in the lower portion of the project in 1980 and 1981. Raw data were not available for the 1980-1981 surveys, and observations are interpreted from published cross-sections (Ricks, 1995). Channel morphology changes between 1997 and 2009 are interpreted from detailed cross-sections surveyed throughout the project area in 1997, 2000, 2002, and 2004-2005 and 2007-2009. The development of bars and channel development are shown on channel cross-sections as unequal distribution of sedimentation across the cross-section as shown in Figure 2.

8 The average channel bed elevation was computed at each surveyed cross-section between 1997 and 2009 as a distance-weighted average of the elevations of the surveyed points between the toes of the design channel, which define the 250 foot design bed width (Figure 3). The net deposition or erosion at each cross-section is the difference between the average bed elevations of the survey and design. Reach-average changes are computed for each year using a distance- weighted average of all surveyed cross-sections in the project area. Net deposition and erosion are also computed for two sub-reaches, which are delineated by active gravel extraction.

50

Channel 40 Development

30 Growth

20 Elevation Elevation (ft, NAVD88)

10 0 40 80 120 160 200 240 280 320 360 400 440

Distance (ft)

Design Surveyed Cross-section

Figure 2. Example adjustments of channel morphology through bar growth and channel development depicted by surveyed cross-sections.

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40 Average Bed Calculation

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20 Elevation Elevation (ft, NAVD88)

10 0 40 80 120 160 200 240 280 320 360 400 440

Distance (ft)

Design Surveyed Cross-section

Figure 3. Schematic of average bed elevation computation at a cross-section.

9 4.2 Pre-Project Channel

4.2.1 Channel Gradient Pre-project bed elevations were extracted from the pre-project contour maps (Figure 4) and the centerline profile in the GDM (COE, 1966). The centerline profile appears to have been surveyed at a finer resolution than the two-foot contour maps and captures the major topographic breaks on the design channel centerline. The centerline profile also contains data below the water surface. The accuracy of the centerline survey is not reported. The centerline survey follows the centerline of the design channel, which is generally within the pre-project bankfull channel. Surveyed points that fell within the wetted area of the channel as depicted on the GDM planform maps were used to represent the pre-project bed elevation. These elevations are in arbitrary locations within the wetted channel and do not represent the channel thalweg or average bed elevation and are simply referred to as “wetted bed elevation” for the remainder of this memorandum. The pre-project channel gradient was estimated from the lowest surveyed points that fell within the wetted area of the channel.

Pre-project channel gradient in Redwood Creek was approximately 0.14% from Station 18950 (22.4 feet) to 7450 (6.3 feet) and decreased to 0.046% measured along the pre-project channel centerline. The pre-project gradient measured along the design centerline is 0.075% between Station 7450 (6.3 feet) and 1584 (1.9 feet) (Figure 5). The reduction in channel gradient near Station 7450 corresponds with an increase in channel sinuosity and is near the mean higher high water tidal datum (6.5 feet) indicating the reach is tidal influenced.

4.2.2 Channel Morphology Channel morphology was interpreted from contours reproduced from maps provided in the GDM (Figure 4). The contour maps have a two-foot contour interval above the water edge as defined on the maps. The planform maps do not contain information below the water surface. The coverage of the channel is complete with the exception of 4,320 feet of channel (the most downstream meander bend) that was cut off by the levee system at the downstream end of the project reach.

The pre-project channel was an alluvial meandering channel with point bars, lateral bars, and mid-channel bars that occasionally split the low flow channel (Figure 4). Channel width ranged from 175 to 330 feet. Sinuosity of the channel increased in the downstream direction as the valley broadened and was influenced by both coastal and .

4.2.3 Sediment Storage Sediment storage within the pre-project channel occurred in point bars, lateral bars, and mid- channel bars and on . Elevations of bar surfaces were extracted from the centerline profile at locations where the design centerline crossed bar surfaces (Figure 6). Bar elevation was measured from the pre-project channel grade to the bar surface. The quantity and quality of data available is not sufficient to compute storage volume in the bars, but provides a gross approximation of the height of the bars in the pre-project channel. The best estimate of the pre- project in-channel storage volume is the volume of material that was removed during excavation (463,000 cubic yards). However, this number is also only a rough approximation because there

10 was excavation below the thalweg in the lower portion of the reach and channel widening in some locations.

Figure 4. Pre-project contour map digitized from COE (1966).

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20

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10 Elevation Elevation NAVD88)(ft, 5

0

-5 0 3000 6000 9000 12000 15000 18000 21000

Station (ft) Pre-project Wetted Bed Elevation Estimated Chanel Grade

Figure 5. Estimated channel grade based on pre-project centerline within wetted channel. Stationing and slopes are based on design centerline, which cuts off 2,520 feet of the pre-project channel.

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25 ) 20

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10 Elevation Elevation NAVD88(ft, 5

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-5 0 3000 6000 9000 12000 15000 18000 21000 Station (ft) Pre-project Wetted Bed Elevation Estimated Chanel Grade Estimated Chanel Grade Pre-project Bar Elevation

Figure 6. Pre-project bar elevations. Stationing and slopes are based on design centerline, which cuts off 2,520 feet of the pre-project channel.

12 The adjacent floodplains of the pre-project channel received and stored significant amount of sediment. Flood deposits from the 1964 storm resulted in large areas of sand deposits greater than 30 centimeters (12 inches) thick on the north of the channel between approximately Station 11500 to 4500 and south of the channel between 19200 and 14000 (Ricks, 1995). The remainder of the floodplain received sand and silt deposits less than 30 centimeters (12 inches) in thickness (Ricks, 1995).

4.3 Redwood Creek Design

4.3.1 Channel Gradient The design channel has a constant gradient of 0.14%. The GDM (COE, 1966, page B-8) specifies that “the invert grade closely follows the existing thalweg.” However, this design criterion was only applied in the upper portion of the project reach (Station 18210 to 6629). In the downstream portion of the project area, the pre-project channel had a gradient of 0.046% measured along the pre-project channel centerline, significantly flatter than the design gradient. The downstream end of the project reach was shortened by 2,520 feet by cutting off the last meander (Figure 4) which would have resulted in an increase in the channel gradient to 0.075% if pre-project bed elevations were maintained at the two tie in points (Figure 5, Figure 7). In order to achieve a design gradient of 0.14% through this reach, the bed elevation of the channel was cut approximately 3.3 feet lower than the pre-project bed elevation at the downstream end of the levee (Station 17+00) (Figure 7). Shortening the length of the channel and cutting the channel below the existing bed elevation resulted in a 200% increase in channel gradient.

The steeper channel gradient in the lower portion of the project decreased the predicted water surface elevations for the design flood, resulting in a corresponding reduction in elevations of the adjacent levee embankments. The levee top elevations were based on providing three feet of freeboard above the estimated design flood profile. The GDM states the following, with the key items underlined (COE, 1966, page B-8, Section B-11.e.):

“Levee grades were established 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. Additional height of levees is provided for superelevation of the water surface caused by flows around bends and upstream from the bridge at Orick to allow for higher backwater caused by obstructions due to debris.”

If the pre-project channel grade had been maintained through the entire reach of the project as stated in the GDM, the predicted water surface elevations would have been significantly higher due to higher bed elevations and a lower channel gradient, and the heights of the levee embankments in the lower portion of the project would have had to be increased to achieve the design capacity.

Both coastal and fluvial processes control the gradient of Redwood Creek at the downstream end of the Flood Control Project, and the base level that controls the channel grade is the ocean. Artificially altering the natural channel grade in an alluvial channel often leads to channel instability because alluvial channel gradient and form are adjusted to convey the imposed discharge and sediment supply through sediment deposition and erosion. The sill was designed to

13 limit potential of the channel, but the design did not account for potential deposition and a return to the pre-project base level.

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25 ) 20

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10 Elevation Elevation NAVD88(ft, 5

0 Design channel cut below pre-projectbed elevation -5 0 3000 6000 9000 12000 15000 18000 21000

Station (ft) Design Pre-project Wetted Bed Elevation Deviation from Natural Grade

Figure 7. Design grade and pre-project wetted bed elevation.

4.3.2 Channel Morphology The design channel geometry is a trapezoidal cross-section, with a 250 foot flat bottom width, and one vertical to three horizontal (1:3) channel side slopes. The design thalweg bed elevation and average bed elevation are equivalent because the design cross-section has a flat-bed.

Channel forming factors were modified by replacing natural bank materials with riprap and building levees that confine high flows. The installation of the levees and riprap altered potential channel responses to variable discharges and sediment supply by restricting the lateral evolution of the channel, removing sediment storage in floodplains, and increasing water surface elevation and velocities during high flow events. The restriction of lateral adjustment shifts the expected morphological response to varying discharge and sediment supply from one dominated by channel widening and lateral migration typical of wide alluvial valleys to predominately vertical bed adjustments typical of confined reaches. Removal of overbank flows and sediment storage in floodplains increases the sediment carried within the main channel which can promote increased sediment deposition within the main channel, while increased velocities and water depth due to levee confinement during high flow events may increase the sediment transport capacity of the channel at high flood stages.

The design modified the channel form of Redwood Creek by removing bars; decreasing channel roughness by removing vegetation, bank irregularities, and bedform roughness; and locally steepening the channel gradient at the downstream end of the Flood Control Project. Decreasing channel roughness and locally steepening the channel gradient have the potential to increase transport capacity of the channel, while removal of channel bars may decrease the transport

14 capacity of the channel. A flat-bed channel has a higher likelihood of bed material deposition at moderate flows than a channel with a bar-pool morphology because the flow spreads across a wider area, decreasing flow velocity and shear stresses acting on the bed, reducing sediment transport (Brookes, 1988). At a higher flood stage, the increased velocity and flow depth due to levee confinement is expected to increase the transport capacity of the design channel compared to the pre-project channel.

The Flood Control project was intended to provide a specific, fixed geometry that could convey the design flood capacity of 77,000 cubic feet per second (cfs) (COE, 1966, page 6). However, the GDM does not contain any qualitative or quantitative analyses to demonstrate that the design would be stable or in equilibrium. The primary indicator that the imposed geometry and gradient changes would not be maintained is that the design only changed one of the controlling factors on channel form (levee construction). The remaining controls on channel form (sea level, stream flow, upstream sediment supply, bed material and valley gradient) were unchanged. The design relies solely on the construction of the levees to: (1) maintain an imposed channel form that is not associated with gravel-bed rivers, (2) create a static bed condition at the outlet of a watershed that has experienced recent basin wide aggradation, and (3) maintain a fixed base level below the pre-project base level.

4.4 Post-Construction Redwood Creek Flood Control Channel

4.4.1 Sediment Transport in Redwood Creek Flood Control Project Area Measurements of sediment transport reported for the USGS Station 11482500 Redwood Creek at Orick are collected at the HWY-1 Bridge (STA 127+00). is composed of the grain sizes that make up appreciable quantities of the , generally the sand and gravel components that are coarser than 0.062 millimeters in diameter (Knighton, 1988). The bed material load is the most important portion of the total load in affecting channel form. Bed material load may be transported as or bedload depending on the size of the grain and flow strength. Grains transported as suspended load are generally suspended within the water column and occasionally come into contact with the stream bed. Transport distances are relatively long compared to transport distances of bedload. Grains transported as bedload slide, roll, and skip along the bed surface. Grains may be suspended within the water column for short periods of time.

Annual bedload and suspended sediment discharge values were compiled from several sources; the composite data set is provided in Table 3. Daily suspended sediment discharge (tons/day) for Water Year 1970-1992 and 2001 and daily bedload sediment discharge (tons/day) 1989-1992 and 2001 are available through the National Water Information System. Klein (2010) computed provisional annual suspended sediment and bedload discharge (tons/year) for the last 20 years using daily average flow. Graphical Constituent Loading Analysis System (GCLAS) was used to compute suspended sediment discharge loads (1990-2006) and the bedload discharge rating curve (Figure 8) was used to compute bedload discharge load (1990 and 2010) (Klein, Provisional, unpublished data, 2010). Annual bedload discharge data from 1974 to 1989 were computed by Redwood National and State Park (RNSP) and reported in Moffatt & Nichol Engineers (2003).

15 100000

10000 1977 data point

1000

100

10 Rating Equation: 1990-2010:

Bedload Dischargetons/day) (Qb, Bedload Qb = 0.0123Qw1.4289 R2 = 0.84 1 100 1000 10000 100000 Discharge (Qw, cfs) Figure 8. Bedload sediment discharge data collected 1990-2010 and 1977 and provisional rating curve (Klein, Provisional, unpublished data, 2010). The median annual suspended sediment discharge was approximately 463,700 tons/year between 1970 and 2006. Suspended sediment discharge was highest in the early 1970’s, reaching a maximum of 3,799,755 tons in 1972 (Figure 9, Table 3). After 1976, suspended sediment discharge varied between 1,403,944 and 5,763 tons/yr. Periods of relatively low suspended sediment discharge occurred from 1987 to 1994 and 2000 to 2005 with an intervening period of relatively higher suspended sediment discharge between 1995 and 1999.

The median annual bedload discharge was approximately 118,400 tons/year between 1974 and 2010. The highest bedload discharge values occurred during the first 10 years of monitoring (Figure 10, Table 3). Relatively low bedload discharges followed the same patterns as suspended sediment discharges between 1987 and 1994 (low) and 1995-1999 (high). Between 2000 and 2010, bedload discharges remained near the median value with the exception of a low bedload discharge in 2001 and high bedload discharge in 2006.

Annual water yield in the basin ranged from 139,076 to 1,249,428 acre-feet with a median value of 744,878 acre-feet between 1970 and 2009 (Figure 11). For the purposes of this report, years with water-yields larger than the median are referred to as “wet” and lower than the median are “dry”. A more sophisticated, statistics-based classification of water years has not been developed to date. Annual sediment loads are correlated with the annual water yield. Higher sediment discharges occur in water years with higher water yields (Figure 12), however, sediment loads can vary by nearly an order of magnitude for a given water yield.

16 Sediment loads and water yields cannot be correlated with changes in sediment storage (deposition or erosion). Sediment storage is controlled by the relative quantity of sediment delivered to the channel and exported from the channel (input- output). In any given water year, whether wet, normal or dry, a channel may aggrade or degrade depending on the local inputs and outputs. Long-term channel aggradation occurs if the transport capacity of a reach is consistently lower than an upstream reach. Conversely, channel degradation may occur if the transport capacity of the downstream reach is consistently higher than the upstream reach. In both of these cases, the channels will continue to adjust its form and gradient until the supplied sediment can be conveyed without aggradation or degradation.

The magnitude of sediment transport rates and succession of wet and dry years will be discussed within the context of sediment storage changes in the following sections that describe channel changes between major channel excavations 1968-1988 and 1988-2009. The general pattern of sediment transport and water yield between these two periods was a decrease in the average water yield of 11% (1968-1988; 1989-2009), decrease in average annual suspended sediment discharge of 59% (1971-1988; 1989-2006) and a decrease in average annual bedload discharge of 37% (1974-1988; 1989-2009).

4,000,000

3,500,000

3,000,000

2,500,000 (tons/yr) 2,000,000

1,500,000 Suspended Sediment Discharge 1,000,000

500,000

0 1970 1975 1980 1985 1990 1995 2000 2005 2010

Year

Annual suspended sediment discharge Median

Figure 9. Annual suspended sediment discharge from 1970 to 2006 (Klein, 2010).

17 400,000

350,000

300,000

250,000

200,000 (tons/yr)

150,000 Bedload Bedload Discharge

100,000

50,000

0 1970 1975 1980 1985 1990 1995 2000 2005 2010

Year

Annual Bedload Discharge Median

Figure 10. Annual bedload discharge from 1974 to 2010 (Klein, 2010).

1,400,000

1,200,000 ft) - 1,000,000

800,000

600,000 WaterYield (acre

400,000

200,000

0 1970 1975 1980 1985 1990 1995 2000 2005 2010

Year

Median Water Yield

Figure 11. Water yield at USGS 11482500 Redwood Creek at Orick, CA from 1970 to 2009.

18 Table 3. Annual Sediment Discharge Data for Redwood Creek at Orick (USGS 11482500).

Water Year Annual Suspended Annual Bedload Source Sediment Discharge (tons) Sediment Discharge (tons) 1970 5,135 -- USGS 1971 2,177,712 -- USGS 1972 3,799,775 -- USGS 1973 757,634 -- USGS 1974 2,228,626 (371,800) USGS (RNSP) 1975 2,768,664 (249,010) USGS (RNSP) 1976 745,314 (100,550) USGS (RNSP) 1977 22,567 (2,049) USGS (RNSP) 1978 948,518 (325,620) USGS (RNSP) 1979 292,989 (43,459) USGS (RNSP) 1980 704,585 (188,250) USGS (RNSP) 1981 187,877 (170,350) USGS (RNSP) 1982 1,276,880 (377,890) USGS (RNSP) 1983 1,327,107 (370,050) USGS (RNSP) 1984 625,810 (279,372) USGS (RNSP) 1985 280,541 (139,376) USGS (RNSP) 1986 1,010,226 (148,237) USGS (RNSP) 1987 103,782 (61,790) USGS (RNSP) 1988 156,514 (62,255) USGS (RNSP) 1989 463,771 60,934 USGS 1990 191,317 62,252 (62,215) USGS (Klein) 1991 34,965 22,941 (22,967) USGS (Klein) 1992 18,184 10,884 (10,884) USGS (Klein) 1993 388,111 149,952 Klein 1994 73,070 39,559 Klein 1995 751,906 188,998 Klein 1996 1,080,307 221,735 Klein 1997 1,563,727 222,343 Klein 1998 1,004,797 183,719 Klein 1999 1,099,292 199,009 Klein 2000 268,395 104,389 Klein 2001 5763 (5763) 11,870 (17,407) Klein (USGS) 2002 262,700 115,562 Klein 2003 135,150 161,801 Klein 2004 89,600 116,671 Klein 2005 95,400 92,705 Klein 2006 843,000 260,483 Klein 2007 -- 118,431 Klein 2008 -- 101,787 Klein 2009 -- 99,027 Klein 2010 -- 66,315 Klein Data Sources: Klein: Klein (2010) provisional sediment loads computed from daily average discharge record. Development of sediment loads computed from 15-minute discharge record is in progress. USGS: Records published within the U.S. Geological Survey National Water Information System. RNSP: Redwood National and State Park, data on file, published in Moffat & Nichol (2003).

19

10,000,000

1,000,000

100,000

(tons/yr) Sediment Discharge 10,000

1,000 100,000 1,000,000

Water Yield (acre-ft)

Figure 12. Correlation between water yield and sediment discharge at USGS Station 11482500 Redwood Creek at Orick.

4.4.2 Channel Design Performance (1966-1987) The channel began to deviate from the design specifications the first winter following construction. In 1968, the Humboldt County Public Works Director sent a letter to the Corps of Engineers reporting three to four feet of gravel deposits that had accumulated in several locations on the newly constructed channel bottom. This letter describes the first reported observation that the channel design was not performing as anticipated. According to Humboldt County’s letter (Humboldt County Public Works, 1968):

“During a recent inspection…. it was noted that in three areas… large deposits of gravel exist varying in depth from approximately three to four feet above the newly constructed channel bottom…. We would like to know from your design considerations what affect this silting action may have on the levee’s function during flooding conditions.”

The District Engineer for the Corps of Engineers responded in a letter (COE, 1968):

“Your concern about gravel deposits has been analyzed. The flood flows on Redwood Creek have been moderate since construction…. Deposition of river gravel is to be expected during these flows, however, the amount of silting that has occurred should not adversely affect the functioning of the project. During large floods the higher velocity of flow should remove the accumulated deposits, therefore, it is not required to clear these deposits at this time. Periodically, however, snags and vegetation in the channel should be removed….”

Aerial photos of the Redwood Creek channel show redevelopment of bar-pool morphology within the Flood Control Project channel in 1974 (the earliest set of photos reviewed). Bar development on the inside of major channel bends is evident. Ricks (1995) documented more than one meter (three feet) of aggradation between approximately Station 1700 and 4000 by 1980. Surveyed channel cross-sections in the aggraded area show the development of a confined

20 low-flow channel and bar development, which is more consistent with the pre-project morphology than the design geometry (Figure 13).

By 1987, at least 210,000 cubic yards of sediment had accumulated within the Flood Control Project. This amount of material was removed in 1987 and 1988 to obtain aggregate for the Redwood National Park road bypass project, returning the channel to near-design specifications (Collins and Dunn, 1990; Redwood National Park, 1992; Cannata, et al., 2006).

The description of the expected geomorphic response of the design channel by the Corps of Engineers District Engineer (COE, 1968) is inconsistent with sedimentation patterns that were observed in Redwood Creek during major storms of 1953, 1955, and 1964. These storms produced exceptionally large volumes of sediment that resulted in basin wide channel aggradation (Nolan & Marron, 1995), rather than flushing the channel of excess sediment. Channel recovery is attributed to moderate flows events. Sediment is incrementally eroded from upstream reaches and deposited in downstream reaches resulting in a pattern of recovery from upstream to downstream. Since the Flood Control Project occupies the downstream-most channel segment in the Redwood Creek basin, the observed deposition within the Flood Control Project is consistent with continued channel aggradation through the lower reaches of the basin.

A similar flood control channel was constructed on the San Lorenzo River in central coastal California in 1959 (Griggs & Paris, 1982). The channel was excavated below the natural grade and subsequently accumulated 400,000 cubic yards of sediment by 1975. Accumulated sediments were not dredged because officials reportedly assumed the sediment would be scoured by high winter flows (Griggs & Paris, 1982). However, the sediment was not flushed from the channel and the channel continued to aggrade to the pre-project channel grade, substantially reducing flood capacity.

Deposition within the Redwood Creek Flood Control Project may have been exacerbated by the imposed channel geometry (flat-bed channel). Conceptually, a trapezoidal channel has a higher likelihood of bed material deposition than a channel with a bar-pool morphology at moderate flows because the flow spreads across a wider area, decreasing flow velocity and shear stresses acting on the bed and reducing sediment transport (Brookes, 1988). In contrast, bars concentrate the flow, locally increasing velocity and shear stresses. Given that more than 50% of the total sediment load in Redwood Creek is transported at discharges well below bankfull and the most effective discharge for sediment transport is less than bankfull (Nolan, Lisle, & Kelsey, 1987), the trapezoidal channel would likely promote sediment deposition during the periods of time when the most sediment is being delivered from the upstream reach.

Sediment transport in the Flood Control Project may increase during high flow events due to levee confinement. However, there is uncertainty whether sediment deposition remains higher within levee systems due to the increased sediment load that remains in the channel and lack of available floodplain sediment storage (Brookes, 1988). The response of a given system is a complex problem and depends on number of factors including the width of the levees, the effects on flow duration, and sediment load.

21

Figure 13. Surveyed channel cross-sections in 1980 and 1981 as published in (Ricks, 1995). Reproduced with permission from C.L. Ricks.

22 4.4.3 Channel Re-excavation and Response (1987-2009) In 1987 and 1988, approximately 210,000 cubic yards of sediment was excavated from the channel within the Flood Control Project resulting in a return to near-design specifications (Collins and Dunn, 1990; Redwood National Park, 1992; Cannata et al., 2006). Approximately 33% of the gravel was replenished by the following year and 44% had returned within three years (Moffatt & Nichol Engineers, 2003)

Ten years later (in 1997), channel surveys indicate that the channel thalweg had deepened upstream within the Flood Control Project reach, producing a slight reduction in overall channel gradient (Figure 14), while the average bed elevation at the surveyed cross-sections rose, on average, 1.9 feet. The peak flow in 1997 was 40,276 cfs (approximately a 10-year recurrence interval) based on recurrence interval computed by Moffatt & Nichol Engineers (2003). This event transported a large annual sediment load within the project site. However, no surveys of the channel were conducted prior to 1997 so there is uncertainty whether the 1997 water year increased or decreased sediment storage within the project area. However, the storm was documented to contribute large volumes of sediment to the reaches upstream of the project area resulting in channel aggradation in several reaches (Madej M. A., 1999).

The difference between the 1997 thalweg and average bed elevations and the channel design geometry indicates that the increase in sediment storage is occurring through the re-formation of bar-pool morphology (Figure 15). The 1997 thalweg elevation is slightly lower than the design bed elevation indicating scour of a low flow channel and pool development, while the 1997 average bed elevation is higher than the design which must be accommodated by deposition of higher bars. The re-development of bar-pool morphology between the major channel excavation in 1987-1988 and the 1997 channel surveys provides further evidence that the design channel is not self-maintaining and adjusts toward pre-excavation morphology in as little as 10 years.

30 25 20 15 10

5 Elevation Elevation NAVD88)(ft, 0 -5 1500 4500 7500 10500 13500 16500 Station (ft)

1997 Average Bed Elevation 1997 Thalweg Elevation Design

Figure 14. Average bed elevation and thalweg bed elevation surveyed in 1997 and design thalweg and average bed elevation.

23 40

Channel Channel 30 Development Development Bar Growth 20

10 Elevation Elevation (ft, NAVD88)

0 0 40 80 120 160 200 240 280 320 360 400 Distance (ft) Design XS 74+75 Figure 15. Bar and channel development at XS 74+75 in 1997. Small-scale gravel extraction occurred four times between 1996 and 2000, and larger extractions began in 2004 and have continued annually through 2009 (County of Humboldt, 2010) (Table 4). The purpose of the gravel extraction from 1996 through the present has been to increase and maintain flood capacity that has been lost as bed material has continued to accumulate in the channel.

Gravel extraction is regulated by a variety of environmental permits which place limits and conditions on where and how the work can be conducted. To date, gravel extraction has primarily occurred in the upper reaches. Reasons for site selection have included: location of largest gravel deposits; better equipment access and available staging areas adjacent to extraction areas; avoidance of areas affected by stream mouth closure and back-water; and hypotheses that upstream bars may act as sediment traps and limit downstream deposition, and that enhancing capacity in the upper reach may have a downstream benefit, while gravel removal downstream would have little or no upstream benefit.

Changes in average bed elevation are referenced to the design bed elevations. The change in average bed elevation in a given year is computed by differencing the average bed elevation at a cross-section and the design average bed elevation at that cross-section. The reach-average change in bed elevation is computed using a distance-weighted average of change in bed elevation at each surveyed cross-section between Station 17145 and 1700 (Table 5).

The most rapid gravel accumulation occurred between 1997 and 2002. Aggradation persisted with a small increase through 2004, reaching a peak increase in average bed elevation of three feet (Table 5). Sediment loads between 1997 and 1999 were high and associated with wetter water years, while four out five years between 2000 and 2004 were relatively dry with one water year close to the median value (Figure 11). All water years between 1997 and 2009, with the exception of water year in 2001, had at least one storm that exceeded the effective discharge for Redwood Creek.

24 Table 4. Gravel extraction quantities for 1996-2009.

Year Quantity Location Source

1996 Unknown, no extraction surveys Bar 10 Basis: summary page in file 1997 0 -- -- Reference to 9,000 cy from Bar 1998 Bars 7, 8, 9 Basis: summary page in file 9, no extraction surveys Basis: summary page in file, 1999 Unknown, no extraction surveys Bars 4, 8, 10 photos from Sept/Oct 99 2000 2,700 Bar 10 Basis: summary page in file 2001 0 -- -- 2002 0 -- -- 2003 0 -- -- 2004 33,470 Bar 7, 8, 9, 11 Basis: extraction surveys 2005 8,348 Bar 3, 11 Basis: extraction surveys 2006 22,806 Bar 6, 11 Basis: extraction surveys 2007 26,196 Bar 3, 4, 7, 8 Basis: extraction surveys 2008 40,003 Bar 3, 4, 5, 7, 10 Basis: extraction surveys 2009 32,863 Bar 3, 4, 5, 11 Basis: extraction surveys

The reach-average bed elevation steadily declined between 2004 and 2009 (Table 5). Between 2004 and 2007 the reach-average bed elevation declined by 0.5 feet. Channel surveys in 2008 and 2009 were conducted after gravel extraction occurred, thus, the apparent decline in reach- average bed elevation within the gravel extraction reach for these years may have been affected by this activity. Downstream of the gravel extraction reach, the average bed elevation dropped an additional 0.7 feet between 2007 and 2009, but remains 2.4 feet above the design channel grade.

The average bed elevations in 2004 are roughly equivalent to the top of bars in the pre-project channel (Figure 16), which suggests that: (1) the in-channel storage volume in 2004 approached or exceeded the pre-project sediment storage volume, and (2) sediment storage distribution in 2004 was similar to the pre-project channel.

COE (2008) estimated that 430,000 cubic yards would need to be excavated (as of December 2008) to return the levee system to design conditions, which is very close to the sediment volume (463,000 cubic yards) that was excavated in 1966/1967 to meet the design specifications. Similarly, excavations in 1987-1988 required the removal of 214,000 cubic yards to reset the channel to design conditions. This repeated long-term aggradation of the levee channel clearly demonstrates that the design channel is not stable and, therefore, does not meet the design objectives.

The decline in average bed elevation over the past five years has been moderate, and the channel capacity remains severely impacted by aggradation. The decline in average bed elevation may be a result of consistent gravel extraction, a reduction in sediment supply, the frequency and

25 magnitude of storm events that are conducive to sediment export from the Flood Control Project, or may indicate that the storage volume has reached a dynamic equilibrium around which the bed elevation will have smaller fluctuations.

Table 5. Increase in reach-average bed elevations above the COE (1966) design grade.

Gravel Extraction Downstream of Gravel Survey Before Project Reach Year Reach Extraction Reach or After Gravel (171+45 to 17+00) (171+45 to 83+50) (83+50 to 17+00) Extraction Increase in Average Increase in Average Increase in Average Bed

Bed Elevation (ft) Bed Elevation (ft) Elevation (ft) 1997 1.76 2.02 1.43 None 2000 2.14 1.76 2.65 2002 2.96 2.52 3.54 -- 2004 3.00 2.64 3.48 Before 2005 2.79 2.31 3.41 Before 2007 2.53 2.11 3.09 Before 2008 1.93 1.10 3.04 After 2009 1.71 1.15 2.44 After

30

25

20

15

10

5 Elevation Elevation NAVD88)(ft, 0

-5 0 3000 6000 9000 12000 15000 18000 21000

Station (ft) Design Pre-project Wetted Bed Elevation Deviation from Natural Grade 2004 Average Bed Elevation

Pre-project Top of Bar Figure 16. Peak aggradation in 2004 compared to design average bed elevation and pre-project wetted bed elevation and top of bars.

26 The shift in average bed elevation over the last six years (2004-2009) corresponds directly with the onset of annual gravel extraction. The volume of gravel extracted from the channel between 2004 and 2009 was 163,686 cubic yards, approximately 37% of the annual bedload load for the same period. Suspended sediment loads are only available for 2004-2006. Considering only this period, the gravel extraction volume represents only 8% of the total sediment load. The significance of these numbers is uncertain because the sediment supply to the reach is unknown and sediment transport can respond non-linearly to changes in channel form and supply. It is possible for relatively small changes in channel form and supply to have much larger cumulative effects.

Water years were generally a mix of dry and normal in the five years prior to peak aggradation and the five years following peak aggradation, with the notable exception of 2006, which does not appear to disrupt the decreasing trend in average bed elevation. It is possible that the sediment supply was elevated following the 1997 event and persisted until 2002-2004 and has subsequently begun to decline. However, no data exist to confirm a reduction in sediment supply during this period.

A combination of direct bed material removal through gravel extraction, lower sediment supply, and favorable flow conditions are likely working in concert to promote a decline in sediment storage within the Flood Control Project. These qualitative observations and hypotheses should be examined with quantitative tools to assess the relative roles of sediment supply, gravel extraction, and flow magnitude and duration to explain the observed trends.

4.5 Stability and Equilibrium in Redwood Creek The most likely state of Redwood Creek from 1955 to the present is disequilibrium. While channel recovery has begun, an equilibrium condition has not been reached and aggradation persists throughout much of the lower watershed. However, during this period of disequilibrium, several channel attributes have been identified that have developed and persisted despite levee construction and major channel excavations; these include bar formation, channel development, and base level adjustment. These channel attributes will be retained as the Redwood Creek channel continues to adjust toward a new equilibrium that is controlled by discharge, sediment supply, bed material, gravel management, and levee confinement.

This historical analysis cannot predict the future channel geometry, grade, and roughness of the Redwood Creek channel, or determine whether an equilibrium condition can be achieved within the levee system and the degree of natural bed fluctuations that may be expected through time. However, it is clear that the current trajectory of the Redwood Creek Flood Control Project is a substantial deviation from the design, which was intended to maintain a static design channel shape and grade in order to meet conveyance requirements with minimal maintenance. The repeated infilling and re-shaping of the channel is evidence that the channel design proposed in the GDM is neither stable nor an equilibrium channel form for Redwood Creek.

27 5 Summary

5.1 Current Conditions The flood capacity of the Redwood Creek Flood Control Project has diminished substantially from its design level of 77,000 cubic feet per second due to sediment deposition. Based on the NHE 2008 Model described in NHE (2010), flood capacity is highest at the upstream extent of the levee system (70,000 cfs in Reach 3) and decreases progressively downstream (52,500 cubic feet per second in Reach 1).

In 2007, the Corps of Engineers placed the Redwood Creek levee system on a national list of 122 levees with maintenance deficiencies that present an increased risk to the public due to the overall loss of capacity (County of Humboldt, 2010). In 2008, the Corps of Engineers determined that the levee had not been brought back to an acceptable condition and placed the Flood Control Project on inactive status under the federal Rehabilitation and Inspection Program. The evaluation in this technical memorandum indicates that fundamental design deficiencies, rather than insufficient maintenance, are the root causes for the diminished levee performance.

5.2 Design Deficiencies The channel design requires: (1) static bed condition, (2) trapezoidal channel geometry, and (3) constant channel gradient with a base level of -1.3 feet NAVD88 in order to achieve the target flood capacity. These three design requirements cannot be achieved within the Flood Control Project.

Redwood Creek is an alluvial channel with highly variable discharge and sediment supply. A static bed condition is not achievable in stream channels that have large, episodic sediment pulses that aggrade the channel during major storms and are subsequently re-shaped by moderate flows as documented in Redwood Creek by Nolan & Marron (1995).

A trapezoidal channel geometry is not a stable channel geometry in gravel-bed rivers. Gravel-bed rivers are typically composed of a sequence of bars and pools which vary in density, location, and shape based on sediment supply, bed and bank materials, and stream flow. In addition, a trapezoidal channel has a higher likelihood of bed material deposition than a channel with a bar- pool morphology at moderate flows because the flow spreads across a wider area, decreasing flow velocity and shear stresses acting on the bed and reducing sediment transport (Brookes, 1988). Sediment deposition during moderate flows (less than bankfull) is important in Redwood Creek because the effective discharge is less than bankfull and these flows are responsible for more than 50% of the total sediment discharge in Redwood Creek (Nolan, Lisle, & Kelsey, 1987). Therefore, the trapezoidal channel geometry would likely promote sediment deposition with the Flood Control Channel during the periods of time when the most sediment is being delivered from the upstream reach.

Both coastal and fluvial processes control the gradient and base level of Redwood Creek at the downstream end of the Flood Control Project. Base level at the downstream end of the project is likely to fluctuate based on short-term ocean conditions, stream flows, and sediment loads. Therefore, artificially lowering the channel base level is not likely to be sustainable and the expectation of a static base level to achieve a specified flood capacity is unreasonable within this

28 dynamic environment. The project design included the installation of a sill to limit potential downcutting of the channel, but did not account for potential sediment deposition and a return to the pre-project base level.

Multiple lines of evidence from reports, letters, and a time-series of topographic data between 1968 and 2009 indicate that the Flood Control Project has never performed as designed:

1. Large-scale sediment removal was not expected to be part of regular maintenance in the Corps of Engineers design document (COE, 1966) and operation and maintenance manual (COE, 1969). The first winter following construction (1968), three to four feet of sediment deposition was observed at several locations within the Flood Control Project. The Corps of Engineers stated that high velocity flows would flush the sediment from the channel and that no maintenance action was necessary. However, high flows are typically coupled with the delivery of large volumes of sediment which result in channel aggradation, rather than sediment removal. Topographic surveys in 1980 and aerial photos document the continued accumulation of sediment and by 1987, at least 210,000 cubic yards of sediment had deposited within the Flood Control Project. 2. In 1987 and 1988, the channel of the Flood Control Project was re-excavated to near design conditions as part of an aggregate mining project. Ten years after excavation, the average bed elevation throughout the flood control channel had increased by 1.9 feet. Repeated infilling of the channel following a resetting of design conditions in 1987 and 1988 indicates that reach-scale sediment accumulations were not an isolated occurrence due to unique conditions and relatively high sediment loads in the late 1960’s and early 1970’s. 3. Topographic surveys 12 years after the initial project construction (1980 and 1981) and nine years after re-excavation to near design conditions (1997) document the re-development of bars, the typical morphology of a gravel-bed river. These results demonstrate that the design geometry is not self-maintaining and reach-wide adjustments toward the pre-project morphology occur in as little as nine years. 4. A comparison of pre-project contours and design bed elevations in the lower portion of the Flood Control Project (Station 6629 to 1700), which currently provides the lowest flood capacity, indicates that the design bed elevations reach 3.3 feet below the pre-project wetted bed elevation. A time-series of topographic surveys after project construction indicate that the bed elevation is approaching the pre-project bed elevation. These results demonstrate that deepening of the channel below pre-project bed elevations is not self-maintaining. 5. Deepening of the channel to achieve a constant slope was a deviation from the stated design criteria that the design channel would follow the natural stream grade. This deviation resulted in lower levee elevations at the downstream end and contributed to reduced channel capacity in the lower reach.

This historical analysis cannot predict the future channel geometry, grade, and roughness of the Redwood Creek channel, nor determine whether an equilibrium condition can be achieved within the levee system or the degree of natural bed fluctuations that may be expected through time. However, it is clear that the current trajectory of the Redwood Creek Flood Control Project is a substantial deviation from the design, which was intended to maintain a static design channel geometry and grade in order to meet conveyance requirements with minimal maintenance. The

29 repeated infilling and re-shaping of the channel is evidence that the channel design proposed in the GDM is neither stable nor an equilibrium channel form for Redwood Creek.

5.3 Actions Taken To Date Humboldt County has performed commercial mining-scale gravel extraction from 2004 through 2009, removing a total of 164,000 cubic yards, in an effort to increase flood capacity. Gravel extraction has primarily occurred in the upper reaches. Reasons for site selection have included: the size of gravel deposits; equipment access and available staging areas adjacent to extraction areas; avoidance of areas affected by stream mouth closure and back-water; and hypotheses that upstream bars may act as sediment traps and limit downstream deposition, and that enhancing capacity in the upper reach may have a downstream benefit, while gravel removal downstream would have little or no upstream benefit.

The decline in average bed elevation during the 2004-2009 gravel extraction period has been moderate but consistent. The observed decline in average bed elevation may be a result of consistent gravel extraction, a reduction in upstream sediment supply, favorable flow conditions, or may indicate that the storage volume has reached a dynamic equilibrium around which the bed elevation will have smaller fluctuations. These qualitative observations and hypotheses should be examined with quantitative tools to assess the relative roles of sediment supply, gravel extraction, and flow magnitude and duration to explain the observed trends.

5.4 Recommendations A major re-evaluation of the levee system is needed to provide adequate, sustainable flood protection for the community of Orick and the Highway 101 corridor. Moffatt & Nichol Engineers (2003) developed and analyzed several alternative levee configurations, but the study did not address the relative roles of sediment supply, gravel extraction, levee confinement, and coastal and estuarine processes, nor did it predict the trajectory of channel evolution and the possible range of expected conditions within this dynamic system. A detailed geomorphic and sediment transport study that addresses these issues is critical to the development of future flood risk reduction measures that will be effective within this dynamic system in the long-term.

30 6 References Andrews, E. D. (1980). Effective and bankfull discharges of streams in the Yampa River basin, Colorado and Wyoming. Journal of Hydrology , 46 (3-4), 311-330.

Brookes, A. (1988). Channelized rivers: Perspectives for environment management. New York: John Wiley & Sons.

Cannata, S. R., Henly, J., Erler, J., Falls, J., McGuire, D., & Sunahara, J. (2006). Redwood Creek Watershed Assessment Report. Coastal Watershed Planning and Assessment Program and North Coast Watershed Assessment Program. Sacramento, CA: California Resources Agency and California Environmental Protection Agency.

Collins, B., & Dunne, T. (1990). Fluvial geomorphology and river-gravel mining: A guide for planners. Special Publication 98, California Division of Mines and Geology , Sacramento.

County of Humboldt. (2010). Project Description for Section 404 Permit Renewal.

Griggs, G. B., & Paris, L. (1982). Flood Control Failure: San Lorenzo River, California. Environmental Management , 6 (5), 407-419.

Humboldt County Public Works. Letter to U.S. Army Corps of Engineers, 1968, June 21. Eureka, CA, USA.

Kelsey, H. M., Madej, M. A., Pitlick, J., Stroud, P., Coghlan, M., Best, D., et al. (1981). Sediment source areas and sediment transport in the Redwood Creek basin: A progress report. Arcata: Redwood National Park Research and Development Technical Report 3.

Klein, R. D. (1991). Physical monitoring of the Redwood Creek estuary 1991 Progress Report.

Klein, R. D. (2010). Provisional, unpublished data. CA, Arcata: Redwood National and State Parks.

Knighton, D. (1988). Fluvial forms & Processes A New Perspective. New York: Oxford University Press, Inc.

Leopold, L. B., Wolman, M. G., & Miller, J. P. (1964). Fluvial Processes in Geomorphology. San Francisco: W.H. Freeman.

Mackin, J. H. (1948). Concept of a graded river. Bulletin of the Geological Society of America , 59, 463-512.

Madej, M. A. (1999). Temporal and spatial variability in thalweg profiles of a gravel-bed river. Earth Surface Processes and Landforms , 24, 1153-1169.

Madej, M. A., & Ozaki, V. (1996). Channel response to sediment wave propogation and movement, Redwood Creek, California, USA. Earth Surface Processes and Landforms , 21, 911- 927.

31 Moffatt & Nichol Engineers. (2003). Hydraulic analysis of alternative levee configurations for lower Redwood Creek Humboldt County, California. Prepared for: Department of Public Works County of Humboldt.

Nolan, K. M., Lisle, T. L., & Kelsey, H. M. (1987). Bankfull discharge and sediment transport in northwestern California. Erosion and Sedimentation in the Pacific Rim: Proceedings of the Corvallis Symposium. IAHS Publ. no. 165.

Nolan, M., & Marron, D. C. (1995). History, causes and significance of changes in the channel geometry of Redwood Creek, northwestern California, 1936 to 1982. In K. M. Nolan, H. M. Kelsey, & D. C. Marron (Eds.), Geomorphic Processes and Aquatic Habitat in the Redwood Creek Basin, Northwestern California (pp. N1-N22). U.S.G.S. Professional Paper 1454.

Northern Hydrology and Engineering (NHE). (2009). Technical Memorandum: Review of Army Corps of Engineers, San Francisco District, Redwood Creek Channel Capacity Analysis and Associated FEMA Restudy Work. Prepared for: County of Humboldt.

Northern Hydrology and Engineering (NHE). (2010). Technical Memorandum: Outstanding Work Products (Tasks) Pertaining to Redwood Creek Flood Control Project. Prepared for: County of Humboldt.

Redwood National and State Parks (RNSP). (n.d.). Data on file. Arcata, CA.

Redwood National Park. (1992). Monitoring and evaluation of gravel extraction on Redwood Creek, CA. Progress Report.

Ricks, C. L. (1995). Effects of channelization on sediment distribution and aquatic habitat at the mouth of Redwood Creek, northwestern California. In K. M. Nolan, H. M. Kelsey, & D. C. Marron (Eds.), Geomorphic Processes and Aquatic Habitat in the Redwood Creek Basin Northwestern California. U.S. Geological Survey Professional Paper 1454.

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