Lorenz Gulch Geotechnical Investigation Trinity River
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LORENZ GULCH GEOTECHNICAL INVESTIGATION TRINITY RIVER RIVER MILE 89.4 to 90.2 TRINITY COUNTY, CALIFORNIA Prepared for: U.S. Bureau of Reclamation Trinity River Restoration Program 1313 South Main Street Weaverville, CA 96093 and Yurok Tribal Fisheries Department 190 Klamath Way Klamath, CA 95548 Prepared by: Rocco Fiori, PG 8066 Fiori GeoSciences PO Box 387 Klamath, CA 95548 February 2011 INTRODUCTION A geotechnical investigation was conducted within the Lorenz Gulch restoration site (Figure 1) to characterize geologic materials, depth to bedrock, and surface water and groundwater conditions. Additional analysis and interpretation was conducted to evaluate the thickness of alluvium and depth to bedrock at a proposed Engineered Log Jam (ELJ) site at river mile 89.68. This information was needed to inform river restoration designers and contractors of conditions that may affect the constructability and outcome of proposed river restoration designs. Lorenz Gulch Project Location Figure 1. General location map of the Lorenz Gulch Restoration Site, Trinity River near Douglas City, California. LOCATION and BACKGROUND The Lorenz Gulch restoration site is located west of Steiner Flat Road, downstream of the Steiner Flat primitive campground and Douglas City (Figure 1). The restoration site extends from Trinity River mile 89.4 to 90.2 and covers both banks of the River. In 2010, the Yurok Tribal Fisheries Program was contracted by the Trinity River Restoration Program (TRRP) to design one of the Phase II channel restoration sites with construction anticipated to occur during the summer of 2012. SITE GEOLOGY Large scale mapping (1:50,000 scale) by Irwin (2009) shows the Lorenz Gulch Environmental Study Limits (ESL) is underlain by Quaternary alluvium (Qal) and Bragdon Formation rocks of the Mississippian sub‐period. Bragdon Formation rocks are comprised of mudstones, sandstone and conglomerates and are part of the Eastern Klamath Terrane. However, bedrock exposures evaluated during this investigation identified the majority of the bedrock within project area as graphitic schist. This rock type is more likely Abrams Mica Schist (Dms) of the Devonian period and part of the Central Metamorphic Terrane. While Irwin’s (2009) map indicates the Bragdon Formation and Abrams Mica Schist is in fault contact in the Lorenz Gulch vicinity, it’s possible that the schist outcrops identified in the field were not observed at Irwin’s mapping scale. APPROACH This geotechnical investigation was conducted in three parts: 1) characterization of surface and subsurface geologic conditions; 2) review of existing reports and datasets; and 3) a geophysical seismic refraction survey. The results of this report incorporate information obtained from all three aspects of this investigation. Characterization of surface and subsurface geologic conditions included: i) logging of test pits and trenches excavated at selected locations at the Lorenz Gulch ESL; ii) measurement of groundwater levels at peizometers installed within a subset of the excavation sites; and iii) a total station survey to tie key features to a common horizontal and vertical datum to allow spatial analysis of the associated data. Test pits and trenches were excavated August 9th and 10th 2010, with a 321B Caterpillar Excavator operated by Dwayne Proctor of the Yurok Tribe Watershed Restoration Department. Test pit logging and groundwater measurements were conducted by Rocco Fiori of Fiori GeoSciences (FGS) and David Bandrowski, Trinity Restoration Program (TRRP). Constrained by time, test pits on the right bank were primarily dug in an effort to locate bedrock and therefore geologic logs were not recorded. A total station survey of key features was conducted by Tony O’Rourke of Trinity Valley Consulting Engineers (TVCE) and tied to benchmarks established by the California Department of Water Resources (CDWR). Ground surface profiles were derived from LiDAR digital terrane model (DTM) prepared by Woolpert (2009). Due to time constraints and the availability of the LiDAR DTM, total station based cross‐section and longitudinal profile surveys were not conducted. Appendix A provides the geotechnical test pit logs prepared as part of this investigation. Appendix B provides the NORCAL Geophysical Consultants report for the seismic refraction survey. RESULTS and DISCUSSION Geologic materials were characterized within fourteen test pits and groundwater measurements were taken at peizometers installed at a subset of eight pits with complementary water surface elevations surveyed at the river edge (Figure 2 and Table 1). Most of the alluvial materials described in the test pits appeared to be fluvially deposited, consistent with the geomorphic setting, and were comprised of coarse gravels, sands and silts (Table 1 and Appendix A). Although, Brandt Gutermuth (TRRP) considered the clean sand unit (SP) located at the base of LB9 may be from historic dredge operations at the site. A sample of this material was taken for further evaluation. Bedrock was encountered at shallow depths in three of the left bank test pits (with Map ID of LB1 at 3 ft., LB8 at 7 ft., and LB9 at 10 ft.). During excavation groundwater quickly equilibrated to levels within a few feet of the ground surface in most of the pits. The exceptions were at the pits where bedrock (LB1 and LB2) or thick alluvium (LB4, LB7 and LB8) limited the ability to excavate to the elevation of water bearing alluvium. Comparison of the river water‐surface elevations with groundwater elevations indicates the river had both gaining and losing reaches within the Lorenz Gulch ESL during the measurement period (Table 1 and Figure 3). NORCAL’s (2010) seismic survey indicated that in‐stream alluvium was likely to be < 10 ft. thick at the location of the proposed ELJ (Appendix B). Seismic velocities at depth were found to be consistent with a harder bedrock type of the mica schist found in field exposures; rather than softer sedimentary rocks indicated by Irwin’s (2009) mapping. Refer to appendix B for additional details from NORCAL’s (2010) seismic survey. A comparison of NORCAL’s (2010) estimate for the thickness of in‐stream alluvium and depth to bedrock at the ELJ site with adjacent test pits is provided in figures 3 and 4. Based on NORCAL’s (2010) estimate, bedrock was about 2 to 3 feet below the depth of pit LB5 at an elevation of approximately 1548 feet. Figure 2. Project location map of the Lorenz Gulch Restoration Site, Trinity River near Douglas City, California. Test pit sites are referenced in the text according to their Map ID as shown in table 1 and figure 2. Table 1. Summary of subsurface geotechnical conditions at the Lorenz Gulch exploration pits. Groundwater and river surface water elevations measured on 08‐10‐2010 by David Bandrowski. Test pit sites are referenced in the text according to their Map ID as shown in table 1 and figure 2. River Relative Pit Feature Groundwater Depth to USCS Material Map ID Feature Water Water Number Elevation Elevation Bedrock Classes Present Elevation Level RB1 RB1 Pit with no Obs Well 1,563.86 - - - NA GW RB2 RB2 Pit at Well Head 1,567.16 1,561.62 1,561.55 -0.07 NA GW RB3 RB3 Pit at Well Head 1,565.91 1,561.23 1,561.66 0.43 NA GW RB4 RB4 Pit with no Obs Well 1,561.32 - - - NA GW RB5 RB5 Pit at Well Head 1,563.70 1,559.06 1,559.01 -0.05 NA GW LB1 LB1 Pit with no Obs Well 1,565.21 - - - 3 GM LB2 LB8 Pit with no Obs Well 1,567.56 - - - 7 SM, GW, SM-SC LB3 LB9 Pit with no Obs Well 1,567.74 - - - 10 GM, SM-SC LB4 LB7 Pit at Well Head 1,571.64 1,560.16 1,559.01 -1.15 NA GM, SM, SC LB5 LB2 Pit at Well Head 1,563.69 1,557.26 1,558.34 1.08 NA GM, GW LB6 LB4 Pit at Well Head 1,566.73 1,558.86 1,559.81 0.95 NA GW, SW LB7 LB3 Pit at Well Head 1,560.64 1,555.86 1,555.69 -0.17 NA GW LB8 LB6 Pit with no Obs Well 1,568.13 - - - NA GW, GM 1,555.70 1,555.52 -0.17 LB9 LB5 Pit at Well Head 1,563.02 NA GW, SP 1575 LB4 1570 LB2 LB3 Top of proposed ELJ surface LB1 EL. 1567.5’ 1565 Pit at ground surface LB5 LB9 EL. 1562.85’ River water surface LB6 1560 Groundwater 1555 <10’ est. thickness of streambed alluvium Pit max. depth Elevation (ft, NAVD88) 1550 EL. 1550.85’ Base of proposed ELJ Est. depth to bedrock (EL. 1548.0’) EL. 1547’ from seismic survey keyed to LiDAR 1545 streambed elevation of 1558.0’. 1540 -200 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Distance (ft) Figure 3. Simplified geologic profile showing relationships of surface and subsurface conditions on the left bank of the river adjacent to the proposed ELJ site at Lorenz Gulch, Trinity River. Inferred bedrock profile line shown as solid black where constrained from test pit exposures; where bedrock was not encountered the maximum pit depths are connected by a solid grey line for visualization purposes. Distance along profile was set at test pit LB1 as the start point. Pits LB7 and LB8 were not shown to simplify the figure. 1600 A A’ 1590 Existing ground surface Bedrock exposure 1580 Proposed ELJ location 1570 ? Qal LB‐5 (cobbles, gravels & sand) Pit max. depth 1560 ? Dms ? EL. 1550.85’ (highly deformed, <10’ Est. depth to bedrock 1550 fractured & foliated ? (EL. 1448.0’) from Elevation (ft, NAVD88) (ft, Elevation micaceous schist) seismic survey keyed 1540 to LiDAR streambed elevation of 1558.0’ 1530 0 100 200 300 400 500 600 700 800 900 1000 Distance (ft) Figure 4.