04 - Nimbus Dive Report_1.pdf

U.S. Department of the Interior Bureau of Reclamation Pacific Northwest Region August 2017

2017 Underwater Examinations

Nimbus Dam Central Valley Project, California

Nimbus Dam, Central Valley Project, California August 2017

FOR OFFICIAL USE ONLY

General On August 29 and 30, 2017, Pacific Northwest (PN) Regional Underwater Inspection Team members performed an underwater inspection at Nimbus Dam, near Folsom, California. The underwater examination was requested by Senior Civil Engineer, Mark Lewis of the Central California Area Office, Folsom, California. Site specific access assistance, access to as-built drawings, and examination requirements were coordinated with personnel from the Central California Area Office. Mr. Lewis served as contact for the underwater inspections and was onsite for consultation of findings. The purpose of the examination was to inspect the power plant trash racks, inspect the upstream side of the dam, inspect the downstream side of the dam, and inspect the powerplant tailrace.

Reclamation personnel who performed the inspection were: Brian Hamilton (PN-1774) from the Salmon Field Office, Salmon Idaho; Ryan Hedrick (MSF-6213) from the Snake River Area Office, Boise, Idaho; Robert McAffee (PN-3142) and Ben Radchuk (PN-3240) from the Pacific Northwest Regional Office in Boise, Idaho. Mr. Hamilton was the dive master and author of this report.

Access to the various dive locations at Nimbus dam was via the Dive Team Sea Arc boat. Dive team members served as boat operators during the inspection. The boat was launched at the ramp on the left bank upstream of the dam for the upstream inspections, and from the gravel beach downstream of the dam for the downstream inspections.

A hazardous energy control program was in effect at the facility at the time of the underwater examination. Gates and valves were closed, and locks were placed to lock out operation of the units. Prior to commencement of diving activities, the dive master verified hazardous energy clearance procedures, and locked onto a common lock box. The dive hazard analysis was discussed with all participating personnel. Clearance locks were removed upon the completion of diving activities. Bulkhead gates were placed in front of the powerplant intakes prior to diving upstream and downstream of the powerplant.

The 2018 inspection was carried out over 2 days. On August 29, all flows were being diverted through the powerplant, with radial gates 1 - 4 being left unlocked for use in an emergency.

Divers inspected the areas upstream and downstream of radial gates 6 - 18 on this day. On August 30, the flows were diverted through radial gates 13 - 18 with radial gates 1 - 9 and the powerplant intakes locked out. Divers inspected the areas upstream and downstream of gates 1 - 5, along with the powerplant intake and tailrace.

The upstream diving operations and the powerplant tailrace inspection were performed using one-man SSA dive equipment with a standby diver using SCUBA equipment. The inspection of the shallow area downstream of gates 1 - 18 was conducted using a two-man buddy team on SCUBA equipment. Diver to surface voice communication was utilized during all inspections, and diver to diver voice communication was also utilized when two divers were in the water together. Go-Pro cameras were mounted on the divers’ masks during the inspections.

Underwater photos included in this report came from the Go-Pro videos.

On the days of the inspection the weather was sunny and hot, with maximum temperatures over 100 degrees. Water temperature was 60 degrees and the maximum diver depth during inspections was 40 feet upstream of the dam, 30 feet in the powerplant tailrace, and 12 feet downstream of the radial gates.

Bureau of Reclamation reference drawings utilized were:

• Drawing No. 485-D-393, Nimbus Dam and Powerplant – General plan and sections

• Drawing No. 485-D-711, Nimbus Power Plant, Intakes, Trashrack – Seats – Support

Beam Installation – List of Parts

• Drawing No. 485-D-712, Nimbus Power Plant, Intakes, Trashrack – Seats – Support

Beam Details

Figure 1 Aerial view of Nimbus Dam.

Radial Gate 1

Unit 2

Unit 1

Radial Gate 18

Stilling basin

Powerplant trashracks

Powerplant tailrace

Background Nimbus Dam was completed in 1956 and is located on the American River approximately 7 miles downstream of Folsom Dam near Folsom, California. Nimbus is a concrete, gravity dam 1,093 feet long with a structural height of 87 feet. The spillway is comprised of 18 radial gates, each 40 feet wide by 24 feet high, with a combined capacity of 300,000 ft /s. Each gate is located between piers on an ogee section with a concrete apron that extends 122.5 feet upstream of the piers. Each pier has a steel rail embedded in the upstream nose. Bulkhead gates are placed against the upstream side of the piers during maintenance of the spillway gates.

The stilling basin apron extends downstream 56 feet and is comprised of a 5-foot-thick apron with chute, apron, and sill blocks extending across the apron. The powerplant, with two generating units, is located between the rightmost spillway radial gate and the right abutment looking downstream.

Numerous underwater inspections have been conducted at Nimbus Dam with the first inspection completed in 1965. During a routine inspection in 1968, a transverse crack and vertical offset were found in the upstream apron of the dam. As a direct result of these findings, additional inspections of the cracked area were carried out in 1970 and 1971. During underwater inspections conducted in 1996, 2002, 2006, 2011, and 2015, the upstream apron crack was found to be covered with several inches (up to 6 inches) of fluffy organic deposits. These deposits were easily disturbed and reduced the underwater visibility to zero, which has made detailed inspections and documentation of the crack difficult. During the 2015 inspection divers released dye near the crack upstream of gate number 1, but the dye was not detected downstream of the dam.

Recent inspections have found large quantities of woody debris and trash on the powerplant intakes. Large quantities of lead fishing sinkers have been found in the powerplant tailrace, but damage occurring due to the movement or presence of the sinkers has not been noted. The concrete chute blocks, apron blocks, and sill blocks downstream of the spillway gates have been found to be in generally good condition, with only minor concrete damage. A contractor has been on-site refurbishing the radial gates for several years and was onsite working on gate number 6 during the 2018 inspection. A bulkhead gate was installed at the upstream side of the radial gate, as shown in Photo 1.

Examination Results Spillway Apron - Upstream of Radial Gates 1 - 18 Past inspections have found the upstream apron covered with 4 to 6 inches of fine organic particles. These particles are easily disturbed and reduce underwater visibility to zero. Detailed inspection of the apron has been difficult, and photos are typically not possible. During the 2018 inspection, however, the bottom was relatively clean, covered only in a thin layer of fine silt.

This material was still easily disturbed, but the small amount present could be cleared away to allow for photos and detailed observations.

Upstream of the pier noses approximately 6 inches is a construction joint in the concrete apron.

This joint is typically tight, with minimal erosion along the joint. There is a soft, flexible material, approximately 3/4-inch-thick, in the center of the joint. Drawing 485-D-393 shows the concrete footer extends down (vertically) 24 feet on the downstream side of the joint, while the upstream apron floor is 12 inches thick. The apron extends approximately 120 feet upstream.

There is a crack in the concrete apron that runs most of the way across the dam. This crack is typically located 12 to 18 inches upstream of the construction joint. In some locations gravel and cobbles cover the floor, and the crack is not visible. Near some of the transverse (upstream to downstream) floor joints, the crack is not present. The floor upstream of the crack is typically 1 to 2 inches lower than the floor downstream of the crack. The dimensions and physical characteristics of the crack vary. At some locations there is erosion along the crack, with the top width being up to 6 inches, and up to 6 inches deep. At other locations, it appears the crack is occurring due to separation between the apron downstream of the floor joint and a separate, 12 - 18 inches wide, concrete placement upstream of the joint. Divers were able to insert a measuring tape horizontally into the crack, and extend it downstream to the joint, in several locations. No areas were found where it appeared as if there was flow going into the crack.

Photos 2 through 9 show the floor joint and the crack at various locations across the dam.

The concrete apron upstream of the crack is almost entirely covered with debris, consisting of river cobbles, gravels, trees, and smaller woody debris. Divers did not inspect the apron to determine the depth of the buildup, but only inspected the area immediately upstream of the pier noses and the bays between the piers. Typically, the debris covered the entire upstream section of the apron almost to the location of the crack and the construction joint. The floor was typically clear near the pier noses, and then the interior of the bays were covered with debris.

Some of the clear areas were likely due to the ongoing placement of bulkhead gates and radial gate repairs. The concrete floor, where visible, was found to be in good condition with less than 1/2 inch of relief throughout the upstream side of the dam (with the exception of the area immediately next to the crack). Inside the bays, the floor is typically covered with 1 foot of small woody debris, less than 6 inches in diameter, and river cobbles from 2 to 20 inches in diameter. Photos 10 through 14 show typical debris buildups in this area.

The bay in front of Radial Gate 6 was blocked by a bulkhead gate and could not be inspected.

On the upstream side of the gate divers found several large sections of concrete, up to 3 feet long and 12 inches thick. One of these sections of concrete had a membrane attached, similar to the membrane found in the joint. The November 2008 underwater inspection found a hole in the concrete floor in the area downstream of this bulkhead gate that was 5 feet in diameter, 18 inches deep. The concrete pieces may be from this hole, but that could not be verified with the bulkhead gate in place.

The bulkhead guides on the upstream of the pier noses were all found to be in place. The guides are rusted, with rust nodules up to 1/2 inch in diameter covering up to 50 percent of the guides.

The rust nodules are hard to remove, but there is minimal metal loss occurring beneath the nodules. Photo 15 shows the typical guide condition.

Radial Gates The gate hoists on the radial gates were found to be covered with small woody debris. The galvanic corrosion bars that had been attached to the gate hoists were in poor shape. Many missing bars were noted, and wires connecting the bars to the gate hoist were broken. A section of the galvanic corrosion system from one of the gates was brought to the surface and given to project personnel. Photos 16 through 20 show the typical condition of the gate hoists and the galvanic corrosion protection system.

The gate hoists themselves were found to be in good condition wherever they could be inspected beneath the woody debris buildup. All cotter pins were found to be in place, as shown in Photo

21. Some of the pins are installed on the upstream side of the gate hoist, and some of the pins are installed on the downstream side of the gate hoists. Although the galvanic corrosion protection bars did not appear to be functional at most of the gate hoists, no corrosion or other damage to the hoist connections was observed.

Powerplant Intake There are two units on the powerplant. The intakes, located on the right side of the dam, are each covered with trashrack structures, 16 feet high and 41 feet wide. Large amounts of woody debris are present on both intake trashracks. This woody debris typically consists of 3 to 12-inch diameter logs and branches, with a layer of up to 3 feet thick resting against the trashrack bars.

On Unit 2, the upper 9 feet of the rack is clear on the left side, decreasing to the upper 3 feet clear on the right side. This results in only 37 percent of the rack being clear of debris.

On Unit 1, the bottom half of the rack is completely covered with woody debris. The left half of the upper part of the rack is approximately 20 percent covered, and the right half of the upper rack is nearly 100 percent clear. This results in approximately 45 percent of the rack being clear of debris.

The concrete floor upstream of the powerplant intakes is nearly completely clear of debris. The concrete is in good condition, with less than 1/4 inch of relief throughout. There is a deflector wall in the area upstream of the intakes that is also in good condition. No damage was noted to the wall. Three concrete anchors for the missing surface buoys were located on the upstream side of the wall. Cables or chains long enough to attach to a surface buoy were not found on the anchors.

Powerplant Tailrace Flow through the powerplant is discharged into a tailrace downstream of the dam. The tailrace is 86 feet wide by 120 feet long. The floor slopes upward in the downstream direction at a 6:1 slope, rising 20 feet in the 120-foot long basin. Maximum depth in the tailrace during the inspection was 30 feet. Flow from each of the two units discharges into the basin through 3 outlet bays, so there are 6 outlet bays total. These bays are numbered from 1 to 6 from left to right, looking downstream. Divers did not go into the outlet bays, inspecting from the mouth of the outlets to the downstream end of the basin.

There is a construction joint in the floor at the downstream end of the pier noses that separate the outlet bays. There was typically a small amount of erosion at this joint, with up to 1 inch of relief along the joint. Lead fishing weights were found accumulated in the joint and in the area immediately downstream of the piers. Downstream of bay 4, there was a small erosion pocket measuring 4 inches wide, 12 inches long, and 5 inches deep located on the joint (Photo 25).

Downstream of bay 6 there was a smaller erosion pocket located along the joint. This eroded area measured 3 inches wide, 5 inches long, and 1 inches deep (Photo 26).

Along the right wall, downstream of bay 6, there was some erosion along the wall joint at the base of the wall. The eroded area along the wall joint was measured at 10 inches tall, up to 5 inches wide, and up to 4 inches deep (Photo 27).

In the center of the tailrace there is a large accumulation of lead fishing weights and cobbles up to 12-inch diameter (Photo 28). This bar is approximately 25 feet wide, 40 feet long, and 6 inches thick. Estimated volume of the accumulation is 10 cubic yards.

Approximately 90 feet downstream of the outlet bays along the right wall there is an accumulation of cobble up to 12-inch diameter and large boulders up to 3 feet in diameter (Photo 29). This accumulation of debris extends from the right wall out in to the center of the tailrace.

Estimated volume of the accumulated debris is 10 cubic yards.

The floor within the rest of the basin was mostly clear, with only scattered cobbles and lead fishing weights found. The floor along the joint at the upstream end of the basin typically showed up to 1/4 inches of relief. This relief extended approximately 10 feet downstream. The floor within the rest of the basin was smooth, with approximately 1/8-inch relief, even around the areas with accumulated rock debris. No exposed rebar was found. The drawings show the concrete floor is 6 feet thick in the basin.

Spillway Stilling Basin – Downstream of Radial Gates 3 through 18 The spillway stilling basin has 3 sets of erosion control blocks. There are 6 chute blocks, 3 feet- 6 inches tall and 3 feet-4 inches wide, located at the toe of the slope below each gate. There are 5 apron blocks below each gate, located 15 feet further downstream, centered in the gaps between the chute blocks. These apron blocks are 4 feet-6 inches tall and 4 feet-6 inches wide.

Sill blocks are located 30 feet further downstream, 5 downstream of each gate. The sill blocks are 4 feet-6 inches tall and 4 feet-6 inches wide. The invert elevation for the stilling basin downstream of gates 3 through 18 is 74 feet, 4 feet higher than the invert for the stilling basin downstream of gates 1 and 2.

Downstream of Radial Gates 3 through 18 (Invert Elevation 74) The chute blocks at the toe of the slope were typically in good condition with sharp edges and corners and less than 1/2-inch erosion (Photo 30). There are 96 chute blocks in this section of the stilling basin, and only 6 damaged blocks were found. This damage typically consisted of chipping along the top edges of the blocks (Photo 31), or on the downstream face (Photo 32), or possibly related to the original concrete placement (Photos 33 and 34). The damage was found to be 1 to 4 inches deep. No exposed rebar was found. The damaged blocks were found downstream of gates 10 and 11 (2 each), 15, and 16 (1 each). All chute block drains were found to be open, but no flows were noted (Photo 33).

The mid-basin apron blocks were in good condition, with sharp edges and corners and less than 1/2 inch of concrete erosion. Only very minor damage was found on any of the 80 apron blocks found in this section of the spillway (Photo 36).

The sill blocks at the downstream end of the basin were also in good condition, with sharp edges and corners and less than 1/2 inch of erosion. Only very minor damage was found on any of the 80 sill blocks, mainly minor chipping along the edges (Photo 37). The contact downstream of the sill blocks was tight. Vertical concrete between 0 and 2 feet was exposed, with gravel and large riprap found downstream of the concrete. No undercutting or eroded areas were found.

The floor of the stilling basin at elevation 74 was typically smooth, with less than 1/2 inch of relief (Photo 38). Downstream of Radial Gate 11, between the chute blocks and the apron blocks, the floor is rougher, with up to 2 inches of relief. Damaged concrete was limited to the area downstream of this gate. No rebar was found in the eroded area. Only minor erosion was found around any of the floor joints.

Downstream of Radial Gates 1 through 2 (Invert Elevation 70) Downstream of Radial Gate 2, the chute blocks at the toe of the slope were typically in good condition with sharp edges and corners and less than 1/2-inch erosion. Four of the six chute blocks downstream of Radial Gate 1 showed concrete damage (Photos 39 – 41). Erosion was typically on the edges, corners and downstream face of the blocks. The concrete was eroded up to 6 inches deep in spots. No rebar was found.

The mid-basin apron blocks were also in good condition downstream of Radial Gate 2 (Photo

42) and showed concrete damaged downstream of Radial Gate 1 (Photo 43). The erosion was typically on the edges, corners, and upstream faces of the apron blocks. Up to 8 inches of concrete was eroded on the most severely damaged block.

The sill blocks at the downstream end of the basin were also in good condition, with sharp edges and corners and less than 1/2 inch of erosion. Only minor damage was found on any of the sill blocks.

Two large concrete blocks were found on the floor of the stilling basin between the chute blocks and apron block, downstream of Radial Gate 1 (Photo 44). There was no apparent function for these blocks, but they are not causing any apparent damage to the basin.

The floor of the stilling basin at elevation 70 was typically smooth, with less than 1/2 inch of relief. The contact downstream of the sill blocks was tight, with no undercutting or erosion noted (Photo 45). Up to 2 feet of vertical concrete was exposed.

Conclusions and Recommendations Divers were able to conduct a thorough inspection of the crack running across the apron in front of the radial gates. The crack was found to be located upstream of a formed joint in all locations.

Drawing 485-D-394 shows the concrete apron upstream of the joint to be 12 inches thick, and 24 inches thick immediately downstream of the joint. The cutoff wall, located just downstream, is 24 feet deep. Divers found no indications that flow was going into the crack. The crack appears to be limited to the 12-inch-thick apron upstream of the joint. The size and condition of the crack does not appear to be changing when compared to photos and descriptions from past inspection reports.

The galvanic corrosion system that had been recently installed on most of the radial gates was severely damaged. The bars were bent or missing on multiple structures. The wires that attached the galvanic corrosion to the gates were broken. Large amounts of woody debris were entangled in the gate attachments. Portions of this system were removed by the divers and given to Nimbus Dam personnel during the inspection. This system appeared to be non-functional.

The trashracks covering the intakes to the powerplant were partially blocked with large and small woody debris. The debris covered roughly 40 percent of the trashrack structures, mainly on the bottom of the trashracks.

There is a large accumulation of lead fishing weights in the powerplant tailrace. There is also an accumulation of large cobble and boulders. The debris does not appear to be doing significant damage to the basin, as only a couple of isolated pockets of erosion were found. This erosion was mainly on the floor joint immediately downstream of the discharge tubes, while the floor throughout the rest of the basin was in good condition. In 2011, the tailrace basin was found to be clear of all debris.

Concrete chute blocks, apron blocks, and sill blocks in the spillway stilling basin are mostly in good condition. Isolated damaged blocks were found, mainly downstream of Radial Gates 1, 10 and 11. The floor of the basin is in good condition, with the only eroded area (up to 2 inches) found downstream of Radial Gate 11. The damaged concrete downstream of Radial Gates 10 and 11 was not noted during the 2011 inspection.

In accordance with FAC 01-07, Reclamation Review/Examination Program for High and Significant Hazard Dams, unless sufficient justification is provided to the facility review team, based on the factors, process, and referred requirements contained in Paragraph 7.E.(4), the maximum time interval between visual examinations (considered “hands on” or essentially equivalent) of all normally inaccessible features at significant and high hazard dams is 8 years.

Photographs

Photograph 1 Upstream side of Nimbus Dam. Bulkhead gate is installed upstream of Bay No. 6, as shown by the arrow. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 29, 2015.

Photograph 2. Spillway Apron. View shows the joint located upstream of the pier noses, and the crack in the concrete apron upstream of the joint. This photo was taken upstream of gate 2 but is typical of conditions across the dam. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 30, 2017.

Flow

Joint

Crack

Photograph 3. Spillway Apron. View shows the floor joint and upstream crack. This photo taken upstream of gate 7. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 29, 2017.

Photograph 4. Spillway Apron. Vertical offset of up to 3 inches was measured along the crack, with the upstream end always being lower. Photo taken upstream of gate 7. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 29, 2018.

Flow

Flow

Photograph 5. Spillway Apron. View shows the floor joint and upstream crack. The joint is partially covered with tied up rags that were used by the contractor to seal the bottom of the bulkhead gate when it was in place. A minor accumulation of small woody debris covers the floor upstream of the crack. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 29, 2018.

Photograph 6. Spillway Apron. Diver was able to insert a stick tape up to 12 inches horizontally into the crack, which was the approximate distance to the joint. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 29, 2018.

Photograph 7. Spillway Apron. A flexible membrane, approximately 3/4 inch thick, is located in the floor joint. This membrane was in place along the entire joint. This photo taken in front of gate 10. The stick tape is resting on the membrane. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 29, 2018.

Photograph 8. Spillway Apron. View, looking downstream, shows the intersection between a longitudinal joint in the apron and the transverse joint that runs across the dam. There was no crack in the apron near the longitudinal joint. This photo taken upstream of bay 11. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 29, 2018.

Joint Intersection

Flow

Photograph 9. Spillway Apron. View shows the crack and the floor joint upstream of the pier between gates 10 and 11. The joint is barely visible beneath a thin layer of fine sediment. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 29, 2018.

Photograph 7. Spillway Apron. Large piece of concrete, 3 feet x 1-foot x 1 foot, located upstream of the bulkhead gate in bay 6. A section of flexible membrane (arrow), similar to the membrane found in the floor joint across the dam, was attached to the side of the concrete block. The orange line follows the edge of the concrete block. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 29, 2017.

Pier

Photograph 11. Spillway Apron. Cobbles and small woody debris located on the apron upstream of the bulkhead gate in bay 6. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 29, 2017.

Photograph 12. Spillway Apron. View shows woody debris and cobbles up to 12-inch diameter found at the downstream end of bay 8. The cobbles were piled up to 3 feet deep in the corner of this bay. This was the only bay where cobbles were found piled up in the corner, although multiple bays had accumulations of cobbles up against the downstream wall. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 29, 2017.

Bulkhead Gate

Photograph 13. Spillway Apron. 4 inch and smaller cobbles and small woody debris found upstream of the pier between gates 9 and 10. This is typical of debris found across the upstream side of the dam. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 29, 2017.

Photograph 14. Spillway Apron. Cobbles up to 12-inch diameter, small woody debris, and some metal walkway debris found in bay 10. This is typical of debris found across the upstream side of the dam. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 29, 2017.

Pier

Photograph 15. Spillway Apron. Bulkhead guides were rusted, with nodules up to 1/2 inches in diameter. This photo, taken on the pier between gates 13 and 14, shows the typical condition of guides across the dam. Nimbus Dam, Central Valley Project, Folsom Unit, American River

Photograph 16. Radial Gates. The gate hoists on the radial gate were covered with small woody debris. This photo shows the left gate hoist on gate 10 but is fairly typical of all gate hoists.

Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 29, 2017.

Photograph 17. Radial Gates. The galvanic corrosion system installed on the gate hoists was found to be broken in numerous places. The system was installed with 6 vertical bars attached to a flat plate on both the upstream and downstream sides of the connection. Multiple bars were broken, missing, or had broken connecting wires. Diver is holding onto a broken bar. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 29, 2017.

Photograph 18. Radial Gates. Diver is holding onto a broken connecting wire on the gate 14 hoist. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, Photograph 19. Radial Gates. The galvanic corrosion system on multiple hoists were missing the vertical bars, like this photo of the gate hoist on the right side of gate 15. On the upstream side of this hoist, only 3 bars are in place (Arrows) and 3 are missing. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 29, 2017.

Photograph 20. Radial Gates. View shows the condition of the galvanic corrosion system on the left side of gate 16. The flat bar across the top is bent, skewing the angle of some of the rods.

Photograph 21. Radial Gates. The cotter pins (arrows) connecting the gate hoist pieces were sometimes found on the upstream side of the gate hoist, and sometimes on the downstream side of the hoist, but were found to be in place and in good condition. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 29, 2017.

Photograph 22. Powerplant Intake. View shows woody debris impinged on the trashrack in front of Unit 2. This photo taken near the top -center of the trashrack. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 30, 2017.

Photograph 23. Powerplant Intake. View shows the accumulation of woody debris near the bottom of the trashrack in front of Unit 2. The bottom half of the trashrack had much more debris than the top. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 30, 2017.

Photograph 24. Powerplant Intake. View shows minor amounts of trash and woody debris caught on the upper left section of the trashrack in front of Unit 2. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 30, 2017.

Photograph 25. Powerplant Tailrace. Lead fishing weights were found accumulated downstream of the piers and in small erosion pockets in the floor joint. This erosion pocket, downstream of Bay 4, measured 4 inches wide, 12 inches long, and 5 inches deep. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 30, 2017.

Photograph 26. Powerplant Tailrace. View shows the small erosion pocket along the joint downstream of Bay 4, and the small erosion pocket along the joint in the right wall. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 30, 2017.

Floor Joint

Erosion on Wall Joint

Photograph 27. Powerplant Tailrace. Close-up view of the erosion along the joint in the right wall, downstream of Bay 6. Diver could insert the stick tape xx inches into the erosion pocket. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 30, 2017.

Photograph 28. Powerplant Tailrace. Large cobble, up to 12 inches in diameter, and lead fishing weights accumulated in the center of the powerplant tailrace. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 30, 2017.

Wall

Floor

Photograph 29. Powerplant Tailrace. Several boulders, up to 3 feet diameter, and cobble located midway down the right wall in the powerplant tailrace. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 30, 2017.

Photograph 30. Spillway Basin. Chute blocks downstream of the radial gates. Chute blocks were typically in good condition, with sharp edges and corners. All drains were clear, but no flow was noted. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, Photograph 31. Spillway Basin. Chute block downstream of Radial Gate 10. Note chipped concrete along the right edge of the chute block (arrows). Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 29, 2017.

Photograph 32. Spillway Basin. Chute block downstream of Radial Gate 11. Note the chipped concrete on the downstream face, upper right corner of the chute block (arrow). Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 29, 2017.

Photograph 33. Spillway Basin. Chute block downstream of Radial Gate 15. Note the damaged concrete on the downstream face (arrow). Horizontal lines across the face are likely form board marks from construction. Nimbus Dam, Central Valley Project, Folsom Unit, American River

Photograph 34. Spillway Basin. Chute block downstream of Radial Gate 16. Note the damaged concrete across the bottom of the downstream face (arrow). Nimbus Dam, Central Valley Project, Photograph 35. Spillway Basin. Chute block drains were all found to be open and in good condition. Note the condition of the concrete on the downstream face of the chute block, typical for most chute blocks. Nimbus Dam, Central Valley Project, Folsom Unit, American River

Photograph 36. Spillway Basin. Apron blocks located downstream of the chute blocks were in good condition. These blocks measured 4 feet-6 inches tall and 4 feet-6 inches wide. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 29, 2017.

Photograph 37. Spillway Basin. Sill Blocks downstream of the Apron Blocks were generally in good condition. This block shows the typical condition, with minor chipping along the edges.

Photograph 38. Spillway Basin. The floor at elevation 74, downstream of Radial Gates 3 – 18, was typically in good condition, with less than 1/2 inch relief. Nimbus Dam, Central Valley Project, Photograph 39. Spillway Basin. Damage to a chute block, downstream of Radial Gate 1 (arrow).

Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 30, Photograph 40. Spillway Basin. View shows damage to chute blocks, downstream of Radial Gate 1 (arrows). Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 30, 2017.

Photograph 41. Spillway Basin. View shows damage to the downstream face of a chute block, downstream of Radial Gate 1 (arrow). Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 30, 2017.

Photograph 42. Spillway Basin. Downstream of Radial Gates 1 and 2, the spillway floor is 4 feet lower. This photo shows an apron block on the upper level in the background, and the lower level in the foreground. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, Photograph 43. Spillway Basin. View shows damage to the upstream face of an apron block downstream of Radial Gate 1. This was the most severely damaged block in the stilling basin.

Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 30, Photograph 44. Spillway Basin. 2 large concrete blocks were found downstream of Radial Gate 1, between the chute blocks and the apron Blocks. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, California, August 30, 2017.

Photograph 45. Spillway Basin. The contact between the concrete stilling basin and the river downstream was tight, with no undercutting or erosion noted. Vertical concrete was typically exposed 0 to 2 feet. Nimbus Dam, Central Valley Project, Folsom Unit, American River Division, Appendix A

• Drawing No. 485-D-393, Nimbus Dam and Powerplant – General plan and sections;

• Drawing No. 485-D-711, Nimbus Power Plant, Intakes, Trashrack – Seats – Support

Beam Installation – List of Parts;

• Drawing No. 485-D-712, Nimbus Power Plant, Intakes, Trashrack – Seats – Support

Beam Details