Attachment_7_Scope_of_Work_Sediment_Removal_and_Condition_Assessment.pdf

SEDIMENT REMOVAL AND CONDITION ASSESSMENT

Summary of Work

This scope is associated with the Contract Price Schedule, Contract Line Item Numbers 0002 and 0003. The purpose of this scope is to clean accumulated sediment out the Indian Garden Surge Tank and to perform a condition assessment of this tank and associated appurtenances.

Background

The water supply the Grand Canyon’s South Rim is delivered via 14-mile 1960’s era TransCanyon Pipeline from a natural spring located 3,500 ft. below the North Rim. Water flows by gravity from the springs to the Indian Garden Pump House Surge tank, where it is then pumped up to the South Rim.

At Indian Gardens, the 30-ft diameter surge tank is located adjacent to the pumphouses and is utilized to remove sediment from the water supply to improve water quality and to protect the Indian Gardens pumps from damage. Sediment is removed by providing a low flow, non-turbulent environment that allows the sediment to settle to the bottom of the sedimentation tank.

In addition, fine screening is provided at the entrances to the tank effluent pipe.

Description of Work

1. Remove the estimated 65 cu. yds of sediment from bottom of surge tank:

a. Placement of all removed sediment material in sediment dewatering bags, or similar, at locations shown on the attached sketch, as approved by the Contracting Officer.

2. Conduct a condition assessment of the tank and associated appurtenances:

a. Conduct a condition assessment of the interior tank surfaces.

b. Conduct a condition assessment of the exterior tank surfaces that are accessible without excavation.

c. Conduct a condition assessment of the tank appurtenances: influent and effluent piping, hatches, ladders, roof, vent, etc.

3. The surge tank shall be considered a confined space and all work inside the tank shall be undertaken accordingly.

Requirements

Completed work shall meet the following requirements:

1. The removal of sediment and the interior cleaning of the surge tank shall be undertaken in a manner that removes all of the sediment and readily removable scale or other material build-up from the interior of the tank.

2. The removal of the sediment and its placement at the approved locations shall be accomplished in close coordination with, and with the approval of, the Contracting

Amendment 0003 / 140P2018R0105 /

GRCA 182403

Officer and Park Management in a manner that minimizes to the greatest extent possible the impacts to Park operations, Park visitors, and the passage of mule trains.

3. Complies with all Division 1 Technical Specifications and other Contract requirements, this includes Technical Specification 01 11 00 – 1.8C regarding interruptions to the utility operations.

Submittals and Deliverables

Submit the following for review and approval.

1. Sediment Removal Plan that includes:

a. Proposed method to remove sediment from tank including what tools and materials will be used.

b. Proposed storage/staging location of removed sediment and other material.

c. Proposed method to transport removed sediment and other material between tank and approved placement location, including how this transport will be accomplished in areas accessible to the public, Park staff, and mule trains.

d. Proposed method to clean sediment, scale, and other material built up from inside the tank.

e. Proposed method of staging/storing bags/containers full of sediment.

f. Schedule for sediment removal and condition assessment.

2. Proposed sediment dewatering bags, or similar products, intended to be used to store the removed sediment.

3. Condition Assessment Report.

Appendices:

O&M Manual Section 2.4 Sedimentation Tank System Map of Indian Garden Sediment Tank Cleaning Project o Note: This area map identifies the tank and the general location to store dewatering bags

Sediment Tank

Grand Canyon National Park Arizona

IG Sediment Tank Cleaning Project

Produced by GRCA FMD GIS July 2018

National Park Service U.S. Department of the Interior

0 9 18 27 364.5 Yards

Garden Creek

Bright Angel Trail

Shaded Area Represents the General Area Stored Sediment Material is to be Located

OPERATION AND MAINTENANCE MANUAL

r-RANSCANYON WATER TRANSMISSIO"I SYSTEM

COLORADO RIVER TO SOUTH RIM

Grand Canyon National Park - Arizona

Denver Service Center

National Park Service - United States Department of the Interior

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Red Text Sedimentation Tank Information

2.4 SEDIMENTATION TANK SYSTEM

2.4.1 General Purpose

The sedimentation tank system removes sediment from the Transcanyon Transmission Flow from Roaring Springs to improve water quality and protect the South Rim Transmission Pumps from damage. Sediment is removed by providing a non-turbulent environment and low flow that will allow the sediment carried by the Transcanyon Transmission Line to settle to the bottom of the sedimentation tank. In addition, fine screening is provided at the entrances to the tank effluent pipe.

2.4.2 Component Description

The sedimentation tank system consists of the following major components:

1. Sedimentation tank;

2. Tank influent line and center feed well;

3. Tank perimeter effluent line and screen system;

4. Tank overflow line;

5. Tank drain system; and

6. Local spring source and isolating valve.

These major components are briefly described in Table 2-6 and are covered in greater detail in the following sections.

2.4.2.1 Sedimentation Tank

The tank has a concrete floor, walls, and roof and is circular. The tank inside diameter is 76 feet and the height from tank floor to ceiling is 10 feet. The tank has a useable storage capacity of 300,000 gallons when filled to the overflow level. The minimum storage capacity when the water in the tank drops to the level of the outlet is 110,000 gallons. A 6-foot high concrete wall running north-south down the center of the tank divides the tank into two main compartments with a small stilling well on the north end.

2.4.2.2 Tank Influent Line and Center Well

The tank influent line is constructed of 6-inch steel pipe and starts in the pipe pit in the North Pump House at Valve No. 102. This line exits the North Pump House through the south wall, then deflects to the east to go around the electrical transformers. The line is laid up around the sedimentation tank and enters the tank on its south side. After entering the tank, the line size increases to 8-inch steel pipe. The 8-inch pipe is secured to the top of the center north/south dividing wall inside the sedimentation tank. The influent line runs along the dividing wall and ends when it connects to the steel center well located at the center of the sedimentation tank. The center well is a 3-foot diameter by 6-foot long cylindrical steel baffle which reduces the flow velocity from the influent pipe and diverts the flow down towards the floor.

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I COMPONENT I

Tank Influent Line

Center Well

Tank Perimeter Effluent Line

Screen Assemblies

Tank Overflow Line

Tank Drain Sys-tern, Valve Nos.

302, 303, and 304

Local Springs Source Isolation Valve No. 301

Sedimentation Tank

TABLE 2-6

SEDIMENTATION TANK SYSTEM DESCRIPTION

LOCATION I SIZE I TYPE

From North Pump 611 and 811 Steel and ductile House to south side iron pipe of tank

Mounted on dividing 3' dia., 6' Steel tube wall at center of high tank

Mounted around 611 pipe, 1 1/811 Polyethylene pipe inside wall of tank to 1 3/8 11 dia. SDR26 3' off of floor holes drilled

12 11 on centers

Mounted over each 411 dia., 8 slot PVC Vee-Wire design hole in tank well screen 15 11 perimeter effluent long line

North end of tank 611 pipe with 611 Ductile iron pipe flare at top

North end of tank 611 pipe and Ductile iron pipe and in vault north valves and handwheel oper-of tank ated gate valves

South end of tank 311 Lever operated butterfly valve

South of pump 76' diameter, Circular concrete houses 300,000 gall on tank with center storage dividing wall capacity

I PURPOSE I

Carry Transcanyon Trans- I mission Water to center of sedimentation tank

Encourage radial flow from center of tank i

Evenly draw the radial flow from center of tank and carry it to pump suction feed line

Screen out .00811 parti-cles and larger to pro-teet South Rim Transmis-sion Pumps. Provides low inlet velocities.

Carry excess water to Garden Creek

Drain the two compart-ments of the tank

Isolates local spring source from tank

Water storage and solids settling for South Rim Transmission Pump suction feed

2.4.2.3 Tank Perimeter Effluent Line and Screen System

The tank perimeter effluent line is a 6-inch diameter polyethylene pipe supported 3 feet off of the sedimentation tank floor along the inside wall of the tank. Two separate sections of line start at the south end of the tank.

Each section starts with a blind flange tapped with a 2-inch check valve. The check valves allow the line to be drained. The sections follow the tank wall around opposite sides. The two sections join into a common 8-inch ductile iron tee at the north end of the tank. The 8-inch line exits the tank on the north end and feeds the suction line for the South Rim Transmission Pumps.

Water for the transmission pumps is withdrawn from the sedimentation tank by the perimeter pipe through a system of .008-inch opening screens. These screens are mounted over holes drilled into the top of the polyethylene pipe.

Refer to Figure 2-2 for details of a typical screen assembly mounted over an inlet hole.

The holes drilled into the polyethylene pipe are on approximately 12-inch centers. The holes increase in size as their distance from the 8-inch ductile iron tee increases. The increasing sizes of holes encourages a more uniform radial flow into the perimeter effluent line from the center well. This will help sediment in the water settle to the bottom of the tank. Hole size ranges from 1 1/8 inches near the 8-inch tee to 1 3/8 inches near the blind flange. The screens are PVC well screens and are sized to reduce the inlet velocity to between .2 and .3 feet per second. The screens are of a vee-wire design which discourages entrapping particles within the slots. Particles too large to enter are trapped at the surface of the screen and held there usually by the inlet flow. When flow stops, these particles will generally fall off.

2.4.2.4 Tank Overflow Line

A 6-inch overflow line carries excess flow to Garden Creek. The overflow riser pipe is located inside the sedimentation tank in the small stilling well at the north end of the tank with the top of the overflow pipe inlet 13 inches below the tank ceiling. Since the normal Transcanyon Flow is roughly 700 gpm and the South Rim Transmission Pumps pump between 420 gpm and 650 gpm (depending on the pump and discharge pipe configuration used), there will always be overflow at the tank. The overflow pipe enters the drain valve vault just outside the north end of the tank where it connects with the drain lines. The common drain line/overflow line carries water to the east side of the pump houses and ends about 45 feet north of the North Pump House. The overflow 1 ine can carry the maximum flow capacity of the Transcanyon Transmission Pipeline. · · ·

2.4.2.5 Tank Drain System

Three separate drain 1 ines and valves drain the three compartments inside the sedimentation tank formed by 6-foot high dividing walls. The outlet side of the valves connect to a common discharge pipe. Figure 2-3 shows the drain system configuration. These handwheel-operated gate valves are located in a vault just north of the sedimentation tank. Valve No. 302 drains the east side of the tank below the 6-foot high dividing wall and Valve No. 304 drains the west side. Valve No. 303 drains the small stilling well on the north end of the tank.

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CENTERS

II PVC CAP (TYP)

PVC SLIPx THREAD

ADAPTOR (TYP.)

I I

~~~~~ 4 11 x2"HEX BUSHING

(TYP.}

2"BRONZE Nl PPLE

{TYP.)

SCREEN AND DRAIN ASSEMBLY

FIGURE 2-2

N

()J ()J

EAST SIDE

OVERFLOW /DRAIN LINE

TO GARDEN CREEK

SEDIMENTATION

DRAIN VALVE VAULT

WEST SIDE

TANK DRAIN SYSTEM

FIGURE 2-3

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2.4.2.6 Local Spring Source and Isolating Valve

One of the original Indian Garden Springs is still active and connected to the sedimentation tank through a 3-inch influent pipe located on the south side of the tank. Three-inch lever-operated butterfly Valve No. 301 is located on this line. During normal operation, the valve is closed and isolates this water source from the tank. This water source could potentially be utilized in an emergency should Transcanyon Flow be disrupted for an extended period of time.

However, there is no current data on the water quality of this source. The Indian Gardens source produces roughly 60 to 70 gpm.

2.4.3 General Operation

No special procedures are required for normal operation of the sedimentation tank once all valves are properly positioned. Gravity flow from the Transcanyon Transmission Line flows into the tank and the water flows by gravity to the suction side of the Transmission Pumps when the pumps are operating. Excess water is automatically discharged through a gravity overflow system.

An electronic level control system automatically monitors water level in the sedimentation tank. The level sensor is located at the tank, and tank level controls are located in the South Pump House. Digital readouts indicating tank level are located in both the South and North Pump Houses. If water level in the tank drops to one foot above the perimeter effluent pipe, the level controls will prevent the transmission pumps from operating and a "Low Tank Level" alarm will activate at the annunciator panel in the North Pump House. The level controls are covered in greater detail in Section 3.2.3.7.

The water source is generally clear except in the early spring when a large sediment load is carried. The sediment load is composed of sand and silt carried by the Roaring Springs flow into the Transcanyon Line. The period of heavy sediment load varies in duration and intensity and depends on the amount of snowpack at the North Rim and how quickly the snowpack melts off in the spring. While the desanding equipment at the Roaring Springs Pump House is designed to remove a large percentage of the sediment, some sediment carries through the system. Occasionally some larger rocks and debris can enter the pipeline and tank from pipeline breaks. This larger material can find its way into the sedimentation tank.

The sedimentation tank will need to be drained and cleaned as sediment builds up on the floor.

2.4.4 Start-up

The sedimentation tank is usually on-line in normal system operation so start-up and shutdown are related primarily to draining and cleaning operations.

2.4.4.1 Start-up After Cleaning Tanks

Use the following procedure to place the sedimentation tank back on line after it has been removed for cleaning.

1. Close tank drain Valve Nos. 302, 303, and 304.

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2. Start inlet piping system - refer to Section 2.3.4.

3. Start other systems dependent on sedimentation tank.

2.4.5 Shutdown

The sedimentation tank will need to be shutdown when cleaning the tank.

Use the following procedure. ·

1. Shutdown all transmission pump systems that are dependent on water in the sedimentation tank.

2. Shut down inlet piping system - refer to Section 2.3.5.

3. Open tank drain Valve Nos. 302, 303 and 304.

2.4.5.1 Cleaning Tank

The sediment buildup on the tank floor should be removed before accumulations begin to affect the screening systems. Sand accumulations may require removal by hand. It is possible to flush sediments out using the drain system but care should be taken to keep enough water flowing so that drain valves and lines do not become plugged. The frequency with which the tank may need to be cleaned depends on the amount of sediment load. This can vary from year to year. At a minimum, the operator should inspect the tank annually at the end of the heavy sediment load season. Actual operating experience will determine frequency of cleaning.

Because the tank has separate sections controlled with different drain valves, it is possible to keep some water in one side and drain the rest of the tank. A small portable pump could then be used, pumping the confined water to help flush the drained side.

2.4.6 Troubleshooting

Troubleshooting guide TG-3 outlines some symptoms and possible solutions to problems that many occur in the sedimentation tank system.

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TROUBLESHOOTING GUIDE

PROBLEM

1. Water level in Sedimentation Tank Low (Little or no Overflow)

2. Water Level in Tank too High

3. Sediment in Transmission Pump Suet ion Line

I POSSIBLE CAUSES I

Tank leaking water

Reduced water flow into tank

Overflow discharge pipe blocked

Sediment accumulating at screen assemblies

TG-3 SEDIMENTATION TANK SYSTEM

CHECK OR MONITOR I SOLUTIONS I

Check that drain valves Close drain valve if are closed and check open for sound of water

Open and closed valve to running through valve flush any sediment out of valve seat

Repair or replace valve

Check ground around tank Inspect concrete for for signs of leaks signs of deterioration.

Drain tank if necessary.

Schedule repairs.

See troubleshooting See troubleshooting guide TG-1 guide TG-1

Overflow inlet and riser Clear blockage pipe inside tank

Overflow/drain outlet Clear blockage

Sedimentation tank for Drain tank and clean out sediment level sediment