B08__A03_Statement_of_Work_Lateral_73.pdf

Lateral 73-36 Check One Replacement

Statement of Work

I. Purpose

The Bureau of Indian Affairs (BIA) requires the services of a construction contractor to replace Check One of Sublateral 73-36. The structure is an important component of the

Colorado River Irrigation Project (CRIP). The existing concrete structure failed during the month of July 2023. At that time voids formed beneath and around the east half of the structure. The formation of these voids caused the east half of the structure to crack and drop in elevation. After the structural failure, water levels upstream of the structure could no longer be maintained as required. The existing structure must be demolished and removed, and a new structure serving the same purpose must be constructed a short distance downstream from its current location. The work will involve construction of a temporary check structure, which will be installed at the head of a temporary bypass channel that will be used to divert canal flows around the location of the new check structure to accommodate construction.

Additionally, the installation of a new sub-sublateral turnout headgate structure is required to replace the existing turnout structure located adjacent to the existing Check 1 structure.

In order to accomplish the project objectives, the BIA seeks to hire a reputable construction contractor familiar with sheet pile installation, irrigation gate installation, as well as concrete structure and earthwork construction.

II. Background

The Sublateral 73-36 Check 1 structure is located approximately 5.5 miles south of Poston, Arizona, at the intersection of Mark Road and 10th Ave. The structure also lies within the

Colorado River Indian Reservation in western Arizona. The structure has approximate UTM

Zone 11 coordinates of 741309.1 m E and 3755415.8 m N.

The existing check structure is composed of a low head concrete check that spans the width of Sublateral 73-36. A long-crested weir is also an important component of the structure and allows Sublateral water levels to be raised to the elevation necessary to turn out the required flows through the adjacent Sub-sublateral 73-36-7 turnout. The existing check structure is also equipped with two large sluice gates.

On or around July 15, 2023, the erosion of fill around Check 1 resulted in the formation of voids beneath and adjacent to the structure. At this time canal flows passed through the voids around and beneath the structure instead of over the long-crested weir. This caused water levels upstream of the structure to drop and water deliveries into Sub-sublateral 73-367 could no longer be made.

As a temporary measure, CRIP maintenance staff made rudimentary repairs to the damaged check structure and the structure was placed back in service. The structure has reportedly functioned adequately in this condition since the temporary repairs were made. However, there is a chance that the repairs could fail. Consequently, a replacement check structure is needed as soon as possible.

The CRIP system operates on a near year-round basis. Maintenance on the system is performed during brief dry-up periods during which flows through the system are halted. A system-wide two-week dry-up period typically occurs annually in January. An additional one week dry-up of

Lateral 73 typically occurs annually in February. Sublateral 73-36 and Sub-sublateral 73-36-7 must remain operational outside of the dry-up periods.

The flow capacity of Sublateral 73-36 upstream of Check 1 is 160 cfs. The flow capacity of

Sub-sublateral 73-36-7 is 60 cfs.

III. Scope of Work

The Lateral 73-36 Check 1 Replacement project shall be carried out in accordance with the contract provisions, technical specifications, and design drawings. The project includes the following principal components:

1) Construct a temporary bypass channel composed of earthen fill to dewater the area where the replacement Check 1 structure will be constructed.

2) Install sheetpile flow barriers at the entrance and exit of the temporary bypass channel to isolate the temporary bypass work area from flows within Sublateral 73-36.

3) Construct a temporary check structure composed of sheetpiles at the head of the bypass channel to maintain an upstream water surface elevation needed to turn out flows required by CRIP operating personnel into Sub-sublateral 73-36-7 while the replacement Check 1 structure is being constructed.

4) Construct or install cofferdams upstream and downstream of the location where the replacement Check 1 structure will be constructed to direct flows through the temporary bypass channel.

5) Construct a replacement Check 1 structure, including a long-crested weir, downstream of the existing Check 1 structure in accordance with project plans and specifications.

6) Temporarily widen an existing access road and place fill required to raise the canal banks adjacent to the location of the replacement Check 1 structure as shown in the project plans.

7) Carry out earthwork required to place the project features (including the temporary project features) depicted in the plans and restore the temporary project features to the conditions shown in the plans after the replacement Check 1 structure is placed in service.

8) Remove the temporary bypass channel, temporary check structure, and any cofferdams after the replacement Check 1 structure is constructed.

9) Restore the temporarily widened access road to the conditions indicated in the project plans as the temporary bypass channel is removed, and place coarse aggregate on new roadways and access roads.

10) Demolish the existing Sublateral 73-36 Check 1 structure.

11) Demolish the existing Sub-sublateral 73-36-7 turnout structure.

12) Install a new precast headgate structure and outlet pipe at the head of the Sub-sublateral 73-36-7 turnout in accordance with project plans and specifications.

The following are important factors to consider as the aforementioned principal project components are carried out:

1) Irrigation water deliveries are made through Sublateral 73-36 nearly continuously throughout the year. Flows through the CRIP system are typically only halted during a two-week dry-up period, which occurs annually in January. Flows through Lateral

73-36 are halted during the aforementioned system dry-up period and during an additional one week dry-up that typically occurs in February. It is likely that certain construction activities, such as demolition of the existing Check 1 structure and installation of the Sub-sublateral 73-36-7 turnout facilities, can only be carried out during the dry-up periods.

2) It is likely that cofferdam construction or placement within Sublateral 73-36 will need to be carried out while flows are run through the Sublateral.

3) The majority of work associated with construction of the replacement Check 1 structure will occur while flows are run through Sublateral 73-36. Consequently, active dewatering may be required as the replacement structure is constructed.