Crow Irrigation Project – Construction Activity Descriptions
Construction Activities - Structures
Drops and Checks: Drops are concrete structures of various types that span a canal in order to drop the water level in the canal from a higher elevation to a lower elevation while dissipating the energy of the water in a way to prevent erosion. Typically drops are designed with pools or basins where the water dissipates its energy into itself as opposed to soil or concrete directly. Checks are similar concrete structures that span a canal but have the intention to regulate the canal water surface to a specific elevation in order to provide sufficient water “head” to meet the requirements of the upstream farmland and downstream flow. Drops and checks are often combined into one structure as one can easily be designed and built to serve both functions.
- Removing: Removing a concrete drop or check structure involves the use of an excavator to remove soil, gravel, riprap or any other debris from around the structure, hydraulic jack hammering using an excavator to break the concrete into pieces, and removing rubble including any reinforcing steel or stoplog debris from the site. The soil immediately surrounding the structure will be disturbed but no significant disturbance will be caused outside of the canal banks or maintenance road unless unusual circumstances exist. The removal of a check structure without replacement will decrease the water elevation in the canal and thus reduce the potential seepage through the canal banks and limit dependent vegetations water supply. Sediment transport will increase as the barrier provided by the structure will no longer exist. Structures will not be removed without some form of replacement or updated structure that serves the same or similar purpose.
- Replacing: The replacement of the check structure will involve excavation for the placement of concrete forms and casting in place of the structure or the placement of a precast structure. The ground around the canal banks will be disturbed by heavy equipment and the updating of maintenance roads to allow access for vehicles. Rehabilitation of a check structure will not involve significant ground disturbance in the case that the structure does not require replacement.
- Rehabilitating: Updating of canal bank and prism reinforcement such as placement of membrane liner, riprap or concrete aprons or wing walls are typical activities that will take place during rehabilitation. Some minor excavation of soil in the canal prism will take place but no significant earth or vegetation disturbance would be expected beyond the extent of the canal banks or maintenance road.
Cross: Crosses are used to convey canal water under roads, railroads, existing streams and natural drainages and other canals. Pipe conduit is generally used for these purposes. In accomplishing the crossing, pipe may have a straight line profile, or a profile with vertical bends. Inverted siphons, pipe chutes or piped drops fall under this category of canal structure but will be described separately.
- Replacing, Rehabilitating: Replacing a cross will involve disturbing an area under the road, stream or natural drainage where the canal path travels, in order to excavate the existing structure and prepare the subgrade for new construction.
Flumes: Flumes are used to convey canal water along steep sidehill terrain or to convey canal water over other waterways, or natural drainage channels. Flumes are also used at locations where there is a restricted right-of-way or where lack of suitable material makes construction of canal banks undesirable or impractical. Flumes supported on a bench excavated into a hillside are called bench flumes, and flumes supported above the ground with reinforced concrete, structural steel or timbers are called elevated flumes.
- Removing, Replacing: Removal and replacing of a bench flume requires the excavation of soil along the structure and demolition of the structure itself. Replacing the flume will involve the excavation and preparation of the surface to be constructed upon, building of the flume itself, typically concrete for a bench flume, and backfill and protection of the structure. Removal and replacement of an elevated flume will involve the use of an excavator to demolish the structure and remove the rubble and debris from the site. Construction of an elevated flume will require earth disturbance beneath the desired path to develop adequate foundation for supports, and to key in the structure where the crossing terminates. Bank stabilization or grading above a bench flume will be required in most applications to minimize slope instability. Grading or stabilizing may disturb an area above the flume.
- Rehabilitating: Rehabilitation activities will depend on the type of flume and materials used as well as the general situation and structures role within the irrigation system. Rehabilitation of a concrete flume will involve removal of compromised or deteriorated sections, replacement of those sections, and grading and stabilization of the soil around the site. Elevated flumes are used only used in small lateral crossing applications on the Crow Irrigation Project.
- Replacing or rehabilitating: Headworks are generally constructed in conjunction with diversion dams in a river or major stream. Removing headworks without replacement is not expected on the CIP. Working in the river bed or stream bed typically necessitates diversion of the flowing water to create dry access to the concrete structure itself typically submerged. Situations requiring the replacement of headworks will be where there is significant erosion around or under the concrete structure and rehabilitation is not a viable option. Construction activities associated with removing headworks includes; construction of coffer dams or other water diversion practices, dewatering the dammed area necessary for construction, excavation of banks and soil around the structure, removal of sediment and debris obscuring the structure, demolition and removal of concrete, excavation of soil to reach acceptable foundation material, compaction of soils and subgrade for the concrete structure. Building cast in place forms, building steel reinforcing, casting the concrete, finishing the structure including the attachment of gates, catwalks, placement of geotextiles and membrane liners, and placement of bank reinforcements such as riprap or concrete wing walls and aprons. Following the activities on the structure itself, repairing the river/stream flow through the structure is accomplished by removing the diversion mechanism in place.
Spillway: Spillways are provided for storage and detention dams to release surplus water or floodwater that cannot be contained in the allotted storage space, and for diversion dams to bypass flows exceeding those turned into the diversion system.
- Replacement, rehabilitation: Construction activities associated with replacing or rehabilitating a spillway will be much the same as those for headworks and diversion dams. On the CIP, typically work will be done on a spillway in conjunction with diversion dams.
Inverted Siphon: Inverted siphons are used to convey canal water under roads, railroads, other structures, various types of drainage channels, and depressions. A siphon is a closed conduit designed to operate full and under pressure.
- New Construction, replacement, rehabilitation: Constructing activities associated with an inverted siphon will involve excavation of a trench wide enough to accommodate construction of the siphon and deep enough to allow the existing feature to cross the constructed conduit at its normal elevation. Often the excavation is significant. Heavy equipment will be operated in the area surrounding the excavation. In the case where a siphon will cross a natural drainage, the existing vegetation in the drainage will be removed along with the native soil. The placement of concrete conduit and backfilling the excavation will be done so as to allow the natural elevation of the existing feature to be restored. Grading and stabilizing will be done to preserve the structure and prevent erosion following construction.
Wasteway: A wasteway is comprised of an overflow or gate structure, in combination with a drop or chute structure and wasteway channel. The overflow and gate structure are often combined to provide for various flow conditions of canal operation.
- Replacement: Replacement of a wasteway will involve essentially the same construction activities as those for drops and checks. Replacement of the structure may be necessary in the event where a structure is significantly deteriorated and rehabilitation is not acceptable.
- Rehabilitation: Rehabilitation of a wasteway will generally focus on remediation and prevention of erosion around the structure and will involve the placement of riprap in the waste stream channel, and potentially flowable fill concrete to reinforce the riprap. Updates to the structure such as the addition of new concrete surfaces, concrete aprons or wing walls would be expected.
Chutes: Chutes are used to convey water from a higher elevation to a lower elevation. A chute structure may consist of an inlet, a chute section, an energy dissipater, and an outlet transition. Chutes are similar to drops except that they carry water over longer distances, over flatter slopes, and through greater changes in grade. Construction activities for chutes will be essentially the same as those for flumes or drops.
Turnouts: Turnouts are used to divert water from a supply channel to a smaller channel or lateral. The structure will usually consist of an inlet, a conduit or means of conveying water through the bank of the supply channel and, where required, an outlet transition. Concrete turnouts will typically be precast offsite to minimize the excavation of the site required for cast in place construction activities. Scenarios where turnouts must be cast in place, the excavation and preparation activities will be essentially the same as for crosses, flumes or chutes.
Automated Structures (Water Measurement, Motorized Gates): The installation of automated gate controls at key diversion points was also identified as a key means of allowing for more efficient water management throughout the CIP, including automated flow measurement, automated reservoir storage monitoring, motorized gate operators, and automated gate controls. Flow and storage would be monitored at a computerized control center at the CIP Headquarters and diversions into key canals or laterals would be adjusted accordingly through automated gate controls.
New turnout and headgate structures installed across the CIP under the Master Plan will be designed to be easily retrofitted in the future for gate motorization and automation if the full funding of water measurement devices is not feasible or decided upon at that time the structures are designed and built. A water measurement plan as well as an operation and maintenance guide for water measurement is proposed for provision in design reports at a later date.
Fish Protection/Passage Structures: The need for fish protection at the intersection of certain streams and canals as well as reservoir outlets was identified as another deficiency with current CIP operations. Only one fish passage structure, a fish ladder at the diversion for the Soap Creek Unit, has been constructed for the CIP by Trout Unlimited, but is no longer in place or functioning. Currently, minimum instream flows are not mandated on the CIP. Future enhancements to irrigation infrastructure to improve fisheries may be considered for the CIP. Protection of the Bighorn Fishery would be the highest priority.
Other Construction Activities during Structure Rehabilitation
Diversion during Construction: Construction of a diversion dam and or headworks typically necessitates the construction of river/stream diversion to facilitate a dry construction area. The optimum scheme is to select an alternative that maximizes practicability while minimizing cost, negative impacts and risk involved. Common practice for diverting streams during construction involves conduits through or under the dam, or temporary channels, flumes or pipelines bypassing the flow around the dam. In any case, barriers are constructed across the stream channel so that the site or portions thereof may be unwatered and construction can proceed without interruption.
Removal of Water from Excavations: Dewatering is the removal and control of groundwater from pores and other open spaces in soil or rock formations to allow construction activities to proceed as intended, and includes the relief of groundwater pressure. Dewatering will be accomplished by using a sufficient number of properly screened wells or equivalent methods. The pumping will be done to prevent the loss of fines from the foundation, maintain stability of excavated slopes and bottom excavations. Pumped water will be disposed of as to not damage adjacent property, cause settling or any other form of unnatural damage. Unwatering is the control and removal of ponded, seeping, or flowing surface water or emerging subsurface water from excavated surfaces and from precipitation within and adjacent to excavations and construction zones using channels, ditches, gravel blankets, pipe, sumps, and discharge lines. Channels, ditches and sumps will be constructed to collect seepage and runoff in work areas and to minimize impact to the site.
Clearing, Stripping, and Grubbing: Preparing sites around canals and structures for construction involves removing of trees, snags, stumps, shrubs, brush, limbs, sticks, branches, and other vegetative growth in and around the canal right-of-way. Rocks, lumps of concrete, fencing, and all trash or rubbish will also be removed. After the clearing of vegetation, organic sod, topsoil to a depth of 6 inches, and grass and grass roots will be removed from the right-of-way. The topsoil will be stockpiled on site to be used for dressing backfill after construction and before seeding of disturbed areas. Wood or root matter remaining after clearing and stripping will be removed to a depth of 12 inches below the ground surface. Clearing, stripping and grubbing will be done in construction areas only to the extent of the outside toe of canal bank. Disposal of clearing and grubbing may include burning if permitted, and disposal of stripped sod and topsoil may be stockpiled or disposed of at contractor secured site.
Earthwork: Earthwork to take place includes, excavation, preparation of foundation subgrade, preparation for placing fill or backfill, placing and compacting fill and structural fill, moisture control, site grading, placing drain rock, and installing vapor barrier. Excavations will have sloping, sheeting, shoring and bracing to conform to safety standards. Where unacceptable subgrade exists, overexcavation shall include the removal of all unacceptable material beneath the structure and replacement with structural backfill. Excavations will be to the depths and widths necessary to accomplish the construction, including concrete forms, working space, structural backfill, and site grading. Placing and compacting of backfill will not be done when the fill material or excavation surface is frozen. Where dry soil is encountered during compaction of backfill etc, water may be added to achieve optimal water content. Site grading will be done to shape, trim, and finish slopes of channels to conform to design grades and minimize erosion. Where applicable, vapor barrier sheets will be used between concrete structures and compacted subgrade.
Riprap Placement: Riprap will be used to protect soils around structures and canal banks from erosion. Stone for riprap will be quarry stone, well graded and angular. Material shall be clean and free from deleterious impurities including alkali, earth, clay, refuse, and adherent coatings. Concrete rubble may be used in places where the size of pieces meets the minimum requirements, as well concrete grout (flowable fill) will be placed over the rubble to secure it place and increase longevity. Stone bedding or geotextiles fabric will be used beneath riprap to minimize potential for erosion of fines beneath the stones.
Seeding and Soil Supplements: Seeding, fertilizing and mulching or a combination of sorts will take place where earth has been disturbed outside of the canal prism. Seeding will mostly take place on the outside of steep canal banks. Certain forms of vegetation are essential in the stabilization of soils on steep surfaces and prevention of erosion due to storm water runoff. Seedbed preparation includes the removal of any noxious weed or infested topsoil, scarification or harrowing of topsoil, removal of stiff clods, lumps, roots, liter, stones, and other foreign material greater than 6 inches from the surface, and filling of rills, gullies and depressions. Fertilizing includes the application of nitrogen and phosphate fertilizer. Seeding will typically be done by the broadcast, drill, or hydroseed method.
Canals: Canals are open earth channels used to convey irrigation water from the river or stream source to the farmland requiring water. Canals are typically shaped to meet specific flow conditions and are lined with clay soil, membrane, or concrete to reduce water loss through the canal banks to seepage and to minimize flow irregularities.
- Cleaning/reshaping: Cleaning and reshaping activities entail cleaning and reshaping the canal prism to its original design dimensions. This serves to remove flow restrictions within the canal and allow for original flow design capacities. Canal cleaning and reshaping involves, the use of an excavator to enter the canal or reach from the canal banks or maintenance road to remove excess soil, vegetation, sediment, silt/sand bars or other debris such as beaver dams from the canal banks and prism to restore the canal to original flow design capacities. The soil in the immediate area of the access road or banks will be disturbed by the passing of the excavator or other earthmoving machinery. The material that is removed from the canal and banks is typically either stockpiled and hauled to a remote location or spread along the outer edge of the banks.
- Lining: Lining a canal involves the placement of either clay, concrete or a membrane liner. Placing clay liner in the canal will involve the operation of heavy equipment in the canal prism and on the banks and maintenance road. Clay lining material will typically be used from a borrow source as near to the area as possible to minimize transport. The clay soil will be placed in the canal and shaped with an excavator and compactor. Lining a canal with a membrane will generally involve a similar process of shaping and compacting the canal prism with an excavator and laying the membrane over the entire canal. The membrane will be anchored in a trench at the top of the canal banks. Soil will be disturbed on either side of the canal in this event. The lining of canals with either a membrane or clay soil will inherently reduce seepage through the canal banks and reduce water supply to vegetation relying on this water supply. The cleaning and grubbing of vegetation from the canal banks reduces the chance of seepage pathways via roots systems through the banks.
Maintenance and Access Roads: Canal O&M roads will also be factored into the rehabilitation and betterment process for canal reaches. In general, O &M road upgrades will be completed at the time of the adjacent canal rehabilitation and betterment, except for cases when O&M road improvements to provide access to a specific CIP structure is warranted for the completion of a structure rehabilitation and betterment project.
- New Construction: New construction of a maintenance road involves clearing and grubbing of vegetation in the road alignment, stripping topsoil and clays from the subgrade, grading and compacting subgrade and surface material, placing drainage and erosion controls, and seeding/mulching/fertilizing the banks and ditches. The area impacted by maintenance road construction is typically directly adjacent to or on the irrigation canal banks. Access roads are an exception and cross a variety of terrain but are typically the same size as maintenance roads. Road alignment width varies by location but typically extends 30-40 feet from the top of canal bank. Native material is used for subgrade and surface of the road if possible but gravel is often hauled in from a remote location to provide adequate road durability.
- Replacement, Rehabilitation: Replacing and rehabilitation construction activities are often very similar but depend on the nature of disrepair for each situation. Clearing and grubbing encroaching vegetation, grading and compacting new or existing surface material (such as gravel), placement of drainage and erosion controls, and seeding/mulching/fertilizing are common construction activities associated with road rehabilitation.
Livestock Damage Repair: Measures for reducing/mitigating damage due to livestock include the installation of fences to prevent livestock access to the canal/lateral. If warranted for certain locations, fences may be utilized in conjunction with the construction of earthen canal access/ramp points (which may be constructed during cleaning and reshaping activities).
Seepage Area Remediation: Remediation efforts for identified seepage areas include canal cleaning and installation of an exposed geomembrane lining system. Canal cleaning is required for removal of organic material from the canal banks prior to liner installation and to provide a suitable foundation for installation of geomembrane material. Another lining alternative is the utilization of an earthen liner. Earthen liners are generally constructed by over-excavating native material within the canal prism (generally several feet), followed by replacing the over-excavated reach with the earthen lining material and placing and compacting in layers to match the design canal prism. Other lining options depending on the proposed application include PVC lining, polyester lining, polypropylene lining, concrete lining, bentonite lining, piping, etc. Construction steps would include canal cleaning, shaping, and compaction; liner and geotex installation or other type of lining system; incorporation of drain inlets, underdrains, or other CIP infrastructure as needed; and keying-in and backfilling of liner if warranted.
Piping of Select Canal Reaches: The piping of canals can be a good alternative in some scenarios. In general, most piping projects require the following construction steps: Removal of trash and vegetation, digging of the ditch; Bedding material, laying of pipe; Installation of check valves, pressure release valves, and other appurtenances associated with either a closed or open system depending on the chosen alternative; Compaction, backfill, and final cleanup and seeding if warranted.
Alternative On-Farm Improvements
Land Leveling: The process of leveling land involves re-grading and re-sloping of the surface material to allow for uniform distribution of water across the field surface. Construction activities are typically the operation of earth moving equipment such as scraper, dozer, or leveling plows. The land type and amount of earthmoving varies with location and if the land has been previously used for crops. Appropriate ditches, structures and drains will be constructed according to the specific layout of the annual crop rotation and irrigation system. In the case where pasture or other native land is leveled and brought into use for crops, a notable change in the surface hydrology will be expected.
Gated Pipe: Gated pipe will be selected for fields with sufficient head to deliver water to the field. Gated pipe falls under the category of surface irrigation, where the water is applied to a portion of the field and allowed to gravity feed across the field. Surface irrigation is most applicable for relatively level fields with consistent slopes. Gated pipe is more specifically a form of furrow irrigation which utilizes pipe to convey and apply irrigation water across the head of field where it can travel the length of the furrows. Ground disturbance is minimal outside of existing fields and irrigation systems as the pipe is typically placed in the disturbed portion of cropland. The piping materials are placed and removed annually on basis of which type of crop rotation the land owner utilizes. The type of crop will determine placement of pipe and waste water ditches. Head water and waste water ditches are annually dug and refilled typically by a small backhoe or similar equipment. Earthen material from existing ground will be used to fill ditches. The placing of ditches will be incorporate with the crop rotation of selected varieties of agriculture species selected.
Surge Irrigation: Surge irrigation is a subcategory of surface irrigation which applies water in set cycles over certain intervals of time. Depending on the soil type, surge irrigation can increase irrigation efficiency by influencing the wetting and drying cycle resulting in greater irrigation water advance rates across the field. Conversion to surge irrigation is an option for all fields selected for gated pipe, or with gated pipe systems currently in place. However, soil characteristics will be investigated across all fields to determine surge irrigation applicability prior to its implementation as certain soil types experience much greater efficiencies through the use of surge irrigation than others.
Lining Applications: The lining of on-farm ditches will be selected for fields utilizing surface irrigation but lacking sufficient head for the utilization of gated pipe without pumping to minimize losses through infiltration. An EPDM liner is typically selected for on-farm ditch lining applications; however, additional lining options such clay soil or concrete may be investigated for practicality. Construction associated with EPDM liner is minimal but involves small amounts of earthwork to ensure liner is secure in applied location.
Center Pivots: Center pivots are a form of sprinkler irrigation for which water is supplied through pipes from the center of a field, and then fed to an automated sprinkler system which rotates around the field. Sprinkler irrigation sprays water across a field’s surface and is generally more suited for steeper land with uneven slopes. Construction activities involve excavating, placing, and backfilling over water supply lines across the field to be irrigated.
Wheel Lines: Wheel lines are a form of sprinkler irrigation which supplies water through pipes to a sprinkler system mounted on wheels which moves down the length of the field (or a portion of the field). Wheel lines will be chosen for fields with slopes that are not “very mild” (i.e., not well suited for surface irrigation). Construction associated with implementation of wheel lines can be minimal to none as the water supply piping and machinery can be surface only. No excavation is typically required to implement wheel lines.