United States Department of Agriculture 320-CPS-1

NRCS, NHCP

October 2020

NRCS reviews and periodically updates conservation practice standards. To obtain the current 

version of this standard, contact your Natural Resources Conservation Service State office or 

visit the Field Office Technical Guide online by going to the NRCS website at 

https://www.nrcs.usda.gov/ and type FOTG in the search field. 

USDA is an equal opportunity provider, employer, and lender.

Natural Resources Conservation Service 

CONSERVATION PRACTICE STANDARD 

IRRIGATION CANAL OR LATERAL 

CODE 320 

(ft)

DEFINITION 

A permanent channel constructed to convey irrigation water from the source of supply to one or more

irrigated areas. 

PURPOSE 

This practice is used to accomplish the following purpose: 

• Facilitate the efficient distribution and use of water on irrigated land 

CONDITIONS WHERE PRACTICE APPLIES 

• Where a canal or lateral and related structures are needed as an integral part of an irrigation water

conveyance system. 

• Where water supplies for the area served are sufficient to make irrigation application practical for

the crops to be grown. 

Use NRCS Conservation Practice Standard (CPS) Irrigation Field Ditch (Code 388) for onfarm irrigation

water conveyance or distribution of less than 25 cubic feet per second. 

CRITERIA 

General Criteria Applicable to All Purposes 

Capacity requirements 

Design canals and laterals to safely convey the required flows based on the maximum probable

retardance conditions. For capacity design, select the value of Manning’s roughness coefficient “n”

according to the— 

• Material in which the canal or lateral is constructed, 

• Alignment, 

• Hydraulic radius, 

• Expected vegetative growth, and 

• Planned operation and maintenance. 

The capacity of canals or laterals will be— 

• Capable of conveying surface runoff that is allowed to enter the channel. 

• Sufficient to meet delivery demands for all the irrigation systems served and the amount of water

needed to cover the estimated conveyance losses in the canal or lateral, or 

• Sized to convey the available water supply in water-short areas where water is not normally
available to meet the irrigation demands. 

Velocities 

Design the velocity in canals and laterals that are nonerosive for the materials through which the channel

passes. For unlined canals, use local information, if available, on the velocity limits for specific soils. If

such information is not available, the maximum design velocities will not exceed those shown in the NRCS

National Engineering Handbook (NEH) (Title 210), Part 654, Chapter 8, “Threshold Channel Design,”

Figure 8–4, or other equivalent method. For unlined canals and laterals constructed with earthen

materials, use a value of Manning’s roughness coefficient “n” no greater than 0.025 to check that

velocities do not exceed permissible values. 

Freeboard 

Include freeboard above the maximum design water that is at least one-third of the design flow depth and

not less than 0.5 feet. 

Water surface elevations 

Design water surface elevations to provide enough hydraulic head for successful operation of all ditches

or other water conveyance structures diverting from the canal or lateral. 

Side slopes 

Design canals and laterals to have stable side slopes, based on specific soils or geologic materials used.

Do not design side slopes for the banks of canals or laterals steeper than those shown in the 210-NEH,

Part 650, Chapter 14, “Water Management (Drainage),” Section 650.1412(d)(3). 

Top width 

Design the top width of the canal or lateral banks to ensure stability, prevent excessive seepage, and

facilitate maintenance. The minimum acceptable bank top width is 2 feet and will equal or exceed the flow

depth. 

Protection from surface waters 

Convey runoff from adjacent areas over or under the canal wherever practical. If runoff is permitted to

enter the canal or lateral, protect the side slopes from erosion, and make provisions for its disposal.

Where sediment-laden water is allowed to enter the canal or lateral, include in the design provisions to

transport the sediment through the canal or lateral, or install measures to trap and remove the sediment. 

Related structures 

Designs canals or laterals with adequate turnouts, checks, crossings, and other related structures needed

for successful operation of the facility. Design all structures in accordance with the applicable NRCS

conservation practice standard. Install structures needed for the prevention or control of erosion before

the canal or lateral is put into operation. 

Linings 

On sites with soils of moderately rapid to very rapid permeability, or where erosive water velocities will

occur, line the canals and laterals or use pipe according to the appropriate NRCS conservation practice

standards for ditch and canal linings or pipelines. 

Maintenance access 

Provide provisions, as required, for maintenance operations. If the top of the bank or berm is used for a

roadway, design the width wide enough to allow safe equipment travel and operation. 

Spoil disposal 

Spread any spoil as soon as feasible using NRCS CPS Spoil Spreading (Code 572). If the spoil is placed

along the canal or lateral, ensure the spoil banks have stable side slopes. Locate the spoil banks so the

spoil material does not erode back into the canal or lateral. Where necessary, make provisions to channel

runoff water through the spoil bank and into the canal or lateral without causing excessive erosion. 

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320

October 2020
CONSIDERATIONS 

When planning this practice, consider the following, as applicable: 

• Features need to incorporate safety elements. 

• The movement of sediment and soluble and sediment-attached substances carried by runoff to

surface waters, and the movement of dissolved substances to ground water. 

• Using buffers or filters to remove sediment from runoff water. 

• When revegetating disturbed ground, consider including plant species that support pollinators and

other beneficial insects. Also, note that in general, native plant species support fewer crop pests

compared to nonnative or invasive plants. Larger vegetation could be restricted to one side of a

canal to allow access for canal management. 

PLANS AND SPECIFICATIONS 

Prepare plans and specifications that describe the requirements for applying the practice according to the

requirements of this standard. As a minimum, in the plans and specifications include— 

• A plan view or map showing the location and extent of the planned canal or lateral. 

• Cross section details. 

• Profiles or channel grades. 

• Embankment/bank details. 

• Spoil placement details. 

• Structural details for apputenant structures. 

• Specifications that describe in writing detailed requirements for installation. 

• If applicable, revegetation details, including recommended species of vegetative cover,

establishment method, and maintenance. If applicable, incorporate NRCS CPS Critical Area

Planting (Code 342) specifications. 

 

OPERATION AND MAINTENANCE 

Provide a site-specific operation and maintenance plan to landowners, and review it with them before the

practice is installed. As a minimum, include in the plan— 

• Performing periodic and post-storm inspections to detect and minimize damage to the canal or

lateral. 

• Performing prompt repair or replacement of damaged components. 

• Removing debris and foreign material that hinder system operation. 

• Maintaining recommended vegetative cover on all slopes and watercourses. When possible,

mowing or other disturbance of vegetation should be scheduled outside of the primary nesting

season for grass-nesting species. 

REFERENCES 

USDA NRCS. 2001. National Engineering Handbook (Title 210), Part 650, Engineering Field Handbook,

Chapter 14, Water Management (Drainage). Washington, D.C. https://directives.sc.egov.usda.gov/

USDA NRCS. 2007. National Engineering Handbook (Title 210), Part 654, Chapter 8, Threshold Channel

Design. Washington, D.C. https://directives.sc.egov.usda.gov/

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October 2020