S01 Attachment 2 - Conservation Practice Standards.pdf

United States Department of Agriculture -CPS-1582

NRCS, MN

February 2016

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

OPEN CHANNEL

CODE 582

(ft)

DEFINITION

An open channel is a natural or artificial channel in which water flows with a free surface.

PURPOSE

This practice may be applied to support one or more of the following purposes:

Construct, improve, or restore an open channel to convey water required for flood prevention, • drainage, wildlife habitat protection or enhancement, or other authorized water management purpose

CONDITIONS WHERE PRACTICE APPLIES

This standard applies to the construction of open channels or modifications of existing streams or ditches with drainage areas exceeding one (1) mi2 (1.6 km2). This standard does not apply to Natural Resources

Conservation Service (NRCS) Conservation Practice Standards (CPSs) Codes 362, Diversions; 412, Grassed Waterways; 388, Irrigation Field Ditches; 607, Surface Drain, Field Ditch; or 320, Irrigation Canal or Lateral.

CRITERIA

General Criteria Applicable to All Purposes

Use of this standard will comply with all applicable federal, state, and local laws and regulations.

Use NRCS Engineering Technical Releases (TR), 210-25, Design of Open Channels; NRCS National

Engineering Handbook (NEH), Part 653, Stream Corridor Restoration: Principles, Processes, and

Practices; and NRCS NEH, Part 654, Stream Restoration Design, as applicable in surveys, planning, site investigations, and design of channel work.

Do not modify the horizontal or vertical alignment of a channel to the extent of endangering the stability of the channel or its laterals.

Capacity

Determine the capacity for open channels according to procedures applicable to the purposes of the channel and according to related engineering standards and guidelines in approved references and handbooks. Designs must consider low flows, average flows, frequent storm flows, and high (infrequent) storm flows.

Determine the water surface profile or hydraulic grade line for design flow using guidelines for hydraulic design in NRCS TR-210-25 and/or NRCS NEH, Part 654. Select a Manning’s n value for the condition representing an aged channel. Base the selection on the expected vegetation and other factors such as the level of maintenance prescribed in the operation and maintenance plan. Establish the required flow capacity by considering volume-duration removal rates, peak flow, or a combination of the two, as determined by the topography, purpose of the channel, desired level of protection, and economic feasibility. Design conditions cannot result in flood impacts to adjacent properties without addressing through the appropriate authorities.

Cross section

Determine the required channel cross section and grade by the plan objectives, the design capacity, the channel materials, the vegetative establishment program, and the requirements for operation and maintenance. As necessary, provide a minimum depth to allow adequate outlets for subsurface drains, tributary ditches, or streams. In urban areas, consider the design impacts on high- value developments.

Channel stability

A stable channel has the following characteristics:

The channel neither aggrades nor degrades beyond tolerable limits •

The channel banks do not erode to the extent that an appreciable change in channel cross-section • results

Excessive sediment bars do not develop •

Gullies do not form or enlarge because of the entry of uncontrolled surface flow to the channel •

Design all channel construction and modification (including clearing and snagging) to result in a stable channel with reasonable maintenance costs. Use vegetation, riprap, revetments, linings, structures, or other measures if necessary to ensure stability.

Use the methods in NRCS TR-210-25 and/or NRCS NEH, Part 654 to determine the stability of proposed channel improvements.

Bank-full flow is the discharge that fills a channel to an elevation where flow begins to spill onto the active floodplain.

Channels must be stable under conditions existing immediately after construction (as-built condition) and under conditions existing during effective design life (aged condition).

Determine channel stability for discharges under the following conditions:

As-built condition.—Bank-full flow, design discharge, or 10-year frequency flow, whichever is • smallest, but not less than 50 percent of design discharge.

The designer may increase the allowable as-built velocity (regardless of type of stability • analysis) in the newly constructed channel by a maximum of 20 percent if—

The soils at the site in which the channel is to be constructed are suitable for rapid • establishment and support of erosion-controlling vegetation.

Species of erosion-controlling vegetation adapted to the area and proven methods of • establishment are known.

The channel design includes detailed plans for establishing vegetation on the channel side • slopes.

Aged condition.—Bank-full flow or design discharge, whichever is larger, except that it is not • necessary to check stability for discharge greater than the 100-year frequency.

Stability checks that are flow related are not required if the velocity is 2 ft/s (0.6 m/s) or less.

For newly constructed channels in fine-grained soils and sands, determine the Manning’s n values according to procedures in Chapter 6 of NRCS TR-210-25. Use caution selecting values greater than

0.25. In channels modified by clearing and snagging, determine the Manning’s n value according to the expected channel condition following completion of the work. Guidance is also available in NRCS NEH, Part 654.

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NRCS, MN

February 2016

Appurtenant structures

Include all structures required for proper functioning of the channel and its laterals, as well as travel ways for operation and maintenance. Minimize the erosion or degradation from inlets and structures needed for entry of surface and subsurface flow into channels. Provide necessary floodgates, water-level-control devices, bays used in connection with pumping plants and any other appurtenances essential to the functioning of the channels. If needed, use protective structures or treatment at junctions between channels, to ensure stability at these critical locations.

Evaluate the effect of channel work on existing culverts, bridges, buried cables, pipelines, irrigation flumes, inlet structures, surface drainage systems, and subsurface drainage systems.

Assure that culverts and bridges modified or added as part of a channel project meet reasonable standards for the type of structure and have a minimum capacity equal to the design discharge or state agency design requirements, whichever is greater. Increase the capacity of culverts and bridges above the design discharge as necessary to assure the channel and associated floodway meet design capacity.

In natural channels, evaluate the effect of the grade control structure on channel and bank stability.

Determine backwater effects and the effects of modification of sediment transport through the reach.

Disposal of spoil

Dispose of spoil material from clearing, grubbing, and channel excavation in a manner that will—

Not modify flows or cause channel instability when the discharge is greater than the bank-full flow. •

Provide for the free flow of water between the channel and floodplain unless the presences of • continuous dikes establish the basis for the valley routing and water surface profile.

Not hinder the development of travel ways for maintenance. •

Leave the right-of-way in the best condition for the project purposes and adjacent land uses. •

Direct water accumulating on or behind spoil areas to protected outlets. •

Maintain or improve the visual quality of the site to the extent feasible. •

Vegetation of channel

Establish vegetation on all channel slopes, berms, spoil, and other disturbed areas according to CPS

Codes 342, Critical Area Planting; or 580, Streambank and Shoreline Protection.

Cultural resources

Consider existence of cultural resources in the project area and any project impacts on such resources.

Consider conservation and stabilization of archeological, historic, structural, and traditional cultural properties when appropriate.

Additional Criteria for Conversion to Two-Stage Ditch

Where an existing agricultural drainage ditch will be converted to a two-stage ditch (wider ditch with benches), additional criteria apply.

The total bench width of the two-stage ditch will be between 2 and 4 times the existing low channel flow

(bank) width. Split the bench width evenly on each side of the low flow channel unless it is necessary to distribute the bench width unevenly, or provide the entire width on one side only. One-sided construction will only be used if it is necessary to avoid protected or inhibitory areas (such as but not limited to trees, wetlands and/or cultural resources).

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Figure 1. Typical Two-sided Two-stage Ditch

The bench height will be determined by using regional curves or other accepted methods to size the low flow channel to carry between 1 and 2-year, 24 hour storms or by approximating the elevation of natural bench formations.

Outside bank slopes will be 2:1 or flatter. Erosion control blanket will be used where conditions are not suitable for rapid vegetative establishment.

If possible, existing drainage tile outlets will be repaired and outlet onto the newly created bench. Riprap or other erosion protection methods will be installed at outlets to protect the bench. Existing structures or other appurtenances will be reconstructed as necessary to fit the new ditch configuration. Underground outlets and other appurtenances shall meet the requirements of conservation practice standards

Subsurface Drainage (606), Underground Outlet (620), or other practices as appropriate.

All bench and bank areas will be seeded according to conservation practice standard Critical Area

Planting (342) and mulched or blanketed. All disturbed areas outside of top of bank will be seeded to the appropriate NRCS standard, planted to crop within 30 days or temporary seeded if the areas is to be planted to a crop at a later time.

CONSIDERATIONS

General Considerations

Visual resource design

Carefully consider the visual design of channels in areas of high public visibility and those associated with recreation. The underlying criterion for all visual design is appropriateness. The shape and form of channels, excavated material, and plantings are to relate visually to their surroundings and to their function.

Fish and wildlife

This practice may influence important fish and wildlife habitats such as streams, creeks, riparian areas, floodplains, and wetlands. Evaluate aquatic organism passage concerns (e.g., velocity, depth, slope, air entrainment, screening, etc.) to enhance positive impacts and minimize negative impacts.

Select project location and construction methods that minimize the impacts to existing fish and wildlife habitat.

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Include measures necessary to mitigate unavoidable losses to fish or wildlife habitat in the design.

Maintain the quality of the landscape by both the location of channel works and plantings, as appropriate.

Vegetation

Stockpile topsoil for placement on disturbed areas to facilitate revegetation.

Consider placement and selection of vegetation to improve fish and wildlife habitat and species diversity.

Consider limiting construction to late summer to allow establishment of appropriate vegetation.

Water quantity and quality

Consider the effects of—

Erosion and the movement of sediment, pathogens, and soluble and sediment-attached • substances that runoff carries.

Short-term and construction-related effects of this practice on the quality of downstream • watercourses.

Wetlands and water-related wildlife habitats. •

Increasing runoff. Concentrating the flow in an open channel may reduce the opportunity of • infiltration.

Ground water recharge. •

Infiltration of small amounts of soluble pesticides used to control vegetation in and along open • channels, which may affect ground water quality.

Two-stage ditch

Consider rerouting tile drainage outlets entering a two-stage ditch to outlet on top of the bench and onto a riprap pad.

Where phosphorus is of particular concern in a two-stage ditch, refer to the Agricultural Waste

Management Field Handbook, Table 6-6 for plant uptake values of alternative plantings.

PLANS AND SPECIFICATIONS

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

As a minimum, include the following items:

Plan View •

Profile •

Cross section (typical or other) •

Location of spoil •

Seeding rates, dates, and establishment procedure •

OPERATION AND MAINTENANCE

Plan

Prepare an operation and maintenance plan for the operator for each channel system. Minimum requirements for operation, maintenance, and the replacement shall be consistent with the design objectives. This includes consideration of fish and wildlife habitat, quality of the landscape, water quality, mitigation features, methods, equipment, costs, stability, function for design life, frequency, and time of year for accomplishing the work. Detailed provisions for operation and maintenance must be made if complex features, such as water-level-control structures and pumping plants, are required.

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Maintenance Access

Travel ways for maintenance generally shall be provided as part of all channel work. This requirement may be met by providing reading access points to sections of the channel if this will permit adequate maintenance in conformance with the operation and maintenance plan.

A travel way shall be provided on each side of large channels if necessary for use of maintenance equipment. Travel ways must be adequate for movement and operation of equipment required for maintenance of the channel. The travel way may be located adjacent to the channel on a berm or on the spread spoil. In some places the channel itself may be used as the travel way. The travel way, including access points, must blend into the topography, the landscape, and adjacent land uses.

Safety

Open channels can create a safety hazard. Appropriate safety features and devices should be installed to protect people and animals from accidents such as falling or drowning.

A maintenance program will be established by the landowner/user to maintain capacity and vegetative cover. Item to consider are:

Do not graze protected area during vegetative establishment and when soil conditions are wet. •

Fertilize to maintain a vigorous vegetative cover. Caution should be used with fertilization to • maintain water quality.

Mulch, spray or chop out undesirable vegetation periodically to prevent growth of large woody • stemmed weeds, water plants such as cattails or trees (such as willows) which impede flow.

Promptly repair eroded areas. •

Remove silt and sediment accumulations in the channel cross-section as soon as practical to • prevent buildup and growth of undesirable vegetation.

Reestabilsh vegetative cover immediately where scour erosion has removed established seeding. •

Keep inlets to side drainage structures open. •

Keep subsurface drain outlet pipes open and protected. Maintain animal guards in proper • operation.

Periodically inspect area for signs of undermining or instability and, if any are observed, take • immediate action to protect from further damage.

Planning Considerations for Quantity and Quality

Quantity

Effects on components of the water budget, especially on volumes and rates of runoff and • infiltration.

Quality

Effects of erosion and the movement of sediment and soluble and sediment-attached substances in • runoff during and immediately after construction

Effects of the use of chemicals during vegetation control. •

Effects of changes in channel vegetation on downstream water temperature. •

Potential for temporary and long-term effects on the visual quality of downstream waters. •

REFERENCES

USDA Natural Resources Conservation Service. Engineering Technical Releases, TR-210-25, Design of

Open Channels. Washington, DC.

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USDA Natural Resources Conservation Service. National Engineering Handbook (NEH), Part 653, Stream

Corridor Restoration: Principles, Processes, and Practices. Washington, DC.

USDA Natural Resources Conservation Service. NEH, Part 654, Stream Restoration Design. Washington, DC.

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NRCS, MN

United States Department of Agriculture -CPS-1587

March 2017

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

STRUCTURE FOR WATER CONTROL

CODE 587

(no)

A structure in a water management system that conveys water, controls the direction or rate of flow, maintains a desired water surface elevation or measures water.

PURPOSE

This practice is used to accomplish the following purpose:

The practice may be applied as a management component of a water management system to • control the stage, discharge, distribution, delivery or direction of water flow.

CONDITIONS WHERE PRACTICE APPLIES

This practice applies wherever a permanent structure is needed as an integral part of a water-control system to serve one or more of the following functions:

Convey water from one elevation to a lower elevation within to, or from, a water conveyance system • such as a ditch, channel, canal or pipeline designed to operate under open channel conditions.

Typical structures: drops, chutes, turnouts, surface water inlets, head gates, pump boxes and stilling basins.

Control the elevation of water in drainage or irrigation ditches. Typical structures: checks, • flashboard risers and check dams.

Control the division or measurement of irrigation water. Typical structures: division boxes and water • measurement devices.

Keep trash, debris or weed seeds from entering pipelines. Typical structure: debris screen. •

Control the direction of channel flow resulting from tides and high water or back-flow from flooding. •

Typical structures: tide and water management gates.

Control the water table level, remove surface or subsurface water from adjoining land, flood land for • frost protection or manage water levels for wildlife or recreation. Typical structures: water level control structures, flashboard risers, pipe drop inlets and box inlets.

Convey water over, under or along a ditch, canal, road, railroad or other barriers. Typical structures: • bridges, culverts, flumes, inverted siphons and long span pipes.

Modify water flow to provide habitat for fish, wildlife and other aquatic animals. Typical structures: • chutes, cold water release structures and flashboard risers.

Provide silt management in ditches or canals. Typical structure: sluice. •

Supplement a resource management system on land where organic waste or commercial fertilizer • is applied.

Create, restore or enhance wetland hydrology •

CRITERIA

General Criteria Applicable to All Purposes

Vegetation complying with practice standard Critical Area Planting (342) shall be established on all disturbed earth surfaces. Where soil, climate or site specific conditions preclude establishing permanent vegetation, other protective means such as mulches or gravels, shall be used.

The structure shall be fenced, if necessary, to protect the vegetation.

Structures shall not be installed that have an adverse effect on septic filter fields.

The water level upstream of water control structures shall not be raised on adjacent landowners without their permission.

Where manufactured structures are used, the hydraulic design shall be provided by the manufacturer.

Additional Criteria for Drainage WaterManagement

The drainage water management system shall be designed in accordance with practice standard

Drainage Water Management (554).

The rate of outflow and the level of the water table shall be controlled by structures or pumps. Structures or pumps shall be located where they are accessible and subject to convenient control. Designs of critical components shall be in accordance with pertinent NRCS Practice Standards.

Water level control structures shall be sized to provide adequate drainage flow and not to restrict drainage capacity. The water level control structure shall be designed so as to allow the water table to fluctuate to satisfy the intended purpose.

For subsurface systems, connect at least 20 feet of solid pipe to the control structure on both the upstream and downstream sides.

Additional Criteria for Water ImpoundmentStructures

All water impoundment structures built under this practice shall meet the requirements of practice standard Pond (378).

CONSIDERATIONS

When planning, designing, and installing this practice, the following items should be considered:

Effects on the water budget, especially on volumes and rates of runoff, infiltration, evaporation, • transpiration, deep percolation and ground water recharge.

Potential for a change in the rate of plant growth and transpiration because of changes in the • volume of soil water.

Effects on downstream flows or aquifers that would affect other water uses or users. •

Effects on the field water table to ensure that it will provide a suitable rooting depth for the • anticipated crop.

Potential use for irrigation and drainage management to conserve water and/or to minimize • negative impacts to offsite water quality.

Effect of construction on aquatic life. •

Effects on stream system channel morphology and stability as it relates to erosion and the • movement of sediment, solutes and sediment-attached substances carried by runoff.

Effects on the movement of dissolved substances below the root zone and to ground water. •

Effects of field water table on salt content in the root zone. •

Short term and construction-related effects of this practice on the quality of downstream water. •

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Effects of water level control on the temperatures of downstream waters and their effects on aquatic • and wildlife communities.

Effects on wetlands or water-related wildlife habitats. •

Effects on the turbidity of downstream water resources. •

Existence of cultural resources in the project area and any project impacts on such resources. •

Conservation and stabilization of archeological, historic, structural and traditional cultural properties • when appropriate.

Design alternatives presented to the client should address economics, ecological concerns and acceptable level of risk for design criteria as it relates to hazards to life or property.

PLANS AND SPECIFICATIONS

Plans and specifications for installing structures for water control shall be in keeping with this standard and shall describe the requirements for applying the practice to achieve its intended purpose.

The plan shall specify the location, grades, quantities, dimensions, materials, and hydraulic and structural requirements for the individual structure. Provisions must be made for necessary maintenance. Care must be used to protect the surrounding visual resources. If watercourse fisheries are important, special precautions or design features may be needed to facilitate continuation of fish migrations.

OPERATION AND MAINTENANCE

An operation and management plan shall be provided to and reviewed with the land manager. The plan shall be site specific and include but not be limited to the following: Structures will be checked and necessary maintenance, including removal of debris, shall be performed after major storms and at least semi-annually. Water level management and timing shall be adequately described wherever applicable.

REFERENCES

USDA-NRCS, National Engineering Handbook (NEH), Part 650, Engineering Field Handbook (EFH), Chapters 3-Hydraulics, 6-Structures and 14- Water Management (Drainage).

USDA-NRCS, National Engineering Handbook (NEH), Part 624, Chapter 10, Water Table Control.

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United States Department of Agriculture -CPS-1606

March 2017

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

SUBSURFACE DRAIN

CODE 606

(ft)

A conduit installed beneath the ground surface to collect and/or convey excess water

PURPOSE

This practice is used to accomplish one or more of the following purposes:

Remove or distribute excessive soil water •

Remove salts and other contaminants from the soil profile •

CONDITIONS WHERE PRACTICE APPLIES

This standard applies to agricultural land where a shallow water table exists and where a subsurface drainage system can mitigate the following adverse conditions caused by excessive soil moisture:

Poor health, vigor and productivity of plants. •

Poor field trafficability. •

Accumulation of salts in the root zone. •

Health risk and livestock stress due to pests such as flukes, flies, or mosquitoes. •

Wet soil conditions around farmsteads, structures, and roadways. •

This standard also applies where collected excess water can be distributed through a subsurface water utilization or treatment area.

CRITERIA

General Criteria Applicable to All Purposes

Capacity

Design capacity shall be based on the following, as applicable:

1. Application of a locally proven drainage coefficient for the acreage drained.

The drainage coefficient is the rate at which excess water must be removed from cropland in order to provide the proper degree of protection for crops to be grown. Coefficients are expressed in inches

(millimeters) of water to be removed in 24 hours. Minimum coefficients are as follows:

Subsurface Drainage Only (no open inlets) •

This condition assumes that surface drainage is adequate. The selected drainage coefficient applies to the entire area being drained. If the runoff from an upland area spreads over the area to be drained and is likely to increase the drainage problem, the acres used in determining the drain (tile) size shall be proportionately increased.

Subsurface and Surface Drainage (surface inlets in subsurface drains) •

Note: A ½ inch (12.7 mm) coefficient may be used if the organic soil occurs only as a small pocket in the vicinity of the inlet.

The selected drainage coefficient will apply to the entire watershed contributing runoff to the surface inlet, except where only a small amount of runoff will be impounded at the location of the inlet with the remainder flowing away in a confined channel. For the latter case, the drain (tile) shall be large enough to remove the impounded water in 24 hours, plus providing additional capacity for the required internal drainage. Blind inlets should only be used in areas where surface drainage will handle most of the surface water.

Determine the minimum capacity using Figure 14-33 of the Engineering Field Handbook Chapter 14 •

(EFH14)(2001). Conduit size can be determined using Figures 14-34 and 14-35, as appropriate, in

EFH14.

2. Yield of groundwater based on the expected deep percolation of irrigation water from the overlying fields.

3. Comparison of the site with other similar sites where subsurface drain yields have been measured.

4. Measurement of the rate of subsurface flow at the site during a period of adverse weather and groundwater conditions.

5. Application of Darcy’s law to lateral or artesian subsurface flow.

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6. Contributions from surface inlets based on hydrologic analysis or flow measurements

Size

The size of subsurface drains shall be computed by applying Manning’s formula, using roughness coefficients recommended by the manufacturer of the conduit. The size shall be based on the maximum design flow rate and computed using one of the following assumptions:

The hydraulic grade line parallel to the bottom grade of the subsurface drain with the conduit 1.

flowing full at design flow (normal condition, no internal pressure).

Conduit flowing partly full where a steep grade or other conditions require excess capacity. 2.

Conduit flowing under internal pressure with hydraulic grade line set by site conditions, which differs 3.

from the bottom grade of the subsurface drain. This procedure shall be used only if surface water inlets or nearness of the conduit to outlets with fixed water elevations permit satisfactory estimates of hydraulic pressure and flows under design conditions.

For assumptions 1 or 2 above, the minimum size of subsurface drains may be determined using the drainage charts in EFH14. Use Figure 14-34 on page 14-66 for corrugated plastic tubing, or Figure 14-35 on page 14-67 for clay and concrete tiles.

All subsurface drains shall have a nominal diameter that equals or exceeds 3 inches. When 3-inch (76

mm) diameter drains are used, any single line shall not exceed 1000 ft (305 m) in length.

Four-inch (102 mm) diameter drains shall not exceed 1320 ft (402 m) in length. In peat and muck soil, when using clay or concrete tile, consider using the longest individual section length possible

Internal Hydraulic Pressure

Drains are normally designed to flow with no internal pressure, and the flow is normally classified as open channel. The design internal pressure of drains shall not exceed the limits recommended by the manufacturer of the conduit.

Horizontal Alignment

A change in horizontal direction of the subsurface drain shall be made by one of the following methods:

The use of manufactured fittings. 1.

The use of junction boxes or manholes. 2.

A gradual curve of the drain trench on a radius that can be followed by the trenching machine while 3.

maintaining grade.

Location, Depth, and Spacing

Base the location, depth, and spacing of the subsurface drain on site conditions including soils, topography, ground water conditions, crops, land use, outlets, saline or sodic conditions, and proximity to wetlands.

The minimum depth of cover over subsurface drains may exclude sections of conduit near the outlet or through minor depressions, providing these sections of conduit are not subject to damage by frost action or equipment travel.

In mineral soils, provide a minimum depth of cover over subsurface drains of 2.0 ft.

In organic soils, provide a minimum depth of cover after initial subsidence of 3 feet. If water control structures are installed and managed to limit oxidation and subsidence of the soil, the minimum depth of cover may be reduced to 2.5 ft.

For flexible conduits, maximum burial depths shall be based on manufacturer’s recommendations for the site conditions, or based on a site-specific engineering design consistent with methods in NRCS National

Engineering Handbook (NEH), Part 636, Chapter 52, Structural Design of Flexible Conduits.

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For computation of maximum allowable loads on subsurface drains of all materials, use the trench and bedding conditions specified, and the compressive strength of the conduit. The design load on the conduit shall be based on a combination of equipment loads, trench loads, and road traffic, as applicable.

Equipment loads shall be based on the maximum expected wheel loads for the equipment to be used, the minimum height of cover over the conduit, and the trench width. Equipment loads on the conduit may be neglected when the depth of cover exceeds 6 ft. Trench loads shall be based on the type of backfill over the conduit, the width of the trench, and the unit weight of the backfill material.

Continuous pipe shall be used where it is not feasible to obtain cover as specified in the national standards, such as where drain lines cross waterways, or roads, the outlet end of mains, or near structures. The continuous pipe shall be of sufficient strength and durability to withstand expected loadings and weathering.

The minimum average depth of any drain through an area requiring continuous internal drainage shall be

3 ft (0.9 m).

The depth of laterals to intercept hillside seepage will vary according to the depth of the impervious layer.

The drain line must be placed so that it intercepts the seepage flow.

The depth of mains will be adjusted so that the laterals can be joined to the main with a center-to-center connection or at a higher point. Where laterals are significantly above the main or more than one lateral joins the main at a point, refer to the section on “Auxiliary Structures and Protection.” A flowline-to-flowline connection is permissible when a center-to-center connection is not practical.

The maximum depth of cover for standard duty corrugated plastic tubing shall be 10 ft for trench widths of

2 ft or less (measured at tubing and to 1 ft above top of tubing). Heavy duty tubing shall be specified for depths greater than 10 ft, trench widths more than 2 ft, or in rocky soils.

For computation of maximum allowable loads on subsurface drains, use the trench and bedding conditions specified and the crushing strength of the kind and class of drain. The design load on the conduit shall be based on a combination of equipment loads and trench loads.

Equipment loads are based on the maximum expected wheel loads for the equipment to be used, the minimum height of cover over the conduit, and the trench width. Equipment loads on the conduit may be neglected when the depth of cover exceeds 6 ft. Trench loads are based on the type of backfill over the conduit, the width of the trench, and the unit weight of the backfill material. A safety factor of not less than

1.5 shall be used in computing the maximum allowable depth of cover for a particular type of conduit.

Minimum Velocity and Grade

In areas where sedimentation is not a hazard, minimum grades shall be based on site conditions and a velocity of not less than 0.5 ft per second. If a sedimentation hazard exits, a velocity of not less than 1.4 ft per second shall be used to establish the minimum grades. Otherwise, provisions shall be made for preventing sedimentation by use of filters or by collecting and periodically removing sediment from installed traps, or by periodically cleaning the lines with high-pressure jetting systems or cleaning solutions.

Minimum grades where no sediment hazard exists are as follows:

Drain Size, Grade, Inches (mm) Percent

3 to 6 (102 to 152) 0.10

8 to 10 (203 to 254) 0.07

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12 and larger (>305) 0.05

Maximum Velocity

Design velocities for perforated or open joint pipe shall not exceed those given in Table M-3, unless special protective measures are installed. Design velocities with protective measures shall not exceed manufacturer’s recommended limits.

On sites where topographic conditions require drain placement on steep grades and design velocities greater than indicated in Table M-3, special measures shall be used to protect the conduit or surrounding soil.

Protective measures for high velocities shall include one or more of the following, as appropriate:

Enclose continuous perforated pipe or tubing with fabric type filter material or properly graded sand 1.

and gravel.

Use non-perforated continuous conduit or a watertight pipe, and sealed joints. 2.

Place the conduit in a sand and gravel envelope, or initial backfill with the least erodible soil 3.

available.

Select rigid butt end pipe or tile with straight smooth sections and square ends to obtain tight fitting 4.

joints.

Wrap open joints of the conduit with tar-impregnated paper, burlap, or special fabric-type filter 5.

material.

Install larger diameter drain conduit in the steep area to help assure a hydraulic grade line parallel 6.

with the conduit grade.

Install open air risers for air release or entry at the beginning and downstream end of the high 7.

velocity section.

Releases from drainage water management structures shall not cause flow velocities in perforated or open joint drains to exceed allowable velocities in Table M-3, unless protective measures are installed.

Thrust Control

Follow pipe manufacturer’s recommendations for thrust control or anchoring, where the following conditions exist:

Axial forces that tend to move the pipe down steep slopes. •

Thrust forces from abrupt changes in pipeline grade or horizontal alignment, which exceed soil • bearing strength.

Reductions in pipe size. •

In the absence of manufacturer’s data, thrust blocks shall be designed in accordance with NEH, Part 636, Chapter 52, Structural Design of Flexible Conduits.

Outlets

Drainage outlets shall be adequate for the quantity and quality of water to be discharged.

Outlets to surface water shall be designed to operate without submergence under normal conditions.

For discharge to streams or channels, the outlet invert shall be located above the elevation of normal flow and at least 1.0 foot above the channel bottom.

Outlets shall be protected against erosion and undermining of the conduit, entry of tree roots, damaging periods of submergence, and entry of rodents or other animals into the subsurface drain.

A continuous section of pipe without open joints or perforations, and with stiffness necessary to withstand expected loads, shall be used at the outlet end of the drain line. Minimum lengths for the outlet section of

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NRCS, MN

conduit are provided in Table M-4. Single-wall Corrugated Plastic Pipe is not suitable for the section that outlets into a ditch or channel.

For outlets into sumps, the discharge elevation shall be located above the elevation at which pumping is initiated.

The use and installation of outlet pipe shall conform to the following requirements:

If burning vegetation on the outlet ditch bank is likely to create a fire hazard, the material from which 1.

the pipe is fabricated must be fireproof.

At least two-thirds of the pipe section shall be buried in the ditch bank, and the cantilever section 2.

must extend to the toe of the ditch side slope, or the side slope shall be protected from erosion. The minimum length of the pipe shall be 10 ft (3 m). Under certain conditions shorter sections are appropriate; e.g., steep-sided main and laterals (1:1 or less) with a narrow bottom width of 3 ft, commonly referred to as “minimum ditches,” for outletting individual subsurface drain laterals. For

15 in. and 18 in. diameter pipe, use 16 ft minimum length. Use 20-ft outlet pipe for all diameters larger than 18 inches.

If ice or floating debris may damage the outlet pipe, the outlet shall be recessed to the extent that 3.

the cantilevered part of the pipe will be protected from the current of flow in the ditch or channel.

When surface water must enter the channel at the location of the outlet pipe, some approved type 4.

of structure shall be installed to safely lower the surface water into the ditch and protect the outlet.

Headwalls used for subsurface drain outlets must be adequate in strength and design to avoid washouts and other failures. At least 8 ft (2.4 m) of rigid pipe shall be used to connect the drain line with the structure.

The minimum thickness for metal pipe shall be 0.064 inches (1.63 mm). 5.

An adequate outlet shall be available and shall meet thefollowing requirements:

Open Channel

The outlet channel shall be large enough to remove surface runoff from the watershed in a period of time sufficient to prevent serious crop damage. Required capacity will be based on NRCS design criteria for open channels. The channel shall be deep enough to provide the minimum of one foot of clearance between the invert of the drain at its outlet and low-water stage in the channel. This clearance may be reduced: (1) where the outlet channel is on such a grade that silting will not occur; (2) where the tile will flow freely within 24 hours after a storm; or (3) where definite scheduled plans have been made for outlet improvement within the next 12 months.

Existing Mutual or Private Drains

When existing mutual or private tile mains are used as an outlet, the tile shall be in good condition, free of failure, and working properly. The minimum capacity of an existing mutual tile main shall be ¼ inch (6.4

mm) per 24 hours for the area to be served (wet acres if no inlets). The minimum capacity of a private tile main (one that is under the control of a farmer for which the proposed system will serve) must have the capacity as shown in Table M-1 or Table M-2, as the case may be.

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NRCS, MN

Legal Closed Drain

A legal closed drain (county or judicial) intended to serve as an outlet for the proposed drainage system shall be administratively considered adequate, provided it is in good repair, free of failure, working properly and the owner has secured required permits to use the legal drain as an outlet.

Sink Holes and Wells

Sink holes and wells are not considered adequate outlets.

Pump Outlet

The pump capacity and setting shall be capable of providing the appropriate drainage coefficient for the area drained.

Surface Drains

The area to be drained must have adequate surface drainage or provisions must be developed, if feasible and practical, for removing surface water overland to the outlet channel. When it is not feasible or practical to remove impounded surface water by surface drains, surface inlets to subsurface drains may be used.

The outlet must be protected against erosion and undermining of the conduit, entry of tree roots, damaging periods of submergence, and entry of rodents or other animals into the subsurface drain. A continuous section of rigid pipe without open joints or perforations will be used at the outlet end of the line and must discharge above the normal elevation of low flow in the outlet ditch. Corrugated plastic tubing is not suitable for the outlet section. Minimize the visual impact of projecting outlets.

Continuously submerged outlets will be permitted for water table control systems if planned and designed according to the standard for Drainage Water Management (554).

Watertight conduits strong enough to withstand the expected loads will be used if subsurface drains cross under irrigation canals, ditches, or other structures. Conduits under roadways must be designed to withstand the expected loads. Shallow subsurface drains through depressed or low areas and near outlets must be protected from damage caused by farm machinery and other equipment and from freezing and thawing.

Alignment

The change in horizontal direction of the tile shall be made by one of the following methods:

The use of manufactured fittings. 1.

The use of junction boxes or manholes. 2.

A gradual curve of the drain trench on a radius that can be followed by the trenching machine and 3.

maintain grade. A gradual curve may be made by hand shaping the inner side of the trench, but in no case shall the radius be less than 5 ft (1.5 m). In either case, rigid tile must then be shaped or chipped so that no crack between tile exceeds 1/8 inch, unless adequately covered.

Protection from Biological and Mineral Clogging

Drains in certain soils are subject to clogging of drain perforations by bacterial action in association with ferrous iron, manganese, or sulfides. Iron ochre can clog drain openings and can seal manufactured

(fabric) filters.

Manganese deposits and sulfides can clog drain openings.

Where bacterial activity is expected to lead to clogging of drains, access points for cleaning the drain lines shall be provided.

Where possible, outlet individual drains to an open ditch to isolate localized areas of contamination and to limit the translocation of contamination throughout the system.

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NRCS, MN

Drain cleaning provisions should be installed in such a way that the drains can be cleaned in an upstream or rising grade direction. If possible, drains in ochre-prone areas should be installed during the dry season when the water table is low and the iron is in its insoluble form.

Where possible, in areas where the potential for ochre problems is high, protection against ochre development can be provided by designing an outlet facility to ensure permanent submergence of the drain line.

Protection from Root Clogging

Problems may occur where drains are in close proximity to perennial vegetation. Drain clogging may result from root penetration by water-loving trees, such as willow, cottonwood, elm, soft maple, some shrubs, grasses, and deep-rooted perennial crops growing near subsurface drains.

The following steps may reduce the incidence of root intrusion:

Install a continuous section of non-perforated pipe or tubing with sealed joints, through the root • zone.

Remove water-loving trees for a distance of at least 100 ft on each side of the drain, and locate • drains a distance of 50 ft or more from non-crop tree species.

Provide for intermittent submergence of the drain to limit rooting depth by installing a structure for • water control (e.g. an inline weir with adjustable crest) that allows for raising the elevation of the drain outlet

Water quality

Septic systems shall not be directly connected to the subsurface drainage system, nor shall animal waste be directly introduced into the subsurface drainage system.

Materials

Subsurface drains include flexible conduits of plastic, bituminized fiber, or metal; rigid conduits of vitrified clay or concrete; or other materials of acceptable quality.

The conduit shall meet strength and durability requirements for the site. All conduits shall meet or exceed the minimum requirements of the appropriate specifications published by the American Society for Testing and Materials (ASTM), American Association of State Highway Transportation Officials (AASHTO), or the

American Water Works Association (AWWA).

Foundation

If soft or yielding foundations are encountered, the conduits shall be stabilized and protected from settlement. The following methods are acceptable for the stabilization of yielding foundations:

Remove the unstable material and provide a stable bedding of granular envelope or filter material. •

Provide continuous cradle support for the conduit through the unstable section. •

Bridge unstable areas using long sections of conduit having adequate strength and stiffness to • ensure satisfactory subsurface drain performance.

Place conduit on a flat, treated plank. This method shall not be used for flexible (e.g. Corrugated •

Plastic Pipe) without proper bedding between the plank and conduit.

Loading

See the EFH14 Table 14-6 on page 14-63 for determining maximum trench depths for corrugated plastic tubing buried in loose, fine-textured soils. Special designs using reinforced concrete pipe or other pipe of similar strength will be used where drainage tile or tubing will not provide adequate strength.

Filters and Filter Material ilters shall be used around conduits, as needed, to prevent movement of the surrounding soil material into the conduit. The need for a filter shall be determined by the characteristics of the surrounding soil material, -CPS-8

NRCS, MN

site conditions, and the velocity of flow in the conduit. A suitable filter shall be used if any of the following conditions exist:

Local experience with soil site conditions indicates a need. •

Soil materials surrounding the conduit are dispersed clays, silts with a Plasticity Index less than 7, • or fine sands with a Plasticity Index less than 7.

The soil is subject cracking by desiccation. •

The method of installation may result in inadequate consolidation between the conduit and backfill • material.

If a sand-gravel filter is specified, the filter gradation shall be designed in accordance with NEH, Part 633, Chapter 26, Gradation Design of Sand and Gravel Filters or the following:

D15 size smaller than 7 times d85 size but not smaller than 0.6 mm, •

D15 size larger than 4 times d15 size, •

Less than 5% passing No. 200 sieve, •

Maximum size smaller than 1.5 inches, where D represents the filter material and d represents the • surrounding base material.

The number following each letter is the percent of the sample, by weight, that is finer than that size. For example, D15 size means that 15 percent of the filter material is finer than that size.

Specified filter material must completely encase the conduit such that all openings are covered with at least 3 inches of filter material, except where the top of the conduit and side filter material are be covered by a sheet of plastic or similar impervious material to reduce the quantity of filter material required. In all cases, the resulting flow pattern through filter material shall be a minimum of 3 inches in length.

Geotextile filter materials may be used, provided that the effective opening size, strength, durability, and permeability are adequate to prevent soil movement into the drain throughout the expected life of the system.

Geotextile filter material shall not be used where the silt content of the soil exceeds 40 percent.

Envelopes and Envelope Material

Envelopes shall be used around subsurface drains if needed for proper conduit bedding or to improve flow characteristics into the conduit.

Materials used for envelopes do not need to meet the gradation requirements of filters, but they must not contain materials that will cause an accumulation of sediment in the conduit, or materials that will render the envelope unsuitable for bedding of the conduit.

Envelope materials shall consist of sand-gravel, organic, or similar material. 100 percent of sand-gravel envelope materials shall all pass a 1.5-inch sieve; not more than 30 percent shall pass a Number 60 sieve; and not more than 5 percent shall pass the Number 200 sieve. ASTM-C-33 fine aggregate for concrete has been satisfactorily used and is readily available.

Organic or other compressible envelope materials shall not be used below the centerline of…