Watershed management field manual



Rome 1998






Conversion Table : English - Metric Units


1.1. General Introduction

1.2. Interaction of Roads and Environment

1.3. Erosion Processes

1.4. Assessment of Erosion Potential

1.4.1. Surface Erosion

1.4.2. Mass Soil Movement


2.1. Route Planning

2.1.1. Design Criteria

2.1.2. Design Elements Number of Lanes and Lane Width Road Width Turnouts Turn-arounds Curve Widening Clearance Speed and Sight Distance Horizontal and Vertical Alignment Travel Time

2.2. Economic Evaluation and Justification

2.2.1. Economic Analysis Method

2.2:2. Analysis of Alternative Routes

2.3. Route Reconnaissance and Location

2.3.1. Road Reconnaissance

2.3.2. Faults

2.3.3. Indicators of Slope Stability


3.1. Horizontal and Vertical Alignment

3.1.1. Horizontal Alignment Considerations

3.1.2. Curve Widening

3.1.3. Vertical Alignment

3.2. Road Prism

3.2.1. Road Prism Stability

3.2.2. Side Cast - Full Bench Road Prism

3.2.3. Slope Design

3.2.4. Road Prism Selection

3.3. Road Surfacing


4.1. General Considerations

4.2. Estimating runoff

4.3. Channel Crossings

4.3.1. Location of Channel Crossings

4.3.2. Fords

4.3.3. Culverts

4.3.4. Debris Control Structures

4.3.5. Bridges

4.4. Road Surface Drainage

4.4.1 Surface Sloping

4.4.2. Surface Cross Drains

4.4.3. Ditches and Beans

4.4.4. Ditch Relief Culverts

4.5. Subsurface Drainage


5.1. Introduction

5.2. Surface Protection Measures

5.2.1. Site Analysis

5.2.2. Site Preparation

5.2.3. Seeding and Planting

5.2.4. Application Methods

5.2.5. Waffling and Filter Strips

5.2.6. Brush Layering

5.2.7. Mechanical Treatment

5.3. Mass Movement Protection


6.1. Road Construction Techniques

6.1.1. Construction Staking

6.1.2. Clearing and Grubbing of the Road Construction Area

6.2. General Equipment Considerations

6.2.1. Bulldozer in Road Construction

6.2.2. Hydraulic Excavator in Road Construction

6.3. Subgrade Construction

6.3.1. Subgrade Excavation with Bulldozer

6.3.2. Fill Construction

6.3.3. Compaction

6.3.4. Subgrade Construction with Excavator

6.3.5. Filter Windrow Construction for Erosion Prevention


7.1. Introduction

7.2. Drainage System Maintenance

7.3. Road Surface Maintenance

7.4. Emergency Storm Response

7.5. Road Abandonment and Reclamation


  1. Sediment production in relation to road density (Amimoto, 1978).

  2. Infinite slope analysis for planar failures.

  3. Relationship between frictional resistance (F) and driving force (E) promoting downslope movement. (Burroughs, et al., 1976).

  4. Sixty cm of soil with 15 cm of ground water will slide when the slope gradient exceeds 58 percent. (Burroughs, et al., 1976).

  5. (a) Subsurface rainwater flows in the direction of the slope when geologic strata dip toward the slope. (b) Subsurface rainwater percolates downward and out of the root zone when geologic strata dip in that direction (Rice, 1977).

  6. Road structural terms.

  7. Turnout spacing in relation to traffic volume and travel delay time.

  8. Typical turnout dimensions.

  9. Relationship between curve radius and truck speed when speed is not controlled by grade (U.S. Forest Service, 1965).

  10. Relationship between grade and truck speed on gravel roads (U.S. Forest Service, 1965).

  11. Stepped backslope (no scale).

  12. Tag line location and center line location of proposed road. Sideslopes are typically less than 40 to 50 percent.

  13. Tag line location and center line location of proposed road. Sideslopes are typically greater than 50%.

  14. Selection of the road alignment in the field by "stretching the tag line". This "stretched", or "adjusted" tag line is surveyed and represents the final horizontal location of the road.

  15. Position [1] shows tag line ribbon at approximately eye-level. The feet of the road locator are "on grade". Position [2] shows the ribbon on-location over the center line or tangent as selected in the field after stretching. The ribbon has been moved horizontally, thereby allowing an estimate of required cut or fill at center line.

  16. Example of the effect of shortened center line through a draw or around a sharp ridge. This situation develops when running the tag line into the draw or around a sharp ridge without allowing for proper curve layout and design location.

  17. Curve layout by deflection method, a useful approach during the original road location phase.

  18. By sighting across draw at 0 percent grade, the desired curve is laid out without increasing the grade.

  19. Cut and fill apportioning through a switchback to maintain a given grade.

  20. Suspected fault zones are indicated by the alignment of saddles in ridges and by the direction of stream channels. Geologic map is found in upper left corner. Major faults are shown as heavy dark lines on geologic maps (Burroughs, et al.,1976).

  21. Stereogram of a possible fault zone. The location of the fault is indicated by the dashed line. (Burroughs, et al., 1976).

  22. Approximate boundary between serpentine (metamorphic rock) material and the Umpqua formation is shown by the dashed line. The determination is based primarily on the basis of vegetation density. Timber on portions of the Umpqua formation have been harvested which accounts for a reduction in vegetation density, particularly in the northwest corner of the photo (Burroughs, et al., 1976).

  23. "Hummocky" topography with springs, curved or tilted trees, and localized slumps, characterize land undergoing active soil

  24. Empirical headwall rating system used for shallow, rapid landslides on the Mapleton Ranger District, Region 6, Oregon.

  25. Non-geometric and conventional p-line traverses (MacDonald and Greening, 1982).

  26. Design adjustments.

  27. Full bench design.

  28. Self-balanced design.

  29. Basic vehicle geometry in off -tracking.

  30. Example of truck-trailer dimensions.

  31. Step 1 for the graphical solution of curve widening.

  32. Step 2 and 3 for the graphical solution for curve widening.

  33. Graphical solution for off-tracking of a stinger-type log truck.

  34. Curve widening and taper lengths.

  35. Curve widening guide for a two or three axle truck as a function of radius and deflection angle. The truck dimensions are as shown.

  36. Curve widening guide for a truck-trailer combination as a function of radius and deflection angle. The dimensions are as shown.

  37. Curve widening guide for a log-truck as a function of radius and deflection angle. The dimensions are as shown.

  38. Curve widening guide for a tractor / trailer as a function of radius and deflection angle. The tractor - trailer dimensions are as shown.

  39. Typical vertical curves.

  40. Vertical curve elements.

  41. Log truck geometry and dimensions.

  42. Translational or wedge failure brought about by saturated zone in fill. Ditch overflow or unprotected surface are often responsible.

  43. Fill failure caused by backward erosion at the toe of the fill due to excessive seepage and unprotected toe.

  44. Elements of road prism geometry.

  45. Required excavation volumes for side cast and full bench ,construction as function of side slope. Assumed subgrade width 6.6 m and bulking factor K = 1.35 (rock).

  46. Erodible area per kilometer of road for side cast construction and full bench/end haul construction as a function of side slope angle and cut slope angle. The values shown are calculated for a 6.6 m wide subgrade. The fill angle equals 37 degrees.

  47. Erodible area per kilometer of road for full bench/end haul construction as a function of side slope angle and cut slope angle. The values shown are calculated for a 6.6 m wide subgrade.

  48. Maximum cut slope ratio for coarse grained soils with plastic fines (low water conditions). Each curve indicates the maximum height or the steepest slope that can be used for the given soil type (After USFS, 1973).

  49. Maximum cut slope angle for coarse grained soils with plastic fines (high water conditions). Each curve indicates the maximum cut height or the steepest slopes that can be used for the given soil type (After USFS,1973).

  50. Maximum cut slope angle for fine grained soils with slowly permeable layer at bottom of cut. Each curve indicates the maximum vertical cut height or the steepest slope that can be used for the given soil type (After USFS 1973).

  51. Maximum cut slope angle for fine grained soils with slowly permeable layer at great depth (>= 3 x height of cut) below cut (After USFS, 1973).

  52. Interaction of subgrade dimension and roadwidth, ballast depth, ditch width and fill widening.

  53. Fill widening added to standard subgrade width where fill height at centerline or shoulder exceeds a critical height. Especially important if sidecast construction instead of layer construction is used.

  54. Template and general road alignment projected into the hill favoring light to moderate cuts at centerline in order to minimize fill slope length. Fill slopes are more susceptible to erosion and sloughing than cut slopes.

  55. Illustration of the very considerable reduction in excavation made possible on a steep slope by the use of cribbing. Crib proportions shown are suitable for log construction; if crib was built of concrete or steel, shorter spreaders could be used in upper 3 m as indicated by the dashed line (Kraebel, 1936).

  56. Ballast thickness curves for single wheel loads (from Steward et al 1977). Conversion factors: 1 inch = 2.5 cm ; 1 kg/cm2 = 14.22 psi.

  57. Ballast thickness curves for dual wheel loads (from Steward et al 1977). Conversion factors: 1 inch = 2.5 cm ; 1 kg/cm2 = 14.22 psi.

  58. Ballast thickness curves for tandem wheel loads (from Steward et al 1977). Conversion factors: 1 inch = 2.5 cm; 1 kg/cm2 = 14.22 psi.

  59. Slope shape and its impact on slope hydrology. Slope shape determines whether water is dispersed or concentrated (US Forest Service, 1979).

  60. Culvert locations have modified drainage patterns of ephemeral streams 2 and 3, an undesirable practice. Locations A and B become potential failure sites. Stream 3 carries more water below B, hence it has more erosive power.

  61. Determining high water levels for measurement of stream channel dimensions

  62. Ford construction stabilized by gabions placed on the downstream end.

  63. Profile view of stream crossing with a ford. Adverse grade provides channeling, preventing debris accumulation from diverting the stream on to and along the road Surface. The profile of the ford along with vehicle dimensions must be considered to insure proper clearance and vehicle passage. (After Kuonen, 1983).

  64. Hardened fill stream crossings provide an attractive alternative for streams prone to torrents or debris avalanches.

  65. Culvert alignment (USDA, Forest Service, 1971).

  66. Proper culvert grades (Highway Task Force, 1971).

  67. Definition sketch of variables used in flow calculations.

  68. Hydraulics of culverts (Highway Task Force, 1971).

  69. Sample work sheet for culvert dimension determination.

  70. Monograph for concrete pipes, inlet control (U.S. Dept. of Commerce, 1963).

  71. Monograph for corrugated metal pipe (CMP), inlet control, (U.S. Dept. of Commerce, 1963).

  72. Monograph for corrugated metal arch pipe (CMP), inlet control (U.S. Dept. of Commerce, 1963).

  73. Monograph for box - culvert, inlet control (U.S. Dept. of Commerce, 1963).

  74. Monograph for corrugated metal pipe (CMP), outlet control, (U.S. Dept. of Commerce, 1963).

  75. Proper pipe foundation and bedding (USDA, Forest Service, 1971).

  76. Debris control structure-cribbing made of timber.

  77. Debris control structure-trash rack made of steel rail (I-beam) placed over inlet.

  78. Inlet and outlet protection of culvert with rip-rap. Rocks used should typically weigh 20 kg or more and approximately 50 percent of the rocks should be larger than 0.1 m3 in volume. Rocks can also be replaced with cemented sand layer (1 part cement, 4 parts sand).

  79. Road cross sections for surface drainage.

  80. Design of outsloped dips for forest roads. A to C, slope about 10 to 15 cm to assure lateral flow; B, no material accumulated at this point - may require surfacing to prevent cutting; D, provide rock rip-rap to prevent erosion; E, berm to confine outflow to 0.5 m wide spillway (FAO, 1978).

  81. Design of insloped dips for forest roads. A to C, slope about 10 to 15 cm to assure lateral flow; B, no material accumulated at this point - may require surfacing to prevent cutting; D, provide rock rip-rap to prevent erosion; E, berm to prevent overflow; F, culvert to carry water beneath road; G, widen for ditch and pipe inlet (FAO, 1978).

  82. Installation of an open-top culvert. Culverts should be slanted at 30 degrees downslope to help prevent plugging (Darrach, et al., 1982).

  83. Cross ditch construction for forest roads with limited or no traffic. Specifications are average and may be adjusted to gradient and other conditions. A, bank tie-in point cut 15 to 30 cm into roadbed; B, cross drain berm height 30 to 60 cm above road bed; C, drain outlet 20 to 40 cm into road; D, angle drain 30 to 40 degrees downgrade with road centerline; E, height up to 60 cm, F, depth to 45 cm; G, 90 to 120 cm.

  84. Spacing standard for open-top culverts on forest road surfaces, Japanese Islands (Minematsu and Minamikata, 1983).

  85. Guides for locating cross drains. Several locations require cross drains independent of spacing guides. A and J, divert water from ridge; A, B, and C, cross drain above and below junction; C and D, locate drains below log landing areas; D and H, drains located with regular spacing; E, drain above incurve to prevent bank cutting and keep road surface water from entering draw; F, ford or culvert in draw; G, drain below incurve to prevent water from coursing down road; I, drain below seeps and springs (FAO, 1978).

  86. Ditch interception near stream to divert ditch water onto stable areas instead of into the stream (EPA, 1975).

  87. Minimum ditch dimensions.

  88. Monograph for solution of Manning's equation (U.S. Dept. of Commerce, 1965).

  89. Minimum berm dimensions.

  90. Ditch relief culvert installation showing the use of headwall, downspout and a splash barrier/energy dissipater at the outlet. Minimum culvert grade is 3 to 5 %. Exit velocities should be checked (EPA, 1975).

  91. Ditch relief culvert in close proximity to live stream, showing rock dike to diffuse ditch water and sediment before it reaches the stream (EPA, 1975).

  92. Energy dissipaters (Darrach, et al., 1981).

  93. Size of stone that will resist displacement by water for various velocities and ditch side slopes. 1 ft.= 30 cm (U.S. Dept. of Commerce, 1965).

  94. Selection criteria for slope stabilization methods.

  95. Selection criteria for surface cover establishment methods in relation to erosion risk.

  96. Preparation and installation procedure for contour wattling, using live willow stakes (after Kraebel, 1936).

  97. Brush layer installation for slope stabilization using rooted plants for cut slope and green branches for fill slope stabilization.

  98. Types of retaining walls.

  99. Forces acting on a retaining wall.

  100. Example of a standard crib wall design (Wash. State Dept. of Highway).

  101. Low gabion breast walls showing sequence of excavation, assembly, and filling. (From White and Franks, 1978).

  102. Road cross section showing possible construction information.

  103. The effect of improperly starting the cut as marked by the slope stake. Starting the cut too high results in excess excavation and side cast. Starting the cut too low leaves an oversteepened cut bank.

  104. Construction grade check. Engineer stands on center of construction grade and sights to RP tag. Measured distance and slope allow for determination of additional cut.

  105. Clearing limits in relation to road bed widths. Significant organic layers are removed between B and E. Stumps are removed between B and D. Stumps may be left between D and E. Organic debris and removed stumps are placed in windrows at F to serve as filter strips (see Chapter 6.3.1).

  106. Pioneer road location at bottom of proposed fill provides a bench for holding fill material of completed road.

  107. Maximum production rates for different bulldozers equipped with straight blade in relation to haul distance (after, Caterpillar, 1984).

  108. Adjustment factors for bulldozer production rates in relation to grade (Caterpillar Performance Handbook, 1984).

  109. Fill slope length reduction by means of catch waif at toe of fill.(see also Fig. 55).

  110. Three basic road prism construction activities.

  111. Road construction with a bulldozer. The machine starts at the top and in successive passes excavates down to the required grade. Excavated material is side cast and may form part of the roadway.

  112. Sliver fills created on steep side slopes where ground slope and fill slope angles differ by less than 70 and fill slope height greater than 6.0 meters are inherently unstable.

  113. Trench excavation to minimize sidehill loss of excavated material. Debris and material fall into trench in front of bulldozer blade. Felled trees and stumps are left until removed during excavation construction. They act as temporary retaining walls.

  114. Fills are constructed by layering and compacting each layer. Lift height should not exceed 50 cm. Compaction should be done with proper compaction equipment and not a bulldozer (from OSU Ext. Service 1983).

  115. Fills which are part of the roadway should not be constructed by end dumping. (from OSU Ext. Service 1983).

  116. Excavation cost comparison for three different embankment construction techniques( 1 cu.yd. = 0.9 m3) (after Haber and Koch, 1983).

  117. First pass with excavator, clearing logs and stumps from construction site. Working platform or pioneer road just outside of planned road surface width.

  118. Second pass with excavator, removing or stripping overburden of unsuitable material and placing it below pioneer road.

  119. Third pass, finishing subgrade and embankment fill over pioneer road. Road side ditch is finished at the same time.

  120. Typical filter windrow dimensions built of slash and placed on the fill slope immediately above the toe. The windrow should be compressed and the bottom part embedded 15 cm in the fill slope (after Cook and King 1983).

  121. "Kanisku" closure is effective on side cast constructed roads on slopes up to 60 percent in areas with low to moderate precipitation.

  122. Relief dip reduces the potential impact of culvert failure by reducing the amount of potential sediment.


  1. Unified Soil Classification System.

  2. Guide for placing common soil and geologic types into erosion classes (Forest Soils Committee of the Douglas-fir Region of the Pacific Northwest, 1957).

  3. Traffic service levels definitions used to identify design parameters (from U.S. Forest Service, Transportation Eng. Handbook).

  4. Example of a roads objective documentation form (from U.S. Forest Service, Transportation Eng. Handbook).

  5. Traveled way widths for single-lane roads.

  6. Lane widths for double-lane roads.

  7. Recommended turnout spacing-all traffic service levels.

  8. Turnout widths and lengths.

  9. Curve widening criteria.

  10. a) Relationship between round trip travel time per kilometer and surface type as influenced by horizontal and vertical alignment; adverse grade in direction of haul (U.S. Forest Service, 1965).

    b) Relationship between round trip travel time per kilometer and surface type as influenced by horizontal and vertical alignment; favorable grade in direction of haul (U.S. Forest Service, 1965)

  11. Comparison of single-lane versus double-lane costs at three different use levels.

  12. Comparison of annual road costs per kilometer - 10,000 vehicles per year.

  13. Comparison of annual road costs per kilometer for 20,000 and 40,000 vehicles per year.

  14. Cost summary comparison (5 vehicles per hour-1/2 logging trucks, 1/2 other traffic); assumes 8-hour hauling day, 140 days/year use, 20 year road life, 23.8.m3 (6.0 M bd. ft.) loads for logging trucks, cost of operating logging trucks including driver's wage--$0.25/min, cost of operating other vehicles-$0.04/minute, 5,535 m3 (1 1/2 MM bd. ft.) timber harvested (Gardner,1978).

  15. Deflection angles for various chord lengths and curve radii.

  16. Maximum grades log-trucks can start on from rest.

  17. Values of friction angles and unit weights for various soils (from Borroughs et. al., 1976).

  18. Maximum cut slope ratio for coarse grained soils (USFS, 1973).

  19. Maximum cut slope ratio for bedrock excavation (USFS, 1973).

  20. Minimum fill slope ratio for compacted fills (US Forest Service, 1973).

  21. Required subgrade width (exclusive of fill widening) as a function of road width, ballast depth and ditch width. Roadwidth = 3.0 m, ditch = 0.9 m (1:1 and 2:1 slopes), shoulder-slopes 2:1.

  22. Calculated sediment yield per kilometer of road for various road types and use levels (Reid, 1981; Reid and Dunne, 1984).

  23. Engineering characteristics of soil groups for road construction (Pearce, 1960).

  24. Required depth of ballast for three design vehicles. Road to withstand large traffic volumes ( > 1,000 axle loads) with less than 5 cm of rutting.

  25. Flood recurrence interval (years) in relation to design life and probability of failure (Megahan, 1977)*.

  26. Values of relative imperviousness for use in rational formula (American Iron and Steel Institute, 1971).

  27. Manning's n for natural stream channels (surface width at flood stage less than 30 m (Highway Task Force, 1971).

  28. Relationship of peak flow with different return periods. (Nagy, et al, 1980).

  29. Values for coefficient of roughness (n) for culverts. (Highway Task Force,1971; 1 foot = 0.3 m; 1 inch = 2.5 cm).

  30. Effect of in-sloping on sediment yield of a graveled, heavily used road segment with a 10 % down grade for different cross slopes (Reid, 1984)*.

  31. Cross drain spacing required to prevent rill or gully erosion deeper than 2.5 cm on unsurfaced logging roads built in the upper topographic position[1] of north-facing slopes[2] having gradient of 80 % [3].

  32. Maximum permissible velocities in erodible channels, based on uniform flow in straight, continuously wet, aged channels. For sinuous channels, multiply allowable velocity by 0.95 for slightly sinuous, 0.9 for moderately sinuous, and 0.8 for highly sinuous channels. (U. S. Environmental Protection Agency, 1975).

  33. Manning's n for open ditches.

  34. Ditch velocities for various n and grades. Triangular ditch with side slope ratio of 1:1 and 2:1, flowing 0.30 'meters deep; R=0.12.

  35. Guide for maximum spacing (in feet) of lateral drainage culverts by soil erosion classes and road grade (2% to 18%) (Forest soils Com., Douglas Fir Reg., PNW, 1957).

  36. Erosion control and vegetation establishment effectiveness of various mulches on highways in eastern and western Washington. Soils: silty, sandy and gravelly loams, glacial till consisting of sand, gravel and compacted silts and clays (all are subsoil materials without topsoil addition). Slope lengths: approximate maximum of 50 m (165 ft). Application rates: Cereal straw - 5,500 kg/ha (2 t/ac); Straw plus asphalt - 5,500 kg/ha (2 t/ac) plus asphalt at a rate of 0.757 I/kg (200 gal/t) of straw; (968 gal/ac); Wood cellulose fiber - 1,345 kg/ha (1,200 Ibs/ac); Sod - bentgrass strips 46 cm (18 in) by 1.8 m (6 ft) pegged down every third row.

  37. Windrow protective strip widths required below the shoulders of 5-year old forest roads built on soils derived from basalt, having 9 m cross-drain spacing, zero initial obstruction distance, and 100 percent fill slope cover density (US Environmental Protection Agency, 1975).

  38. Road construction equipment characteristics (from OSU Extension Service, 1983).

  39. Job condition correction factors for estimating bulldozer earth moving production rates. Values are for track-type tractor equipped straight (S) blade ( Caterpillar Handbook, 1984).

  40. Approximate economical haul limit for a 185 hp bulldozer in relation to grade Production achieved are expressed in percent of production on a 10 percent favorable grade with 30 m haul (100 %). (Pearce, 1978).

  41. Average production rates for a medium sized bulldozer (12-16 tonnes) constructing a 6 to 7 m wide subgrade (Caterpillar, 1984).

  42. Production rates for hydraulic excavators in relation to side slopes, constructing a 6 to 7 m wide subgrade.

  43. Fill slope erosion volume for windrowed and nonwindrowed slopes during a 3 year period following construction (Cook and King,1983).

  44. Typical road maintenance activities appropriate for a given road use level.

The designations employed and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of the Food and Agriculture Organization of the United Nations concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries.

M-34 ISBN 92-5-102789-X

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© FAO 1998