Ditch Maintenance.pdf

ROAD AND STREAM

CROSSING DESIGN

A. INTRODUCTION

1. BASIC DESIGN CONSIDERATIONS

Road design is often a combined result of economic and environmental factors that influence construction, operating and maintenance costs. Unfortunately, because construction costs are felt immediately, economics was once the most important consideration employed in choosing a road’s final design. However, excessive hauling expenses and difficulty of travel on a road, high road maintenance costs, as well as high environmental costs may have a far greater effect on the long-term economics of forest or ranch operations, or rural development, than the savings in initial construction of a low standard, poorly located or inadequately designed road.

Roads are designed for the expected type and frequency of use. Most forest, ranch and rural roads are designed as “low volume roads” that carry an average of less than 400 vehicles per day (Keller and Sherar, 2003), and have an appropriate design geometry for that level of use. Over-design can be a costly mistake. For example, construction costs for a 14-foot wide road on a steep sideslope may be as much as 30% higher than for a 12-foot wide road in the same location, simply because of the extra 2 feet of excavation width. Long-term mainte-nance costs are also likely to be higher for the wider road. For this reason, it is important to determine the main types of vehicles and the expected volume and speed of traffic so that the required, minimum road standards can be estab-lished well before actual construction begins.

Both road length and road width should be designed to minimum standards for the antici-pated uses of the road. Narrow roads without inside ditches dramatically reduce excavation and sidecast volumes, thereby reducing cutbank height and decreasing the likelihood of slope failures (Table 15). For example, building a 12-foot wide insloped road with a 3-foot wide inside ditch requires moving almost 60 percent more material during construction than if the road were outsloped with no ditch (Table 15).

Road design begins with planning for the road’s location. Selection of the final route will constrain many future design deci-sions. Two important design questions that need to be answered early in the planning process are 1) road prism design and 2) road surface design. Routing the alignment through or around various obstacles and hazards may dictate the use of certain road prism designs. In addition to these, there are special situations that often arise and require special road design considerations.

When considering road prism design, it is impossible to over-emphasize the importance of drainage in maintain-ing stable roads and protecting water quality. Roads should be designed and constructed to cause minimal disruption of natural drainage patterns. Provisions for two components of road drainage should be included in every road project: 1) road surface drainage (including drainage which originates from the cutbank, road surface and fill slope) and 2) hillslope drainage (includ-ing drainage from large springs, gullies and streams which cross the road alignment).

Finally, because hillslope morphology and hydrology varies greatly across the land-scape, rarely can a single type of road prism and road surface drainage design be uni-formly applied to a new or reconstructed road. One size does not fit all circumstances, and both road prism and road surface design is best varied to meet local conditions!

2. DESIGNING ROADS TO

MINIMIZE HYDROLOGIC

CONNECTIVITY AND

PROTECT WATER QUALITY

Roads can adversely impact streams, water quality and aquatic habitat in several ways, including erosion and sediment delivery, altering hillslope and stream hydrology, and discharg-ing chemical spills to streams and water bodies.

Sediment is delivered to streams as a result of both episodic (usually storm related) and chronic (every runoff event) erosion processes.

The most common storm-triggered erosion includes stream crossing washouts (gullying), stream diversions (and resultant hillslope gullying and slope failures), culvert outlet

TABLE 15. Excavated soil volume for full bench roads with various road widths and hillslope gradients

Hillslope gradient

Excavated volume to construct a full bench road assuming ½:1 cutbanks

(yd3 excavated per ft of road) Road width 12 ft Road width 14 ft Road width 16 ft Road width 18 ft

30 0.94 1.28 1.62 2.17

35 1.13 1.54 2.01 2.55

40 1.33 1.82 2.37 3.00

45 1.55 2.10 2.75 3.48

50 1.78 2.42 3.16 4.00

55 2.02 2.81 3.60 4.55

60 2.28 3.11 4.06 5.14

65 2.57 3.50 4.57 5.78

70 2.87 3.91 5.11 6.46

75 3.20 4.36 5.69 7.20

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erosion, hillslope gullying, bank erosion, and road and landing (turnout) fill slope failures.

In chronic erosion, sediment is delivered to streams every time there is a rainfall and runoff event sufficient to cause erosion of bare soil areas, concentrated runoff on compacted road surfaces and ditches, and sediment transport to nearby streams. The most common road-related bare soil areas include unpaved road surfaces, as well as unvegetated fill slopes, cutbanks, ditches and landslide surfaces. If the runoff path from one of these bare soil areas delivers surface runoff and eroded sediment (even turbid water) to a stream it is termed “hydrologically connected” to the stream system. Hydrologic connectiv-ity refers to the length or proportion of a road or road network that drains runoff directly to streams or other water bodies.

A hydrologically connected road or road segment has been defined as: “Any road segment that has a continuous surface flow path to a natural stream channel during a ‘design’ runoff event” (Fumiss et al., 2000).

A suitable “design” runoff event for many purposes has been suggested to be the 1-year, 6-hour storm, with antecedent moisture condi-tions corresponding to the wettest month of the year. This is the type of frequent rainfall and runoff event that is likely to generate surface runoff from most or all compacted and bare soil surfaces. During runoff events, a hydrologi-cally-connected road becomes an extended part of the natural stream network. Inboard ditches that drain to stream crossings and roadside ditches draining to gullies below ditch relief culverts are the most common road segments that are hydrologically connected to streams (Figure 21). Other road drainage structures (e.g., rolling dips and waterbars) may also discharge runoff and sediment to nearby waterbodies. Wherever a hydrologic connection exists, concentrated runoff, eroded sediments and road-associated chemicals have a direct route to the natural channel network and surface waters.

Compared to unroaded hillslopes, roads always increase hydrologic connectivity.

Hydrologic connectivity is affected by natural processes and human intervention.

It will increase with increasing intensity and duration of precipitation or snowmelt. In contrast, connectivity will be reduced where there are sediment obstructions and natural depositional areas, soils are deep and permeable, hillslopes are gentle and ungullied, streams are far away, and cross-drains are spaced closely together on the road. All these road and hillslope factors are important in predicting sediment travel distances below cross-drains, FIGURE 21.

Road approaches to bridges are often hydrologi-cally connected.

This through cut approach to a temporary bridge drains directly into the stream.

HANDBOOK FOR FOREST, RANCH AND RURAL ROADS

ROAD AND STREAM CROSSING DESIGN

and in planning and designing for reduced connectivity and sediment pollution.

Simple road location, design, construc-tion and maintenance measures can be employed to reduce hydrologic connectiv-ity. Connectivity can be reduced by good road location, such as upper hillslope locations far from streams. Hydrologic connectivity is also increased with increasing stream crossing fre-quency; so roads should be located where there will be fewer crossings. Hydrologic connectiv-ity on existing roads can be greatly reduced by designs that remove ditches and convert road shapes from insloped to outsloped, as well as by constructing frequent road surface and ditch drainage structures to disperse road runoff so it cannot reach a stream. Reducing the length of road that is hydrologically connected to streams will directly and immediately improve water quality and protect downstream aquatic habitat.

B. ROAD PRISM AND

ROAD SURFACE DESIGN

1. ROAD PRISM DESIGN

Road prisms may be designed to be full bench, partial bench (part cut and part fill, also called balanced cut and fill) or full fill (Figure 22).

Roads which are constructed without endhaul-ing are partial bench roads where spoil gen-erated during initial grading is used to widen the roadbed and fill depressions and stream channels crossed by the road. This has been the most commonly used construction practice for rural forest and ranch roads. The fill is placed and compacted, or (more commonly) sidecast loosely into the desired location.

However, there are many circumstances where sidecasting is no longer acceptable and alter-native designs and methods are needed and required to reduce environmental impacts and to provide a stable roadbed. The fourth design type is a through cut road where the align-ment is cut through a ridge and for a short section the road has a cutbank on both sides.

Through cut road sections, if needed, should be short because they are not easily drained.

Roads may need to be full bench on steep slopes (those over about 55%), especially where the road is close to or approaches a watercourse, or where water quality could be impacted by road work (Figure 23)1. Full bench construction requires that all the spoil generated by cutting into the hillside must be either used in filling local stream crossings and low spots in the new road, or endhauled to a stable storage site where spoil materi-als have no risk of entering a watercourse.

Outside of stream crossings, which are always full fill, road segments constructed with full fill techniques are less common, but may be employed on lowlands, wet valley bottoms, 1 See Appendix C for California Forest Practice Rules requirement for full bench slopes.

FIGURE 22.

Idealized diagrams depicting (a) a partial bench or cut-and-fill road (with both cut and sidecast);

(b) full bench road (all material endhauled—no sidecasting); (c) a through cut sec-tion (all cut, with spoil material endhauled); and

(d) a full (through) fill road (no cut— all fill placed and compacted in shal-low layers). Road surface shapes can be outsloped, outsloped with a ditch, crowned with a ditch, or insloped with a ditch, depending on local conditions (Modified from:

Kramer, 2001).

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extended wet areas, or wherever the road needs to be elevated to provide proper sub-surface drainage, or where it is inadvisable to cut into the hillside. Roads using this technique are usually confined to short reaches where slopes are potentially unstable and cuts into the slope could trigger soil movement. Full fill sections of road are often supported by struc-turally engineered fills with steep fill faces.

Cut-and-fill design: For most forest, ranch and rural landowners, use of cut-and-fill road construction has been preferred because it minimizes the amount and cost of earth moving. In other words, less soil moved gen-erally means less expense (Figure 24). Most older roads on the landscape have probably been built with this construction technique, using only a bulldozer for road building.

However, the indiscriminate use of sidecast road construction (the simplest method of cut-and-fill construction) has probably caused more problems for landowners than any other type of road building.

Sidecasting construction techniques should not be used on slopes over 55 to 60 percent because this results in fill slopes of about 67 percent, the average angle of repose (stabil-ity) for most loose soil materials (Figure 25).

For this reason, sidecast construction should be limited to gently sloping areas where streams are far from the road prism.

Cut-and-fill construction techniques can be used on slightly steeper slopes when exca-vated keyways and subgrade benching are employed to develop stable, well compacted road fills. In general, cuts should equal the needed fill volume, plus about 20 percent to allow for expansion and settling of loose fill.

That is, the loosened, excavated soil will take up about 20% more space than when it was “in-place” (before it was excavated). During the process of cutting and filling, it is critical to avoid letting sidecast or waste material enter streams or watercourses, or placing it on unstable or steep slopes where it might erode and be delivered to a nearby water body.

The angle or steepness of both cut and fill slopes is very important in building stable

FIGURE 23. Full bench road. The height of the cut-bank, the slope of the natural hillside and the small amount of sidecast indicates that this road is full bench and cut entirely into native hillslope materials.

Note that the road has no inboard ditch.

roads. There is a tradeoff in determining the optimum cut slope angle to construct (Table 16). Cutbank slopes should be designed to achieve maximum stability as well as a minimum exposure of bare soils.

Cut and fill slopes are usually expressed as horizontal-to-vertical ratios, such as ½:1, 1:1, or 1½:1 (Table 1). Road banks can be cut as steep as the stability of the material will permit, ranging from ¼:1 for very stable rock materials to 3:1 for erodible or unstable soils (Table 17). The stability of cut slopes is also highly dependent on soil types, groundwater conditions and local climate. Observations of cut slopes in settings similar to your proposed road project will provide useful information on in-the-field performance of cuts of various heights and angles. Use this field information to help you design the best cuts possible. A general guide for the maximum steepness of road cuts in various rock and soil materials is

FIGURE 24. In contrast to the road in Figure 23, this partial bench road was built by extensive side-casting. At least half the roadbed is built on fill materials.

FIGURE 25. For most earth materi-als, sidecasting on natural slopes over about 55% in steepness will result in steep, loose, unstable sidecast slopes that are easily eroded or prone to sliding. The face of an uncompacted sidecast slope should not exceed about 67% with most materials (less with granular or non-cohesive soils). This is the maximum angle of stability for most uncompacted, sidecast soil material.

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shown below (Table 17). Note that wet slopes, unstable or erodible soils, and highly fractured or bedded rocks may require gentler slope cuts.

Cut height and cut angle also affect the sta-bility of the final cut slope. Cuts which are stable at ½:1 at a 6-foot height may not be stable when the cut height is twice as high at 12 feet. Higher cuts lead to increased gravita-tional force and reduced stability at the face of the slope. Tall (deep) cuts are also more likely to intercept emerging soil water that can weaken the cut slope and cause failures that block the road or result in persistent ditch and roadbed maintenance problems.

Fill slopes can be built to a variety of angles depending on the properties of the material used, the amount of properly applied compac-tion, soil moisture and the type and density of vegetation that is established on the surface. In contrast to cut slopes, a typical fill slope angle of 1½:1 or 2:1 will be stable under normal soil and site conditions. The greater the rock fragment content in the soil, the steeper the fill slope can be and still remain stable. A slope angle of 2:1 is most easily vegetated and provides the best long term stability to most fills. In general, thick accumulations of loose, dry, side-casted soil that is not com-pacted will not usually hold a slope over about 60 percent, whereas many fill materi-als that are placed and properly compacted in thin, less than 1-foot horizontal layers (lifts) may be stable at slopes up to or slightly greater than 1½ to 1 (67%), or steeper. While a thin veneer of angular sidecast materi-als may hold on a slope steeper than 65%, a thick wedge of loose sidecast may not be stable even at a 50% slope (Figure 26).

TABLE 16. Advantages and disadvantages of steep cut slopes1

Advantages of steep cut slopes Disadvantages of steep cut slopes

1. Less right-of-way 1. Difficult to revegetate

2. Less excavated material 2. Prone to raveling

3. Less sidecast material 3. Prone to tension cracks and failure

4. Shorter slope exposed to surface erosion 4. Slightly greater risk of a rotational failure

1BCMF (1991)

TABLE 17. Common soil/rock types and stable slope ratios1

Soil/Rock type Slope Ratio (horizontal:vertical)

Most rock ¼:1 to ½:1

Most in-place rocks ¾:1 to 2:1

Very fractured rock 1:1 to 1 ½:1

Loose coarse granular soils 1 ½:1 to 3:1

Heavy clay soils 2:1 to 3:1

Soft clay-rich zones or wet seepage areas 2:1 to 3:1

Fill of most soils 1 ½:1 to 3:1

Fill of hard angular rock 1 1/3:1 or flatter

Low cuts and fills (<6–10 feet high) 2:1 or flatter (for revegetation) 1From Keller and Sherar, 2003. Note that slope stability is also very particular to the soil type and slope angle, and somewhat to rainfall, soil moisture, and other factors.

55ROAD AND STREAM CROSSING DESIGN

Stable road fills can be built on moderate and steep slopes by using benching, keyways and layered compaction methods.

Here, a bench is excavated at the base of the proposed fill, and layers of moist, compacted soil are built up on this stable bench. The stability of the fill can be further increased by starting with an insloped bench that helps anchor the base of the fill into the native hillslope. A keyway is sometimes used to “lock” the fill into the denser or more stable underlying native soils or bedrock materials.

Fill can then be compacted in lifts on top of this bench and steeper fill slope angles are possible. A keyed and benched fill depends on the fill being as strong as, or stronger than, the soil removed in the excavation. This bench and keyway locks the fill in place on the slope and prevents the fill from devel-oping a failure plane where it is placed on the natural ground surface (Figure 26).

In critical areas, engineered fills that utilize reinforcing geotextiles or other internal supports can be constructed with nearly vertical faces.

These are especially useful in short road sections where other fills would be unstable or erode and sediment could then enter a watercourse. In such cases, it may be neces-sary or prudent to employ a qualified, experi-enced geotechnical engineer or geologist to design a stable cut and fill road. Depending on the stability of the cut slope rock and soil materials, it may be simpler and cheaper to construct a full bench road where all the exca-vated material is simply endhauled off-site and deposited in a stable storage site.

2. ROAD SURFACE

(SHAPE) DESIGN

Road surface design is really road surface drainage design, and should be chosen based on both maintaining safety for the intended uses, protecting the integ-rity of the road, and minimizing erosion and sediment pollution in streams. All three design standards should be met. Road surfaces can be designed as outsloped, crowned, or insloped (Figure 27). Often, more than one of these road surface designs is used along the road length. A road should never be graded with a flat road shape since this has no drainage. A flat road shape that does not drain to one side or the other is prone to puddling and pot holes in areas of no road grade, or to ruts and surface erosion if it is sloping up or down a hill.

Flat, poorly drained roads often require a high level and frequency of maintenance.

Outsloped roads are considered the best and most preferred road shape for most circumstances. Insloped and crowned roads require inside ditches, and ditches generally require regular maintenance. In addition to construction costs (ditched roads require con-siderably more excavation and construction costs—see Table 15), it is important to consider long term maintenance requirements and costs when deciding whether to construct an out-sloped road or an insloped/crowned road.

FIGURE 26. Over-loading steep slopes with uncompacted side-cast material can result in landslid-ing that damages streams hundreds of feet downslope (Modified from:

BCMF, 1991).

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FIGURE 27. Road surface shapes include outsloped, insloped and crowned. The diagram depicts an outsloped road with no ditch (top), an insloped road with the inside ditch (center), and a crowned road with an inside ditch (bottom).

Outsloped road shapes are gen-erally preferred because of lower construction and maintenance costs. Where cutbanks are wet with spring flow an outsloped road shape can be combined with an inside ditch. Note that insloped and crowned roads generally require more hillslope cutting and have higher cutbanks than outsloped roads because of the extra width needed for a ditch (Modified from:

Adams and Storm, 2011).

Road shaping for proper drainage is not an all-or-nothing proposition. For example, roads which contour the landscape may alternate from outsloped to insloped as the road tra-verses the hillside. Roads that are outsloped for much of their length may also be locally insloped to deal with local conditions (e.g., a sharp outside curve). While some wet cutbanks may require the construction of an inside ditch (or French drain) for drainage, the roadbed itself may still be a worthy candidate for outsloping. Ultimately, it is critical to properly design road surfaces to minimize erosion of the roadbed, ditch, cutbank and fill slope surfaces, while minimiz-ing sediment delivery to streams.

a. Outsloped roads, with or without an inside ditch

It is generally recommended that most forest, ranch and rural roads be con-structed as single lane (minimum width), outsloped roads with minimal cut-and-fill, wherever conditions are suitable.

Intervisible turnouts can be provided to allow passing. An outsloped road cross section is likely to cause the least disturbance and soil movement, create less environmental impact and have lower maintenance costs than other designs. Outsloped roads disperse and drain runoff along the entire outside edge of the road (Figure 28). They are less expensive to construct and less difficult and expensive to maintain than insloped roads, provided they are constructed in appropriate hillslope locations.

If hillslopes are dry, and cutbanks along existing roads display little or no evidence of emergent water (springs or seeps) during the wet season, there is no reason to construct or maintain an inboard ditch along a road. Analyzing cutbank and hillslope hydrologic characteris-tics will allow you to determine whether an inside ditch is necessary along your road.

Roads built wherever the surface can be kept dry and free draining should generally be outsloped to disperse runoff. Conditions that might limit road outsloping include: 1) steep road grades (≥20%) which may make adequate outsloping difficult; 2) winter use of

FIGURE 28. Well built, outsloped road displaying minimum cut, smooth free drain-ing surface, and no outside berm.

The road contours the topography and its rolling grade and rolling dips disperse sur-face runoff.

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an unsurfaced road (snow or muddy conditions on a steep, outsloped road may be hazardous);

or 3) upslope runoff or excessive spring-flow from the cutbank or roadbed (which might make an inside drainage ditch necessary).

However, many ditched roads are also candidates for surface outsloping, thereby draining surface runoff to the outside and not into the ditch. The inside ditch will carry relatively clear water flows from seeps and springs, while the outsloped road surface ensures that turbid road runoff and fine sediment eroded from the roadbed will be drained to the outside edge of the road where it can be safely discharged into vegeta-tion and onto undisturbed soils. Outsloping thereby minimizes flows in the inside ditch and reduces the potential for erosion and sediment delivery from the road surface.

Clearly, if conditions permit, roads should be constructed with an outsloped surface, no ditch and no berms along the outside edge of the road. If berms are needed for safety, they should be frequently breached along their length to allow for dispersed road surface drainage. Table 18 shows design criteria for the degree of outslop-ing needed to drain road surfaces on differ-ing grades. The design of outsloped road surfaces, especially on steeper road grades, should also consider safety where the road surface may be slippery (e.g., in rain or snow conditions) during parts of the year.

Where fill slopes are stable, roads should be designed and constructed with minimum width and with a mild, 3 to 5% outslope (Figure 29). However, on most roads, especially those with grades in excess of eight percent (8%), outsloping is not always enough to get surface runoff out of wheel ruts and off the road quickly. Here, in addition to outslop-ing, waterbars (for seasonal or temporary roads) or rolling dips (for permanent and seasonal roads) are necessary to divert surface runoff off outsloped roads.

Where an outsloped road turns on an outside curve the outslope is frequently tapered to a flat or insloped road shape, depending on the direction of the turn, and then back to an outslope in the following straight stretch.

These safety measures are commonly called super-inslope (turning around a ridge) or super-outslope (turning around a depres-sion or swale). The shape of the road keeps vehicles slightly “banking” around turns in the road and allows them to maintain constant speeds without increasing the risk of skidding off the turning road. The short sections of alternating road shape also allow for dispersed road surface drainage.

TABLE 18. Outsloping “pitch” for roads up to 8% grade1

Road grade Outslope “pitch” for unsurfaced roads

Outslope “pitch” for surfaced roads

≤ 4% 3/8” per foot 1/2” per foot

5% 1/2” per foot 5/8” per foot

6% 5/8” per foot 3/4” per foot

7% 3/4” per foot 7/8” per foot

≥ 8% 1” per foot 1 ¼” per foot

1California Department of Forestry and Fire Protection (2008)

b. Crowned roads

A crowned road surface is one which tradition-ally slopes gently away from the centerline of the road and drains to both sides of the crown (Figure 30). Crowning is most commonly used where roads are wide enough for two lane traffic.

Crowning may also be employed for safety purposes to keep traffic separated and where road grades are steep or snow is common; the crowned shape helps keep vehicles from sliding off the road.

The inside portion of a crowned road drains inward to the cutbank and ditch, while the outside portion drains out across the fill slope, thereby reducing the volume of road surface runoff (and fine sediment) that flows into the inside ditch. Crowning can be peaked at the center of the road, essentially dividing road surface drainage in half, or offset from the center so that more of the road surface drains to one side or the other. For example, crowning at the inside 1/3 of the road surface results in most

FIGURE 29. Road shape changes as the road trav-els through the landscape. For example, an out-sloped road will have a steep or “banked” outslope through inside curves, a con-sistent outslope through straight reaches and a flat or slightly insloped shape as it goes through an outside curve. The road may have an outslope of 2-3% across the travel surface while the shoulder is more steeply outsloped to ensure runoff and sediment will leave the roadbed.

FIGURE 30.

Crowned roads are peaked near the center of the roadbed and each side of the road surface drains to a ditch. This stable, crowned road reach is through cut into the hill and the ditches are drained fre-quently to prevent ditch erosion.

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surface runoff and sediment being discharged to the outside of the road bench, much like an outsloped road. Regardless of the location of the crown, an inside ditch is still required.

On rock and native surfaced roads crowning requires frequent maintenance grading and/ or the use of rolling dips to drain water off the road before it can produce ruts (Figures 31a, 31b). The recommended cross-slope is between 4% and 6%, or ½” to ¾” of fall per hori-zontal foot of width. The steeper cross-slope means less potential for water to penetrate and weaken the road and, therefore, longer intervals between maintenance grading operations.

c. Insloped roads

Insloped roads should be constructed only where road surface drainage needs to be kept off the fill slope (because it is unstable or the road is located right next to a stream), or where outsloping would create unsafe driving conditions. Insloped roads drain surface runoff to the inside of the roadbed, usually into a ditch, where it is combined with flow from the cut

FIGURE 31B. Centerline crown has become misshapen over time. Road surface drainage is trapped on the road by gravel movement, grader berms or by road rutting under heavy traffic. Road drainage travels down the rutted road surface causing erosion, loss of road material, and increased maintenance. The road can be reshaped to restore cross drainage, but rolling dips are needed for effective, permanent surface drainage (Center for Dirt and Gravel Road Studies, 2005).

Windrow of displaced material

Windrow of displaced material

Water confined to road surface

Water confined to road surface

FIGURE 31A. Centerline crown with proper cross-slope for an unpaved road. Road drainage flows without obstruction off the road surface into surrounding vegetation. Note the slight grade increase at the road shoulder to encourage off-road drainage (Center for Dirt and Gravel Road Studies, 2005).

Water flows off road surface slope and upslope hillside areas and discharged to nearby stream crossing culverts or ditch relief culverts (Figure 32). The ditch causes the water table beneath the road to be lowered as it flows out of the soil and into the ditch.

Depending on the type of traffic and the road surfacing materials, roads steeper than about 8 to 10 percent may be too steep for safely outsloping. In this setting, insloping with a ditch may be necessary, although the potential for gullying in the ditch increases with road grade. Inside ditches should be drained at intervals sufficient to prevent ditch erosion and outlet gullying, and at locations where water and sediment can be filtered before reaching a watercourse. “Filtering” can be accomplished by thick vegetation, gentle slopes, constructed settling basins, or filter windrows of woody debris and mulch placed and secured on the slope.

As with outsloped roads, steep insloped road surfaces may be difficult to quickly drain. Rolling dips (for permanent roads and seasonal roads) or waterbars (for seasonal or temporary, unsur-faced roads) should be constructed at frequent intervals sufficient to disperse road surface runoff from steep road segments (Tables 3 and 19). Ditches and culverts need occasional maintenance to operate correctly and to carry the flows they were designed to handle. When

FIGURE 32. This insloped ranch road drains to an inside ditch that shows evidence of past downcutting.

Inside ditches need to be drained with ditch relief culverts frequently enough to prevent ditch erosion, as well as erosion of the slope where the culvert is discharged.

TABLE 19. Recommended maximum rolling dip and ditch relief culvert spacing, in feet, based on road gradient and soil erodibility 1, 2

Soil erodibility Road gradient (%) and drainage structure spacing (feet)

0–3 4–6 7–9 10–12 >12

High to moderate 250 160 130 115 100

Low 400 300 250 200 160 1Based on Keller and Sherar, 2003. Also suggested by California Board of Forestry and Fire Protection in Technical Rule Addendum No. 5 (see Appendix C).

2Table distances are designed to prevent ditch erosion, not to eliminate hydrologic connectivity. If road surface drainage is hydrologically con-nected to a stream crossing, install first a rolling dip and/or ditch relief culvert close to the crossing, but such that it drains onto the fill slope or hillslope and will not deliver runoff to the watercourse. The next (second) drainage structure should be placed so that it too will not discharge to the stream. Add additional drainage relief treatments along the road according to the approximate spacing recommended in this table.

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ditches become blocked by cutbank slumps, they need to be cleaned. However, exces-sive maintenance of ditches (mostly grading) can cause continuing and persistent erosion, sediment transport and sediment pollu-tion to local streams during storm runoff.

3. ROAD DRAINAGE

STRUCTURES

Road drainage structures include those features of a road, other than road shape, designed to drain road surface and cutbank runoff off or away from the road prism. Road drainage structures include rolling dips, water-bars, drainage berms, ditches and ditch relief culverts (Table 20). The purpose of all drainage structures is to get water off of, and away from, the roadbed as quickly as possible so roadbed materials do not become saturated, and roadbed/ditch erosion is minimized.

a. Rolling dips

Rolling dips and a smooth, sloped road surface are critical to maintaining a well-drained, out-sloped road. On climbing (or falling) roads, especially on outsloped road shapes, the road surface can be drained using rolling dips or waterbars. Unlike abrupt waterbars, rolling dips should be able to be driven at prevailing speeds on the road where they are installed.

Rolling dips are smooth, angled depressions constructed in the roadbed (Figures 33a, 33b).

Typical design dimensions for rolling dips are shown in Table 21. It is important to use rolling dips, rather than waterbars, on roads with even infrequent use because traffic will quickly break down and/or breach

TABLE 20. Comparison of drainage structures used on dirt and gravel roads

Structure type Ditch relief culverts Rolling dips Water bars Cross road drains

Purpose Drains the road’s inside ditch

Drains the road surface;

Only drains the ditch if dip is deep and intersects the ditch

Drains the road surface

Drains road surface, ditch and springs on decommissioned or closed roads

Construction costs

High Medium Low Low to Medium

Maintenance

Medium Low High None

Needs frequent inspection and inlet cleaning

Needs occasional repair or reshaping

Needs frequent cleaning, reshaping and replacement

Should not need any maintenance

When to use

On all road grades On low and moderate grades

On all road grades On all closed or decommissioned roads, especially at springs and seeps

On high or low traffic roads with frequent maintenance

On high or low traffic roads

On low traffic roads or seasonal roads

When not to use

On infrequently maintained roads; or wherever they would discharge to streams or onto unstable areas

On steep grades (>12% to 18%), depending on traffic type

On high traffic roads

Where the cross road drain would feed water onto an unstable area or deliver eroded sediment to a streamBelow unstable or raveling cut slopes On curves waterbars. Waterbars should be reserved for unsurfaced seasonal roads that are to have little traffic and/or no wet season use.

Rolling dips are usually used on outsloped roads to drain road surface runoff to the outside of the road, but may be built on either insloped, crowned or outsloped roads to drain runoff in either direction. However, keep in mind the goal of effective road drainage is to disperse rather than collect and concentrate road runoff.

Drainage structures that drain to the inside of the road will likely require a greater number of ditch relief culverts to prevent ditch erosion and/or the formation of hillslope gullies.

Rolling dip design—In general, broad rolling-dips are usually built perpendicular to the road alignment, with a cross slope of 3 to 5 percent greater than the grade of the road.

FIGURE 33B.

This side view of an outsloped road shows that the rolling dip does not have to be deep or abrupt to reverse road grade and effectively drain the road surface. This out-sloped forest road has rolling dips that allow all traf-fic types to travel the route without changing speed.

FIGURE 33A.

Rolling dip con-structed on a rock surfaced rural road.

The rolling dip rep-resents a change-in-grade along the road alignment and acts to discharge water that has collected on, or is flowing down, the road surface. This road was recently converted from a high maintenance, insloped, ditched road to a low main-tenance, outsloped road with rolling dips.

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The cross grade slope ensures proper drainage to the outside of the dip. If the outslope in the axis of the rolling dip is insufficient, water will not drain, sediment will be deposited, and puddles and potholes will form. The morphology of the dip results in an up-and-down or slight rolling movement when driven. Some rolling dips are built at a 30 to 45 degree angle to the road alignment, but if the road is to receive com-mercial truck and trailer traffic (e.g., log trucks or cattle trailers) this angle can cause a signifi-cant rocking and twisting action to heavy truck loads and trailers that may not be acceptable.

Rolling dips are built with a long, shallow approach on their up-road side and a more abrupt rise or reverse grade on their down-road side (Figure 34, Table 21). Dips should be constructed deep enough into the road subgrade so that traffic and subsequent road grading will not obliterate them. Their length and depth should provide the needed drainage, but not be a driving hazard (Figure 35).

Rolling dips can be broken down into three types, depending on the existing road gradient and conditions of the outboard edge of the road. Figure 36 provides the general design characteristics of the three rolling dip types.

■ A Type 1 rolling dip is the standard rolling dip design for roads that do not have a through cut or large berm that would prevent the dip from draining onto the adjacent outboard fill slope. Type 1 rolling dips are built on roads with road gradients less than 12–14%, and with or without a small outboard berm that can be easily removed. If an outboard berm is present make sure to remove the berm through the entire length of the dip.

■ Type 2 rolling dips are designed for roads with gradients less than 12–14% within a small through cut, or that have a large (i.e. tall and/ or wide) berm on the outboard edge of the road. This type of dip requires “breaching” or excavating the outboard through cut or large berm through the axis of the dip. The width of the breach is dependent on the road conditions (e.g., width of berm, road steepness, and road subgrade materials).

■ Type 3 rolling dips are suggested for roads with gradients that exceed 12–14% where road steepness prevents the construction of a rolling dip with a reverse grade. Instead of building a dip with a reverse grade, a Type 3 rolling dip is constructed by building an aggressive 6–8% outslope from the inboard to the outboard edge of road to ensure that runoff travels obliquely across the road and exits the road within the rolling outslope.

This outslope is developed by ripping the roadbed and pushing road fill from the outboard half to the inner half of the road.

TABLE 21. Table of rolling dip dimensions1

Road grade

Upslope approach2

(distance from up-road start of rolling dip to trough) (ft)

Reverse grade2

(distance from trough to crest) (ft)

Depth below average road grade at discharge end of trough2 (ft)

Depth below average road grade at upslope end of trough2 (ft)

<6 55 15–20 0.9 0.3

8 65 15–20 1.0 0.2

10 75 15–20 1.1 0.1

12 85 20–25 1.2 0.1

>12 100 20–25 1.3 0.1

1USDA-SCS (1981)

2See also Figure 36

FIGURE 35. This outsloped forest road is used by commercial log-ging trucks and was constructed with frequent rolling dips to pro-mote road surface drainage. The dips were built as a part of planned road construction for use by truck and trailer traffic.

Note that the cut-banks are rocky, dry and stable, and there is no inside ditch.

FIGURE 34. A classic Type I rolling dip, where the excavated up-road approach (B) to the rolling dip is several percent steeper than the approaching road and extends for 60 to 80 feet to the dip axis. The lower side of the structure reverses grade (A) over approximately 15 feet or more, and then falls down to rejoin the original road grade. The dip must be deep enough that it is not obliterated by normal grading, but not so deep that it is difficult to negotiate or a hazard to normal traffic. The outward cross-slope of the dip axis should be 3% to 5% greater than the up-road grade (B) so it will drain properly. The dip axis should be out-sloped sufficiently to be self-cleaning, without triggering excessive downcutting or sediment deposition in the dip axis (Modified from: Best, 2013).

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FIGURE 36. Rolling dip types

Type 1 Rolling Dip (Standard)

Type 2 Rolling Dip (Through-cut or thick berm road reaches)

Type 3 Rolling Dip (Steep road grade)

Type 1 rolling dips are used where road grades are less than about 12-14% and road runoff is not confined by a large through cut or berm. The axis of the dip should be perpendicular to the road alignment and sloped at 3-4% across the road tread. Steep roads will have longer and more abrupt dip dimensions to develop reverse grade through the dip axis. The road tread and/or the dip outlet can be rocked to protect against erosion, if needed.

Type 2 rolling dips are constructed on roads up to 12-14% grade where there is a through cut up to 3 feet tall, or a wide or tall berm that otherwise blocks road drainage. The berm or native through cut material should be removed for the length of the dip, or at least through the axis of the dip, to the extent needed to provide for uninterrupted drainage onto the adjacent slope. The berm and slope material can be excavated and endhauled, or the material can be sidecast onto native slopes up to 45%, provided it will not enter a stream.

Type 3 rolling dips are utilized where road grades are steeper than about 12% and it is not feasible to develop a reverse grade that will also allow passage of the design vehicle (steep road grades require more abrupt grade reversals that some vehicles may not be able to traverse without bottoming out).

Instead of relying on the dip’s grade reversal to turn runoff off the roadbed, the road is built with an exaggerated outslope of 6-8% across the dip axis. Road runoff is deflected obliquely across the dip axis and is shed off the outsloped section rather than continuing down the steep road grade.

Rolling dip spacing—The frequency or “spacing” of rolling dips and grade breaks (Figure 37), and the amount of “outslop-ing” needed to drain the road surface, depends on the grade of the road, as well as the road surfacing (Table 19). To design drainage structure spacing, it is useful to look at local roads to determine the maximum spacing that is likely to work for the soils and climate in your specific area.

Example design criteria for drainage struc-ture spacing to minimize road surface and ditch erosion (waterbars and rolling dips) are listed in Table 3 and, alternately, Table 19 (rolling dips and ditch relief culverts).

Drainage tables provide guidance and are common in the literature, but local observa-tions are key to determining the most appro-priate spacing in your particular area. In general, the spacing of road drainage struc-tures is appropriate when you can observe minor rilling (incision) on the road fill slope where the road runoff is occurring, but hill-slope rills and gullies are absent or do not extend continuously on native slopes below the drainage structure outlet (Figure 38).

The basic spacing guidance of Table 19 must be tempered by the proximity of the discharge points to streams and other waterbodies, and considering factors that might increase the probability of runoff and sediment being transported to the stream, lake or wetland. Those things that are likely to increase the probability of sediment delivery include such factors as: a short distance to the nearest stream or water body, steep slopes, unstable terrain, the presence of gullies or channels that could collect and efficiently transport road runoff and sediment, bare soils or low vegetation density, and shallow or clay rich soils with low infiltration rates. To account for hydrologic connectivity in the suggested drainage spacing tables you must signifi-cantly and progressively reduce the spacing of drainage structures as you get closer to streams and other waters, accounting for the proximity of stream or lake, slope steep-ness and the other contributing factors.

b. Waterbars, rubber waterbars and open top box culverts

Waterbars can also be used to drain a road surface. These are shallow, abrupt, excavated dips or troughs with an adjacent, downslope hump or mounded berm that are built at an oblique angle across the road (Figure 39). To maintain the greatest effectiveness, the axis of the waterbar (including where it drains onto the adjacent hillslope) should be con-structed at a gradient slightly steeper than the road gradient it is intended to drain.

This prevents deposition within or at the outlet of the structure and maintains flow and sediment transport along its length.

Waterbars are useful only on low standard seasonal or temporary, unsurfaced roads where winter or wet season use will not occur, because traffic easily cuts through the soft berm and fills the adjacent dip.

Waterbars should be constructed at proper

FIGURE 37.

Installing rolling dips, or “rolling the grade” of an outsloped road helps guarantee that surface runoff will not concen-trate on the road surface and erode the roadbed, and that road runoff will be dispersed across the hillside.

Spacing of rolling dips depends on road grade, soil erodibility and proximity to the nearest water body (Kramer, 2001).

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spacing according to the grade of the road (Figure 40; Tables 3 and 19). Waterbars are usually regraded (smoothed out) at the beginning of each operating season in which the road is to be used and opened to traffic, and then reconstructed prior to the begin-ning of each winter or wet season period.

Waterbars are high maintenance drainage struc-tures that are prone to failure if not properly built and maintained. Unauthorized winter traffic is likely to break down waterbars and result in serious road surface erosion and water pollution. Roads that are drained with waterbars should be restricted from most traffic, especially during the wet season when soils are softest.

On seasonal rocked roads and roads where waterbars cannot be built and maintained each year, thick rubber flaps or “rubber water-bars” are occasionally constructed into the roadbed. The rubber waterbar is most useful where frequent road grading is not necessary but the road surface needs better drainage.

These drainage diversion devices are some-times made of thick rubber strips or salvaged conveyor belt fabrics, and are dug at least 12

FIGURE 38. Roll-ing dips should be spaced on outsloped roads so that the road surface is well drained and free from erosion, and the slopes below each dip show minimal erosion.

Three broad roll-ing dips (see arrows) are visible in this upgraded road reach used by both commer-cial and residen-tial traffic.

FIGURE 39.

Waterbars are often used to drain sur-face runoff from seasonal, unsurfaced roads. Because they are easily broken down by vehicles, waterbars are only used on unsurfaced roads where there is little or no wet weather traffic. In this photo, a waterbar and ditch relief cul-vert are used to drain all road surface and ditch runoff from the insloped road prism.

inches into, and anchored in, the roadbed at an angle oblique to the road alignment, much like a waterbar. They stick up about 4 inches above the running surface and divert surface runoff to the side of the road. The flap bends down as vehicles pass over the waterbar and then immediately springs back to deflect runoff.

Unlike waterbars, vehicles can drive over the flap without having to slow down; it folds over and pops back up when the vehicle passes.

The main shortcomings include the labor inten-sive installation required to build each diversion device, and the difficulty of grading the road surface that contains frequent rubber waterbars.

Open top box culverts (usually made of wood or metal) can also be used to drain the road surface, but they often fill with soil and rock, are difficult to grade over, and usually require higher levels of maintenance to keep open and functional. They should have a relatively steep grade so they self-clean during runoff events, and are often fitted with a surface grate on top to prevent large rocks from entering the top of the culvert and obstructing flow.

Like waterbars, and for maximum effective-ness and minimal maintenance, these less common road drainage structures should be constructed obliquely across the road such that their slope is slightly greater than the grade of the road they are draining.

c. Drainage berms

Road berms are generally defined as a continuous row of fill and/or aggregate, FIGURE 40. Waterbars are constructed on unsurfaced forest and ranch roads that will have little or no traffic during the wet season. The waterbar should be extended to the cutbank to intercept all ditch flow (1) and extend beyond the shoulder of the road. A berm (2) must block and prevent ditch flow from continuing down the road during flood flows. The excavated waterbar

(3) should be constructed to be self-cleaning, typically with a 30° skew to the road alignment with the excavated material bermed on the downhill grade of the road (4). Water should always be discharged onto the downhill side on a stable slope protected by vegetation.

Rock (shown in the figure) should not be necessary if waterbars are spaced close enough to prevent serious ero-sion.