Chapter7

Chapter 7

Drainage of Low-Volume

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drainage, drainage, and drainage”

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OAD LOCATION and drainage of roads, rainy periods to see how the water is actually moving, areas, and other areas of activity where it is concentrated, what damage it may cause, Rare the most significant factors that can affect and what measures are needed to prevent damage

water quality, , and road costs. Drainage and keep the drainage systems functioning properly.

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olume R olume olume R olume includes controlling surface water and adequately passing water under roads in natural channels. ROADWAY SURFACE DRAINAGE CONTROL Drainage issues that must be addressed in road design The roadway surface needs to be shaped to dis- and construction include roadway surface drainage; perse water and move it off the road quickly and as control of water in and

at pipe inlets/outlets;

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oads oads crossings of natural channels oads and streams; wet area crossings; subsurface drainage; and selection and design of (Chapter 8), low water crossings (Chapter 9), and (Chapter 10). Three of the most important aspects of road design are drainage, drainage, and drainage!

Adequate road drainage requires careful attention to detail. Drainage conditions and patterns must be studied on the ground. Drainage Photo 7.1 Design roads that move water rapidly off the surface of the should be observed during road and minimize water concentration with the use of rolling grades and outsloped, insloped, or crowned roads. LOW-VOLUME ROADS BMPS : 53 difficult to drain and can feel un- safe. 3-5% 3-5% Insloped roads best control 2:1 the road surface water but con- 2:1 centrate water and thus require a Crown Section system of ditches, cross-drains, and extra road width for the . Cross-drains, using either rolling 1:1 dips (broad-based dips) or Brush 3-5% 2:1 pipes, must be spaced frequently Outslope Section enough to remove all the expected road surface water before erosion 1:1 occurs. The maximum recom- 3-5% 2:1 mended distances (listed in Table 2:1 Armored 7.1) should be used for guidance Inslope with Ditch Section Ditch on spacing of cross-drains and ditch relief structures. Specific lo- cations should be determined in Figure 7.1 Typical road surface drainage options. the field based upon actual water frequently as possible (Photo need for an inside ditch, and mini- flow patterns, rainfall intensity, 7.1). Water standing in potholes, mizes costs. Outsloped roads road surface erosion characteris- ruts and sags will weaken the with rich, slippery road sur- tics, and available erosion resis- subgrade and accelerate damage. face materials often require tant outlet areas. Water concentrated in ruts or kept surface stabilization or limited use on the road surface for long dis- during rainy periods to assure traf- Crown section roads are ap- tances can accelerate erosion as fic safety. On road grades over 10 propriate for higher standard, two well as wash off the surface ma- to 12 percent and on steep hill lane roads on gentle grades. They terial. Steep road grades cause slope areas, outsloped roads are also require a system of inside surface and ditch water to move rapidly, and make surface drain- age difficult to control. Steep grades accelerate erosion unless surfaces are armored or water is dispersed or removed frequently.

Roadway surface water should be controlled with positive drainage measures using outsloped, insloped, or crown sections of road, as shown in Fig- ure 7.1. Outsloped roads best disperse water and minimize road width, but may require roadway surface and fill slope stabilization. An outsloped road minimizes Photo 7.2 Use rolling dip (broad-based dip) cross-drains to move concentration of water, minimizes water off the road surface efficiently and economically, without the needed road width, avoids the use of culvert pipes. Rolling dip cross-drains are not susceptible to plugging.

LOW-VOLUME ROADS BMPS:54 Table 7.1 ditches and cross drains. It is dif- Recommended Maximum Distance Between Rolling Dip ficult to create and maintain a or Culvert Cross-Drains (meters) crown on a narrow road, so gen- erally insloped or outsloped road Low to drainage is more effective for ru- Road Grade % Non-Erosive (1) Erosive Soils (2) ral roads.

0-3 120 75 Culvert cross-drains are used to move ditch water across the 4-6 90 50 road. They are the most common type of road surface drainage, and 7-9 75 40 are most appropriate for high- standard roads where a smooth 10-12 60 35 road surface profile is desired. However the pipes are expensive, 12+ 50 30 and the relatively small culvert pipes used for cross-drains are suseptible to plugging and require cleaning.

Table 7.2 Rolling dip cross-drains Recommended Water Bar Spacing (meters) (broad-based dips) are designed to pass slow traffic, while also dis- Road/ Low to persing surface water (Photo 7.2). Grade % Non-Erosive soils (1) Erosive Soils (2) Rolling dips usually cost less, re- quire less maintenance, and are 0-5 75 40 less likely to plug and fail than culvert pipes. Rolling dips are 6-10 60 30 ideal on low volume, low to mod- erate speed roads (20-50 kph). 11-15 45 20 Spacing is a function of road grade and type, as seen in 16-20 35 15 Table 7.1. Other types of roadway surface cross-drain structures oc- 21-30 30 12 casionally used include open top wood or metal flumes, and rub- 30+ 15 10 ber water deflectors.

Steep road grades are unde- Note: (1) Low Erosion Soils = Coarse Rocky Soils, , and sirable and problematic, but oc- Some Clay casionally necessary. On grades up (2) High Erosion Soils = Fine, Friable Soils, Silt, Fine to 10%, cross-drains with culverts Sands or rolling dips are easy to use. Be- tween 10 and 15%, frequently Adapted from Packer and Christensen (1964) spaced culvert cross-drains work, & Copstead, Johansen, and Moll (1998) often in conjunction with armored ditches. On grades over 15%, it is difficult to slow down the wa-

LOW-VOLUME ROADS BMPS : 55 RECOMMENDED PRACTICES

ROADWAY SURFACE frequently enough to remove inside ditch and moderately DRAINAGE CONTROL all surface water. Table 7.1 fast vehicle speeds. • Design and construct roads gives recommended cross- so that they will move water drain spacing. • Construct water bars on rapidly off the road surface infrequently used roads or to keep the surface drained • Protect cross-drain outlets closed roads to control and structurally sound. with rock (riprap), brush, or . Construct logging slash to dissipate frequently spaced waterbars • Avoid steep road grades in energy and prevent erosion, angled at 0-25 degrees with excess of 12 to 18%. It is or locate the outlet of cross an outslope of 3-5% and a very difficult and expensive drains on stable, non-erosive depth of 0.3 to 0.6 meters. to properly control drainage soils, rock, or in well veg- Install water bars as shown on steep grades. etated areas (Figure 7.2b). in Figure 7.5. Spacing of waterbars is shown in Table 7.2. • Maintain positive surface • Construct rolling dips drainage with an outsloped, rather than culvert cross- Use catch water ditches insloped, or crown roadway drains for typical, low- • section using 3 - 5 % cross volume, low speed roads (intercept ditches) across the slopes (up to 5% is best) with grades less than 12%. natural ground above a cut (Figure 7.1). Construct rolling dips deep slope only in areas with high enough to provide adequate intensity rainfall and over- land flow. These ditches are • Roll grades or undulate the drainage, angled 0-25 useful to capture overland road profile frequently to degrees from perpendicular sheet flow before it pours disperse water, particularly to the road, with a 3-5% over the cut slope and into and out of stream outslope, and long enough erodes or destabilizes the crossings (Figure 7.2a, (15 to 60 meters) to pass cut. However, be aware that Photo 7.1). vehicles and equipment (See Photo 7.2). In soft soils, catch water ditches are that armor the mound and dip are not properly maintained • Use frequently spaced with gravel or rock, as well can become a counter- leadoff ditches (Figure 7.2b as the outlet of the dip productive pool for water and Figure 7.8) to prevent (Figure 7.3). above the slope, increasing accumulation of excessive the probability of a slope water in the roadway • Install culvert cross-drains failure. ditches. with an angle of 0-30 de- grees perpendicular to the • Avoid the use of outside • Use roadway cross-drain road, using an outslope of ditches, along the outside structures (either rolling 2% greater than the ditch edge of the road, except in dips, pipe culverts, or open grade to prevent plugging. specific areas that must be top culverts (flumes)) to (Figure 7.4). (See Chapter protected from sheet flow move water across the road 8 for more information on off the road surface. Prefer- from the inside ditch to the culverts). Use culvert cross- ably, use berms. Note that an slope below the road. Space drains on roads with an outside ditch or berm neces- the cross-drain structures sitates additional road width.

LOW-VOLUME ROADS BMPS:56 PRACTICES TO AVOID abney level, or survey • Long sustained road grades erosive, unprotected soils. equipment to ensure that that concentrate flows. • “Eyeballing” grades in flat you have proper slopes or grades. • Discharging water onto . Use a clinometer,

Figure 7.2

Cut slope

Fill slope

Reinforced dip

Ground slope

a. Basic road surface drainage with outsloping, rolling grades, and reinforced dips.

b. Basic road surface drainage with leadoff ditches and culvert cross-drains exiting into vegetation or a streamside buffer area. (Adapted from Montana State Univ. 1991)

LOW-VOLUME ROADS BMPS : 57 Figure 7.3 Rolling (broad-based) dip cross-drains.

Inslope or Outslope

Mound

Spacing 30 - 150 m

Riprap at Dip o Exit 0-25

Armored Dip

a. Perspective View Dip

Dip Average Road Grade 2-5% Outslope

2-5% Outslope

b. Profile

For Insloped Road – Slope to Depth of Inside Ditch Armor Dip and Mound For Outsloped Road – 3-5 cm Deep Surface as Needed with or Match Depth of Inside Ditch at Road Grade 2-12% 5-15 cm Aggregate Reverse Slope Entrance – 15-30 cm Deep at Exit 3-6%

Average Road Grade

8-30m 7-12m 8-30m

c. Rolling Dip Profile Detail

LOW-VOLUME ROADS BMPS:58 Figure 7.4 Culvert cross-drains.

Inlet Structure as Needed

Spacing 30–150 m

Berm o 0o -30

Place Outlet Pipe at Natural Ground Level or Riprap Armor the Fill Material.

Figure 7.5 Water bar construction. (Adapted from Wisonsin’s Forestry Best Management Practices for Water Quality. 1995, Publication FR093, Wisconsin Department of Natural Resources)

Berm Tied into Embankment Spacing 10–75 m

o

0-25

Exit onto Stable or Armored Ground

a. Perspective View

Road Grade

30-60 cm 30-60 cm ○○○○○○○○○○○○○○○○○○○

1 - 2 m 1 - 2 m 1 - 2 m

b. Cross-Section

LOW-VOLUME ROADS BMPS : 59 CONTROL AT INLETS AND OUTLETS OF CROSS-DRAINS AND DITCHES Water should be controlled, directed, or have energy dissi- pated at the inlet and outlet of culverts, rolling dips, or other cross-drainage structures. This can ensure that water and debris enters the cross-drain efficiently without plugging, and that it ex- its the cross-drain without - aging the structure or causing ero- sion at the outlet.

Culvert inlet structures (drop Photo 7.3 Use masonry, , or metal inlet structures to inlets) are usually placed in the in- control water in the ditch, direct the water into the cross-drain pipe, side ditchline at the location of a and prevent ditch down-cutting. culvert cross-drain. They are com- monly constructed of concrete, ter or remove it from the road Water bars are used to con- masonry (Photo 7.3), or from surface rapidly. On such steep trol drainage on closed or inac- round metal pipe, as seen in Fig- grades, it is best to use frequently tive roads, 4-wheel drive roads, ure 7.6. They are typically used spaced cross-drain culverts, with skid roads, and skid . Water where the ditch is eroding and armored ditches. Also, the road bars are frequently spaced (see downcutting, so that the structure surface will need armoring or sur- Table 7.2) for maximum erosion controls the ditch elevation. Inlet facing with some form of pave- control and can be shaped to pass structures are also useful to ment to resist erosion. For sea- high clearance vehicles or to block change the direction of water sonal or low use roads, interim traffic. flowing in the ditch, particularly drivable waterbars could also be on steep grades, and they can help constructed. stabilize the cut bank behind the pipe inlet.

The outlet of pipes and dips are ideally located in a stable, non- erosive soil area or in a well-veg- etated or rocky area. The accel- erated velocity of water leaving a roadway can cause severe erosion or gullying if discharged directly onto erosive soils (Photo 7.4). The pipe, dip, or drain outlet area can be stabilized, and the energy of the water dissipated, by dis- charging the water onto 1-2 cu- bic meters of a graded rock riprap, as seen in Figure 7.7. Other en- ergy dissipation measures include Photo 7.4 Add outlet protection and/or energy dissipators to pre- the use of stilling basins, rein- vent erosion and the formation of gullies.

LOW-VOLUME ROADS BMPS:60 slow down the velocity of water, as shown in Figure 7.8. Ditches are commonly armored with grass, matting, rock, or masonry /concrete paving (Photo 7.6). Grasses can resist flow ve- locities to 1-2 meters per second. A durable armoring such as graded rock riprap or concrete is recom- mended on grades over 5 percent in erosive soils or for velocities over a few meters per second.

Ditch dikes will prevent ditch erosion, and dikes can serve to catch sediment, but they require Photo 7.5 Protect the outlet of culvert pipe and rolling dip cross- maintenanceneed in that they need drains with riprap or a masonry splash apron, or choose areas of to be periodically cleaned out. bedrock or dense vegetation. Common ditch dike construction forced splash aprons, or use of in erosive soils, and with flow ve- materials include loose rock, ma- dense vegetation or bedrock locities over one meter per sec- sonry, concrete, bamboo, and (Photo 7.5). ond may require armoring or the wooden posts. Each dike structure use of small ditch dike or dam should be keyed into the banks of Ditches on steep road grades, structures placed in the ditch to the ditch and a notch is needed over each structure to keep the RECOMMENDED PRACTICES flow in the middle of the ditch.

CONTROL AT INLETS & OUTLETS 1.0 meters beyond the toe of • When ditch grade control is the fill slope to prevent PRACTICES TO needed, use drop inlet struc- erosion of the fill material. tures with culvert cross-drains AVOID • In erosive soils, armor to prevent ditch down-cutting • Discharging a cross-drain or where space is limited roadway ditches and leadoff pipe or dip onto any unpro- against the cut bank (Figure ditches with rock riprap tected fill slope or barren, 7.6). Alternately, use catch (Photo 7.7), masonry, erosive soil. basins excavated into firm concrete lining or, at a minimum, grasses. Ditch soil. dike structures can also be • Discharging cross-drain • Discharge culverts and cross- used to dissipate energy and pipes mid-height on a fill drain dips at natural ground control ditch erosion. slope. level, on firm, non-erosive (Figure 7.8).

soil or in rocky or brushy • Discharging cross-drain areas. If discharged on the fill • Discharge roadway drains in pipes or dips onto unstable slopes, armor outlets with an area with infiltration natural slopes. riprap or logging slash, or use capability or into filter strips down-drain structures. (Fig- to trap sediment before it ures 7.3, 7.4, 7.7 and Figure reaches a waterway. Keep 8.1). Extend the pipe 0.5 to the road and streams hydro- logically “disconnected.”

LOW-VOLUME ROADS BMPS : 61 Photo 7.6 Armor ditches with vegetation, rock, masonry, or concrete to resist ditch erosion and carry the water to a stable exit point.

Figure 7.6 Typical drop inlet structure types (with culvert cross-drains).

Use drop inlet structure to control the level of water, turn water into the pipe, and prevent downcutting and erosion of the ditch.

Roadbed surface Energy Grass for Dissipation Erosion with Riprap, Control Concrete Apron or Splash pool with water

General Information

Roadbed Surface Slope 45 cm Slope Bottom of ditch

30 cm Window 45 cm

"1 m

Culvert Pipe (30-60 cm diameter)

Drop inlet 30 cm structure Sand trap

0.6-1.2 m 90 cm Masonry Metal Design and Installation Detail

LOW-VOLUME ROADS BMPS:62 Figure 7.7 Culvert outlet protection.

Fill Slope Rocks: 15-50 kg 5% greater than 25 kg

0.5 m minimum 15-30 cm minimum depth 1-2 m Ground Line

Photo 7.7 A rock armored ditch and metal drop inlet to control the water and prevent down-cutting of the ditch.

LOW-VOLUME ROADS BMPS : 63 Figure 7.8 Ditches and ditch armoring.

Roadway Exit ditch in stable, vegetated area

Flow Excavate ditch into firm soil. Armor ditch in erosive soil areas.

a. Ditch Layout and Leadoff (Adapted from Wisconsin’s Forestry Best Management Practices for Water Quality, 1995)

Insloped Roadbed Armor the ditch with rock, masonry or grass

1 m Cut slope

30 cm min.

b. Typical Ditch Armoring and Shape

Cut slope

Roadway

Ditch dikes made of rock or wood to reduce flow velocity

Weir shape to keep flow mid-ditch

c. Use of Ditch Dikes

LOW-VOLUME ROADS BMPS:64 NATURAL STREAM CROSSINGS nel scour, traffic delays, and costly existing undersized pipes, there is Road crossings of natural repairs if a structure fails. Also, a high risk of a culvert pipe plug- drainage channels and streams re- structures can greatly impact fish, ging and the site washing out or quire hydrologic and hydraulic de- as well as other aquatic species, failing. In such areas, or in par- sign expertise to determine the at all stages of life. Stream cross- ticularly sensitive watersheds, proper size and type of structure, ings should be as short as possible overflow protection is desirable. as discussed in Chapters 5 and 6. and cross perpendicular to the A low point in the fill and an ar- Structures for small drainages can channel (Photo 7.8). The road and mored overflow “spillway,” as be sized using Table 8.1. The ditches should be armored, ditches shown in Figures 7.11a & b, will choice of structure includes cul- should divert surface water before protect the fill and keep the flow vert pipes, arch or box culverts, it reaches the stream channel, and in the same drainage, thus reduc- low water fords, or bridges, as construction should minimize the ing diversion potential and usually shown in Figure 7.9. area of disturbance, as shown in preventing a failure. A plugged Figure 7.10. Large drainage pipe that diverts the stream water Because drainage crossings crossings should receive site-spe- down the road can cause a great are at areas of running water, they cific analysis and design input, ide- deal of off-site damage or gully- can be costly to construct and can ally by an experienced hydraulic ing or cause landslides, as seen in have major negative impacts on engineer and other specialists. Figures 7.11c & d. Overflow water quality. Impacts from im- structures should not be used as a proper design or installation of In drainages with uncertain substitute for good hydraulic de- structures can include degraded flow values, large quantities of de- sign, but they can offer “cheap water quality, bank erosion, chan- bris in the channel, or on sites with insurance” against failure at cul- vert crossings.

Figure 7.9 Structural options for crossing natural streams. (Adapted from Ontario Ministry of Natural Resources, 1988)

a. b. Low-Water Crossing

c. Arch Pipe d. Culvert with Single or Multiple Pipes

LOW-VOLUME ROADS BMPS : 65 RECOMMENDED PRACTICES

NATURAL STREAM CROSSINGS grade as practical. Roll yond a large fill to return • Use drainage structures that grades into and out of the water to the drainage and crossing to disperse water. best conform to the natural prevent off-site damage (Figure 7.11). channel and that are as wide • Stabilize disturbed soil as the active stream channel around crossings soon after • Stabilize roadway ap- (bankfull width). Minimize construction. Remove or proaches to bridges, fords, natural channel changes and protect fill material placed in or culvert crossings with the amount of excavation or the channel and floodplain. gravel, rock, or other fill in the channel. suitable material to reduce • Use bridges, low-water fords • Limit construction activity to road surface sediment or improved fords, and large from entering the stream periods of low flow in live arch pipes with natural streams. Minimize use of (Figure 7.12). Install stream bottoms wherever cross-drains on both sides equipment in the stream. possible to maximize flow Stay out of the stream! of a crossing to prevent capacity, minimize the road and ditch runoff from • Design structures and use possibility of a plugged pipe, entering the drainage and minimize impacts on construction practices that channel. aquatic species. minimize impacts on fish and • Construct bridges and other aquatic species or that • Locate crossings where the can enhance fish passage. culvert fills higher than the stream channel is straight, road approach to prevent • Cross drainage channels as stable, and not changing road surface runoff from shape. Bedrock locations are infrequently as possible. draining directly into the desirable for concrete When necessary, cross stream -- but ONLY if structures. streams at right angles likelihood of culvert failure is VERY small. except where prevented by • For overflow protection, (Figure 7.13). Typically, terrain features (Figure construct fills over culverts the crossing should be 7.10). with an armored low point designed to minimize the near the pipe in low fills or • Keep approaches to stream amount of fill. add an armored rolling dip crossings to as gentle a on native ground just be-

PRACTICES TO AVOID

• Working with equipment in • Adversely impacting fisheries an unprotected natural with a stream crossing streambed. structure. • Locating stream crossings • Allowing runoff from road- in sinuous or unstable side ditches to flow directly channels. into streams.

LOW-VOLUME ROADS BMPS:66 Figure 7.10 Natural drainage crossings. Minimize the area of disturbance with a perpendicular stream crossing alignment, and armor the roadway surface.

Poor Stream Crossing Better Stream Crossing

Cross- drain

Road

Road

Cutslopes Large Road cutslope Road area

Crossings near parallel to the drainage cause a Drainage crossings perpendicular to the creek large disturbed area in the channel, streambank, minimize the area of disturbance. Armor the and approach cuts. stream crossing and roadway surface.

Photo 7.8 Avoid natural drainage crossings that are broad and that are not perpendicular to the drainage. Stay out of the stream! This broad channel is a good site for a vented ford.

LOW-VOLUME ROADS BMPS : 67 Figure 7.11

Culvert Installed with Protection using an Armored Overflow Dip to Prevent Washout and Fill Failure

A C D B

(A) Roadway Cross Drain (Dip) (B) Culvert (C) Overflow Protection Dip (D) High point in the road profile

Road Profile Across the Drainage and Dip A D Fill C

B Armored dip

Pipe

a. Overflow dip protection at a fill stream crossing. (Adapted from Weaver and Hagans, 1994)

LOW-VOLUME ROADS BMPS:68 Figure 7.11 (continued)

Good Installation

b. Armored dip over a low fill to prevent stream diversion.

Poor Installation

c. Sketch of a stream diverted down the road, forming a new channel.

Plugged Culvert

Abandoned Channel

Slumping

New Channel in a Gully

d. Consequence of stream diversion out of its natural channel. (Adapted from M. Furniss, 1997)

LOW-VOLUME ROADS BMPS : 69 Figure 7.12 Protection measures at stream crossings.

Ditch Rolling dip or Gravel cross-drain or stone 15-30 m stone Hardened surfacing or gravel stream Riprap (rock) approach bottom

Debris

Armor or stabilize the actual stream crossing (ford), add surfacing to the roadbed, and drain water off the road surface before reaching the crossing. Set stream channel armoring at the elevation of the natural stream bottom.

Figure 7.13 High point over the crossing. (Adapted from Wisconsin’s Forestry Best Management Practices for Water Quality, 1995)

Road

If a plugging failure is unlikely to occur, place fill directly over a culvert higher than the road approach to prevent surface road runoff from draining toward the crossing structure and into the stream.

LOW-VOLUME ROADS BMPS:70 WET AREAS AND MEADOW can provide some reinforcement eas, subsurface drainage will of- CROSSINGS, USE OF UNDERDRAINS strength as well as provide sepa- ten not be effective. Here, either Road crossings in wet areas, ration to keep aggregate or other the roadway platform needs to be including damp meadows, materials from punching into the raised well above the , swamps, high areas, weak subgrade. such as with a turnpike roadway and spring sources are problem- section, or the surfacing thickness atic and undesirable. Wet areas are Subsurface drainage, through design may be based upon wet, ecologically valuable and difficult use of underdrains or aggregate weak subgrade conditions that for road building, logging, or other filter blankets, is commonly used will require a relatively thick operations. Soils in these areas are along a road in localized wet or structural section. A thick aggre- often weak and require consider- spring areas, such as a wet cut gate layer is commonly used, with able subgrade reinforcement. bank with seepage, to specifically the thickness based upon the Drainage measures are expensive remove the groundwater and strength of the soil and anticipated and may have limited effective- keep the roadway subgrade dry. traffic loads. ness. Wet areas should be A typical underdrain design uses avoided! an interceptor trench 1-2 meters deep and backfilled with drain PRACTICES If wet areas must be crossed rock, as shown in Figure 7.16. TO AVOID and cannot be avoided, special Subsurface drainage is typically • Crossing wet areas unnec- drainage or construction methods needed in local wet areas and is essarily. should be used to reduce impacts much more cost-effective than from the crossing. They include adding a thick structural section • Concentrating water flow multiple drainage pipes (Photo to the road or making frequent in meadows or changing 7.9) or coarse permeable rock fill road repairs. Design and filtration the natural surface and to keep the flow dispersed, requirements for underdrains are subsurface flow patterns. subgrade reinforcement with discussed in Chapter 6 and other coarse permeable rock, grade con- references. • Placing culverts below the trol, and the use of filter layers and meadow surface elevation. , as shown in Figure In extensive swamp or wet ar- 7.14. The objective is to maintain the natural groundwater level and flow patterns dispersed across the meadow and, at the same time, provide for a stable, dry roadway surface.

Local wet areas can be tem- porarily crossed, or “bridged” over, using logs, landing mats, tires, aggregate, and so on. (see Figure 7.15). Ideally, the tempo- rary structure will be separated from the wet area with a layer of . The geotextile helps fa- cilitate removal of the temporary material and minimizes damage to the site. Also, a layer of geotextile Photo 7.9 Avoid crossing wet meadow areas. When necessary to cross, use multiple drainage pipes to keep water flow dispersed across the meadow.

LOW-VOLUME ROADS BMPS : 71 Figure 7.14 Wet meadow road crossing options. (From Managing Roads for Wet Meadow Ecostream Recovery by Wm. Zeedyk, 1996)

PERMEABLE FILL WITH CULVERTS (for periodic high flows on flood plains and meadows)

a.

Culverts Berm

Flow Flow

Meadow Roadway

b. Roadway

Berm Geotextile Meadow surface Flow 10-15 cm minus rock

ROCK FILL WITHOUT CULVERTS (for minimal overland flow) c. 15 cm Thick Aggregate Base Course Roadway 15 cm Minus Rock Geotextile Course Placed Approx. 30 cm Thick Flow Flow

LOW-VOLUME ROADS BMPS:72 Figure 7.15 Pole or plastic pipe fords for wet area and bog crossings. Pole fords must be removed immediately after use or before the upstream end becomes clogged with debris and impedes stream flow. (Adapted from Vermont Department of Forests, Parks and Recreation, 1987)

RECOMMENDED PRACTICES

WET AREAS AND MEADOW soil with a filter layer of link fencing or wire under CROSSINGS, UNDERDRAINS geotextile or gravel. the logs can help facilitate • For permanent road cross- removal of the logs. ings of meadows and wet- • For temporary crossing of lands, maintain the natural small, wet drainages or • In spring areas, use drainage patterns swamps, “corduroy” the measures such as by the use of multiple pipes road with layers of logs underdrains or filter blankets set at meadow level to placed perpendicular to the to remove local groundwa- spread out any overland road and capped with a soil ter and keep the road flow (See Photo 7.9). or gravel driving surface. subgrade dry (Figure 7.16, Alternatively, a coarse, PVC pipe, landing mats, Photo 7.10). permeable rock fill can be wood planks, tire mats and used where overland (sur- other materials have also • Use underdrains behind face) flow is minimal (see been used (see Figure 7.15). retaining structures to Figure 7.14). Place a layer of geotextile prevent saturation of the between the saturated soil backfill. Use underdrains or • In areas with local wet spots and logs or other material filter blankets behind fills and limited road use, rein- for additional support and to (embankments) placed over force the roadway with at separate the materials. springs or wet areas to least 10-30 cm of coarse Remove logs from any isolate the fill material and graded rock or a very coarse natural drainage channel prevent saturation and granular soil. Ideally, sepa- before the rainy season (see possible subsequent fill rate the coarse rock and wet Photo 8.8). A layer of chain- failure.

LOW-VOLUME ROADS BMPS : 73 Figure 7.16 Typical road underdrain used to remove subsurface water.

Cut

Roadbed Ditch 15 cm Cap of Impermeable Soil Geotextile Enveloping the Filter Material

Ground Water

Variable Depth (Typically Filter Material, Permeable Sandy +/- 1.5 m Gravel, Well Graded Deep) NOTE: With Geotextile, use clean, coarse gravel. Without Geotextile, use fine, clean sand. Pipe, Perforated 5-10 cm 15 cm dia. (min) 60 cm min.

Photo 7.10 Use subdrains or filter blankets when necessary to remove groundwa- ter from the roadway subgrade in local wet or spring areas. Note that this design needs a second layer of geotextile between the soft subgrade soil and the coarse filter rock to keep the rock clean.

LOW-VOLUME ROADS BMPS:74