chapter 7 JM - International Road FederationLOW-VOLUME ROADS BMP S:56 RECOMMENDED PRACTICES ROADWAY SURFACE DRAINAGE CONTROL • Design and construct roads so that they will move water

53LOW-VOLUME ROADS BMPS :

Chapter 7

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ROAD LOCATION and drainage of roads,construction areas, and other areas of activityare the most significant factors that can affect

water quality, erosion, and road costs. Drainageincludes controlling surface water and adequatelypassing water under roads in natural channels.Drainage issues that must be addressed in road designand construction include roadway surface drainage;control of water in ditches andat pipe inlets/outlets;crossings of natural channelsand streams; wet areacrossings; subsurfacedrainage; and selection anddesign of culverts (Chapter8), low water crossings(Chapter 9), and bridges(Chapter 10). Three of themost important aspects ofroad design are drainage,drainage, and drainage!

Adequate road drainagerequires careful attention todetail. Drainage conditionsand patterns must be studiedon the ground. Drainageshould be observed during

rainy periods to see how the water is actually moving,where it is concentrated, what damage it may cause,and what measures are needed to prevent damageand keep the drainage systems functioning properly.

ROADWAY SURFACE DRAINAGE CONTROL

The roadway surface needs to be shaped to dis-perse water and move it off the road quickly and as

“Three of the most important aspects of road design --drainage, drainage, and drainage”

Photo 7.1 Photo 7.1 Photo 7.1 Photo 7.1 Photo 7.1 Design roads that move water rapidly off the surface of theroad and minimize water concentration with the use of rolling grades andoutsloped, insloped, or crowned roads.

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54LOW-VOLUME ROADS BMPS:

frequently as possible (Photo7.1). Water standing in potholes,ruts and sags will weaken thesubgrade and accelerate damage.Water concentrated in ruts or kepton the road surface for long dis-tances can accelerate erosion aswell as wash off the surface ma-terial. Steep road grades causesurface and ditch water to moverapidly, and make surface drain-age difficult to control. Steepgrades accelerate erosion unlesssurfaces are armored or water isdispersed or removed frequently.

Roadway surface watershould be controlled with positivedrainage measures usingoutsloped, insloped, or crownsections of road, as shown in Fig-ure 7.1. Outsloped roads bestdisperse water and minimize roadwidth, but may require roadwaysurface and fill slope stabilization.An outsloped road minimizesconcentration of water, minimizesneeded road width, avoids the

need for an inside ditch, and mini-mizes costs. Outsloped roadswith clay rich, slippery road sur-face materials often require rocksurface stabilization or limited useduring rainy periods to assure traf-fic safety. On road grades over 10to 12 percent and on steep hillslope areas, outsloped roads are

FFFFFigurigurigurigurigure 7.1 Te 7.1 Te 7.1 Te 7.1 Te 7.1 Typical rypical rypical rypical rypical road surfoad surfoad surfoad surfoad surface drace drace drace drace drainaainaainaainaainaggggge optionse optionse optionse optionse options.....

difficult to drain and can feel un-safe.

Insloped roads best controlthe road surface water but con-centrate water and thus require asystem of ditches, cross-drains,and extra road width for the ditch.Cross-drains, using either rollingdips (broad-based dips) or culvertpipes, must be spaced frequentlyenough to remove all the expectedroad surface water before erosionoccurs. The maximum recom-mended distances (listed in Table7.1) should be used for guidanceon spacing of cross-drains andditch relief structures. Specific lo-cations should be determined inthe field based upon actual waterflow patterns, rainfall intensity,road surface erosion characteris-tics, and available erosion resis-tant outlet areas.

Crown section roads are ap-propriate for higher standard, twolane roads on gentle grades. Theyalso require a system of inside

Crown Section

Outslope Section

Inslope with Ditch Section

Brush

ArmoredDitch

3-5%

2:12:1

2:1

2:12:1

1:1

1:1

3-5%

3-5%

3-5%

Photo 7.2 Photo 7.2 Photo 7.2 Photo 7.2 Photo 7.2 Use rolling dip (broad-based dip) cross-drains to movewater off the road surface efficiently and economically, without theuse of culvert pipes. Rolling dip cross-drains are not susceptible toplugging.


ditches and cross drains. It is dif-ficult to create and maintain acrown on a narrow road, so gen-erally insloped or outsloped roaddrainage is more effective for ru-ral roads.

Culvert cross-drains are usedto move ditch water across theroad. They are the most commontype of road surface drainage, andare most appropriate for high-standard roads where a smoothroad surface profile is desired.However the pipes are expensive,and the relatively small culvertpipes used for cross-drains aresuseptible to plugging and requirecleaning.

Rolling dip cross-drains(broad-based dips) are designedto pass slow traffic, while also dis-persing surface water (Photo 7.2).Rolling dips usually cost less, re-quire less maintenance, and areless likely to plug and fail thanculvert pipes. Rolling dips areideal on low volume, low to mod-erate speed roads (20-50 kph).Spacing is a function of roadgrade and soil type, as seen inTable 7.1. Other types of roadwaysurface cross-drain structures oc-casionally used include open topwood or metal flumes, and rub-ber water deflectors.

Steep road grades are unde-sirable and problematic, but oc-casionally necessary. On grades upto 10%, cross-drains with culvertsor rolling dips are easy to use. Be-tween 10 and 15%, frequentlyspaced culvert cross-drains work,often in conjunction with armoredditches. On grades over 15%, itis difficult to slow down the wa-

Low toRoad Grade % Non-Erosive soils (1) Erosive Soils (2)

0-3 120 75

4-6 90 50

7-9 75 40

10-12 60 35

12+ 50 30

TTTTTaaaaabbbbble 7.1le 7.1le 7.1le 7.1le 7.1

Recommended Maximum Distance Between Rolling Dip or Culvert Cross-Drains (meters)

Road/Trail Low toGrade % Non-Erosive soils (1) Erosive Soils (2)

0-5 75 40

6-10 60 30

11-15 45 20

16-20 35 15

21-30 30 12

30+ 15 10

TTTTTaaaaabbbbble 7.2le 7.2le 7.2le 7.2le 7.2

Recommended Water Bar Spacing (meters)

Note: (1) Low Erosion Soils = Coarse Rocky Soils, Gravel, andSome Clay

(2) High Erosion Soils = Fine, Friable Soils, Silt, FineSands

Adapted from Packer and Christensen (1964)& Copstead, Johansen, and Moll (1998)


RECOMMENDED PRACTICES

ROADWAY SURFACE

DRAINAGE CONTROL

• Design and construct roadsso that they will move waterrapidly off the road surfaceto keep the surface drainedand structurally sound.

• Avoid steep road grades inexcess of 12 to 18%. It isvery difficult and expensiveto properly control drainageon steep grades.

• Maintain positive surfacedrainage with an outsloped,insloped, or crown roadwaysection using 3 - 5 % crossslopes (up to 5% is best)(Figure 7.1).

• Roll grades or undulate theroad profile frequently todisperse water, particularlyinto and out of streamcrossings (Figure 7.2a,Photo 7.1).

• Use frequently spacedleadoff ditches (Figure 7.2band Figure 7.8) to preventaccumulation of excessivewater in the roadwayditches.

• Use roadway cross-drainstructures (either rollingdips, pipe culverts, or opentop culverts (flumes)) tomove water across the roadfrom the inside ditch to theslope below the road. Spacethe cross-drain structures

frequently enough to removeall surface water. Table 7.1gives recommended cross-drain spacing.

• Protect cross-drain outletswith rock (riprap), brush, orlogging slash to dissipateenergy and prevent erosion,or locate the outlet of crossdrains on stable, non-erosivesoils, rock, or in well veg-etated areas (Figure 7.2b).

• Construct rolling dipsrather than culvert cross-drains for typical, low-volume, low speed roadswith grades less than 12%.Construct rolling dips deepenough to provide adequatedrainage, angled 0-25degrees from perpendicularto the road, with a 3-5%outslope, and long enough(15 to 60 meters) to passvehicles and equipment (SeePhoto 7.2). In soft soils,armor the mound and dipwith gravel or rock, as wellas the outlet of the dip(Figure 7.3).

• Install culvert cross-drainswith an angle of 0-30 de-grees perpendicular to theroad, using an outslope of2% greater than the ditchgrade to prevent plugging.(Figure 7.4). (See Chapter8 for more information onculverts). Use culvert cross-drains on roads with an

inside ditch and moderatelyfast vehicle speeds.

• Construct water bars oninfrequently used roads orclosed roads to controlsurface runoff. Constructfrequently spaced waterbarsangled at 0-25 degrees withan outslope of 3-5% and adepth of 0.3 to 0.6 meters.Install water bars as shownin Figure 7.5. Spacing ofwaterbars is shown in Table7.2.

• Use catch water ditches(intercept ditches) across thenatural ground above a cutslope only in areas with highintensity rainfall and over-land flow. These ditches areuseful to capture overlandsheet flow before it poursover the cut slope anderodes or destabilizes thecut. However, be aware thatcatch water ditches are thatare not properly maintainedcan become a counter-productive pool for waterabove the slope, increasingthe probability of a slopefailure.

• Avoid the use of outsideditches, along the outsideedge of the road, except inspecific areas that must beprotected from sheet flowoff the road surface. Prefer-ably, use berms. Note that anoutside ditch or berm neces-sitates additional road width.


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

Cut slope

Reinforced

dip

Fill slope

Ground slope

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

PRACTICES TO AVOID

• Long sustained road gradesthat concentrate flows.

• Discharging water onto

erosive, unprotected soils.

• “Eyeballing” grades in flatterrain. Use a clinometer,

abney level, or surveyequipment to ensure thatyou have proper slopes orgrades.

FFFFFigurigurigurigurigure 7.2e 7.2e 7.2e 7.2e 7.2


ArmoredDip

Mound

Inslope or

Outslope

Riprapat DipExit

Dip Average Road Grade

Dip

2-5%Outslope

2-5%Outslope

c. Rolling Dip Profile Detail

Average Road Grade

Armor Dip and MoundSurface as Needed with5-15 cm Aggregate

For Insloped Road – Slope to Depthof Inside DitchFor Outsloped Road – 3-5 cm Deepor Match Depth of Inside Ditch atEntrance – 15-30 cm Deep at ExitReverse Slope3-6%

Road Grade2-12%

7-12m8-30m 8-30m

0-25o

b. Profile

a. Perspective View

Spacing 30 - 150 m

FFFFFigurigurigurigurigure 7.3 e 7.3 e 7.3 e 7.3 e 7.3 Rolling (broad-based) dip cross-drains.


InletStructureas Needed

Place Outlet Pipe atNatural GroundLevel or RiprapArmor the FillMaterial.

Berm

Spacing 30–150 m

0o -30o

Exit onto Stableor ArmoredGround

Berm Tied intoEmbankment Spacing 10–75 m

Road Grade

30-60 cm 30-60 cm○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○

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

0-25

o

a. Perspective View

FFFFFigurigurigurigurigure 7.4 e 7.4 e 7.4 e 7.4 e 7.4 Culvert cross-drains.

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

b. Cross-Section


ter or remove it from the roadsurface rapidly. On such steepgrades, it is best to use frequentlyspaced cross-drain culverts, witharmored ditches. Also, the roadsurface will need armoring or sur-facing with some form of pave-ment to resist erosion. For sea-sonal or low use roads, interimdrivable waterbars could also beconstructed.

Water bars are used to con-trol drainage on closed or inac-tive roads, 4-wheel drive roads,skid roads, and skid trails. Waterbars are frequently spaced (seeTable 7.2) for maximum erosioncontrol and can be shaped to passhigh clearance vehicles or to blocktraffic.

Photo 7.3 Photo 7.3 Photo 7.3 Photo 7.3 Photo 7.3 Use masonry, concrete, or metal inlet structures tocontrol water in the ditch, direct the water into the cross-drain pipe,and prevent ditch down-cutting.

Photo 7.4 Photo 7.4 Photo 7.4 Photo 7.4 Photo 7.4 Add outlet protection and/or energy dissipators to pre-vent erosion and the formation of gullies.

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 ofculverts, rolling dips, or othercross-drainage structures. Thiscan ensure that water and debrisenters the cross-drain efficientlywithout plugging, and that it ex-its the cross-drain without dam-aging the structure or causing ero-sion at the outlet.

Culvert inlet structures (dropinlets) are usually placed in the in-side ditchline at the location of aculvert cross-drain. They are com-monly constructed of concrete,masonry (Photo 7.3), or fromround metal pipe, as seen in Fig-ure 7.6. They are typically usedwhere the ditch is eroding anddowncutting, so that the structurecontrols the ditch elevation. Inletstructures are also useful tochange the direction of waterflowing in the ditch, particularlyon steep grades, and they can helpstabilize the cut bank behind thepipe inlet.

The outlet of pipes and dipsare 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 aroadway can cause severe erosionor gullying if discharged directlyonto erosive soils (Photo 7.4).The pipe, dip, or drain outlet areacan be stabilized, and the energyof 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 includethe use of stilling basins, rein-


forced splash aprons, or use ofdense vegetation or bedrock(Photo 7.5).

Ditches on steep road grades,

RECOMMENDED PRACTICESCONTROL AT INLETS & OUTLETS

• When ditch grade control isneeded, use drop inlet struc-tures with culvert cross-drainsto prevent ditch down-cuttingor where space is limitedagainst the cut bank (Figure7.6). Alternately, use catchbasins excavated into firmsoil.

• Discharge culverts and cross-drain dips at natural groundlevel, on firm, non-erosivesoil or in rocky or brushyareas. If discharged on the fillslopes, armor outlets withriprap or logging slash, or usedown-drain structures. (Fig-ures 7.3, 7.4, 7.7 and Figure8.1). Extend the pipe 0.5 to

1.0 meters beyond the toe ofthe fill slope to preventerosion of the fill material.

• In erosive soils, armorroadway ditches and leadoffditches with rock riprap(Photo 7.7), masonry,concrete lining or, at aminimum, grasses. Ditchdike structures can also beused to dissipate energy andcontrol ditch erosion.(Figure 7.8).

• Discharge roadway drains inan area with infiltrationcapability or into filter stripsto trap sediment before itreaches a waterway. Keepthe road and streams hydro-logically “disconnected.”

PRACTICES TOAVOID

• Discharging a cross-drainpipe or dip onto any unpro-tected fill slope or barren,erosive soil.

• Discharging cross-drainpipes mid-height on a fillslope.

• Discharging cross-drainpipes or dips onto unstablenatural slopes.

Photo 7.5Photo 7.5Photo 7.5Photo 7.5Photo 7.5 Protect the outlet of culvert pipe and rolling dip cross-drains with riprap or a masonry splash apron, or choose areas ofbedrock or dense vegetation.

in erosive soils, and with flow ve-locities over one meter per sec-ond may require armoring or theuse of small ditch dike or damstructures placed in the ditch to

slow down the velocity of water,as shown in Figure 7.8. Ditchesare commonly armored with grass,erosion control matting, rock, ormasonry /concrete paving (Photo7.6). Grasses can resist flow ve-locities to 1-2 meters per second.A durable armoring such as gradedrock riprap or concrete is recom-mended on grades over 5 percentin erosive soils or for velocitiesover a few meters per second.

Ditch dikes will prevent ditcherosion, and dikes can serve tocatch sediment, but they requiremaintenanceneed in that they needto be periodically cleaned out.Common ditch dike constructionmaterials include loose rock, ma-sonry, concrete, bamboo, andwooden posts. Each dike structureshould be keyed into the banks ofthe ditch and a notch is neededover each structure to keep theflow in the middle of the ditch.


Slope

Culvert Pipe (30-60 cm diameter)

Sand trap

0.6-1.2 m

Roadbed Surface

Drop inletstructure

Bottom of ditch

Roadbed surface EnergyDissipationwith Riprap,ConcreteApron orSplash poolwith water

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

Design and Installation Detail

"1 m

General Information

MetalMasonry

Window

45 cm

45 cm

30 cm

90 cm

30 cm

Slope

Grass forErosionControl

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

Photo 7.6 Photo 7.6 Photo 7.6 Photo 7.6 Photo 7.6 Armor ditches with vegetation, rock, masonry, or concrete toresist ditch erosion and carry the water to a stable exit point.


Fill Slope

0.5 mminimum

15-30 cmminimumdepth

Ground Line1-2 m

Rocks:15-50 kg5% greater than 25 kg

FFFFFigurigurigurigurigure 7.7 e 7.7 e 7.7 e 7.7 e 7.7 Culvert outlet protection.

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


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

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

Exit ditch instable, vegetatedarea

Insloped Roadbed Armor the ditch withrock, masonry or grass

1 m

30 cm min.

Cut slo

pe

b. Typical Ditch Armoring and Shape

Cut slope

Ditch dikes made of rock orwood to reduce flow velocity

c. Use of Ditch Dikes

Weir shape to keepflow mid-ditch

Roadway

Flow

Roadway

FFFFFigurigurigurigurigure 7.8 e 7.8 e 7.8 e 7.8 e 7.8 Ditches and ditch armoring.


NATURAL STREAM CROSSINGS

Road crossings of naturaldrainage channels and streams re-quire hydrologic and hydraulic de-sign expertise to determine theproper size and type of structure,as discussed in Chapters 5 and 6.Structures for small drainages canbe sized using Table 8.1. Thechoice of structure includes cul-vert pipes, arch or box culverts,low water fords, or bridges, asshown in Figure 7.9.

Because drainage crossingsare at areas of running water, theycan be costly to construct and canhave major negative impacts onwater quality. Impacts from im-proper design or installation ofstructures can include degradedwater quality, bank erosion, chan-

a. Bridge b. Low-Water Crossing

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

FFFFFigurigurigurigurigure 7.9 e 7.9 e 7.9 e 7.9 e 7.9 Structural options for crossing natural streams. (Adapted from Ontario Ministry of NaturalResources, 1988)

nel scour, traffic delays, and costlyrepairs if a structure fails. Also,structures can greatly impact fish,as well as other aquatic species,at all stages of life. Stream cross-ings should be as short as possibleand cross perpendicular to thechannel (Photo 7.8). The road andditches should be armored, ditchesshould divert surface water beforeit reaches the stream channel, andconstruction should minimize thearea of disturbance, as shown inFigure 7.10. Large drainagecrossings should receive site-spe-cific analysis and design input, ide-ally by an experienced hydraulicengineer and other specialists.

In drainages with uncertainflow values, large quantities of de-bris in the channel, or on sites with

existing undersized pipes, there isa high risk of a culvert pipe plug-ging and the site washing out orfailing. In such areas, or in par-ticularly sensitive watersheds,overflow protection is desirable.A low point in the fill and an ar-mored overflow “spillway,” asshown in Figures 7.11a & b, willprotect the fill and keep the flowin the same drainage, thus reduc-ing diversion potential and usuallypreventing a failure. A pluggedpipe that diverts the stream waterdown the road can cause a greatdeal of off-site damage or gully-ing or cause landslides, as seen inFigures 7.11c & d. Overflowstructures should not be used as asubstitute for good hydraulic de-sign, but they can offer “cheapinsurance” against failure at cul-vert crossings.


PRACTICES TO AVOID• Working with equipment in

an unprotected naturalstreambed.

• Locating stream crossingsin sinuous or unstablechannels.

RECOMMENDED PRACTICESNATURAL STREAM CROSSINGS

• Use drainage structures thatbest conform to the naturalchannel and that are as wideas the active stream channel(bankfull width). Minimizenatural channel changes andthe amount of excavation orfill in the channel.

• Limit construction activity toperiods of low flow in livestreams. Minimize use ofequipment in the stream.Stay out of the stream!

• Design structures and useconstruction practices thatminimize impacts on fish andother aquatic species or thatcan enhance fish passage.

• Cross drainage channels asinfrequently as possible.When necessary, crossstreams at right anglesexcept where prevented byterrain features (Figure7.10).

• Keep approaches to streamcrossings to as gentle a

grade as practical. Rollgrades into and out of thecrossing to disperse water.

• Stabilize disturbed soilaround crossings soon afterconstruction. Remove orprotect fill material placed inthe channel and floodplain.

• Use bridges, low-water fordsor improved fords, and largearch pipes with naturalstream bottoms whereverpossible to maximize flowcapacity, minimize thepossibility of a plugged pipe,and minimize impacts onaquatic species.

• Locate crossings where thestream channel is straight,stable, and not changingshape. Bedrock locations aredesirable for concretestructures.

• For overflow protection,construct fills over culvertswith an armored low pointnear the pipe in low fills oradd an armored rolling dipon native ground just be-

yond a large fill to returnwater to the drainage andprevent off-site damage(Figure 7.11).

• Stabilize roadway ap-proaches to bridges, fords,or culvert crossings withgravel, rock, or othersuitable material to reduceroad surface sedimentfrom entering the stream(Figure 7.12). Installcross-drains on both sidesof a crossing to preventroad and ditch runoff fromentering the drainagechannel.

• Construct bridges andculvert fills higher than theroad approach to preventroad surface runoff fromdraining directly into thestream -- but ONLY iflikelihood of culvertfailure is VERY small.(Figure 7.13). Typically,the crossing should bedesigned to minimize theamount of fill.

• Adversely impacting fisherieswith a stream crossingstructure.

• Allowing runoff from road-side ditches to flow directlyinto streams.


Poor Stream Crossing Better Stream Crossing

Drainage crossings perpendicular to the creekminimize the area of disturbance. Armor thestream crossing and roadway surface.

Cross-drain

Cutslopes

Road

Road

FFFFFigurigurigurigurigure 7.10 e 7.10 e 7.10 e 7.10 e 7.10 Natural drainage crossings. Minimize the area of disturbance with a perpendicularstream crossing alignment, and armor the roadway surface.

Crossings near parallel to the drainage cause alarge disturbed area in the channel, streambank,and approach cuts.

Largecutslopearea

Road

Road

Photo 7.8Photo 7.8Photo 7.8Photo 7.8Photo 7.8 Avoid natural drainage crossings that are broad and that arenot perpendicular to the drainage. Stay out of the stream! This broadchannel is a good site for a vented ford.


B

C

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

D

A

Road Profile Across the Drainage and Dip

DCB

Pipe

Armored dip

Fill

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

A

FFFFFigurigurigurigurigure 7.11e 7.11e 7.11e 7.11e 7.11

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


FFFFFigurigurigurigurigure 7.11 (contine 7.11 (contine 7.11 (contine 7.11 (contine 7.11 (continued)ued)ued)ued)ued)

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

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

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

Good Installation

Poor Installation

AbandonedChannel

Slumping

New Channelin a Gully

Plugged Culvert


FFFFFigurigurigurigurigure 7.12e 7.12e 7.12e 7.12e 7.12 Protection measures at stream crossings.

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

15-30 m stoneor gravelapproach

Hardened

streambottom

Ditch

Riprap (rock)

Gravelor stonesurfacing

Debris

Rolling dipor

cross-drain

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

Road

FFFFFigurigurigurigurigure 7.13 e 7.13 e 7.13 e 7.13 e 7.13 High point over the crossing. (Adapted from Wisconsin’s Forestry Best ManagementPractices for Water Quality, 1995)


WET AREAS AND MEADOW

CROSSINGS, USE OF UNDERDRAINS

Road crossings in wet areas,including damp meadows,swamps, high groundwater areas,and spring sources are problem-atic and undesirable. Wet areas areecologically valuable and difficultfor road building, logging, or otheroperations. Soils in these areas areoften weak and require consider-able subgrade reinforcement.Drainage measures are expensiveand may have limited effective-ness. Wet areas should beavoided!

If wet areas must be crossedand cannot be avoided, specialdrainage or construction methodsshould be used to reduce impactsfrom the crossing. They includemultiple drainage pipes (Photo7.9) or coarse permeable rock fillto keep the flow dispersed,subgrade reinforcement withcoarse permeable rock, grade con-trol, and the use of filter layers andgeotextiles, as shown in Figure7.14. The objective is to maintainthe natural groundwater level andflow patterns dispersed across themeadow and, at the same time,provide for a stable, dry roadwaysurface.

Local wet areas can be tem-porarily crossed, or “bridged”over, using logs, landing mats,tires, aggregate, and so on. (seeFigure 7.15). Ideally, the tempo-rary structure will be separatedfrom the wet area with a layer ofgeotextile. The geotextile helps fa-cilitate removal of the temporarymaterial and minimizes damage tothe site. Also, a layer of geotextile

can provide some reinforcementstrength as well as provide sepa-ration to keep aggregate or othermaterials from punching into theweak subgrade.

Subsurface drainage, throughuse of underdrains or aggregatefilter blankets, is commonly usedalong a road in localized wet orspring areas, such as a wet cutbank with seepage, to specificallyremove the groundwater andkeep the roadway subgrade dry.A typical underdrain design usesan interceptor trench 1-2 metersdeep and backfilled with drainrock, as shown in Figure 7.16.Subsurface drainage is typicallyneeded in local wet areas and ismuch more cost-effective thanadding a thick structural sectionto the road or making frequentroad repairs. Design and filtrationrequirements for underdrains arediscussed in Chapter 6 and otherreferences.

In extensive swamp or wet ar-

eas, subsurface drainage will of-ten not be effective. Here, eitherthe roadway platform needs to beraised well above the water table,such as with a turnpike roadwaysection, or the surfacing thicknessdesign may be based upon wet,weak subgrade conditions thatwill require a relatively thickstructural section. A thick aggre-gate layer is commonly used, withthe thickness based upon thestrength of the soil and anticipatedtraffic loads.

Photo 7.9Photo 7.9Photo 7.9Photo 7.9Photo 7.9 Avoid crossing wet meadow areas. When necessary tocross, use multiple drainage pipes to keep water flow dispersedacross the meadow.

PRACTICESTO AVOID

• Crossing wet areas unnec-essarily.

• Concentrating water flowin meadows or changingthe natural surface andsubsurface flow patterns.

• Placing culverts below themeadow surface elevation.


ROCK FILL WITHOUT CULVERTS(for minimal overland flow)

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

CulvertsBerm

Berm

Flow

Mea

dow

Flow

Road

way

Meadowsurface

Geotextile

Flow

Roadway

10-15 cm minus rock

Roadway

FlowFlow

15 cm ThickAggregateBase Course

15 cm Minus RockCourse Placed Approx.30 cm Thick

FFFFFigurigurigurigurigure 7.14 e 7.14 e 7.14 e 7.14 e 7.14 Wet meadow road crossing options. (From Managing Roads for Wet Meadow EcostreamRecovery by Wm. Zeedyk, 1996)

c.

b.

a.

Geotextile


FFFFFigurigurigurigurigure 7.15e 7.15e 7.15e 7.15e 7.15 Pole or plastic pipe fords for wet area and bog crossings. Pole fords must be removedimmediately after use or before the upstream end becomes clogged with debris and impedes streamflow. (Adapted from Vermont Department of Forests, Parks and Recreation, 1987)

RECOMMENDED PRACTICESWET AREAS AND MEADOW

CROSSINGS, UNDERDRAINS

• For permanent road cross-ings of meadows and wet-lands, maintain the naturalgroundwater flow patternsby the use of multiple pipesset at meadow level tospread out any overlandflow (See Photo 7.9).Alternatively, a coarse,permeable rock fill can beused where overland (sur-face) flow is minimal (seeFigure 7.14).

• In areas with local wet spotsand limited road use, rein-force the roadway with atleast 10-30 cm of coarsegraded rock or a very coarsegranular soil. Ideally, sepa-rate the coarse rock and wet

soil with a filter layer ofgeotextile or gravel.

• For temporary crossing ofsmall, wet drainages orswamps, “corduroy” theroad with layers of logsplaced perpendicular to theroad and capped with a soilor gravel driving surface.PVC pipe, landing mats,wood planks, tire mats andother materials have alsobeen used (see Figure 7.15).Place a layer of geotextilebetween the saturated soiland logs or other materialfor additional support and toseparate the materials.Remove logs from anynatural drainage channelbefore the rainy season (seePhoto 8.8). A layer of chain-

link fencing or wire underthe logs can help facilitateremoval of the logs.

• In spring areas, use drainagemeasures such asunderdrains or filter blanketsto remove local groundwa-ter and keep the roadsubgrade dry (Figure 7.16,Photo 7.10).

• Use underdrains behindretaining structures toprevent saturation of thebackfill. Use underdrains orfilter blankets behind fills(embankments) placed oversprings or wet areas toisolate the fill material andprevent saturation andpossible subsequent fillfailure.


FFFFFigurigurigurigurigure 7.16e 7.16e 7.16e 7.16e 7.16 Typical road underdrain used to remove subsurface water.

Photo 7.10 Photo 7.10 Photo 7.10 Photo 7.10 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 designneeds a second layer of geotextile between the soft subgrade soil and the coarsefilter rock to keep the rock clean.

VariableDepth(Typically+/- 1.5 mDeep)

5-10 cm60 cmmin.

Pipe, Perforated15 cm dia. (min)

Filter Material, Permeable SandyGravel, Well GradedNOTE: With Geotextile, use clean,coarse gravel.Without Geotextile, use fine, cleansand.

Ground Water

Geotextile Enveloping theFilter Material

Ditch15 cm Cap ofImpermeable Soil

Roadbed

Cut

Documents

chapter 7 JM - International Road FederationLOW-VOLUME ROADS BMP S:56 RECOMMENDED PRACTICES ROADWAY SURFACE DRAINAGE CONTROL • Design and construct roads so that they will move water