Donald Walker, T.I.C. Director, author

Lynn Entine, Entine & Associates, editor

Susan Kummer, Artifax, design

Local Road Assessment and Improvement

Drainage Manual

Transportation Information Center

University of Wisconsin–Madison

This manual is designed to help local officials evaluate drainage conditions along rural and urban roadways. It is part of the PASERWARE pavement management system. This manual can also be used to plan for road maintenance and improvements.

The manual includes background information on the importance of drainage and the impacts of poor drainage on roadway performance. It covers both ruralroads with ditches and culverts and urban sections with curb and gutter andstorm sewer. Drainage elements are explained and accompanied byrepresentative photographs to aid field inspection.

Evaluation tools are a necessary part of pavement management systems. Theyaid in developing cost estimates and setting strategies for maintenance andimprovement. This manual has a rating and evaluation section which you can use with PASERWARE, with another pavement management system, or simply as an inspection tool for your maintenance and improvement programs.

Most local officials can use visual inspection and common sense to evaluate and develop an effective roadway maintenance and improvement program.However, we strongly recommend using professional advice in redesigning andmaking major improvements in culvert sizes and in assessing the causes andsolutions for flooding conditions. The cost and potential severity of problemscaused by flooding and improper drainage design and construction makeprofessional assistance a wise investment.

The state has regulations on erosion control and on constructing and maintaining local roads near wetlands and navigable streams. When you areplanning work near navigable streams and wetlands and on larger projects thatwill uncover more than five acres of soil, you should contact the Department ofNatural Resources (DNR) transportation liaison for your county. They welcomequestions about the regulations and will help you meet state requirements.Sediment run-off and other non-point source pollution regulations are evolving.It is wise to keep abreast of these changes.

Local Road Assessment and Improvement

Drainage Manual

Mowing to ditch linehelps drainage.

Good round-bottom ditch.Seed for erosion control.

Maintaining proper drainage

Even on roads built with all the properdrainage elements, neglecting mainte-nance is likely to result in flooding,washouts, and potholes. To keep aroad in good condition, maintenanceto the road surface and shouldershould retain and restore the originaldesign as much as possible. On gravelroads, this involves smoothing andreshaping with a motor grader.Surfaced roads may need periodicpatching or overlays.

Other conditions requiring mainte-nance to improve drainage include:

• Ditches clogged with debris orsediment need cleaning to avoidoverflowing and washouts.Excavated sediments which are ofthe same quality as the aggregatemix on the road can be put backonto a gravel road and bladedinto the surface. Well designedand built ditches have gentle sideslopes which a grader can travelto clean the ditch bottom.

3

Rock-linedditch helpspreventerosion.

Goodurbandrainagesystem.

Cleaning ditches with steepslopes requires a backhoe whichis more expensive and timeconsuming to use than a grader.

• Vegetation and brush thatobstruct water flow need to bemowed or cut. However, whenremoving sediment from ditchesbe careful to disturb vegetation as little as possible to limiterosion. It may be necessary to re-seed, mulch, or use othererosion protection methods onsteep slopes or in areas sensitiveto severe erosion. Sediments from eroding slopes can filldownstream road ditches andculverts or pollute streams and lakes.

• Culverts need to be kept free ofsediment so water flows freelyand doesn’t wash out roads andflood adjacent property. Inspectperiodically for sediment buildupand for cracks or corrosion thatmight lead to culvert failure. Clear clogged culverts with hand shovels or mechanized

equipment. However, the bestmaintenance technique is toprevent sediment buildup inditches so there is no material to run into and clog culverts.

• In urban areas, ponding behindcurbs that saturates the streetbase needs to be corrected.Regrade soil in the terrace behindthe curb to protect the streetstructure from localized pondingdue to lawn watering or runoff.For more severe or persistentconditions, install underdrainbehind the curbing. Repair orreplace sunken inlets whichcollect standing water.

The text and photos that followdescribe each drainage componentand depict inadequate drainageconditions that can lead to roaddamage. This manual should serve as a guide to help you locate, assess,rate, and improve drainage conditionson your roads.

Maintaining properdrainage

Assessing drainage systems4

Poor crown allowspavement saturation.

The problem with this method isthat usually water can’t penetratebeneath and through the shoulder’ssubsurface material. These imperme-able shoulders keep water fromdraining out of the roadway’s base. Itis trapped and weakens the roadway.

Assessing drainagesystemsA drainage system includes the

pavement and the water handlingsystem. They must be properlydesigned, built, and maintained. The water handling system includes:shoulders, ditches and culverts; curb,gutter and storm sewer. When a roadfails, whether it’s concrete, asphalt orgravel, inadequate drainage often is a major factor.

Shoulders and embankmentsdamaged by heavy rain or floods canallow water to stand on the road orseep back into the base, saturating it.Surface cracks allow water to pene-trate and weaken the base. Poordesign can direct water back onto theroad or keep it from draining away.Too much water remaining in thesurface, base, and subgrade combinewith traffic action to cause potholes,cracks and pavement failure.

The basics of drainage are similar inboth rural and urban settings. Someissues specific to certain drainageelements are discussed separately.

CrownThe road surface should be crownedso water will run off to the shoulders.As a general rule, the center of theroad on paved surfaces should be 21⁄2 inches higher than the shoulder, 5 to 6 inches higher for gravelsurfaces. Shoulders should slope asmuch or more than the road to keepwater moving to the ditches. Forexample, a paved roadway with an 11 foot lane and 4 foot shouldershould have a total crown (fromcenterline to outside edge of shoulder)of not less than 4 inches.

Gravel roads need special attentionbecause they are more susceptible torain damage. They will need highercrowns than paved surfaces to prevent the surface from absorbingtoo much water, becoming saturated,and not drying out. Traffic action on a saturated surface causes potholesand ruts.

A good quality gravel surfaceabsorbs minimal amounts of water,sheds the rest, and dries out quickly.Poor drainage may be caused bygravel with a poor gradation ofstones, sand and fines. You canpartially compensate for poor qualitygravel with a higher road crown.

Steep roads may also require highercrowns since the water will tend toflow down the road flooding trafficlanes, rather than across the crown.

ShouldersShoulders extend the road surface,directing water flow to the ditches ifthey slope as much or more than thecrown. If they slope less, water willbuild up during heavy rain at the joinbetween shoulder and road, floodingtraffic lanes. Make sure the shouldercontinues the road crown smoothly.

Springs or seepage areas willrequire special treatment. You can usefrench drains (rock filled trenches) orperforated pipes to drain this sub-surface water into ditches or streams.

One common method for construc-ting gravel roads, the trench techni-que, causes poor drainage. It involvesmaking a shallow excavation of justthe intended road surface, then fillingit with sub-base and base material.The shoulders are not fully excavatedand the original soil is covered with athin layer of gravel.

Poorshoulderslope trapswateragainstpavement.

Lack ofshoulderand ditch,and poorcrown leadto pavementfailure.

Assessing drainage systems 5

For proper drainageand longer roadway life,excavate the shouldersto the same depth as theroadway and use thesame sub-base and basematerial. Use a gravel orcrushed rock that drainswell to remove anywater which soaksthrough the surface orenters the subsurface.

DitchesDitches carry water awayfrom the roadway andinto streams or othernatural waterways. To do this, ditchesmust be properly shaped for safety,maintenance, waterflow, and erosioncontrol. The ditch should be at leastone foot below the bottom of thegravel base in order to drain thepavement. Deeper ditches may benecessary to provide positive drainagepatterns.

Use a smooth transition to theditch. Sides that are too steep maycause errant vehicles to roll over. Side slopes of 4:1 are desirable whilethe maximum slope should be 21⁄2:1.A gentle slope makes mowing andditch cleaning easier,faster and cheaper, but,of course, they require awider right of way.

It’s very important thatwater flow along ditchesand not stand in them.Standing water maysaturate the subsurfacematerial beneath theroadway, preventing theroad from drainingduring the next storm.Standing water alsoreduces the ditch’scapacity to handlerunoff. As a result, thenext storm could washout the roadway.

Ditches with 1% gradient aredesirable (1⁄ 2% minimum) to insure proper flow. The flow ofwater in ditches should not erodethe ditch itself or weaken theadjoining shoulder. Vegetation inditches is necessary to help keep thesoil in place and minimize erosion.Use rubble, riprap, or fabric to slowwater flow on steep slopes, or pavethem to prevent serious erosion. You may also consider installing ashort section of storm sewer.

High shouldercreates a secondaryditch and damagespavement.

No separate ditch.Pavement cut into roadside,trapping water.

Shallow andnarrow ditch.Likely to causeroad floodingand futuredamage.

Poor ditch slopetraps water.

No ditch orshoulderdrainage cancause severepavementdamage.

Lack of curb, ditch or drivewaycauses poor drainage, pavementdamage and parking problem.

Assessing drainage systems6

Roadway culvertsCulverts channel water under theroadway from one side to the other.They help control water flow and slow it down to control erosion. Indesigning culverts consider loads and cover, durability and capacity,placement, and slope.

A culvert must be strong enough tosupport the fill material above it andthe traffic that moves over it. Concreteculvert strength depends on its wallthickness and the amount of steelreinforcement it has. Steel culvertstrength depends on the depth ofcorrugations, gauge of steel used, and,to a great extent, on the quality andcompaction of backfill material on thesides and haunches of the pipe.

Culverts should be covered with atleast 12 inches of soil from the top ofthe pipe to the top of the subgrade.Arched and elliptical pipes or shallowbox culverts can be helpful wherecover depth over the culvert is limited.

A culvert must be durable and havesufficient hydraulic capacity to carryaway a predetermined quantity ofwater in a given time. Design chartsare available for each type of culvert. A complete design involves reviewingthe topography, predicting runoff,sizing the waterway and culvert, andcomparing culvert cost to the risk offlood damage. For roadway cross cul-verts, the minimum recommended sizeis 18 inches. A professional designerwith local experience can save youconstruction costs and damage claims.

Altering the entrance configurationcan significantly improve culvertcapacity. Beveling the edge of the inletor using side-tapers helps considerably.

Culverts should slope enough sowater will flow at about 21⁄2 feet persecond. A minimum drop of 6 inchesacross the road is desirable. This willkeep sediment from accumulating inthe pipe but will not cause extensiveerosion at the discharge end. Metalaprons or concrete headwalls improvethe capacity, reduce erosion, and canshorten culvert length. For safety,headwalls should not project abovethe level of the roadway surface.

Place culverts so they match existingcontours, or in the existing channel, ifpossible. Be extremely careful aboutchanging culvert locations, capacitiesor drainage patterns. Section 88.87 ofthe Wisconsin Statutes requires thathighways not impede the general flowof surface water. Drainage Districtsmust be notified if any changes ormajor maintenance work are beingplanned (Wis. Stat. 86.075). Beforereplacing culverts located in estab-lished flood plains you must alsosecure prior approval from the DNR.

The state has regulations on erosioncontrol and on constructing and main-taining local roads near wetlands andnavigable streams. When you areplanning work near navigable streamsand wetlands and larger projects thatwill uncover more than five acres ofsoil, contact the county transportationliaison from the Wisconsin DNR.

Culvert lacks propercover. No endtreatment. Ditchcleaning needed.

Pipe cleaningrequired. Determinesource of silt andcorrect to preventfuture problems.

No end treatment,inadequate ditchcause erosion andpoor inlet capacity.

No end treatment.Extend culvert toincrease shoulder andsafety at intersection.

Pipe too long. Dischargenear top of side slopewill cause erosion.

Assessing drainage systems 7

Driveway culvertsDriveways can block drainage andcause flooding. Culverts should berequired to maintain normal ditchdrainage. A minimum 15-inchdiameter is recommended.

Driveways should be built so thatthey either slope away from theroad or are graded with the lowpoint over the culvert. This preventswater from washing onto the roadfrom driveways.

Curb and gutterGood drainage and maintenancepractices are similar in both ruraland urban areas. However, usingcurb and gutter and storm sewerraises additional considerations.

Curb and gutter may be prefer-able to an open ditch in areas withlimited right-of-way or where openditches are unacceptable. Shortsections of curb and gutter may be used at spot locations withoutrequiring storm sewer.

Drainage from improperlyconstructed driveway cancause road damage.

Cleaning and replacingdriveway culverts isan ongoing priority.

Broken andfailed curb.

Gutter coveredwith asphaltoverlays reducesflow capacity.

Silt from adjacentproperty causesflooding.

Curb settlementcauses pondingand pavementdamage.

Lack of positivegradient causesflooding.

Remove siltand debristo improvedrainage.

Pavementsettlementcauses ponding.

Assessing drainage systems8

Storm sewers and inletsStorm sewer systems collect waterfrom the street and adjoining propertyand deliver it to open surface water-ways—streams, rivers, lakes. Shortsections of storm sewer may be useful in rural areas with steep slopeswhere runoff is eroding open ditches,causing a problem. Storm sewer isalso helpful at intersections and other locations.

It is important to maintain curb,gutter, inlets, and storm sewersystems. They should be inspectedevery five years. Inspecting stormsewer either visually or through TVremote systems is obviously moredifficult and expensive than inspectingsurface facilities. Consider schedulingthis on a regular basis or in areas withvisible surface problems. Catch basins,and manholes must be cleaned onceor twice a year as part of maintainingan urban storm water system.

Storm water pollution is receivingincreased attention nationally and inWisconsin. Surface water collects awide range of pollutants as it travelsover a roadway surface, across lawns,and into ditches and storm sewersystems. Unfortunately, many of thesepollutants are carried directly intowaterways and ground water. Since control and monitoring of storm water quality is becoming morecomplex, local agencies must reviewtheir practices and be aware ofcontrols, regulations, and effectivepollution abatement practices.

In general it has been found thatopen drainage systems with vegetatedditches are helpful in reducing thepollutants in runoff from roadways.This suggests that open ditch sections,where possible, are preferable tostorm sewers for pollution abatement.A more detailed discussion of thistopic is beyond the scope of thispublication.

Pavement settlementcauses ponding in thispoor parking lot inlet.

Routine cleaningimproves drainage.

Cloggedinletcausesflooding.

Rating and evaluating roadway drainage 9

Rating and evaluating roadway drainage

Periodic inspection, rating andevaluation of roadway drainage isrequired as a part of pavementmanagement. It is considered goodpractice even without a formalpavement management program. A regular inspection program allowsmanagers to identify and schedulenecessary improvements on a timelyand cost-effective basis.

Routine maintenance can oftenavert more serious drainage-relatedproblems. While casual observationis frequently used, a scheduled andorganized evaluation systemproduces more consistent results.

These more formal evaluations alsopromote good recordkeeping whichis very helpful in planning projectsand reducing time and informationloss due to staff turnover.

The basic rating system used in thismanual is based on common senseand is intended to be easy to use. It describes four rating categories:excellent, good, fair, and poor. Theratings are described by the generalcondition, typical defects, and therecommended improvements.

Each category is illustrated by aseries of photographs. It is unlikelythat all defects will be present.There may be only one or two in aspecific section of road. The extentof work required should helpdetermine if the rating is poor, fair,or good. Annual costs associatedwith the necessary maintenanceand improvements for each ratingcan be developed and used with apavement management system for programming both short-rangeand long-range improvements.

Rating Condition Improvement

Good

Wide adequate ditches or like-new curb, gutter and storm sewer system. All culverts clean and sound.

Overall, pavement and shoulder have adequate crown,ditching or storm sewer on the majority of the section.May need localized cleaning of ditches, storm sewers andculverts; minor repairs to curbs, inlets and culverts.No drainage-related pavement damage.

No improvementnecessary.

Minor or localizedrepairs.

Fair Minimal crown on pavement. Some areas need shoulderslope improvement. Ditching improvement or cleaningneeded on up to 50% of ditches. Pavement distress from localized flooding or ponding indicates improve-ments are needed in some storm sewer, inlets or ditching.Some culverts need cleaning or minor repairs.

Several improvementsnecessary.

Poor No pavement crown. Shoulders create secondary ditch.Frequent ponding. Significant ditching improvementsneeded on more than 50% of roadway. Frequent localizedflooding or erosion with pavement distress or failure.Significant improvement in storm sewer, curb or inletsand/or major culvert replacement or improvement needed.

Major improvementin drainage required.

Excellent

Rating and evaluating roadway drainage10

■■✔ ExcellentWide adequate ditches or like-new curb, gutter and storm sewer system. All culverts clean and sound.

No improvement necessary.

Excellent culvert.

Excellent rural drainage.

Excellent urbandrainage.

Excellent culvertand end protection.

Rating and evaluating roadway drainage 11

■■✔ Good Overall, pavement andshoulder have adequatecrown, ditching or stormsewer on the majority ofthe section. May needlocalized cleaning of ditches, storm sewers andculverts; minor repairs tocurbs, inlets.

Minor or localized repairs.No pavement damagerelated to poor drainage.

Good urban drainage.Local curb failure needsreplacement.

Good shoulder andditching as part ofnew asphaltoverlay. Erosioncontrol needed.

Good rural ditchand drivewayculvert. Culvertend needscleaning.

Good culvertwith apronend walls.

Adequateditch andgood erosioncontrol.

Rating and evaluating roadway drainage12

■■✔ Fair Minimal crown on pavement. Some areas need shoulder slopeimprovement. Ditching improve-ment or cleaning needed on up to 50% of ditches. Pavementdistress from localized flooding orponding indicates improvements areneeded in some storm sewer, inletsor ditching. Some culverts needcleaning or minor repairs.

Several improvements necessary.

Localized pondingand deterioratingcurb and gutter.

Culvert cleaningrequired.

Clean drain andsewer to preventlocalized flooding.

Needs localizedcurb repair.

Reshape terracebehind curb torestore drainageand prevent damageto curb and street.

Rating and evaluating roadway drainage 13

High shoulder and noditch lead to pavementdamage. Needs majorditch improvement fora short distance.

Secondary ditch trapswater. Actual ditchadequate. Minorgrading will restorecrown and drainage.

Shallow ditch will notadequately drainpavement. Needs ditchcleaning.

High water table andlocalized floodingcause prematurepavement failure fora short distance.

Good culvertbut ditch needscleaning.

Rating and evaluating roadway drainage14

Curb and gutterneed functionalrepair. Removeasphalt in gutterand restore streetsection when streetis reconstructed.

■■✔ Poor No pavement crown. Shoulderscreate secondary ditch. Frequentponding. Significant ditchingimprovements needed on morethan 50% of roadway. Frequentlocalized flooding or erosion withpavement distress or failure.Significant improvement in stormsewer, curb or inlets, and/or majorculvert replacement or improve-ment needed.

Major improvement in drainagerequired.

Totally deterioratedcurb. Needs newcurb and gutter tokeep water out ofpavement base.

Flooding. Curband gutter needreconstruction.

Poor curb andgutter withdamaged pavement.Need completereplacement.

Rating and evaluating roadway drainage 15

No ditch forextensive portionof roadway.

No crown or ditching.Reconstruction required.

No drainage leadsto failedpavement.

Inadequate drive-way culvert. Willcause local flooding.

High water hasdamaged pavement.

Severe roadsideerosion causes siltingdownstream.

Ditch erosion atculvert end. Cleanditch and use apronend walls.

Inspectculvert forcorrosionfrequently.Repair whenneeded.

Summary16

SummaryThe value of proper drainage design and maintenance on roads cannot beover-emphasized. The drainage system includes the roadway; the shoulders,ditches, and culverts; and the curbs, gutters and storm sewer systems. Theseelements work together as a system to prevent water from infiltrating theroad surface, remove it from the driving lanes to the side ditches or gutter,and carry it away from the roadway. Even roads with all the proper drainagedesign elements will flood, wash out, and develop cracks and potholes ifmaintenance is neglected.

• Build and maintain a roadway crown to drain water from the surface: 1⁄4 inch per foot of width for paved roads, 1⁄2 inch per foot of width for gravel roads, more under certain conditions.

• Avoid the trench technique of construction. Extend the roadway base to the outer shoulder edge.

• Use ditches with gentle side slopes for vehicle safety, to minimize erosion, and to aid maintenance.

• Design culverts to handle soil and traffic loads with appropriate drainagecapacity. Good design saves money. Professional help is recommended.

• Maintain the pavement and culverts to perform as originally intended.

• Keep ditches clean for efficient water flow.

• Inspect culverts regularly. Inspection after a heavy rain will give the most information on your drainage problems.

• Maintain natural surface water flow conditions and coordinateimprovements with local drainage boards and with the DNRtransportation liaison.

• Avoid placing asphalt overlays on concrete gutter. Mill so overlay matches gutter level.

• Inspect and clean storm sewer inlets and sewers.

Regular annual evaluation of drainage systems is an important part ofmaintaining and managing our roadways. Using the simple evaluationsystem outlined here will help local officials in their pavement managementresponsibilities. This can be incorporated into a formalized pavementmanagement system such as PASERWARE or done manually to have asimple ongoing record of drainage maintenance needs. Before investing inpavement surface improvements, make drainage improvements. It is mosteconomical and effective to plan and upgrade drainage as part of roadsurface improvements.