U.S. Department of 

Agriculture

Forest Service

National Technology & 

Development Program

0877 1807—SDTDC

7700 Transportation

Management

2500 Watershed

December 2008

Soil and Water 

Road-Condition

Index -

Desk Reference

U.S. Department

of Transportation

Federal Highway

Administration

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Soil and Water

Road-Condition

Index -

Desk Reference

Carolyn Napper, Soil Scientist

USDA Forest Service

San Dimas Technology and Development Center

December 2008

i

Table of Contents

Introduction ......................................................................................................................................................... 1

Section 1: Characterizing the Road .................................................................................................................. 1

 1. Road-Surface Shape ..................................................................................................................................... 1

 2. Hillslope Position ........................................................................................................................................... 3

 3. Road Gradient ............................................................................................................................................... 4

 4 Hillslope Gradient ........................................................................................................................................... 5

 5. Road-Surface Material .................................................................................................................................. 6

Section 2: Road-Condition Indicators .............................................................................................................. 7

 1. Road-Surface Drainage ................................................................................................................................. 7

 2. Stream-Crossing Structure Condition ..........................................................................................................11

 3. Road-Subsurface Drainage ......................................................................................................................... 12

 4. Diversion Potential at Crossings .................................................................................................................. 14

 5. Road-Stream Connectivity .......................................................................................................................... 15

 6. Road-Surface Condition .............................................................................................................................. 17

 7. and 8. Cutslope and Fillslope Condition ..................................................................................................... 18

Section 3: Other Road Condition Evaluation Considerations ..................................................................... 20

References ......................................................................................................................................................... 23

1

INTRODUCTION

Section 1: Characterizing 

the Road

The San Dimas Technology and Development Center (SDTDC) of 

the Forest Service, U.S. Department of Agriculture, developed the 

soil and water road-condition index (SWRCI) to provide a roadcondition assessment tool for watershed- and project-scale analysis. 

SWRCI is intended to be a rapid-assessment tool for soil scientists 

and hydrologists to use to identify effects of roads to soil quality and 

function, as well as impacts to water quality and downstream values. 

The SWRCI desk reference is a companion document to the SWRCI 

field guide and rating form. The desk reference: 

• Provides a description of each road attribute (road-surface 

shape, road-stream connectivity). 

• Identifies the questions the attribute addresses for a 

project- or watershed-scale road analysis. 

• Identifies related indicators and the usefulness of the 

attribute in identifying road impacts to soil and water 

resources with referenced research findings.

Once the SWRCI forms have been completed for a road, project 

area, or forest, the desk reference provides the watershed specialist 

(soil scientist, hydrologist, geologist) with information to more fully 

disclose and document the cause-and-effect relationship of a roadcondition rating and the potential impacts to soil and water resources.

The desk reference is formatted to emulate the fields within the 

SWRCI rating form. The first section addresses a description of 

each indicator used to characterize the road segment as in step 1 of 

the SWRCI form. The second section follows step 2 of the SWRCI 

form and presents information on the indicators used to determine 

the road-condition evaluation. Finally, in the third section there is a 

discussion on how traffic levels and soil texture may be used in the 

SWRCI process.

1. Road-Surface Shape

Description

Road-surface shapes include insloped, outsloped, crowned, 

entrenched, turnpiked, and user-created. Design elements of a road 

include the traveled-way width, shoulders, road gradient, curve 

widening, and pavement structures (Forest Service Handbook (FSH) 

7709.56 Chapter 4-Design). An entrenched road has a constructed 

berm or throughcut on both sides of the road prism preventing 

runoff from leaving the road prism except at designated locations. 

Turnpiked-road templates are constructed above the natural 

topography of the adjacent landscape. Turnpiked roads are used 

where soil strength is low and where surface ponding may occur for 
2

significant portions of the year. The user-created road evolves from 

use of an area. The road does not have any design elements and 

may be limited to wheel tracks only.

The road-template design may vary with changes in topography, 

hillslope position, hillslope gradient, road gradient, and surface 

and subsurface drainage features. The road design criteria 

includes environmental and resource considerations, safety, traffic 

requirements, and traffic-service levels.

Changes in the design of the road template are logical road-segment 

break points.

Questions potentially addressed

• How and where does the road template modify the surface 

and subsurface hydrology of the area?

• How and where does the road template cause surface 

erosion?

• How and where does the road system affect mass wasting?

• How and where is the road hydrologically connected to the 

stream system?

• How do the connections affect water quality?

• How and where does the road-template design cause 

diversion potential?

• How and where does the road reduce soil quality?

Related indicators

• Road-surface drainage.

• Stream-crossing condition.

• Diversion potential at crossings.

• Road-stream connectivity.

• Road-surface material.

• Road gradient.

• Hillslope gradient.

• Cutslope and fillslope condition.

Utility (Usefulness of the indicator or attribute)

The appropriate road-surface shape (road-design template) is 

determined by topography, climate, and access. The surface shape 

identifies how the road prism affects runoff and potential impacts to 

soil and water resources. 

Measure

Each road-surface shape has an associated set of road-drainage 

features that can be anticipated. For example an insloped road will 

generally have cross-drained culverts and a ditch. An insloped road 

may be used on steep hillslopes to ensure driver safety. Conversely, 

outsloped roads are generally located where hazards from driving off 

the road are not a concern, for example on flat-to-moderate gradient 
3

roads (less than 8-percent gradient), and may use drainage dips to 

break up the contributing road-drainage area.

In some cases a road-surface shape may be undefined due to lack of 

maintenance, or may change throughout the length of the road. Use 

the road-surface shape as a tool to segment the road and thereby 

categorize associated road-drainage features.

2. Hillslope Position

Description 

The hillslope position of a road segment can determine sediment 

potential and drainage design. Hillslope-position classes include: 

ridge (upper one-third), midslope (middle one-third), and bottom 

(lower one-third), or within a streamside-management zone (SMZ) or 

riparian area designation, and a climbing segment from the bottom to 

the ridge. 

Questions potentially addressed

• How does the road-segment location affect erosion and 

sediment production?

• How and where does the road-segment location intercept 

subsurface drainage?

• How and where is the road segment hydrologically connected 

to the steam system?

Related Indicators

• Road-surface drainage.

• Diversion potential.

• Road-stream connectivity.

• Road gradient.

• Hillslope gradient.

• Cutslope and fillslope condition.

Utility

The hillslope-position indicator is associated with erosion and the 

sediment production potential of a road segment. Studies in Oregon 

identified road segments in the midslope location have the potential 

to deliver the most sediment over other slope locations. Croke (2001) 

identified road-to-stream linkages that occurred at relief culverts 

draining cut-and-fill roads in midvalley positions. However, bottom 

locations—or segments within streamside management areas—have 

the highest proportion of road segments hydrologically connected 

to streams (Skaugset 1998). Studies on the Bluff Creek watershed 

in California found road failure rates 30 times higher in the lower 

(bottom) position of the hillslope compared to the upper position 

(Furniss 1999).
4

Measure

Circle the hillslope position of the road segment being evaluated. 

Hillslope position is another indicator used to segment a road.

3. Road Gradient

Description 

Road gradient is the slope of the road along its longitudinal axis. 

Road gradient is a key design element affecting the type of vehicles 

that use the road, vehicle speed, and safety. The road gradient also 

affects soil and water resources in terms of erosion potential, runoff, 

and sediment delivery. “Erosion potential increases as a function of 

the square of the slope and of the third power of the water velocity” 

(FSH 7709.56, Ch. 4.32). Studies identify sediment generation as a 

function of the traffic intensity, road gradient, and road contributing 

length.

Road gradient also determines the type of road-drainage structure, 

road-template design, and road-surface requirements. Forest Service 

guidance on road grades is identified in the Forest Service Manual 

(FSM) 7700 and FSH 7709.56 chapters 2 and 4. Steep road-gradient 

segments are common throughout National Forest System roads 

and require review to ensure that adverse impacts to soil and water 

resources do not occur.

Questions potentially addressed

• What percentage of a road is within a given road-gradient 

class?

• What type of road-template design is used on steep road 

grades and is it effective?

• What type of road-drainage structure is used on steep road 

grades and is it effective?

• What erosion- or sediment-production rates are associated 

with different road gradients?

Related indicators

Additional indicators that help to identify the effect of road gradient on 

soil and water resources include:

• Soil texture.

• Hillslope gradient.

• Diversion potential.

• Road-stream connectivity.

• Road-surface shape.

• Road-surface drainage.

Utility

The gradient of a road segment will change depending on how 

frequently the road is segmented. More frequent segmentation can 
5

identify significant erosional processes associated with road gradient 

especially when combined with slope length. In a study by Luce and 

Black (1999), the interaction between length and gradient found little 

effect to sediment production when the gradient was low regardless 

of how long the segment was. However, when combining long 

segment lengths with higher gradients there was significant sediment 

production. Kahklen (2001) identified gradient and road-surface 

material particle size to be the two most important variables that 

influence the development of rills on the road surface.

Measure

Identify the road gradient with a clinometer for the segment being 

traversed. Road gradients range from 0 to 8 percent, 9 to 15 percent, 

and greater than 15 percent.

4. Hillslope Gradient

Description 

Hillslope gradient is the uphill and downhill slope where the road is 

located. The hillslope gradient is important from several aspects. 

First, any hillslope runoff onto the road needs to be accounted for in a 

road-drainage structure. The steeper the hillslope gradient, the larger 

the cutslope tends to be, which may increase erosion and/or the 

interception of subsurface flows. Secondly, it is difficult to disperse 

runoff on steep hillslopes without causing fillslope erosion. Gentler 

hillslopes provide more opportunities to disperse runoff, whereas 

steep slopes often develop concentrated flowpaths. Routing road 

runoff to natural stream channels through inside ditches is common 

on steep hillslopes.

Questions potentially addressed

• What kind of road-design template is used on steep-hillslope 

gradients?

• What if any problems are associated with this?

Related indicators

The influence of hillslope gradient on soil and water resource impacts 

is most helpful when evaluated using these indicators:

• Road gradient.

• Road-template design.

• Road-surface shape.

• Condition of culverts and/or drainage facility.

• Hydrologic connectivity.

• Diversion potential.
6

Utility

Hillslope gradient is a significant topographic feature that often 

dictates the road-construction methodology. Steep slopes—over 35 

percent gradient—may have full-bench construction where the road 

is cut into the hillside with no fill. Other roads—on somewhat gentler 

slopes—may be a combination of cut and fill, where approximately 

half the road prism is constructed from compacted fill material 

excavated from adjacent areas. The steeper the hillslope gradient, the 

greater the cutslope area and the increased potential for intercepting 

subsurface flows. The soil profile exposed in a cutslope is difficult 

to revegetate due to low organic-matter content and nutrient 

composition. Road segments with steep hillslopes can be costly and 

difficult to maintain.

Key design considerations for a road on a steep hillslope include 

runoff from the hillslope above, potential interception of subsurface 

flows, and identifying suitable locations to disperse runoff without 

creating gullies below the road prism. Many roads on steep hillslopes 

use overside metal drains or culvert extensions to route runoff below 

the road to protect the fillslope. Gullies can form at the outlet of the 

structures without adequate energy dissipaters.

Wemple et al. (1996) found that gully incision is significantly more 

likely below culverts on steep hillslopes (greater than 40 percent 

gradient). Croke and Mockler (2001) found a correlation between 

midslope roads and length of contributing areas. The midslope roads 

drained a greater area, and the gradient of the discharge hillslope 

was much steeper than ridgetop roads. 

Measure

Use a clinometer to field verify both uphill and downhill slope gradient. 

To guarantee an accurate reading, ensure that the measurement is 

taken away from the road’s cut and fill.

5. Road-Surface Material

Description 

The SWRCI focuses on native-surface roads, but can be used 

to inventory an entire watershed rapidly. Roads with a higher 

maintenance level may be aggregate surfaced or paved.

Questions potentially addressed

• How and where does the road-surface material affect erosion 

and sediment potential?

• How and where does the road-surface material adversely 

affect soil quality through road widening or braiding? 

• How and where does the road-surface material intercept 

subsurface flows?
7

Related indicators

• Traffic.

• Road-surface drainage.

• Road gradient.

• Soil texture.

• Surface condition.

Utility

Studies indicate that forest roads are a source of sediment production 

(Croke 2001; Forman 2003; Furniss 1999; Luce 1999; Reid 1984). 

Road-surface material can help reduce erosion and sediment 

delivery by preventing rutting on roads used during wet conditions.

The type and condition of surface material used, combined with 

surface traffic, affect the erosion and sediment potential of a 

segment. McDonald and Coe (2005) found that as little as 10 

centimeters of coarse aggregate on the road reduced the sediment 

production rate by an order of magnitude over a native surfaced road. 

Foltz (1996) studied sediment production rates comparing different 

aggregate quality and found that aggregate quality affected sediment 

production. The study determined that the quality of the aggregate 

was more important with higher traffic and higher rainfall.

Measure

Circle the road-surface material, such as native, aggregate (gravel), 

or paved for the road segment evaluated.

1. Road-Surface Drainage

Description of indicator

Surface drainage provides for the interception, collection, and 

removal of water from the surface of roads and slope areas. Surface 

drainage features may need to accommodate debris movement, 

mud flows, and runoff. For a proper drainage system, combine the 

appropriate design elements to protect adjacent resources. Roadsurface drainage is intimately associated with the road-surface 

shape, topography, and climate. Key design considerations include 

the length of the contributing road area and the contributing hillslope 

above the road.

Surface-drainage structures commonly include ditch-relief culverts, 

drainage ditches, drainage dips, lead-off ditches, and overside drains. 

Culverts in natural stream crossings are not considered a component 

of the road-surface drainage structures but are addressed in the 

stream-crossing condition indicator.

Section 2: Road-Condition 

Indicators
8

Questions potentially addressed

• How and where does the road drainage modify the surface 

and subsurface hydrology of the area? 

• How and where does the drainage structure cause surface 

erosion? 

• How and where is the road hydrologically connected to the 

stream system? 

• How and where does the road drainage cause diversion 

potential? 

• What is the typical length and size of the contributing area?

Related indicator

• Road-surface shape.

• Stream-crossing condition.

• Diversion potential.

• Soil texture.

• Road gradient.

• Hillslope gradient.

• Fillslope condition.

• Cutslope condition.

Utility

Road-surface drainage directly affects soil and water quality 

properties from the time of construction throughout the life of the 

road. Roads reduce infiltration and modify hillslope drainage. 

Therefore, surface-drainage structure design and subsequent 

structure maintenance is critical to the degree of impact a particular 

road segment has on soil and water resources. Proper design and 

maintenance of road-drainage structures can minimize the effects 

of the road and limit onsite and offsite impacts to soil and water 

resources.

The road surface is a main source of erosion and sediment 

generation (Croke and Hairsine 2001; Croke and Mockler 2001; 

Grace 2004; Luce 2002; Luce and Black 2001; Reid 1984). Welldesigned roads with functioning surface-drainage structures can 

minimize erosion and sediment delivery.

Studies by Borga (2005) evaluated the influence of roads in steep 

forested areas prone to landslides. Roads concentrate flow, creating 

shallow landslides. Sources of runoff include overland flow from the 

compacted road surface and subsurface flow intercepted by road 

cutslopes. In some instances increased road runoff occurred from 

partially plugged or blocked culverts causing shallow landslides 

below the road.
9

Croke and Hairsine (2001) identify the importance of road-drainage 

spacing as a key design variable for effective roads management. 

Drainage spacing is linked to road gradient and regional rainfall 

characteristics. The position of the drainage discharge or outfall point 

in the landscape can create erosion at the outlet.

Evidence of scour in the drainage ditch or at the outlet of a culvert 

indicates an exceedance of threshold stream power (Croke 2001). At 

this point the ditch and area below the culvert outlet become a source 

of sediment, which affects the road-condition rating.

The road-drainage structure combined with slope position also 

can affect the road segments’ impact on soil and water condition. 

Croke and Mockler (2001) found statistically significant differences 

between ridgetop roads with winged ditches versus midslope roads 

with ditch-relief culverts. The contributing road length was up to 

three times longer on midsloped roads. Midsloped roads discharged 

onto hillslopes that were twice as steep. The study found that 

linkages between midsloped roads with ditch-relief culverts can have 

deleterious impacts to water quality.

Road segments that are entrenched concentrate surface runoff and 

may intercept subsurface flows. In some topography it is difficult to 

avoid short (25- to 50-foot) entrenched road segments. Avoid long 

entrenched segments to reduce road-derived sediment.

Measure

Drainage structures cause adverse effects to soil and water 

resources by increasing drainage density, accelerating erosion, 

creating diversion potential, increasing hydrologic connectivity, 

plugging with debris, and increasing drainage-ditch scour and 

erosion.

Field evaluation of drainage structures using physical features, 

including rill or gully erosion, sediment deposition, and blocked 

structures, identifies how well the structures function.

Road-Surface Drainage Types

Ditch-Relief Culverts. Ditch-relief culverts periodically relieve the ditch 

flow by piping water to the opposite side of the road, where flow can 

disperse away from the roadway without creating erosion. Spacing 

depends on road gradient, road surface and ditch soil types, runoff 

characteristics, and the effect of water concentrations on slopes 

and streams below the road (FSH 7709.56 Road Preconstruction 

Handbook).
10

A survey conducted for the Oregon Department of Forestry 

determined that “sediment blockage and crushing of culvert inlets are 

the most common factors reducing the capacity of stream and roadcrossing culverts”(Skaugset 1998). Approximately 28 percent of the 

ditch-relief culverts had inlet capacities reduced by at least half.

Figure 1—Components of a road.

Drainage Ditch. The drainage ditch transports water leaving the road 

surface or cutslope to the nearest ditch-relief culvert, lead-out ditch, 

or natural channel. The ditch is constructed between the traveled way 

and the adjacent terrain. Drainage ditches are found on insloped, 

outsloped, and crowned roads. Drainage-ditch effectiveness varies 

with length, slope, and armor.

Lead-Off Ditch–Winged Ditch. A lead-off ditch (or winged ditch) is 

designed to carry water away from the road. This ditch is used in flat 

or gentle topography and often is associated with a crowned road 

design. The lead-off ditch is located at the point where a roadside 

ditch daylights onto natural ground.

The road-section length and width contributing to a lead-off ditch 

determines the distance sediment travels downslope (Grace 2005).

Drainage Dips. Drainage dips intercept and remove surface water 

from the traveled way and shoulders. When properly constructed, 

they provide uninterrupted flow of traffic and are relatively 

maintenance-free drainage structures. Drainage dips are not 

recommended on road gradients greater than 10 percent.
11

Overside Drains. Overside drains are used to protect the fillslope from 

increased erosion. Overside drains are commonly used in conjunction 

with drainage dips or entrenched road prisms. All structures must be 

located, installed, and maintained properly to be effective.

2. Stream-Crossing Structure Condition

Description of indicator

Proper road drainage is critical in preventing adverse impacts to 

water and soil resources. Stream-crossing structures are designed 

with consideration of topographic, climatic, hydrologic, and other 

environmental conditions (sensitive soils, springs, unstable geology). 

Evaluation of the indicator identifies the physical condition of the 

structure as well as its ability to pass debris and water over time.

Questions potentially addressed

• Does the structure provide passage of runoff and associated 

sediment and debris? 

• How and where does the stream crossing cause erosion? 

• Does the crossing have diversion potential? 

• Is there the potential for cascading effects with other drainage 

structures within the segment? 

• Is the road hydrologically connected to the stream-crossing 

structure?

Related indicators 

The stream-crossing structure condition is related to the road-surface 

drainage indicator. Combining the two indicators provides a synopsis 

of how surface drainage is removed from the surface of roads and 

slopes, and the physical condition of the structure. 

Evaluation of stream-crossing structure condition is linked to the 

diversion-potential indicator. 

Utility

A survey conducted for the Oregon Department of Forestry 

determined that “sediment blockage and crushing of culvert inlets are 

the most common factors reducing the capacity of stream and roadcrossing culverts”(Skaugset 1998). 

Measure

Field review of the stream crossing identifies its physical condition 

and evidence of its ability to handle runoff and provide the debris 

passage required for a road-segment rating. 
12

3. Road-Subsurface Drainage

Description 

Subsurface drainage is designed to intercept, collect, and redirect 

ground water that may otherwise flow into the road subgrade. 

Subsurface-drainage features also lower water tables and drain 

isolated water pockets.

Subsurface-drainage design may use a ditch and ditch-relief culverts 

to intercept and redirect ground water into a stream. Permeable 

fills are constructed to allow ground water to percolate through the 

subgrade of the road. For isolated water pockets, use French drains 

or design an engineered drainage system to keep water away from 

the road surface. This will provide stability and increase load-bearing 

capacity.

Questions Potentially Addressed

• How and where does the road segment intercept subsurface 

flow? 

• How and where does the road modify or change subsurface 

flow? 

• How and where does the road change adjacent vegetation?

Related Indicators

• Road-surface drainage.

• Cutslope condition.

• Fillslope condition.

• Hydrologic connectivity.

• Road-segment location.

Utility

Roads that intercept subsurface flows create adverse affects on soil 

and water resources in several ways. The degree to which a road 

intercepts subsurface flow—and subsequently redirects it—varies 

from location to location.

Wemple (2003) tested a theoretical model to evaluate how roads 

interact with hillslope flows dominated by subsurface flow and the 

hydrologic response during storms. The model can target road 

segments that need restoration or removal because of subsurfaceflow interception. Findings showed variability as each segment of a 

forest road acts as a subcatchment for the hillslope above the road. 

Therefore the road catchment includes the hillslope above the road, 

the road surface, and the roadside ditch draining to the culvert. Road 

segments responded differently with hillslope length, soil depth, 

and cutbank depth as key variables for determining road-runoff 

production. Findings demonstrated that as road segments intercept 
13

subsurface flows and redirect them to ditches and streams, the timing 

of water delivery in the catchment is altered. The key is in identifying 

which road segments intercept subsurface flows and whether they 

significantly augment the hydrograph and increase the potential 

flooding.

La Marche and Lettenmaier (2001) developed a model to determine 

how forest roads contribute to streamflow based on:

(1) Interception of subsurface flows by road cutslopes.

(2) Road-generated runoff to the stream network via the roaddrainage system.

Their study, conducted on the Deschutes River in Washington, tried 

to discern the impacts of increased runoff from forest roads versus 

increased runoff from vegetation removal at different scales. Findings 

from the predictive model and field testing identified a synergistic 

response of increased runoff where vegetation removal occurred 

above forest roads. The road cutslope height did not affect ditch 

runoff. 

Tague and Band (2001) built on Wemple’s (1996) studies of 

connectivity and developed another model. The study was to identify 

the hydrologic affects to areas below roads where subsurface flows 

were intercepted and redirected away by ditches, and areas below 

cross-drain culverts that were not hydrologically connected to the 

stream. Results showed that redirecting subsurface flows can 

produce a significant reduction in downslope soil moisture and may 

lead to adverse affects on vegetation over time. Vegetation health 

in water-limited environments may be an indicator of change in 

subsurface water storage and possibly a change in water quality.

These studies were conducted in the Pacific Northwest and illustrate 

the road’s effect in the interception and redirection of subsurface 

flows. On forests with roads that intercept and redirect subsurface 

flows, staff should be cognizant of the potential adverse offsite effects 

of increased runoff and flooding that may occur. This indicator, as 

tested by Wemple, helps identify road segments that require further 

analysis, reconstruction, or potential road removal.

Measure

Evaluations of the effect of functioning subsurface drainage include 

factors, such as erosion of ditches from the change of subsurface 

to surface flows in a ditch, change in vegetation community, and/or 

adverse impacts from concentration of flows and flooding. 
14

4. Diversion Potential at Crossings

Description of indicator

Diversion potential predicts how a stream responds if the capacity 

of the stream-crossing structure is exceeded, or if the structure 

becomes plugged. If the stream overtops the crossing and remains 

in its natural channel, there is no diversion potential. If the stream 

overtops the crossing and flows down the road, there is diversion 

potential.

Diversion potential exists on roads that have a continuous climbing 

grade across the stream crossing or where the road slopes 

downward away from a stream crossing in at least one direction 

(Furniss 1997).

Diversion potential affects the SWRCI when the road-design 

elements do not adequately account for diversion. In many cases, 

designed diversion-prevention dips below culverts ensure that a 

stream that overtops its culvert will stay in its natural channel.

Questions potentially addressed

• Do the road segment design elements (physical characteristics 

of the road) account for diversion potential? 

• What is the diverting feature (road or ditch)? 

• What is the potential diversion distance (how far will the water 

flow before entering its original channel or another existing 

channel)?

Related indicators

The following indicators help assess the potential adverse effects 

of stream diversion. A diversion potential on an outsloped road with 

a road gradient of less than 5 percent may have localized erosion. 

The diversion potential can be eliminated easily with a diversionprevention dip. On an insloped road with a road gradient of 12 

percent, the extent of diversion potential may be greater and the 

solution is more difficult and costly.

• Road-template surface shape.

• Road gradient.

• Road-surface drainage.

• Road-surface material.

Utility

Diversion potential at road-stream crossings is relatively easy to 

identify and inexpensive to repair (Furniss 1997). The potential effect 

of a stream diversion to soil and water resources can be significant 

and very costly—if not impossible—to repair in many locations. The 

soil lost during the incision process of the new stream channel is 
15

rarely accounted for in the damage-survey report. Repairs generally 

modify the road-prism geometry to prevent diversion potential, 

but in some cases, the fix comes after a significant soil loss and 

deterioration of water quality.

Inventory data collected in California, Oregon, and Washington, 

determined that 56 percent of the 1,922 road-stream crossings on 

Federal lands would divert with exceedance events. The potential 

diversion distance varied with 36 percent exceeding 31 meters in 

distance (Furniss 1997). Road-stream crossing assessments have 

been the focus on many forests to alleviate the threat of diversion 

potential. (Scaife, personal communication).

Measure

Diversion potential in a road segment is determined by identifying 

the interaction of drainage structures (road-stream crossings and 

ditch-relief culverts) with the road-design template or road-surface 

shape. Reviewers evaluate the low point of the road over the crossing 

structure compared to surroundings to determine where water will 

flow should the crossing pond water and overtop. The number of 

crossings with diversion potential on the road segment is then divided 

by the total number of crossings to identify a percentage of crossings.

5. Road-Stream Connectivity

Description of indicator

Connectivity is the extent to which the drainage features of a road 

system are linked directly to the channel network. Research shows 

that roads intercept surface and subsurface flow and reroute runoff 

to the channel network through ditches (Forman 2003). Connectivity 

generally is expressed as the percentage of the total road length 

connected to the channel network rather than the percentage of a 

given road segment.

A road’s hydrologic connectivity includes flows between crossdrain culverts on an insloped or crowned road, and any continuous 

flowpath from the culvert outlet to the stream via a gully. Steeper 

hillslope gradients (greater than 40 percent) have a higher 

predictability of gullying at cross-drain outlets (Wemple 1996).

Questions potentially addressed

• How and where is the road hydrologically connected to the 

stream? 

• What are the characteristics of the segment that is 

hydrologically connected to the stream? 

• What percentage of the road network in a watershed is 

hydrologically connected?
16

Related indicators

The effect of hydrologic connectivity can be better assessed in 

combination with these related indicators:

• Road-template design/surface shape.

• Road-surface drainage.

• Condition of culvert/drainage facility.

• Diversion potential.

• Road gradient.

• Hillslope gradient.

• Cutslope condition.

• Fillslope condition.

Utility

The hydrologic connectivity of roads is a simple indicator to identify 

and provides valuable information on the amount of total roadstream connectivity in a watershed. Where road-stream hydrologic 

connectivity exists, accelerated runoff, sediment, and roadassociated chemicals have a direct route to surface waters (Furniss 

2000).

Wemple (1996) studied surface runoff from roads by analyzing the 

amount (volume) and timing effect from increased drainage efficiency. 

Approximately 57 percent of the road length surveyed functioned as 

flowpaths and was hydrologically connected to the stream network. 

The type of flowpath varied and included road segments directly 

draining to streams, and road segments draining to culverts with 

gullies incised below.

Wemple tested a model to predict gullies below ditch-relief culverts. 

Gullies were predicted accurately using the length of road segment 

contributing to the culvert and the hillslope gradient where the road is 

located. 

In contrast, La Marche (2001) found that the hillslope curvature 

and distance to the natural stream channel are the most important 

variables in determining connectivity of ditch-relief culverts to the 

channel network. Hillslope curvature relates to the “concentration of 

subsurface flow and, therefore, to the occurrence of surface flow and 

channel initiation” (La Marche 2001).

Croke et al. (2001) studied gully initiation and road-to-stream 

linkages in Australia and found that 18 percent of the surveyed road 

was hydrologically connected to the stream via gully development 

from culverts. Most hydrologically connected road segments were 

in the midslope position (on the hillslope) and had three times 

larger contributing areas than the roads on the ridgetop position. 

Additionally, gully development could be predicted 79 percent of the 

time from a statistical functional analysis using contributing road 
17

length or area and hillslope gradient. Croke et al. also found that the 

size of sediment produced from the roads fell into the less than 2.0 

millimeter in diameter class. Other studies link deleterious effects to 

aquatic organisms from fine sediment.

The utility of the hydrologic connectivity indicator is that—once 

identified—the ability to “disconnect” road segments can be achieved 

through reconstruction, maintenance, and relocation. Croke (1999) 

suggested that runoff delivery in nonchannelized pathways could be 

controlled by manipulating the runoff volume and better distributing 

runoff across a hillslope. Croke found that sediment concentrations in 

runoff entering a gully will persist within the gully pathway without any 

deposition. Therefore, practices that reduce hydrologic connectivity 

through gullies and focus on dispersive road-surface drainage 

designs, such as outsloping, may substantially cut sediment delivery 

to streams.

Measure

Road-stream connectivity is determined two ways: (1) Identify ditch 

lengths and road segments that drain directly into a stream. Ditches 

on insloped roads commonly drain to the inlet of a stream-crossing 

structure. (2) Identify ditch lengths that drain to ditch-relief culverts 

that show scour at the outlet, which result in gullies that eventually 

lead to a stream channel. Measure those ditch lengths with gully 

erosion greater than 10-meters below the culvert. Identify the length 

of contributing area for each category of culvert and divide by the 

total length of the road segment to rate.

6. Road-Surface Condition

Description of indicator

Road-surface condition identifies active erosion or rutting on the 

road surface. Rill and gully formation within the traveled way may 

adversely affect soil and water quality. Potential erosion of surface 

fines from heavily used roads, which result in soft powdery materials 

susceptible to erosion, also may adversely affect soil and water 

quality.

There are different types of erosion: sheet, rill, and gully. Sheet 

erosion is a uniform detachment of soil particles from the soil surface. 

Rill erosion is the detachment and transport of soil by a concentrated 

flow of water. Rills are a minimum depth of 1 inch, a minimum length 

of 12 feet, and a depth change of at least 25 percent over the 12-

foot length. A gully is defined as an erosion channel where water 

accumulates in narrow channels and, over short periods, removes 

the soil from this narrow area to considerable depths, ranging from 1 

to 2 feet to as much as 75 to 100 feet. (Brady 1984)
18

Questions potentially addressed

• How does the road-surface condition affect erosion rates? 

• How does the road surface channelize surface flow?

Related indicators

• Road gradient.

• Soil texture.

• Road-surface drainage.

• Road-surface material.

• Traffic level.

Utility

Rills, gullies, and ruts are detected easily while surveying a road 

segment. Loose road material also can be a key factor in the surface 

condition. Roads receiving high levels of traffic or experiencing 

droughty conditions become dusty and have excessive material that 

is mobilized easily in a storm event. Road-gradient and road-surface 

material particle size are two key variables influencing sediment 

production from roads (Kahklen 2001).

Clues to how a road responds to runoff are not always apparent. 

Field review of a road segment during the summer may not reflect 

rutting from wet-season use. Maintenance activities, such as road 

grading, may remove or repair evidence of rill-and-gully patterns. 

Ziegler (2004) found that flow-path changes caused by ruts were later 

remedied with maintenance. Similar results are seen with loose road 

material. The first storm provides an initial flush of sediment with a 

gradual decline in material transported. Sediment transport is limited 

by the detachment of new material, which is a function of surface 

erodibility. Adjust field visits accordingly to target segments that may 

be vulnerable to rills, gullies, ruts, and loose road material. 

Measure

Surveyors determine the surface condition by identifying the 

presence, absence, and extent of rills, ruts, and gullies along the road 

segment.

7. and 8. Cutslope and Fillslope Condition

Description of indicator

The footprint of a road on the landscape is measured from the top 

of the cutslope to the toe of the fillslope. The amount of material 

removed or disturbed during road construction affects revegetation 

and overall hillslope stability. Subsurface flows on cutslopes may 

cause instability and long-term maintenance needs. Fillslopes provide 

buffers and can disperse runoff and trap sediment effectively if wellvegetated and stable.
19

Questions potentially addressed

• How and where does the road intercept subsurface flow? 

• How and where does the road affect cutslope or fillslope 

stability? 

• How and where does the cutslope modify erosion rates?

Related indicators

The cutslope- and fillslope-indicator is examined in the context of the 

following indicators:

• Road gradient.

• Hillslope gradient.

• Road-surface drainage.

• Road-stream connectivity.

• Soil texture.

Utility

The cutslope and fillslope condition affects sediment production. 

Cutslope and fillslope stability is observed by determining the amount 

of surface vegetation and cover on the slope. Luce and Black 

(1999) found that road segments cleared of vegetation produced 

seven times more sediment than road segments where vegetation 

was retained. Researchers (Appelboom 2002; Burroughs 1990; 

Burroughs and King 1985; Croke 2006; and Wemple 2003) identified 

the importance of the cutslope in sediment-production capability and 

the effectiveness of different mulches and vegetation in reducing 

sediment.

Burroughs (1990) found that filter windrows are cost effective in 

reducing sediment yield by 88 percent compared to untreated 

fillslopes. Burroughs found sediment-reduction decreased as slopegradient and silt-content increased.

Measure

Identify the effective soil cover and stability rating for both the 

cutslope and fillslope. Use tools, such as aerial photography to 

identify roads with large cuts and fills, and geographic information 

systems to overlay hillslope gradient, roads, and sensitive soil types 

to detect potentially unstable areas.
20

Traffic Level 

Traffic is the amount of use a road receives. Traffic is defined by 

type, frequency, and seasonality. Unfortunately, Forest Service traffic 

counts are seldom collected over an entire watershed. However, 

information on commercial haul is accounted for in timber receipts 

and estimated from road-use permits.

Questions potentially addressed

• How and where does traffic affect the erosion and sediment 

potential of the road? 

• How does the seasonality of use affect the road condition?

Related indicators

• Road-surface material.

• Road-surface condition.

• Road gradient.

• Soil texture.

Utility

Studies link the traffic level on roads to erosion and sediment 

production from roads with high-traffic use (defined as over four 

logging trucks per day) (Cederholm et al. 1980). Studies by Croke 

(1999) in the Cuttagee Creek watershed found traffic volume 

influenced the amount of sediment available for mobilization. 

Estimates of traffic were ranked similarly to those of Cederholm et al. 

(1980) and Reid (1981). Croke’s work complements earlier studies by 

Reid and Dunne (1984) and found the following:

1. Sediment concentrations in road runoff were between 

five- and eight-times higher on well-used roads than 

abandoned ones.

2. Roads with higher intensity traffic have greater volumes of 

loose material available at the surface.

3. Roads used infrequently or abandoned have little 

sediment and are minor sources of sediment (Croke 

1999).

Measure

Traffic on a road segment is divided into three categories: high, low, 

and closed. The three categories serve as an initial screening tool 

despite the relative subjectivity that may occur between designating 

a high or low surface-traffic level. Prior to conducting SWRCI, each 

forest staff should document the criteria for assessing surface traffic. 

In many areas, traffic levels adjust seasonally based on access and 

recreational opportunities. In other situations, commercial log-haul or 

chip-van access can occur for just one to two seasons with low traffic 

thereafter.

Section 3: Other Road 

Condition Evaluation 

Considerations
21

A closed road has a barrier to prevent vehicle access. A level 1 road 

has barriers that prevent vehicle traffic. The SWRCI assessment 

must be done by walking the road.

A road may be rated as high from administrative and commercial 

traffic associated with a forest health project; oil and gas exploration; 

or recreational traffic to a popular destination, such as a campground, 

fishing area, or day-use facility. Generally, collector and arterial roads 

have more volume, and low-traffic volume roads are associated with 

local and spur roads.

Soil Texture

Description

Soil erosion from a road’s surface, cutslope, or fillslope can adversely 

impact water quality. Erosion reduces soil quality, and vegetation 

establishment on degraded soils is more difficult. Keep soil erosion 

to a minimum to reduce a road’s potential adverse impact on soil and 

water resources.

Different erosional processes occur on roads due to reduced 

infiltration. The response of roads to runoff varies with soil-particle 

size, slope length, and road gradient. Roads constructed with large 

cuts and fills may use road-surface material from another soil horizon 

or imported from offsite. Identifying the soil texture can help in 

locating and designing a road to prevent accelerated erosion and to 

ensure adequate bearing capacity of the road.

Questions potentially addressed

• How and what erosional processes occur on the road? 

• What soil-particle size is dominant throughout the road 

segment? 

• What segments of road have the higher erosion rates? 

• What type of road-template design and road-surface drainage 

is present on roads with high erosion rates? 

• What is the length of the contributing area? 

• How does the road drainage affect erosion on or off the road?

Related indicators

Indicators directly related to the erosional process on a road include 

the following:

• Road gradient.

• Road-surface material.

• Cutslope condition.

• Fillslope condition.
22

Utility

Soil texture provides information on potential adverse effects to 

downstream values from erosion and subsequent sediment delivery. 

Fine-textured soils may adversely affect spawning gravels, or reduce 

water clarity. Soil texture is a component of the WEPP model. The 

WEPP road model (Elliott 2004) data are entered into the Web-based 

model to estimate sediment production and delivery. Forest staff 

and private landowners collect information on soil texture as a part 

of the WEPP model to estimate the sediment loads from unpaved 

roads entering streams (Watershed Conservation Resource Center, 

unpublished paper).

Luce and Black (1999) found that soil texture has a strong effect on 

sediment yield. The finer textured soil produced more sediment than 

coarser, sandy soils. Even with gravel (aggregate) placed on roads, 

silty-clay loam soils produced nine times more sediment than roads 

constructed on a gravelly loam. Burroughs (1992) using laboratory 

tests and rainfall simulation found low erodibility in clay- and sanddominated soils, with higher erodibility as the silt-fraction increased.

Measure

Identify the dominant soil texture: clay loam, sandy loam, silt loam, 

or loam. Soil texture can be preidentified from the soil-resource 

inventory for the forest and used to pre- or post-stratify road 

segments. The soil textures can be included in the Web-based WEPP 

model to identify sediment production from different road segments.
23

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26

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SDTDC’s national publications are available on the Internet at:

http://www.fs.fed.us/eng/pubs/

Forest Service and U.S. Department of the Interior, Bureau of Land Management employees also can 

view videos, CDs, and SDTDC’s individual project pages on their internal computer network at:

http://fsweb.sdtdc.wo.fs.fed.us/

For additional information on the Soil and Water Road Condition Index project, contact Carolyn Napper at 

SDTDC. Phone: 909–599–1267 ext. 229. E-mail: cnapper@fs.fed.us