Forest Road Sediment and Drainage Monitoring Project Report for

Private and State Lands in Western Oregon

February 20, 1998

Prepared for:

Oregon Department of Forestry

Salem, Oregon

By:

Arne Skaugset

Marganne M. Allen

Forest Engineering Department

Oregon State University

Corvallis, Oregon
1

Forest Road Sediment and Drainage Monitoring Project Report for Private and

State Lands in Western Oregon

Summary

This is the second report completed as part of a four-year project to investigate the

effectiveness of forest road drainage practices designed to minimize sediment delivery to

streams. This investigation is expected to yield a list of recommended road drainage and

construction practices for private and public forest land managers and agencies that

regulate forest management activities in western Oregon. This report summarizes data

collected during the summer and fall of 1995 and 1996, years two and three of this

project. Road drainage and sediment delivery data were analyzed in a regional context,

as well as broken into categories based on best management practices (BMP’s). A final

technical paper will be produced at the end of this project.

Objectives

1. Develop field methods that determine the sediment delivery potential from forest

roads.

2. Survey discharge structures and their potential for sediment delivery to waters of the

state on forest roads in each of the five western Oregon georegions.

3. Develop a forest road inventory and erosion hazard inventory protocol for

landowners.

4. Develop a road management guidebook.

5. Develop a technical issue paper for the Board of Forestry.

Background

Forest roads are recognized as the greatest potential source of accelerated erosion

associated with forest management. The original intent of this four year monitoring

project was to evaluate the effectiveness of Oregon’s forest practice standards to

minimize delivery of surface erosion from forest roads to waters of the state.

The February 1996 storm caused many landslides, channel changes, and other

effects to natural resources and private property in northwestern Oregon. The storm was

an unusual, but extremely important event that provided a unique opportunity to test the

effectiveness of forest practices developed over the last two decades.

This report summarizes monitoring activities conducted during the summer and

fall of 1995 and 1996. The data set reflects years when the relative importance of chronic

surface erosion versus episodic landslide erosion as sources of sediment may have

differed.
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Methods

This report includes data that was collected during the summer and fall of 1995

and 1996. The method used to select areas to be surveyed differed each year. In 1995,

all accessible roads within a randomly selected township, range and section were sampled

to obtain at least 10 miles of road length. Where the randomly selected area (township,

range and section) did not contain 10 miles of road, the sampling boundary was extended

until that length of road was encompassed. Land in federal ownership, agricultural or

urban landuse or dominated by large waterbodies was not sampled.

Areas sampled in 1996 were chosen using two different methods. First of all,

three 10 square-mile areas of the highest landslide density observed after the February

1996 storm were selected non-randomly by the Oregon Department of Forestry (ODF)

and are called the “red” zones. Secondly, areas of similar landscape, climate and geology

but with low-landslide density were identified and three areas were randomly selected

from these areas, one to pair with each of the “red” zones.

Forest roads in each of the five western Oregon georegions were sampled in 1995.

During the 1996 survey, only three georegions were sampled (Coast Range, Interior,

Western Cascades), thus the South Coast and Siskiyou georegions are represented by

only the 1995 data. The Oregon forest practice rules (ODF 1997) use the term

“geographic region” to describe large areas with similar climate, geomorphology and

natural vegetation. There are five geographic regions in western Oregon:

1) Coast Range – Characterized by marine sandstone geology, high

annual rainfall and steep coastal mountains.

2) Western Cascades – Characterized by volcanic geology, high annual

rainfall and is generally mountainous.

3) Interior – The foothills on either side of the Willamette Valley.

4) South Coast – An area of unique geology and hydrology in Oregon

that consists mainly of sheared metamorphic rocks and has the highest

instantaneous maximum streamflows in the state.

5) Siskiyou – Highly variable geology and average annual rainfall ranges

from 20 to over 100 inches.

Information gathered

The same methods were used to collect data in both 1995 and 1996 although the

codes used for some road classifications and characteristics differed slightly. The data

gathered included:

1) Road characteristics;

2) Condition of sample road segments (between surface discharge points); and

3) The location of surface water discharge points, including their potential to

deliver sediment to waters of the state.
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1) General Characteristics

The following data was collected to characterize the road:

a) Legal description (section, township, range) and name;

b) Forest practices maintenance status (active, inactive or vacated); and

c) Road surfacing material (clean rock, dirty rock or dirt).

2) Source Area

The source area of a surface water discharge point is the length of road draining to

that point. Information was gathered for every road segment in the survey area. The

information collected was used to evaluate the potential of the road to generate sediment

for delivery to a surface water discharge point. This information was also used to

evaluate road BMP’s for erosion control and dispersal of surface waters.

The length and slope of each road segment was measured. Road design was

categorized as crowned, insloped or outsloped. The condition of the ditch and the height

and vegetative cover of the cutslope were also determined.

3) Discharge

Surface drainage points from the road include cross-drainage culverts, live stream

crossings, waterbars, rolling dips, grade breaks, saddles, and random points of discharge.

Information on drainage characteristics and potential sediment delivery at all surface

drainage points was collected.

The diameter of all cross-drainage and live stream crossing culverts was

measured. Culvert condition was estimated as the amount of the culvert inlet remaining

open expressed as a percent. Where the culvert inlet was reduced, the cause was

recorded as mechanical crushing, filling by debris or age related deterioration.

The culvert outlet or other discharge point was examined for evidence of sediment

delivery to waters of the state. Discharge directly into a stream or a gully connected with

a stream was rated a “yes” for delivery. Discharge points with either no evidence of

erosion or deposition at the outlet or no evident pathway for sediment to enter the waters

of the state were rated “no” for sediment delivery. All other discharge points rated

“possible” for sediment delivery.

4) Data Collection

The length of each road segment was measured using a distance measuring

instrument (DMI) that was installed in the vehicle and recorded distance in feet. A hip

chain was used for distance measurements when roads could not be driven. A clinometer

was used for slope measurements.
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Results

The analysis addressed the following questions:

1) What is the proportion of the forest road system that potentially delivers sediment to

streams;

2) What is the spacing of surface runoff discharge points;

3) What is the distance from stream crossing structures to the first drainage point

upslope; and

4) What are the characteristics of forest road drainage systems and road segments where

evidence of sediment delivery to waters of the state exists.
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Summary of road survey information by georegion

Table 1: Summary of road survey information by georegion.

Georegion Percent

Road

Grade

No. Road

Segments

Sum of Length

(ft)

Average

Length (ft)

Percent of Length in

Georegion

0 – 4 405 188,056 464 40

5 – 8 246 129,861 528 27

9 – 12 158 70,348 445 15

13 – 18 182 79,357 436 17

>18 30 5,212 174 1

Coast Range

Total 1,021 472,834 463 100

0 – 4 496 198,977 401 40

5 – 8 305 195,760 642 39

9 – 12 113 70,815 627 14

13 – 18 44 25,028 569 5

>18 10 5,475 548 1

Interior

Total 968 496,055 512 100

0 – 4 138 37,530 272 28

5 – 8 77 35,178 457 26

9 – 12 77 33,661 437 25

13 – 18 67 28,025 418 21

>18 3 385 128 0

South Coast

Total 362 134,779 372 100

0 – 4 135 27,432 203 38

5 – 8 41 13,985 341 20

9 – 12 52 18,449 355 26

13 – 18 13 9,523 733 13

>18 4 2,282 571 3

Siskiyou

Total 245 71,671 293 100

0 – 4 319 123,545 387 37

5 – 8 177 80,221 453 24

9 – 12 104 52,614 506 16

13 – 18 112 74,348 664 22

>18 0 0 0 0

Western

Cascades

Total 712 330,728 465 100

0 – 4 1,493 575,540 385 38

5 – 8 846 455,005 538 30

9 – 12 504 245,887 488 16

13 – 18 418 216,281 517 14

>18 47 13,354 284 1

Entire

Database

Total 3,308 1,506,067 455 100
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Table 2: Summary of road survey information by georegion. ESS = Excessively spaced segment by

Arnold’s (1957) criteria for road drainage spacing. Y or P = “Yes” and “Possible” connectivity with

waters of the state.

Georegion Percent

Road

Grade

Percent Length in

Georegion Rated

(Y or P) for

Sediment Delivery

Percent Length

in Georegion

ESS

Percent Length in Georegion ESS

and Rated (Y or P) for Sediment

Delivery

0 – 4 14 7 2

5 – 8 10 12 4

9 – 12 3 10 2

13 – 18 6 13 5

>18 0 0 0

Coast Range

Total 33 42 13

0 – 4 8 4 1

5 – 8 9 16 4

9 – 12 4 12 3

13 – 18 2 5 2

>18 1 1 1

Interior

Total 24 42 11

0 – 4 8 0 0

5 – 8 8 8 4

9 – 12 6 16 4

13 – 18 1 17 1

>18 0 0 0

South Coast

Total 23 41 9

0 – 4 13 2 0

5 – 8 11 6 6

9 – 12 17 14 8

13 – 18 5 13 5

>18 1 3 1

Siskiyou

Total 47 38 20

0 – 4 17 2 1

5 – 8 8 6 2

9 – 12 5 11 4

13 – 18 7 21 6

>18 0 0 0

Western

Cascades

Total 37 40 13

0 – 4 12 4 1

5 – 8 9 11 4

9 – 12 4 12 3

13 – 18 5 12 4

>18 1 1 0

Entire

Database

Total 31 40 12
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1995 Survey Comparison

This report summarizes the combination of the 1995 and 1996 ODF forest road

drainage and sediment delivery surveys. The 1995 data is summarized in a 1996 report to

the Department of Environmental Quality (ODF 1996a). To the length of road surveyed

in 1995, 61 miles (324,141 feet) were added to the Coast Range georegion, 72 miles

(380,876 feet) were added to the Interior georegion and 28 miles (148,278 feet) were

added to the Western Cascade georegion by the 1996 survey. No survey data was

collected in 1996 in the Siskiyou or South Coast georegions. The number of surface

drainage discharge points surveyed approximately doubled from 1,398 to 2,810.

Delivery to Streams

Of the 285 miles (1,506,067 feet) of forest road surveyed in the five georegions,

25 percent drained directly to streams and another 6 percent were rated “possible” for

sediment delivery. A total of 31 percent of the road length surveyed had a “yes” or

“possible” delivery rating. Within the georegions, the combined “yes” and “possible”

ratings ranged from 23 percent of the length of roads surveyed in the South Coast up to

47 percent in the Siskiyou georegion.

Spacing of Drainage Systems

The spacing between surface drainage discharge points ranged from 0 feet

(culvert installed at a high point in the road) to 7,516 feet. The average spacing between

surface drainage discharge points was 455 feet for all georegions and ranged from 293

feet in the Siskiyou georegion to 512 feet in the Interior georegion. The average segment

length of roads with a grade greater than 18 percent was 284 feet and increased to 538

feet on roads with grades of 5-8 percent.

The spacing between points of surface road drainage (road segment length

between drainage points) was compared with the spacing recommended by Arnold

(1957). Arnold’s spacing criteria was developed based on experience in the Western

Cascades. The spacing of inventoried drainage structures was compared with Arnold’s

recommended spacings for a silt-loam soil and a 1-2 in/hr rainfall intensity.

Table 3: Arnold's (1957) recommended forest road drainage spacings for a silt loam soil receiving 1-2

inches/hr of rainfall.

Slope Class Excessively Spaced Segment (ESS) Criteria (ft)

0 – 4 1500

5 – 8 865

9 – 12 480

13 – 18 335

>18 250
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Of the road length surveyed, 40 percent were in segments that were longer than

Arnold’s spacing criteria. This value was consistent throughout the georegions, ranging

from 38 percent in the Siskiyou georegion to 42 percent in the Coast Range georegion.

Of the total length of excessively spaced segments (ESS), 88 percent have slopes

between 5 and 18 percent. Thirty-one percent of the length of ESS’s is in the 13-18

percent road grade class. Of the total road length surveyed, 12 percent was classified as

both ESS and rated “Yes” or “Possible” for sediment delivery to waters of the state.

Figure 1 shows the ratio of the average road segment length to the maximum

length according to Arnold’s criteria by road grade category and georegion. On average,

segment lengths remain below the limits set by Arnold’s criteria except in the 13 – 18

percent road grade category where the average of all georegions is at least 25 percent and

up to 119 percent greater. Though the Coast Range had the highest percent of road

segments exceeding Arnold’s spacing criteria, Figure 1 suggests that the spacing criteria

was not exceeded by very much. The georegions that exceed the criteria by the most, on

average, are the Interior and Siskiyou georegions.

Culvert Spacing Relative to Arnold's Criteria

0

0.5

1

1.5

2

2.5

0 - 4 5 - 8 9 - 12 13 - 18 >18

Road Grade Class (%)

Ratio (Segment Length/Arnold's Criteria Length)

C R

INT

S C

S K

W C

Figure 1: Ratio of segment length to Arnold’s criteria recommended length expressed as an average by road gradient

class and georegion. CR = Coast Range, INT = Interior, SC = South Coast, SK = Siskiyou and WC = Western

Cascades.
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Distance Above Stream to First Drainage

Table 4: Summary of stream-crossing segment data.

Georegion Road Stream Crossing Segments Rated “Y” or “P” for Sediment

Grade No.

Segments

Total

Segment

Length

(ft)

Avg.

Segment

Length

(ft)

Percent of Total

“Y” or “P”

Length Upslope

of Stream

Crossings

Percent of Total

“Y” or “P”

Length Upslope

of Stream

Crossings and

ESS

0 – 4 99 45,301 458 29 8

5 – 8 73 37,274 448 23 13

9 – 12 13 13,855 543 5 5

13 – 18 33 5,071 561 12 16

>18 1 111 111 0 0

Coast Range

Total 219 106,642 487 69 42

0 – 4 89 31,228 351 26 4

5 – 8 55 36,749 668 31 18

9 – 12 19 13,855 729 12 14

13 – 18 8 5,071 634 4 5

>18 1 507 507 0 1

Interior

Total 172 87,410 508 73 42

0 – 4 40 9,407 235 30 0

5 – 8 15 9,640 643 30 21

9 – 12 11 5,625 511 18 14

13 – 18 3 1,093 364 3 3

>18 0 0 0 0 0

South Coast

Total 69 25,765 373 81 38

0 – 4 10 1,309 131 4 0

5 – 8 6 4,157 693 12 26

9 – 12 5 2,653 531 8 14

13 – 18 1 2,566 2,566 8 22

>18 1 856 856 3 7

Siskiyou

Total 23 11,541 502 35 69

0 – 4 72 27,663 384 24 0

5 – 8 47 19,364 412 17 6

9 – 12 23 13,351 580 11 13

13 – 18 27 18,260 676 16 22

>18 0 0 0 0 0

Western

Cascades

Total 169 78,638 465 68 41

0 – 4 310 114,908 371 25 4

5 – 8 196 105,559 539 23 14

9 – 12 71 42,540 599 9 11

13 – 18 72 45,515 632 10 13

>18 3 1,474 491 0 0

All

Georegions

Grand

Total

652 309,996 475 67 42

Of the 88 miles (462,108 feet) of road segments that were rated as either “Y” or

“P” for sediment delivery to waters of the state, 67 percent (309,996 feet) were

immediately upslope of live stream crossings. This percentage ranges from 35 percent

of all “Y” or “P” rated road segments in the Siskiyou georegion to 81 percent in the
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South Coast georegion. The first two segments upslope of live stream crossings

accounted for 74 percent of the length connected to waters of the state, though this

proportion ranged from 90 percent in the South Coast georegion to 41 percent in the

Siskiyou georegion. The average length of road segments that were rated “Y” or “P” and

were located upslope of stream crossings was 475 feet for the whole database, 373 feet

for the South Coast georegion and 502 feet for the Siskiyou georegion. This average

length is only 20 feet more than the average road segment length for the entire database.

Road segments that were rated “Y” or “P” for sediment delivery, were directly

upslope of stream crossings and were excessively spaced comprised 9 percent of the

entire database. These excessively spaced segments constitute 42 percent of segments

rated “Y” or “P” for sediment delivery that are adjacent to streams, which is not

substantially different from the proportion of excessively spaced road segments in the

entire road survey database (40 percent). Thus, the occurrence of drainage spacing that

exceeds Arnold’s spacing criteria is similar for both segments rated “Y” or “P” for

sediment delivery upslope of stream crossings and for elsewhere in the road network.

However, the small sample size of this group of data suggests that such conclusions be

drawn with caution.

Drainage System Characteristics

There were 2,810 surface drainage discharge points surveyed that drained 3,308

road segments (some surface drainage discharge points drained two road segments). Of

these surface drainage discharge points, 714 (25 percent) were cross-drain culverts, 498

(18 percent) were live stream-crossing culverts and 18 (1 percent) were bridges.

Additional drainage points included 306 waterbars, 305 ditch relief points, 210 grade

breaks and 54 “other” drainage features. Non-engineered surface drainage occurred at

398 points and represented 14 percent of the data.

The majority (58 percent) of the drainage points rated “Y” or “P” for sediment

delivery to waters of the state were live-stream crossing culverts. Another 19 percent of

the drainage points that delivered to streams were cross-drain culverts. Non-engineered

or random drainage points were less than 10 percent of the drainage points that delivered

sediment to streams. The average road segment length that was drained by cross-drain

culverts or live-stream crossing culverts was not appreciably different from the average

for the entire database.

Over half (56 percent) of the live-stream crossing culverts were completely open and 10

percent were at least 51 percent obstructed. The degree of obstruction ranged from 91

percent of live-stream crossing culverts in the South Coast georegion that were

completely open to 39 percent in the Coast Range georegion. Only 46 percent of all

cross-drain culverts were completely open and 14 percent were at least half blocked.

Nineteen percent of the cross-drain culverts surveyed in the Siskiyou georegion were

completely open and 28 percent at least half blocked as compared to 78 percent being

completely open in the South Coast georegion. However, this data represents a small
11

sample size of cross-drain culverts and live-stream crossing culverts in the Siskiyou

georegion compared to the other georegions.

Table 5: Summary of road drainage features inventoried.

Drainage Type Number Percent

Cross-Road Culvert (Single) 714 25

Cross-Road Culvert (Multiple) 3 0

Stream-Crossing Culvert (Single) 498 18

Stream Crossing Culvert (Multiple) 10 0

Bridge 18 1

Ditch Relief 305 11

Water Bar 306 11

Non-Engineered Relief 398 14

Grade Break 210 7

Road Junction 290 10

Outsloped Road Construction 4 0

Other 54 3

Total 2,810 100

Table 6: Summary of road segment data by drainage feature type for the entire database and for

segments definitely or possibly connected to waters of the state.

Entire Database Possible and Positive Sediment Delivery to Channels

Discharge Length (ft) Drainage

Feature (DF)

Discharge Length

Drainage

Feature

No. Rd.

Segments

Total

Length

(ft)

Avg

(ft)

%

of

Total

No.

DF

% of

Total

(of DF)

No. Rd.

Seg.

Total

Length

(ft.)

Avg.

(ft.)

% of

Total

(of DF)

Cross-Road

Culvert (Single)

861 424,269 493 28 158 5 (18) 181 89,544 495 6 (19)

Cross-Road

Culvert

(Multiple)

4 2,601 650 0 2 0 (0) 2 601 301 0 (0)

StreamCrossing

Culvert (Single)

619 292,015 472 19 493 15 (58) 613 287,277 469 19 (62)

Stream Crossing

Culvert

(Multiple)

15 11,097 740 1 10 0 (1) 15 11,097 740 1 (2)

Bridge 22 11,629 529 1 13 0 (2) 14 5,762 412 0 (1)

Ditch Relief 324 180,046 556 12 22 1 (3) 23 11,812 514 1 (3)

Water Bar 309 67,429 218 4 55 2 (6) 55 11,994 218 1 (3)

Non-Engineered

Relief

499 164,588 330 11 74 2 (9) 89 32,168 361 2 (7)

Grade Break 285 141,913 498 9 4 0 (0) 6 2,526 421 0 (1)

Road Junction 299 190,434 637 13 14 0 (2) 15 7,228 482 0 (2)

Outsloped Road

Construction

4 1,109 277 0 0 0 (0) 0 0 0 0 (0)

Other 67 18,937 282 1 9 0 (1) 11 2,099 191 0 (0)

Total 3,308 1,506,067 455 100 854 31 (100) 1,024 462,108 451 31 (100)
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Table 7: Inlet of all surveyed stream-crossing and cross-drainage pipes as a percent of the original

open area.

Streams Cross

Drains

Streams Cross Drains

% Open

# % # % # % # %

0 29 6 41 6 21 12 16 11

1 – 24 12 2 34 5 5 3 7 5

25 – 49 10 2 22 3 6 3 4 3

50 – 79 80 16 156 22 20 12 36 25

80 - 99 88 18 125 18 53 31 24 17

100 279 56 336 46 68 39 57 40

Total

Entire

Database

498 100 714 100

Coast

Range

173 100 144 100

Streams Cross

Drains

Streams Cross Drains

% Open

# % # % # % # %

0 5 4 13 7 1 2 2 2

1 – 24 3 2 9 5 1 2 1 1

25 – 49 1 1 5 3 1 2 0 0

50 – 79 23 21 52 26 1 2 8 7

80 - 99 15 12 29 15 1 2 16 13

100 71 59 89 45 48 91 96 78

Total

Interior

121 100 197 100

South

Coast

53 100 123 100

% Open

Streams Cross

Drains

Streams Cross Drains

# % # % # % # %

0 1 5 2 6 1 1 8 4

1 – 24 0 0 4 13 3 2 13 6

25 – 49 0 0 3 9 2 2 10 5

50 – 79 3 16 2 6 30 23 58 27

80 - 99 3 16 15 47 16 12 41 19

100 12 63 6 19 80 61 88 40

Total

Siskiyou

19 100 32 100

Western

Cascades

132 100 218 100

Fifty-five percent of all live-stream crossing culverts that were observed to be

partially blocked were blocked by sediment and another 19 percent of those culverts were

partially crushed. These same factors were responsible for the blockage of cross-drain

culverts as well, with 60 percent of the blockages due to sediment and 26 percent due to

crushed openings. However, only 19 percent of this data is from the South Coast and

Siskyiou georegions. Thus, the dominant mechanisms that caused culvert blockage come

predominately from the Coast Range, Interior and Western Cascade georegions which

may not adequately represent other georegions.
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Road Segments That Deliver Sediment

In previous sections, it was reported that 31 percent of the total road length was

rated as “Y” or “P” for delivery of sediment to waters of the state. Twelve-percent was

classified as both delivering sediment (“Y” and “P”) and being excessively spaced. Of

the road segments rated to deliver sediment (“Y” and “P”), the percent of length in ESS’s

was identical to the percentage of the entire database (40 percent). Sixty-seven percent of

the length of segments rated as “Y” or “P” for sediment delivery were adjacent to stream

crossings, but the average length of these segments was not different from the average

segment length for the entire database. Excessively spaced segments were not more

common in the road segments adjacent to stream crossings either. What follows is an

analysis of other characteristics of road segments rated to deliver (“Y” or “P”) sediment

to streams.

The greatest number of road segments rated to deliver sediment (“Y” or “P”) were

on midslope roads (Figure 2) (72 percent) and the least number were on ridgetop roads (7

percent). It is likely, however, that the surveyors classified only the extreme cases as

valley and ridgetop roads, artificially increasing the number of segments classified as

midslope. Fifty-one percent of the length of road segments rated as “Y” or “P” for

sediment delivery were upslope of midslope live-stream crossing culverts and 36 percent

had road grades between zero and eight percent. Midslope roads also comprise 74

percent of all roads, but the proportion of midslope roads that are rated to deliver

sediment (30 percent) is not different from the overall average of 31 percent. However,

59 percent of all valley bottom roads deliver sediment to streams although they constitute

only 11 percent of the database. This suggests that locating roads parallel and directly

Road Slope Position and Sediment Delivery

0

500

1000

1500

2000

2500

3000

M idslope Ridge Valley

Slope Position

Number of Segments

Total Segments

Sediment Delivery

Figure 2: Connectivity with waters of the state by road location.
14

adjacent to or crossing streams increases the potential for sediment delivery. These

trends hold across all georegions except the Siskiyou georegion where midslope roads

still have the greatest number of drainage points that directly deliver to waters of the state

but only 20 percent of midslope road segments rated to deliver sediment were associated

with live-stream crossing culverts. In this georegion, waterbars accounted for 36 percent

of the midslope sediment sources, 17 percent were non-engineered drainage points and

16 percent were cross-drain culverts.

Any relationship between road prism construction (crowned, insloped, outsloped,

other) and sediment delivery varied substantially between georegions. Thus, conclusions

regarding such a relationship are difficult. Additionally, crowned roads made up 78

percent of the length of all roads surveyed and all construction types were not represented

in all georegions. Less than 20 percent of the length of road draining to streams in all

georegions had potholes or ruts. Relationships with sediment delivery may have been

obscured by inconsistencies in the classification of road prism construction and surface

condition between the 1995 and 1996.

Seventy-two percent of midslope road segments rated to deliver sediment (“Y” or

“P”) had cutbank heights at least five feet high as compared to 60 percent of midslope

segments not rated to deliver sediment. The type of road surfacing material did not differ

from other midslope roads, with approximately 80 percent of all roads being surfaced by

dirty rock. The occurrence of culvert blockage and culvert outlet conditions did not

differ between midslope road segments rated to deliver sediment and the entire data set.

Overall, no clear pattern in differences of road surface material or condition, increased

culvert obstruction or culvert outlet erosion emerged with midslope road segments rated

to deliver sediment even though cutbank heights were slightly greater.

Approximately 60 percent of all roads rated “Y” or “P” for sediment delivery to

streams of the South Coast and Siskiyou georegions were less than 300 feet long (Figure

3). In the other georegions, 60 percent of the segments were greater than 300 feet long.

Since only 41 percent of road segments rated to deliver sediment in the Siskiyou

georegion occur within the first two road segments upslope of stream crossings, this

suggests that non-stream crossing drainage structures in this georegion have greater

connectivity with streams than in other georegions despite shorter road segments.

Excessively Spaced Segments (ESS)

As reported in previous sections, 40 percent of the total length of roads surveyed

had excessive spacing between surface drainage points compared to Arnold’s spacing

criteria. This is identical to the proportion of ESS within the subset of road segments

rated to deliver sediment (“Y” or “P”) to waters of the state. The proportion of ESS

within road segments both rated to deliver sediment and were upslope of live-stream

crossing culverts was not different from that value for the entire database (42 percent).

The proportion of road length rated to deliver sediment within ESS and non-ESS

segments was also identical (31 percent). This suggests that using Arnold’s criteria for
15

drainage spacing is not a reliable best management practice by itself to reduce

connectivity between road drainage points and streams.

Like segments rated to deliver sediment (“Y” or “P”), the majority (80 percent) of

ESS were located on midslope roads except for the Siskiyou georegion where 45 percent

were midslope, 37 percent were valley bottom roads and 19 percent were on ridges. The

Western Cascades georegion also differs slightly from the norm with only 68 percent of

ESS on midslope locations and 23 percent on ridges. Within each slope location, the

highest percentage of ESS occurs with midslope or ridge top road locations although the

Siskiyou georegion again differs with valley bottom and ridge top roads having the

greatest proportion of ESS.

Any relationship between road prism construction (crowned, insloped, outsloped,

other) and ESS varied substantially between georegions. Thus any conclusions regarding

this relationship are difficult because crowned roads made up 78 percent of the length of

all roads surveyed and not all construction types were found in all georegions. Less than

15 percent of the length of ESS in all georegions had road surfaces with potholes or ruts.

Except for the South Coast georegion where road surfaces classified as “Other” were

common, the majority of ESS had smooth surfaces. This suggests that long distances

between drainage points does not in itself lead to poor road conditions.

There is no indication that the amount of culvert inlet blockage differs or that

cutting or fillslope failures are more common below culvert outlets of excessively spaced

segments as compared to the entire database. Road surfacing material and cutbank height

distributions also do not differ from that of the entire database.

Discharge Length to Live Streams 

(Rated "Y" or "P" for Sediment Delivery)

18% 21%

29% 28%

8 %

24% 21%

29% 34%

31%

18% 16%

19% 11%

21%

14%

13%

11%

11%

14%

8 %

10%

6 %

8 %

6 %

7 % 7 %

0 %

8 %

11% 13%

5 % 5 %

2 % 12%

0 %

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

C R INT S C SK W C

Georegion

Segments by Length Class (Percent of All Segments Connected 

to Streams)

900+

750 - 900

600 - 750

450 - 600

300 - 450

150 - 300

0 - 150

Length 

Class (ft)

Figure 3: Connectivity with waters of the state by segment length class.
16

Discussion

In contrast to accepted road construction BMP’s, the average road segment

length, both for the entire database and above stream crossings, increased with increasing

road grade. Only the Coast Range georegion deviated from this trend with average road

segment lengths generally decreasing with increasing road grade.

Of the entire length of road segments sampled, 31 percent were definitely or

possibly connected to waters of the state and thus potentially delivered sediment. This

ranged from 24 percent in the Interior to 47 percent of the road length sampled in the

Siskiyou georegion. Sixty-seven percent of segments rated as delivering sediment (“Y”

or “P”) to waters of the state were immediately upslope of live-stream crossings. The

first two segments upslope of live-stream crossings accounted for 74 percent of the total

segment length rated to deliver sediment (“Y” or “P”). This trend is consistent for all

georegions except the Siskiyou georegion where 59 percent of the segments rated to

deliver sediment are farther than two road segments away from live-stream crossings.

Waterbars, cross-drain culverts and non-engineered drainage points were a high

proportion of the road segments rated to deliver sediment in this georegion. Because of

the sheer number of roads classified as midslope, most road segments rated to deliver

sediment occur in midslope locations although valley bottom roads had the highest

proportion of road segments connected with streams.

When the 1996 and 1995 data are combined, the result is an overall lower

estimated proportion of road connectivity with streams (31 vs. 39 percent). The greatest

difference was in the Interior georegion where the 1995 survey had estimated 40 percent

of the road length was rated “Y” or “P” to deliver sediment versus the 24 percent

observed in the combined data. The difference in road connectivity with streams in the

Coast Range and Western Cascade georegions differed by 6 and 12 percent, respectively.

These stream-connectivity values are lower than the value of 57 percent reported

by Wemple et al. (1997) in a study of channel network extension by forest roads in the

western Cascades of Oregon. Reid and Dunne (1984) reported an even higher value of

75 percent stream connectivity in the Clearwater basin of Washington. The values in this

report, however, do compare well with the 34 percent connectivity reported by Bilby et

al. (1989) in southwest Washington and the 39 percent value estimated for the Kilchis

watershed in northwest Oregon (ODF 1996b).

The spacing of surface road drainage discharge points that were in excess of

Arnold’s spacing criteria occurred on 40 percent of the road length sampled. This

percentage was consistent across georegions and within the subset of road segments

directly upslope of live-stream crossings. However, excessive segment lengths were

most common in the 13 – 18 percent road grade class. The Interior and Siskiyou

georegions had the largest average segment lengths that exceeded Arnold’s spacing

criteria. Again, the greatest number of excessively spaced segments were classified as

being on midslope locations, although most ESS in the Siskiyou georegion were on ridge
17

top and in valley bottom locations. The proportion of road segment length rated to

deliver sediment (“Y” and “P”) was also constant at 31 percent regardless of whether

drainage points were excessively spaced or not. Only 12 percent of the entire database

was both rated to deliver sediment to streams and observed to have excessive drainage

point spacing.

Piehl (1988) in a review of 515 cross-drain culverts in the Oregon Coast Range

found that culvert spacing on state and private lands were 1.4 and 1.7 times that

recommended by Arnold (1957) as opposed to the average of 0.6 observed in this

inventory. However, the 42 percent of the total road length that exceeded Arnold’s

spacing criteria found by the ODF road drainage and sediment delivery survey in the

Kilchis watershed agrees with the 40 percent reported in this study (ODF 1996b).

Combining the 1995 and 1996 databases resulted in an increase in the proportion

of road length in excess of Arnold’s spacing criteria (40 vs. 36 percent), except within the

Western Cascade georegion where a decrease of 4 percent occurred. The greatest

increase (15 percent) was observed in the Coast Range geogregion while the Interior

georegion increased by 13 percent.

Non-engineered road drainage points accounted for only 14 percent of the

sampled drainage points and less than 10 percent of drainage points connected to streams.

Most connectivity with streams occurred at live-cross stream or cross-drain culverts,

representing 43 percent of the sampled drainage points. Over half (56 percent) of the

live-stream crossing culverts were completely open, although this was true for only 40

percent of the cross-drain culverts. In the South Coast georegion, 78 percent of crossdrain culverts were completely open, although in the Siskiyou georegion almost half were

at least 80 percent blocked. However, the 32 cross-drain culverts sampled in the Siskiyou

georegion may be insufficient to base conclusions on. Sediment and crushing of the

culvert inlet were the most common blockage factors. Only 19 percent of the drainage

points sampled were in the South Coast and Siskiyou georegions. Thus, the primary

culvert blockage factors identified were dominated by processes most common in the

Coast Range, Interior and Western Cascade georegions.

Piehl et al. (1988) also found sediment deposition and physical damage or denting

to be the most common factors reducing culvert inlet capacity. The 1995 Oregon

Department of Forestry road drainage and sediment delivery survey, which makes up 43

percent of this inventory’s road length, and the Kilchis Watershed survey also identified

these two factors as the main contributors to reduced culvert capacity (ODF 1996a, ODF

1996b). Piehl et al. (1988) reported a damage rate of 74 percent, but this inventory found

only 60 percent of cross-drain culverts in the Coast Range to have reduced intake

capacities. The greatest difference in culvert condition observed with the increased

sample size of the 1995 survey in the Coast Range, Western Cascade and Interior

georegions was a slight reduction in the proportion of culverts completely open and an

increase in the proportion of culverts only 50 – 79 percent open.
18

Approximately 60 percent of road segments rated to deliver sediment (“Y” or

“P”) in the South Coast and Siskiyou georegions were less than 300 feet long. In

contrast, the same proportion of road segments rated to deliver sediment in the other

georegions were greater than 300 feet long. The Siskiyou georegion has a relatively low

proportion of stream-crossings, thus this suggests that road segments not linked to stream

crossings have a higher degree of connectivity with streams compared to the other

georegions despite short segment lengths. This may be partly explained by the high

degree of obstruction of cross-drain culverts and thus the potential for re-routing the flow

from several segments onto one drainage point.

The two subsets of road segments rated to deliver sediment and excessively

spaced segments were further investigated for special characteristics. No patterns of road

prism construction, surface material or condition, frequency or degree of culvert blockage

or culvert outlet erosion were observed with these subsets.

As previously noted, the two years of data compiled in this report represent time

periods both before and after the February, 1996 flood. Conceivably, then, differences in

some parameters (i.e. connectivity between drainage points and streams, culvert

blockage) that exist between findings in this report and the 1996 report (ODF 1996a) that

summarizes the 1995 data may be related to flood impacts. A storm event of that

significance might be expected to increase the proportion of road segments connected to

streams although the value of this parameter was actually observed to decrease. More in

line with the expected consequences of a flood event was a slight increase in the

proportion of culverts with inlets only 50 – 79 percent open. It is not at all clear,

however, that such differences are related to the 1996 flood or simply reflect different

inventory crews, different site selection methods, or different locations sampled.

Recommendations

Stream crossing segments still exhibit the greatest connectivity with and thus the

highest potential for sediment delivery to streams. Management practices that either

reduce that connectivity or maximize the filtering of water from the road prism would

help diminish the potential for sediment delivery to streams. Valley bottom roads also

show a high degree of connectivity with streams and would benefit from the same

practices.

This survey and others show that sediment blockage and crushing of the culvert

inlet are the most common factors reducing the capacity of stream- and road-crossing

culverts. Road-crossing culverts in the Siskiyou georegion have a substantial proportion

(28 percent) with inlet capacities reduced by at least half. Construction and maintenance

practices that maintain clear and undamaged culverts should alleviate these problems.

This survey provides information as to the spatial extent of sediment delivery to

streams, but it gives no indication of the amount of sediment being produced. Estimating

sediment production would be a difficult and highly inaccurate addition to this survey

protocol. Several factors have being linked, however, to sediment production from road
19

surfaces including surface material, surface condition and traffic levels (Bilby et al.

1989, Foltz and Burroughs 1990, Reid and Dunne 1984). The survey protocol currently

incorporates all of these parameters except traffic level. Reid and Dunne (1984)

estimated that heavily used roads produced 130 times the sediment of an abandoned road.

Including a rough estimate of traffic level in the survey protocol, such as heavy,

moderate, light, or abandoned, might help landowners to prioritize road segments or

drainage points for maintenance or repair.

As no difference in the proportion of connectivity with streams was observed

between excessively- and non-excessively spaced road segments this suggests that

Arnold's spacing criteria in itself is not sufficient as a means of reducing connectivity

with streams. Further investigation of the relationship between road segment length,

grade class and sediment production may be necessary to determine the ideal use of such

criteria as a best management practice.

Enough data has been collected in the Coast Range, Western Cascade and Interior

georegions to support the general conclusions made regarding road drainage and

sediment delivery potential in these georegions. Further surveys should be conducted in

the South Coast and Siskiyou georegions, however, to validate the stated characteristics

of these georegions. In particular, it would be valuable to confirm the high degree of

connectivity between streams and relatively distant road segments in the Siskiyou

georegion and the responsible process or processes.

Literature Cited:

Arnold, J. 1957. Chapter XIII. Engineering aspects of forest soil. In An introduction

to forest soils of the Douglas-fir region of the Pacific Northwest. Western

Forestry and Conservation Assoc., Portland, Oregon. Pp. 1-15.

Bilby, R. E., K. Sullivan and S. H. Duncan. 1989. The generation and fate of roadsurface sediment in forested watersheds in southwestern Washington. Forest

Science, 35(2):453-468.

Foltz, R. B. and E. R. Burroughs. 1990. Sediment production from forest roads with

wheel ruts. In Watershed Planning and Analysis in Action. Symposium

Proceedings of IR Conference, Watershed Mgt/IR Div/ASCE. Durango, CO/

July 9-11, 1990. pp. 266-275.

Piehl, B. T., R. L. Beschta and M. R. Pyles. 1988. Ditch-relief culverts and low-volume

forest roads in the Oregon Coast Range. Northwest Science 62(3):91-98.

Oregon Department of Forestry. 1996a. Road sediment monitoring project report

Survey of road drainage in Western Oregon. Report for the Department of

Environmental Quality.
20

Oregon Department of Forestry. 1996b. Forest roads, drainage and sediment

Delivery in the Kilchis River Watershed. Report for the Tillamook Bay National

Estuary Project.

Reid, L. M. and T. Dunne. 1984. Sediment production from forest road surfaces.

Water Resources Research, 20(11):1753-1761.

Wemple, B. C., J. A. Jones and G. E. Grant. 1997. Channel network extension by

logging roads in two basins, Western Cascades, Oregon. Water Resources

Bulletin 32(6):1195-1207.