United States Department of Agriculture Forest Service Technology & Development Program		Water/Road Interaction: Examples from Three Flood Assessment Sites in Western Oregon
7700—Transportation Systems 2500—Watershed and Air Management September 1998 9877 1805P—SDTDC Revised for Internet July 2003

Water/Road Interaction: Examples from Three Flood Assessment Sites in Western Oregon

Ronald L. Copstead, P.E.
USDA Forest Service
Pacific Northwest Research Station

David Kim Johansen, P.E.
USDA Forest Service
Willamette National Forest

• Acknowledgments
• ABSTRACT
• Summary — Learning from flood events
• Introduction
• Background
• Site Descriptions and Damage Assessment
• Breitenbush Site
• K-Creek Site
• Devil's Creek Site
• Discussion
• Plugged Stream Culverts
• Culvert Inlet Design
• Culvert Spacing
• Ditch Erosion
• Fill Failure
• Traveled-way Surface and Ditch Shape
• Road-surfacing Materials
• Conclusions, Summary, and Recommendations
• Literature Cited
• Unit Conversions
• Culvert Size Conversions

Acknowledgments
Information and technical review of this manuscript were provided by the resource staff of the Willamette National Forest. Background information regarding flood events was provided by Larry Cronenwett, (retired), formerly of the Engineering Staff, Pacific Northwest Region, USDA Forest Service.

ABSTRACT
Copstead, Ronald L.; Johansen, David Kim. 1998. Water/Road Interaction: Examples from Three Flood Assessment Sites in Western Oregon. Report 9877 1805—SDTDC. San Dimas, CA: U.S. Department of Agriculture, Forest Service, Technology and Development Program. 15 p.

Assessments of damage from storms in 1995 and 1996 to three forest road segments on the Detroit Ranger District of the Willamette National Forest are reported. Consequences to roads and road-related structures are discussed. Changes for the three road segments in the designs and materials used for road surfacing and road drainage structures including ditches and cross drains are suggested. Information is based on that developed and reported for other publications in the Water/Road Interaction Technology Series.

Key words: Forest roads, drainage, floods, ditches, culverts, road erosion

Summary — Learning From Flood Events
From the fall of 1995 through the spring of 1996, intense storms caused widespread flooding in areas of the Pacific Northwest of the United States. This flooding provided an opportunity for U.S. Department of Agriculture (USDA) Forest Service staff to see where the “weak links” were in the integrity of the system of national forest roads for which they have responsibility. Following the storms, assessments were made so that road segments could be repaired, rebuilt, decommissioned, or obliterated. Of particular interest for the Water/Road Interaction series of publications are examples, afforded by these events, of how forest road drainage facilities fail when stressed. The following key observations were made:

Providing “fail-safe” road drainage and stream crossing designs will minimize risk to downstream values during intense runoff periods.

Damage from surface runoff during storms did not tend to be initiated on dense and well-graded gravel road surfaces. Rather, sediment and rock debris from eroded, unlined ditches were initially scattered onto road surfaces. Erosion of these ditches progressed to the point where severe gullying resulted. Ditches eroded rapidly because of the erosive nature (fine-grained, noncohesive) of the soil.

Integrity of the road surface can be enhanced during high-runoff periods by using rock sources that produce well-graded material with adequate plasticity.

For the three sites surveyed, forest vegetation tended to buffer the flow of debris and sediment so that it did not reach large streams. Relatively broad, flat areas adjacent to the eroded section of road (especially at the K-Creek site) caused water to pool and sediment to drop out.

In locations where obstructed crossings carry a high risk to downstream values, it may be necessary to consider structures other than pipe crossings, such as low-water fords.

Culvert inlet areas that had an abundance of vegetation, and that were wide and shallow, had the effect of slowing stream flows, thereby causing debris and sediment to settle and accumulate at the pipe entrance. These inlets often became obstructed.

Shallow fills over bedrock surfaces that are parallel to the slope and adjacent to roads are at high risk to initiate slides during periods of intense runoff. During construction of full-bench roads, preventing these shallow fills over bedrock will reduce the risk of slide initiation. Existing steep, wet, relatively shallow fills should be watched closely by maintenance personnel.

A regular function of routine maintenance of forest roads is to assess road segments for conditions that could lead to damage during periods of high runoff. All too often there is a temptation to repair road damage by simply restoring the road and associated structures to a condition similar to what existed before the storm. In many cases this may be appropriate, but careful evaluation may also suggest improvements that could reduce risk of future damage.

Introduction
Between November 1995 and April 1996, the Pacific Northwest experienced a series of intense storms. Some of the effects of these storms included high runoff into drainage systems associated with forest roads. While there was widespread and extensive property damage and thousands of people were unfortunately affected in adverse ways, these storms provided natural resource professionals with opportunities to see firsthand how roads and road-related structures performed in response to high-intensity events.

During the assessment of effects from the weather events of November 1995 and February 1996, patterns began to emerge regarding damage to roads and adjacent sites. This report describes and discusses examples of road damage caused by storms using three forest road segments in the Detroit Ranger District of the Willamette National Forest. The primary purpose of this publication is to describe the road and road-related structures that were in place at the time of the storm events, highlight what happened to them, and discuss what is needed to improve the design and maintenance of road drainage.

Background
Forest roads and surrounding areas of the Pacific Northwest experienced various types of storm-caused damage, depending on factors such as soil type, land steepness, vegetation, storm intensity, and road construction details. Various natural phenomena, such as channel erosion, slope movement, snow avalanche, and surface erosion caused damage to roads. Conditions related to activities such as timber harvest, drainage modification, and harvest and road-related stream channel modification also caused damage to roads. Damage to forest and streams was also attributed to culvert plugging, stream diversion, erosion and sedimentation, and road-fill failures.

Although damage to facilities was costly and in some cases caused inconvenience and hardship, the percentage of land area or length of road that was damaged to the point of requiring repair was relatively small (less than 1 percent of total land area or length of road). National forests that were affected by these storms concluded from their assessments that recent restoration and maintenance efforts probably reduced the number of road-related slides that deposited large amounts of sediment to streams. The flood damage, nevertheless, presented an opportunity to observe road-drainage features that were the most vulnerable to failure and to study the cause of those failures.

Surveys of damage to roads and to local areas surrounding roads on the Detroit Ranger District showed that plugged culverts and road-surface and road-fill erosion accounted for most of the damage to roads and road-stream crossings (figure 1).

Culvert plugging by stream bedload and woody debris was the most common type of failure overall (28 percent). Often a small branch is caught in the culvert inlet, resulting in stream bedload accumulation and eventual burying of the inlet. Culverts that were 600 mm (24 in) in diameter or less accounted for 81 percent of the plugged culverts.

The cause of flood damage to roads from erosion of road surfaces and stream channels adjacent to roads, fill failures, and forest slides contributed about equally to road damage, each comprising 14 to 18 percent of the number of sites that were damaged. Cutslope failures contributed only 6 percent of sites that were damaged.

Damage to roads in the form of fill failures, fill erosion, road surface erosion, and debris piles were all found at about the same frequency and contributed to 16 to 20 percent of the damage.

Figure 1—Road related storm damage by type (352 sites on Detroit RD 1996).

Site Descriptions and Damage Assessment
Road surface materials for each of the three sites considered in this report are similar (table 1). The basic geology in the area consists of mostly andesite and basalt lava flows intermixed with volcanic deposits of ash and tufts. Soils are derived from this material and include coluvium and glacial deposits. In terms of engineering soil gradation, these soils are mostly sand and silt mixtures (Unified Soil Classifications SM and ML), containing rock fragments up to cobble size. Low-plasticity soils derived from volcanic deposits tend to be quite erodible. Surfacing for all roads was derived from local andesite lava flows and was crushed to 20-mm (3/4-in) minus, dense-graded aggregate. This material also has low plasticity. Roadside ditches were unlined and partially vegetated, primarily with grass, ferns, or other small plants.

Breitenbush Site
The damage on this site began where two adjacent small streams carrying high flows and sand- to cobble-sized bedload plugged road culverts (figure 2). Both of these 1.0- to 1.5-m- (3.3- to 4.9-ft-) wide stream channels crossed the road with 450-mm (18-in) diameter corrugated metal pipes lying on a 5 percent (stream) gradient. The upstream end of the culverts projected into widened inlet basins. Roadside ditches were open uphill and downhill to these inlet basins. Stream bedload accumulated at both of the culvert inlet basins, filling them until their inlets were buried. Storm flow was then diverted to the road ditch. The flow and stream sediment from the first culvert flowed down to the second culvert, where they combined and continued down the road ditch. The flow from the slope above the road and road-surface drainage also accumulated in the ditch. The high-volume and high-velocity flow—the road and ditch grade ranged from 8 to 11 percent—began eroding the road ditch to a depth limited only by bedrock lying from 0.5 to 3 m (1.6 to 9.8 ft) below the originally constructed ditch bottom (figure 3). At the entrances to ditch-relief culverts, either inlet basins were severely eroded or the entrances were plugged by sediment so that none of the culverts were transporting water across the road. The accumulated flow could not escape the ditch and continued down the road for 1.2 km (0.75 mi).

Storm flow eventually eroded through softer road-fill material, forming a 1.3-m-(4.3-ft-) deep gully. Debris was carried into a small, intermittent drainage where some of it was deposited. The remainder was carried further downhill into a larger stream drainage. This larger drainage crossed the road near the junction via the forest highway with a 900-mm (36-in) culvert. The entrance basin to this crossing was filled by debris and the pipe entrance was buried. The stream flow overtopped the road and eroded through the fill approximately 1 m (3.3 ft) down to bedrock, carrying the material 100 m (328 ft) to the forest highway below. Debris deposited on the highway, and the flow spread over the road surface and was dispersed into the forest below.

The dense, well-graded gravel road surface received little damage from surface runoff during the storm. Sediment and rock debris were scattered onto the road surface at some locations. A summary of drainage features and observations of what happened during and after storm events is shown in table 2. Virtually all of the material that eroded and moved offsite came from the unlined ditches and resulting gullying that occurred during the storm (figure 3).

K-Creek Site
Damage at this site began at the 2.7-km (1.7-mi) stream crossing, where debris filled a stream crossing culvert inlet basin, plugging the 600-mm (24-in) corrugated metal pipe and diverting the stream into the road ditch (figure 4). As the road ditch was eroded, debris was scattered on the road surface in numerous places. Ditch relief culverts were plugged by coarse-grained sediment. A portion of the water was deflected off the road by this debris, while the remainder flowed down the road ditch and traveled on the surface, causing additional damage. Eventually the stream eroded through the road template, creating a gully up to 3-m (9.8-ft) deep by 3-m (9.8-ft) wide (figure 5). This gully followed the road until it eroded through the fill, diverting the stream and depositing sediment in a broad, shallow basin adjacent to the road. The forest vegetation served as a buffer strip to help capture sediment before it reached stream channels. The basin allowed water to pool and drop suspended sediment. Location of cross drains and stream crossing culverts are shown in table 3.

Debris generated from the diverted stream 0.5 km (0.3 mi) away was deposited in the inlet basin for a 600-mm (24-in) diameter culvert near the bottom of the site (at location 3.2 km), plugging the inlet. The stream flow pooled and overtopped the road, eroding the fill and exposing the culverts. A summary of drainage features and observations of what happened during and after storm events at this site is shown in table 3.

Devil’s Creek Site
This site involved the failure of a thin sidecast road fill that scoured the soil off a drainage headwall on the hillside and deposited it on a lower segment of the road, within the lower segment of the drainage and within Devil’s Creek (figure 6). The failure was caused by water being diverted by a small cutslope slide (estimated volume was 5 m3) that dammed a ditch, causing ditch water to run out onto the road surface, down wheel tracks, and eventually over the fill slope edge of the road (figure 7). The fill slope apparently failed as a result of saturation caused by rainfall combined with the overtopping ditch water eroding the toe of the fill. The failed fill slope became a fast-moving earth flow that combined with wet 1- to 5-m- (3.3- to 16.4-ft-) thick surface soil in the drainage headwall, gathering volume as it moved and not stopping until it encountered the road below. The debris then filled the inlet basin of the road culvert, covered the road with about 800 m3 (28,251.7 ft3) of debris, and sent the bulk of the debris into the drainage channel below the road and into Devil’s Creek. The channel debris swept up a forest slope, knocking down trees and burying the channel. The drainage eventually eroded through the channel debris to reconnect with Devil’s Creek. It was estimated that, of the approximately 4,200 m3 (148,321.6 ft3) that were eroded and subsequently slid from the site, about 25 percent was stopped by the road, with the remaining volume ending up within the stream bed below. The road fill was partially eroded, damaging the outlet end of two 750-mm- (30-in-) diameter stream culverts located at the site. Apparently, one of the culverts had been damaged in the past by a debris slide, left in place, and replaced with a new culvert of the same size.

Subsequent maintenance at the site removed debris from the road surface, but did not re-establish flow into the cross drain. Instead, water flowed into the road ditch and continued about 70 m (229.7 ft) down to the next culvert. The sediment carried by the diverted drainage eventually plugged the next culvert, then diverted over the road surface, eroding approximately 3,000 m3 (105,944 ft3) of road fill. The fill material was deposited in a flattened area below the road and within a forested buffer strip area below. Very little road fill reached Devil’s Creek drainage below. The road culvert was severely damaged and the road subgrade was destroyed.

Figure 2—Breitenbush site map.

Figure 3—Severe gullying at Breitenbush flood-assessment site.

Figure 4—K-Creek site map.

Figure 5—Severe gullying at K-Creek flood-assessment site.

Figure 6—Devil's Creek site map.

Figure 7—Initiation of debris slide at Devil's Creek flood-assessment site.

1. Hydraulically undersized stream culverts
2. Hydraulically inadequate spacing of ditch relief culverts
3. Poor inlet/channel relationship, allowing debris and bedload to accumulate and plug culverts
4. Lack of fail-safe drainage features—ditch dams, rolled grade or drain dips, and inadequate maintenance frequency—allowing local failures to initiate damage affecting larger areas and impact higher-valued resources.

• A hydrologic and hydraulic analysis to design all stream crossings. Such analysis would have resulted in stream crossings designed to provide adequate capacity for passage of debris and bedload during storms.
• A cross-drain spacing analysis based on soil type, road grade, hydrologic input, and location of the road on the slope, which would have indicated the need for more frequent ditch relief culverts.
• Ditch erosion-control measures such as riprap or grass.
• Emergency overflow surface drain features at stream crossings to allow water to pass across the road near a stream crossing if the culvert becomes plugged—features such as road grade shape, using broad-based dips and hardened surfaces at stream crossings, or more erosion-resistant surface material.

The three flood-assessment sites considered illustrate the need for planning and designing road-related drainage according to the following general guides:

1. Know the soil types of the subgrade, cut and fill slopes, and nearby areas, and use this information for planning drainage and erosion control measures.
2. Use cross-drain spacing guidelines that assess native, subgrade, traveled-way surface, and ditch surfacing materials.
3. Use hydrologic and hydraulic analyses to determine correct culvert sizes to prevent overtopping a site at all stream and drainage sites, even small ones. By creating a lower limit for culvert sizing, such analyses match culvert dimensions to the stream channel requirements. The recommended minimum-size culvert for any stream is 600 mm (24 in).
4. Consider the debris and bedload characteristics of the site and the associated drainage area, and increase the size of culverts to enable the pipe to allow storm flows to pass when partially plugged.
5. Examine all stream crossings to determine the possible consequences of a large-storm event on stream bedload movement and debris potential. The cost of improved drainage can be much less than the cost of repairing damage to drainage facilities inadequate to the demands of storm events. Consider construction of a diversion-prevention dip to ensure that overtopping flows are directed back into the channel (Copstead, et al. 1998, Furniss, et al. 1997).

Literature Cited
Baeder, Larry and Jere Christner. 1981. Revision of the guide for spacing relief culverts for the Willamette National Forest. Eugene, OR: USDA, Forest Service, Pacific Northwest Region, Willamette National Forest; 18 p.

Copstead, Ronald, David Kim Johansen, and Jeffry Moll. Water/Road Interaction: Introduction to surface cross drains. In: Water/Road Interaction Technology Series, Publication 9877 1806. San Dimas, CA.: USDA, Forest Service, San Dimas Technology & Development Center. 15 p.

Furniss, Michael J., Michael Love, and Sam A. Flanagan. 1997. Diversion potential at road-stream crossings. In: Water/Road Interaction Technology Series, Publication 9777 1814. San Dimas, CA.: USDA, Forest Service, San Dimas Technology & Development Center. 12 p.

Harris, D. D, Larry L. Hubbard, and Lawrence E. Hubbard. 1979. Magnitude and frequency of floods in western Oregon. Open-File Report 79-553. Portland, OR: U.S. Department of the Interior, U.S. Geological Survey. 29 p.

Normann, Jerome M., Robert J. Houghtalen, and William J. Johnson. 1985. Hydraulic design of highway culverts. Hydraulic Design Series No. 5, FHWA-IP-85-15. McLean, VA: Turner-Fairbank Highway Research Center, Office of Implementation. 272 p.

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