1 

Monitoring Road Decommissioning in the Mann 

Creek Watershed: Post-storm Report 

Payette National Forest 

February 14, 2011 

Nathan Nelson1, Richard Cissel1, Tom Black1, and Charlie Luce2

1Hydrologist

2

Research Hydrologist

US Forest Service

Rocky Mountain Research Station 

322 East Front Street, Suite 401

Boise, Idaho, 83702
2 

Table of Contents

Executive Summary ............................................................................................................... 4

Acknowledgements ............................................................................................................... 5

1.0 Background ................................................................................................................. 6

2.0 Study Objectives ......................................................................................................... 6

4.0 Monitoring Locations .................................................................................................. 7

Regional Monitoring Sites ........................................................................................................... 7

Mann Creek Sites ......................................................................................................................... 9

5.0 Storm Event .............................................................................................................. 11

6.0 Storm Damage Characteristics ................................................................................... 13

Treatment Roads ....................................................................................................................... 13

Control Roads ............................................................................................................................ 16

7.0 Inventory and Model Results ..................................................................................... 20

Sediment Production and Delivery ............................................................................................ 20

Mass Wasting Risks .................................................................................................................... 21

8.0 Gullies ....................................................................................................................... 25

9.0 Stream Crossing Analysis ........................................................................................... 26

10.0 Conclusions ............................................................................................................... 29

References .......................................................................................................................... 31

List of Tables 

Table 1: The locations and types of road treatments monitored. ................................................ 9

Table 2: Decommissioning treatments applied by road number. ............................................... 11

Table 3: Modeled sediment production and delivery characteristics. ........................................ 21

Table 4: Summary of GRAIP model risk predictions for the Mann Creek road decommissioning 

project. .......................................................................................................................................... 30

List of Figures 

Figure 1: Locations of monitored sites in Region 4. ...................................................................... 8

Figure 2: Map of road locations within the Mann Creek watershed. ......................................... 10

Figure 3: Damage from the April storms.. ................................................................................... 12

Figure 4: Wieser River hydrograph. ............................................................................................. 13

Figure 5: Rills and small gullies on the recontoured road surface. ............................................. 14

Figure 6: Gully crossing recontoured road. ................................................................................. 15

Figure 7: Slump in recontoured fill.. ............................................................................................ 16

Figure 8: Gullied flowpath on control road 500100500. ............................................................. 17

Figure 9: Non-engineered drain formed along control road. ...................................................... 18
3 

Figure 10: Gully and rills along control road. ............................................................................... 19

Figure 11: Cutslope failure along control road 50029/50473/50007?. ....................................... 20

Figure 12: Natural slope stability index in the area of the control section of the 

50029/50473/50007 road. ............................................................................................................ 22

Figure 13: The most significant slope stability changes along the control section of road 50029.

....................................................................................................................................................... 23

Figure 14: Changes in slope stability risk along the control section of road 50029 where the 

terrain was unstable before and after the storm. ........................................................................ 24

Figure 15: Background slope stability and the changes in slope stability. .................................. 25

Figure 16: Map showing excavated stream crossings with numeric labels. ............................... 27

Figure 17: Grain size distributions for and pictures of crossing 1. .............................................. 28

Figure 18: Sediment size distributions from four other crossings. ............................................. 29
4 

Executive Summary 

The USFS, Rocky Mountain Research Station (RMRS) and Intermountain (INT) Region, Pacific 

Northwest Region (PNW), Pacific Southwest Region (PSW) and the Northern Region (NR) are 

monitoring a sample of the road decommissioning and maintenance projects to assess their 

effectiveness in reducing impacts and risks to key watershed processes. Risk profiles are being 

developed and compared, before and after road treatments, with the Geomorphic Road Analysis 

and Inventory Package (http://www.fs.fed.us/GRAIP). This suite of robust inventory and analysis 

tools evaluates the following road impacts and risks: road-stream hydrologic connectivity, fine 

sediment production and delivery, shallow landslide risk, gully initiation risk, stream crossing failure 

risk, and drain point condition. 

Since FY 2009, inventories have been conducted at eight sites in the Intermountain Region. A site 

consists of a group of road segments totaling four miles treated with either decommissioning or 

Storm Damage Risk Reduction (i.e., stormproofing). Post-storm inventories have been collected 

at four of these sites during FY2010. This report focuses on how decommissioning work 

implemented by the Payette National Forest in the Mann Creek watershed compared to 

untreated control roads following a significant storm event. At the Mann Creek sites, treatments 

included removal of culverts and fills at stream crossings and recontouring of the road prism. 

Following a significant storm event, hydrologic flowpaths on treated roads remained diffuse, 

and storm-related damage consisted of rills and small gullies within the recontoured road 

prism. Storm damage on untreated roads involved gullied wheel tracks and ditches, formation 

of non-engineered drains and erosion of fill materials, gully formation, and general degradation 

of the road surface. 

Predictions of sediment production and delivery confirm the reductions predicted following 

treatment on the treated roads; however, along control roads, modeled sediment delivery 

increased from 16.6 Mg/yr to 54.5 Mg/yr following the storm due to a 23% increase in stream 

connectivity. Storm-related changes in the control road’s drainage structures, namely the 

formation of non-engineered drains and by-passes of existing drainage, resulted in local 

decreases in modeled slope stability. 
5 

Summary of GRAIP road risk predictions for the Mann Creek watershed decommissioning 

project. 

Impact/Risk Type Effect of Treatment: Effect of Treatment: 

Initial GRAIP Prediction Post-storm validation

Road-Stream Hydrologic 

Connectivity

-97%, -2,923 m of connected road -98%, -2937 m of connected road

Fine Sediment Delivery -98%, -40.7 Mg/yr -99%, -41.4 Mg/yr

Landslide Risk Restored to near natural condition Restored to near natural condition

Gully Risk Reduced from low to negligible Reduced from low to negligible

Stream Crossing Risk

 - plug potential -100% (eliminated at 13 sites) -100% (eliminated at 13 sites)

 - fill at risk -100% (807 m3removed) -100% (807 m3 removed) 

 - diversion 

potential

-100% (eliminated at 8 sites) -100% (eliminated at 8 sites)

Drain Point Problems -100% (0% vs. 35% of drain points) + 12% (0% to 12%)

Impact/Risk Type Control Roads Treatment Roads

Effects of Storm Effects of Storm

Road-Stream Hydrologic 

Connectivity

+ 1136 m (+ 12 connected drains) - 14 m (- 3 connected drains)

Fine Sediment Delivery + 38.0 Mg/yr - 0.5 Mg/yr

Landslides + 1 (1,700 Mg) + 2 (285 Mg)

Gullies + 9 (4.9 Mg) + 1 (0.8 Mg)

Drain Point Problems + 25% (23% to 48%) + 12% (0% to 12%)

Acknowledgements 

We would like to thank the field crews (Laura Hutchinson, Jim Peterson, Matt Steiger, and Jim 

Bitzenberg) for collecting the data. We would also like to thank Dave Kennell and Tom 

Crawford of the Payette National Forest for help in selecting these sites and logistical support 

during a very busy field season. Thanks also to Rick Hopson and the Region 4 staff for 

supporting this monitoring effort. Post-storm inventories on treated roads were completed by 

Katelin Aldritt, Michael Barr, Ian Bell, and Scott Bergandorf from the RMRS; the post-storm 

inventory of the control roads was completed by Paul Micheletty and Matt Taylor from the 

Boise National Forest.
6 

1.0 Background 

The National Forest Transportation System is vast and represents an enormous investment of 

human and financial capital. This road and trail network provides numerous benefits to forest 

managers and the public, but can have adverse effects on water quality, aquatic ecosystems, 

and other resources. There is currently a large backlog of unfunded maintenance, 

improvement, and decommissioning work on national forest roads, and many critical 

components of the network (e.g., culverts) are nearing or have exceeded their life-expectancy. 

This significantly elevates risks to aquatic resources. Many Intermountain Region forests have 

been actively addressing known road issues in critical resource areas. Various road treatment 

techniques and restoration activities are being applied throughout the region to address the 

resource risks posed by forest roads. 

The USFS, Rocky Mountain Research Station (RMRS), Intermountain (INT) Region, Pacific 

Northwest Region, Pacific Southwest Region, and the Northern Region are implementing a 

roads monitoring project to evaluate the effectiveness and to learn from the successes of 

road restoration treatments being implemented on national forests throughout the regions. 

As of October 2010, post-storm event data has been collected at six sites, post-treatment 

data has been collected on 33 sites with partial datasets collected at 13 additional sites. 

This report briefly describes the overall objectives of the regional-scale study and the 

methods being used. Specific results presented herein, however, are focused only on road 

decommissioning work completed by the Payette National Forest (PNF) in the Mann Creek 

watershed in FY2009. As other data become available, similar reports will be developed for 

additional sites. In addition, syntheses of results at multiple sites will be produced 

throughout and at the end of this monitoring project. 

2.0 Study Objectives 

The monitoring project is designed to assess the effectiveness of decommissioning and 

maintenance projects in reducing road impacts and risks to several key watershed processes. 

Specifically, the project is intended to address the following questions. 

How effective are USFS road restoration projects in: 

1) reducing or eliminating: 

a. the risk of increased peak flows resulting from road-stream connectivity? 

b. fine sediment production and delivery to stream channels? 

c. shallow landslide risk? 

d. gully initiation risk? 

e. the risk and consequences of stream crossing failures? 

2) improving the performance of the road drainage system
7 

3.0 Methods 

The Geomorphic Road Analysis and Inventory Package (GRAIP, Prasad et al. 2007, 

http://www.fs.fed.us/GRAIP) is being used to inventory and model the risk profile of each of the 

road segments included in the study. The GRAIP system consists of a detailed, field-based road 

inventory protocol combined with a suite of geographic information system (GIS) models. The 

inventory is used to systematically describe the hydrology and condition of a road system using 

Geographic Positioning System (GPS) technology and automated data forms (Black et al., 2009; 

Cissel et al., 2009). The GIS models use these data to analyze road-stream hydrologic 

connectivity, fine sediment production and delivery, shallow landslide potential with and without 

road drainage, gully initiation risk, and the potential for and consequences of stream crossing 

failures. Detailed information about the performance and condition of the road drainage 

infrastructure is also supplied. 

 

Risk profiles are being developed and compared at untreated control segments and treated 

segments before and after road projects. At a given site, monitored road segments typically 

comprise 4 miles of both treated and control sites. Control sites were selected based on their 

similarity to treated sites with respect to road construction methods, maintenance levels, geology, 

and hydrologic regimes. Each site also includes a final validation evaluation at both treatment and 

control sites following a substantial storm event (minimum 5 – 10 year recurrance interval). This 

will allow testing of the initial GRAIP risk predictions and provide an unbiased comparison between 

the treated and the untreated roads. 

4.0 Monitoring Locations 

Regional Monitoring Sites 

In FY2009, pre-treatment evaluations were completed at five sites1 on four national forests in 

the Intermountain Region. Decommissioning was implemented at four of these sites and one 

other site was treated with “storm damage risk reduction2” (Figure 1, Table 1). Four posttreatment inventories were also completed in FY2009. In FY2010, post-storm surveys were 

completed at three sites (Mann Creek, Calf Creek, and Squaw Creek), and four new sites were 

completed.

 

1

 Each site will include the following evaluations: pre-treatment, post-treatment, and post-storm validation 

on treated road segments; and pre-treatment and post-storm validation on control segments. 

2

 “Storm Damage Risk Reduction (SDRR) is used to refer to relatively low-cost treatments applied across 

extensive portions of the road network with the objective of protecting aquatic resources and 

infrastructure. These treatments are intended to reduce the chronic effects of roads (e.g., fine sediment 

delivery) and significantly reduce the likelihood and consequences of catastrophic failures (e.g., diversion 

of stream flow onto roads) associated with large storm events. A variety of tools may be used to achieve 

these objectives, depending on site-specific conditions. These include diversion potential dips at roadstream crossings, water bars, and broad-based drain dips. These simple, extensive treatments are 

intended to compliment the use of more intensive treatments (e.g., decommissioning, road realignments) 

that are typically implemented on relatively small segments of the network. 
8 

Figure 1: Locations of monitored sites in Region 4. 
9 

Table 1: The locations and types of road treatments monitored. 

National 

Forest

Start Year Treatment Watershed

Payette 2009 Decommissioning Mann Creek

Payette 2009 Decommissioning Calf Creek

Boise 2009 Decommissioning Squaw Creek

CaribouTargee2009 Other Treatments Island Park

2009 Storm Damage Risk 

Reduction

Island Park

Payette 2010 Long-Term Closure Little Weiser

Boise 2010 Storm Damage Risk 

Reduction

Rice Creek

Dixie 2010 Decommissioning Mammoth Creek

Fish Lake 2010 Decommissioning Monroe Mountain

Mann Creek Sites 

During the summer and fall of 2009, field crews inventoried decommissioning sites in the 

Intermountain Region, including the Mann Creek watershed (Table 1, Figure 1). This watershed 

is principally underlain by basalts, with Columbia River Group basalts dominating the midelevation band. The higher elevations are underlain by sedimentary rocks of the Olds Ferry and 

Izee terranes as well as mafic plutons of the Blue Mountains island arc terrane. The average 

annual precipitation for the basin ranges from 10 - 30 inches per year. The watershed is 

managed for multiple uses including timber harvest, grazing, and recreation. The inventoried 

sites are located between 4,800 and 6,000 feet above sea level just east of the Snake River. 

The Mann Creek watershed is located in Washington county between U.S. Highway 95 and the 

Snake River. Mann Creek Reservoir, located near the mouth of the watershed, is about halfway 

between Weiser and Cambridge. 

Data were collected on roads in the spring of 2009 before the decomissioning treatments 

began, and once again in summer of 2009 once the treatments were completed (Figure 2). Pretreatment roads were native or gravel surface roads that were generally in good shape. With 

the exception of road 501641000, the roads were classed as maintenance level 2 or 3 (Table 2). 

The maintenance class of road 501641000 is unknown, as the forest did not classify it, though it 

appears to be similar to maintenance level 2 based on photographs and comparisons to 

descriptions of road maintenance classes. Flow on the roads was generally contained in wheel 

tracks or a ditch. Both treatment and control sites included roads on a range of hillslope 

positions, though dominantly valley-bottom, and included frequent live stream crossings. The 

watershed has moderately steep topography, so stream crossing fills are not typically large. 
10

Decommissioning treatments were performed by USFS equipment and staff, and involved 

removing stream crossing culverts and fills and partially to fully recontouring the road prism. 

Figure 2: Map of road locations within the Mann Creek watershed.
11

Table 2: Decommissioning treatments applied by road number. 

Decommissioned Road Control Road

Road # Maintenance 

Level Treatment Road # Maintenance Level Treatment

50007 2

Stream crossing extraction, 

recontouring, and culvert 

removal

50029 

50473

50007?

2 None

50029 2

Stream crossing extraction, 

recontouring, and culvert 

removal

50029 

50473

50007?

2 None

50470 1

Stream crossing extraction, 

recontouring, and culvert 

removal

50029 

50473

50007?

2 None

50019 

Old 51189 3

Stream crossing extraction, 

recontouring, and culvert 

removal

50019 

Old 51189 3 None

50164100

0

Unclassified

Stream crossing extraction, 

recontouring, and culvert 

removal

500100500 Unclassified None

5.0 Storm Event 

In April of 2010 a series of small rain-on-snow events occurred within the Mann Creek drainage. 

While these events generally had recurrence intervals of 2 – 3 years, the resulting flooding, 

especially along Mann Creek, caused extensive damage to stream-side roads, especially where 

diversions occurred (Figure 3). These rain-on-snow events affected a fairly narrow elevation 

band below the treated roads as the snow line retreated. Another similar event in May might 

have affected some portions of the study site. 

On June 2, 2010, a strong series of thunderstorms passed through the area. These storms 

brought heavy rains over a ripe snow pack, and even though the precipitation intensities have 

less than 5 year recurrence intervals, the resulting floods, measured at the USGS gage in 

Cambridge, Idaho, have a recurrence interval of ~8 years (Figure 4). The river crested at 7,440 

cfs on June 5. 
12

Figure 3: Damage from the April storms. The author is standing in a gullied wheeltrack where 

flood water was diverted down the road from the stream crossing behind the author’s head. 
13

Figure 4: Wieser River hydrograph. 

6.0 Storm Damage Characteristics 

Treatment Roads 

Crews surveyed the decommissioned roads in late June and early July to determine what 

changes were caused by the storms. Most storm-related damage consisted of rills and small 

gullies in the recontoured surface (Figure 5); these rills were, with few exceptions, limited to 

the fill material and did not extend onto the hillslope below the recontoured road. Where 

these met size criteria for gullies, eroded sediment volumes were estimated and recorded. 

Most of these rills and gullies were concentrated between 4,500 and 5,000 ft elevation. The 

estimated volume of these rills and gullies is 600 ft3(~28.6 metric tons). This type of erosion 

was expected given that the recontoured roads consisted of unconsolidated fill with less 

vegetation than surrounding hillslopes. 

Crews found one gully that extended beyond the recontoured surface where the road crossed a 

swale (Figure 6). They also found two small landslides where recontoured fill had slumped 

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

6500

7000

7500

8000

Discharge (cfs)

Weiser River at Cambridge (cfs) 2-Year Flood 

5-Year Flood 7-Year Flood

10-Year Flood
14

(Figure 7). The combined estimated volume of the gully and both landslides is ~6,000 ft3 (~286 

metric tons). 

Figure 5: Rills and small gullies on the recontoured road surface. 
15

Figure 6: Gully crossing recontoured road. 
16

Figure 7: Slump in recontoured fill. View is from top of scarp. 

Control Roads 

A crew surveyed the control roads on June 10 and 11. Repair work had already been completed 

on the lowest portion of the 50009 road; hence this portion was not surveyed after the storm. 

Damage generally consisted of longitudinal rills or gullies along the road surface or ditch (Figure 

8), new non-engineered drains (Figure 9) and fill erosion (528 ft3 or ~25.2 metric tons), and gully 

formation (Figure 10). In 2009, 13.4% of the road surface was recorded as being in other than 

“good” condition (13.1% rutted, 0.3% rocky); after the storm, 53.8% of the road surface was in 

less than “good” condition (52.3% rilled or eroded, 1.5% rocky).

Nine gullies were found totaling 100 ft3, or ~4.9 metric tons, along with one large cutslope 

failure (~35,600 ft3 or ~1,700 metric tons; Figure 11).
17

Figure 8: Gullied flowpath on control road 500100500. 
18

Figure 9: Non-engineered drain formed along control road. 
19

Figure 10: Gully and rills along control road. These extend from the road surface across the fill 

and onto the hillslope below. 
20

Figure 11: Cutslope failure along control road 50029/50473/50007?. 

7.0 Inventory and Model Results 

Sediment Production and Delivery 

Comparison of the treatment and control roads prior to treatment indicates that the two sets 

of roads were likely to produce similar amounts of sediment (Table 3). While the control roads 

were expected to produce somewhat more sediment and have virtually the same portion of the 

road connected to the stream, sediment delivery from the control roads was predicted to be 

about 40% of that from the treatment roads. This was due to differences in vegetation 

densities in the road flowpaths; control roads were more likely to have greater than 25% 

vegetation cover in at least one flowpath than were the treatment roads. 

Treatments were expected to reduce annual sediment production by 56% and annual sediment 

delivery by 98% (Table 3). Post-storm surveys indicate these reductions may be closer to 73% 

for sediment production and 99% for delivery. Recontouring the road surface eliminated the 

longitudinal, concentrated flowpath and replaced it with a transverse, diffuse flowpath, 

drastically reducing sediment production. Sediment delivery is further reduced because the 

diffuse flowpath is less likely to allow stream connections. 
21

The post-storm survey of the control roads was shorter by 1 km than the pre-storm survey 

because repairs had already been completed on the kilometer of road omitted, which is why 

sediment production appears to be reduced following the storm. When averaged over the 

length of the surveyed road, sediment production increased slightly (17.0 kg/m/yr – 18.1 

kg/m/yr). The increase in sediment delivery is significant (2.0 kg/m/yr – 7.3 kg/m/yr) and is 

due, in large part, to an increase in connected road length (38% - 61%). In addition, some local 

areas have seen large increases in sediment production (up to 17.5x prior levels). Areas that 

saw the greatest decreases (to 80% of prior levels) in sediment production, weren’t stream 

connected, while areas that saw the biggest increases were stream connected. 

Table 3: Modeled sediment production and delivery characteristics. 

Sediment 

Production 

(Mg/yr)

Sediment 

Delivery 

(Mg/yr)

Delivering 

Road Length 

(m)

% Sediment 

Delivery

% Delivering 

Road Length

Pre-treatment 136.6 41.7 3,000 31% 37%

Post-treatment 60.5 1.0 77 2% 1%

Post-storm 

Treatment 37.5 0.3 63 1% 1%

Pre-storm Control 144.5 16.6 3,262 11% 38%

Post-storm Control 135.4 54.5 4,598 40% 61%

Mass Wasting Risks 

Landslide risks within the Mann Creek watershed appear to be minimal; prior to the storm, no 

landslides were located on either the control or treatment roads. On treatment roads, 

landslide risks were predicted to have returned to near the natural state following recontouring 

and conversion to diffuse drainage. This appears to have held true, as the two small landslides 

located along the treated roads after the storm event consisted entirely of loose, 

unconsolidated, recontoured fill material. These two failures were limited to recontoured fill 

material which was likely past its saturated angle of repose. The one landslide located along 

the control roads was much larger, but resulted from, and was confined to, a cutslope failure. 

Flowpaths along the decommissioned roads did not change and were still diffuse; hence 

landslide risk along treatment roads did not increase after the storm. While landslide risks may 

be low, damage resulting from the storm appears to have increased those risks along the 

control roads, as discussed below. 

The risk of shallow landslide initiation is predicted using SINMAP 2.0 (Pack et al., 2008, 

http://hydrology.neng.usu.edu/sinmap2/), modified to account for contributions of road 

runoff. SINMAP has its basis in the infinite plane slope stability model and produces raster grids 

that illustrate slope stability based on hillslope and specific catchment area at each DEM grid 
22

cell. While it is possible to calibrate SINMAP to account for local geology, the data necessary 

was not available; therefore this analysis uses SINMAP’s default values and may over-predict 

unstable areas. Pre- and post-storm landslide risk grids are subjected to a series of 

mathematical operations that result in grids that show the important changes to landslide risk 

due to the storm. These change grids are compared to the natural landslide risk grid (Figure 12) 

to show how the storm affected slope stability in the context of the background risks (i.e. the 

risks without the influence of the road drainage). Important grid cell changes are those pre- to 

post-storm differences that show a risk change from stable to unstable, unstable to stable, or 

that become more or less stable while remaining unstable following the storm. 

Figure 12: Natural slope stability index in the area of the control section of the 

50029/50473/50007 road. The yellow, blue, and green cells are generally classified as stable, 

while the pink, red, and tan cells are generally classified as unstable. 

A second stability index (SI) run was performed to address the effects of road water 

contribution to drain points on the control road prior to the storm. A third model run was 

performed to illustrate the risk of shallow landsliding after the storm damage occurred. In 

Figure 13, the areas along the 50029 road where the storm changed the risk from the unstable 
23

category (defended, upper threshold, and lower threshold from Figure, above) to the stable 

category (quasi-stable, moderately stable, and stable) are shown in green, and areas where the 

storm changed the risk from the stable category to the unstable category are shown in red. 

These are the areas where risk has been sufficiently reduced (green), or where risk has been 

increased significantly (red). The storm-related damage along the road changes the hydrologic 

structure of the road; this results in changes in the locations and quantities of water drained 

from the road. If a gullied flowpath bypasses existing drainage, the area below the bypassed 

drain may experience a relative increase in stability (green); however, because of the additional 

water delivered to some other drainpoint, whether new or existing, the hillslope below that 

drain will become less stable (red). 

Figure 13: The most significant slope stability changes along the control section of road 50029. 

The risk of the areas in green was sufficiently reduced, while the risk in the red areas was 

significantly increased.

Figure 14 shows the areas where the risk of shallow landsliding was high (unstable grid cells) 

both before and after the storm. The light blue cells are areas where the risk decreased 

(became more stable), but the terrain was still unstable after treatment. This was generally due 
24

to a decrease in the length of road that drained to each point, but was not enough of a 

reduction to move the risk category to stable. The orange cells are areas where the risk 

increased (became less stable) after the storm, and the terrain was unstable before the storm. 

This is generally due to the addition of drainage over slopes that were unstable without 

considering the effects of road drainage. 

Figure 14: Changes in slope stability risk along the control section of road 50029 where the 

terrain was unstable before and after the storm. The blue areas are locations where the risk 

was lowered, and the orange areas are where the risk increased. 

In some locations, a drain point that was located above a slope that was unstable without road 

drainage consideration received less flow after the storm, and so became more stable. In these 

locations, there was no way to reduce the overall shallow landslide risk to be stable. These 

locations are shown in Figure 15, where the cross-hatch areas were unstable without 

consideration of road drainage and cross-hatch over blue shows the areas that also experienced 

reduced risk. In some of these locations, the storm may have reduced the stability category to 

nearly background (natural) levels. Reduction to fully background levels would require the 

removal of the drain points over those unstable slopes. Areas where the underlying risk 
25

(without a road) was unstable and the stability risk increased after the storm (cross-hatch over 

orange) show that drainage was added over a slope that was unstable without consideration of 

road drainage. 

Figure 15: Background slope stability and the changes in slope stability. The cross-hatch 

pattern is all of the area in the map that was unstable without consideration of road drainage. 

8.0 Gullies 

During pre- and post-treatment visits, only one gully was located (along one of the control 

roads); this “gully” is likely a deeply incised stream and was not recorded as a gully during poststorm visits. The post-storm inventory describes this as a natural ford across a small stream. 

Following the storm event, only one gully was located along the treated roads and nine gullies 

were recorded along the control roads. All of these were new gullies. The gully found along 

the treatment roads (along the decommissioned 50019) is located where a wet swale crosses 

the road and likely delivered concentrated flow during the storm event. This gully begins just 
26

above the recontoured road and extends onto the hillslope just below recontoured road. It has 

a volume of ~17 ft3. 

Nine gullies were located along the control roads. All of these gullies extend across the road fill 

slope onto the hillslope below the road and initiate at drain points (two ditch relief culverts and 

seven non-engineered drains) along the road. The total volume of these gullies is ~103 ft3. 

9.0 Stream Crossing Analysis 

Adjustments and channel formation were also recorded at excavated stream crossings (Figure 

16). Crews measured channel incision within the excavated crossings as well as bed particle 

size distributions within the crossing and in an adjacent upstream reach. Nine of the twelve 

excavated crossings had not experienced flow prior to the post-treatment site. An estimated 

2,600 ft3

 (~125.7 metric tons) of material has been removed by the stream during adjustment 

at these nine crossings. This is separate from the fill removed by equipment during the 

excavation. These crossings are mostly 1st-order streams. 

Crossings 1, 4, and 12 had all received some flow prior to the post-treatment inventory; since 

the crews did not record incision measurements during the post-treatment inventory, the 

amount of post-storm incision could not be calculated at these locations. Photographs indicate 

that much of the ~6,000 ft3 of material from these three crossings was removed prior to the 

post-treatment inventory. Without accurate measurements immediately following the 

treatment, prior to any adjustment, it is impossible to determine how much of this ~6,000 ft3 of 

material was removed by equipment at the time of treatment or removed by the stream during 

adjustment following the treatment. Crossings 1 and 4 are 4th-order streams; crossing 12 is a 

3

rd-order stream. 

Side slopes on the excavated crossings appear to be stable, no failures were noted on any of 

the side slopes within the study area. The three perennial stream crossings (1, 4, and 12) did 

not show evidence of lateral adjustments following the storm event. 

The bed particle size distributions can be used to help determine if adjustments at the crossing 

are complete; if the distributions in the crossing and upstream are similar after the storm, it is 

less likely that further adjustments to the channel morphology within the crossing will occur. 
27

Figure 16: Map showing excavated stream crossings with numeric labels. 

Excavated stream crossings are at various states of adjustment following the storm event and 

spring runoff. Most of the examined crossings follow the expected response where excess fines 

in the crossing following excavation are being, or have been, removed. This indicates that the 

bedload more closely resembles that of the natural channel upstream of the crossing. The 

pattern is more advanced and more clearly defined in higher-order channels than in lowerorder channels because of greater stream power and longer flow durations. Increased stream 

power does not predict greater erosion, only quicker sorting and removal of excess fines from 

the stream bed within the excavated crossing. Three of the 1st-order channels show an 

opposite pattern where the bedload has become significantly finer than post-treatment, but 

also remain significantly finer than upstream reaches. Some likely reasons include a lack of 

significant adjustment (crossing 5), scour through the disturbed sediment into undisturbed 

colluviums (crossing 6), or entrapment of fines by large anchor clasts (crossing 8). 

Crossing 1, along the main stem of Mann Creek, shows an example of a well-adjusted stream 

crossing (Figure 17). This crossing has rapidly adjusted by removing fines left behind following 

excavation of the fill and culverts. This is one of two 4th-order stream crossings in the study. 
28

Figure 18 shows the bedload grain-size distributions for four different 1st-order stream 

crossings, including the three described above, that are not adjusting as expected. 

Figure 17: Grain size distributions for and pictures of crossing 1. Note the shift in the “in 

crossing” size distributions indicating removal of fine sediment and restoration of natural bed 

characteristics observed upstream. 
29

Figure 18: Sediment size distributions from four other crossings. Crossing 7 shows incomplete 

adjustment along the expected trend and textural coarsening of the upstream reach following 

high flows. Crossing 5 shows a lack of any significant adjustment to bed sediment. Textural 

fining observed at crossing 6 was due to incision into underlying colluviums; at crossing 8 

textural fining was due to entrapment of fines by large keystone clasts. 

10.0 Conclusions 

Recontouring of treatment roads was expected to convert longitudinal concentrated flowpaths 

to lateral diffuse flowpaths and cause the former road to behave similarly to the surrounding 

hillslopes. Observations following a significant storm event support these expectations (Table 

4). Observations further indicate that the recontoured treatment roads sustained less stormrelated damage than untreated control roads. Storm-related damage on treatment roads 

consisted of rills and small gullies similar to those that form on unconsolidated, unvegetated 

slopes; these rills and gullies were limited to the recontoured fill surface. Storm related 

damage on the control roads consisted of longitudinal gullies formed on existing flowpaths 

(wheel tracks or ditches), formation of non-engineered drains with fill erosion, and formation of 

gullies below drains; there was also a marked (40.4% of total road length) increase in the 
30

amount of road classified as having problems associated with scour occurring along the road 

flowpath. 

Model runs using the post-storm inventories confirmed post-treatment model predictions for 

the treated roads or indicated that reductions may be greater than predicted by the posttreatment model runs. Control roads, however, are predicted to have greatly increased 

sediment delivery because storm-related damage resulted in greater stream connection, 

especially in areas where sediment production was locally increased. Changes in the control 

roads hydrologic structure also caused local decreases in slope stability. 

Observations at the excavated stream crossings show that most of the crossings are adjusting 

as expected. Higher-order crossings have nearly completed adjustment and bedload 

characteristics are now similar to upstream reaches. Some 1st-order crossings did not follow 

the expected pattern of increased sorting and removal of finer sediment. 

Table 4: Summary of GRAIP model risk predictions for the Mann Creek road decommissioning 

project. 

Impact/Risk Type Effect of Treatment: Effect of Treatment: 

Initial GRAIP Prediction Post-storm validation

Road-Stream Hydrologic 

Connectivity

-97%, -2,923 m of connected road -98%, -2937 m of connected road

Fine Sediment Delivery -98%, -40.7 Mg/yr -99%, -41.4 Mg/yr

Landslide Risk Restored to near natural condition Restored to near natural condition

Gully Risk Reduced from low to negligible Reduced from low to negligible

Stream Crossing Risk

 - plug potential -100% (eliminated at 13 sites) -100% (eliminated at 13 sites)

 - fill at risk -100% (807 m3removed) -100% (807 m3removed) 

 - diversion 

 potential

-100% (eliminated at 8 sites) -100% (eliminated at 8 sites)

Drain Point Problems -100% (0% vs. 35% of drain points) + 12% (0% to 12%)

Impact/Risk Type Control Roads Treatment Roads

Effects of Storm Effects of Storm

Road-Stream Hydrologic 

Connectivity

+ 1136 m (+ 12 connected drains) - 14 m (- 3 connected drains)

Fine Sediment Delivery + 38.0 Mg/yr - 0.5 Mg/yr

Landslides + 1 (1,700 Mg) + 2 (285 Mg)

Gullies + 9 (4.9 Mg) + 1 (0.8 Mg)

Drain Point Problems + 25% (23% to 48%) + 12% (0% to 12%)
31

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33