Sediment Sources

Overall Evaluation: Some Action Needed /Closely Monitor

Figure 1. Road and stream locations in the South Slough and Coastal Frontal Watersheds

Issue Summary:
Eleven of the 69 culverts evaluated in the project area are deemed at-risk of causing the release of significant volumes of sediment into project area streams. Five of those culverts are deemed high risk. High-risk culverts are also sized or positioned inadequately for draining 50 year rain events and could cause upstream flooding.

Why do we care:
Roads are chronic sources of sediments that can overwhelm watershed streams. Plugged and undersized culverts may wash out in storms resulting in large amounts of sediment going into streams, harming important aquatic habitat.

What’s happening?
A sediment sources assessment was conducted for the South Slough and Coastal Frontal watersheds in 2010 and 2011 to evaluate three potential sediment sources: 1) Slope stability, in which each sub-basin was evaluated for % of area at risk of slope failure in four risk categories from low to extremely high; 2) Road and landing surveys, in which roads and road drainage features were examined for erosion potential and compared to Oregon Department of Forestry Best Management Practices (ODF 2003); and 3) Stream crossing capacity evaluation, in which stream crossing sites were rated for their flow capacity compared to a 50-year event and their risk of failure.

Sediment
Because particles of silt and organic matter, collectively referred to here as sediment, are easily transported by flowing water, sources of these particles, such as landslides, earthflows, or collapsing banks can affect large areas of habitat downstream from the source. Fine sediment, beyond natural background levels, is detrimental to fish and their habitat in many ways. For example, sediment can cover salmonid spawning gravels, often causing high rates of egg mortality. More than 10-15% fine sediment (silt/organics) reduces the flow of oxygenated water to the eggs (FRS 2003). In the case of adult salmon, high concentrations of suspended sediment may delay or divert spawning runs (Mortensen et al. 1976). Additionally, as pools collect sediment, depth decreases and solar heating occurs more rapidly. Aggradation, or raising of the streambed, can influence flow levels, flooding and erosion.

Slope Stability
Unstable slopes often lead to shallow slope landslides and deep-seated soil creeps. It is important to note that landslides are an important natural process that delivers beneficial gravel, boulders, and large woody debris into the stream channel. However, an overabundance of avoidable landslides caused sometimes by poorly considered human activities in the watershed, can overwhelm aquatic ecosystems. Slope, vegetation, and geology all have direct relationships to the slope stability of an area.

Presence of mature vegetation is important component of stable slopes. “There is some evidence that the removal of trees on steep slopes (greater than 80%) can make the area vulnerable to shallow landslides, and can lead to temporary acceleration of the landslide rate. This vulnerability begins when many of the finer roots of the harvested trees become rotten (about 4 years) and ends once the replacement stand has developed a dense root network (about 30 years for wet portions of the state)” (OWEB, 1999). Many of the upland slopes in the assessment area are commercial forests on short harvest rotations, most are harvested in 30 or 40 year rotations. Because of this, there may be chronic slope problems from this type of land management. Adhering to Best Management Practices during forest harvesting is important to minimize loss of soil on unstable steep slopes.

Our slope stability analyses indicate that 79.2% of the study area (South Slough and Coastal Frontal watersheds, shown in Figure 1) is in the low risk category for landslide potential, and approximately 15.5% of the area is at moderate risk, see Figures 2 and 3. High risk is 4.1% of the area and extremely high risk is 1.2% of the area. High and extremely high risk areas total 5.3% of the South Slough and Coastal Frontal sub-basins. The most unstable slopes are located in the headwaters of Big Creek, located in the northwest part of the sub-basin. Not surprisingly, there are also steep, potentially unstable slopes along the coast associated with the Coastal Frontal sub-basins.

Figure 2. Map showing hillslope risk classes, streams (black) and roads (blue) in the South Slough and Coastal Frontal watersheds. Slopes between 0% and 30% are considered Low risk for failure (green); slopes between 30% and 50% and considered Moderate risk of failure (yellow); slopes between 50% and 70% are considered High risk of failure (red); and slopes above 70% are considered Extremely High risk of failure (red).

Figure 3. Area in square kilometers for each of the four slope risk categories in the South Slough and Coastal Frontal watersheds

Road Sediment
Road sediment surveys were conducted for three primary purposes: 1) identify fish passage impediments at road stream crossings; 2) determine the degree of road failure risk; and 3) identify locations where hydrologic connectivity of road drainage ditches to live stream networks could be altered to filter road sediment before it reaches the stream.

Hydrologic connectivity occurs when road drainage is discharged directly into channels via culvert outflow or drainage ditch relief near stream channels (assumed to be within 100 feet). Either one of these conditions will potentially increase sediment transport volumes and flood stage elevations downstream.

Table 1. Site type summary

The South Slough and Coastal Frontal watersheds road and landing survey was conducted between June 2010 and August 2011. All private roads were surveyed where landowner permission was granted. Table 1 shows the number and type of drainage features, and the min, max, avg. of the length of each road segment leading to each site type.

A total of 81.3 miles of road were surveyed in the South Slough and Coastal Frontal watersheds. The total number of sites surveyed was 589. The average number of drainage sites per mile was 7.3. Within the forested roads in this survey, there are 69 stream crossings, 220 ditch relief culverts, and 300 ditch outs. Figure 4 shows the location of all the sites surveyed in and around the assessment area.

Figure 4. Drainage Feature Sites

Stream Crossing Drainage Evaluation

The 69 stream crossing culverts studied in the road and landing survey, shown in Table 1, were ranked for their ability to properly drain the area upstream during a fifty-year rain event. Sixteen percent (16%) of the stream crossings in this survey are considered at risk for improper drainage or failure because they are undersized.

At-risk culverts are ranked in Table 2, below, based on the percentage of associated drainage area they can properly drain during a 50-year rainfall event. The number of culverts in each failure risk level (left column) spread across the table depending on the associated fill volume size class. It is important to consider both failure risk and fill volume since it is the fill that becomes the sediment source upon failure of the crossing.

Table 2. 50-Yr. Rainfall Fill Failure Risk

In the assessment area, three of the 69 culverts were ranked as having very high risk of failure, potentially releasing 236 yrd3 of fill. Five were ranked as having high risk, potentially releasing 362 yds3 of fill. Two ranked moderate, potentially releasing 173.7 yds3 of fill. One of them ranked low, potentially releasing 326 yds3. There is a total of 1097.7 yds3 of fill at these 11 at-risk culverts.

Recommended Actions
Sediment loading, best treated at its source, can be addressed in many ways. Careful consideration should be taken when planning land use activities that disturb the already erosion-prone soil. Carefully directing run-off through proper culverts, road-side ditches and away from road surfaces will reduce its erosion potential.

Table 3 shows treatment recommendations based on the South Slough and Coastal Frontal sub-basins road sediment survey analysis. “New structures needed” are based on Oregon Department of Forestry (ODF, 2003). Best Management Practices addressing ditch lengths. “Replacement structures needed” address all road drainage features, and are based on the Pacific Watershed Associates Road and Landing Survey Protocol (Haga, 1997) adapted by the Coos WA.

Table 3. Sediment source Treatment recommendations

Installation of 358 new ditch relief culverts is recommended to reduce road related sediment in the assessment area. Of the existing 69 stream crossing structures, we suggest replacing 23 undersized culverts with properly sized culverts (many of those culverts need to be replaced anyway). Six sites are listed as fish passage barriers. Of the 121 existing ditch relief culverts, 60 are damaged and need to be replaced. Drainage sites with the greatest potential for future erosion from excessive ditch lengths, unstable fill, culvert failure, and/or active erosion should be addressed first. Figure 5 shows the locations of recommended treatment sites.

Figure 5. Map showing treatments based on South Slough and Coastal Frontal road erosion survey information gathered in 2010 and 2011.

Literature Cited

FRS Freshwater Laboratory, 2003. How Groundwater Can Affect the Survival Rate of Salmon Eggs. Fact Sheet. Scotland, UK Accessed 2-23-05 http://www.frs-scotland.gov.uk/FRS.Web/Uploads/Documents/poster 1.pdf

Hagans, Dan K. 1997. Field Training in Watershed/Road Sediment source Assessments and Erosion Prevention. Pacific Watershed Associates, Arcata, California.

Mortensen, D. G., B. P. Snyder, and E. O. Salo. 1976. An analysis of the literature on the effects of dredging on juvenile salmonids. Contract Number N-68248-76-C-0011. Special report to the Department of Navy, University of Washington, College of Fisheries, Fisheries Research Institute, Seattle.

Oregon Department of Forestry 2003. Forest Practices Technical Note Number 8: Installation and Maintenance of Cross Drainage Structures on Forest Roads.

Oregon Watershed Enhancement Board (OWEB).1999. Oregon Watershed Assessment Manual. Prepared by Watershed Professionals Network. Salem. OR.

Ott, Robert A. 2000. Factors Affecting Stream Bank and River Bank Stability, With an Emphasis on Vegetation Influences. Compiled for the Region III Forest Practices Riparian Management Committee, Fairbanks, Alaska.