Restoration Program

Program Administration

Session: Fine Sediment Measurement and Modeling

Wednesday, February 7, 2007 - 10:55 AM to 11:40 AM

Method for Estimating Fine Sediment Storage on the Mainstem Streambed

Dave Gaeuman, Trinity River Restoration Program, dgaeuman@mp.usbr.gov, (530) 623-1813

Presentation [PPS - 0.8 mb]

The Trinity River Restoration Program identifies a loss of substrate quality associated with excessive fine sediment inputs to the Trinity River as an important factor in the decline of the Trinity River fishery. The Trinity River Record of Decision instructs the Program to reduce fine sediment storage in the mainstem Trinity River via 1) watershed actions designed to decrease tributary inputs of fine sediment, and 2) managed flow releases that will transport fine sediments downstream at greater rates than it is delivered from the tributaries. A method for assessing the success of these management actions is therefore required. It has been proposed that assessing the quantity of fine sediment on the surface of the streambed is an efficient means to evaluate storage changes. Large amount of fine sediment on the surface indicates that the local supply of fine sediment has recently exceeded the transport capacity needed to remove it. Surface fines also suggest that the interstices in the gravel substrate are filled, since fines on the surface tend to infiltrate into voids in the substrate. A rapid assessment of the bed area covered with fines can be obtained via visual estimation. However, estimating storage requires a method convert area coverage to volumes. This presentation describes an analysis of the geometry of a gravel streambed specified by a median particle size and a measure of spread, here selected as the 90 th percentile particle size. Bed particles are assumed to be ellipsoids with an aspect ratio of ½ and a log-normal particles size distribution. Expressing the particles sizes in dimensionless form allowed the development of generalized equations that quantify the frequency distribution of elevations of randomly generated bed surfaces. Dimensionless bed heights were found to be Weibull-distributed, where the distribution parameters are functions of grain size sorting. This distribution allows the fill volumes and areal coverages corresponding to a given height of fill in bed surface interstices to be computed numerically.

Presentation notes:

Surface assessments of sand volume may be the best done visually as it is easy and accurate. Core samples would be effort-intensive. Gaeuman proposes a method that uses median D50 and D90 plus visual estimates of sand cover. Gaeuman showed the distribution of heights of individual, idealized bed material particles to follow a mathematical distribution and that this allows the volume filled by sand to be calculated. If one can then visually estimates the D50 and D90 and the percent of sand cover, they can derive sand volume on the bed surface of a stream.

Questions: Visually estimating D50 is relatively easy but it takes more time to develop skill to estimate D90. This method has not been validated in the river yet.

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Watershed Restoration Overview and Effectiveness

Joshua Allen, Assistant Planner, Trinity County Natural Resources Division,
jallen@trinitycounty.org or (530) 623-1351 x. 3411

Presentation [PPS - 10 mb]

Watershed restoration and sediment reduction has been an ongoing effort in the Trinity River basin for over 30 years. The Trinity River Record of Decision (ROD) specifically identified a watershed restoration program was to be implemented as part of the Preferred Alternative. The ROD specified that the "Trinity Management Council (TMC) will guide an upslope watershed restoration program to address the problems of excessive sediment input from many of the tributaries of the Trinity River resulting from land use practices. The watershed protection program of the Preferred Alternative includes road maintenance, road rehabilitation, and road decommissioning on private and public lands within the Trinity River basin below Lewiston Dam, including the South Fork Trinity River basin ." (ROD, pg14)

After many years of enduring studies, projects began to be implemented before the advent of the current Trinity River Restoration Program, beginning with the Grass Valley Creek Watershed Restoration Program (GVCWRP). Grass Valley Creek is a major contributor of sediment due its geological makeup of highly decomposed granitics. The GVCWRP installed Buckhorn dam to catch sediment from the upper watershed, pools and ponds located close to the Trinity River to catch the remaining sediment coming downstream, and instituted revegetation and management plans. The project cost was approximately $60 million and is considered complete, with an expected 160 year lifetime for Buckhorn dam, and annual dredging of Hamilton ponds by the Trinity County Resource Conservation Service (TCRCD). There is a need, however, for watershed restoration work in other portions of the Trinity River basin.

Many agencies and NGO's within the Trinity River basin are implementing restoration, such as the 5 Counties Salmonid Conservation Program in conjunction with the Trinity County Natural Resources Division and Trinity County Road Department in the upper basin, TCRCD in conjunction with the US Forest Service in the South Fork Trinity River basin, and the Hoopa Valley and Yurok Tribes in their respective areas of sovereignty. Most projects revolve around reducing sediment inputs related to roads and crossings, thus many roads in forested areas are being decommissioned or redesigned, and crossings are either eliminated or upgraded with fish friendly passages such as natural bottom arched culverts and bridges. Most of this work is funded with internal agency funds, grants, and the diminutive amount of funds budgeted by the TMC. The TMC funds are generally used to seek more funding via leveraging of grants for predetermined projects, but often times they are used for adaptive management, i.e. to react to unexpected emergencies such as landslides caused by wildfire and poor land management practices.

Watershed restoration is an important component of the ROD and requires significant funding to implement projects to meet that goal.

Presentation notes:

Allen gave a review of programs and efforts by various groups to reduce sediment deliveries to the Trinity River . He noted the work in the Grass Valley watershed, the Hamilton Ponds in the lower Trinity and work in the South Fork Trinity River. Buckthorn Dam in Grass Valley captures about half the sediment transport. Five County group do road assessments and fish passage projects. Trinity County RCD is working in the South Fork Trinity watershed on decommissioning of roads.

Question about the methods used to estimate sediment reductions could not be answered.

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Evaluation of Fine Sediment Production in the Rush Creek Watershed

Mary Ann Madej, U.S. Geological Survey Western Ecological Research Center
mary_ann_madej@usgs.gov, (707) 825-5148

Presentation [PPS - 2.4 mb]

Two sediment management goals of the Trinity River Restoration Program are to reduce delivery of fine bedload sediment (0.5 to 8 mm in diameter) to the mainstem Trinity River and to comply with the sediment Total Maximum Daily Load (TMDL) objective of reducing sediment delivery to 125% of background. Sources and delivery of fine sediment in the Rush Creek watershed were evaluated through a combination of air photo analysis, slope stability modeling and field measurements. The predominant bedrock types in this basin are the Weaverville Formation, the Shasta Bally batholith and the Bragdon Formation. These bedrock types produce abundant fine material upon weathering, and the granitics have the highest sand and fine gravel fractions. Soils are generally shallow to moderately deep loams to gravelly loams. In the fraction of the soil that is less than 2 mm in diameter, the Rush Creek soil horizons have an average of 24% clay. This size fraction is transported as wash load and is generally not found in storage in the channel beds.

From 1924 to 1998, landslides accounted for about 60% of erosion in the Rush Creek basin, and road-related erosion was about 30% of the total erosion. The Rush Creek basin has 160 km of roads with more than 400 stream crossings. Only 30% of the road length is under public management (federal, state or county). Erosion of road cutbanks contributes material to inboard ditches, which can then be transported to streams. About 25 to 40% of the road network is hydrologically connected to streams. Erosion pins documented cutbank surface lowering rates on non-granitic soils during Water Year 2006, which had one of the wettest winters on record. Four grids of pins had average erosion rates of 1, 2, 3 and 6 mm, respectively. These rates were all less than the '9 mm/yr' considered as the 'moderate' rate in previous road inventories in the Trinity River basin, which had been based on literature reviews. Fine sediment delivery from cutbanks can be minimized by hydrologically disconnecting roads and by revegetation of raw banks. Localized stream bank erosion rates are as high as 110 m 3/km of stream length per year, but streambank stabilization is difficult because of access and cost. Areas susceptible to landslides were modeled using SHALSTAB. The headwaters of Rush Creek have the steepest and most unstable slopes, but this is a mountainous wilderness area with no management control. Another potentially unstable area is in the downstream eastern portion of Rush Creek on the metasedimentary rocks of the Bragdon Formation.

Currently Rush Creek delivers an average of about 1140 Mg (about 800 yd³) of fine bedload sediment to the mainstream Trinity River every year. Presently, the mainstem Trinity River stores about 30,200 Mg of fine bedload within its active channel bed between the mouth of Rush Creek and the confluence with Grass Valley Creek, the next large contributor of fine sediment. Main channel storage of fine sediment in this reach is equivalent to about 25 years of fine sediment output from Rush Creek .

Presentation notes:

TMDL targets are being met for fine bedload outputs. The erosion model for the Rush Creek watershed showed that much of the landslide potential exists in the steep areas in the Trinity Wilderness but cannot be addressed due to the wilderness designation. Given the relatively small bedload input from Rush Creek versus the large channel storage volume in the mainstem (down to Grass Valley Creek), sediments from Rush Creek may not be as bad as originally thought. Furthermore, many of the Rush Creek roads are on private lands, and it is unlikely that efforts to improve public roads in Rush Creek would have significant effects on fine sediment in the mainstem.

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Discussion Session: Fine Sediment Measurement and Modeling

Panelists: Joshua Allen, Dave Gaeuman & Mary Ann Madej

Is the Grass Valley project having an effect? Yes, the sediment volumes delivered to the mainstem from Grass Valley used to be very large. Hamilton Ponds are not necessarily a good monitor of sediment loads; the river is better, but we're just starting to monitor the river.

Large amounts of sediment in the upper river (upstream of Poker Bar) were transported out this year. But there are also large amounts of sand in the banks that are being mined out. There may still be many releases. We don't know the impacts of having "monolithic flows" just before spawning. There are large amounts of sediments in the system and they may act to fill beds.

Gaeuman thought his sand estimation method works well since the underlying voids are generally filled so the bed surface become the barometer. The void spaces in the gravel are generally 20-30 % and these are generally filled by sands in the Trinity.

What are the effects of Sierra Pacific activities? We don't know for sure, as they don't let surveyors on their properties. Other areas of sediment sources are the Lowden burn area. A large slide occurred there and moved sediment downhill. Area of forest harvest in the Trinity basin is being evaluated and may be available from the TCRCD.

Can flows move the sediment? Are the egg to fry stage being impacted? It is not clear what we need to be looking at.are volumes less important than size distributions? How sediment monitoring/results relate to biological responses are not clear. Literature suggests the focus should be more on post-emergence life stages. Juvenile coho use areas with sand bottoms, so some level of sand is tolerated.

Arnold Whitridge asked the scientists what they were going to produce in order to "relieve us of these unsatisfactory conversations about sediment." (i.e., too much uncertainty). Gaeuman answered, "We do probably know that we don't like sand. but we don't know how to handle this yet." The sediment people need to talk to the biologists more.

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