Abstract

Ephemeral gully restoration techniques are a simple, cost effective solution for landowners who would like to minimize sediment discharge and down cutting on their gullies. This report covers several strategies for accomplishing sediment reduction through the use of sediment baffles.

Introduction

As the hydrologic cycle impacts the earth, rivers serve to carry away the products of erosion-sediment and runoff. This cycle has repeated itself for over 4 billion years (Mount, 1995). During this time frame two exceptional events occurred. Vascular plants colonized the earth about 400 million years ago. Many of the early plants are associated with the riparian zone and their impact was to restrict the wandering of stream channels, thereby reducing sediment loads within streams.

The second major change in river dynamics is a result of human evolution. Early hunter gathers tended to colonize floodplains where food resources were abundant. In California, human impacts associated with increase sediment load began with the Gold Rush. From the Gold Rush to the present, man’s impact on river systems is both dramatic and far reaching. Since that time humans need for water has created a state with over 1400 dams, thousands of miles of levees, watershed clear cut of trees, and a network of roads crossing rivers (Mount, 1995). The result of these impacts is a decrease of biological diversity, decreased water quality and a diminishment of ecological processes.

Under natural conditions earthquakes, landslides and floods interact to define the geomorphic surface of earth. As mountains grow the atmosphere mitigates their growth. The water-soaked atmosphere in association with oxygen chemically and physically reduces mountain ranges. With sufficient volume and intensity, rainfall aided by gravity moves sediments downhill toward the river. It is the river that completes the process of carving valleys and transporting the products of erosion to the sea.

Soil erosion and the natural recovery processes interact to form a condition referred to as a dynamic equilibrium, where sediments pass through the system at a rate stable to recovery (Downie, 1998). However, human induced impacts often disrupt this balance causing excess sediments to enter the system.

Within the South Dobbyn Creek watershed (see Figure 2), located in southwestern Trinity County and western Humboldt County, California much of the increased sediment load is the result of previous land use practices including rural subdivision roads, timber harvest practices, and cattle ranching operations.

Methods

This project addresses a small portion of the erosion problem, specifically that portion associated with ephemeral gullies. All of the gullies addressed in this project are the result of water runoff from rural residential roads (Prunuske, 1987). These roads were originally constructed for timber harvest use and at a later time converted to rural residential use. Various techniques involving the use of baffles to reduce gully downcutting and sediment flow were incorporated. This presentation will explore the techniques used in this project.

Variations of siltation baffles are suitable treatment techniques for gully erosion (Flosi, 1998). These baffles can be of two general types: secured and unsecured. Secured baffles are affixed to the sidewall of the gully and are appropriate for higher flow or gradient situations. Unsecured baffles rely on the proper use and combination of vegetation and rocks to stabilize gully down cutting. Both types incorporate a porous design allowing water to pass through the baffle while retaining sediment. These structures are soft and therefore do not divert the watercourse or create additional bank scour. The baffles can incorporate both living and dead vegetation. Figures three through six are typical of the sediment baffles used in this project.

Results

Working under a grant from the U. S. Fish & Wildlife Service, Six Rivers Restoration, a restoration contractor joined with four contiguous landowners to stabilize road associated gullies along South Dobbyn Creek in Trinity County, California. The site restoration incorporates a variety of baffle techniques.

UNSECURED SILTATION BAFFLES

An important aspect of gully restoration is to first repair the headcut associated with the gully erosion (Flosi, 1998). Usually this is associated with repair of a road surface. This can include road realignment, regrading, culvert installation and/or increased diversion opportunities. Without correcting the headcut the construction of baffles will usually fail. Therefore, only after corrections of the headcut problem are baffles an appropriate technique.

This gully is over six feet deep (Figure 7). Several unsecured siltation baffles were incorporated so that the fall of the watercourse was limited to less than two feet per baffle (Figure 8).

This site was not accessible by vehicle and an important consideration included nearby construction materials. Douglas fir branches and rock boulders between 6 and 12 inches were used. Selected baffle sites within the gully were chosen based on two criteria. First, a hard fixture was selected, if available, at which to construct the baffle. This may include a large boulder or tree root protruding from the sidewall. This helps to anchor the structure in place. Second, the ideal structure will have the top of one structure level with the water flow from the above structure. This is ideal and not always an option in the field.

To construct the baffle the following steps are required.

1. The gully bottom is cleared of vegetation
   
   
2. A bench to support the rock face is constructed
   
   
3. Leaf litter is distributed on the newly constructed bench to aid in sediment retention.
   
   
4. Several Douglas fir branches, in this case, were woven into a mat and placed on the gully bottom. This step is repeated until the desired height of the structure is achieved. This is the most critical part of the baffle and care should be take to construct enough woven mats to support a adults weight without dropping more than two inches.
   
   
5. Rocks are used randomly to hold the woven mats in place.
   
   
6. A dam face is constructed to assist in holding sediment. The face is not designed to hold back the sediment. It is the woven baffle that will accomplish this purpose. In conjunction with the rock face the woven structure will slow the stream velocity and arrest the sediment.
   
   
7. The structure needs sediment to complete its construction. This comes as the fall and winter rains fill the woven area with sediment. This can be somewhat problematic if hard rains result. High velocities and flows into the unfilled structure will wash them away. However, fall rains in this area are historically light and serve well in delivering the correct amount of sediments to fill the baffle. The result is a stable siltation baffle that has mitigated further downcutting.
   
   

SECURED SILTATION BAFFLES

Secured siltation baffles differ from unsecured siltation baffles in their purpose of use and the manner in which they are integrated with the gully banks (Figure 9). They are a more appropriate structure for use when velocity and slope are such that high flows would cause the destruction of unsecured baffles.

Here erosive downcutting has reached a depth of 10 to 12 feet. The gully was cleaned and benched to accommodate secure anchor points. Slits were dug into the sidewall about 12 to 18 inches to hold crossmembers. The sites for crossmemebers are selected to take advantage of natural constrictions. Usually some hard point in the sidewall, a tree root or large boulder.

The restoration area is then graded to achieve a stair step sequence designed to slow the velocity and allow for sediment entrapment.

After the site has been graded the securing crossmembers are put in place. Boulders and rock can be incorporated to secure the cross member in the sidewall. Then baffle mats are installed in much the same manner as unsecured baffles. The baffles are woven and interlocked to avoid being carried down stream during the initial sediment entrapment period.

Analysis

Several areas of siltation baffle construction require analysis. The success of sediment entrapment devices requires that the headcut issues be addressed. Without correction of the headcut problem, the success of a baffle is severely reduced. In most instances, the original gully was created from improper road surface runoff. Highly erosive soils, steep slopes and high surface runoff volumes combine to spell failure for this technique. Consideration of these conditions assist in the selection of the proper baffle type.

Ideally, baffles should be placed so that the angle of flow is as close to flat as possible. That is to say the slope should approach zero. This greatly enhances the probability of success as it reduces the downcutting velocity associated with gully erosion. However, this can often be problematic in the field due to various terrain composition as well as access to construction materials. To construct a stair step design numerous baffles may be required for steeper terrain.

Constructed baffles require the filling with sediment produced from rains in order to be complete. This is often the threshold issue in a baffles success. Unfortunately the restoration contractor has little control over this important step. If initial rains are heavy baffles may be washed away and need to be reconstructed the following season. If rains are gentle sediment will fill the baffle and the structure will last for years to come.

Conclusion

Erosion, within the South Dobbyn Creek watershed is a major problem for watershed health. Siltation baffles address a small portion of the sediment load entering the system. However, siltation baffles are simple structures to build and can be constructed with nearby materials. They are excellent sediment entrapment options for landowners to construct themselves. They can be built one baffle at a time and can easily accomplished in a weekend.

Additional landowner outreach is necessary to accomplish the technology transfer to landowners. Hands on workshops are a viable option to accommodate the technology transfer (Figure 11).

References

Downie, S., and D. Halligan, R. Taylor. 1998. Rural Road Erosion Hazard Assessment.

Flanagan S., J. Ory, and M. Furniss. 1997. Handbook for the Inventory and Assessment of Culverts on Forest Roads. Six Rivers National Forest – Watershed Group, Eureka, California.

Flosi, G., S. Downie, J Hopelain, M. Bird, R Coey, and B. Collins. 1998. California Salmonid Stream Habitat Restoration Manual. California Department of Fish & Game, Inland Fisheries Division, Sacramento, California.

Handley, B. 2001. South Dobbyn Creek Headwater Restoration Project. Wild River Conservancy, Sacramento, California.

Mount, Jeffrey. 1995. California River and Streams – The Conflict Between Fluvial Process and Land Use. University of California Press.

Prunuske, L. 1987. Groundwork, a Handbook for Erosion Control in North Coastal California. Marin County Resource Conservation District.

Weaver, W. E., and D. K. Hagens. 1994. Handbook for Forest and Ranch Roads. Mendocino County Resource Conservation District.

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