Restoration of a Stream Channel Using Native Materials
By Scott McGill and Jim Gracie
 
INTRODUCTION

The Maryland State Highway Administration's (SHA) proposal to extend Warren Road to Interstate 83 in northern Baltimore County involved crossing a small trout stream, which had been channelized in the past by a previous owner of the site. Because of the alignment of the bridge and the degraded condition of the stream, SHA proposed restoration of 1100 feet of channel using native materials and natural stream channel geometry to mitigate for lost fish habitat and, wetland impacts associated with the bridge crossing.

Stream Morphology

Stream channels develop their shape, size, slope and other morphological features as a result of the interaction of flowing water on the materials in the stream's valley. Streams carve channels that have remarkable similarities throughout the world. This is because water follows the laws of physics everywhere. A stream or river is the manifestation of a process of energy transformation in which the potential energy c-f elevation is transformed into the kinetic energy of flowing water.

In this process of energy use and transformation there are eight variables. There are discharge, width, depth, velocity, slope, roughness, particle size and sediment quantity.

The independent variables are discharge and sediment load. Discharge can he modified by changes in climate, which affect the amount of precipitation or alterations in the hydrologic regime, which affect the fate of precipitation, such as changes in infiltration, evapotranspiration, storage or runoff. Sediment quantity is affected by the type and distribution of rocks, the effect of weathering and transport.

The interactions of these variables follow the laws of physics, including conservation of mass and energy, the relationship offriction to velocity, depth and slope, and the relationship of sediment load to power. Over geologic time, the continual interaction of these parameters tends toward minimum work and minimum variance among the parameters and results in the patterns that can be observed.

Without disturbance, a state of dynamic equilibrium exists in which sediment supply and sediment transport are in equilibrium and erosion rates are in equilibrium with deposition rates such that patterns are maintained. Disturbance which creates a change in one variable sets up a series of concurrent changes in the others, resulting in altered channel patterns. Stream morphology is the result of an integrative process of mutually adjusting variables. In other words, streams with different properties will adjust differently.

Assessment of Existing Conditions

In order to design a channel which would provide self-maintenance, would provide habitat for aquatic organisms (including trout), and would reduce bank erosion, the key parameters of the stream channel were examined, including the bankfull width, depth, slope of the water surface, meander radius, sinuosity, channel substrate, confinement, and ratio of bankfull width to bankfull depth. The existing channel, previously channelized, was out of pattern and was undergoing adjustment. Most banks were eroding, supplying excess sediment to downstream reaches.

The stream was classified using the Rosgen Classification system, developed by David Rosgen. The Rosgen Classification System is a system of classifying stream channel, using delineative criteria which integrate the key morphological variables which form, shape and maintain the channels. It enables one to predict the behavior of streams based upon stream type. It provides a consistent frame of reference and allows extrapolation of properties and responses from one stream or stream reach to another of the same type. The system helps one prescribe appropriate restoration measures by giving ranges of parameters such as sinuosity, slope, substrate, width/depth ratio, and confinement, which are stable for each stream type. The classification system has been used to prescribe restoration measures for streams from 10 to 500 feet in width.

Design

Detailed cross-sections of the existing channel were taken to insure the new design channel had the capacity to carry the 2-year storm, the frequency storm which shapes and maintains stream channels. The design of the relocated channel was based on a C4 channel in the classification system. Meander radius and length were based on bankfull widths and depths. The restoration of stable geometry is a key ingredient in this approach. Establishment of a stable radius for the meander (approximately 2.9 - 2.)3 times bankfull width is recommended).

The technique recommended for hank stabilization involved the use of root wads and native rock. Trees were obtained front the area cleared for the highway extension and the stream relocation areas. Trees with a diameter at breast height (nBH) of at least 12" were nagged and later knocked down with a backhoe during the clearing phase of the road construction, and the trees were cut approximately 12 feet above the base of the trunk. Species used included oak, willow, maple, tulip poplar, walnut, and hickory. The rest of the tree was cut in 12-18 foot lengths to be used for footer and cutoff logs. Native rock, mostly limestone, was obtained on-site from cut areas for the road construction. Both were set aside to he used later for the relocation of the stream.

Construction

Before construction of the bridge or the relocated channel could begin, a diversion channel was constructed to carry flows during the construction of the new channel. Channel construction was accomplished using a track-hoe, bulldozer, and a front-end loader.

The channel was first cut to grade, making sure the slope was consistent, and the point bars were sloped at 5-10% The outside of each bend was then reverted with root wads using the following sequence of installation: Trenches were dug for the trees and the root mass was placed so that it was perpendicular to the water flow at each point in the bend. This was done to create velocity dissipation on the outside of the bend. Cutoff logs and rock were placed on top of the root wad to keep the root wad in place and protect the bank against erosion at high flows. Work proceeded from upstream to downstream and footer logs for each root wad were placed behind the upstream footer logs to create a shingle effect. Finally, the areas behind the root wads were back-filled to grade.

A vortex rock, weir structure, first designed and used by David Rosgen, was placed at the upstream and downstream end of each bend. Rocks were bedded in the bank to direct flows into each bend and out of each bend into a cross over reach. Footer rocks were placed beneath the vortex rocks to prevent scouring which, would move the vortex rocks during high flows.

The diameter between vortex rocks was crucial to the design. Rocks were spaced approximately 1/2 diameter apart and the center rock could extend up to only 20% of bankfull depth.

The weirs have several other benefits, which should develop over time. These benefits include the formation of small eddies below the weirs, which provide habitat for trout, and the scour created by the weirs creates substrate suitable for spawning gravel.

The use of root wads for bank stabilization is extremely cost effective when compared to traditional engineering techniques, such as the use of rip rap and gabions. When combined with stable channel geometry, the technique is maintenance free and allows natural vegetation to establish itself, which will provide stabilization when the root wads begin to decay. The root wads provide a vertical bank, which is more natural than 1 to 1 slopes needed for rip rap. Most importantly, the root wads provide excellent fish and invertebrate habitat, while also providing a natural looking, aesthetically pleasing stream system. Because materials can be obtained on-site, restoration can he accomplished in remote watersheds where access may be a problem.

During the cutting of the relocated channel, bedrock was encountered so footer logs were not needed and the root wads were placed directly on top of the bedrock. Large rock was placed on the intersection of the root wads so that the structure was counter-weighted.

Toward the end of the project, the elevation of the existing channel at the tie-in location was 1 foot above the new channel. To prevent head-cutting of the channel upstream, two vortex rocks weirs were placed close to one another to create a step pool system. This created a grade control system, which provided a stable natural tie-in.

There was a considerable amount of skepticism regarding the root wad technique and the channel geometry design. Some thought that the first large storm would rip out the root wads and create a straight gully directly through the meanders. Approximately 3G hours after completion of the new channel, a tropical storm hit the east coast and dumped 3.6 inches of rain on the site (a rain gauge was set up on-site) equivalent to a two year storm - the design storm.

There was virtually no damage to any of the stream work, and the channel functioned to effectively move its sediment through the system. The areas below the rock weirs were scoured out to allow the convergence of flows to the center of the channel, and material on the root wads was not washed off as many had predicted. There was considerable deposition on the point bars as expected.

The majority of the project was completed in mid September. A planting plan for the stream banks will soon he completed and will utilize red ample, river birch, and dogwood. All work requiring a waterway construction permit in natural trout streams in Maryland must be completed between May 1 and September 30 to avoid impacting trout during the spawning season.

Kline Construction was the prime contractor for the work. Brightwater, Inc. was retained by SHA for the design and supervision of the stream restoration portion of the project. Whitney, Hailey, Cox, and Magnani, Inc., provided survey, engineering and drafting services. The Maryland State Highway Administration has utilized the root wad technique in several projects.

For more information contact Jim Gracie at Brightwater, Inc., 9330 C Red Branch Road Columbia, MD 21045, (410) 964-9429 Brightwater, Inc. is an environmental consulting company specializing in stream restoration.