Columbia Parks and Recreation
Creating Community through People, Parks and Programs
This stream bank stabilization project is located in a tributary to Harmony Creek in the southwest corner of Cosmo Park. The primary area of concern was the portion of the channel which parallels Business Loop 70 to the west of the park's main entrance. The stream flows to the west along the north side of Business Loop 70. The drainage area (with the west end of the channel as the point of study) is comprised of approximately 83 acres of commercially developed land, park land and highway right-of-way. The storm water flow data used for design are summarized in the following table:
|Storm Event||At Park's Main Entrance Storm Water Flow (cfs*)||At west end where channel bends away from Bus. 70 (cfs*)|
*cubic feet per second
†A 100-year storm event is the rain storm which has 1% chance of happening any given year. (The percent chance is determined by taking the reciprocal of the given number. For example, 100-year => 1/100 = 0.01 = 1% or 2-year => ½ = 0.5 = 50%.
One way to picture the flow amounts given above: 1 cubic foot = 7.5 gallons. Now think of gallon milk jugs. Ninety-five cubic feet per second = 713 gallon jugs of water rushing by every second.
In many places, the stream banks had lost their
vegetative cover and most of the channel itself was
bare earth. Without rock or plant roots to protect
it against the erosive force of storm water, notches,
called head cuts, began to form in the stream bed.
Once started, the head cuts slowly worked upstream
contributing to a cycle of erosion and silt deposition
that was harming water quality and threatening utilities
There were three primary goals for this project: to stabilize the channel, to learn more about bioengineering and stabilization and to demonstrate the process and techniques for the general public.
Several methods are available to deal with stream bank erosion problems. A large portion of the water can be piped underground, portions of the channel can be lined with hard armor such as rip rap, concrete pavement or some form of concrete block. Or portions of the channel can be stabilized utilizing a combination of some of the above methods with vegetation, a process known as bio stabilization.
Bio stabilization. was chosen for this project after careful consideration of site conditions, costs, aesthetics and water quality. Also this channel, due to easy access and visibility, was thought to be a good location for a demonstration of bio stabilization. techniques. There are many bio stabilization. methods and techniques other than those used for this project. The methods differ primarily as a result of the materials used to accomplish project goals. Most methods have in common an attempt to better understand how streams and drainage channels interact with their surroundings.
Interfluve, Inc. of Hood River, Oregon
was hired to formulate a bio stabilization. design,
to conduct bioengineering seminars for City staff
and to provide training in the necessary construction
The channel was stabilized using 2 basic methods. Where possible, steep banks were cut back to make them less steep. This reduces the chance of slope failure (embankments caving in), allows storm water to spread out more and helps to slow the water and reduce the depth both of which make the flow less erosive.
A series of pools and riffles were also designed for the channel. The riffles (also known as grade control structures) allow the water to flow more rapidly without damaging the stream bed or banks. Riffles also add oxygen to water making it possible to support more life. Pools are areas where water moves much more slowly and is therefore less erosive. (Natural streams in this part of the country are typically made up of a series of pools and riffles.) In the riffles, water moves fast and is more erosive so rock is placed in the channel and stream banks to resist the erosive force. In pools, water moves slowly enough that vegetation alone is often able to withstand the erosive force and no rock is needed.
In areas where erosive forces were particularly high, a rock lining/foundation was designed across the bottom of the channel. The height to which this lining was extended up the sides of the channel was determined by analyzing the force that the water from a 100-year storm exerts on the bank. Where that force was such that vegetation alone could withstand it, rock was stopped and vegetation was used to protect the banks. On both sides of the channel in these areas, stream banks were designed to be rebuilt on top of the rock lining/foundation using topsoil wrapped with erosion control fabrics which were then staked down using 2"x4" wood stakes. Live willow stakes were placed below the wrapped soil on 1-foot centers and grass seed was spread on the soil before the fabrics were wrapped over it.
All along the channel plants were specified to provide structural support to the stream banks. Willows were chosen for the sides of the channels because they are inexpensive, easy to install and they root quickly and deeply. They will also provide shade for the channel which is necessary for many creek dwelling bugs and other wildlife. Prairie grasses were chosen for the stream banks above the willows because of their ability to grow deep root systems in difficult conditions. These plants will help provide deep slope stability to the stream banks at a reasonably low cost. Some wild flowers were included in the prairie grass seed mix to improve the aesthetics of the stabilization and to attract birds and butterflies as well as other wildlife.
The grasses planted include Sideoats Grama, Canada
Wild Rye, Little Bluestem, Big Bluestem, Fowl Mannagrass,
Switchgrass and Prairie Cordgrass. Interspersed among
the grasses, observers can expect to see Lance-leaved
Coreopsis, Purple Coneflower, Spiked Grayfeather,
Showy Goldeneye and Black-eyed Susan.
A City Street Division crew constructed the project with initial guidance in the unfamiliar construction techniques provided by the design engineer from Interfluve, Inc. Construction began in late fall of 1999. Fall is a good time to sow seed and install plants since everything is dormant and will remain so for a while and it is generally one of the driest times of the year which makes it good for stream work. Also, the Street Division's routine maintenance work generally begins to slow down toward the end of the year because of low temperatures making it an ideal time for them to schedule extra work.
Unfortunately, the late fall and early winter of 1999 were unusually wet. This caused delays in construction and less than optimal working conditions through the en of winter. It also provided one of the most important lessons from this demonstration project: extra care must be taken to plan well and minimize actual construction time during projects of this type. Windows of opportunity in the weather are often small and unpredictable and plants need to be installed as soon as possible after they are purchased. Long term storage of plants can be difficult and if they are not taken care of they will not thrive when planted. Since they are integral to the structural strength of the stream banks, it is vital that they thrive.
The most unfamiliar construction aspect of the project was the building of the soil fabric units. With some tips from the consultant and a little practice, though, the construction crew found the work relatively easy. Erosion control fabrics must be very strong to withstand flowing storm water and provide a good place for plant seeds to germinate and grow. Generally a densely woven but fairly light fabric is used next to the soil in order to hold seed in place and provide a good environment for germination. A very coarse, strong fabric is place on top of the finer, weaker fabric to withstand rushing water. The fabrics must be firmly staked using stout wood stakes to hold the soil and fabric unit (called a lift or wrap) in place. The stakes are also placed closely enough together to push the fabrics firmly onto the seed and soil. It is important that there be no gaps between the fabric and the soil so that the seed stays in place and makes good contact with the soil. It helps to rake the soil smooth before sowing seed and staking the fabric.
The fabrics and the stakes are expected to biodegrade in approximately 3 years. By that time the vegetation will have had a chance to put down sufficient roots to hold the stream banks in place.
The prairie grasses used in this project are warm season grasses. This means that they do most of their growing when the weather is hot, in July and August. Consequently, springtime, with relatively cool temperatures provides an opportunity for invasive weeds to get a jump on newly seeded warm-season grasses. Once the warm season grasses are well established, they will crowd weeds out of their midst. But early maintenance of a planting of this sort requires some way to get rid of the weeds and to allow the grasses to grow. Sometimes an herbicide is used to kill the weeds before the grasses start growing. Mowing is often used to cut the weeds to a height that allows the grasses to compete for sunlight. Well-controlled burning is often used in established stands of prairie grass to mimic the natural conditions that allowed prairie grasses and flowers to thrive for millennia. If native plants are to be used in a project, someone who has experience with them should be consulted in the planning phase in order to plan both the short-term and long term maintenance of the planted area.
For this project it was decided that selective weeding by hand would be used to reduce competition for the warm season grasses and wild flowers. The weeding has focused on just a few aggressive plants such as ragweed which adversely shade the more desirable plants. By being so selective, the task is made much less daunting. This is also a great way to get a good view of what is actually growing in the newly stabilized areas. Being close enough to pull or cut weeds, it is easier to see the beneficial plants coming up.
In the fall of 2000, the performance of the stabilization
measures will be evaluated. Areas where the vegetation
has not done well will receive supplemental plantings.
Fresh willow cuttings will be readily available on
site from some of the plantings which have done extremely
The first two goals for this project, stabilizing the channel and learning more about the bio stabilization. process, have been met to a great degree. While more work is required, the channel is stable now and the cost of the stabilization work alone, leaving aside educational costs, has been competitive with other methods of accomplishing the same goals. The lessons learned from this project have already been applied to other storm drainage work around Columbia. The project will continue to be observed with particular attention paid to aesthetic and water quality benefits and maintenance costs.
This web page posting is part of an attempt to
meet the third goal, demonstrating bioengineering
and stabilization to the general public.
This page last updated on 7/25/00.