Erosion / Sediment Control Demonstration Project--Nichols Arboretum
Nichols Arboretum, University of Michigan
Basin Program Funds:
Uncontrolled discharges into School Girls Glen ravine are causing bank erosion and severe head cutting and down cutting in the ravine. This erosion in turn has undermined and scoured the ravine banks, which has resulted in slumping and mass wasting of the ravine sides. Many large trees in the ravine have been undermined and toppled by this activity, further destabilizing the banks of the ravine.
School Girls Glen, located in the Nichols Arboretum, is a large, steep ravine that drains into the Huron River. Most of the surface runoff that flows into the glen comes from the adjoining Forest Hills Cemetery, which occupies the headwater area of a steep 20-acre watershed. The runoff generated from this area in a 10-year storm has been estimated at 16 cu. ft. per second. The runoff is funneled and concentrated by a series of roads that converge near the boundary fence between the cemetery and the arboretum. Storm water runoff from Washington Heights Road and the adjacent parking lots and roofs also contribute to the problem. The water discharges at the head of the ravine without benefit of any holdup or temporary storage. A recent study indicated some 13,530 cubic yards of soil were lost over a 28-year period.
A series of cross-channel grade stabilization structures were installed at the head of the ravine. Both log and rock structures were considered, in addition to vegetated gabion walls. Channel armoring and bank protection were employed along critical reaches of the ravine. Concentrated and erosive flows of water entering the ravine from the sides were diverted, spread out by means of diversion dikes, waterbars, rolling dips, cross drains and downslope drains. Sediment barriers were employed in places where runoff carries substantial sediment. Invasive woody species were removed to permit the recovery of native ground cover. Trees removed during this process were used in the bioengineering stabilization practices.
The major purpose of this project has been to arrest erosion and slope failures occurring in School Girls’ Glen and to reduce or eliminate major sediment discharges to the Huron River at the mouth of School Girls’ Glen. Over the past two years, we undertook a number of specific activities to meet these goals:
- Constructed a series of stepped pools at the top of School Girls’ Glen to slow storm water and replaced sections of drain inlets and a sewer capturing the runoff from 20 acres of Forest Hill Cemetery above the Arboretum
- Extended storm sewer outlet pipes to base of School Girls’ Glen to stop erosion along the slide slopes that was creating large gullies
- Built a series of crib wall structures in eroding ravines at an overlook area and filled them with soil and installed transplanted plants, turning a former eyesore into an attractive gathering area
- Repaired a large cavity left from a downed tree by filling with layers of sand and gravel as a demonstration of “soil wrap” techniques
- Built and maintained a series of boulder check dams along the channel of School Girls’ Glen
- Constructed gabion dam at degraded portion of ravine, successfully stabilizing adjacent eroding slopes and trapping sediments behind the dam
- Used boulders found in the ravine to armor the toe of slumped banks in the glen
- Created interpretive “Clean Water Trail” through School Girls’ Glen and adjacent areas of Nichols Arboretum and prepared a display and brochure for visitors
- Worked on removing invasive plants throughout the glen
- Established photomonitoring system for systematically recording changes within School Girls’ Glen
- Brought public recognition to the project through articles in the Nichols Arboretum newsletter Treeline, and the Ann Arbor News, various public walks and bulletin board displays in the arboretum, university class studies and participation in the project, visits by local elementary schoolchildren, and promotion of the Clean Water Trail with its interpretive brochure
As result of these various activities, we have trapped considerable sediment that would have entered the Huron River. In the approximately 1,200-foot long channel of School Girls’ Glen, we estimate that we stabilized some 1.5 acres of eroding and potentially eroding slopes through our efforts in armoring the toe of slopes, planting and vegetative management, and placement of gabion and other check dams along the channel. We also estimate that we made approximately .35 acres of streambank improvements in this project.
During the two-year life of this project, we estimate that we have trapped some 27.4 cubic yards (40.7 tons) of soil behind our gabion dams and check dam structures in the ravine. This represents approximately 407.0 lbs. of phosphorus and 81.4 lbs. of nitrogen. In addition, assuming that our erosion control efforts in the .35 acres of the School Girls’ Glen channel would have reduced other channel erosion from moderate levels (assumed 1.0 feet/year) to slight levels (.5 feet/year), we estimate we have prevented 41.9 tons of additional sediment from entering the Huron River each year. Over two years, this would be 83.8 tons of sediment and 838.0 lbs. of phosphorus and 167.6 lbs. of nitrogen.
The combined totals are:
- 124.5 tons of sediment
- 1345.0 lbs. of phosphorus
- 249 lbs. of nitrogen
Through reducing the inputs of both nutrients and sediments into the Huron River, we are assuming that this project has helped contribute to the overall health of the benthic and fish populations the Huron River, although we have no data to support that claim. In addition, our ongoing removal of non-native invasive plants in the School Girls’ Glen area and restoration of a native herbaceous layer of vegetation will ultimately create a more stable habitat for a variety of animal and insect species.
One of the key benefits of this demonstration project has been the increased public awareness of School Girls’ Glen, the erosion problems there over the past 50-60 years and the efforts Nichols Arboretum has taken to address those problems. The ravine has become much more visible to the casual visitor to the arboretum. Through our interpretive displays and “Clean Water Trail” brochures, visitors have become much more aware of our efforts to manage erosion. With the installation of our new Gateway Garden with its stepped pools, all visitors are much more aware of the water that flows into the head of the ravine.<
Contact: Robert E. Grese, 734-763-0645