Case Study 1 Data Table
Buckhorn summit site is located on highly weathered "decomposed granite" (DG) with quartz diorite mineralogy. The site was originally cut to about a 1:1 slope in about 19xx, and a remnant of the old slope face is visible on the right hand 10 % of the "before" picture (Figure 1). Soon after construction, the cutslope face was destabilized by groundwater flow originating from an old logging landing located 10 to 50 m behind the top of the slope. Ground water flow caused slumping in the mid-slope region of the original slope, and surface flow eroded the crest of the cut. The slope retreated about 5 to 15 m away from the road, with sediment moving off site. An estimated xx cu yd per yr were removed from the shoulder of the site before treatment.
As part of a revegetation research project (RTA # xxx), a 1.2 m high gabion wall was constructed and backfilled so as to construct a 2:1 fill slope (6 m slope length) using DG from the SHA 299 1.0 slope repair. The plot fill material was slightly finer than the existing slope face material at 0.06 because of repeated handling, mixing with compost, or because of the different source location. The constructed 2:1 slope included treatments with 0, 6, 12, and 24 %(vol/vol) (0, 2, 4, and 8 % by dry weight) of unscreened, yard waste compost that was mixed with DG by backhoe bucket. The compost source was the City of Redding's municipal recycling facility (90 days of US EPA 503 thermophilic process followed by 90 days aerobic curing; unscreened, with up to 15 cm shredded wood fragment length). The treatment depths were 60 cm depth over unconsolidated DG. Data from rainfall simulation indicated that the 24 % compost mixture had comparable infiltration to a disturbed, but revegetated reference site located 1.6 km (1 mi) west on 299. This finding was used to model the probability that an amended slope would produce negligible erosion over the various rainfall intensities experienced in that region. Findings were used to plan a follow-up series of test plots, as well as to design a treatment for the entire slope.
The second year study plots were constructed at a slope angle of 2:1, and with varying incorporation depths, to evaluate optimizing construction costs and efficiencies. At the same time, a design was developed to stabilize the remaining unstable slope into a series of benches that had a compacted portion nearest the remaining slope, and an uncompacted, compost incorporated portion filled onto the exposed edge of the bench. The overall slope angle after filling the benches was to be 1.5:1 (H:V). The thickness of the compost amended horizon was 300 mm when measured perpendicular to the slope face. A heavy (900 g/m2) coir fabric covered the compacted bench portion and was anchored into the slope by the overlying lift. The outer edge of the fabric was draped over the compost-incorporated slope face, providing short-term surface erosion control. Native grasses, shrubs and trees were planted in the uncompacted, compost-amended layer.
Additional design and construction information for this and several other slopes information is contained in the Final Construction Report (McCujllah, 2003).
Roots of native perennial grasses (Elymus elyjmoides) reached 2 m depth at the end of the first growing season and set viable seed. The first year there were no differences in plant shoot biomass cover between compost treatment plots. The second year, however, the till/no compost plots grew 3.17 g biomass per plant while plants on the till/24 % compost plots grew 14.17 g per plant. On plots with no tillage (plants installed directly into the very porous but untilled DG matrix), plant growth was 0.44 g/plant while plants on the tilled/24 % compost treatments grew 4.0 g/plant. This comparison is analogous to conifers planted into small planting holes at the Kelsey site located south across the canyon. Evidently, the undisturbed DG is very porous to water flow (infiltration rates of 100.8 mm/hr) but they are not penetrable to root growth.
|SRE Step||Ref site||Constructed|
|1. Site Locations|
|ref site is regraded|
logging landing reveg'd in
|parent material||DG||DG||DG|| |
|2. Slope stability|
|seepage from upslope|
|erosion||none, 15 yr|
from first yr
|installed diversion ditch|
the first year, no erosion
|3. Soil water relations|
|50.6||29.0||60.0||measured 3 yrs after|
|16.8||51.1||38.6||ref site had compaction|
layer from construction
|plant avail water %||14.0||9.3||14.5||to approx. -5.0 MPa|
|4. Organics, C, N pools|
|carbon pools|| ||near zero||loaded to|
|measure end of season|
|nitrogen pools|| ||near zero||loaded to|
|5. non-N nutrients|
|chemical cond||sufficient||sufficient||sufficient|| |
|macronutrients||sufficient||sufficient||sufficient||measure end of season|
|6. Soil biology|
|7. Surface erosion stability (mulch, waterflow)|
|g sed per 15 min|
storm per m2
|5.01||41.55||0.62||60 mm/hr storm, 15 min|
|nitrogen loss|| || || || |
|8. Plant response|
|shoot biomass g/pl|| ||3.17||14.17||no growth difference first|
yr; these data for 3rdyr
|root depth m|| ||>2.0||>1.3||measured after first year|
|root density cm/ml|| ||0.88||1.75||measured after 2yr growth|
The construction process was intensive, partly because the slope area was constrained and the oversteepened slope required movement of a large volume of material from head of the cut, storing it on the bench above the slope, and bringing it back down during reconstruction. The small working environment meant that backhoe and bulldozer equipment size was also constrained. The intensive process was successful through the first three seasons, and resulted in the "only slope to stay up through the first rainy season in the last 30 years" according to Milt Apple, Caltrans District 2 Maintenance Supervisor.
A main result of the improved infiltration is the increase in the rainfall intensity that the slope can experience before overland flow starts. The significance of overland flow is, of course, the start of sediment transport and erosion, but it is also an indicator of whether a slope will regenerate soil fertility or whether it will chronically lose the nutrient rich surface duff and fine weathered particles that are critical for soil regeneration. The following table indicates the results from infiltration capacity modelling, coupled with long term rainfall data from representative weather stations.
A secondary result of related work on the same Soil Resource Evaluation project is the finding that deep cuts into DG materials have a potential for fixation of ammonium fertilizers or organic matter decomposition products, so that the ammonium is unavailable for plant uptake. Detailed measurements on the neighboring slope at SHA 299 0.6 (Old Faithful) indicate that over 55 % of the ammonium released from fertilizers or from organic matter decomposition is fixed (bound) in the mineral interlayers. This requires a compensating increase in the amount of fertilizer applied or the compost release rate so that plants acqure adequate nutrients during the first year of establishment. This effect occurs in other DG substrates as well Figure 16.
Figure 16. Fixation of ammonium in interlayer minerals in vermiculites (weathered micas) in decomposed granite rocks. Location key: INY, Inyo; MER, Merced; MON, Mono; SD, San Diego,
Remaining questions regarding this type of installation are the ability of the slope substrate to make the transition between compost-enhanced infiltration and natural soil structure that is generated by plant growth on site. Third-year infiltration data documented an increase in infiltration on vegetated plots, which is a positive sign. Continued fertility will be required in order to support adequate plant growth and maintain infiltration rates. The long-term behaviors of compost materials, however, has not been documented. Additionally, a large amount of organic matter is stabilized in soils as soils develop good aggregation. The soil aggregation process (accumulation of humic materials and microbial biomass residues generates a nutrient requirement in addition to that required for vigorous plant growth. Failure of the slope from nutrient deficiency will not be sudden, but will be observable over several years by an increase in plant spacing, less litter cover on the soil surface, and possibly by an increase in N-fixing species or by plant leaf indications of nutrient deficiency. A surface amendment of approximately 30 to 50 kg N/ha of slow release nitrogen would regenerate plant growth on the slope for another period of several years. If the ammonium fixation capacity is not saturated, these fertilization rates should be increased by 50 to 100 %. Eventually, the slope can be expected to become permanently self-sustaining, if infiltration remains high enough to prevent overland flow. and if nutrients (especially N) remain at adequate levels required for vigorous plant growth.
Elymus multisetus(Bigsquirreltail) were plug-planted on a grid pattern (20 cm spacing) for experimental purposes. Plants seeded on the slope included Elymus multisetus (Bigsquirreltail) and Bromus carinatus (California brome), Achillea millefolium (yarrow) and Eriophyllum lanatum (woolly sunflower) (locally collected). The second year, seedlings of Pinus ponderosa (Ponderosa pine), Pinus lambertiana (sugar pine), and Quercus kelloggii (Black oak) were planted randomly on the slope to provide a woody component.
Other suitable plant species include Poa secunda (Bluegrass),Ceanothus lemmonii (Lemmon's Ceanothus), Ceanothus prostratus var. laxus (Mahala Mat), Acer macrophyllum (Big Leaf Maple), Aster oregonensis (Oregon Aster), Lotus crassifolius (Big Deervetch), Calocedrus decurrens (Incense Cedar), Pseudostuga menziesii (Douglas Fir).
Permitting: Chris Cummings, Project Engineer, Shasta Trinity Counties, Redding, CA. Grant administrator: Jack Broadbent, HQ Landscape Architecture, Sacramento, CA. Erosion control and slope design: Salix Applied Earth Care, Redding, CA. Contractor: Cross Country Construction, Douglas, CA. Soils: Soils and Revegetation Lab, University of California, Davis. Matt Curtis, lead hydrologist; Ryan O'Dell, lead botanist; Eric Rider, lead soil chemist.
McCullah, J. 2003. Final Construction Report CTSW-RT-03-058.33.12, June, 2003.