Steve Johnson, Principle, Natural Resource Services, Inc. and Tom Roberts, Senior Landscape Architect, Norris Design
|Photo 1: View of San Juans from Cornerstone|
Successful high altitude wetland mitigation in semi-xeric conditions with highly seasonal hydrology can be somewhat of a challenge to say the least. Natural Resource Services, Inc. (NRSI) of Boulder, Colorado undertook that challenge in 2006 when the company was awarded a contract by Cornerstone Montrose, LLC (the Owner) to design, construct and monitor fifteen acres of wetland mitigation for 6.82 acres of permanent impacts to wetlands and ephemeral streams on the Uncompahgre Plateau about twenty miles southwest of Montrose, Colorado.
The project site is known as Cornerstone Colorado and is being developed as a year round, high-end residential development (412 planned residences) which will incorporate a championship 18-hole golf course, an equestrian center, other seasonal outdoor recreational activities, and an extensive limited use trail system. The development is located within a 6,057 acre tract of open rangeland which lies within parts of Montrose and Ouray counties (1,575 acres and 4,482 acres, respectively) in southwestern Colorado. More than 3,000 acres of the project site will remain as deed-restricted open space which will be maintained through the implementation of a formal wildlife management plan. According to Burns and McDonnell Engineering, Inc. (2005), the golf course was designed to incorporate as much of the existing natural geologic and vegetative features as possible to minimize construction costs and environmental impacts. Aesthetics and incorporation of wildlife habitat throughout the site are important goals of the project.
The varied terrain within the project site and the surrounding Uncompahgre Plateau provides a unique environment for the formation of wetlands (Fig. 1). The Plateau is a nearly flat-topped highland or “cuesta” which averages about 25 miles wide and 100 miles long and begins just north of the San Juan Mountain Range. It consists of a twice uplifted block of 600 million year old Precambrian rocks overlain by Triassic and Jurassic pink sandstone and Mesozoic sedimentary layers known as Dakota sandstone and Mancos shale which are between 65 and 240 million years old. The last uplift of the Plateau occurred between 10 and 28 million years ago and was associated with intense volcanic activity to the east and southeast (Chronic 1980).
|Photo 2: Construction of Wetland Site A – 2007|
Elevations at the Cornerstone site range from 7900 feet msl on the east side to over 9800 feet on the west side. Annual precipitation in Montrose averages 9.7 inches but is slightly higher at the project site and is highly seasonal. A variety of vegetative communities are supported throughout the site resulting from highly varied topography, elevational temperature differences, and differences in precipitation. Vegetative communities range from lush mature and immature aspen (Populus tremuloides) stands and associated herbaceous ground cover at the higher elevations to mature Ponderosa pine (Pinus ponderosa) and Gambel’s oak (Quercus gambelii) scrub stands at the mid elevations to Pinyon pine (Pinus edulis)/Juniper (Juniperus scopularum)/sagebrush (Artemesia sp.) communities at the lower elevations. A large variety of herbaceous grasses, sedges, rushes, and forbs is associated with each of these communities. Much of the wetland and stream habitat at the site has been degraded by historic intensive grazing. The riparian areas of the site were also historically inhabited by beaver (Castor canadensis) at a number of locations as evidenced by remnant willow stands and the remains of old beaver dams. Beaver were extirpated from most of the site many years ago.
Required environmental permitting for the project included a Clean Water Act Section 404 Individual Permit (IP), the application for which was submitted by Cornerstone Montrose LLC to the Grand Junction office of the Sacramento District of the U.S. Army Corps of Engineers (USACE) in February 2006. The Owner proposed to mitigate for permanent infrastructure construction impacts to drainages and wetlands which included 1.81 acres of palustrine emergent wetlands, 0.24 acre of palustrine forested/scrub-shrub wetlands, 0.93 acre of palustrine forested wetland, 0.91 acre of open water/palustrine unconsolidated bottom wetland, and 2.93 acres of ephemeral stream corridor. The owner further proposed to mitigate for these impacts by developing 15.46 acres of in-kind wetlands at numerous dispersed sites and 1.05 acres of ephemeral stream mitigation in two connected drainages.
NRSI was contracted by Cornerstone in March of 2006 to finalize the mitigation site selection and design process and to complete the construction and implementation of the mitigation as well as the post installation monitoring and maintenance to fulfill the 404 permit requirements. NRSI contracted three firms to serve on the mitigation team. Norris Design of Denver, Colorado was contracted to complete finished construction drawings for the mitigation wetlands and provide construction oversight as well as to provide as-built drawings when construction was complete. Confluence Engineering, LLC of Boulder, Colorado was contracted to complete construction drawings for the ephemeral stream mitigation. American Civil Constructors, Inc. (ACC) of Littleton, Colorado was contracted to complete the actual construction, finished grading and planting. Most plants were contracted through Rocky Mountain Native Plants Company of Rifle, Colorado.
In May 2006, the project team conducted two site visits to select suitable mitigation sites and finalize construction drawings for submission to USACE. On June 6, 2006 construction drawing sets for 15 wetland mitigation construction sites including 36 individual cells and 6425 linear feet of ephemeral stream were completed and submitted, along with a proposed Mitigation Plan, to USACE for approval. The plan was approved and the 404 permit was issued on August 11, 2006.
A number of problems were encountered in completing the construction drawings. They included 1) the extremely short time frame in which to complete the site assessment and drawing sets, i.e. approximately one month, due to construction scheduling requirements with other ongoing construction at the site; 2) the lack of adequate topographic mapping with greater accuracy than five foot contours over most of the project site where the wetland mitigation would occur, and 3) mitigation would be designed to include no irrigation since the success of the mitigation would be dependent on existing natural hydrology.
|Photo 3: Completed Site I Cell 2 – 2007 Before Planting|
It was decided by the design and construction team, for permitting expediency and the necessity for construction to begin in 2006 to coordinate with other ongoing construction at the site, that construction drawings would be completed using the available less accurate topographic data. The team agreed that approach would require more intensive field coordination between oversight and construction crews and more extensive “field fitting” of the construction into existing site topography which often differed markedly from “mapped” topography, but that the field and construction experience of the team oversight personnel and the construction crew would allow that process to work. After discussions with USACE staff, that plan of action was approved.
Construction was initiated at two of the mitigation sites on September 7, 2006. Five wetland cells totaling 0.32 acres and 3500 linear feet of ephemeral stream were completed by October 30 to include overseeding with an upland native seed mix, hydro-mulching, and planting 300 Bebb willow (Salix bebbiana) and mountain willow (S. monticola) cuttings. Construction in 2006 ceased for the winter at the end of October due to frequent heavy snows.
Construction began in mid-June of 2007 and continued through the first week of September when all earthwork, grading, seeding and hydro-mulching, and planting was completed for 21 wetland sites and 5700 feet of ephemeral stream (Figs. 2, 3, 4). A total of 37 wetland cells were completed. The planting of approximately 20,000 native wetland herbaceous plants and shrubs was also completed at approximately 50 percent of the sites prior to August 15. In addition, the salvage and replanting of existing native herbaceous wetland vegetation, where it existed, was maximized to reduce plant material and labor costs.
|Photo 4: Site C Cell 1 – September 2007|
The difference between the number of wetland sites and individual cells actually constructed and the number originally planned was due to “field fitting” problems identified at some of the original sites and to subsequent site and cell substitutions and relocations to more suitable sites while the construction work progressed. Two examples were Site M and Sites K-1 through K-11. Site M was deleted in 2007 as construction progressed and individual sites were scrutinized more carefully. Site M was at high altitude (approximately 9550 feet msl) and was determined to be environmentally and physically unsuitable to access for construction. It was, therefore, decided by the team to substitute an additional eight cell sites associated with the Site K location for the acreage lost by deleting the three cells associated with Site M.
Another critical issue which was addressed during planning, design and construction of the wetland mitigation sites included ensuring sufficient hydrology would be maintained at the sites for sufficient duration to allow facultative hydrophytic vegetation to become established. This is generally accepted to be 5 to 12.5 percent of the growing season (7 to 20 days) of continuous soil saturation (Wetlands Research Program 1987), but in the xeric conditions of the Montrose area is probably longer. Since the primary water source for all wetlands on the site is spring runoff, it was deemed necessary to hold as much runoff as possible in the wetland cells for as long as possible in the spring and early summer to maximize the length of time of soil saturation. To accomplish this critical goal, construction techniques included the installation of impervious clay cores within the low dikes impounding most wetland cells. After extensive searching, suitable clay was located onsite. Twenty-four inch wide clay cores were installed to bedrock or to a depth of five feet below ground level, whichever was deeper, in most dikes to prevent seepage of water from the impounded cells.
Other issues which are being addressed and which will be addressed in 2008 include 1) planting approximately 20,000 native containerized plants in the remainder of the sites; 2) ongoing noxious weed control to meet the 404 permit Special Condition of less than 10 percent weeds within each site, 3) tweaking the overflow structures associated with the wetland cell dikes to safely maximize water levels and wetland acreage within each cell, and 4) initiation of site monitoring and maintenance to comply with 404 permit requirements.
Questions related to this project can be addressed to Steve Johnson at 303.915.3211 or email at email@example.com or to Tom Roberts at 303.892.1166 or email at firstname.lastname@example.org.
Burns & McDonnell, Inc. 2005. Conceptual wetland mitigation plan for the Cornerstone Golf and Equestrian Community, U.S. Army Corps of Engineers File No. 200575469. Prepared for Cornerstone Montrose LLC, Ridgeway, Colorado by Burns & McDonnell Engineering, Inc. Kansas City, Missouri.
Chronic, Halka. 1980. Roadside geology of Colorado. Mountain Press Publishing Co., Missoula, Montana.
Environmental Laboratory. 1987. “Corps of Engineers Wetlands Delineation Manual,” Technical Report Y-87-1, US Army Engineer Waterways Experiment Station, Vicksburg, Mississippi.