by Blair Hurst, Walsh Environmental
Successful design, permitting and implementation of a stream and riparian corridor enhancement project always present a unique set of challenges. Such challenges typically result from site-specific conditions, land ownership situations and desired project outcomes. These challenges and many others have been encountered in McIntyre Gulch, where the Rocky Mountain Region of Centura Health will complete voluntary enhancements as a part of their larger project of constructing a new St. Anthony’s Hospital at the Denver Federal Center (DFC). Walsh Environmental is the general contractor on the project, providing enhancement plans in McIntyre Gulch (Gulch) as well as remediation and site development design.
Challenges began before foot was set on site, as this area was historically used as a landfill and has a long history of landscape alteration. The wastes in the landfill have been characterized for a wide range of hazardous constituents during site investigations, and determined to be solid waste known to contain asbestos. Alterations to the landscape include channelization and fill of portions of the Gulch, as well as the placement of fill in other surface drainages and historic wetlands on the site.
Due to the variety of historical activities at the DFC and its long history, the U.S. General Services Administration (GSA) is required by the Colorado Department of Public Health and Environment (CDPHE) to investigate and clean up environmental contamination in the project area, known as the Hospital Phase II Area. The City of Lakewood has entered into a contract with the U.S. Government to take ownership of the Hospital Phase II Area within 30 days following receipt by GSA of the CDPHE approval of the Corrective Measures Closure Report.
As the new property owner, the City of Lakewood will seek maintenance eligibility of McIntyre Gulch enhancements through Urban Drainage and Flood Control District (UDFCD). All enhancement designs and activities must therefore be completed in accordance with UDFCD standards. In addition to required design approval by UDFCD, McIntyre Gulch is designated by the U.S. Army Corps of Engineers (Corps) as a jurisdictional water under Section 404 of the Clean Water Act. Due to its jurisdictional status, a Section 404 Permit is necessary for all enhancement activities. The Corps has authorized this project under the Nationwide Permit No. 39.
The numerous constraints associated with the Hospital Phase II Area are described more fully below. These constraints coupled with the regulations of multiple jurisdictional entities created a unique challenge for design and implementation of enhancements to McIntyre Gulch. Addressing environmental legacies of the past and meeting goals of the future requires creative and collaborative solutions, as well as an understanding of the natural systems, site history, existing conditions, required Corrective Action, and enhancement goals.
Site History and Existing Conditions
Located north of Alameda Avenue, between Union Boulevard and Kipling Street in Lakewood, the DFC land area is slightly more than one square mile. The property began as a ranch, which was acquired by the Federal Government in 1941. The Denver Ordnance Plant (DOP) was constructed on the property. Under contract with the Federal government, Remington Arms operated the DOP as a small-arms ammunition plant from 1941 until the end of World War II. After the end of World War II, the DOP was converted to the DFC, a Federal Government facility operated by GSA. Over the years, following the closure of the DOP, several of the Federal agencies at the DFC disposed of miscellaneous wastes, including building and road demolition debris, in the southwest portion of the site, now known as the Southwest Landfill. The DFC is currently used by multiple Federal agencies for a variety of uses, including laboratories, materials testing and storage, maintenance facilities, and offices.
McIntyre Gulch is a perennial drainage that flows west to east along the northern boundary of the Hospital Phase II Area. Historically, the drainage meandered across the northern portion of the property. Significant site disturbance occurred in 1963 during the construction a large building adjacent to the Gulch. A portion of McIntyre Gulch was straightened and the cut-off meander filled with soil and debris. At about this time, a drainage project was constructed on the South Branch of McIntyre Gulch, which is tributary to McIntyre Gulch. The project concrete-lined the lower reach of the South Branch and changed its confluence with the mainstem to a 90-degree angle via a concrete drop structure.
Currently, the banks of the Gulch are steep and in many areas erosion and visible slumping is occurring, with bank heights up to 15 feet in some locations. Landfill waste occurs to the south of the Gulch, and right up to the edge of the channel in some locations. Banks that are eroded contain exposed roots from the tangle of overgrown woody vegetation that lines the tops of the banks. Woody vegetation contains a mixture of cottonwoods and willows mixed with non-native species. Understory vegetation is intermittent and sparse as a result of shading and a lack of sustainable hydrology, and primarily comprised of upland grasses and weeds, as shown in Photos 1 and 2. Along the channel bed, bedrock has been exposed through continued erosion and channel downcutting along an approximately 420 foot long reach. An existing bedrock cascade is the downstream extent of the bedrock exposure, below which the channel bottom has notable deposition of sands and gravels with some coarser materials.
|Photo 1||Photo 2|
Required Corrective Action
The Corrective Action for the property consists of removal of waste where necessary and construction of a landfill cover. Waste removal is necessary in all areas where the landfill cover will not be constructed. The landfill cover consists of two layers of soil: the Lower Permeability Layer (LPL) and the Vegetative Layer (VL). The LPL is an 18-inch thick soil layer with specific permeability requirements. The VL is an 18-inch thick soil layer that will promote plant growth.
Several factors were used to determine the extent of waste removal, the extent of the LPL, and the constructability of the LPL. The extents and depths of waste placement on the property were estimated based on historic mapping and aerial photos, and comparison to current site grades. LPL placement was configured to achieve the maximum aerial extent of cover placement, and minimum aerial extent of waste excavation. Several constraints on cover extents, however, were also factored in. These included the need to have no cover material and no waste within the McIntyre Gulch right of way (ROW). The LPL layer will end outside of the McIntyre Gulch ROW so that future work in the ROW will not impact the LPL and so that a robust landscaping plan can be implemented. This will minimize erosion, create an amenity, and achieve the goal of improving the natural function of this reach of the Gulch.
McIntyre Gulch Enhancement
The project reach of McIntyre Gulch is bounded by property lines on its upstream and downstream ends and includes approximately 750 linear feet of stream corridor. The intent of the Gulch enhancements is to stabilize overly steep (in some cases vertical) unvegetated banks, restore the native riparian canopy and understory (including preservation of existing mature cottonwoods), and improve the stream corridor’s natural functions. Improvements to the Gulch are designed to be consistent with UDFCD guidelines and standards to ensure that the City of Lakewood may apply for maintenance eligibility for the project reach from UDFCD.
Given numerous project constraints, formulating a design that meets UDFCD criteria, but still achieves the goal of natural function, proved challenging and required several rounds of submittal and technical review by UDFCD. Project constraints include the required Corrective Action, existing Gulch conditions, and proposed land use. In its existing condition, the Gulch is deeply entrenched with very steep and tall sideslopes. Existing sideslopes are often steeper than 1H:1V (horizontal: vertical) and up to 15 feet in height. These physical constraints made it infeasible to regrade to notably milder slopes in many areas, as new grading could not catch the steep existing grades without extensive walling. However, UDFCD criteria requires graded bank slopes to be no steeper than 4H:1V. To meet this condition, the final design utilizes small, localized boulder walls (i.e., natural materials to match the natural setting) to balance the competing desires to reduce sideslopes without “walling in the Gulch”L.
Another existing condition in the Gulch that presents a constraint on the enhancement design is the 90-degree angle of the South Branch confluence. The South Branch is contained within a concrete drop structure, and its orientation has created problematic hydraulics and erosive conditions in this area. The proposed design includes filling in the eroded area and constructing a grouted sloping boulder drop to formalize and protect the confluence. Installation of the structure will occur under design engineer supervision to ensure a naturalized appearance of the boulders. The structure will utilize soil riprap on the upstream side to provide protection, but also allow native vegetation to establish in the vicinity of the drop.
Proposed land uses present additional constraints to the McIntyre Gulch enhancement. These include a bike path paralleling the gulch along its north bank, a vehicular crossing over McIntyre Gulch at the downstream project extent, an extended detention basin with a pipe outlet and an emergency spillway to the downstream portion of the gulch, a parking lot along the upstream half of the gulch, and an emergency surface drainage and pipe outlet to McIntyre Gulch from the proposed hospital facilities. These proposed land uses were accommodated in a way that would impart the most natural function and appearance on the Gulch. The crossing is a three-sided, open bottomed structure that free spans the Gulch, reducing impacts on stream hydraulics. Pipe outlets to the Gulch were designed with natural boulders as energy dissipation in lieu of concrete blocks to minimize additional structures in the Gulch. Emergency spillways down to the Gulch will be vegetated with native grasses and shrubs.
The enhancements have been permitted under the U.S. Army Corps of Engineers Nationwide Permit Number 39. Under this permit, impacts are limited to a total of 0.5 acre and 300 linear feet of the stream. Close attention to impact locations and quantities was necessary, as the permit covers Gulch enhancement activities in addition to site-wide development activities, both of which went through several rounds of revisions. In order to stay within permit limits, site-specific adaptations of UDFCD design standards were necessary. For example, UDFCD prefers placement of soil riprap toe protection along the toe of all newly graded banks. This was not possible due to limitations on linear feet of impact. Soil riprap toe protection is planned for only the downstream extent of the project reach, where the crossing will be located and existing banks are steepest and tallest, and where the channel invert has not yet encountered bedrock.
To address toe protection requirements along all other regraded banks, an alternate interim toe protection was proposed and discussed with UDFCD. Walsh designers and UDFCD reviewers agreed upon the installation of log deflectors at 10-foot intervals along regraded toes that do not have soil riprap toes. The deflectors will act to redirect low flows away from the newly graded slopes while vegetation establishes. The log deflectors replace the interim stability function of the soil riprap toe in a much less impactful manner (i.e., without constituting an impact under the project’s Nationwide Permit). Longevity of log deflectors varies due to species and condition of the logs upon installation, but logs are anticipated to stay in place for approximately 3 years before they decompose. This duration will provide ample interim protection while newly installed vegetation establishes. Rock-log deflectors are planned for the outside of one channel bend, as they will provide added protection at this relatively abrupt existing feature.
The project reach contains important existing features, which are intentionally incorporated into the proposed enhancements. The dense overhead canopy that exists in some areas provides a strong attribute for bank stability, quality riparian habitat, and a pleasing natural aesthetic. Proposed grading has been designed to avoid mature cottonwoods, such that preservation of this resource is maximized. A bedrock cascade, with a vertical drop of approximately 6 feet, exists in the middle of the project reach. This feature will be maintained. Per UDFCD technical review requirements, concrete cutoff walls will be installed at grade upstream of the drop to ensure that this feature does not migrate upstream and cause further erosion in the Gulch.
Revegetation will include plantings to increase lower and mid-level canopy cover along the channel (currently lacking in this reach), and the installation of larger trees and shrubs that will be strategically located to provide shade in any newly exposed areas or localized areas that currently lack overhead cover. Expansion and creation of low benches will specifically improve the herbaceous and shrubby vegetative components in the reach. The restored root matrix will supplement bank stabilization. Additional benefits include reduced sediment loading and improved water quality for the project area and downstream reaches, as well as improved production export to downstream habitat due to increased organic plant materials along the Gulch.
The proposed enhancements will provide a net aquatic benefit to the reach by improving the corridor’s overhead cover function, enhancing shading and cooling functions, as well as increasing the detritus supply for the system. Additionally, installation of native grasses, trees, and shrubs along sections of currently denuded banks will provide a substantially improved natural aesthetic for the stream corridor. Wildlife usage may be increased due to the restoration of native, riparian plants along the gulch. Enhancement of the vegetation communities and improvements to the stream and riparian corridor interspersed with open space habitat will provide increased cover, food and nesting opportunities for locally occurring wildlife and avian species.
While there were several constraints on design, and hurdles during approval and permitting, a project reach with so many factors to consider oftentimes has much to be gained through creative, accommodating and persistent design. To date, waste excavation and tree preservation along the south bank of the Gulch have been completed (Photo 3). Stay tuned to future greenline articles discussing progress and details of construction of this unique project.