Hubbell Trading Post Cultural Landscape Report

TREATMENT RECOMMENDATIONS (continued)

Issue 3: Erosion Control

Historically, erosion has been a continuous problem within the Hubbell landscape as a result of the fine sands, intermittent yet at times extremely heavy precipitation, and generally sparse vegetative cover. Hubbell recognized the potential for problems with erosion when he decided upon an irrigation method for his extensive farm fields. To better control the flow of water across his lands and prevent both water waste and erosion resulting from discharge into the Pueblo Colorado Wash, he selected to use the check or terrace irrigation method. This method allowed him to gradually step the terraces across the field and effectively slow the water as it neared the drop down to the wash.

As evidenced from numerous historic photographs as well as personal accounts from area residents, the Pueblo Colorado Wash has changed from a wide shallow wash to a deeply cut arroyo over the past 95 years. It is assumed that the same is true to some degree, of the adjoining drainages or washes that feed into the Pueblo Colorado.

Beginning in the 1940's the Hubbell family along with a variety of government agencies began to address the erosion problems in the area of the wash — primarily by introducing vegetation in an effort to slow the downcutting processes. By the 1930's the vegetation along the wash had been severely depleted by both overgrazing and trampling and the open sandy wash offered no resistance to the flood of waters that periodically flowed through its channels.

As early as 1976 the National Park Service initiated erosion control measures within the Pueblo Colorado Wash by constructing large steel and rock gabions, primarily as splash blocks and later as wingwalls along the southern embankment. In 1984 erosional activities affecting archeological site HUTR-10 were slowed as the site was buried with sterile soils and stabilized using a covering of filter fabric. A few years later in 1987 stabilization work was undertaken on the Sand Dune site. Following the mitigation of disturbed and unstable sections of the site additional gabions were placed below the embankment in the Pueblo Colorado Wash.

During the early phases of field investigations for this study it was noted that erosion continues to be a problem within the Pueblo Colorado Wash as well as along its adjoining drainage or arroyo to the south (immediately west of the park housing area) . In addition, the gabions are a fairly dominant visual feature as one enters the historic site.

The recommendations provided in this section address four areas and provide:

1) possible alterations to the erosion control structures already existing within the Pueblo Colorado Wash to soften their visual impact and to increase their stabilization efforts;

2) possible implementation measures for use in the side drainage/arroyo west of the park housing area to stabilize existing slumped banks and to prevent further erosion that might result from the rehabilitation of the agricultural field immediately above it to the east;

3) protection and erosion control measures for the exposed tree roots along the embankments adjacent to the Visitor Center/Administrative Offices on the School house/Chapter house tract; and

4) treatment and monitoring measures for archeological site HUTR-10.

Pueblo Colorado Wash

During the summer of 1995 it was determined that excess fill soils were available as a result of the new housing construction project within the park. In lieu of hauling this material off-site it was determined that it could be placed in the gaps behind the steel wire and rock gabions in the Pueblo Colorado Wash. Several tons of fill material were placed between the south embankment and the gabions with some material covering sections of the gabions.

It has recently been noted by park staff that a variety of plant materials has started to become established and will eventually serve to soften the overall visual impact of the engineered structures. With this in mind it is recommended to further improve upon the visual quality of the gabions and increase their stabilization effects by re-introducing native willow to the immediate area by placing bundles, matting, or sprigs into and around the gabions. The bundles, matting, or sprigs can be anchored using a variety of methods (figure 106). This bio-engineering approach is both cost effective and can be installed using either volunteer or day labor provided there is a knowledgeable technician on hand for overall supervision.

An earlier study conducted by Ertec noted that "individual hydraulic structures...represent only one component of a successful treatment system. All of the gullies in the drainage network need to be treated by a combination of geomorphic, vegetation, and engineering measures." [192] The study continues by stating that researchers have "found that some tributary gulleys can be controlled by vegetation management alone, if their base levels are controlled by gulleys that are structurally treated." [193]. This finding has been applied to the following recommendations for the side drainage.

Figure 106. Details of bio-engineered style gabions. Engineering construction details by Richard Westmacott. (click on image for a enlargement in a new window)

Side Drainage / Arroyo West of Housing Area

It is primarily the west side of this drainage that has recently experienced some degree of slumping and active erosion. This is also the area that historically supported the irrigation lateral for the 16 acre field to the west. Several of the historic stone headgates have or are in the process of falling into the drainage as a result of the slumping activity. This ongoing erosional activity should be halted, especially if the park has future plans to rehabilitate the adjacent agricultural field and re-establish the flow of water through the historic irrigation lateral.

It is recommended that some degree of recontouring the severely slumped sections of the embankment be undertaken to create a stable slope. This can be achieved using a combination of both cut and fill and then anchoring the recontoured slope using either a geo-grid or filter fabric with either log cribwork interlayed with live willow brush bundles or with a polymer grid and brush packing (refer to figure 106). Even a combination of these two approaches might be appropriate.

Field investigations noted that there are currently two varieties of native willow growing within this drainage. The masses of willow could easily be trimmed for selective branches to be woven into bundles or matting, and it is likely that the drainages in the surrounding area could provide additional cuttings for this use. A quick review of area aerial photographs would reveal these locations.

Regardless of the exact technique selected, it is strongly recommended that the final appearance be "soft" and in keeping with the historic scene. This is important to maintain the integrity of the landscape. Such an approach can be more cost effective, environmentally friendly and innovative as well as being visually sensitive to the historic landscape resource. It is more compatible with the vernacular and seems to be more effective than the more heavy handed erosion control measures that are used across reservation lands (figure 107). A strong consideration in favor of the bio-engineered approach is that volunteer labor crews can be recruited from native plant societies, permaculture groups, and the community at large to assist in the installation process.

Protection and Erosion Control Measures for Exposed Tree Roots on School House / Chapter House Tract

As a result of years of uncontrolled visitor circulation in and around the school house/chapter house tract following the removal of a post and wire fence that followed the top of the embankment, the large roots of the historic elm trees have been severely exposed and the soils around them compacted. As visitors continued to scramble up and down the embankment and erode the unstable soils, the exposure of the tree roots increased thereby degrading their overall condition and posing safety hazards to visitors.

Figure 107. Tetrahedrons used for arroyo stabilization in vicinity of Cornfields, 1993.

In 1987 a layer of fill was placed over the exposed roots but nothing was done to remedy the uncontrolled traffic that caused the erosional problem. By 1992 the root were again exposed, the surrounding soils compacted, and the trees declining in health as a result. In the summer of 1993 as field work was being initiated on this report, the author was asked to address the visitor circulation problems and subsequent erosion problems in this area and develop a design recommendation that could be installed as soon as possible. After reviewing a variety of historic documentation including numerous historic photographs a design alternative was proposed for an accessible route into the visitor center/administrative office building and stabilization of the erosion in the area of the tree roots.

As per the approved access design (figures 108, 109, 110, 111), additional fill will be placed over the tree roots and surrounding ground plane to facilitate a new pedestrian flagstone walk connecting the trading post and the visitor center building. The historic post and wire fence alignment along the top edge of the embankment will be replaced to provide both a visual and physical barrier to visitors and control their access to the site, thereby preventing them from walking up the embankment and over the tree roots.