San Luis Valley Regional Habitat Conservation Plan Draft for Public Review
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3.3Community InfrastructureVegetation Removal from the FloodwayBackgroundThe City of Alamosa has an existing system of levees along about 3.5 miles of the Rio Grande to prevent urban flooding during high runoff periods. In order to effectively convey river flow through this system and prevent flooding, the City needs to periodically clear vegetation from the floodway area. In the future, other municipalities such as Monte Vista, Del Norte, and South Fork also may need to take similar measures to prevent flooding and maintain the capacity of existing and future flood-control structures. 
Alamosa floodway The Alamosa Floodway and riparian habitat mapping are shown in Figure 12. There are currently about 46 acres of willow habitat within the floodway, which extends along the Rio Grande for 3.5 miles. In average years, the City clears between 7 and 10 acres of willows per year, while in some years the city has cleared up to 20 acres at one time. To allow for typical and routine clearing, incidental take coverage through this HCP is limited to the clearing of 4 acres of native riparian vegetation within any lineal mile of river. This standard is intended to encourage routine, smaller-scale clearing (which reduces the need for large-scale emergency clearing), and to set a standard for other communities if levee systems are developed in the future. Clearing beyond the 4 acre/mile threshold could result from a lack of planning or emergency flood conditions, and would be accounted for as part of the contingency mitigation. For the City of Alamosa, this standard amounts to coverage for up to 14 acres of clearing in (4 acres x 3.5 miles of floodway) in any given year. Coverage applies to units of government (or their designated representatives), and is not extended to individual property owners. General EffectsThe elimination of habitat patches typically occurs through cutting and mowing. Excavation and chemical control also could potentially be used. These activities have the potential to directly impact individual birds, and may result in indirect impacts from the removal of potential habitat. ImpactsPotential impacts of vegetation removal from the floodway was based on the 4 acre/mile threshold that is sufficient to cover typical clearing activities in Alamosa. Additional impacts resulting from clearing beyond the 4 acre/mile threshold or additional coverage for clearing future levees in other communities (Monte Vista, Del Norte, and South Fork) will be within the 6-acre contingency for infrastructure activities with uncertain or negligible impacts (see Section 3.4 below). The calculation of impacts is based on the following assumptions:
Estimated Impacts:
Levee Improvement and MaintenanceBackgroundFuture improvements or upgrades to the existing levee system in Alamosa or proposed new levees in Monte Vista, Del Norte, and South Fork may require the removal of willows and other native riparian vegetation in those areas. These improvements could entail grading, excavation, and the placement of riprap, gabions, boulders, and other hard structures on stream channels. Native riparian vegetation would likely re-colonize impacted areas surrounding the structures after they are installed. The coverage is limited to units of government (or their designated representatives). The construction of new levees, or other large-scale projects with a Federal nexus (including Corps funding or Section 404 wetlands permitting) will need to obtain ESA clearances through a separate Section 7 process, and will not be covered by this HCP. General EffectsThe impacts of levee construction and maintenance include the temporary elimination of habitat, with the potential for direct impacts to individual birds. ImpactsPotential impacts from future levee improvements and maintenance activities were estimated by calculating length of existing and proposed levees (53,797 feet/10.2 miles), the proportion of those levees that are adjacent to riparian habitat (about 25 percent), and then applying a range of impact footprints to the proportion of levees that are adjacent to habitat. Impact footprint estimates (200 square feet/year to 10,000 square feet every three years) were based on input with the Public Works Director for the City of Alamosa (Koskelin 2005). The range of potential habitat impacts was derived by multiplying the annual footprint impact by the average percentage of area containing riparian habitat. The calculation of potential impacts was based on the following assumptions:
Figure 12. Alamosa Floodway
Estimated Impacts:
Sediment Removal and Spoils DisposalBackgroundSediment deposition within the Rio Grande floodplain in general, and the Alamosa floodway system in particular, has contributed to a loss of flood capacity and stability through the system. Similar situations exist along the Alamosa River. In order to manage these systems and prevent flooding, local governments need to periodically remove sediment from the river channel and floodways with heavy equipment. Equipment access and sediment removal would likely disturb some riparian habitat areas. General EffectsThese activities may result in a temporary disturbance to some habitat areas. In addition, sediment removal indirectly eliminates the long-term potential for development of riparian habitat. However, these activities are conducted on an infrequent, ad hoc basis. ImpactsThe impacts of sediment removal and disposal are expected to be negligible, and are covered within a 6-acre contingency pool for uncertain impacts. Infrastructure ConstructionBackgroundThe construction of new types of public infrastructure, including roads and bridges, water and sewer lines, and other infrastructure that may cross through or run alongside riparian habitat areas, where a certain amount of habitat would be permanently eliminated. Coverage is limited to the Applicants and their authorized agents or contractors. Activities conducted by non-Applicant entities or private infrastructure providers are not covered by this HCP. This HCP does not cover activities requiring a Section 404 wetlands permit. General EffectsInfrastructure construction activities have the potential to result in the elimination of potential habitat areas and directly impact individual birds. ImpactsWhile no specific infrastructure projects are planned, it is estimated that the impacts of four projects each year will range between 1,000 and 10,000 square feet per year. This theoretical measure is to ensure coverage for yet unforeseen projects such as road realignments, new bridges, new water/sewer lines, or others that may occur in the next 30 years. Infrastructure construction impacts would be permanent, and would be tracked and mitigated on a continual basis during HCP implementation. The calculation of potential impacts was based on the following assumptions:
Estimated Impacts:
Infrastructure MaintenanceBackground
County road adjacent to willows Coverage for the ongoing maintenance of public infrastructure in the Valley includes activities such as maintenance and repair of roads and bridges, water and sewer lines, and other infrastructure. Maintenance of these infrastructure items may require the removal of native riparian vegetation in some areas to provide access to the work site or to ensure safety. Coverage is limited to the Applicants and their authorized agents or contractors. Activities conducted by non-Applicant entities or private infrastructure providers, and activities requiring a Section 404 wetlands permit, are not covered by this HCP. County road and bridge repairs may require occasional clearing of small areas of native riparian vegetation. The eventual maintenance of a City of Alamosa sewer line that crosses the Rio Grande will likely require some vegetation clearing; however, such maintenance is anticipated to be infrequent (Koskelin 2005). General EffectsVarious infrastructure maintenance activities may result in the temporary removal of habitat, and could potentially disturb or directly impact individual birds. ImpactsThe impacts of infrastructure maintenance activities were not quantified, are expected to be negligible, and are covered within a 6-acre contingency pool for uncertain impacts. |