Environmental Considerations
Topography
The project area lies on the eastern side of the Spokane River Valley, to the northwest of the Spokane City limits. To the immediate west of the site lies the Avista Corp. Nine Mile Dam, a concrete gravity dam constructed in 1906. Surface topography in the vicinity of the project area is typified by relatively flat to gently sloping terraces, separated by steep, bedrock-supported slopes. SR 291 is separated from the Spokane River by a steep bedrock slope that is approximately 50 to 70 feet in height. Downstream of the dam, in the vicinity of Nine-Mile Bridge (Charles Road), cliffs in excess of 80 feet in height flank both sides of the river. From stations SR 316+00 to SR 332+00, a steep embankment of sandy, gravelly loam walls off the eastside of SR 291. The embankment slope varies from vertical rock outcroppings to grades steeper than 2:1 and the embankment height varies from 50 to 120 feet.
Drainage
SR 291 is a major contributor of surface runoff with additional flow from the surrounding hillside. The project proposes to treat all impervious surface runoff. Water quality treatment facilities include biofiltration swales and filter strips. Runoff will be conveyed through a network of roadside ditches, catch basins, grate inlets, and culverts.
Vegetation
Site vegetation consists of a sparse conifer forest. Ponderosa pine is the predominant tree species. Trees range between 6 to 18 inches in diameter. Wooded areas have a sparse understory consisting primarily of low growing deciduous shrubs and field grasses. Open slope areas are vegetated with a mixture of low-growing field grasses.
Sensitive Areas
The Spokane River runs parallel to SR 291 within the project limits. The Environmental Review Summary states any paving improvements, safety restoration, and drainage modifications will not impact the Spokane River. As stated in the drainage section, biofiltration swales and vegetated filter strips will treat all surface runoff.
Adjacent Areas
From MP 8.55 to 8.89 on SR 291, the land use is primarily commercial businesses and residential housing. The Riverside State Park owns the property from MP 8.89 and up north beyond the project. A portion of the proposed Rutter Parkway will be constructed on existing private property. Property owners on the top of Rutter Parkway will be affected mainly by dust during construction. The commercial businesses and property owners along SR 291 may be affected by dust during roadway excavation and miscellaneous grading. The upper west bank of the Spokane River will need to be protected during grade work and controlled blasting to prevent any sediment, by wind or water, from entering the river.
Overall Erosion Potential Assessment
The factors that control erodibility consist of the soil type, precipitation, existing and proposed vegetation, slope length and gradient, and surface texture. As mentioned in the soils section, the project consists of a type B gravelly, sandy loam, which is considered moderately erosive. The precipitation is relatively low, however, the spring runoff from rain and snowmelt may affect the steeper slopes in the area. Along SR 291, the proposed slope cuts range from 20 to 120 feet high and will be laid back at a 1.5H:1V slope. The possibility of erosion is high in this area due to the slope length, gradient, and soil type. The potential for erosion to damage this project, surroundings, properties, and the environment is moderate.
Temporary Erosionand Sedimentation Control
The Temporary Erosion and Sedimentation Control (TESC) Plan is designed to establish when, where, and how specific best management practices (BMPs) will be implemented to prevent erosion and the transport of sediment from the site during construction. Due to the unpredictable nature of weather and construction conditions, the TESC Plan is a "living document" and is subject to additions and modifications to successfully prevent erosion throughout construction.