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Gold Star Canyon Floodplain Information Report Mesa County, Colorado primarily described as 'B' and 'C These soils are generally characterized by moderate infiltration rates with moderately slow to moderately rapid permeability and moderately low runoff and somewhat poorly drained soil with moderately high runoff potential, respectively. The slope ranges from 0.02 National Monument. The report on Gold Star Canyon for in 1997 and 2004, respectively. a basis for this study since it was feet per foot. the Colorado LANDesign prepared both a drainage report and a floodplain delineation two developments, The Preserve Subdivision and The Gigoux Property, The hydrologic data and parameters developed in these studies serves as reviewed by the local Natural Resources Conservation Service slope along the study reach is 0.11 and South Broadway Street to 0.34 on channels are generally unpaved and predominately heavily vegetated I ongi tudinal fIver Related Studies The average between the 2.0 Introduction Gold Star Canyon is a natural drainageway located in western Colorado within the City of Grand Junction in Mesa County, Colorado. Almost half of the Gold Star drainage basin is within the Colorado National Monument located southwest of the City. The canyon was formed by runoff from the rock outcrop plateaus of the National Monument. It is 311z square mile drainage basin tributary to the Colorado River. The canyon is well defined within the undeveloped National Monument Park, but becomes almost non-existent within the development of the City. Near the outfall to the Colorado River, the channel once again becomes well defined and deeply incised. The basin starts at elevation 6,210 feet on the mesa and outfalls to the river at 4,515 feet. The average slope of the channel is approximately 11%, however, some areas are vertical waterfalls over the rock outcrops and other reaches are flatter. 1.0 The previous study used the SCS TR-55 approach. The parameter of concern in the previous calculations was the curve number (CN). Table 2-2a and 2-2d from SCS TR-55 show pinyon-juniper CN for D group soils is 89 for poor ground cover, 80 for fair ground cover, and 71 for good ground cover. "Good" ground cover is defined as greater than 70% ground cover; "Poor" is defined as less than 30% ground cover, and "Fair" is the area in between. The following are quotes from the November 24, 2003 comment letter from Ryan Altenburg, EIT, Agricultural Engineer, Grand Junction Field Office The upper reaches of Gold Star are intermittent drainageways that only carries a surface flow during rainstorm events and snowmelt. The lower reaches, south of Broadway (US Highway 340), contain springs and the resulting growth of wetlands gave rise to the name "The Preserve" land development for the vegetation and habitat. It should also be noted that the lower reach is also used as a wasteway for irrigation water, and a base flow may be evident during dry periods "I have reviewed aerial photographs, visited portions of the sub-watershed, and have spoken with park representatives in regards to the hydrology and agronomic characteristics of the sub-watershed I have concluded that the ground cover for this sub-watershed is less than 50% ... It is my suggestion that a minimum curve number for the Pinyon-Juniper area be a minimum of 80 and the rock outcrop a minimum of 88...1 would like to make note that considerable caution [is warranted) in figuring the hydraulics and hydrology to be used for the Gold Star Canyon watershed It has been noted that unusually high run-off events have been experienced on this drainage, and flood events are of concern to citizens in the area. " Purpose Prior to this study, the Gold Star Canyon floodplain was neither published nor delineated as Zone AE by FEMA. This hydrologic analysis and eventual floodplain analysis was requested by the local governments because of new development pressure and repeated flooding of existing structures 1.1 and to guide local flood hazards and future flood This report was prepared to provide information relative to the occurrence of floods officials in planning the use and regulation of the floodplain areas so that damages are minimized at South Broadway after rise concrete culvert under The LANDesign January 7, 2004 report computed a 100-year flow of 324 cfs revisions to the Curve Number as suggested above. The roughly 10' span by 7 South Broadway was determined to be adequate. The report stated Study Area 1.2 "The capacity of the culvert is not exceeded during the 100-year storm event. However, based on accounts from the property owners and the Redlands Water and Power Company, the capacity of the culvert was exceeded during a storm in 1996. Presumably, the event generated a larger amount of water than a 100-year storm 1 This hydrology report was prepared for approximately 3.52 square miles of Gold Star Canyon beginning on the Colorado National Monument and ending in the Colorado River basin. The Gold Star Canyon basin originates southwest of the Colorado River at a local highpoint elevation of approximately 6,210 feet on the Colorado National Monument. South Broadway Street divides the basin into two portions characterized differently by vegetation, topography, and development. The upper portion, southwest of South Broadway Street lacks urban development and contains significant outcroppings of rock (described as soil hydrologic classification 'D' as defined by the Natural Resource Conservation Service). These soils are generally characterized by low permeability and high runoff. Steep slopes are prominent in this area. The topography dominates flow in the upper basin. However, as the water flows north into the lower basin, flow is dominated by development of residences and roadways. Flow is ultimately directed through these manmade elements to the natural drainageway of Gold Star Canyon which has been preserved and outfalls into the Colorado River. The lower basin is characterized by a mixture of low-density subdivisions and small farms on flatter slopes Soils in the lower basin are