FLOOD11630 - Laserfiche WebLink

Home Browse Search

Red Canyon Floodplain Information Report Mesa County, Colorado The average longitudinal slope along the study reach is 0.06 feet per foot. The slope ranges from 0.02 between the river and South Broadway Street to 0.45 on the Colorado National Monument. The channels are generally unpaved and predominately heavily vegetated. Introduction Red Canyon is a natural drainageway located in western Colorado within the City of Grand Junction in Mesa County, Colorado. About two-thirds of the Red Canyon 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 4.75 square mile drainage basin tributary to the Colorado River. The upper reach of Red Canyon is an intermittent drainageway, or "wash," that only carry surface flow during rainstorm events and snowmelt. 1.0 Related Studies An adjacent study (Gold Star Canyon) also flowing off the Colorado National Monument used the SCS TR- 55 approach. 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: 2.0 The canyon is well defined and incised within the undeveloped National Monument Park, but the middle portion through proposed new development and the golf course, the drainageway becomes more broad and shallow. Near the outfall to the Colorado River, the channel once again becomes well-defined and deeply incised. The basin starts at elevation 7,025 feet on the mesa and outfalls to the river at 4,540 feet. The average slope of the channel is approximately 6%, however, some areas are vertical waterfalls over the rock outcrops and other reaches are flatter. 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. " Zone AE by governments Purpose Prior to this study, the Red Canyon floodplain was neither published nor delineated as FEMA. This hydrologic analysis and hydraulic analysis was requested by the local because of new development pressure and better management of the floodplain. 1.1 Four related studies have been prepared for the Red Canyon area. LANDesign prepared both a Hydrologic and Hydraulic Analysis and a Drainage Report on Red Canyon for two filings within the Monument Valley development, Filing 5 in 1994 and Filing 6 in 1998, respectively. The Hart Group prepared the Supplementary Drainage Report for Monument Valley Filing 5 in 1993. Additionally Paragon Engineering prepared the Drainage Report for Monument Valley Filing 3 in 1982. This report was prepared to provide information relative to the occurrence of floods and to guide local officials in planning the use and regulation of the floodplain areas so that flood hazards and future flood damages are minimized. The studies that were preformed by Paragon Engineering and the Hart Group used the rational method to determine peak runoff values. The two studies performed by LANDesign used the policies outlined in the Mesa County Storm Drainage Criteria Manual as a basis to determine the study methodologies, techniques and hydrologic data. Their hydrology was preformed using the SCS TR-55 approach with the u.S. Army Corps of Engineers HEC 1 Flood Hydrograph Package. These independent LANDesign hydrologic analyses produced similar results to the analysis Matrix preformed using the SCS TR-55 approach with the u.S. Army Corps of Engineers HEC-HMS computer program. The design points in the LANDesign studies and the Matrix study are not in the same locations. However, the South Camp Road design point in the LANDesign Filing 5 report is located between Junctions 2 and 3 in the Matrix analysis. It can be seen in the Hydrologic Summary Table in section 3.0 below; that the peak flow rate determined in the LANDesign Filing 5 report at the South Camp Road design point is between the peak flow rates at Junctions 2 and 3 in the Matrix analysis. Based upon this independent analysis, the same flow rate at South Camp Road can be used as in the previous development studies. Study Area Hydrology was analyzed for the approximately 4.75 square miles of tributary area to the Red Canyon drainageway beginning on the Colorado National Monument and ending in the Colorado River basin. The Red Canyon basin originates southwest of the Colorado River at a local highpoint elevation of 7,025 feet on the Colorado National Monument. The boundary between the Colorado National Monument and the Redlands divides the basin into two portions characterized differently by vegetation, topography, and development. The upper portion 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 whole basin. The lower basin is characterized predominately by desert shrub with a mixture of low-density subdivisions and small farms on flatter slopes. Soils in the lower basin are 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. 1.2