2018-12-26_REVISION - M2008078 (28)

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Creek within the Town of Holly and adjacent areas of Prowers County; and the Arkansas River and Willow Creek within the City of Lamar, were obtained from new topographic maps developed by Synergy Mapping, Inc. from aerial photographs (Reference 19). Below water sections were obtained from field survey performed by Wagner Land Surveying, Inc. The Highway 385 Bridge, Atchison Topeka & Santa Fe (AT&SF) Railroad, and Highway 50 Bridge in Granada; the Highway 89 Bridge, Atchison Topeka & Santa Fe (AT&SF) Railroad, Highway 50 Bridge, and County Road 34 Bridge in Holly; and the County Road 11 and 12 and Colorado Road 9 Bridges, Atchison Topeka & Santa Fe (AT&SF) Railroad, Highway 50 Bridge, Memorial Drive Bridge, several residential access bridges, and culverts beneath County Road Hh, Willow Valley Road, Woodland Lane, and several Golf Course crossings in Lamar, were surveyed to obtain elevation data and structural geometry. Water Surface elevations for floods of the selected recurrence intervals were computed with the USACE HEC-RAS v. 4.0.0 step backwater computer program (Reference 20). The starting water surface elevations for the Arkansas River, Wild Horse Creek, Wolf Creek, and Willow Creek, were determined using normal depth. Initial Manning's `n' values used in these detailed hydraulic analyses were determined from field observation and aerial photography. In general, the field conditions reflected areas of vegetation, housing and business developments, and open space. Manning's `n' values varied from location-to-location within the study reach. Manning's `n' values used for the Arkansas River in Granada and Holly ranged from 0.02 to 0.075 in both the main channel and overbank areas; in Lamar, values ranged from 0.03 to 0.065. Significant growth of tamarisk vegetation exists alongside the low flow portion of the channel. For Wolf Creek, Manning's `n' values ranged from 0.03 to 0.07 for the main channel and 0.05 to 0.75 in overbank areas. For Wild Horse Creek, Manning's `n' values ranged from 0.02 to 0.035 for the main channel and 0.02 to 0.70 in overbank areas. For Willow Creek, Manning's `n' values ranged from 0.04 to 0.045 for the main channel and 0.05 to 0.65 in overbank areas. Calibration of Manning's `n' values in the channel area was completed for the Arkansas River using stage versus flow data collected from the United States Geological Survey (USGS) at the downstream side of the Highway 385 Bridge in Granada, Colorado. In Granada, insufficient Wolf Creek channel capacity upstream of Highway 50 results in overtopping of the east bank and shallow overland flow to a spill flow channel just south of Highway 50. Lateral structures were added to the hydraulic model to represent overtopping of the east bank. Zone X shaded zones and an area of Zone AO (1 foot depth) represent the spills from Wolf Creek overland to the Highway 50 Split Flow channel. Most of the spill from Wolf Creek to the Highway 50 Split Flow returns north over Highway 50 back to the main channel of Wolf Creek at the upstream end of the Highway 50 Split Flow reach, and the remainder return at the confluence just south of the Atchison Topeka and Santa Fe 11