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. '. I '. . I '. I I I . I . I :1 . ,. I '. North St. Vrain & St. Vrain Creeks Floodplain Update North St. Vrain & SI. Vrain Creeks Floodplain Update Town of Lyons Town of Lyons SECTION 2 - STUDY AREA DESCRIPTION April 2, 1957. mean monthly high temperatures range from 76 degrees in July to 35 degrees in January. Mean monthly low temperatures range from 45 degrees in July to 14 degrees in January. 2.1 Drainage Basin Characteristics 51. Vrain Creek, a left bank tributary of the South Platte River, originates on the eastern slopes of - the Rocky Mountains near the Continental Divide. North 51. Vrain and South St. Vrain Creeks, which drain approximately 211 square miles of mountain area, join in the foothills at Lyons, Colorado to form the main stream of the 51. Vrain. From Lyons, the 51. Vrain flows in a southeasterly direction through Longmont, Colorado. The St. Vrain flows in a northeasterly direction to its confluence with the South Platte River near Milliken and Platteville, Colorado. Elevations within the basin range from approximately 4,760 feet at the mouth to over 14,000 feet on Longs Peak. The St. Vrain basin above Lyons, Colorado is primarily forested, mountainous terrain, while the St. Vrain basin below Lyons is primarily pasture and crop lands. 2.3 Studv Reach Description The study reach consists of two sub-reaches: one on the main stream of the St. Vrain and the other on the North St. Vrain Creek. The sub-reach on the main stream of the St. Vrain starts at US Highway 36 and extends approximately 1.3 river miles upstream to Lyons to the confluence of the North and South St. Vrain Creeks. The topographic mapping provided in the 1987 FIR was used for this study. St. Vrain Creek: This study includes the 51. Vrain Creek from U.S. Highway 36 east of Lyons to the confluence of the North and South St. Vrain at Lyons, and the North St. Vrain Creek within Lyons. Drainage basin areas for St. Vrain Creek and its tributaries at a number of locations are listed later in this report in Table 2, Section 4. The main channel for St. Vrain Creek is fairly well-defined; however, the overbank area becomes a relatively flat alluvial floodplain. Throughout this study reach, the banks of the St. Vrain are bordered by pasture and crop lands, with the banks themselves lined with brush apd cottonwoods. The channel is irregular in cross section through the main stream study reach, consisting of high banks approximately 150 to 300 feet apart with the low flow channel meandering between these limits. The channel bed consists of shifting sand and gravel bars strewn with cobbles and small boulders. The average bed slope for the main stream study reach is approximately 40 feet of drop per mile (0.76%). A number of small lakes and reservoirs exist in the upper portions of the basin including Buttonrock Reservoir, Longmont"s major raw water storage reservoir. Complete in 1969, Buttonrock lies on North St. Vrain approximately six miles west of Lyons. Buttonrock was constructed for municipal water supply storage and was not intended to serve as a flood controls structure. However, Buttonrock does provide some residual flood control effects by attenuating peak flood flows as they are routed through the reservoir. Numerous smaller offstream reservoirs, constructed primarily for agricultural purposes are located throughout the lower St. Vrain Creek basin, and provide some residual flood control benefits, but to a lesser degree than that provided by Buttonrock Reservoir. North St. Vrain Creek: The North St. Vrain sub-reach begins at the confluence of the North and South St. Vrain Creeks and extends approximately 2.0 river miles upstream. North St. Vrain Creek is a typical mountain stream with a well-defined channel through the canyon and a steep slope (approximately 66 feet of drop per mile or 1.3%). The channel is irregular in cross section, with bank widths of between 90 and 150 feet. The channel bed through the North St. Vrain reach consists primarily of cobbles and small boulders, with sandbars appearing near the mouth of the North St. Vrain. 2.2 Climate Precipitation and temperature records are available for Longmont and Allenspark, Colorado. These locations appear to adequately describe the range of climate variation within the study area. The 100-year floodplain and profile are shown on the enclosed drawings. A cursory review comparing the 1987 FIR floodplain delineation to the revised floodplain shows that the floodplain limits did not change significantly from the 1987 FIR However, when the flood profiles are compared, it should be noted that the flood profiles did change significantly in places from the original study. The average annual precipitation at Longmont is approximately 13 inches. Longmont receives approximately 9.4 inches of precipitation during the months of April to September. The maximum rainfall depth accumulated over a 24-hour period was 4.04 inches, which occurred on May 9, 1957. Mean monthly high temperatures range from 89 degrees F in July to 41 degrees F in January. Mean monthly low temperatures range from 58 degrees F in fuly to 12 degrees F in January. Obstructions to flood flows exist along the entire study reach. Trees, brush, and other vegetation growing along the streambank tend to impede floodwaters, causing backwater conditions that increase flood depths upstream. Manmade obstructions such as buildings, bridges and culverts also impede floodwaters, causing backwater conditions that increase the flood flow depths upstream of the obstruction and high velocities downstream of the obstruction. Brush and other debris washed downstream during flooding may collect at The average annual precipitation at Allenspark is approximately 20.6 inches. Allenspark receives approximately 17.5 inches of precipitation during the months of April to September. The maximum 24-hour rainfall depth recorded at Allenspark is 4.3 inches, which occurred on Floodplain biformatioll Report 6 K:\356001D04\FLOODPLAIN INFORMATION REPORT.doc Floodplain lnformatioll Report 7 K:1356001004\FLOODPLAIN INFORMA nON REPORT. doc June 1998 June 1998