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'998. CtI/BnuJo FItJod Haunl Mitigatilln PI", (409) tit reported just west and northwest of Fort Collins totally saturating the ground, producing major flooding in Laporte, and setting the stage for the evening flood event. . On the evening of July 28, 1997, intense rains began around 6:30 p.m. in the foothills west of Fort Collins. Winds from the east and southeast continued to pump mOisture mto the storm system throughout the evening. The core of the storm was very small but remained nearly stationary over the headwaters of Spring Creek, the Fairbrooke Channel, Clearview Channel, the CSU Drainage Basin and the West Vine Drainage Basin. Rainfall intensity increased and reached a maximum between 8.30 p.m. and 10:00 p.m. before ending abruptly. A subsequent analysis of rainfall conducted by CSU showed a maximum of 10.2 inches of rainfall in less than five hours near the intersection of Drake Rd. and Overland Trail. On July 29, 1997, slow-moving thunderstorms dumped large amounts of rainfall aver the Pawnee Creek Basin in Weld and Logan counties and over the Schaefer DI<IW Basin in Morgan County north of WeI- dona Floodwaters from Schaefer Draw entered the unincorporated Town of Weldona on the evening of July 29 while similar damaging floodwaters from Pawnee Creek entered the unincorporated Town of Alwood early July 30th (west of Sterling and north of U.S. Hwy 6). Additionally, floodwaters fIowjng east from Atwood entered the City of sterling. During the Presidential Declaration Incident Period (July 28 - August 12, 1997) storm systems drenched other areas in northeastem Colorado as well a several counties in southeastem Colorado. In addition, the Denver-metro Area received flooding rains as did the Clear Creek County area to the west of Denver. 2.2.1.2.2 1997 - Surface and Hydrology Larimer, Logan, Weld, Morgan and Clear Creek coun- ties have drainage tributaries to the South Platte River. In addition, portions of Elbert and Uncoln counties have tributaries to the South Platte River. The river basin has a drainage area of about 24,300 square miles and is located in three states, Colorado (79 percent of the basin); Nebraska (15 percent of the basin); and Wyoming (6 percent of the basin). The basin has a continental climate modified by topog- raphy, in which there is large temperature ranges and irregular seasonaJ and annual precipitation. Mean tem- peratures increase from west to east and on the plains from north to south. Areas along the Continental Divide average 30 inches or more of precipitation annually, which includes snowfall in excess of 300 inches in contrast, annual precipi1ation on the plains east of Denver, Colorado, and in the South Park area in the southwest part of the basin, ranges from 7 to 5 tit . inches. Mo.st of the precipitation on the plains occurs as ram, which falls between April and September. Rangeland is present across all areas of the basin except over the high mountain forests. Agricultural land i.s restricted mostly to the plains. Urban or built-up land IS present primarily along the Front Range urban corridor in Colorado. Phillips County and parts of UncoIn and Elbert coun- ties have drainage tributaries to the Republican River. The Republican River is, in tum, tributary to the Kansas River in Kansas. The Republican River Basin in Colorado consists primarily of rangeland with some farming and ranching communities scattered through- out the basin. The Arkansas River Basin is very similar to the South Platte River Basin in topography, geology and hydrol- ogy. Annual mean temperatures are slightly higher than the Platte Basin. Annual rainfall amounts average between 7 and 15 inches, except in the mountainous areas of the basin. Land use is similar, as well, and . consists mainly of agriculture. Crowley, Kiowa, otero and Prowers counties are drainage tributaries to the Arkansas River. In addition, portions of Elbert, Un coin and Baca counties are within the Arkansas River Basin, A very small part of the incident area lies within the Cimmaron River Basin. The southern portion of Baca County has drainage tributaries to the Cimmaron River. The Cimmaron River flows from Colorado into Kansas and then into Oklahoma where it ultimately joins the Arkansas River in Tulsa. The Cimmaron River Basin is similar in topography and climate to the Arkansas River Basin. 2,2.1.3 Snowmelt Floods Snowmelllloods result from the melting of the winter snowpack in the high mountain areas. Snowmelt floods typically begin as spring runoff appears, after the first spring warming trend. If the trend continues up to 8-10 consecutive days in a basin where the snowpack has a water content more than about 150 percent of average, serious flooding can develop. The total duration of snowmelt floods is usually over a period of weeks rather than days. They yield a larger total volume in comparison to other varieties of floods in Colorado. Peak flows, however, are generally not as high as flows for the other types. A single cold day or cold front can interrupt a melting cycle causing the rising water to decline and stabiliZe until the cycle can begin again. Once snowmelt floods have peaked, the daily decreases are moderate, but fairly constant Snowmelt flooding usually occurs in May, June, and early July. File:FHMP~.2.pab Chapter 2 - 6 1/ZTJlI6:14 AM