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III. SCOPE OF WORK A. Background Information Climatic Information: It is common in Colorado for nooding to occur in the spring and summer from rainfall events. The highest rates of storm water runoff are caused by intense rainfall on a watershed. The intensity of rainfall can vary considerably during each storm event. Summer rainstorms that cause flooding of the communities of Dacono, Frederick, Evanston, and Firestone are generally associated with frontal systems involving cold air from the northwest and warm moist air from the Gulf of Mexico. From approximately May 15th, May 30th and July 15th, August 15th, monsoon moisture typically enters Colorado from the Gulf of Mexico. The rainstorms vary in duration from a few hours to several days depending on the extent of the storm and its speed. One or more storm cells of high rainfall intensities generally exist within a given rainstorm event and the path of these storm cells often determine the location of nooding. Highest recorded rainfall events in Colorado typically occur during the last week of July and the first week of August. Storm Drainage Situation: A copy of portions of the report titled [,25 Corridor Master Drainage Plall Godding Hollow and Tri,Town Basins South Weld County, Colorado by Anderson Consulting Engineers, Inc, draft dated June 24, 1999, were available for public distribution and are included in Appendix I, The communities of Dacono. Frederick, Evanston, and Firestone have a long history of flooding problems. In these communities, there is currently no significant existing detention storage for storm water runoff. These communities outfall storm water runoff into the Tri,Town Drainageway (shown on Figure 2) which originates near Dacono, The drainageway conveys storm water runoff northerly toward Frederick, west along the south side of Frederick. and north along the west side of Frederick, Evanston, and Firestone. The drainageway is currently a small earthen channel with a capacity to convey somewhere between a 2'year and a 10-year existing condition storm event runoff. Storm events greater than the existing channel capacity flow overland through the communities causing flooding of residential homes, businesses, and roadways. The District is very proactive and sees the need for sl:orm drainage improvements, They have adopted very strict criteria for localized detention ponds for new developments within the communities of Frederick. Evanston, and Firestone by 5