2011-06-30_REVISION - M1981302 (13)

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BACKGROUND In August 1997, Ayres was asked by Flatiron Sand and Gravel Company (Flatiron) to assist in the technical evaluation of an existing levee at the site of a construction aggregates mining oper2tion l nO n as the Deepe Pit. Deepe Pit is located on land unrest of South Boulder Creek between Marshall Road and South Boulder Road in Boulder County immediately adjacent to the city of Boulder. At the time we were asked to assist, the mined area was undergoing reclamation construction activity within2n2rea bounded generally by a levee on the south and east, U.S. Highway 36 on the north and high ground along State Highway g (South Broadway) on the unrest. An undisturbed natural buffer between 5002nd 1,500 feet wide separates the levee from South Boulder C reek on the east. The pit area inside the levee is generally lower than the adjacent natural ground and slopes to the north. The scope of the Ayres investigation concerns the _adequacy of the levee erosion protection along the east or South Boulder Creek side of the levee between Station 105+00 and 1 48+65. This portion of the levee is immediately adjacent to the levee internal area w hich is currently undergoing reclamation and where the 1 00-year floodplain of South Boulder Creel extends to the edge of the levee from the east. The I 00-year flood w ater depth along the levee in this reach varies from o -- 5 feet and the velocity from 4.5 to g. 5 fusee indicating that levee erosion protection may be necessary. The regulatory flood study for this area is the Mouth Boulder Creek Flood Hazard area Delineation" 1986). That study shows the levee as @ ground surface, higher than the adjacent natural ground which establishes the west edge of the loo -year floodplain limits in this reach. That study provides the regulatory flood elevations, limits, and velocities which are the basis for establishing the need for levee erosion protection. INVESTIGATION A review of available information and physical inspection of the levee took place in late August. The following general conditions and observations were ,node: • Relocated Dry Creek Ditch fro. 2 and one of its laterals parallels the levee at its toe along almost the entire reach of interest. Dry Creep Ditch Igo+ 2 flows in a moderately well armored gravel lined channel at a normal depth of between 2 and 6 inches and three feet wide (during the irrigation season). The average flog velocity is approximately I foot per second, which is well belo w the 5 foot per second velocity where erosion protection would be considered. Overbank flooding is rare along this reach of South Boulder creep having very rarely exceeded channel capacity and nearer gotten even close to the levee during its approximately 20 -year life. The levee physical condition, particularly its erosion resistance characteristics permitted its logical breakdown into three distinctive reaches for more detailed evaluation. Ayres Associates