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I I I I . I I I I I I I I Ii I I I I I 3. Drawing GH-3 (Sheet 5 of 30), Grange Hall Creek Sta. 250+00 to 220+00 The channel in the lower part of this reach is narrow and deep with a steep bottom slope. Grange Hall Creek is crossed by Irma Drive in a 60" RCP and the railroad spur embankment in a 108" CMP, which has been shored with timber to prevent collapsing. The proposed improvements for this reach consist of the following: (I) Continuation of the grass-lined channel indicated on Drawing GH-2, extending to Irma Drive. One additional check drop is proposed in this reach of the grass-lined channel. (2) An 84" RCP culvert is proposed as an additional culvert to be constructed under Irma Drive. (3) A 94" diameter steel liner plate or equivalent is to be constructed inside the existing 108" CMP under the railroad spur dike. The space between the 94" liner plate and the structurally damaged 108" CMP is to be grouted. 4. Drawing GH-4 (Sheet 6 of 30), Grange Hall Creek Sta. 220+00 to 190+00 The channel in this reach is narrow and deep with a steep channel bottom slope. An existing 108" CMP culvert under the railroad shows evidence of significant crown deflection due to the embankment load. The flood flows downstream of the Union Pacific Railroad are carried in the channel with high velocities, resulting in a narrow flood plain. The flood plain above the Union Pacific Railroad is determined by the backwater from the culvert under the railroad. The main channel for Tributary N intersects Grange Hall Creek at Station 191 +90. A portion of this reach below the rai Iroad spur dike would be inundated by the Carpenter Dam and Northglenn Dam Reservoir, if built. There are no improvements proposed for this reach and the area is to be regulated by flood plain zoning. The condition of crown deflection of the 108" CMP under the railroad does not appear to be critical to the structural integrity of the culvert. 5. Drawing GH-5 (Sheet 7 of 30), Grange Hall Creek Sta. 190+00 to 160+00 The channel in this reach is narrow and deep with a steep bottom slope. The majority of the flood flows are carried in the channel with the associated high velocities and a resulting narrow flood plain. In the lower portions of this area depicted on the drawing the channel consists of a small main channel and a relatively broad overbank area. The main channel for the NE tributary intersects Grange Hall Creek in the lower portions of the reach at approximately Station 162+ I O. There are no improvements proposed for this reach and the area is to be regulated by flood plain zoning. 6. Drawing GH-6 (Sheet 8 of 30), Grange Hall Creek Sta. 160+00 to 130+00 The channel in this reach consists of a small main channel and a broad overbank area. The flood plain varies from a broad, deep, low velocity section near Colorado Blvd. to a relatively narrow, deep section at the upper end of the reach depicted on this drawing. The main channel below Colorado Blvd. is narrow and incised with a relatively steep, longitudinal slope. The majority of the storm flows are carried in the main channel with high flow velocities. Some overbank flooding also occurs which increases the flood plain width. There are no improvements proposed for the area above Colorado Blvd. (Station 160+00 to 143+65) and the area is to be regulated by flood plain zoning. The proposed improvements to the Colorado Blvd. crossing consist of the construction of a 100' - 10'S x 6'H RCBC and a 60' extension of the existing 23.5' x 8.8' bridge (30' each side of Colorado Blvd.). The remaining portion of the reach below Colorado Blvd. to Station 130+00 is to be regulated by flood plain zoning. Engineering design work is currently underway for the extension of East 108th Avenue across Grange Hall Creek (approximately Station 135+00); however, the cost of the improvements are not contained within this report. 15