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I I I I I I I I I I I I I II I I I I I feet north of Grange Hall Creek along I07th Avenue. This flood plain results from the storm runoff overtopping 1-25 due to the inadequacy of the culverts under 1-25 to pass the 100-yeor storm. The overflow runoff continues eastward down 107th Avenue to Grant Street and then southward to Grange HolI Creek. The median barriers along 1-25 couse the highway overflow storm runoff to pond to depths of 2Y2 feet deep or more behind the barriers during the 100-year storm, which would make 1-25 virtually impassable. The capacity of the existing culverts is approximately 550 cfs ( 5-year storm), which is based on 1-25 just becoming inundated. The area along the channel below Grant Street (Sta. 294+65) consists of a detention pond with a low level outlet and 0 concrete emergency spillway just west of Washington Street converging to 0 narrow, deep channel passing by the school. An existing 72" CMP crosses under Grant Street. The channel longitudinol slope is relatively steep adjacent to the school area. The flood ploin is broad with low velocity flow in the pond oreo and narrows down with high velocity flows in the deep, narrow channel. A secondary flood plain, resulting from the overtopping of 1-25 and subsequent flow down 107th Avenue, occurs along Garlond Street in the vicihity of Hulstrom School. A portion of the flows coming down Grant Street will overtop the curb at Garland Street and flow towords the school with a flow depth of less than 6 inches. The improvements shown on Drowing GH-I are the following: (I) Replacement of median barrier on 1-25 with a new barrier that wi II allow storm woter to pass through the barrier reducing the flooding problem on 1-25. (2) Improvement of the existing stilling basin east of 1-25 by grouting and repairing the gab ion structure. Also, additional concrete work is recommended to improve the intake structure for the irrigation system (see detail on Drawing D-I, Sheet 30 of 30). The low flow or trickle chonnel through the park area is to be replaced by a concrete trickle channel of the type shown on tre drawing. (3) The improvements at Grant Drive consist of adding a 78" RCP culvert (125 feet) parallel to the existing 72" CMP, constructing a concrete tronsition structure, and constructing 600 feet of 90" RCP culvert from the tronsition structure post the school area. A portion of the 100-year flood will continue to overtop Grant Drive ot a depth of less than 6 inches but will be contained in an overflow swale over the storm sewer below Grant Drive. (4) Channelization is shown on the drowing for the area through the detention pond to convey the low flows. (5) The detention pond embankment is to be raised 2 feet along with an extension of the concrete spillway walls. 2. Drawinq GH-2 (Sheet 4 of 30), Granqe Hall Creek Sta. 279+00 to 250+00 The channel in this reach is characterized by a narrow, deep main channel ond 0 broad, shallow overbank flow area. The channel longitudinal slope is also relatively steep. The flood plain consists of high velocity flows in the main channel and lower velocity flows in the overbank area. Grange Hall Creek is crossed by Marion Street in a 48" CMP, Larson Drive in a 42" CMP, and Washington Street in a 36" CMP. The improvements proposed for this reach consist of the following: (I) Replace the existing 36" CMP under Washington Street with three 4' x 8' reinforced concrete box culverts (RCBC). (2) Replace the existing 42" CMP under Larson Street with a 96" RCP culvert. (3) Reploce the existing 48" CMP under Marion Street with a 96" RCP culvert. (4) Construct a grass-lined chonnel with a paved maintenance road/bike troil and 0 concrete trickle channel (J 5 cfs capacity at a 0.5% slope). Also, along the channel reach nine 3' concrete check drop structures are proposed. Additionally, rip-rapped stilling basins are proposed at the outlets of each of the culverts under Washington, Larson, and Marion Streets. 14