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I I I I I I I I I I I I I II I I I I I representing obout I % of the 100-year flood peaks, were used for design purposes. The channel bottom is to be sloped at I to 2 percent toward the trickle chonnel. Check drops have been designed with an apron and local side slope protection to prevent the storm flows from undercutting the check drop. The check drop structures ore to be constructed with concrete. 2. Clased Conduits Street crossing culverts have been designed with headwalls at the inlet and outlet as well as local slope protection such as rip-rap or revet mattress to prevent local erosion and to increase culvert efficiency. Where high outlet velocities occur rip-rap stilling basins have been recommended. 3. Spillways The various spillways were designed to pass the 100-yeor peak flow for future conditions with improvements with essentiolly zero freeboard as presently occepted by the State Engineer's Office for minor structure rated reservoirs. The minor structure rating should be verified during final design. An energy dissipating pool is required at the spillwoy outlets. The upstream faces of the dams, where improvements are proposed, ore to be rip-rapped to minimize wave action erosion. In some cases outlet works are also recommended as part of the improvements. 4. Detention Facilities The existing detention facilities were designed to pass the 100-yeor storm runoff and improvements were recommended on this basis. Co Operations and Maintenonce One of the mojor considerations given to the design of the improvements was the operotion and maintenance aspect of the improvements. Several factors were considered during the selection and design of the alternative plan, such as: erosion control, debris removal, structure repair, chonnel shooling, grass reseeding, bank stabilization, ond drainageway access. The design of the channel minimizes erosion by reducing the channel slope and providing lined channel sections and drop structures for steeper channel portions. A maintenance access road has been included in the design to focilitate debris removal, structure repair, and regular maintenance work. The channel bottoms hove been designed to slope to the invert to minimize shoaling. D. Description of Master Plan Drawinqs The Phase B report contains the II" x 17", I" = 100' (reduced) scale master plan drawings for the Grange HolI Creek basin, including the four tributaries in the major drainageway study area. The drawings present preliminary and conceptuol engineering of the alternative improvement plan selected as a result of the Phase A analysis, but are not suitable for construction purposes. The drawings show the plan and profile of the future development 100-yeor flood plain with and without the proposed improvements, as well as the existing and proposed drainage facilities. Typical existing and proposed channel sections are also shown. Referenced sections are illustrated for the purpose of defining the channel floodway for the reach in question (see Table 111-5, Floodwoy Data). Sheet I of 30 presents the index of the drawings, and Sheet 2 of 30 presents the drainage basins used for the onalysis. A discussion of each of the remaining drawings is presented herein. I. Drawinq GH-I (Sheet 3 of 30), Granqe Hall Creek Sta. 309+60 to 279+00 The channel in this reach consists of a grass-lined channel with check drops (Grange Hall Creek Park). The channel crosses 1-25 by woy of a 42" RCP from the north side of the shopping center and a 48" RCP and a 60" RCP from the south side of the shopping center. The outlet structure for the 48" and 60" RCP is utilized both as a stilling basin and as a sump for the pumping of water for the sprinkler irrigation of the park area. The flood plain in this area consists of two parts. The first part is along the pork area, which is 0 norrow, low velocity flood plain. The second part is approximately I, I 00 13