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I I I II I I I I 1\ I I I I I I II I I I v. DESIG N CONSIDERATIONS AND CRITERIA The master plan presented in this report was developed using criteria set forth by the local entities and the Urban Drainage and Flood Control District. This chapter describes the design criteria used and considerations observed in arriving at the final Phase "B" plan for flood control, and which should be carried over into the final design. Channel Desi~n General - The preliminary design of the proposed channel and related improvements is based on criteria set by the Sponsors of the Study. These criteria are generally based on the Urban Storm Drainage Criteria Manual, Reference D, and on the Notice to Proceed, Appendix A, and have been adapted to fit the local needs and conditions. Listed in Table V-I are the criteria on which the hydraulic design of the Direct Flow Area 0054 storm drainage facilities improvements have been based. TABLE V-I Design Criteria G rass-iined Channels Underground Conduits Mannings 'n' Max. Permissible Vel. Min. Permissible Vel. Side Slopes Minimum Curvature Free Board .030 7.0 fps 2.0 fps 4:1 100' radius l'min. .012 2.0 fps 2 x diameter Hydraulic G .L. Below Surface Grass-Lined Channel - Where land availability permitted and where environmental and aesthetic aspects were of prime importance, a grass-lined channel has been proposed to convey flood waters of either the 2- or 100-year storm. To facilitate mowing, channel side slopes should be no steeper than 4 to 1. An exception to this criteria occurs in Reach 3 where an existing right-of-way necessitates some leniency, and three to one side slopes are used for an overflow channel. In the lowest two reaches, interception of runoff by irrigation ditches which traverse the basin and agricultural activities have eradicated all traces of the historic channel. The proposed flowline for the 2- and 100-year channel was lowered approximately 2 to 4 feet below the existing ground on centerline. This V-I minimizes channel berming and increases the efficiency of lateral and overland flow interception. Drop structures are incorporated into the reaches of grass channel to main- tain velocities at 7 feet per second or less. The improved channel is to be seeded with natural, drought resistant grasses that require only occasional mowing. Each local entity will have the option of using a grass type that would require irrigation and extensive mowing. Upon completion of grading and seeding, the channel banks may require a temporary bank stabilization material such as jute mesh. Trickle Channel - Trickle flow is considered to be the runoff from events such as ground water, snowmelt, lawn watering, and short duration low-intensity storms. This flow is approximated at 1% of the design storm flow. Subsurface drains can be used to convey trickle flows through detention facilities, thereby minimizing swampiness and making facilities suitable for park uses. Structures Various hydraulic structures are proposed to control storm water. Suitable channel transitions are to be provided as necessary at all structures and appropriate maintenance access should be provided so that each structure can be inspected and repaired regularly to assure its integrity. The following structures are proposed for use in the Direct Flow Area 0054 drainageway: 1. Concrete Box Culverts - Concrete box culverts are utilized at street cros- sings where design discharges are moderate and where box culverts may provide less expensive alternatives to bridges. Riprap is included at all culvert inlets and outlets to minimize the effects of scour. Minimum velocities are to be limited to 2 feet per second for the 2-year event to minimize sediment deposition in culvert bottoms. 2. Drop Structures - The effect of a drop structure is to lessen the channel slope, thereby reducing channel velocities to acceptable limits. Concrete drop structures, generally 2 to 3 feet in height, with sloping faces are recommended for the final phase of the 100-year channel improvements in Reaches 1 and 2. Since the initial phase is an interim measure, a more temporary type drop with salvageable materials is suggested. Such a drop could be made of sheet piling with riprap layed against the face of the structure to form a sloping 2 to 1 (horizontal to vertical) face.