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V. BASIS OF DESIGN The recommended alternative plan is a combination of det.ention pond storage and flood channel improvements. The design con- sists of open channels, closed conduits, street crossings, and deter.tion storage. The followin9 para'lra'~hs describe the design criteria used in this report. Qpen Channell~ Open channels were used for t:l1e Vi'ester:J.y Creek improvE,ments wherever possible because of t:heir rEllclt.i.vely low cost., lars'e capacity and potential for l~creational use and detention s.torage. Grass-lined channel@ WE,re c''losen for the follm..lng' reasons: (1) relatively low construction costs, (2) fewer ~aintenance problems, (3) g~l@s-lined channels provide a de- sirable green belt and recreational area adding siginficant social benefits. '1'he Urban Storm Drainage Crit:erie. J'1anuc.l of the Urban Dra.inage and Flood Control District Wolf; uE:ed Eli3 a 'luidE~ for designing .the graSS-lined channels. 'I'he Chanm',l CJ:oss-section n.sed was a 3 percent sloped bottom wit.11 a rectTc.gular concrete .trickle channel. The maximum side slope was :3:1 <3 horizontal to 1 vertical) for maintenance purposes. A Manninq's n value of I) - 035 was chosen for an averacre :roug:"mess coefficient for flow depths of 4 to 5 ft. .. Some of the design criteria described in the Drainaqe Criteria Hanual were not followed. 'fhe bottom widi::h .to channel depth l~atio used for Wes.terly Creek channel improvement:s is less than 1:hat t:"ecommended in the Drainage Crib:!ria Manual because Westerly Creek flows through a highly urban:iz,"d area. A lower bottom Id.dth to channel depth ratio was ",:hosen to reduce the righ1:-of- I,ay costs and to prodUCE, a better i::~HlSi tion bet,^reen channels and closed conduits at street. crossinqs. A cross-section with a sloped bottom and trickle channel as required in the manual v,as selected 1:0 prevent: channel deposition durinq low flow periods.. The trickle cha.nnel desi"n criteria described in the Drainaqe Criteria Manual could n01: be followed because the width calcuj.atedwould bEl 1:00 narrow to assure ade- quate maintenance. Therefore, ins1::EoaC of determining a width based on 0.5 percent. t.O 1 percent: of the 1.0-ye:ar peak discharge as stated in the manual, a croSS'.."BcU.on of 6-ft [minimum) wide by l~-ft (minimum) deep was selected. .Supercrit:ical flow in an opE'n channel in an urban area creates certa:..n hazards. Because of the ,;t.eE'[:ness of West.erly Creek, :s.upercritical flow could occur at. several locations wit.h existing candi tions. The open channels in thi,; report: were designed to J:.ave subcritical flow with drop sl:ruct:ure:3 located where necessary. --16..