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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />u. bridges and Culverts <br />The vari ous cross i ngs of the st udy st reams are descri bed in <br />the Specific Reach Description Section of this report, and also are <br />s ummaY'i zed in the accompanyi ng tab I es. <br />All crossings have been rated for capacity and overflow, wf1ere <br />appropriate. Ratings for bridge crossings which have no substructure <br /> <br />or piers within the channel section are based on the slope-area method <br />of calculation, using either a high 'n' val ue or red uced sect ions area <br />( or combination thereof) to reflect the debr'j s blockage potential. <br /> Box culverts and pipe crossings have been rated to determi ne <br /> <br />. <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />whether inlet or outlet control prevails. Effect of headwall sand <br />wingwalls has been included in the assessment of hydt"aulic efficiency. <br />The HEC-13 design tables have been used for box culvert ratings. <br />Various standard rating tables incorporating the effect of entrance <br />conditions have been used to determine pipe capacitiE's (see <br />References 38, 39, 40). Impact of debris blockage potential is <br />i nc 1 uded, as appropri ate, by reduci ng ent ranee area. <br />Overflow has been determined based on the limiting sections <br />defined from the mapping provided, and calculated for weir flow. The <br />weIr coefficient utilized is 3.1, reflecting the general gradual <br />approach condition. The 3.1 weir coefficient also reflects flow in the <br />non,supercritical flow regime. <br />The treatment of the small i rr'i gat i on dam on Mont rose Arroyo near <br />the County Road 6700 alignment deserves mention. The reservoir embank- <br />ment is of earthfill construction and will 'likely wash out at the 50- <br />year frequency storm level. Due to the relative sma'll size of this <br />reservoir, the number of stream crossings before urbanization is <br />encountered, and the Ii ke ly gradua I fa il ure mode, th,e effect of any <br />surge in water caused by a breach of the 12mbankment 'I'/i 11 be damped out <br />within a short distance. Therefore, the mapping sholrls the reservoir as <br />remaining in place, representing the conservative situation for defini- <br />tion of flood plain event. <br /> <br /> <br />- <br />