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,~lE!vation of approximately 5560' is built int:o the embankment to paE:s the flow from South Oak Creek when there is rur:,()ff. This culvert is assumed to be COJ11plete:,y plugged in the dan break ana lysis even though there was no evi'lence this had happEned in the past based on the site visit. The te p area of ponded watElr if the eml:ankment was full to the top would be approximately 12 acres and would hold approximately 150 acre-ft. ThE: south Oak Creek watershed (s: taller subarea upstream of the embankment) was routed through the earn breach, through the South Oak Creek channel reach to Rockvale, then routed through the Oa}; Creek channel reach to Florence whE 1'e it was comoined ,tli th thEl design hydrograph of the remaining ~art: of the watershed (larger subarea). 8-point channel cr05S sect ions for t:he two reaches werE' estimated from tapa maps. A Manning's 'n' value of 0.045 was used for oath channel reaches. In the dam breach model iC was issumed that the breach would oe 1:rapezoidal ,lith side slopes of 2 lOrizontal to }. vertical with the bottom of the breach at the inver: of the channel. The top of the embankment was set to an eleva'..ion of 5590 and a spillway elevation of 5589.9 was set even the ugh there is not "ct:ually a spi:.lway on the embankment. .A spil1way elevation and width is required in HEC-l when a dam preach model is run. It: Has also assumed in the model that t:here ;.as water held behind the embankment at an elevation of 5589.5 before the start of runoff. Several dam breach plans were run W] t:h HEC-l varying the bottom widt:h of the breach and varying the ti ne for the breach to develop. The final breach plan chosen Wi s one with a brElach botton width of 200' developing fully in 30 minutes. This scenario was run with the 100-yr 24-hr rainfall ald with the 6-hr local storm P~lP. The model was run once with the dam breach paraneters included and once with the dam breach parameters not included but all other parameters kept the same, fer both rainfall events. For the 100-yr 24-hr rainfall the peak f:ow at Florence increased by approximately 3% with the dam b:eeach included in the model and the peak flow increased by less than 1% wfen the 6-hr local storm PMP was used in the model. I , CONCLUSIONS The 100-yr discharge computed by the HEC-l model for this stud:f is 16,000 cfs, which is much lower than the 20,000 cfs repo:~ted in reference 12. Thai: study used rainfall-:cunoff rela~ionships based on storm studies for watersheds much larger than Oak creek. Al though the current study retains the unit hydrograph developed in reference 12, thE! hyetog)~aph and infi:.tration rates were based on SCS rnethodoloe,y, believ.ed to be the most appropriate relationships available for watersheds of this size, The lack of stream flow gaging station data for the foothill 10