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v 1-5 Besides replacing culverts, erosion control measures and bank stabi 1 i- zation measures would be taken. A sedimentation pond would be located above the trailer court to control debris and sediment that discharges from the mountainous area above. The approxImate costs of thIs alternative iare measure,d In terms of damages and Improvement costs. In a 40..ye"ar period It Is estimated that the level of damage after culvert 'improvements ,,"ootId be reduced 20%, to 2.6 m111 Ion dollars. The costs of the Improvements and re.. moval of ImmedIate hazard structures would be approxImately 1.7 mIllion dollars, brIngIng the total cost to approxImately 4.3 mIllion dollars. Detention Pond. A modifIcation of the fmproved flood plain scheme Is the Implementat Ion of the detention pond s,cheme that was found most efficient In the hydrological portIon of this study. Two large on" stream ponds, one located above the t ra lIE,. courts and the other be 1 ow, would signIficantly reduce the damages In the trailer s,reas above U.S. 6 and some of the damages below. The damelges would be approximately 2.2,m1111on dollars. The costs of capItal construction would be approx- Imately 2.0 millIon dollars, bring the totial to 4.2 mIllIon dol'lars. Estimated OptImum Combination. The estlmelted optimum ellternatlve was determined by comparIng the other proposed alternatives on a n~ach by reach basIs. That Is, for any gIven reach. one may determine which one of the alternatIves would be morEl sult.~ble on a cost basis. There are three schemes whIch would be Implement,~d In frv'~ le1ngths of the Gulch. The fl rst sIgnIfIcant Improvement length would be the tral'ler court area of reach 2 and the upper portIon of n~ach 3. The concept here would be Implementation of strict flood plaIn zonIng. That Is, trail- ers will not be a 11 owed to move I n to vacated spaces. whether the space Is vacated by floodIng or by choIce. Private traIler court owners would be encouraged to help In a cost sharing program o.f chan- nel Improvements designed to remove the flc)od plain zoning designation. A series of sedimentatIon/debrIs ponds would also provIde detention storagE! so that peak flows would be conslcl,~rably abated. The prlvllte owners ,"ould share In the costs of channel Improvements and prl)portlon- ally share the cost of detentIon storag'~ whIch benefIts the OWl1ers. The local entItles would also share In ths"se costs, SIS well as the various culvert and debrIs storage pond cc"sts common to other plans. If the owners choose not to make lmprovemsl11ts then the flood plain zonIng would remaIn, wIth the possibilIty ,)f the entIre, flood plalr1 area becomIng eventually voId of tral1el.s. Such a plan would provIde for the safety and welfare of potentIal arid exIstIng downstream resIdents.