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<br />OCT-30-96 WED 14:02 <br /> <br />BRR BLD 53 DFC <br /> <br />FAX NO. 2365034 <br /> <br />P. 05 <br /> <br />Ii <br /> <br />was relerring to the duration 01 the llood/debrls wave. II so, It proVides support that the event <br />was a debrl. flow rather than a water-domlnated flood. Interestingly, he recently buUt a house <br />and hor.e shelter. in the Iow.f1ow area of on Cedar Guk:h. Mr. Pentz indicated that he (and <br />others, based on onsite Invutigatlons) have eldenalvely bulldozed the debris fans at the base of <br />the canyons. Thus, paleollood rec:onstructlona primarily were made in undisturbed sections of <br />channela up$lream from the apex of each tributary'e fan. <br /> <br />Paleoflood dischargH were 88tlmated for 13 alt.. In the Morgan area (lable 1). Not all sites <br />Ire presented In table 1; altes not included are replicate eatlmal.. having a similar discharge on <br />the same stream or on small basina upetream from aites llated. Drainage area and stream <br />gradient also were estimated (lab Ie 1) from topographk: maps, but Ire for comparative <br />purposes. Within the study area, the maximum peak discharge of 1,200 ft3Is occurred In Cedar <br />Gulch. Assuming Mr. Pentz Is correet, this was assoc:ialecl with a alorm about 1951. For the <br />1958 storm, the maximum peak discharge of 850 ft3I' oc:curred In Vence Hollow at site 1 (unit <br />disc:harge of about 530 ft3Islmi2). In 1958, a peak disc:harge of about 80 ft3Ie (as shown on <br />the original isohyetal map) was estimated for Vence Hollow about 0.5 mile upstream from site 1 <br />(table 1; the net drainage area between these sit.. la approximately 0.2 mI2). In 1958, It was <br />concluded that thl. sIte defined the approximate northern limit for the 1958 rainstorm <br />(elevation about 8,000 fl at the 1958 Vence Hollow flood site). The differenco in tho 1958 and <br />1998 flood estimall. could be explained by: 1) elther 01 the.. flood eslimates is Incorrecl (note, <br />there were 5 paleoflood siles oyer a distance of aboul 0.5 mile along Vence Hollow with sImilar <br />dlsc:harges): 2.) both estimated discharges are correct and the unIt diSC:harge was about 2,700 <br />ft3Islmi2 from rainfall runoff and some ..dlments for the 0.2 mi2 area; and 3) a debris flow <br />Inltiated on the ste.p slopes between the two sit.. and the peak discharges made in 1998 are <br />incorrect It seem. unlikely that a debris flow would hay. b.en Initiated so near the boundary of <br />the storm, but not In other similarly steep baaln. having more rainfall. It also seems unlikely <br />that other basins having more rainfall (near.r the ranch) did not hays similar or larger unit <br />discharge. That peak disc:hBrge did not increase in a downstream dIrection (actually decreasing <br />on the fan a. Is typical for debris flows) al80 suggests extremely high sediment concentralions <br />and perhaps a debris flow. Using the paleoflood estlmatea and 1958 rainfall and flood estimates, <br />the areal extent of substantial runoff (water or debris flows) la about 10 mi2, and the total <br />areal extent of runoff in 1958 is perhaps 15 to 20 mi2 (figure 3). <br /> <br />4 <br />