Laserfiche WebLink
<br />2974 <br /> <br />JARRETT AND TOMLINSON: REGIOI\AL INTERDISCIPLINARY PALEOFLOOD METIlOD <br /> <br />\ <br />.~ <br /> <br />6 <br /> <br />5 <br /> <br />Colluvtll (Ilnlilu11r) ~ <br /> <br />/ /boUlder'~~II~~t:~ltfe~ ~i <br />a :1~~i~le:~:~pe\: <br />(NI) CJ <br />............uiA.;;:i>sl... ..... <br />FB--' <) <br />? ....,0 0000 _~ <br />__ 'DOc> _- 0 <br />~o 0-00";;" q,o"<;l -C1 000 <br />oo~PO~O~~oO &.o.} <br />oooOou:;''tYqooooo <br />Pleistocene-outwash gravels <br /> <br /> <br />E 4 <br />" <br />.2 <br />.. <br />> <br />.. 3 <br />w <br /> <br /> <br />2 <br /> <br />1 <br />o <br /> <br />30 <br /> <br />40 <br /> <br />10 <br /> <br />20 <br />Stationing, m <br /> <br />Figure 7. Graph of channel cross section for Eikhead Creek <br />downstream from North Fork Elkhead Creek shown in Figure 6. <br /> <br />During on-site visits, peak discharge using 1995 HWMs also <br />was estimated for seven sites at streamflow-.gaging stations <br />(Table 2, type is "gage"). These estimates using the critical- <br />depth and slope-conveyance methods were compared to the <br />peak discharge for 1995 obtained from gage records to help <br />assess the accuracy of paleodischarge estimation methods. The <br />estimated discharges generally are within about 10% of the <br />gaged discharge (Table 2, "Q" and "Q"go Difference" col- <br />umns) thus showing that the methods used to estimate paleo- <br />flood discharge are reliable. However, additi.)nal sources of <br />uncertainties (e.g., channel change) are more difficult to quan- <br />tify. Attempts to estimate other uncertainties i.n discharge are <br />reflected in terms of discharge uncertainty (Table 2, "Q, %" <br />column) and age (Table 2, age). <br />Paleoflood estimates in bedrock channels generally were <br />assigned an uncertainty of 25%, whereas alluvial channels were <br />assigned an uncertainty of 30%. Paleoflood estimates in allu. <br />vial channels with minor arroyo development, though the age <br />of NI surfaces are very old, were assigned an age of 100 years <br />to account for historical arroyo development. Additional con- <br />fidence in paleoflood estimates is exhibited when multiple sites <br />are used and results are similar. For example, for Elkhead <br />Creek downstream from Elkhead Reservoir, paleoflood esti- <br />mates ranged from 127 m' s'[ to 135 m' s'[ (sites 47 and 48 <br />in Table 2), a difference of about 3%. Simihrly, paleoflood <br />estimates along Elkhead Creek upstream from the reservoir <br />increase consistently from 79 mJ S-l to 135 m3 S-l with in- <br />creasing drainage area. <br />Streams in northwestern Colorado have few coarse flood <br />deposits on the floodplain. Where present, the deposits are <br />either associated with the record snowmelt flooding in 1984, or <br />the deposits are very old (Table 2). The fact that streams in <br />northwestern Colorado have no substantial paleoflood evi- <br />dence is important, because it indicates the lack of substantial <br />flooding in Elkhead Creek basin is not due to chance. The <br />paleoflood data then were used to help define the regional <br />maximum flooding and for flood-frequency analyses in north- <br />western Colorado. <br /> <br />5.2. Regional Analyses of :vIaximum Rainfall and Flood Data <br /> <br />5.2.1. Maximum rainfall. A relation betNeen maximum <br />24-hour rainfall and elevation for the study area in northwest- <br />ern Colorado is presented in Figure 8; this rdation was con- <br />structed from documented rainstorms from about 1900 <br /> <br />through 1990 [Jam/!, 1990bJ and was updated through 1997 <br />with recent data [McKee and Doesken, 1997]. Although there <br />has been an extensive program for documenting extreme rain- <br />storms in Colorado, there have been few intense flood- <br />producing rainstorms documented in northwestern Colorado <br />(triangles on Figure 8). The maximum 24-hour rainfall data for <br />181 stations and bucket surveys in southwestern Colorado also <br />are shown on Figure 8 for comparison to help define the <br />northwestern Colorado region. The maximum 24~hour amount <br />for northwestern Colorado is 82 mm (Meeker). Maximum <br />monthly values for northwestern Colorado only slightly exceed <br />record maximum 24-hour amounts for southwestern Colorado <br />(Figure 8), dramatic evidence of large relative difference in <br />flood-producing rainfall from northwestern to southwestern <br />Colorado. The maximum rainfall amount for southwestern <br />Colorado is about 150 mm in a few hours for Sweetwater <br />Creek (1976) and for Dove Creek in 24 hours (1972) [McKee <br />and Doesken, 1997J; these areas are influenced by the flow of <br />moist air from the southwest [Collins el aI., 1991J. Maximum <br />24-hour rainfall in southwestern Colorado is somewhat larger <br />than in northwestern Colorado but is substantially less than the <br />maximum 6-hour rainfall amounts of up to 610 mm in eastern <br />Colorado. Of particular interest in western Colorado, maxi. <br />mum 24-hour precipitation amounts fell as snow and are pre. <br />sented as snow-water equivalent (SWE) (Figure 8, open <br />squares). <br />A subjective, indirect indicator of the occurrence of intense <br />rainfall and associated flooding is the development of rill <br />(light) and or gully (deep) erosion on hillslopes [JarTert, 1990b; <br />Ja"ett and Browning, 1999]. Generally, erosion potential in- <br />creases as slope steepens; steeper slopes usually require com- <br />paratively small amounts of rainfall before substantial erosion <br />occurs, although erosiveness also depends on the type of soil, <br /> <br /> . NW co 24-hr <br />24-hr PMP . NW co month <br /> . SWco 24-hr <br /> , W CO 24-hr SWE <br /> -10D-yr,24-hr <br /> ......1D-yr.24-hr <br /> 1976 Sweetwater <br /> 1/ Dove Creek <br />6-hr PMP <br /> . . <br /> . , , <br /> OOOOC>"'Boo~gow ~ , <br /> . . .~~-at! . .,. ~~,.......... , <br /> ~.~fJ'.., i.; o:lb eo , <br /> . . . <br /> <br />600 <br /> <br /> SOD <br /> 400 <br />E <br />E <br /> 300 <br />~ <br />c <br />.. <br />0: 200 <br /> 100 <br /> <br />o <br />1000 <br /> <br />2000 3000 <br />Elevation. m <br /> <br />4000 <br /> <br />Figure 8. Ma-amum 24-hour and maximum monthly precip- <br />itation for northwestern (NW) Colorado and maximum 24- <br />hour precipitatioo for southwestern (SW) Colorado [JarTell, <br />1987, 1990b; McKee and Doesken, 1997J. Two of the largest <br />southwestern Colorado rainstorms (Sweetwater Creek and <br />Dove Creek) are noted. It is important to note that numerous <br />large snowstorms reported as snow-water equivalent (SWE) <br />account for some of the largest 24-hour precipitation amounts <br />in all of western (W) Colorado. The lO-year and 100-year, <br />24-hour duration rainfall amounts [Miller el al., 1973J are <br />shown to place contemporary rainfall data into a frequency <br />context. The 6.hour and 24-hour duration probable maximum <br />precipitation (PMP) values [Hansen et aI., 1977] are also shown <br />for comparison. <br />