Estimates of Peak Discharge for the September 2013 Floods in Boulder County, Colorado. John Moody 17 November 2015 AWRA Meeting--Denver

Transcription

1 Estimates of Peak Discharge for the September 2013 Floods in Boulder County, Colorado John Moody 17 November 2015 AWRA Meeting--Denver

2 Location Colorado Front Range Boulder County

3 Streams, rainfall and measurement sites North St. Vrain Creek above Apple Valley bridge North St. Vrain Creek above Highway 7 bridge South St. Vrain Creek below confluence with Middle St. Vrain Creek South St. Vrain Creek above confluence with Middle St. Vrain Creek Little James Creek above confluence with James Creek James Creek above Jamestown Lefthand Creek at Buckingham Park James Creek below Jamestown Lefthand Creek below Nugget Gulch Fourmile Creek above Long Gulch Long Gulch above Loretta-Linda & Loretta-Linda at flume 9-1 Fourmile Canyon Creek at 491 Wagonwheel Gap Fourmile Creek at Logan Mill Twomile Creek at 215 Linden Sugarloaf Boulder Creek at mouth of Boulder Canyon Gregory Canyon below Long Canyon & at Rest Area Bluebell Canyon above Mesa Trail crossing Bear Canyon above Bear Mtn. Drive Cumulative Rain Dark blue = 0 mm Dark red = 400 mm

4 Motivation Provide estimates of peak flood discharge caused by unusual rainfall rather than the usual snowmelt Find a relation between peak discharge and average rainfall intensity Test a sediment transport method of estimating peak discharge Compare to maximum worldwide floods

5 Methods Average rainfall intensity: Averaging interval = T c T c = time-to-concentration (NRCR, 2010) T c = L 0.8 (M+1) Y 0.5 L= longest hydrologic path (m) M= maximum potential retention (M~0) Y= average basin slope (percent)

6 Methods 21 mountain sites: Drainage area (A): km 2 Mean bed slope (S): Bed roughness (D 84 z-axis): mm Boulder Creek at mouth of Boulder Canyon A=337 km 2 S=0.027 D 84 = 450 mm

7 Methods Reach characteristics: ~ straight reaches minimal evidence of erosion & deposition ~10 stream widths long James Creek below Jamestown A= 43.7 km 2 S=0.036 D 84 = 217 mm

8 Method Used several methods to provide an ensemble average and estimate of the uncertainty. Jarrett Cowan Empirical Critical flow Slope-area

9 Sediment transport method 0.15 Smart 1984: 10.5 mm Smart 1984: 2.0 mm non-uniform Smart, 1984: 4.2 mm Smart, 1984: 4.3 mm, non-uniform Oak Creek: 2-30 mm Cao 1985 Manning's n, m -1/3 s n = 0.025q s ; R 2 =0.53 n = 0.026q s ; R 2 = Sediment transport rate per unit width, q s, kg s -1 m -1

10 Results Photo by Mark Williams

11 Peak discharge--rainfall intensity relation Unit peak discharge, m 3 s -1 km Q u = 0.26(I Tc -8.6); R 2 = Average rainfall intensity during time-to-concentration, I Tc, mm h -1

12 Comparison of methods 1000 Jarrett Q(Ensemble) = 1.18Q(Jarrett); R² = 0.99 Ensemble average discharge, m 3 s Cowan Empirical SAC Sediment transport Critical flow Q(Ensemble) = 0.92Q(Cowan); R² = 0.99 Q(Ensemble) = 0.75Q(Empirical); R² = 0.99 Q(Ensemble) = 1.29Q(SAC); R² = 0.99 Q(Ensemble) = 0.81Q(Sed.Transp); R² = 0.98 Q(Ensemble) = 1.01Q(Critical) R² = Peak discharge method, m 3 s -1

13 Comparison with maximum worldwide floods Peak discharge September floods Peak Discharge, m 3 s Bluebell Canyon World's Largest 1996 Spring Creek: Thunderstorm after wildfire 1894 Boulder Creek 1965: Jimmy Camp Creek, Fountain, CO 1965 Bijou Creek nr Wiggins,CO Other measurements for Basin area, km 2

14 Comparison with maximum worldwide floods Unit peak discharge 1000 Unit Peak Discharge, m 3 s -1 km Front Range 2013 Bluebell Canyonl World Boulder Creek: May 29-June 2; inches 1996 Spring Creek: Summer Thunderstorm after wildfire 1965 Jimmy Camp Creek, Fountain, CO 1965 Bijou Creek nr Wiggins, CO Other measurements for Basin area, km 2

15 Conclusions 1. Mean COV of Ensemble average ~ +32%. 2. Peak discharge for these floods in steep mountain stream is ~ critical flow (F~1). 3. Sediment transport may limit flow velocities such that F ~1.

16 Conclusions 4. A relation exists between unit peak discharge and average rainfall intensity. 5. Rainfall was extreme but floods were not. 6. Low flood magnitudes were caused by high accumulation rainfall pattern being oriented ~perpendicular to drainage pattern of Colorado Front Range.

17 THANK YOU

18 Critical Flow Assumption Froude number = 1 = v/(gh) 1/2 v = (gh) 1/2 v = S 1/2 h 3/2 n n = Manning s roughness parameter which changes with depth