Weller_Hydrology_Hydraulics.pdf

HYDRAULICS REPORT

Weller Trail Bridge Replacement

Roaring Fork River

DESIGN DATA SUMMARY

Table 1. Summary of water surface and proposed low chord elevations.

Summary of Hydraulic Analyses

Section No. 1 Computed WSE ( ft )

Minimum Low

Chord Elev.

2-Year Flood 100-Year Flood

Proposed Bridge 97.6 -- 95.6

SITE DESCRIPTION

The Weller bridge site is located in Pitkin County, Colorado approximately 7 miles east of Aspen in the White River National Forest at an elevation of approximately 9,300 feet.

The existing bridge is a 61-foot long steel truss frame structure with wood decking. The hydraulic capacity of the existing bridge is influenced by transient beaver dams in the low gradient reach both upstream and downstream of the crossing. In order to avoid future beaver-related channel adjustments, the replacement bridge will be moved downstream approximately 300 meters, below the geomorphic break, to where the channel is steeper and more incised.

HYDROLOGY

The design discharges for the structure were initially calculated using USGS regression equations developed for the mountain regions of Colorado (Vaill 1999). For this flood region, the predicted discharges are a function of drainage area and mean basin slope.

The drainage area of 67.4 square miles and the mean basin slope of 0.458 ft/ft were determined with the USGS StreamStats tool.

The USGS equations (Vaill 1999) for the design floods are as follows:

Q2 = 11.0 (DA)

0.633

(S + 1) 3.465

= 663 cfs

Q100 = 39.5 (DA)

0.706

(S + 1) 1.577

= 1399 cfs

Where S = mean basin slope of 0.458 ft/ft

DA = drainage area of 67.4 square miles

In order to corroborate the regression predictions, nearby stream gages were examined.

The nearest gages with similar drainage areas, aspects, geology, vegetation cover, etc. are shown below in Table 2.

Table 2. Stream flow gages near the Weller trail bridge replacement site.

Station

No.

Station Name Drainage

Area (sq.

mi.)

Years

Record

Elev.

(feet)

Precip.

(inches)

Basin

Slope

(ft/ft)

09073500 Roaring Fork River at

Aspen, CO

109.0 13 11,000 30.0 0.369

09073300* Roaring Fork River above Difficult Creek near Aspen, CO

75.8 22 11,500 29.0 0.452

09074000 Hunter Creek near

Aspen, CO

41.1 13 10,700 32.0 0.302

09078000 Fryingpan River at

Norrie, CO

90.6 31 10,500 29.0 0.403

*This gage was not used by Vaill (1999) in peak flow regressions due to influence of transbasin diversions.

Discharge estimates from the gages in Table 2 were translated to the project site using ratios of drainage areas as described by Vaill (1999):

QWeller = Qgage (DAWeller/DAgage) x

( 1 )

Where: DAWeller = 67.4 square mile drainage area

DAgage = drainage area of the stream gage in square miles

X = 0.69 for the Colorado mountain flood region

The predicted values for the Weller site are plotted below in Figure 1, along with the regression estimates and associated standard errors.

Note in Figure 1 that the predictions separate into two groups. The regression estimates

(Vaill 1999) and the predictions from the Hunter Creek and Fryingpan gages are reasonably close together and fall within the error bars of the regression (dark dashed lines). However, the gage predictions from the Roaring Fork gage at Aspen give estimated peak discharges considerably higher than the others.

In order to evaluate the validity of the Roaring Fork gage estimates, other gage data from the Roaring Fork were examined. Gage 09073300 (Roaring Fork River above Difficult

Creek near Aspen, CO) was not used in the Vaill (1999) study because of the influence of transbasin diversions just upstream. However, the data show several measurements that are supportive of the Q100 value predicted by the Roaring Fork at Aspen gage. The peak discharge at the Difficult gage was 2,350 cfs in 1985, followed by 2,310 cfs in 1995 (see

Figure 1). These data suggest that the peak flow predictions scaled from gage 09073500

(Roaring Fork River at Aspen, CO) provide the most realistic estimates.

For the following hydraulic analysis, the design discharge for the 100-year recurrence interval flood at the Weller site, as scaled from gage 09073500 (Roaring Fork River at

Aspen, CO) is

Q100 = 2,684 cfs

Figure 1. Predicted peak discharges at the Weller trail bridge site.

CHANNEL AND FLOODPLAIN ROUGHNESS VALUES

Roughness elements in the design reach are primarily boulders, which makes typical methods of back-calculated roughness from a discharge measurement difficult. Instead, channel characteristics at Weller were matched to photographs of measured reaches in

Hicks and Mason (1998). See Figure 2 in Appendix A.

Note in Figure 3 of Appendix A that as discharge increases, the Manning n values tend to stabilize as roughness elements become a smaller part of the flow depth. The lowest measured value was 0.098, which corresponds to a flow approximately 20 percent larger

1 10 100 1000

D is c h a rg e Q c fs

Recurrence Interval, RI (year)

Peak Flow Predictions - All Methods

Vaill - Regression

Regression - 1 SE

Regression + 1 SE

Gage - RF at Aspen

Gage - Hunter Crk

Gage - Fryingpan Roaring Fk at Difficult - 1985

Roaring Fk at Difficult - 1995

2350 cfs

2130 cfs than the average annual flood. Previous analyses using empirical roughness equations

(Jarrett 1984, Thorne and Zevenbergen 1985) typically show about a 10 percent decrease in roughness between the bankfull discharge and the 100-year recurrence interval flood.

As such, the value of 0.098 will be used for flows approaching bankfull and a value of

0.089 (10 percent decrease) will be used for flows near the Q100 discharge.

Model output for flows around the estimated Q2 corresponds reasonably well with field-identified channel depositional features, suggesting that estimated roughness values are appropriate.

The roughness value for the overbank area is not substantively different than that for the channel because of the lack vegetation and dominance of boulders.

PROPOSED BRIDGE HYDRAULICS

Because the reach is so irregular in shape (from large boulders) and steep, the application of steady-uniform flow assumptions, or even gradually varied flow, is problematic. So rather than attempting to model a continuous water surface slope with HEC-RAS, a single cross section with stable downstream control and no backwater effects was modeled with WinXSPRO (Hardy et al. 2005). Specific field measurements and data are shown in Appendix B.

The stream gradient through the cross section was measured at low flow between successive grade controls. The measured value was 0.085 ft/ft, and is assumed to represent the water surface slope during peak discharges.

The model output was summarized previously in Table 1. Detailed output for a series of flow depths is shown in Appendix B.

Prepared by: Mark R. Weinhold, P.E. 3/02/2010

Forest Hydrologist

White River National Forest

REFERENCES

Arcement, G.J., Jr., and Schneider, V.R. 1989. Guide for selecting Manning’s roughness coefficients for natural channels and floodplains. U.S. Geological Survey Water-Supply

Paper 2339, 38 pp.

Hardy, Thomas; Panja, Palavi; Mathias, Dean 2005. WinXSPRO, a channel cross section analyzer, User's Manual, Version 3.0. Gen. Tech. Rep. RMRS-GTR-147. Fort

Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research

Station. 95 p..

Hicks, D.M. and Mason, P.D. 1998. Roughness characteristics of New Zealand rivers.

Water Resources Publications, LLC, Englewood, CO, 329 pp.

Jarrett, R.D. 1984. Hydraulics of High-Gradient Streams. Journal of Hydraulic

Engineering, Am. Soc. Civil Engr., Vol. 110, No. 11, pp. 1519-1539.

Thorne, C.R. and Zevenbergen, L.W. 1985. Estimating Mean Velocity in Mountain

Rivers. Journal of Hydraulic Engineering, Am. Soc. Civil Engr., Vol. 111, No. 4, pp.

612-624.

Vaill, J.E. 1999. Analysis of the magnitude and frequency of floods in Colorado. U.S.

Geological Survey Water-Resources Investigations Report 99-4190, 35 pp.

APPENDIX A: CHANNEL ROUGHNESS

DETERMINATION

Figure 2. Site photos of the Weller site (left) and a matching channel (right) from

Hicks and Mason (1998).

Figure 3. Roughness values as a function of discharge from Hicks and Mason (1998).

APPENDIX B: PROPOSED BRIDGE HYDRAULICS

Figure 4. Measured cross section and modeled Q100 water surface elevation.

Cross Section Measurements:

Table 2. Measured cross section data at the proposed bridge location.

Station Elevation* Station Elevation Station Elevation

(feet) (feet) (feet) (feet) (feet) (feet)

1 101.82 29 91.17 47 88.62

4 98.58 33 90.86 49 89.26

9 96.8 36 89.76 49.5 91.17

14.5 94.72 36.5 86.53 54 92.26

17 93.13 40 85.94 58 93.63

23 91.55 42 89.35 63 96.68

26 90.78 44.5 87.75 66 99.16

*Elevations based on capped re-bar TBM; elevation assumed at 100.00 feet.

0 10 20 30 40 50 60 70

El ev at io n f ee t

Distance ( feet )

Weller Trail Bridge

Q100 Elev. = 95.6 ft

Detailed Hydraulic Model Output:

*******************************WinXSPRO*********************************

C:\Documents and Settings\mweinhold\Desktop\xsect1.out Input File: C:\Documents and Settings\mweinhold\Desktop\xsect1.sec

Run Date: 03/01/10

Analysis Procedure: Hydraulics Cross Section Number: 1

Survey Date: 02/26/10

Resistance Method: Manning's n Low Stage n 0.098

High Stage n 0.088

STAGE AREA PERIM WIDTH R DHYD SLOPE n VAVG Q SHEAR FROUDE

(ft) (sq ft) (ft) (ft) (ft) (ft) (ft/ft) (ft/s) (cfs) (psf)

6.50 87.05 42.47 34.91 2.05 2.49 0.0850 0.098 7.15 622.67 10.87 0.798

6.60 90.58 43.17 35.58 2.10 2.55 0.0850 0.098 7.29 659.94 11.13 0.804

6.70 94.17 43.87 36.25 2.15 2.60 0.0850 0.097 7.42 698.64 11.38 0.811

6.80 97.83 44.57 36.92 2.19 2.65 0.0850 0.097 7.55 738.80 11.64 0.817

6.90 101.55 45.28 37.59 2.24 2.70 0.0850 0.097 7.69 780.44 11.90 0.823

7.00 105.35 45.98 38.26 2.29 2.75 0.0850 0.097 7.82 823.59 12.15 0.830

7.10 109.21 46.68 38.94 2.34 2.80 0.0850 0.096 7.95 868.29 12.41 0.836

7.20 113.13 47.36 39.59 2.39 2.86 0.0850 0.096 8.09 914.82 12.67 0.842

7.30 117.11 47.85 40.03 2.45 2.93 0.0850 0.096 8.24 965.27 12.98 0.849

7.40 121.14 48.35 40.48 2.51 2.99 0.0850 0.095 8.40 1017.26 13.29 0.855

7.50 125.21 48.84 40.93 2.56 3.06 0.0850 0.095 8.55 1070.80 13.60 0.861

7.60 129.33 49.34 41.38 2.62 3.13 0.0850 0.095 8.71 1125.92 13.90 0.867

7.70 133.49 49.82 41.82 2.68 3.19 0.0850 0.095 8.86 1182.80 14.21 0.874

7.80 137.68 50.20 42.14 2.74 3.27 0.0850 0.094 9.03 1242.93 14.55 0.880

7.90 141.91 50.58 42.46 2.81 3.34 0.0850 0.094 9.19 1304.65 14.88 0.886

8.00 146.18 50.96 42.78 2.87 3.42 0.0850 0.094 9.36 1367.97 15.21 0.892

8.10 150.47 51.34 43.10 2.93 3.49 0.0850 0.093 9.52 1432.91 15.55 0.898

8.20 154.80 51.71 43.42 2.99 3.56 0.0850 0.093 9.69 1499.48 15.88 0.904

8.30 159.16 52.09 43.75 3.06 3.64 0.0850 0.093 9.85 1567.70 16.20 0.910

8.40 163.55 52.47 44.07 3.12 3.71 0.0850 0.093 10.01 1637.58 16.53 0.916

8.50 167.97 52.85 44.39 3.18 3.78 0.0850 0.092 10.18 1709.14 16.86 0.921

8.60 172.42 53.23 44.71 3.24 3.86 0.0850 0.092 10.34 1782.41 17.18 0.927

8.70 176.91 53.61 45.03 3.30 3.93 0.0850 0.092 10.50 1857.38 17.50 0.933

8.80 181.43 54.00 45.37 3.36 4.00 0.0850 0.091 10.66 1933.64 17.82 0.939

8.90 185.99 54.48 45.80 3.41 4.06 0.0850 0.091 10.81 2009.84 18.11 0.945

9.00 190.59 54.95 46.23 3.47 4.12 0.0850 0.091 10.95 2087.87 18.40 0.951

9.10 195.23 55.43 46.66 3.52 4.18 0.0850 0.091 11.10 2167.74 18.68 0.956

9.20 199.92 55.90 47.09 3.58 4.25 0.0850 0.090 11.25 2249.48 18.97 0.962

9.30 204.65 56.38 47.51 3.63 4.31 0.0850 0.090 11.40 2333.12 19.25 0.968

9.40 209.42 56.85 47.94 3.68 4.37 0.0850 0.090 11.55 2418.67 19.54 0.974

9.50 214.24 57.33 48.37 3.74 4.43 0.0850 0.089 11.70 2506.17 19.82 0.979

9.60 219.10 57.80 48.80 3.79 4.49 0.0850 0.089 11.85 2595.63 20.10 0.985

9.70 224.00 58.28 49.23 3.84 4.55 0.0850 0.089 12.00 2687.09 20.39 0.991 ~Q100

9.80 228.94 58.75 49.66 3.90 4.61 0.0850 0.089 12.15 2780.58 20.67 0.996

9.90 233.93 59.23 50.08 3.95 4.67 0.0850 0.088 12.29 2876.11 20.95 1.002

10.00 238.96 59.70 50.51 4.00 4.73 0.0850 0.088 12.44 2973.72 21.23 1.008