1. Copyright [insert date set by system] by [CH2M HILL entity] Company Confidential Hydrologic Evaluation of the Little Thompson River Phase 2: Little Thompson River above Big Thompson River James Woidt, PE / CH2M HILL Cory Hooper, PE / CH2M HILL April 8, 2015

2. Presentation Overview Phase 2 Objectives Overview of Study Approach Overview of Study Area Data Collection Flood Frequency Analysis Model Development and Calibration Model Results Conclusions Q & A

3. Phase 2 Objectives Extend Phase I study to confluence with the Big Thompson River –Estimate peak discharges from September 2013 –Prepare updated flood-frequency analyses which include estimates of September 2013 peak discharge –Extend rainfall-runoff model to confluence with Big Thompson River –Calibrate rainfall-runoff model to 2013 event –Use rainfall-runoff model to estimate predictive peak discharges based on NOAA / NRCS design storms –Assess recurrence interval of September 2013 event along Little Thompson River using predictive hydrologic model

4. Study Approach Foundation of approach is calibrated rainfall-runoff model –US Army Corps of Engineers’ HEC-HMS v3.5 used –Model provides peak discharges at multiple locations along Little Thompson River mainstem –Model predicts runoff volumes and hydrographs –Model reflects relative differences and timing of flows –Model is able to be calibrated: 2013 rainfall calibrated to ground rain gages 2013 peak discharges estimated in multiple locations Time-of-peak discharge available at I-25

5. Project Area Phase 1Phase 2 Berthoud Milliken Johnstown

6. Previous Studies Previous Estimates of 1 Percent Annual Chance Peak Discharge Location Drainage Area (mi 2 ) Estimated 1 Percent Annual Chance Peak Discharge USACE, 1974 USACE, 1977 Ayres Associates, 2009 CDOT, 2011 FEMA Effective Hydrology Little Thompson River at LTCANYO Gage 100N/E 6,962 a N/E Little Thompson River at South County Line Road 132N/E7,2009,50011,305 a 7,200 b 9,500 c Little Thompson River at Interstate 25 166N/E 14,728N/E Little Thompson River upstream of confluence with Big Thompson River 1964,800N/E 4,800 b a While part of the hydrologic model, peak discharge estimates at this location was not a focus of the study b Per Preliminary Weld County FIS (FEMA, 2013) c Per Larimer County FIS (FEMA, 2013)

7. Previous Studies CDWR Dam Safety Branch, 2014 –Study to evaluate failure of five water supply dams –Hydrologic model developed for watershed above X-Bar 7 Ranch –Calibrated to September 2013 event –Determined peak discharge of September 2013 event was caused by rainfall-runoff processes, not dam failure –CH2M HILL and CDWR collaborated and provide review of the other entity’s study report –Comparison of modeled peak discharges provided on subsequent slides

8. Phase 2 Objectives Extend Phase I study to confluence with the Big Thompson River –Estimate peak discharges from September 2013 –Prepare updated flood-frequency analyses which include estimates of September 2013 peak discharge –Extend rainfall-runoff model to confluence with Big Thompson River –Calibrate rainfall-runoff model to 2013 event –Use rainfall-runoff model to estimate predictive peak discharges based on NOAA / NRCS design storms –Assess recurrence interval of September 2013 event along Little Thompson River using predictive hydrologic model

9. Data Collection – Peak Discharge Estimates Bob Jarrett and Dana McGlone Critical Depth Method: High water measured at drops, weirs, and long reaches with bed slopes greater than 0.5%; critical depth assumed to estimate discharge

10. Data Collection – Peak Discharge Estimates Bridge Hydraulics Method: High water marks, approach cross-sections, and bridge information surveyed to use HEC-RAS to back-calculate peak discharge

11. Data Collection – Peak Discharge Estimates 7,800 cfs 15,700 cfs 13,400 cfs 18,000 cfs 15,700 cfs; observed peak afternoon of 9/12 Courtesy of Jarrett, In Press URS, 2015

12. Phase 2 Objectives Extend Phase I study to confluence with the Big Thompson River –Estimate peak discharges from September 2013 –Prepare updated flood-frequency analyses which include estimates of September 2013 peak discharge –Extend rainfall-runoff model to confluence with Big Thompson River –Calibrate rainfall-runoff model to 2013 event –Use rainfall-runoff model to estimate predictive peak discharges based on NOAA / NRCS design storms –Assess recurrence interval of September 2013 event along Little Thompson River using predictive hydrologic model

13. Flood Frequency Analysis Little Thompson River at Canyon Mouth near Berthoud –37 total years of record (17 years of 1929 to 1961; 1993 to 2013) –Bulletin 17B methodology used with Station Skew only Exceedance Recurrence Interval (years) Canyon Mouth near Berthoud (cfs) 2306 51,119 102,204 507,229 10010,992 50025,656 Courtesy of Ayres Associates, 2014

14. Phase 2 Objectives Extend Phase I study to confluence with the Big Thompson River –Estimate peak discharges from September 2013 –Prepare updated flood-frequency analyses which include estimates of September 2013 peak discharge –Extend rainfall-runoff model to confluence with Big Thompson River –Calibrate rainfall-runoff model to 2013 event –Use rainfall-runoff model to estimate predictive peak discharges based on NOAA / NRCS design storms –Assess recurrence interval of September 2013 event along Little Thompson River using predictive hydrologic model

15. Subbasin, stream, and flow path delineation – via 40-foot USGS Topographic Maps Rainfall-Runoff Model Development

16. Infiltration Losses – NRCS Curve Number (CN) methodology with USGS National Land Cover Dataset and USDA Soil Maps used to determine CN based on TR-55 USDA, 2013

17. Rainfall-Runoff Model Development Infiltration Losses USGS, 2006

18. Rainfall-Runoff Model Development

19. Phase 2 Objectives Extend Phase I study to confluence with the Big Thompson River –Estimate peak discharges from September 2013 –Prepare updated flood-frequency analyses which include estimates of September 2013 peak discharge –Extend rainfall-runoff model to confluence with Big Thompson River –Calibrate rainfall-runoff model to 2013 event –Use rainfall-runoff model to estimate predictive peak discharges based on NOAA / NRCS design storms –Assess recurrence interval of September 2013 event along Little Thompson River using predictive hydrologic model

20. Rainfall-Runoff Model Development Rainfall Analysis AWA, 2014 Rainfall Calibration Storm Precipitation Analysis System (SPAS) – NEXRAD calibrated to ground rain gages. 10 days of 5-minute data on 1-km grid Average data at centroid of each subbasin

21. Rainfall-Runoff Model Development Rainfall Analysis

22. Model Calibration Systematic Peak Discharge Calibration –Peak Discharges estimated at different locations throughout the study watersheds –Goal was to obtain the best fit to the majority of peak discharge estimations –In some cases debris flows and landslides resulted in peak discharge surges (USGS 2013) that were higher in magnitude than the rainfall / runoff model could produce

23. Calibration Parameters Calibrated Model Curve Number Snyder’s Peaking Coefficient / Lag Time Manning’s N

24. Calibration Parameters Manning’s N Value –No Effect on Runoff Volume –Minimal Effect on Peak Discharge and Timing Snyder’s Peaking Factor and Lag Time –No Effect on Runoff Volume –Moderate Effect on Peak Discharge –Greater Effect on Time-of-Peak Location Calibration Data Source Observed Time of Peak Discharge Modeled Time of Peak Discharge Little Thompson River at Interstate 25CDOT, In Press9/12 Afternoon9/12 2:20 p.m.

25. Calibration Parameters Curve Numbers –Affected Runoff Volume –Significant Effect on Peak Discharge –Negligible Effect on Time-of-Peak Calibrated Model Manning’s N Snyder’s Peaking Coefficient / Lag Time Curve Number

26. Curve Number Calibration

27. Land Cover Classifications / Aerial Imagery USGS Land CoverTR-55 Classification Phase 2 Plains Phase 2 Mountains – Little Thompson Phase 2 Mountains – North Fork 11 - Open WaterOpen WaterFair Poor 12, 41, 42, 43 - Deciduous ForestOak- AspenFair Poor 21- Developed Open Space Developed Open Space, 2 Acre LotsFair Poor 22 - Developed, Low Intensity Developed Medium Intensity, 1 Acre LotsFair Poor 23 - Developed, Medium Intensity Developed Medium Intensity, 1/4 Acre LotsFair Poor 24 – Developed, High Intensity Developed High Intensity, 1/8 Acre LotsFair Poor 31 - Barren LandBarren LandFair Poor 52 – Shrub/BrushShrub, BrushFair Poor 71, 72, 81 - Grassland/PastureGrassland, pastureFair Poor 82- Row CropsCrops, Row CropsFair Poor 90, 95 - Woody Wetlands, Herbaceous Woody Wetlands, HerbaceousFair Poor

28. Calibration Results Location Drainage Area (square miles) Observed Peak Discharge (cfs) Modeled Peak Discharge (cfs) % Difference Little Thompson River Downstream of Confluence with West Fork Little Thompson River 43.27,800 a 9,28019% Little Thompson River at X Bar 7 Ranch 81.815,73114,343-9% Little Thompson River at South County Line Road 131.713,40015,47916% Little Thompson River at Interstate 25 164.115,70015,173-3% Little Thompson River at County Road 17 184.718,000 b 14,820-18% a - This flow was inaccessible and the observed peak discharge was estimated based on observations along similar, adjacent watersheds. b – Bridge overtopped, (URS, 2015)

29. Comparison to CDWR, 2014 modeling results Calibration Results Location Observed Peak Discharge (cfs) CH2M HILL, 2015 (cfs) CDWR, 2014 Little Thompson River at Pinewood Springs 14,600 a 9,400 10,190 Little Thompson River at X Bar 7 Ranch 15,731 b 14,300 15,999 a – NRCS, 2013 b – CDWR, 2014

30. Phase 2 Objectives Extend Phase I study to confluence with the Big Thompson River –Estimate peak discharges from September 2013 –Prepare updated flood-frequency analyses which include estimates of September 2013 peak discharge –Extend rainfall-runoff model to confluence with Big Thompson River –Calibrate rainfall-runoff model to 2013 event –Use rainfall-runoff model to estimate predictive peak discharges based on NOAA / NRCS design storms –Assess recurrence interval of September 2013 event along Little Thompson using predictive hydrologic model

31. Predictive Model Implementation Used calibrated rainfall-runoff model to predict peak discharges for the 10, 4, 2, 1, and 0.2 percent annual chance events –Predictive model developed by “resetting” CN to AMCII to be consistent with Colorado Floodplain and Stormwater Criteria Manual Other inputs for predictive model: NRCS 24-hr Type II Hyetograph

32. Predictive Model Implementation Site-specific DARF

33. Curve Number Calibration Estimate of the Recurrence Interval of the September 2013 Event Predictive Annual Chance Peak Discharge (cfs) Location Observed Peak Discharge (cfs)10 percent4 percent2 percent1 percent0.2 percent Estimated Recurrence Interval (yr) Little Thompson River Downstream of Confluence with West Fork Little Thompson River8,9556481,3652,2433,4187,504>500 Little Thompson River at US 369,0566511,3762,2643,4557,600>500 Little Thompson River at X Bar 7 Ranch 15,7312,3104,5006,97010,20020,700100 to 500 Little Thompson River at LTCANYO Gage 15,5002,7605,3808,33012,10024,700100 to 500 Little Thompson River at South County Line Road 13,4003,6506,94010,60015,30030,80050 to 100 Little Thompson River at Interstate 25 15,7004,1407,09010,90016,00033,500100 Little Thompson River at County Road 17 18,0004,4807,15010,70015,70032,100100 to 500 Little Thompson River Upstream of Confluence with Big Thompson 14,7004,4507,13010,50015,40031,400100 Note: Italics denotes from the Little Thompson River [Phase 1] Hydrologic Analysis (CH2M HILL, 2014).

34. Conclusion

36. Predictive rainfall-runoff model recommended as best model / method to estimate Little Thompson high-flow hydrology The September 2013 flood on the Little Thompson was… –Greater than a 500-year flood above US 36 –Between a 100- and 500-year flood through the Canyon –On average, a 100-year flood across the plains

37. Questions

38. Conclusion