Ewri conference presentation 052412 a

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Ewri conference presentation 052412 a

  1. 1. EWRI Congress – May 24, 2012 Ian Paton, P.E., M. ASCET. Andrew Earles, Ph.D., P.E., M. ASCE Shannon Tillack, EI Wright Water Engineers, Inc.
  2. 2. • Background • Fourmile Canyon Fire • Watershed Condition Post-Fire• Runoff Conditions - Post-Fire • Model Predictions • Model vs. Measured (2011) • Flooding/Erosion Impacts• Assessment of Future Risk • Probability of Storms and Runoff Response• Mitigation Strategies• Discussion
  3. 3. Fourmile Canyon fire burned from Sept. 6 - 13, 2010 Boulder• Approx. 6181 acres (~ 10 sq. mi)• Destroyed 169 homes• Fire suppression & emergency mgmt: $ 14.1 M• Insurance claims: ~ $ 217 M (most expensive wildfire in Colo. history, prior to High Park fire)
  4. 4. Denver
  5. 5. Burn severity has a direct impact on the post-fire hydrologyBurn Severity Acres % of AreaHigh 684 11 % 60%Moderate 3001 49 %Low/Unburned 2492 40 %Total 6177 100 %
  6. 6. Slope % of Area 0 – 10 % 6%11 – 30 % 40 % 31+ % 54 % Steep slopes – Promotes high runoff rates when understory has been burned
  7. 7. Formation of rills, gullies
  8. 8. Storm Event Depth (inches) (1) Probability of Occurrence in Any Single Year 2-Year, 1-Hr 0.9 ” 50% (1/2 = 50%)10-Year, 1-Hr 1.5 ” 10% (1/10 = 10%)25-Year, 1-Hr 1.7 ” 4% (1/25 = 4%)100-Year, 1-Hr 2.4 ” 1% (1/100 = 1%) Focused on short duration (1-Hr), high intensity storms Note: (1) Storm depths based on NOAA Atlas, except 2-hour event (used depth from USGS study)
  9. 9. 0Initially focused on 9 of thedrainage sub-basins withmoderate/high burn severity 7 3 10 11 18 12 23 16 All sub- basins are less than 0.5 sq. mile. (Except Basin 23: ~0.7 sq. miles).
  10. 10. Model Used: HEC-HMS Curve Number Loss Method Curve Numbers for Burned Areas - Based on Values from USFS Literature - Moderate Burn Intensity: CN = 89 - Severe Burn Intensity: CN = 96 - Anticipated Condition: CN = 92 (Compare with Unburned: CN = 70 – 71) Topographic data: Boulder County GIS
  11. 11. Curve Numbers:Low, Med, HighEstimates Moderate burn intensity: 89 High burn intensity: 96 Note: If unburned forest: Anticipated 2-yr event generates very condition: 92 little surface runoff 2-Yr, 1-Hr Event: 0.9 in.
  12. 12. Reasonableness Check: 2-Year EventUnit Rate of Runoff (cfs/acre)Compare with: Pajarito Canyon burn area~ 2-yr storm (0.75 in. /45 minutes)1.1 cfs/acre 2-Yr, 1-Hr Event: 0.9 in.
  13. 13. 10-Yr, 1-HrEvent: 1.5 in.
  14. 14. Reasonableness Check: 25-Yr EventUnit Rate of Runoff (cfs/acre)Compare With: Buffalo Ck burn area ~25 yr.+ storm (2 in. /1 hr)3.1 cfs/acre 25-Yr, 1-Hr Event: 1.7in.
  15. 15. Also modeledrouted flowsRoute flows fromall sub-basinstogether:- 2-yr-10-yr- 25-yr-100-yr
  16. 16. Pre-storm ~2-Year Event – Post-Fire Flooding
  17. 17. ~ 2-Year Event (7/13/11)Peak Flow Rate at Fourmile Creek and Boulder Creek:- Model-Estimated Peak Flow Rate: 670 cfs- Actual Measured Peak Flow Rate: 770 cfs
  18. 18. Washed Out Road - Ingram GulchPhoto credits: C. Schroeder, Boulder County
  19. 19. ScourPhoto credits: DepositionGROUND Engineering
  20. 20. Blocked Roads Plugged CulvertsPhoto credit: Boulder County
  21. 21. Storm Event (1) Depth (inches) (2) Probability of Occurrence in Any Single Year 2-Year, 1-Hr 0.9 ” 50% (1/2 = 50%) 10-Year, 1-Hr 1.5 ” 10% (1/10 = 10%) 25-Year, 1-Hr 1.7 ” 4% (1/25 = 4%)100-Year, 1-Hr 2.4 ” 1% (1/100 = 1%) Notes: (1) 1-hour duration storms evaluated because high intensity results in high peak runoff rates (2) Storm depths based on NOAA Atlas, except 2-hour event (used depth from USGS study)
  22. 22. Probability of Storm Events Occurring  Example:  2‐Yr Event Probability During Next 10 Years 100% 90% Probability  Curve 80% Near certainty that a 2‐Year Event will occur  during the next 10 years  70% (10 years – anticipated time needed for the watershed  60% to substantially “recover” hydrologically)Probability 50% 75% probability of a 2‐year event within 2 years  40% following the fire 30% 50% probability  20% of a 2‐year event  Similar curves can be generated for  within 1 year  storm events with other return  10% following the  frequencies (10‐yr, 25‐yr, etc.) 0% fire 0 1 2 3 4 5 6 7 8 9 10 Years After Fire
  23. 23. Conceptual Hydrologic Recovery Following Wildfire  Moderate to Severe Burn Intensity 20.0 19.0 “Increases in peak flows can be  18.0 expected to continue for 2 – 3 years  17.0 after the fire and then begin to  16.0 15.0 reduce toward pre‐fire levels.” Ratio: 14.0 13.0 ‐ FEST Report (USFS/BLM)Qburned/Qpre Peak flow rate 12.0 (post-fire) 11.0 -------------------- 10.0 Peak flow rate 9.0 (pre-fire) 8.0 (Runoff ratios 7.0 for 10-year 6.0 storm event) 5.0 4.0 3.0 2.0 1.0 0 1 2 3 4 5 6 7 8 9 10 Years After Fire
  24. 24. 10‐Year Event Conceptual Hydrologic Recovery  Following Wildfire‐‐Moderate to Severe Burn Intensity 20.0 100% Probability of At Least One 10‐year Event During Time Period 19.0 Ratio: Probability of at 18.0 least one 10-year 90% 17.0 Peak flow rate event occurring 16.0 (burned) during 10-year 80% -------------------- time period. 15.0 Peak flow rate 70% 14.0 (unburned) 13.0 Qburned/Qpre 60% 12.0 11.0 50% 10.0Combine  9.0Watershed Recovery Curve (Blue) 40% 8.0and  7.0 30%Storm Probability Curve (Red) 6.0 5.0 20% 4.0Example:  10‐Year Event 3.0 10% 2.0Can assist land managers with decision‐ 1.0 0%making regarding mitigation measures 4 0 1 2 3 5 6 7 8 9 10 Years After Fire
  25. 25. 10‐Year Event Conceptual Hydrologic Recovery  Following Wildfire‐‐Moderate to Severe Burn Intensity 20.0 100% Probability of At Least One 10‐year Event During Time Period 19.0 Ratio: Probability of at 18.0 least one 10-year 90% 17.0 Peak flow rate event occurring 16.0 (burned) during 10-year 80% 15.0 Example: -------------------- time period. 14.0 10 x increase in  Peak flow rate 70% peak flow  (unburned) 13.0Qburned/Qpre compared to pre‐ 60% 12.0 11.0 fire (for 10‐yr event) 50% 10.0 9.0 40% 8.0 7.0 30% 6.0 If the 10‐year event  5.0 20% (1.5”) occurs approx. 4.0 4 to 5 years after fire 3.0 10% 2.0 1.0 0% 0 1 2 3 4 5 6 7 8 9 10 Years After Fire
  26. 26. 10‐Year Event Conceptual Hydrologic Recovery  Following Wildfire‐‐Moderate to Severe Burn Intensity 20.0 100% Probability of At Least One 10‐year Event During Time Period 19.0 Ratio: Probability of at 18.0 least one 10-year 90% 17.0 Peak flow rate event occurring 16.0 (burned) during time 80% Example: -------------------- period. 15.0 10 x increase in  Peak flow rate 70% 14.0 (unburned) 13.0 peak flow Qburned/Qpre 12.0 compared to pre‐ 60% 11.0 fire (for 10‐yr event) 50% 10.0 9.0 40% 8.0 Which has roughly a  7.0 30% 6.0 40 % probability  If the 10‐year event  5.0 within 4 to 5 years  (1.5”) occurs approx. 20% 4.0 post‐fire 4 to 5 years after fire 3.0 10% 2.0 1.0 0% 0 1 2 3 4 5 6 7 8 9 10 Years After Fire
  27. 27. • Managed by Boulder CountyLand Use Department• April 2010 – 2 weeks• ~ 1,960 acres• Combination: • Straw mulch (~ 1.5 tons/ac) • WoodStraw (on 350 ac.) Photo credit: Boulder County
  28. 28. Limitednumber ofinstallations
  29. 29. Culvert Debris Rack
  30. 30. Photo credit: Boulder County
  31. 31. • Post-Fire Change in Hydrology – Substantial • Pre-fire – 2-yr event - virtually no runoff • Post-fire – 2- yr event – damaging flooding/erosion• Land managers must consider hydrologic recovery • First 2 – 3 years post-fire – typically highest flow rates • Consider watershed recovery and probability of storm occurrence together (overlay curves) • Use probability of impacts to assess need for mitigation measures

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