1. EWRI Congress – May 24, 2012
Ian Paton, P.E., M. ASCE
T. Andrew Earles, Ph.D., P.E., M. ASCE
Shannon Tillack, EI
Wright Water Engineers, Inc.

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. 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. Denver

5. Burn severity has a direct
impact on the post-fire
hydrology
Burn Severity Acres % of Area
High 684 11 %
60%
Moderate 3001 49 %
Low/Unburned 2492 40 %
Total 6177 100 %

8. Slope % of Area
0 – 10 % 6%
11 – 30 % 40 %
31+ % 54 %
Steep slopes –
Promotes
high runoff rates when
understory has been
burned

9. Formation of rills, gullies

11. 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)

12. 0
Initially focused on 9 of the
drainage sub-basins with
moderate/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).

13. 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

14. Curve
Numbers:
Low, Med, High
Estimates 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.

15. Reasonableness Check: 2-Year Event
Unit 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.

16. 10-Yr, 1-Hr
Event: 1.5 in.

17. Reasonableness Check: 25-Yr Event
Unit 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.

18. Also modeled
routed flows
Route flows from
all sub-basins
together:
- 2-yr
-10-yr
- 25-yr
-100-yr

20. Pre-storm
~2-Year Event – Post-Fire Flooding

21. ~ 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

22. Washed Out Road - Ingram Gulch
Photo credits: C. Schroeder, Boulder County

23. Scour
Photo credits:
Deposition
GROUND Engineering

24. Blocked Roads Plugged Culverts
Photo credit: Boulder County

26. 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)

27. 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

28. 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

29. 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%
10.0
Combine
9.0
Watershed Recovery Curve (Blue) 40%
8.0
and 7.0
30%
Storm Probability Curve (Red)
6.0
5.0 20%
4.0
Example: 10‐Year Event
3.0 10%
2.0
Can 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

30. 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)
13.0
Qburned/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

31. 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

33. • Managed by Boulder County
Land Use Department
• April 2010 – 2 weeks
• ~ 1,960 acres
• Combination:
• Straw mulch (~ 1.5 tons/ac)
• WoodStraw (on 350 ac.)
Photo credit: Boulder County

34. Limited
number of
installations

37. Culvert
Debris Rack

38. Photo credit: Boulder County

39. • 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