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.

• 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

Boulder

Fourmile Canyon fire burned from Sept. 6 - 13, 2010

• 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)

Denver

Burn Severity Acres % of AreaHigh 684 11 %Moderate 3001 49 %Low/Unburned 2492 40 %Total 6177 100 %

60%

Burn severity has a direct impact on the post-fire hydrology

Slope % of Area0 – 10 % 6 %11 – 30 % 40 %

31+ % 54 %

Steep slopes –

Promotes high runoff rates when understory has been

burned

Formation of rills, gullies

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%)

Note:(1) Storm depths based on NOAA Atlas, except 2-hour event (used depth from USGS study)

Focused on short duration (1-Hr), high intensity storms

0

3

10121811

7

16 23

Initially focused on 9 of the drainage sub-basins with moderate/high burn severity

All sub- basins are less than 0.5 sq. mile. (Except Basin 23: ~0.7 sq. miles).

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

2-Yr, 1-Hr Event: 0.9 in.

Curve Numbers:

Moderate burn intensity: 89

High burn intensity: 96

Anticipated condition: 92

Low, Med, High Estimates

Note: If unburned forest: 2-yr event generates verylittle surface runoff

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.

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

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.

Also modeled routed flows

Route flows from all sub-basins together:

- 2-yr-10-yr- 25-yr-100-yr

Pre-storm

~2-Year Event – Post-Fire Flooding

~ 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

Photo credits: C. Schroeder, Boulder County

Washed Out Road - Ingram Gulch

Photo credits: GROUND Engineering

Scour

Deposition

Photo credit: Boulder County

Blocked Roads Plugged Culverts

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)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 1 2 3 4 5 6 7 8 9 10

Pro

bability

Years After Fire

Probability of Storm Events Occurring Example:  2‐Yr Event Probability During Next 10 Years

Probability Curve Near certainty that a 2‐Year Event will occur 

during the next 10 years 

(10 years – anticipated time needed for the watershed to substantially “recover” hydrologically)

50% probability of a 2‐year event within 1 year following the fire

75% probability of a 2‐year event within 2 years following the fire

Similar curves can be generated for storm events with other return frequencies (10‐yr, 25‐yr, etc.)

1.02.03.04.05.06.07.08.09.010.011.012.013.014.015.016.017.018.019.020.0

0 1 2 3 4 5 6 7 8 9 10

Qbu

rned

/Qpr

e

Years After Fire

Conceptual Hydrologic Recovery Following Wildfire Moderate to Severe Burn Intensity

“Increases in peak flows can be expected to continue for 2 – 3 years after the fire and then begin to reduce toward pre‐fire levels.”

‐ FEST Report (USFS/BLM)

Ratio:

Peak flow rate (post-fire)--------------------Peak flow rate (pre-fire)

(Runoff ratios for 10-year storm event)

0%

10%

20%

30%

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90%

100%

1.02.03.04.05.06.07.08.09.010.011.012.013.014.015.016.017.018.019.020.0

0 1 2 3 4 5 6 7 8 9 10

Pro

bability of A

t Lea

st O

ne 10‐yea

r Eve

nt D

uring Time Pe

riod

Qbu

rned

/Qpr

e

Years After Fire

10‐Year Event Conceptual Hydrologic Recovery Following Wildfire‐‐Moderate to Severe Burn Intensity

Probability of at least one 10-year event occurring during 10-year time period.

Ratio:

Peak flow rate (burned)--------------------Peak flow rate (unburned)

Combine Watershed Recovery Curve (Blue)and Storm Probability Curve (Red)

Example:  10‐Year Event

Can assist land managers with decision‐making regarding mitigation measures

0%

10%

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90%

100%

1.02.03.04.05.06.07.08.09.010.011.012.013.014.015.016.017.018.019.020.0

0 1 2 3 4 5 6 7 8 9 10

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bability of A

t Lea

st O

ne 10‐yea

r Eve

nt D

uring Time Pe

riod

Qbu

rned

/Qpr

e

Years After Fire

10‐Year Event Conceptual Hydrologic Recovery Following Wildfire‐‐Moderate to Severe Burn Intensity

Probability of at least one 10-year event occurring during 10-year time period.

Ratio:

Peak flow rate (burned)--------------------Peak flow rate (unburned)

If the 10‐year event (1.5”) occurs approx.4 to 5 years after fire

Example:10 x increase in 

peak flow compared to pre‐

fire (for 10‐yr event)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1.02.03.04.05.06.07.08.09.010.011.012.013.014.015.016.017.018.019.020.0

0 1 2 3 4 5 6 7 8 9 10

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bability of A

t Lea

st O

ne 10‐yea

r Eve

nt D

uring Time Pe

riod

Qbu

rned

/Qpr

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Years After Fire

10‐Year Event Conceptual Hydrologic Recovery Following Wildfire‐‐Moderate to Severe Burn Intensity

Probability of at least one 10-year event occurring during time period.

Which has roughly a 40 % probability within 4 to 5 years 

post‐fire

Ratio:

Peak flow rate (burned)--------------------Peak flow rate (unburned)

If the 10‐year event (1.5”) occurs approx.4 to 5 years after fire

Example:10 x increase in 

peak flow compared to pre‐

fire (for 10‐yr event)

• 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

Limited number of installations

CulvertDebris Rack

Photo credit: Boulder County

• 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