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
Copyright [insert date set by system] by [CH2M HILL entity] • Company Confidential
Presentation Overview
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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
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
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
Project Area
Milliken
Berthoud
Phase 1
Johnstown
Phase 2
Previous Studies
Previous Estimates of 1 Percent Annual Chance Peak Discharge
Drainage
Area
Estimated 1 Percent Annual Chance Peak Discharge
(mi2)
USACE,
1974
USACE,
1977
Ayres
Associates,
2009
CDOT,
2011
FEMA
Effective
Hydrology
Little Thompson River at LTCANYO Gage
100
N/E
N/E
N/E
6,962 a
N/E
Little Thompson River at South County
Line Road
132
N/E
7,200
9,500
11,305 a
7,200 b
Location
9,500 c
Little Thompson River at Interstate 25
166
N/E
N/E
N/E
14,728
N/E
Little Thompson River upstream of
confluence with Big Thompson River
196
4,800
N/E
N/E
N/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)
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
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
Data Collection – Peak Discharge Estimates
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
Bob Jarrett and
Dana McGlone
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
Data Collection – Peak Discharge Estimates
18,000 cfs
7,800 cfs
15,700 cfs
13,400 cfs
15,700 cfs; observed
peak afternoon of 9/12
Courtesy of Jarrett, In Press
URS, 2015
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
Flood Frequency Analysis
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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)
2
306
5
1,119
10
2,204
50
7,229
100
10,992
500
25,656
Courtesy of Ayres Associates, 2014
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
Rainfall-Runoff Model Development

Subbasin, stream, and flow path delineation – via
40-foot USGS Topographic Maps
Rainfall-Runoff Model Development

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
Rainfall-Runoff Model Development
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Infiltration Losses
USGS, 2006
Rainfall-Runoff Model Development
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Subbasin and streams – 40-foot USGS Topographic Maps
Infiltration losses – Curve Numbers from TR-55
Runoff hydrographs – Snyder Unit Hydrograph / CWCB Criteria
0.33
𝐿 ∗ 𝐿𝑐
𝑇𝐿𝐴𝐺 = 22.1 𝐾𝑛 ∗
𝑆
Channel Routing – Muskingum Cunge with 8-point cross-section
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
Rainfall-Runoff Model Development

Rainfall Analysis
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
AWA, 2014
Rainfall-Runoff Model Development
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Rainfall Analysis
Model Calibration
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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
Calibration Parameters
Snyder’s
Peaking
Coefficient
/ Lag Time
Manning’s
N
Curve
Number
Calibrated Model
Calibration Parameters

Manning’s N Value
– No Effect on Runoff Volume
– Minimal Effect on Peak Discharge and Timing
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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 25
CDOT, In Press
9/12 Afternoon
9/12 2:20 p.m.
Calibration Parameters

Curve Numbers
– Affected Runoff Volume
– Significant Effect on Peak Discharge
– Negligible Effect on Time-of-Peak
Snyder’s
Peaking
Coefficient /
Lag Time
Manning’s N
Curve Number
Calibrated Model
Curve Number Calibration

Initial CN assignments made using standard CNs
– Modeled Q < Observed (under-calibrated)
– Why might this be?
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Antecedent Moisture Condition (AMC)
– Measure of soil moisture content prior to start of rainfall
• “Dry”
• AMC I
“Normal”
AMC II
“Wet”
AMC III
– AMC II Assumes 5 – Day Rainfall < 2.1 inches
– Calibrate with AMC III Conditions
𝐶𝑁𝐴𝑀𝐶𝐼𝐼𝐼 =
23𝐶𝑁𝐴𝑀𝐶𝐼𝐼
10 + 0.13𝐶𝑁𝐴𝑀𝐶𝐼𝐼
Curve Number Calibration
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Land Cover Classifications / Aerial Imagery
USGS Land Cover
11 - Open Water
TR-55 Classification
Open Water
12, 41, 42, 43 - Deciduous Forest Oak- Aspen
Developed Open Space,
21- Developed Open Space
2 Acre Lots
Developed Medium
22 - Developed, Low Intensity
Intensity, 1 Acre Lots
Developed Medium
23 - Developed, Medium Intensity Intensity, 1/4 Acre Lots
Developed High Intensity,
24 – Developed, High Intensity
1/8 Acre Lots
31 - Barren Land
Barren Land
52 – Shrub/Brush
Shrub, Brush
71, 72, 81 - Grassland/Pasture
Grassland, pasture
82- Row Crops
Crops, Row Crops
90, 95 - Woody Wetlands,
Woody Wetlands,
Herbaceous
Herbaceous
Phase 2
Phase 2
Mountains – Mountains
Phase 2
Little
– North
Plains Thompson
Fork
Fair
Fair
Poor
Fair
Fair
Poor
Fair
Fair
Poor
Fair
Fair
Poor
Fair
Fair
Poor
Fair
Fair
Fair
Fair
Fair
Fair
Fair
Fair
Fair
Fair
Poor
Poor
Poor
Poor
Poor
Fair
Fair
Poor
Calibration Results
Location
Drainage Area Observed Peak Modeled Peak
%
(square miles) Discharge (cfs) Discharge (cfs) Difference
Little Thompson River Downstream of
Confluence with West Fork Little Thompson
River
43.2
7,800
Little Thompson River at X Bar 7 Ranch
81.8
Little Thompson River at South County Line
Road
a
9,280
19%
15,731
14,343
-9%
131.7
13,400
15,479
16%
Little Thompson River at Interstate 25
164.1
15,700
15,173
-3%
Little Thompson River at County Road 17
184.7
18,000
14,820
-18%
b
a
- This flow was inaccessible and the observed peak discharge was estimated based on observations along similar, adjacent watersheds.
b
– Bridge overtopped, (URS, 2015)
Calibration Results

Comparison to CDWR, 2014 modeling results
Location
Observed Peak
Discharge (cfs)
Little Thompson River at Pinewood Springs
14,600
Little Thompson River at X Bar 7 Ranch
15,731
a
– NRCS, 2013
b
– CDWR, 2014
a
b
CH2M HILL,
2015 (cfs)
CDWR,
2014
9,400
10,190
14,300
15,999
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
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
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Other inputs for predictive model:
NRCS 24-hr Type II
Hyetograph
Predictive Model Implementation

Site-specific DARF
Curve Number Calibration
Estimate of the Recurrence Interval of the September
2013 Event
Predictive Annual Chance Peak Discharge (cfs)
Location
Little Thompson River Downstream of Confluence with
West Fork Little Thompson River
Little Thompson River at US 36
Little Thompson River at X Bar 7 Ranch
Little Thompson River at LTCANYO Gage
Little Thompson River at South County Line Road
Little Thompson River at Interstate 25
Little Thompson River at County Road 17
Little Thompson River Upstream of Confluence with Big
Thompson
Observed
Peak
Discharge
(cfs)
10 percent 4 percent
2 percent
1 percent
Estimated
Recurrence
0.2 percent Interval (yr)
8,955
9,056
15,731
15,500
13,400
15,700
18,000
648
651
2,310
2,760
3,650
4,140
4,480
1,365
1,376
4,500
5,380
6,940
7,090
7,150
2,243
2,264
6,970
8,330
10,600
10,900
10,700
3,418
3,455
10,200
12,100
15,300
16,000
15,700
7,504
7,600
20,700
24,700
30,800
33,500
32,100
>500
>500
100 to 500
100 to 500
50 to 100
100
100 to 500
14,700
4,450
7,130
10,500
15,400
31,400
100
Note: Italics denotes from the Little Thompson River [Phase 1] Hydrologic Analysis (CH2M HILL, 2014).
Conclusion
Conclusion
Conclusion

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
Questions
Conclusion