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Gross Reservoir Hydroelectric Project Probable Maximum Flood Study
FERC Project No. 2035-030 Colorado DAMID: 060211
FINAL REPORT
Prepared for:
Prepared by:
March 2017
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The following individuals have been directly responsible for the preparation, review and approval of this Report.
Prepared by:
John Haapala, P.E., Senior Hydrologic/Hydraulic Engineer
Approved by:
Brian Hall, P.E., Project Manager
Contributors:
Carmen Bernedo, P.E., Principal Engineer
Christopher Gifford-Miears, EIT, Associate Engineer
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TABLE OF CONTENTS
Table of Contents ...... i List of Tables ...... iii List of Figures ...... iv List of Appendices ...... vii
EXECUTIVE SUMMARY ...... 1 1. PROJECT DESCRIPTION ...... 1-1 1.1 Project Data ...... 1-1 1.2 Basin Hydrologic Data ...... 1-1 1.3 Upstream Dams ...... 1-4 1.4 Field Visit ...... 1-4 1.5 Watershed Description ...... 1-7 1.5.1 Watershed Area-Elevation Data ...... 1-7 1.5.2 Land Use and Land Cover ...... 1-9 1.6 Previous Studies ...... 1-11 2. WATERSHED MODEL AND SUBDIVISION ...... 2-1 2.1 Watershed Model Methodology ...... 2-1 2.2 Sub-Basin Definition ...... 2-2 2.3 Channel Routing Method...... 2-4 3. HISTORIC FLOOD RECORDS ...... 3-1 3.1 Stream Gages ...... 3-1 3.2 Historic Floods ...... 3-1 3.2.1 Flood Frequency ...... 3-5 3.2.2 Seasonal Flood Distribution ...... 3-10 3.3 Precipitation Associated with Historic Floods ...... 3-12 3.4 Snowpack and Snowmelt During Historic Floods ...... 3-15 4. UNIT HYDROGRAPH DEVELOPMENT...... 4-1 4.1 Approach and Tasks ...... 4-1 4.2 Unit Hydrograph Methodology ...... 4-5 4.3 Preliminary Estimates of Clark Parameters ...... 4-6 5. HYDROLOGIC MODEL CALIBRATION ...... 5-1 5.1 Calibration Scenarios ...... 5-1
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5.2 Summer Floods ...... 5-3 5.3 Spring Floods ...... 5-9 5.4 Recommended Parameters to be Used for PMF Analysis ...... 5-14 6. PROBABLE MAXIMUM PRECIPITATION ...... 6-1 6.1 Probable Maximum Precipitation Data ...... 6-1 6.2 Discussion of Temporal Distribution Using Critically Stacked Pattern ...... 6-6 6.3 Candidate Storms for the PMF ...... 6-6 7. LOSS RATES ...... 7-1 7.1 General ...... 7-1 7.2 Loss Rate Values ...... 7-1 8. COINCIDENT HYDROMETEOROLOGICAL AND HYDROLOGICAL CONDITIONS FOR THE PROBABLE MAXIMUM FLOOD ...... 8-1 8.1 Reservoir Level ...... 8-1 8.2 Baseflow ...... 8-1 8.3 Snowpack ...... 8-1 8.3.1 Available Historical Snowpack Data ...... 8-1 8.3.2 Methodology Used to Determine the Estimated PMF Snowpack ...... 8-5 8.3.3 100-Year Snowpack Antecedent to the PMP ...... 8-5 8.4 Snowmelt ...... 8-7 9. PMF HYDROGRAPHS ...... 9-1 9.1 PMF Inflow and Outflow Hydrographs ...... 9-1 9.2 Sensitivity Analysis ...... 9-6 9.3 Additional PMF Hydrograph Considerations Summary ...... 9-8 9.4 Recommended PMF ...... 9-10 9.5 Comparison with Previous Gross Dam PMF Studies ...... 9-11 10. REFERENCES ...... 10-1
List of Tables
Table 1.2-1. Streamflow and Diversion Gages in the Boulder Creek Watershed ...... 1-2 Table 1.5-1. Area in Elevation Bands to Gross Dam ...... 1-7 Table 1.5-2. Area in Elevation Bands to Eldorado Springs ...... 1-7 Table 1.5-3. Watershed Cover ...... 1-9 Table 2.2-1. Sub-basin Drainage Areas ...... 2-2
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Table 3.2-1. Recorded Peak Instantaneous Flows – South Boulder Creek near Eldorado Springs (1896-1995) ...... 3-2 Table 3.2-2. Recorded Daily Maximum Flows – South Boulder Creek near Eldorado Springs (1896-1995) ...... 3-3 Table 3.2-3. Recorded Peak Instantaneous Flows – South Boulder Creek near Rollinsville (1911-1949, intermittent) ...... 3-3 Table 3.2-4. Calculated Daily Maximum Inflows – Gross Reservoir (1958-2016) ...... 3-4 Table 3.2-5. Maximum Peak Flows in the Vicinity of Gross Dam Watershed ...... 3-5 Table 3.2-6. Peak Annual Flows for South Boulder Creek near Eldorado Springs ...... 3-7 Table 3.2-7. Calculated Flood Frequency for South Boulder Creek near Eldorado Springs ...... 3-8 Table 3.2-8. Flow Frequency for Watersheds in the Vicinity of Gross Dam Watershed 3-9 Table 3.2-9. Monthly Distribution of Annual Peak Flows ...... 3-10 Table 3.3-1. Precipitation for Historic Storms (from AWA) ...... 3-12 Table 3.4-1. Earliest and Latest Snowpack at SNOTEL Stations ...... 3-15 Table 3.4-2. Antecedent Snowpack Snow Water Equivalent ...... 3-16 Table 4.1-1. Calibration/Verification Floods...... 4-2 Table 4.3-1. Initial Estimates of Clark Parameters ...... 4-7 Table 5.1-1. Calibration Scenario Descriptions ...... 5-2 Table 5.2-1. September 1938 Simulation Results ...... 5-4 Table 5.2-2. September 2013 Pinecliffe Simulation Results ...... 5-5 Table 5.2-3. September 2013 Gross Reservoir Inflow Simulation Results ...... 5-7 Table 5.2-4. September 2013 Below Gross Dam and Eldorado Springs Calibration Results ...... 5-8 Table 5.3-1. May 1969 Gross Dam Inflow Simulation Results ...... 5-9 Table 5.3-2. May 1969 Eldorado Springs Simulation Results ...... 5-10 Table 5.3-3. June 1995 Pinecliffe Simulation Results ...... 5-11 Table 5.3-4. June 1995 Gross Dam Inflow Simulation Results ...... 5-12 Table 5.3-5. June 1995 Eldorado Springs Simulation Results ...... 5-13 Table 5.4-1. Final Estimates of Clark Parameters ...... 5-15 Table 6.1-1. PMP Depth-Duration Data ...... 6-1
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Table 6.1-2. PMP Seasonality Ratios ...... 6-2 Table 6.1-3. All-Season PMP by Sub-Basin for Various Durations – Gross General Temporal Distribution ...... 6-2 Table 6.1-4. All-Season PMP by Sub-Basin for Various Durations – Critically Stacked General Temporal Distribution ...... 6-3 Table 6.1-5. May 15 PMP by Sub-Basin for Various Durations – SPAS 1253 General Temporal Distribution ...... 6-3 Table 6.1-6. Local Storm PMP by Sub-Basin for Various Durations – Gross Local Temporal Distribution ...... 6-3 Table 7.2-1 Watershed Uniform Loss Rates ...... 7-1 Table 7.2-2. Watershed Loss Rates ...... 7-2 Table 8.3-1. Lake Eldora SNOTEL – Rates of SWE Depletion ...... 8-4 Table 8.3-2. Equivalent Flow Rates to SWE Loss Rates ...... 8-5 Table 8.3-3 100-Year Snowpack at Snow Stations (by AWA) ...... 8-5 Table 8.3-4. 100-Year Snowpack Snow Water Equivalent (by AWA)...... 8-6 Table 8.4-1 Summary of Snowmelt Parameters ...... 8-9 Table 9.1-1. Candidate PMF Base Runs ...... 9-2 Table 9.1-2. Candidate PMF Routing Results at Gross Dam ...... 9-3 Table 9.1-3. PMF Scenario Snowmelt Summary ...... 9-4 Table 9.2-1. PMF Routing Sensitivity Analysis Results ...... 9-7 Table 9.2-2. Candidate PMP Depth-Duration Summary ...... 9-8 Table 9.3-1. MWH PMF Experience Values...... 9-10 Table 9.5-1. PMP and PMF Inflow Comparison ...... 9-12
List of Figures
Figure 1.2-1. South Boulder Creek Watershed Boundary and Streamflow Gage Locations ...... 1-3 Figure 1.4-1. Gross Dam ...... 1-4 Figure 1.4-2. Gross Reservoir ...... 1-5 Figure 1.4-3. Gross Dam Watershed near Rollinsville ...... 1-5 Figure 1.4-4. Gross Dam Watershed at Moffat Tunnel Portal ...... 1-6
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Figure 1.4-5. Gross Dam Upper Watershed ...... 1-6 Figure 1.5-1. Watershed Sub-Basins and Elevation Bands ...... 1-8 Figure 1.5-2. Gross Dam Land Cover Map ...... 1-10 Figure 2.2-1. Gross Dam Watershed Sub-Basins to Eldorado Springs ...... 2-3 Figure 2.2-2. Gross Dam Watershed Grid Cells ...... 2-4 Figure 3.2-1. Log Pearson Type III Flood Frequency Plot for South Boulder Creek near Eldorado Springs ...... 3-8 Figure 3.2-2. Historic Daily Flow Frequency at the USGS Eldorado Springs Gage ..... 3-11 Figure 3.2-3. Historic Daily Flow Frequency at the USGS Rollinsville Gage ...... 3-11 Figure 3.3-1. Flow Data and September 1938 Cumulative Sub-basin Rainfall Depths 3-13 Figure 3.3-2. Flow Data and May 1969 Cumulative Sub-basin Rainfall Depths ...... 3-13 Figure 3.3-3. Flow Data and June 1995 Cumulative Sub-basin Rainfall Depths ...... 3-14 Figure 3.3-4. Flow Data and September 2013 Cumulative Sub-basin Rainfall Depths 3-14 Figure 4.1-1. June 1938 Recorded Daily Flows at Eldorado Springs ...... 4-2 Figure 4.1-2. May 1969 Recorded and Calculated Daily Flows ...... 4-3 Figure 4.1-3. June 1995 Recorded and Calculated Daily Flows ...... 4-4 Figure 4.1-4. September 2013 Recorded and Calculated Daily Flows...... 4-4 Figure 5.2-1. September 1938 Daily Average Flow at Eldorado Springs ...... 5-4 Figure 5.2-2. September 1938 Hourly Flow at Eldorado Springs ...... 5-5 Figure 5.2-3. September 2013 Daily Average Flow at Pinecliffe ...... 5-6 Figure 5.2-4. September 2013 Hourly Flow at Pinecliffe ...... 5-6 Figure 5.2-5. September 2013 Inflow to Gross Reservoir ...... 5-7 Figure 5.2-6. September 2013 Hourly Flow at Eldorado Springs ...... 5-8 Figure 5.3-1. May 1969 Daily Gross Reservoir Inflow ...... 5-10 Figure 5.3-2. May 1969 Flow at Eldorado Springs ...... 5-11 Figure 5.3-3. June 1995 Daily Average Flow at Pinecliffe ...... 5-12 Figure 5.3-4. June 1995 Daily Average Inflow to Gross Reservoir ...... 5-13 Figure 5.3-5. June 1995. Hourly Flow at Eldorado Springs ...... 5-14 Figure 6.1-1. Incremental and Accumulated All-Season PMP – Gross General Temporal Distribution ...... 6-4
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Figure 6.1-2. Incremental and Accumulated All-Season PMP – Critically Stacked General Temporal Distribution ...... 6-4 Figure 6.1-3. Incremental and Accumulated May 15 PMP – SPAS 1253 Temporal Distribution ...... 6-5 Figure 6.1-4. Incremental and Accumulated Local Storm PMP – Gross Local Temporal Distribution ...... 6-5 Figure 7.2-1. Hydrologic Soil Groups ...... 7-0 Figure 8.3-1. Lake Eldora SNOTEL Data – October 1978 to September 2005 ...... 8-3 Figure 8.3-2. Lake Eldora SNOTEL Data – October 2005 to June 2016 ...... 8-4 Figure 8.3-3. 100-Year Snow Water Equivalent for May 1 (by AWA) ...... 8-6 Figure 8.3-4. 100-Year Snow Water Equivalent for June 15 (by AWA)...... 8-7 Figure 8.4-1. Example Temperature at Four Sample Grid Cells for May 15 ...... 8-8 Figure 9.1-1. Candidate PMF Inflow Hydrographs ...... 9-5 Figure 9.1-2. Candidate PMF Outflow Hydrographs ...... 9-5 Figure 9.2-1. PMF Sensitivity Analysis Inflow Hydrographs ...... 9-7 Figure 9.2-2. PMF Sensitivity Analysis Outflow Hydrographs ...... 9-8 Figure 9.4-1. Gross Dam PMF Inflow, Outflow, and Reservoir Elevation ...... 9-11
List of Appendices
Appendix A: Maximum Flood and PMF Data – Background Information Appendix B: Site-Specific Probable Maximum Precipitation Study for Gross Reservoir, by Applied Weather Associates
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EXECUTIVE SUMMARY
The purpose of the study was to develop the Gross Dam inflow design flood, which is the Probable Maximum Flood (PMF). The PMF is an industry standard design criterion that regulatory authorities apply to large dams like Gross Dam. The PMF is the largest flood that may be expected from the most severe combination of critical meteorological and hydrologic conditions that are reasonably possible in the drainage basin tributary to Gross Dam. The PMF results from the Probable Maximum Precipitation (PMP), which was also developed as a part of this study, and other coincident conditions including snowmelt. The candidate PMF inflow hydrographs were routed through the reservoir with the purpose of selecting the critical PMF and providing information for sizing (at a later date) the spillway and selection of a dam crest level to ensure the safe flood passage by the dam.
Project Description
The existing Gross Dam and Reservoir (Project) is an on-stream facility located on South Boulder Creek in Boulder County, Colorado, and in the Arapahoe-Roosevelt National Forest. The dam is owned and operated by Denver Water and provides raw water storage from both a west slope inter- basin diversion and from the South Boulder Creek watershed upstream of Gross Dam. The dam structure is a curved concrete gravity dam formed with a structural height of 345 feet that was completed in 1954. A water diversion through Moffat Tunnel into South Boulder Creek above Gross Dam began operation in about 1936. The existing dam crest level is at El 7290 with a spillway crest level at El 7280. Denver Water proposes to raise Gross Dam by 131 feet to a final height of 471 feet, increasing storage volume from 41,811 acre feet to approximately 119,000 acre- feet.
Watershed Description
The Gross Dam watershed is primarily hilly to mountainous with a relatively mildly sloping central valley floor. The drainage area of the Gross Dam watershed is 93 square miles. About 77% of the watershed lies between elevation 10,000 feet and elevation 13,400 feet. Evergreen forest is the dominant watershed cover type, totaling about 76% of the entire watershed. Grassland/herbaceous covers about 7% of the watershed. Streamflow is highly seasonal with about 90% of the runoff occurring during the 6-month period of April through September.
Historic Floods
Streamflow records exist at two gaging stations within the watershed upstream from Gross Dam (Rollinsville and Pinecliffe), plus calculated inflows to Gross Reservoir. Another streamflow gaging station is the long-term USGS gage downstream at Eldorado Springs (now operated by the
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Colorado DWR), which has a drainage area about 20% greater than at the dam site. In 103 years of record at the Eldorado Springs gaging station, the peak recorded flow was 7,390 cfs. Since 1954, the flow record at the Eldorado Springs gage has been affected by regulation at Gross Reservoir. June is the month during which the annual maximum flows most frequently occur. In 173 station-years of annual maximum flow data, an annual maximum flow has never been recorded during the months of October through April.
Hydrologic Model
The HEC-HMS Hydrologic Modeling System was chosen as the rainfall-runoff model to develop the PMF because it is one of the models recommended by Federal Energy Regulatory Commission (FERC) specifically for this purpose, it is the up-to-date replacement model for the HEC-1 Flood Hydrograph Package, and it includes a temperature index method for snowmelt. The Clark unit hydrograph was used as the runoff model method to develop flood hydrographs. The gridded cell method was used for precipitation and snowmelt. When the Clark method is used with gridded cells, it was modified in HEC-HMS to distribute parameters from a sub-basin to the grid cells and is called ModClark. Gridded cell runoff was collected into seven sub-basins above Gross Dam, with three additional sub-basins between Gross Dam and the USGS gaging station on South Boulder Creek at Eldorado Springs.
Streamflow data for model calibration and verification were available at two stations upstream from Gross Dam (Rollinsville and Pinecliffe) plus a long-term gaging station a short distance downstream from the dam (Eldorado Springs). Because both spring rainfall with snowmelt floods and summer rainfall floods can occur in the Gross Dam watershed, two spring floods and two summer floods were selected for runoff model calibration. Preference was given to selecting floods of the greatest magnitude that had recorded data at the streamflow gaging stations that would also satisfy the spring/summer distribution.
Runoff model calibration challenges included highly variable precipitation over short distances within the watershed, a lack of extreme historic floods for model calibration, a lack of historical precipitation and snowpack data within the watershed tributary to Gross Dam. Uncertainties in the basic precipitation and snowpack data must be acknowledged. Given these limitations, calibration of the watershed model parameters was not stretched beyond normal ranges to improve the calibration, as the preferred approach was to use parameters within a normal range for the PMF simulation.
Probable Maximum Precipitation
Derivation of the site specific PMP is detailed in a separate report prepared by MWH sub- consultant Applied Weather Associates (AWA), which is included as Appendix B to this report.
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The site-specific all-season (maximum) PMP was found to occur during the period June 15 through September and was derived on an hourly basis for a 72 hour (3 day) time sequence for each of the grid cells tributary to the Gross Dam site. Reductions to the all-season PMP were developed for spring period coincident with the seasonal 100-year snowpack.
Alternative temporal distributions for the PMP were evaluated. The basin-wide all-season average PMP depth-duration values were 2.88 inches for 1 hour, 5.38 inches for 6 hours, 11.69 inches for 24-hours, and 17.54 inches for 72 hours. As used in the selected actual general storm, the basin- wide average PMP depths were 0.93 inches for 1 hour, 4.86 inches for 6 hours, 11.63 inches for 24-hours, and 17.56 inches for 72 hours. The local storm PMP depth-duration values were 2.88 inches for 1 hour, 4.40 inches for 3 hours, and 5.40 inches for 6 hours. The data sets for various seasonal time periods and sensitivity runs form cases from which the PMF can be determined.
Snowpack
Snow water equivalent (SWE) values coincident with spring storms were developed by AWA. SWE data was available in the vicinity of the Gross Dam watershed at SNOTEL and GHCN stations. In addition to the point snow water equivalent stations, AWA utilized the National Operational Hydrologic Remote Sensing Center (NOHRSC) SNOw Data Assimilation System (SNODAS) gridded dataset. SNODAS integrates observed, remotely sensed, and modeled datasets into estimated snowpack variables.
Coincident and Antecedent Conditions
The primary coincident conditions evaluated were several cases formed by seasonal combinations of the 100-year snowpack and the PMP. Coincident seasonally varying temperatures were also included to determine snowmelt. Baseflow was assumed to be equal to the historical maximum calculated daily inflow to Gross Reservoir.
For Gross Dam, the initial reservoir level was assumed to be at the uncontrolled spillway crest, as planned for the raised Gross Dam, for all PMF cases. A spillway width was assumed for the purposes of evaluating the PMF cases on a common basis, but this does not imply a recommendation of spillway size or type for the final design.
Probable Maximum Flood Hydrograph
After evaluating all of the candidate cases for the PMF including general and local PMP storms, temporal and spatial distributions of the PMP, alternative seasonal PMP cases combined with concurrent 100-year snowpacks, and sensitivity runs, a general storm with the Gross General temporal distribution is recommended as the inflow design flood for Gross Dam. Because a site- specific PMP study was performed for Gross Dam, and the Gross General temporal distribution is
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based on a combination of actual recorded maximum storms, it is considered to represent a reasonably conservative, but historically based, temporal distribution that best represents actual storms in the transposable vicinity.
Figure ES-1 is a plot of the PMF inflow, total outflow, and reservoir elevation. The PMF peak inflow was 40,398 cfs and the peak outflow was 36,711 cfs. The PMF routing assumed a 322-ft wide spillway with an ogee shaped crest, which resulted in a peak PMF reservoir elevation at 7,415.6 feet. It is emphasized that the 322-ft wide spillway is not a recommended width, as the actual spillway width, type, and configuration will be determined in the Final Design phase of the proposed raising of Gross Dam.
45,000 7422
40,000 7420
35,000 7418 Inflow (cfs)
30,000 Outflow (cfs) 7416 Reservoir Elevation (feet)
25,000 7414
Flow (cfs) 20,000 7412
15,000 7410 (feet) Elevation Reservoir
10,000 7408
5,000 7406
0 7404 0 12 24 36 48 60 72 84 96 Hours Figure ES-1. PMF Inflow, Outflow, and Reservoir Elevation
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1. PROJECT DESCRIPTION
The existing Gross Dam and Reservoir (Project) is an on-stream facility located on South Boulder Creek in Boulder County, Colorado, and in the Arapahoe-Roosevelt National Forest. The dam is owned and operated by Denver Water and provides raw water storage from both a west slope inter- basin diversion and from the South Boulder Creek watershed upstream of Gross Dam. The Gross Reservoir Expansion Project is crucial to providing water dependability to Colorado’s Front Range, protecting against potential catastrophic events such as fires, landslides, floods, drought and infrastructure failures. For the planned dam raise of Gross Dam, which would incorporate a new spillway, Denver Water hired AWA and MWH to develop the Site-Specific Probable Maximum Precipitation and Inflow Flood Design Study respectively.
1.1 Project Data
The dam structure is a curved concrete gravity dam formed with a structural height of 345 feet that was completed in 1954. The existing dam crest level is at El 7290 with a spillway crest level at El 7280. Denver Water proposes to raise Gross Dam by 131 feet to a final height of 471 feet, increasing storage volume from 41,811 acre feet to approximately 119,000 acre-feet. The facility is regulated by the Federal Energy Regulatory Commission (FERC) and the Colorado State Engineer’s Office (SEO).
In accordance with standards of the industry for a dam if its size, hazard classification, and economic, and water supply importance to Denver, the inflow design flood for Gross Dam is the Probable Maximum Flood (PMF). The determination of the design flood inflow hydrograph and the preliminary outlet capacity at the dam is the subject of this report. The PMF inflow flood hydrograph will inform sizing of the main spillway and the dam crest elevation in future design studies. Preliminary studies have indicated that the raised dam crest level would be at El 7421 feet, with a spillway crest level at El 7406. The vertical datum for Gross Dam is NAVD88 adjusted by -6.399 feet.
1.2 Basin Hydrologic Data
Table 1.2-1 shows the availability of South Boulder Creek and Boulder Creek streamflow gaging data for both the USGS and the Colorado Division of Water Resources. Calculated Gross Reservoir inflow on a daily basis was also provided by Denver Water. The locations of the streamflow gaging stations with tributary area boundaries to the downstream limit of the study at the Eldorado Springs gaging station are shown on Figure 1.2-1.
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Table 1.2-1. Streamflow and Diversion Gages in the Boulder Creek Watershed
Flow Drainage Gage Years Gage Number Data Gage Name Area Datum of Begin Date End Date Notable Maximum Flows or Method Provider (sq.mi) (feet) Record South Boulder Creek 718 cfs - June 12, 1949 06729000 USGS 42.7 8,380 11 10/1/1911 9/30/1949 near Rollinsville 622 cfs - June 21, 1947 South Boulder Creek 1,150 cfs - June 7, 1979 06729300 USGS 72.7 7,930 2 5/18/1979 10/3/1980 at Pinecliff 914 cfs - June 20, 1980 Colo. Div. of South Boulder Creek BOCPINCO ------18 1/1/1998 5/26/2016 780 cfs - Sept. 13, 2013 Water Res. at Pinecliff Calculated Denver South Boulder Creek 869 cfs - June 17, 1995 93 7,000 59 3/1/1958 3/31/2016 Daily Inflow Water at Gross Dam 770 cfs - Sept. 12, 2013 Colo. Div. of South Boulder Creek BOCBGRCO 93.2 ----- 48 10/1/1967 5/26/2016 262 cfs - Sept. 20, 2013 Water Res. below Gross Reservoir South Boulder Creek 7,390 cfs - Sept. 2, 1938 06729500 USGS 111 6,080 96 1-1-1896 9/30/1995 near Eldorado Springs 2,370 cfs - June 18, 1951 Colo. Div. of South Boulder Creek 639 cfs - May 30, 2003 BOCELSCO 111 ----- 20 10/1/1995 9/30/2015 Water Res. near Eldorado Springs (daily avg.) Colo. Div. of South Boulder Creek Diversion 452 cfs - Sept. 4-5, 1959 BOSDELCO N/A ----- 57 10/1/1958 9/30/2015 Water Res. near Eldorado Springs (daily avg.) Boulder Creek at 8,400 cfs - Sept. 13, 2013 06730200 USGS 307 5,160 29 10/1/1986 3/31/2016 N 75th St. near Boulder 2,050 cfs - May 30, 2003 Boulder Creek at 8,910 cfs - Sept. 13, 2013 06730500 USGS 447 4,860 64 4/1/1927 3/31/2016 Mouth near Longmont 4,410 cfs - Sept. 3, 1938
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Figure 1.2-1. South Boulder Creek Watershed Boundary and Streamflow Gage Locations
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1.3 Upstream Dams
There are no dams upstream from the Gross Dam.
1.4 Field Visit
A field visit was performed on August 23, 2016. The probable maximum precipitation (PMP) and PMF Board of Consultants (BOC) experts and consultants performed a watershed tour in a van provided by Denver Water. Stops were made at Gross Dam and Reservoir, the Moffat Tunnel portal, and several locations along South Boulder Creek.
Figure 1.4-1 through Figure 1.4-5 show a variety of watershed terrain within the Gross Dam watershed. The photos were all taken during the field trip on August 23, 2016.
Figure 1.4-1. Gross Dam
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Figure 1.4-2. Gross Reservoir
Figure 1.4-3. Gross Dam Watershed near Rollinsville
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Figure 1.4-4. Gross Dam Watershed at Moffat Tunnel Portal
Figure 1.4-5. Gross Dam Upper Watershed
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1.5 Watershed Description
1.5.1 Watershed Area-Elevation Data
In mountainous regions, snowpack can vary widely with elevation. To account for the variation of snowpack with elevation, the watershed area is divided into 1,000-foot elevation bands. The 1,000-foot elevation bands tributary to Gross Dam and to the USGS gaging station at Eldorado Springs are graphically depicted on Figure 1.5-1. Selected sub-basin boundaries are also depicted on Figure 1.5-1.
Table 1.5-1 provides the detailed results of the area by 1,000-foot elevation bands to the Gross Dam. Table 1.5-2 provides the areas in 1,000-foot elevation bands to Eldorado Springs.
Table 1.5-1. Area in Elevation Bands to Gross Dam
Area in Elevation Bands (sq. mi.) % of Downstream Point 6-7000 7-8000 8-9000 9-10000 10-11000 11-12000 12-13000 13-14000 Total Total Moffat Tunnel 0.00 0.00 0.00 0.83 3.58 4.09 0.40 0.01 8.9 9.6% Rollinsville USGS Gage 0.00 0.00 3.62 14.02 12.79 3.18 0.43 0.01 34.0 36.6% Pinecliffe USGS Gage 0.00 0.07 20.64 9.52 0.98 0.00 0.00 0.00 31.2 33.5% Gross Dam 0.00 7.35 10.31 0.67 0.55 0.00 0.00 0.00 18.9 20.3% Gross Dam Total 0.00 7.43 34.57 25.03 17.89 7.27 0.82 0.02 93.0 100.0% 0.0% 8.0% 37.2% 26.9% 19.2% 7.8% 0.9% 0.02% 100.0%
Table 1.5-2. Area in Elevation Bands to Eldorado Springs
Area in Elevation Bands (sq. mi.) % of Downstream Point 6-7000 7-8000 8-9000 9-10000 10-11000 11-12000 12-13000 13-14000 Total Total Moffat Tunnel 0.00 0.00 0.00 0.83 3.58 4.09 0.40 0.01 8.9 8.0% Rollinsville USGS Gage 0.00 0.00 3.62 14.02 12.79 3.18 0.43 0.01 34.0 30.6% Pinecliffe USGS Gage 0.00 0.07 20.64 9.52 0.98 0.00 0.00 0.00 31.2 28.0% Gross Dam 0.00 7.35 10.31 0.67 0.55 0.00 0.00 0.00 18.9 17.0% CO Gage below Gross Dam 0.08 0.35 0.00 0.00 0.00 0.00 0.00 0.00 0.4 0.4% Eldorado Springs USGS Gage 3.65 11.69 2.50 0.00 0.00 0.00 0.00 0.00 17.8 16.0% Eldorado Springs Gage Total 3.73 19.47 37.07 25.03 17.89 7.27 0.82 0.02 111.3 100.0% 3.4% 17.5% 33.3% 22.5% 16.1% 6.5% 0.7% 0.02% 100.0%
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Figure 1.5-1. Watershed Sub-Basins and Elevation Bands
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1.5.2 Land Use and Land Cover
Figure 1.5-3 shows the type and distribution of watershed cover and Table 1.5-3 provides a data summary of cover types for the entire watershed, based on GIS data. Evergreen forest is the dominant watershed cover type, totaling about 76% of the entire watershed. Grassland/herbaceous covers about 7% of the watershed, and perennial ice/snow covers about 4.8%, although visual observations during the August 2016 field trip appeared to show much less ice/snow cover.
Table 1.5-3. Watershed Cover
To Gross Dam Area % of Code Description (sq. mi.) Total 42 Evergreen Forest 70.83 75.77% 71 Grassland/Herbaceous 6.56 7.02% 12 Perennial Ice/Snow 4.52 4.84% 52 Shrub/Scrub 3.47 3.71% 31 Barren Land (Rocks/Sand/Clay) 2.14 2.29% 41 Deciduous Forest 2.06 2.20% 90 Woody Wetlands 1.60 1.71% 11 Open Water 0.75 0.80% 21 Developed, Open Space 0.66 0.71% 22 Developed, Low Intensity 0.36 0.39% 81 Hay/Pasture 0.21 0.23% 95 Emergent Herbaceous Wetlands 0.17 0.18% 43 Mixed Forest 0.11 0.11% 23 Developed, Medium Intensity 0.04 0.04% Total 93.47 100.00%
Watershed cover can affect flood hydrographs. The USACE (1994) has indicated that overland flow rarely occurs in forested soils. Forested areas exhibit the greatest surface losses because of their well-developed canopies and significant surface storage provided by surface litter. In forested areas, lateral movement of the infiltrated water occurs along the lower conductivity layer as either saturated or unsaturated flow until it seeps out to the surface nearer the bottom of the hillslope. Observations have shown that the subsurface movement of water down the hillslope combined with overland flow from the source areas is the flood mechanism in forested areas. In some respects, the apparent rainfall excess in a flood hydrograph in a forested area is a combination of interflow, subsurface flow, and overland flow. Compared to less forested or more urban areas, the forested hydrographs would be longer and less peaky.
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Figure 1.5-2. Gross Dam Land Cover Map
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1.6 Previous Studies
Three previous PMF studies have been performed for Gross Dam and Reservoir as presented in the following three documents:
• DMJM Phillips-Reister, 1977. Gross Dam and Reservoir, Safety Inspection Report, for Denver Board of Water Commissioners.
• DMJM Phillips-Reister-Haley, 1982. Gross Dam and Reservoir, 1982 Safety Inspection Report, for Denver Board of Water Commissioners.
• Goodson & Associates, Inc., October 1987. Safety Inspection Report for Gross Dam, Reservoir and Appurtenant Structures, prepared for Denver Water Department.
The three previous studies provided different results, differentiated in part by the following results:
• The DMJM 1977 study had a 41,150 cfs peak reservoir inflow and a 24-hour PMP depth over the watershed of 11.78 inches.
• The DMJM 1982 study had an 80,450 cfs peak reservoir inflow and a 24-hour PMP depth over the watershed of 18.79 inches.
• The Goodson and Associates 1987 study had an 89,960 cfs peak reservoir inflow and a 24- hour PMP depth over the watershed of 23.9 inches.
The current study is independent and substantially different from any previous study because of watershed sub-basin delineation, calibration and verification of unit hydrographs, the probable maximum precipitation, snowpack and snowmelt, and other parameters.
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2. WATERSHED MODEL AND SUBDIVISION
2.1 Watershed Model Methodology
Two flood hydrology models were considered for performing the PMF study including:
• Flood Hydrograph Package (HEC-1). This model was developed by the Hydrologic Engineering Center (HEC) of the USACE and was previously the most widely used model in PMF studies. HEC-1 is one of the two rainfall-runoff models recommended for PMF studies (FERC 2001). Compared to other models, HEC-1 has the advantage of including the recommended energy budget snowmelt method as well as fully documented equations for calculating snowmelt in the model.
• Hydrologic Modeling System (HEC-HMS). This model was also developed by the HEC and is the Windows-based successor to HEC-1. HEC-HMS contains many of the same methods as HEC-1 and is the other model recommended for PMF studies (FERC 2001). HEC-HMS can perform gridded runoff simulation using gridded precipitation as input. The modified Clark unit hydrograph method, ModClark, is the quasi-distributed unit hydrograph method that can be used with gridded meteorologic data. Snowmelt in the HEC-HMS model is based on a method that uses temperature data as the only meteorologic time-series data input.
Flood hydrology model selection was reviewed with the BOC during the initial BOC meeting on June 22-23, 2016. With BOC input from that review, the HEC-HMS Hydrologic Modeling System was selected as the rainfall-runoff model for developing the PMF inflow and routing of the PMF through the reservoir. HEC-GridUtil was used to process the gridded precipitation data sets and convert into HEC-DSS format (Hydrologic Engineering Center's Data Storage System). HEC- DSS primarily facilitated the use of gridded precipitation from the site-specific PMP study performed by AWA.
The ModClark unit hydrograph method was used along with uniform infiltration losses. The Clark method parameters Tc (time of concentration) and R (a storage coefficient) were developed by calibration and experience in similar studies. The ratio R/(Tc + R) has been found in a number of studies to be fairly constant on a regional basis (ASCE 1997; FERC 2001, pg. 36; and USACE HEC 2001). This relationship was used as a means of initially estimating the parameters. Snowmelt was accomplished within the HEC-HMS program using the temperature index method.
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2.2 Sub-Basin Definition
The segmentation of the watershed into sub-basins included a number of factors, including the following:
• The USGS gaging stations would be included as the downstream boundary of sub-basins to facilitate model calibration.
• The major tributaries should be sub-basins.
• There should be sufficient sub-basins to account for the areal variation of historic precipitation and the probable maximum precipitation.
• There should be sufficient sub-basins to account for the elevation distribution of the watershed.
• The objectives should be accomplished without an excessive number of sub-basins that would cause unwarranted difficulty in model calibration and data preparation.
The gridded cell method was used for precipitation and snowmelt. Using the above factors as guidelines, Table 2.2-1 and Figure 2.2-1 summarize and outline the selected 7 sub-basins tributary to Gross Dam and the 3 additional sub-basins between Gross Dam and the USGS gaging station at Eldorado Springs, which is the downstream limit of the flood hydrology model calibration runs. All flood hydrology model runs for the PMF have a downstream limit at Gross Dam.
Table 2.2-1. Sub-basin Drainage Areas
Sub-basin Sub-basin Area Total Area Sub-basin Downstream Point ID (mi2) (mi2) B1 Moffat Tunnel 8.9 8.9 B2a Moffat Tunnel DS North Basin 7.7 16.6 B2b Moffat Tunnel DS South Basin 13.8 30.4 B3 Rollinsville USGS Gage 12.6 42.9 B4 Pinecliffe USGS Gage 31.7 74.6 B5a Gross Dam Local 12.7 87.3 B5b Gross Dam Reservoir Local 5.8 93.0 B6 CO Gage below Gross Dam 0.4 93.5 B7 Boulder Cr Diversion Structure 13.9 107.3 B8 Eldorado Springs USGS Gage 4.0 111.3
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Figure 2.2-1. Gross Dam Watershed Sub-Basins to Eldorado Springs
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Figure 2.2-2 presents the Gross Dam watershed grid cells as they were represented within HEC- HMS. From among the grid cell sizes available for use in HEC-HMS, a grid cell size of 2 km by 2 km (1.24 miles by 1.24 miles) or 4 square kilometers (1.54 square miles) was selected to most nearly match the grid cell sizes used to generate the precipitation. Where grid cells were partially within a sub-basin, the area of the grid cell within the sub-basin was interpolated. Where grid cells were completely outside the Gross Dam watershed, the precipitation for these cells were zero.
Figure 2.2-2. Gross Dam Watershed Grid Cells
2.3 Channel Routing Method
Channel routing in the Gross Dam hydrologic model was performed using the Muskingum-Cunge methodology. South Boulder Creek carries snowmelt and rainfall-runoff from upland sub-basins to the Gross Dam reservoir and continues below Gross Dam through Eldorado Springs, Colorado. Channel geometry was represented using 8-point cross-sections cut from the Gross Dam digital elevation model (DEM). Other channel parameters (e.g., length, slope, etc.) were estimated using HEC-GeoRAS. Manning’s “n” values were estimated as 0.04 for the calibration runs. Indications are that channel roughness can vary markedly with the depth of flow on high gradient streams (Jarrett 1984). The higher Manning’s “n” values suggested by Jarrett (1984) were checked and
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found to have a negligible effect on results. Thus, no adjustments to the Manning’s “n” values were made in the PMF runs.
Level pool routing was used for routing through Gross Reservoir. Although Gross Reservoir is relatively large, it may not be large enough to have a significant routing effect on the PMF as the inflow PMF volume will be many times greater than the reservoir volume available to attenuate the inflow flood. Routing effects are also dependent on the hydrograph shape near the peak, such that very peak hydrographs will be subject to greater attenuation.
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3. HISTORIC FLOOD RECORDS
3.1 Stream Gages
As previously presented in Table 1.2-1, streamflow records exist at two gaging stations within the watershed upstream from Gross Dam, plus calculated inflows to Gross Reservoir. Another streamflow gaging station is the long-term USGS gage downstream at Eldorado Springs, which has a drainage area about 20% greater than at the dam site.
3.2 Historic Floods
For the two long-term USGS gages upstream or near Gross Dam plus the calculated inflows to Gross Reservoir, the ranked highest peak flows and daily average flows of record have been summarized in Table 3.2-1 through Table 3.2-4. Floods for the same date at different stations have been highlighted in the same color. Floods with the largest recorded peaks at most gages are favored for selection as flood hydrograph calibration floods. As would be expected, there is some variation in the flood rankings from gage to gage, in part due to the period of record available for each gage.
Data in Table 3.2-3 for South Boulder Creek near Rollinsville would include Moffat Tunnel diversions. Records of Moffat Tunnel flows prior to 1956 are unavailable. Data in Table 3.2-4, calculated daily maximum inflows to Gross Reservoir have been deregulated to remove Moffat Tunnel flows.
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Table 3.2-1. Recorded Peak Instantaneous Flows – South Boulder Creek near Eldorado Springs (1896-1995)
Ranked Rank Annual Peak Peak Flow Date (cfs) 1 9/2/1938 7,390 2 6/18/1951 2,370 3 5/7/1969 1,690 4 6/6/1921 1,440 5 6/6/1949 1,430 6 6/20/1909 1,340 7 6/21/1947 1,290 8 5/24/1914 1,240 9 6/3/1895 1,130 10 5/9/1900 1,100 11 6/4/1952 1,080 12 6/15/1953 988 13 6/22/1918 915 14 5/13/1942 913 15 6/16/1965 910 16 7/21/1957 905 17 6/12/1915 885 18 5/29/1956 852 19 6/18/1995 845 20 6/26/1937 780
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Table 3.2-2. Recorded Daily Maximum Flows – South Boulder Creek near Eldorado Springs (1896-1995)
Ranked Rank Annual Peak Peak Flow Date (cfs) 1 6/18/1951 1,390 2 5/7/1969 1,120 3 6/7/1921 1,050 4 6/6/1949 1,020 5 5/24/1914 950 6 6/22/1947 929 7 6/19/1909 910 8 6/4/1952 864 9 5/13/1942 830 10 6/18/1995 760 11 6/16/1965 740 12 6/29/1957 711 13 6/22/1918 702 14 6/12/1915 680 15 6/17/1959 632 16 6/13/1938 626 17 6/13/1924 604 18 6/13/1953 598 19 6/10/1923 594 20 6/26/1937 586
Table 3.2-3. Recorded Peak Instantaneous Flows – South Boulder Creek near Rollinsville (1911-1949, intermittent)
Ranked Rank Annual Peak Peak Flow Date (cfs) 1 6/12/1949 718 2 6/10/1947 622 3 6/2/1914 542 4 5/22/1948 498 5 6/20/1915 484 6 6/8/1912 450 7 6/22/1917 432 8 6/28/1945 410 9 6/10/1946 358 10 6/12/1911 350 11 6/10/1916 324 12 5/31/1913 320
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Table 3.2-4. Calculated Daily Maximum Inflows – Gross Reservoir (1958-2016)
Ranked Rank Annual Peak Daily Flow Date (cfs) 1 6/17/1995 869 2 6/7/1997 838 3 6/17/1965 787 4 9/12/2013 770 5 5/30/2003 674 6 5/30/2014 671 7 5/6/1969 660 8 5/24/1984 624 9 6/12/1983 592 10 5/20/1973 578 11 6/11/2015 577 12 6/1/1991 543 13 6/11/1980 513 14 6/18/1975 512 15 6/13/1959 508 16 6/12/2010 505 17 6/23/1971 487 18 5/17/1987 480 19 6/15/1978 471 20 5/27/1970 468
Because of an apparent lack of large recorded floods in the watershed tributary to Gross Dam, a search was conducted for records of major floods on other similar streams in the vicinity of Gross Dam. This search was limited to watersheds with streamflow gage elevations of at least 6,500 feet because of the notable elevation dependent flood increases noted by Jarrett (1993) to occur below about elevation 7,500 feet. Gaging stations were also excluded where a reservoir could potentially cause significant flow regulation. Table 3.2-5 shows a summary of information obtained from qualifying USGS gaging stations. Middle Boulder Creek at Nederland was included in the table because a small upstream reservoir controls an insignificant part of the watershed. The largest maximum unit flow rate shown in Table 3.2-5 occurs at a much smaller drainage area than for Gross Dam, but still the unit discharge is not large compared to the Gross Dam PMF unit discharge results shown in Section 9.
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Table 3.2-5. Maximum Peak Flows in the Vicinity of Gross Dam Watershed
USGS Gage Drainage Period Years of Recorded Peak Flows Gage USGS Gage Name Elev. Area of Peak Q Peak Q Maximum Max. Unit Q Upstream Number (feet) (sq. mi.) Record Record (cfs) (cfs/mi2) Reservoirs Bear Creek near 06710350 7,150 96.3 1979 - 1990 12 465 4.8 None Evergreen
06716500 Clear Creek near Lawson 8,080 147.0 1946 - 2015 62 6,130 41.7 None Middle St. Vrain C. near 06723000 7,560 28.0 1926 - 1994 21 892 31.9 None Allens Park Middle Boulder Creek at Peterson 06725500 8,186 36.2 1908 - 1995 87 811 22.4 Nederland Lake Coal Creek near 06730300 6,540 15.2 1959 - 1982 24 2,060 135.5 None Plainview Big Thompson River Lawn Lake 06733000 7,493 138.0 1947 - 1998 52 1,870 13.6 at Estes Park Dam (1) Note 1: Lawn Lake Dam was an earthen dam that failed on July 15, 1982 with a peak flow of 5,500 cfs at USGS Gage 06733000.
The Big Thompson River flood of July 31, 1976 was an exceptional flood event for the region that exhibited extreme spatial variability of runoff (Jarrett and Costa, 2006). The Big Thompson River above Drake (drainage area 189 square miles) had a peak flow of 28,200 cfs. Upstream at USGS gage 06733000, Big Thompson River at Estes Park (drainage area 138 square miles), the peak discharge was only 457 cfs. It is noted that the Estes Park gage is at elevation 7,493 feet, the elevation above which significant flood decreases have been noted (Jarrett 1993).
As stated by Jarrett and Costa (1988), “Precipitation, streamflow, and geomorphic evidence indicates that there is a distinct decrease in floods above about 7,500 feet in the foothills of northern Colorado”. This information generally supports a conclusion that the perceived low recorded South Boulder Creek maximum flows above Gross Dam are typical of Colorado Front Range watersheds at a similar elevation.
Historic maximum flood information for an even wider area including lower elevations and additional Big Thompson flood data for comparison purposes is provided in Appendix A. The information in Appendix A was presented at the Gross Dam PMF BOC Meeting #2 on August 4, 2016.
3.2.1 Flood Frequency
Peak annual flows have been recorded by the USGS at Eldorado Springs for the unusually long period of 103 years, as summarized in Table 3.2-6. Peak flow rates provided by the USGS include both average daily values and instantaneous peaks. It is noted that peak flows for time periods that
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were both unregulated (prior to the existence of Gross Reservoir) and regulated (with Gross Reservoir) are included in the peak flow table.
Peak flows for return periods up to 10,000 years were estimated for South Boulder Creek at Eldorado Springs. Peak flows were estimated for various return periods by fitting recorded peak flow data with a Log Pearson Type III distribution according to methods in Bulletin 17B (IACWD, 1982). The station log-skew coefficient of 1.4 was used to develop the flood frequency estimates.
The quality of the fit of the parameterized Log Pearson Type III distribution to the observed data is evaluated by plotting the data and the parameterized distribution together. A good fit is indicated by data points for observed annual peaks which are close to and randomly distributed above and below the computed Log Pearson Type III curve. The probability values assigned to each data point, called plotting positions, and the scale of the x-axis, are selected so that the Log Pearson Type III distribution appears as a straight line when the skew value is zero.
The fitted distribution and resulting estimated peak flows at specified return periods are approximations. The ability to fit a distribution depends on the size and the variability within the sample. Confidence limits around the computed distribution curve provide a measure of the uncertainty for the predicted discharge at a specified exceedance probability.
Figure 3.2-1 shows the fitted Log Pearson Type III distribution as a solid line, 5 percent and 95 percent upper and lower confidence limits on the distribution as dashed lines, the observed annual peak flow data, and return periods for which peak flows were estimated in Table 3.2-7. On Figure 3.2-1, the standard normal variable values on the x-axis represent standard deviations from the mean.
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Table 3.2-6. Peak Annual Flows for South Boulder Creek near Eldorado Springs
Date Peak Flow Date Peak Flow Date Peak Flow (cfs) (cfs) (cfs) June 19, 1888 245 May 22, 1927 343 June 21, 1961 375 May 31, 1889 730 May 27, 1928 490 May 13, 1962 458 May 28, 1890 705 June 6, 1929 310 June 16, 1963 311 May 25, 1891 650 June 19, 1930 536 July 10, 1964 626 June 24, 1892 730 June 8, 1931 427 June 16, 1965 910 June 3, 1895 1,130 May 23, 1932 356 July 15, 1966 360 May 29, 1896 382 May 19, 1933 666 June 6, 1967 469 June 11, 1897 650 May 15, 1934 275 June 7, 1968 468 June 17, 1898 475 June 11, 1935 477 May 7, 1969 1,690 June 20, 1899 700 May 16, 1936 420 May 21, 1970 469 May 9, 1900 1,100 June 26, 1937 780 June 27, 1971 455 June 24, 1901 360 Sept. 2, 1938 7,390 June 12, 1972 416 June 5, 1905 740 June 1, 1939 540 May 6, 1973 450 June 14, 1906 655 July 28, 1940 688 May 30, 1974 475 June 15, 1907 685 May 12, 1941 672 July 3, 1975 344 June 15, 1908 315 May 13, 1942 913 June 9, 1976 282 June 20, 1909 1,340 May 30, 1943 538 June 7, 1977 318 June 3, 1910 245 June 2, 1944 528 May 26, 1978 384 June 9, 1911 440 June 25, 1945 558 July 3, 1979 326 June 25, 1912 645 June 15, 1946 568 June 16, 1980 406 May 29, 1913 350 June 21, 1947 1,290 June 4, 1981 366 May 24, 1914 1,240 May 23, 1948 639 June 19, 1982 435 June 12, 1915 885 June 6, 1949 1,430 June 13, 1983 602 June 11, 1916 350 June 13, 1950 737 July 3, 1984 334 June 18, 1917 563 June 18, 1951 2,370 May 31, 1985 274 June 22, 1918 915 June 4, 1952 1,080 June 4, 1986 286 August 7, 1919 560 June 15, 1953 988 June 4, 1987 350 May 26, 1920 531 May 21, 1954 247 May 20, 1988 474 June 6, 1921 1,440 June 9, 1955 478 June 1, 1989 363 June 13, 1922 397 May 29, 1956 852 June 11, 1990 355 June 9, 1923 646 July 21, 1957 905 June 1, 1991 368 June 14, 1924 625 June 6, 1958 345 May 24, 1992 346 June 22, 1925 186 June 17, 1959 680 June 1, 1993 383 May 24, 1926 561 June 6, 1960 556 June 2, 1994 359 June 18, 1995 845
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Return Period (Years) 2 5 10 20 50 100 500 1,000 10,000
Log Pearson Type III Flood Frequency for the South Boulder Creek near Eldorado Springs
10,000 1888 - 1995 Computed Curve Observed Annual Peaks 5% and 95% Confidence Limit Curves Peak Flow (cfs)
1,000
100 -2.6 -1.6 -0.6 0.4 1.4 2.4 3.4 Standard Normal Variable
Figure 3.2-1. Log Pearson Type III Flood Frequency Plot for South Boulder Creek near Eldorado Springs
Table 3.2-7. Calculated Flood Frequency for South Boulder Creek near Eldorado Springs
Return Period Flow (Years) (cfs) 2 480 5 790 10 1,110 25 1,710 50 2,360 100 3,240 200 4,410 500 6,540 1,000 8,710 10,000 20,600
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Based on the historic flood records presented in Section 3.2 and the flood frequency estimates above, there appears to be a general lack of large recorded floods in or near the Gross Dam watershed relative to previous estimates of the PMF. This raises the question of whether the perceived lack of large recorded floods in the Gross Dam watershed is unusual, or whether it is a characteristic of similar watersheds in the region. Jarrett (1993) performed an analysis of 77,987 station-years of streamflow gaging station data from 3,748 stations in the Rocky Mountains. The analysis indicates that there is a latitude-dependent elevation limit to substantial rainfall-induced flooding. This study included 935 gaging stations and 18,238 station-years of flow data for the Rocky Mountains in Colorado. Although maximum flow values were provided in the Jarrett (1993) paper, details on the specific watersheds (area, location) were not provided. Streamflow data was separated by station elevation, above and below 2,300 m (about 7,500 feet elevation). For Colorado stations above 7,500 feet, the maximum unit discharge found was 101 cfs/sq. mi., or about 9,400 cfs for a drainage area the same as is tributary to Gross Dam. For Colorado stations below 7,500 feet, the maximum unit discharge was 3,475 cfs/sq. mi. (drainage area not given, but probably a small watershed), or about 323,000 cfs for a drainage area the same as is tributary to Gross Dam. The applicable conclusion from the Jarrett study is that the perceived lack of large floods in the drainage area tributary to Gross Dam is characteristic of the similar Colorado Rocky Mountain region based on elevation, and not unique to the Gross Dam watershed. The most exceptional recorded discharge near Gross Dam was the 7,390 cfs peak flow on September 2, 1938 at the Eldorado Springs gage, which corresponds to unit discharge of 66.6 cfs/sq. mi.
A study prepared by the USGS (Kohn, et al, 2016) provides additional flood frequency information in the vicinity of Gross Dam. Table 3.2-8 shows data taken from this USGS report at gages in the vicinity of Gross Dam watershed. Although the gage elevations are much lower than Gross Dam, the unit discharge flood frequencies are still relatively low. No paleoflood information was provided for the area near Gross Dam.
Table 3.2-8. Flow Frequency for Watersheds in the Vicinity of Gross Dam Watershed
USGS Drainage Period Years of 100-Year Flood 500-Year Flood
Gage USGS Gage Name Gage Elev. Area of Peak Q Peak Q Q100 Unit Q100 Q500 Unit Q500 Number (feet) (sq. mi.) Record Record (cfs) (cfs/mi2) (cfs) (cfs/mi2) Boulder C. at mouth 06730500 4,860 447.0 1927 - 2017 64 5,190 11.6 8,480 19.0 near Longmont St. Vrain Creek at 06724000 5,292 216.0 1895 - 1998 104 7,430 34.4 13,900 64.4 Lyons Left Hand Creek near 06724500 5,710 52.0 1929 - 1980 16 1,230 23.7 1,660 31.9 Boulder
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3.2.2 Seasonal Flood Distribution
Table 3.2-9 summarizes the month of occurrence of the annual peak flow at each of the two long- term USGS gages in or near the watershed tributary to Gross Dam plus the calculated daily inflows to Gross Reservoir. June is the month during which the annual maximum flows most frequently occur. In 173 station-years of annual maximum flow data, an annual maximum flow has never been recorded during the months of October through April.
Additional daily flow frequency data for South Boulder Creek at Eldorado Springs is provided on Figure 3.2-2 and for the USGS gage at Rollinsville on Figure 3.2-3. The information presented in the table and two figures in this section indicates that the months of October through April can be eliminated from further consideration as a potentially critical month for the PMF. About three years of data after 1936 included in data used to develop Figure 3.2-3 would include Moffat Tunnel flows. Data for Moffat Tunnel flows prior to 1956 are unavailable.
Table 3.2-9. Monthly Distribution of Annual Peak Flows
Drainage Area = 111 sq.mi. Drainage Area = 93 sq.mi. Drainage Area = 42.7 sq.mi. South Boulder Creek Calculated Daily Inflows South Boulder Creek near Eldorado Springs to Gross Reservoir near Rollinsville Peak % of Peak % of Peak % of Month Years Total Years Total Years Total January 0 0% 0 0% 0 0% February 0 0% 0 0% 0 0% March 0 0% 0 0% 0 0% April 0 0% 0 0% 0 0% May 30 29% 22 38% 2 17% June 64 62% 35 60% 10 83% July 7 7% 0 0% 0 0% August 1 1% 0 0% 0 0% September 1 1% 1 2% 0 0% October 0 0% 0 0% 0 0% November 0 0% 0 0% 0 0% December 0 0% 0 0% 0 0% Total 103 100% 58 100% 12 100%
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1,200 Maximum 1% Exceedance Based on 46 water years of data from 1905 - 1950. 5% Exceedance USGS gaging station 06729500, 1,000 25%Exceedance South Boulder Creek near Eldorado Springs. Drainage area is 111 square miles. 50% Exceedance 90% Exceedance 800
600 Daily Flow(cfs)
400
200
0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 3.2-2. Historic Daily Flow Frequency at the USGS Eldorado Springs Gage
1,200
Maximum 2% Exceedance Based on 7 water years of data from 1915 - 1949. USGS gaging station 06729000, 1,000 5% Exceedance South Boulder Creek near Rollinsville. Drainage area is 42.7 square miles. 25%Exceedance 50% Exceedance
800 90% Exceedance
600 Daily Flow(cfs)
400
200
0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 3.2-3. Historic Daily Flow Frequency at the USGS Rollinsville Gage
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3.3 Precipitation Associated with Historic Floods
The Storm Precipitation Analysis System (SPAS) was used to develop historical precipitation data for the South Boulder Creek watershed upstream from the USGS gage at Eldorado Springs. SPAS is a state-of-the-science hydrometeorological tool used to characterize the magnitude, temporal, and spatial details of precipitation events. A more complete discussion of the development of historic precipitation for use in runoff model calibration is included in Appendix B.
Historical data was acquired to develop temperature time series for use in rain on snow PMF modeling. Information from four storms was used in the runoff model calibration efforts, as summarized in Table 3.3-1 and Figures 3.3-1 through 3.3-4..
Table 3.3-1. Precipitation for Historic Storms (from AWA)
Precipitation (inches) Aug. 30 - May 3 - June 11 - Sept 8 - Area Sub-basin Sept. 5, May 5, June 28, Sept 18, (sq. mi.) 1938 1969 1995 2013 B1 8.9 1.9 6.4 0.3 1.9 B2a 7.7 2 6.9 0.3 5.6 B2b 13.8 2 6.8 0.3 4.9 B3 12.6 2.2 8 0.3 5.7 B4 31.7 2.6 9.1 0.6 6.4 B5a 12.7 3.7 9.7 0.7 9.8 B5b 5.8 3.5 9.7 0.9 9 Gross Dam 93 2.5 8.3 0.5 6.2 Basin Average B6 0.4 4.7 10.1 0.6 12.3 B7 13.9 5 10.6 0.6 12.4 B8 4 5.6 10.3 0.6 14.1 Eldorado Springs Gage 111.3 3 8.6 0.5 7.3 Basin Average
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Figure 3.3-1. Flow Data and September 1938 Cumulative Sub-basin Rainfall Depths
Figure 3.3-2. Flow Data and May 1969 Cumulative Sub-basin Rainfall Depths
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Figure 3.3-3. Flow Data and June 1995 Cumulative Sub-basin Rainfall Depths
Figure 3.3-4. Flow Data and September 2013 Cumulative Sub-basin Rainfall Depths
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3.4 Snowpack and Snowmelt During Historic Floods
Normally three floods are selected for calibration and verification of unit hydrograph parameters and loss rates. Because the South Boulder Creek is subject to two distinctly different types of floods, snowmelt dominated floods in the spring and rainfall dominated floods in the summer, two historic floods of each type were selected for analysis. The flood periods selected for calibration and verification of hydrograph parameters are:
1. September 1938 (summer)
2. May 1969 (spring)
3. June 1995 (spring)
4. September 2013 (summer)
Table 3.4-1 summarizes the earliest and latest recorded dates for snowpack at the SNOTEL stations. To be counted as snowpack, the recorded snow on the ground must persist on a seasonal basis. There is no evidence of a snowpack existing for the August and September calibration storms, other than small permanent snowfields.
Table 3.4-1. Earliest and Latest Snowpack at SNOTEL Stations
Station Continental Elevation Maximum SWE Earliest Day Latest Day Reporting Station Name Number Divide Loc. (feet) (inches) Date with Snowpack with Snowpack Since Lake Eldora 564 East 9,700 24.0 7-Apr-1996 21-Oct-2015 19-Jun-1995 1-Oct-1978 University Camp 838 East 10,300 34.8 4-May-1996 4-Oct-1986 9-Jul-1995 1-Oct-1978 Niwot 663 East 9,910 21.1 4-May-1996 6-Oct-2009 22-Jun-1995 9-Jun-1980 Sawtooth 1251 East 9,620 26.0 10-May-2016 21-Oct-2015 21-Jun-2015 10-Sep-2013 Wild Basin 1042 East 9,560 26.0 5-May-2011 4-Oct-2009 13-Jun-2011 20-Dec-2002 High Lonesome 1187 West 10,620 25.9 15-Apr-2015 5-Oct-2013 20-Jun-2014 6-Jul-1905 Fool Creek 1186 West 11,150 27.2 17-May-2014 5-Oct-2013 21-Jun-2014 1-Oct-2011 Berthoud Summit 335 Crest 11,300 34.8 25-May-2011 22-Sep-2009 7-Jul-1995 1-Oct-1978 Echo Lake 936 East 10,600 16.1 8-May-2007 4-Oct-2009 14-Jun-2015 8-Sep-1998
Table 3.4-2 presents a summary of the antecedent snowpack used for the spring calibration storms, as developed by AWA.
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Table 3.4-2. Antecedent Snowpack Snow Water Equivalent
May 3, 1969 - June 11, 1995 - Sub-basin May 9, 1969 June 28, 1995 B1 4.7 15.9 B2a 1.6 10 B2b 1.2 9.5 B3 0.3 2.4 B4 0 0.1 B5a 0 0 B5b 0 0 Gross Dam 0.8 4.1 Basin Average B6 0 0 B7 0 0 B8 0 0 Eldorado Springs Gage 0.7 3.4 Basin Average
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4. UNIT HYDROGRAPH DEVELOPMENT
4.1 Approach and Tasks
The South Boulder Creek basin was considered to be a case where sufficient streamflow data of satisfactory quality were available for developing unit hydrographs. Three USGS gages have been in operation for various periods within or not far downstream of the area tributary to Gross Dam. Two of the USGS gages, plus the calculated inflows to Gross Reservoir were used in the calibration of unit hydrograph parameters. The USGS gage at Rollinsville was not operational for any of the selected calibration floods. Snowpack data is available at several stations (see section 3.4 and 8.3) and was considered to be adequate. Although long-term precipitation stations are not available within the watershed tributary to Gross Dam, a meteorological model (by AWA) provided estimated data using stations near the watershed. As discussed in Section 2.1, the HEC-HMS Hydrologic Modeling System (USACE HEC, 2015) was chosen as the watershed model to perform the calibration and verification runs and the final PMP runoff and PMF routing runs.
Nine floods were initially considered for runoff model calibration and verification, with four being selected. The South Boulder Creek is subject to annual maximum flows, having two distinctly different predominant origins; snowmelt in the spring and rainfall in the summer. Two floods of each type were selected for calibration and verification. Preference for selection of historic floods for calibration and verification was based on:
• the largest floods of record;
• the floods with data at the most streamflow gages;
• the floods with the most complete flow data near the peak flow;
• distribution of floods in the May through September potential flood season.
The floods selected for calibration are summarized in Table 4.1-1.
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Table 4.1-1. Calibration/Verification Floods
Date Season Comments Storm Process September Only non-May/June annual maximum flood with records at Summer Rainfall-runoff 2013 Gross Dam. September Maximum peak flow in over 100 years of record at Eldorado Summer Rainfall-runoff 1938 Springs. Streamflow records available at Gross Dam and Eldorado May 1969 Spring Springs; very high recorded rainfall depths at Eldorado Springs Rain-on-Snow and Gross Dam. Largest calculated daily inflow in 58 years of record at Gross June 1995 Spring Dam; 19th largest at Eldorado Springs; data available below the Rain-on-Snow dam; latest recorded snowpack at long-term SNOTEL gages.
The available USGS gaging station data for these floods are plotted on Figure 4.1-1 through Figure 4.1-4. These plots provide an indication of the relative magnitude and timing of flows at the various gaging stations for the period both before and after the peak flows. All data on these plots identified as inflow to Gross Dam has been deregulated to remove the effects of Moffat Tunnel flows.
700
600
500 South Boulder Creek near Eldorado Springs USGS Gage 06729500
400
Flow(cfs) 300
200
100
0 8/28/1938 9/2/1938 9/7/1938 9/12/1938 9/17/1938 9/22/1938 Figure 4.1-1. June 1938 Recorded Daily Flows at Eldorado Springs
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1,200
USGS 06729500 near Eldorado
1,000 Inflow to Gross Dam (calculated)
Below Gross Dam CO DWR BOCBGRCO 800
600
400 Average Daily Flow (cfs) Flow Daily Average
200
0 5/1/1969 5/3/1969 5/5/1969 5/7/1969 5/9/1969 5/11/1969 5/13/1969 Figure 4.1-2. May 1969 Recorded and Calculated Daily Flows
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1,000 USGS 06729500 near Eldorado 900 Calculated Inflows at Gross Dam
800 Below Gross Dam CO DWR BOCBGRCO
700
600
500
400 Average Daily Daily Flow(cfs) Average 300
200
100
0 6/11/1995 6/13/1995 6/15/1995 6/17/1995 6/19/1995 6/21/1995 6/23/1995 6/25/1995 6/27/1995 Figure 4.1-3. June 1995 Recorded and Calculated Daily Flows
800
700
600 Inflow at Gross Dam (calculated) Pinecliff CO DWR BOCPINCO 500 Below Gross Dam CO DWR BOCBGRCO
400
300 Daily Average DailyAverage Flow (cfs)
200
100
0 9/1/2013 9/6/2013 9/11/2013 9/16/2013 9/21/2013 9/26/2013 10/1/2013 Figure 4.1-4. September 2013 Recorded and Calculated Daily Flows
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4.2 Unit Hydrograph Methodology
The rainfall to runoff transform method for the sub-basins was based on the ModClark unit hydrograph method. To enable the Clark method to be used with gridded cells, it was modified in HEC-HMS to distribute parameters from a sub-basin to the grid cells, with the resulting method called ModClark. The Clark unit hydrograph method is a featured method in FERC guidelines (FERC 2001), is one of the rainfall to runoff transformation methods available in HEC-HMS (USACE HEC 2015) and is a widely used method by the Corps of Engineers (USACE HEC, 2001). SEO guidelines recommend the following types of synthetic unit hydrographs for estimating inflow design floods in Colorado:
• Dimensionless unit hydrographs such as those in the U.S. Bureau of Reclamation (USBR) Flood Hydrology Manual (Cudworth, 1989);
• S graphs such as those in the USBR Flood Hydrology Manual (Cudworth, 1989);
• Clark unit hydrograph, with limitations.
The SEO Guidelines states that the preferred method for the Rocky Mountain Type of Watershed would be the dimensionless unit hydrograph by USBR but they also may accept the Clark unit hydrograph method.
The ModClark method was selected for the Gross Dam watershed taking into account: • The Clark unit hydrograph is an acceptable method in FERC (2001) PMF guidelines;
• The Clark unit hydrograph is a widely used method for large watersheds that MWH has extensive experience using (see Section 5);
• It is the only unit hydrograph method available in HEC-HMS for gridded models;
• By using a gridded approach in HEC HMS, the gridded site specific PMP developed by AWA can be most directly utilized.
By using the ModClark Method, the watershed is divided into uniform grids cells and each cell represents a small sub watershed. Therefore, it is a linear, quasi-distributed transform method based on the Clark conceptual unit hydrograph.
The Clark (or ModClark) unit hydrograph method requires two parameters, Tc and R (USACE 1994). Equations to estimate these two parameters are presented in SEO guidelines (2008) as follows:
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• Time of Concentration (Tc) - the travel time, during the corresponding period of most intense rainfall excess, for a flood wave to travel from the hydraulically most distant point in the watershed to the point of interest (concentration point). The recommended equation for Rocky Mountain, Great Plains and Colorado Plateau type watersheds is:
= 2.4 . . . . Where: 0 1 0 25 0 25 −0 2 𝑇𝑇𝑐𝑐 𝐴𝐴 𝐿𝐿 𝐿𝐿𝑐𝑐𝑐𝑐 𝑆𝑆 Tc = time of concentration, in hours A = area, in square miles S = watercourse slope, in ft/mile L = length of the watercourse to the hydraulically most distant point, in miles
Lca = length measured from the concentration point along L to a point on L that is perpendicular to the watershed centroid, in miles • Basin storage coefficient (R) - relates the effects of direct runoff storage in the watershed to unit hydrograph shape. The equation for estimating the storage coefficient (R) is:
= 0.37 . . . Where: 1 11 0 80 −0 57 𝑅𝑅 𝑇𝑇𝑐𝑐 𝐿𝐿 𝐴𝐴 Tc = time of concentration, in hours A = area, in square miles L = length of the watercourse to the hydraulically most distant point, in miles The standard Clark unit hydrograph method requires an additional component called a time-area relation. The ModClark method eliminates need for a time-area curve and instead uses a separate travel time index for each grid cell. The travel time index for each cell is scaled by the overall time of concentration (USACE 2015).
4.3 Preliminary Estimates of Clark Parameters
Initial Clark Unit Hydrograph parameters were estimated following Colorado Dam Safety Guidelines (SEO 2016) which calls for equations and procedures outlined in SEO (2008). The initial estimates for Clark parameters were adjusted based on calibration and modification of the R parameter to simulate selected historic floods. A frequently used concept for calibration is that the ratio R/(Tc + R) tends to be fairly constant on a regional basis (FERC, 2001). A summary of initial Clark parameters are shown in Table 4.3-1.
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Table 4.3-1. Initial Estimates of Clark Parameters
Longest Sub-basin Description Flowpath Tc (hours) R R/(Tc+R) Name (miles) B1 Moffat Tunnel Basin 5.11 1.36 0.55 0.29 B2a Moffat Tunnel DS N Basin 7.20 2.02 1.23 0.38 B2b Moffat Tunnel DS S Basin 8.77 2.10 1.07 0.34 B3 Rollinsville, CO Basin 7.29 2.16 1.01 0.32 B4 Pinecliffe, CO Gage Basin 10.82 2.82 1.10 0.28 B5a Gross Dam Reservoir Basin 6.80 2.10 0.92 0.30 B5b Gross Dam Reservoir Inflow 6.42 1.66 1.06 0.39 B6 Below Gross Dam 1.20 0.51 0.33 0.39 B7 S Boulder Cr Diversion Structure 7.70 2.09 0.96 0.31 B8 Eldorado Springs, CO Gage 4.97 1.33 0.83 0.39
The Clark unit hydrograph method is not recommended for watersheds larger than 10 square miles in the SEO (2008) guidelines, not because of any limitations in applicability of the general Clark method (or ModClark) to large watersheds, but because the Clark Tc and R equations in the SEO guidelines were derived for much smaller watersheds (i.e., <5 sq. mi., George Sabol, personal communication). Tc and R values derived for much smaller watersheds might tend to develop a more peaky unit hydrograph than would be applicable to a larger, mostly forested watershed.
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5. HYDROLOGIC MODEL CALIBRATION
Development of unit hydrograph parameters for the ModClark unit hydrograph method involves the two parameters Tc (time of concentration) and R (a storage coefficient). A frequently used concept for calibration is that the ratio R/(Tc + R) tends to be fairly constant on a regional basis. Table 5.1-1 summarizes the calibration scenarios that were simulated. The R/(Tc + R) ratio values of 0.6 to 0.8 are based on both MWH PMF experience in mountainous basins and broadly used Corps of Engineers experience values (USACE HEC 2001). The same final Clark unit hydrograph parameters were used for all floods, both spring and summer.
The USACE Hydrologic Engineering Center has developed hydrologic and hydraulic models of the Sacramento and San Joaquin Rivers for the purpose of developing comprehensive flood control plans (USACE HEC, 2001). The Sacramento and San Joaquin Rivers comprise nearly 60,000 square miles and were simulated in 33 individual models. This comprehensive flood study used the HEC-HMS model with gridded precipitation. The ModClark rainfall to runoff transformation was used to compute the subbasin hydrographs. Clark unit hydrograph parameter optimization was performed for 30 subbasins, ranging in size from 11.6 square miles to 613.8 square miles with longest flow path slopes ranging from 1.9% to 12.8% (similar slopes to Gross Dam watershed subbasins). The R/(Tc + R) ratio for the 30 parameter optimization subbasins ranged from 0.50 to 0.91, with 25 of 30 falling in the range of 0.59 to 0.86. The smallest subbasin (11.6 sq. mi.) had an R/(Tc + R) value of 0.84, and the largest subbasin (613.8 sq. mi.) had an R/(Tc + R) ratio of 0.77. Using the principle that the R/(Tc + R) ratio is constant for hydrologically similar areas, a value of either 0.6 or 0.8 was assigned to subbasins depending on the particular river. Considering the scope of this study that includes the western slope of the Sierra Nevada Mountains in much of California and the narrow range of R/(Tc + R) values used, it supports the R/(Tc + R) value of 0.6 for the Gross Dam watershed as being reasonable to somewhat conservative and confirms the applicability of the ModClark and gridded precipitation approach.
In addition, MWH has previously used the Clark unit hydrograph method for PMF determination on a number of mountainous watersheds. MWH experience values for the R/(Tc + R) ratio are around 0.6 or slightly higher for several watersheds. Additional detailed information is presented in Section 9.3.
5.1 Calibration Scenarios
Calibration of the Gross Dam hydrologic model was performed by applying sets of reasonable rainfall-runoff and snowmelt parameters to best match observed or estimated historic flood events. The precipitation associated with each calibration event is summarized in Section 3.3. Each calibration event basin model was different based on available information and components within
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the Gross Dam watershed. It is emphasized that the simulated hydrographs are the result of both the basic meteorological data input, as well as the HEC-HMS model input parameters, such as unit hydrograph and loss parameters.
Because the ultimate objective of the calibration was to develop a reasonable set of parameters to use for developing the PMF, parameters outside the generally accepted range were not used, regardless of whether they would improve the calibration. For example, where simulated runoff volumes were substantially greater than recorded volumes, loss rates were not raised to unreasonably high values that would be necessary to obtain a good calibration. Using exceptionally high loss rates for calibration, far outside the normal range, would not be acceptable to transfer as input to the PMF simulation runs, so there was no need to even test extraordinary parameters. Tabulated volume and peak flow results as well as hydrographs for each calibration event are summarized and reported in Section 5.1 and Section 5.2. The figures include all scenarios for completeness.
Table 5.1-1. Calibration Scenario Descriptions
Scenario Initial Losses Uniform Losses Tc Method (1) R/(Tc+R) 1 0" Initial Losses Average uniform Losses Tc ~0.3(2) 2 1.2" Initial Losses Average uniform Losses Tc 0.6 3 1.2" Initial Losses Average uniform Losses Tc 0.8 4 0" Initial Losses High uniform Losses Tc 0.6 5 0.5" Initial Losses High uniform Losses Tc 0.6 6 0" Initial Losses Average uniform Losses Tc 0.8 7 0" Initial Losses Average uniform Losses Tc + 20% 0.8 8 0" Initial Losses 0.02 in/hr Uniform Loss Tc 0.8 9 0" Initial Losses High uniform Losses Tc 0.8 10 0" Initial Losses High uniform Losses Tc +100% 0.8 Notes: (1) Tc calculated following procedures in SEO (2008). (2) Tc and R calculated following procedures in SEO (2008).
The initial abstraction and uniform loss rate method of simulating infiltration was used. Ten scenarios were used for model calibration with initial and uniform losses as defined in Table 5.1-1. Initial abstractions (losses at the start of the storm) ranged in the calibration runs from 0.0 to 1.2 inches and uniform loss rates are taken as a function of the sub-basin hydrologic soil group composition. Uniform loss rates are presented as a range of losses based on the hydrologic soil group (see Table 7.2-1). A reference to high uniform losses in Table 5.1-1 would, for example, indicate that the high end of the range of losses for each soil type was used for the scenario.
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5.2 Summer Floods
The September 1938 flood is significant because it had the advantage of occurring before construction of Gross Dam, such that the flow records at the Eldorado Springs gage are not reservoir regulated. The USGS gaging station at Eldorado Springs has the only flow records available for the September 1938 flood, but this event is also significant because it was the largest recorded peak flow (7,390 cfs) in over 100 years of record. Simulated and recorded flows for the period August 30 through September 5, 1938 are summarized in Table 5.2-1 and are plotted on Figure 5.2-1 and Figure 5.2-2. It is noted that for all scenarios, the simulated runoff volume is substantially greater than the observed volume. On Figure 5.2-1, the highest (black line) peak is for the minimum loss rate Scenario 8. On Figure 5.2-2, the observed peak flow is shown with a blue triangle, and the highest peak flow (red line) is for Scenario 1 that most closely uses the SEO (2008) parameter estimates. Scenario 5 produced a peak flow that was 47% above the observed peak flow.
In both the summer and spring hydrologic model calibration runs, simulated peak flows consistently overestimated the recorded peak flows, even after the R (attenuation) values were increased from SEO guideline values. Hydrograph volumes were generally overestimated near the peak day, but some simulated hydrograph volumes were similar to observed volumes when considering a longer period of several days. This gives one indication of reasonable precipitation values, but a hydrograph that is too peaky, but does not eliminate rainfall data as a potential error source.
Calibration results are presented for the September 8-17, 2013 flood event on Table 5.2-2 through Table 5.2-4, and Figure 5.2-3 through Figure 5.2-6. In general, the pattern of overestimating the peak hourly and peak daily flows is similar to the September 1938 event, with Scenario 8 having the largest overestimate of daily values and Scenario 1 having the largest overestimate of hourly peak values. Scenario 5 has similar overall runoff volumes compared to those observed at Pinecliffe and inflows to Gross Reservoir. Eldorado Springs flow volumes were affected by missing data. Simulated peak flows for Scenario 5 were consistently high, including a simulated peak flow of 3,540 cfs at Eldorado Springs compared to the estimated peak flow of 2,100 cfs. As shown on Table 3.3-1, the most intense rainfall fell between Gross Dam and Eldorado Springs gaging station.
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Table 5.2-1. September 1938 Simulation Results
September 1938 Simulation Eldorado Springs, CO Observed Peak Simulated Observed Initial Constant Simulated Simulated Scenario Runoff Discharge Peak Daily Peak Daily Loss Loss R/(Tc+R) Runoff % Diff Peak Flow Volume (USGS) Avg Flow Flow (in) (in/hr) (ac-ft) (cfs) (ac-ft) (cfs) (cfs) (cfs) 1 0 Avg ~0.3(1) 7,539 2,675 182% 19,609 7,390 2,376 595 2 1.2 Avg 0.6(2) 6,455 2,675 141% 10,689 7,390 2,603 595 3 1.2 Avg 0.8(2) 6,331 2,675 137% 5,891 7,390 2,685 595 4 0 Max 0.6(2) 6,446 2,675 141% 10,841 7,390 2,582 595 5 0.5 Max 0.6(2) 6,437 2,675 141% 10,841 7,390 2,582 595 6 0 Avg 0.8(2) 7,413 2,675 177% 6,878 7,390 3,081 595 7 0 Avg 0.8(2) 7,429 2,675 178% 6,876 7,390 3,085 595 8 0 0.02 0.8(2) 12,105 2,675 352% 9,443 7,390 4,440 595 9 0 Max 0.8(2) 6,364 2,675 138% 6,030 7,390 2,687 595 10 0 Max 0.8(2) 6,961 2,675 160% 4,046 7,390 2,604 595 Notes: (1) R/(Tc+R) range based on procedures outlined in SEO (2008). (2) R/(Tc+R) ratio range of 0.6-0.8 used for hydrologically similar basins from different study (USACE, 2001). Also based on MWH experience values for PMF studies
Figure 5.2-1. September 1938 Daily Average Flow at Eldorado Springs
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Figure 5.2-2. September 1938 Hourly Flow at Eldorado Springs
Table 5.2-2. September 2013 Pinecliffe Simulation Results
September 2013 Simulation Pinecliffe, CO Observed Peak Simulated Observed Initial Constant Simulated Simulated Scenario Runoff Discharge Peak Daily Peak Daily Loss Loss R/(Tc+R) Runoff % Diff Peak Flow Volume (CDWR) Avg Flow Flow (in) (in/hr) (ac-ft) (cfs) (ac-ft) (cfs) (cfs) (cfs) 1 0 Avg ~0.3(1) 6,638 4,107 62% 7,739 780 962 699 2 1.2 Avg 0.6(2) 6,001 4,107 46% 4,336 780 1,164 699 3 1.2 Avg 0.8(2) 5,994 4,107 46% 2,381 780 1,194 699 4 0 Max 0.6(2) 4,267 4,107 4% 3,415 780 882 699 5 0.5 Max 0.6(2) 4,195 4,107 2% 3,415 780 882 699 6 0 Avg 0.8(2) 6,649 4,107 62% 2,419 780 1,208 699 7 0 Avg 0.8(2) 6,634 4,107 62% 2,127 780 1,192 699 8 0 0.02 0.8(2) 16,161 4,107 294% 4,053 780 2,245 699 9 0 Max 0.8(2) 4,261 4,107 4% 1,819 780 888 699 10 0 Max 0.8(2) 6,535 4,107 59% 1,518 780 1,067 699 Notes: (1) R/(Tc+R) range based on procedures outlined in SEO (2008). (2) R/(Tc+R) ratio range of 0.6-0.8 used for hydrologically similar basins from different study (USACE, 2001). Also based on MWH experience values for PMF studies (3) Moffat Tunnel volume (1,199 AF) and peak daily flow (79.3 cfs) is included in the Pinecliffe gage record.
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Figure 5.2-3. September 2013 Daily Average Flow at Pinecliffe
Figure 5.2-4. September 2013 Hourly Flow at Pinecliffe
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Table 5.2-3. September 2013 Gross Reservoir Inflow Simulation Results
September 2013 Simulation Gross Dam Inflow (calculated) Observed Simulated Observed Initial Constant Simulated Simulated Scenario Runoff Peak Daily Peak Daily Loss Loss R/(Tc+R) Runoff % Diff Peak Flow Volume Avg Flow Flow (in) (in/hr) (ac-ft) (cfs) (ac-ft) (cfs) (cfs) 1 0 Avg ~0.3(1) 13,077 8,142 61% 10,744 2,270 770 2 1.2 Avg 0.6(2) 11,843 8,142 45% 6,566 2,124 770 3 1.2 Avg 0.8(2) 11,827 8,142 45% 3,969 1,972 770 4 0 Max 0.6(2) 9,493 8,142 17% 5,389 1,661 770 5 0.5 Max 0.6(2) 9,236 8,142 13% 5,389 1,661 770 6 0 Avg 0.8(2) 13,059 8,142 60% 4,003 1,982 770 7 0 Avg 0.8(2) 13,038 8,142 60% 3,626 1,955 770 8 0 0.02 0.8(2) 24,022 8,142 195% 5,863 3,301 770 9 0 Max 0.8(2) 9,480 8,142 16% 3,171 1,537 770 10 0 Max 0.8(2) 12,888 8,142 58% 2,813 1,930 770 Notes: (1) R/(Tc+R) range based on procedures outlined in SEO (2008). (2) R/(Tc+R) ratio range of 0.6-0.8 used for hydrologically similar basins from different study (USACE, 2001). Also based on MWH experience values for PMF studies (3) Gross Dam inflows were deregulated to remove the effects of Moffat Tunnel flows.
Figure 5.2-5. September 2013 Inflow to Gross Reservoir
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Table 5.2-4. September 2013 Below Gross Dam and Eldorado Springs Calibration Results
September 2013 Simulation Below Gross Dam (3) Eldorado Springs, CO Observed Observed Observed Peak Simulated Observed Scenario Initial Constant Simulated Simulated Runoff Peak Daily Runoff Discharge Peak Daily Peak Daily Loss Loss R/(Tc+R) Runoff Peak Flow Volume Flow Volume (Jarrett) Avg Flow Flow (in) (in/hr) (ac-ft) (cfs) (ac-ft) (cfs) (ac-ft) (cfs) (cfs) (cfs) 1 0 Avg ~0.3(1) 471 185 7,487 2,511 9,136 2,100 1,941 635 2 1.2 Avg 0.6(2) 471 185 7,099 2,511 6,298 2,100 1,913 635 3 1.2 Avg 0.8(2) 471 185 7,067 2,511 3,572 2,100 1,755 635 4 0 Max 0.6(2) 471 185 6,503 2,511 5,865 2,100 1,703 635 5 0.5 Max 0.6(2) 471 185 7,442 2,511 3,540 2,100 1,699 635 6 0 Avg 0.8(2) 471 185 7,404 2,511 3,416 2,100 1,752 635 7 0 Avg 0.8(2) 471 185 7,435 2,511 3,112 2,100 1,689 635 8 0 0.02 0.8(2) 471 185 12,320 2,511 4,553 2,100 2,619 635 9 0 Max 0.8(2) 471 185 6,512 2,511 3,294 2,100 1,575 635 10 0 Max 0.8(2) 471 185 7,407 2,511 2,110 2,100 1,434 635 Notes: (1) R/(Tc+R) range based on procedures outlined in SEO (2008). (2) R/(Tc+R) ratio range of 0.6-0.8 used for hydrologically similar basins from different study (USACE, 2001). Also based on MWH experience values for PMF studies (3) Moffat Tunnel volume (1,199 AF) and peak daily flow (100.4 cfs) discounted from Below Gross Dam gage record. (4) Peak observed flow at the Below Gross Dam streamflow gage was 285 cfs at 09/11/2013 23:00. (5) Peak observed flow at the Eldorado Springs, CO streamflow gage was 714 cfs at 09/13/2013 03:00 (hourly) and estimated peak flow by Jarrett. (6) Moffat Tunnel volume (1,199 AF) and peak daily flow (79.3 cfs) discounted from Eldorado Springs gage records and Diversion Canal volume (664 AF) and peak daily flow (0.03 cfs) added to Eldorado Springs.
Figure 5.2-6. September 2013 Hourly Flow at Eldorado Springs
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5.3 Spring Floods
As shown on Table 3.3-1 and Table 3.4-2, the May 3-9, 1969 flood primarily resulted from rainfall with a contribution from snowmelt, while the June 11-28, 1995 flood primarily resulted from snowmelt with a contribution from rainfall. The initial snowpack depths and extents over the watershed, as well as the temperature data time-series that was used to determine snowmelt, were prepared by AWA. Due to the lack of snow measurements within the watershed at the time of the floods, there is unavoidable uncertainty in these estimates.
Both floods repeat the pattern of the simulated scenarios, generally overestimating the flood volumes and peak flows. Although the June 1995 simulated total volumes are similar or even lower than the observed volumes, near the peak, the daily volumes are overestimated. For the May 1969 flood, where recorded outflows from Gross Dam are used, Scenario 5 closely simulates the peak flow at Eldorado Springs. Simulated peak flows are notably overestimated for the June 1995 flood, where flows were not regulated by Gross Reservoir.
Table 5.3-1. May 1969 Gross Dam Inflow Simulation Results
May 1969 Simulation Gross Dam Inflow (calculated) Observed Simulated Observed Initial Constant Simulated Simulated Scenario Runoff Peak Daily Peak Daily Loss Loss R/(Tc+R) Runoff % Diff Peak Flow Volume Avg Flow Flow (in) (in/hr) (ac-ft) (cfs) (ac-ft) (cfs) (cfs) 1 0 Avg ~0.3(1) 16,413 3,979 313% 8,122 3,362 660 2 1.2 Avg 0.6(2) 14,780 3,979 271% 6,425 3,789 660 3 1.2 Avg 0.8(2) 14,306 3,979 260% 5,303 3,906 660 4 0 Max 0.6(2) 10,369 3,979 161% 4,741 2,600 660 5 0.5 Max 0.6(2) 10,212 3,979 157% 4,741 2,600 660 6 0 Avg 0.8(2) 15,341 3,979 286% 5,325 3,923 660 7 0 Avg 0.8(2) 14,869 3,979 274% 4,938 3,817 660 8 0 0.02 0.8(2) 29,808 3,979 649% 8,350 6,468 660 9 0 Max 0.8(2) 10,056 3,979 153% 3,829 2,736 660 10 0 Max 0.8(2) 9,402 3,979 136% 2,833 2,403 660 Notes: (1) R/(Tc+R) range based on procedures outlined in SEO (2008). (2) R/(Tc+R) ratio range of 0.6-0.8 used for hydrologically similar basins from different study (USACE, 2001). Also based on MWH experience values for PMF studies (3) Gross Dam inflows were deregulated to remove the effects of Moffat Tunnel flows.
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Figure 5.3-1. May 1969 Daily Gross Reservoir Inflow
Table 5.3-2. May 1969 Eldorado Springs Simulation Results
May 1969 Simulation Eldorado Springs, CO Observed Peak Simulated Observed Initial Constant Simulated Simulated Scenario Runoff Discharge Peak Daily Peak Daily Loss Loss R/(Tc+R) Runoff % Diff Peak Flow Volume (USGS) Avg Flow Flow (3) (4) (in) (in/hr) (ac-ft) (cfs) (ac-ft) (4) (cfs) (cfs) (cfs) 1 0 Avg ~0.3(1) 3,389 2,562 32% 2,964 1,690 695 996 2 1.2 Avg 0.6(2) 3,294 2,562 29% 1,958 1,690 802 996 3 1.2 Avg 0.8(2) 3,234 2,562 26% 1,320 1,690 893 996 4 0 Max 0.6(2) 2,578 2,562 1% 1,670 1,690 623 996 5 0.5 Max 0.6(2) 2,548 2,562 -1% 1,670 1,690 586 996 6 0 Avg 0.8(2) 3,369 2,562 32% 1,342 1,690 894 996 7 0 Avg 0.8(2) 3,358 2,562 31% 1,262 1,690 897 996 8 0 0.02 0.8(2) 8,831 2,562 245% 2,578 1,690 2,042 996 9 0 Max 0.8(2) 2,535 2,562 -1% 1,041 1,690 669 996 10 0 Max 0.8(2) 2,469 2,562 -4% 787 1,690 641 996 Notes: (1) R/(Tc+R) range based on procedures outlined in SEO (2008). (2) R/(Tc+R) ratio range of 0.6-0.8 used for hydrologically similar basins from different study (USACE, 2001). Also based on MWH experience values for PMF studies. (3) Peak observed daily flow at the Eldorado Springs, CO streamflow gage was 1,120 cfs at 05/07/1969 (daily). (4) Moffat Tunnel volume (1,430 AF) and peak daily flow (124 cfs) discounted from Eldorado Springs gage records and no Diversion Canal records were avaiable for this storm event.
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Figure 5.3-2. May 1969 Flow at Eldorado Springs
Table 5.3-3. June 1995 Pinecliffe Simulation Results
June 1995 Simulation Pinecliffe, CO Observed Simulated Observed Initial Constant Simulated Simulated Scenario Runoff Peak Daily Peak Daily Loss Loss R/(Tc+R) Runoff % Diff Peak Flow Volume Avg Flow Flow (in) (in/hr) (ac-ft) (cfs) (ac-ft) (cfs) (cfs) 1 0 A vg ~0.3(1) 18,222 20,127 -9% 3,560 1,968 764 2 1.2 A vg 0.6(2) 17,819 20,127 -11% 3,288 1,919 764 3 1.2 A vg 0.8(2) 17,666 20,127 -12% 2,758 1,777 764 4 0 Max 0.6(2) 15,400 20,127 -23% 2,712 1,491 764 5 0.5 Max 0.6(2) 15,400 20,127 -23% 2,712 1,491 764 6 0 A vg 0.8(2) 17,767 20,127 -12% 2,761 1,777 764 7 0 A vg 0.8(2) 17,741 20,127 -12% 2,623 1,791 764 8 0 0.02 0.8(2) 28,346 20,127 41% 4,118 3,149 764 9 0 Max 0.8(2) 15,255 20,127 -24% 2,232 1,461 764 10 0 Max 0.8(2) 18,361 20,127 -9% 2,277 1,802 764 Notes: (1) R/(Tc+R) range based on procedures outlined in SEO (2008). (2) R/(Tc+R) ratio range of 0.6-0.8 used for hydrologically similar basins from different study (USACE, 2001). Also based on MWH experience values for PMF studies (3) Moffat Tunnel flows included in Pinecliffe flows were 752 AF from June ll-16, and zero thereafter.
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Figure 5.3-3. June 1995 Daily Average Flow at Pinecliffe
Table 5.3-4. June 1995 Gross Dam Inflow Simulation Results
June 1995 Simulation Gross Dam Inflow (calculated) Observed Simulated Observed Initial Constant Simulated Simulated Scenario Runoff Peak Daily Peak Daily Loss Loss R/(Tc+R) Runoff % Diff Peak Flow Volume Avg Flow Flow (in) (in/hr) (ac-ft) (cfs) (ac-ft) (cfs) (cfs) 1 0 A vg ~0.3(1) 18,411 20,388 -10% 3,540 1,959 869 2 1.2 A vg 0.6(2) 17,866 20,388 -12% 3,280 1,908 869 3 1.2 A vg 0.8(2) 17,712 20,388 -13% 2,747 1,786 869 4 0 Max 0.6(2) 15,474 20,388 -24% 2,709 1,477 869 5 0.5 Max 0.6(2) 15,460 20,388 -24% 2,709 1,477 869 6 0 A vg 0.8(2) 17,954 20,388 -12% 2,749 1,787 869 7 0 A vg 0.8(2) 17,928 20,388 -12% 2,609 1,799 869 8 0 0.02 0.8(2) 28,744 20,388 41% 4,110 3,139 869 9 0 Max 0.8(2) 15,328 20,388 -25% 2,217 1,468 869 10 0 Max 0.8(2) 18,548 20,388 -9% 2,271 1,806 869 Notes: (1) R/(Tc+R) range based on procedures outlined in SEO (2008). (2) R/(Tc+R) ratio range of 0.6-0.8 used for hydrologically similar basins from different study (USACE, 2001). Also based on MWH experience values for PMF studies (3) Gross Dam inflows were deregulated to remove the effects of Moffat Tunnel flows.
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Figure 5.3-4. June 1995 Daily Average Inflow to Gross Reservoir
Table 5.3-5. June 1995 Eldorado Springs Simulation Results
Below Gross June 1995 Simulation Eldorado Springs, CO Dam (3) Observed Observed Peak Simulated Observed Scenario Initial Constant Simulated Simulated Runoff Runoff Discharge Peak Daily Peak Daily Loss Loss R/(Tc+R) Runoff % Diff Peak Flow Volume Volume (USGS) Avg Flow Flow (in) (in/hr) (ac-ft) (cfs) (ac-ft) (ac-ft) (cfs) (cfs) (cfs) 1 0 Avg ~0.3(1) 14,067 14,924 11,489 30% 785 845 726 856 2 1.2 Avg 0.6(2) 14,067 14,900 11,489 30% 785 845 726 856 3 1.2 Avg 0.8(2) 14,067 14,900 11,489 30% 785 845 726 856 4 0 Max 0.6(2) 14,067 14,904 11,489 30% 785 845 727 856 5 0.5 Max 0.6(2) 14,067 14,900 11,489 30% 785 845 726 856 6 0 Avg 0.8(2) 14,067 14,924 11,489 30% 794 845 733 856 7 0 Avg 0.8(2) 14,067 14,924 11,489 30% 795 845 733 856 8 0 0.02 0.8(2) 14,067 15,160 11,489 32% 933 845 795 856 9 0 Max 0.8(2) 14,067 14,904 11,489 30% 787 845 727 856 10 0 Max 0.8(2) 14,067 14,924 11,489 30% 797 845 735 856 Notes: (1) R/(Tc+R) range based on procedures outlined in SEO (2008). (2) R/(Tc+R) ratio range of 0.6-0.8 used for hydrologically similar basins from different study (USACE, 2001). Also based on MWH experience values for PMF studies (3) Observed Below Gross Dam Gage flows input as discharge source from outlet of sub-basin B6.
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Figure 5.3-5. June 1995. Hourly Flow at Eldorado Springs
5.4 Recommended Parameters to be Used for PMF Analysis
Evaluation of the Gross Dam calibration results were based on a combination of suitability of the rainfall-runoff parameters for purposes of modeling the PMF, initial parameter estimates from SEO guidelines with acknowledgment of limitation of their applicability (i.e., SEO [2016], SEO[2008], and SEO[2007]), comparison of modeled and observed results for historic storm events, and engineering judgment. Based on matching of peak flow records and on comparisons with the other scenarios, Scenario 5 is the recommended set of rainfall-runoff parameters. Scenario 5 employed an upper range of NRCS (1986) infiltration rates, an R/(Tc+R) ratio of 0.6, and initial losses of 0.5 inches over the Gross Dam basin.
For the purposes of modeling the PMF, it is recommended to use Scenario 5 basin parameters, except with a modification to include average (instead of high) NRCS (1986) constant infiltration losses. The use of high infiltration losses for a PMF simulation would be atypical as the assumption of more conservative average to low loss rates is the norm for PMF conditions that may occur following periods of significant antecedent rainfall.
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Scenario 5 was the consensus selection among independent calibration evaluators. Although other parameters, particularly higher loss rates, would provide better calibration simulations, they would be outside the normally acceptable range and would not be acceptable for use in the PMF simulations where unusually high loss rates are not the accepted norm. Use of average constant infiltration losses will better represent the infiltration capacity of Gross Dam basin shallow soils and avoid potential underestimation of total PMF runoff volume without being overly conservative. For Gross Dam general storm simulations of 72 hour durations an equivalent of up to 5.3 inches will be observed from infiltration losses by using average NRCS (1986) constant infiltration values against 7.7 inches if using maximum NRCS (1986) constant infiltration values.
The calibration of the Gross Dam hydrologic model was performed following FERC and Colorado Dam Safety guidelines. Calibration was performed for four historic flood events, two spring events (rain-on-snow) and two summer events (rainfall only), and incorporated available streamflow gage information as well as Gross Dam calculated information from Denver Water. Precipitation data was incorporated to the hydrologic model as gridded rainfall, gridded temperature, and gridded initial snowpack depths provided by AWA.
The recommended set of parameters to be used for the PMF analysis is Scenario 5 (see Table 5.4-1), using average NRCS (1986) constant infiltration losses. This recommendation is based partially on the results of the calibration to match observed peak flow values and more so on engineering experience in hydrologic modeling. The recommended rainfall-runoff parameters would be expected to result in a conservative estimation of the PMF runoff (see Section 9.3 for additional discussion).
Table 5.4-1. Final Estimates of Clark Parameters
Longest Sub-basin Description Flowpath Tc (hours) R R/(Tc+R) Name (miles) B1 Moffat Tunnel Basin 5.11 1.36 2.04 0.60 B2a Moffat Tunnel DS N Basin 7.20 2.02 3.03 0.60 B2b Moffat Tunnel DS S Basin 8.77 2.10 3.15 0.60 B3 Rollinsville, CO Basin 7.29 2.16 3.24 0.60 B4 Pinecliffe, CO Gage Basin 10.82 2.82 4.23 0.60 B5a Gross Dam Reservoir Basin 6.80 2.10 3.15 0.60 B5b Gross Dam Reservoir Inflow 6.42 1.66 2.49 0.60 B6 Below Gross Dam 1.20 0.51 0.77 0.60 B7 S Boulder Cr Diversion Structure 7.70 2.09 3.14 0.60 B8 Eldorado Springs, CO Gage 4.97 1.33 2.00 0.60
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6. PROBABLE MAXIMUM PRECIPITATION
The Probable Maximum Precipitation (PMP) is, theoretically, the greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular orographic location at a certain time of the year. The applicable available National Weather Service PMP guidance document is Probable Maximum Precipitation Estimates United States Between the Continental Divide and the 103rd Meridian, Hydrometeorological Report No. 55A (Hansen, et al, 1988). To develop an updated PMP with a detailed focus specifically on the watershed tributary to Gross Dam, a site-specific PMP was developed using current state-of-the- practice methods.
The site-specific PMP was developed by Applied Weather Associates (AWA), working under subcontract to MWH. This section briefly summarizes the results of the site specific PMP analysis. A complete report on development of the site-specific PMP is included as Appendix B.
6.1 Probable Maximum Precipitation Data
The PMP study by AWA resulted in depth-duration data as shown in Table 6.1-1. The Table 6.1-1 data represents average precipitation depths over the 93 square mile watershed tributary to Gross Dam. Unique depth-duration curves were developed for each grid cell in the model domain.
Table 6.1-1. PMP Depth-Duration Data
General Storm Local Storm Basin Basin Duration Avg. PMP Duration Avg. PMP (Hours) (inches) (Hours) (inches) 1 2.88 1 2.88 6 5.38 3 4.40 24 11.69 6 5.31 72 17.54
The applicable PMP for any watershed will vary by season, duration, and areal extent. There is a seasonal variation of the PMP and the month or season having the greatest depth is referred to as the all-season PMP. The all-season PMP applies from June 15 through September 15 for the Gross Dam watershed. The current PMF study also investigated the potential for spring rain-on-snow events to develop the critical PMF reservoir inflows. Seasonality ratios to be applied to the all- season PMP for use in deriving rainfall depths that could occur concurrent with the 100-year snow water equivalent depths were also prepared by AWA for separate times of the year beginning May 1. Concurrence of the 100-year snowpack and a seasonal PMP is the key factor in the seasonality
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ratios. The seasonality ratios of the PMP for other times of the year to the all-season PMP are summarized on Table 6.1-2.
Table 6.1-2. PMP Seasonality Ratios
Seasonality Date Ratio April 15 0.680 May 1 0.745 May 15 0.810 June 1 0.855 June 15 0.900 The Gross Dam watershed PMP was developed for durations up to 72 hours. The all-season general storm PMP depths for the three alternative temporal distributions with the Default spatial distribution for various durations from 1-hour to 72 hours by sub-basin are presented in Table 6.1-3 through Table 6.1-5. The Default spatial distribution is derived using the precipitation frequency data. Table 6.1-3 and Table 6.1-4 are applicable to the all-season general PMP, while Table 6.1-5 is for the May 15 rain-on-snow PMP. Note that the Gross General and the Critically Stacked temporal distributions were synthetically developed, while the SPAS 1253 temporal distribution was developed from a specific storm that occurred in May 1969. The local storm PMP for durations up to 6 hours by sub-basin is presented in Table 6.1-6.
The incremental and accumulated precipitation for the three alternative distributions of the 72- hour general storm PMP is shown on Figure 6.1-1 through Figure 6.1-3. Note that the 72-hour May 15 PMP is reduced by a PMP seasonality ratio coincident with a 100-year snowpack as compared to the all-season PMP and no snowpack. The incremental and accumulated precipitation for the 6-hour local storm is shown on Figure 6.1-4. To facilitate comparison, these figures have been prepared to all have the same incremental and accumulated precipitation y-axis scales, and the same 72-hour x-axis scale, regardless of the precipitation amounts or the storm durations.
Table 6.1-3. All-Season PMP by Sub-Basin for Various Durations – Gross General Temporal Distribution
Drainage Area All Season 1-hr All Season 6-hr All Season 24-hr All Season 72-hr Sub-basin (sq.mi.) PMP (inches) PMP (inches) PMP (inches) PMP (inches) B1 8.9 0.43 2.25 5.39 8.18 B2A 7.7 0.82 4.27 10.24 15.46 B2B 13.8 0.81 4.23 10.12 15.26 B3 12.6 0.85 4.44 10.61 16.00 B4 31.7 0.94 4.91 11.75 17.76 B5A 12.7 1.37 7.10 17.01 25.72 B5B 5.8 1.32 6.86 16.43 24.79 Total/Avg 93.0 0.93 4.86 11.63 17.56
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Table 6.1-4. All-Season PMP by Sub-Basin for Various Durations – Critically Stacked General Temporal Distribution
Drainage Area All Season 1-hr All Season 6-hr All Season 24-hr All Season 72-hr Sub-basin (sq.mi.) PMP (inches) PMP (inches) PMP (inches) PMP (inches) B1 8.9 2.63 4.90 10.66 15.95 B2A 7.7 2.72 5.09 11.03 16.66 B2B 13.8 2.66 4.98 10.82 16.19 B3 12.6 2.80 5.23 11.37 17.13 B4 31.7 2.94 5.48 11.92 17.89 B5A 12.7 3.16 5.91 12.84 19.27 B5B 5.8 3.25 6.08 13.20 19.85 Total/Avg 93.0 2.88 5.38 11.69 17.56
Table 6.1-5. May 15 PMP by Sub-Basin for Various Durations – SPAS 1253 General Temporal Distribution
Drainage Area All Season 1-hr All Season 6-hr All Season 24-hr All Season 72-hr Sub-basin (sq.mi.) PMP (inches) PMP (inches) PMP (inches) PMP (inches) B1 8.9 0.65 2.35 7.03 12.84 B2A 7.7 0.68 2.48 7.40 13.50 B2B 13.8 0.66 2.41 7.20 13.14 B3 12.6 0.70 2.55 7.65 13.96 B4 31.7 0.73 2.67 7.99 14.59 B5A 12.7 0.79 2.90 8.67 15.83 B5B 5.8 0.81 2.97 8.87 16.20 Total/Avg 93.0 0.72 2.62 7.83 14.30
Table 6.1-6. Local Storm PMP by Sub-Basin for Various Durations – Gross Local Temporal Distribution
Drainage Area Local Storm 1-hr Local Storm 6-hr Sub-basin (sq.mi.) PMP (inches) PMP (inches) B1 8.9 2.51 5.02 B2A 7.7 2.59 5.18 B2B 13.8 2.46 4.92 B3 12.6 2.52 5.04 B4 31.7 2.71 5.41 B5A 12.7 3.14 6.27 B5B 5.8 3.13 6.26 Total/Avg 93.0 2.70 5.40
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3.00 18.0
2.75 16.5
2.50 15.0
2.25 13.5
2.00 12.0
1.75 10.5
1.50 9.0
1.25 7.5
1.00 6.0 Incremental (inches) Incremental Precipitation Accumulated (inches) Precipitation Accumulated 0.75 4.5
0.50 3.0
0.25 1.5
0.00 0.0 0 6 12 18 24 30 36 42 48 54 60 66 72 Hour Figure 6.1-1. Incremental and Accumulated All-Season PMP – Gross General Temporal Distribution
3.00 18.0
2.75 16.5
2.50 15.0
2.25 13.5
2.00 12.0
1.75 10.5
1.50 9.0
1.25 7.5
1.00 6.0 Incremental (inches) Incremental Precipitation Accumulated (inches) Precipitation Accumulated 0.75 4.5
0.50 3.0
0.25 1.5
0.00 0.0 0 6 12 18 24 30 36 42 48 54 60 66 72 Hour Figure 6.1-2. Incremental and Accumulated All-Season PMP – Critically Stacked General Temporal Distribution
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3.00 18.0
2.75 16.5
2.50 15.0
2.25 13.5
2.00 12.0
1.75 10.5
1.50 9.0
1.25 7.5
1.00 6.0 Incremental (inches) Incremental Precipitation Accumulated (inches) Precipitation Accumulated 0.75 4.5
0.50 3.0
0.25 1.5
0.00 0.0 0 6 12 18 24 30 36 42 48 54 60 66 72 Hour Figure 6.1-3. Incremental and Accumulated May 15 PMP – SPAS 1253 Temporal Distribution
3.00 18.0
2.75 16.5
2.50 15.0
2.25 13.5
2.00 12.0
1.75 10.5
1.50 9.0
1.25 7.5
1.00 6.0 Incremental (inches) Incremental Precipitation Accumulated (inches) Precipitation Accumulated 0.75 4.5
0.50 3.0
0.25 1.5
0.00 0.0 0 6 12 18 24 30 36 42 48 54 60 66 72 Hour Figure 6.1-4. Incremental and Accumulated Local Storm PMP – Gross Local Temporal Distribution
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6.2 Discussion of Temporal Distribution Using Critically Stacked Pattern
Because of its common significance to both the PMP and the PMF report, this section substantially repeats Section 9.3.3 of the Gross Reservoir PMP report prepared by AWA. Several generalized hydrologic implementation manuals, as well as guidance provided by the Colorado State Engineer's Office, suggest the use of a critically stacked temporal pattern for deriving the PMF from the given PMP depths. In this scenario, the largest one-hour precipitation depth is placed near the center of the storm with incrementally smaller values alternating on either side. This pattern represents a worst-case scenario where all maximum PMP depths are enveloped and adjacent to each other, following the depth-duration curve pattern. However, from a meteorological perspective, this pattern represents an idealized pattern that is not reflected by any actual storm events or any other of the hundreds of storm patterns AWA has analyzed across the region. Therefore, this pattern does not follow the process employed in developing PMP depths for the Gross Dam basin because it is not site-specific and storm based.
AWA recommends using actual storm based patterns from site-specific data whenever possible and justified. However, the recommended Gross General Temporal Pattern (Figure 6.1-1) comes very close to replicating the critically stacked pattern. In this scenario, the major difference is that the largest one-hour is significantly smaller than the critically stacked pattern (Figure 6.1-2). This makes meteorological sense as the one hour value is solely derived from the Big Thompson Canyon, CO July 1976 SPAS 1231 Zone 1 storm. This storm was more of a local storm type than a general storm type. Therefore, it is very conservative to include it with the general storm temporal pattern because the temporal precipitation accumulation characteristics associated with that event would not occur during a general storm PMP event. However, given the uncertainty in overall PMP development process and the significant reductions in PMP depths versus HMR 55A derived in this study, utilizing this critically stacked temporal pattern as a sensitivity during the PMF development process is acceptable. To develop the critically stacked pattern, AWA utilizes the pattern following the guidelines in United States Bureau of Reclamation Flood Hydrology Manual (Cudworth, 1989). Figure 6.1-2 displays the critically stacked pattern provided for a Gross Dam PMF sensitivity run.
6.3 Candidate Storms for the PMF
Candidate storms for the PMF include the all-season general storm PMP and the local storm (thunderstorm) PMP. As previously noted, the all-season PMP applies from June 15 through September 15 for the Gross Dam watershed and would occur without a coincident snowpack. Alternative spatial and temporal distributions of the PMP are tested as described in Section 9.1.
Based on PMF guidelines, (FERC 2001), if snowpack is apt to exist in at least part of the drainage basin in the season when a seasonal PMP would occur, an antecedent 100-year snowpack covering
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the area that could be subject to snowpack should be assumed to exist at the time when the seasonal PMP occurs. During the spring snowmelt season from May 1 through June 15, several factors (temperature, SWE, and seasonal PMP) are varying with time in different ways that makes it difficult to determine in advance whether the critical PMF condition would result from the summer all-season PMP with no snowpack, or the rain-on-snow spring condition with a reduced PMP. Therefore, in addition to the summer all-season PMP, four (May 1, May 15, June 1, June 15) rain- on-snow candidate PMF conditions were developed with seasonally appropriate variations in the 100-year snowpack, coincident temperature time-series data, and seasonal PMP depths. Development of the 100-year snowpack is discussed in Section 8.3.4.
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7. LOSS RATES
7.1 General
The initial abstraction and uniform loss rate method of simulating infiltration was used for development of the PMF. Initial abstractions (losses at the start of the storm) ranged in the calibration runs from 0.0 to 1.2 inches (see Table 5.1-1) and uniform loss rates are taken as a function of the sub-basin hydrologic soil group composition (see Table 7.2-1). As used in a runoff event model such as HEC-HMS, loss rates effectively means any rainfall or snowmelt that does not reach the river within the time frame of the simulation.
As presented in Section 5.4, the candidate PMF scenario runs were based on an initial abstraction of 0.5 inches, and average uniform losses as shown in Table 7.2-2.
7.2 Loss Rate Values
The uniform infiltration rates were estimated from the soil information obtained and calibration with historic data of runoff values. Figure 7.2-1 shows the hydrologic soil group (HSG) information for the Gross Dam Basin obtained from 2003 Natural Resources Conservation Service (NRCS) SSURGO data. An area of 59 square miles of the basin or 53% is comprised of HSG “D” soils. These soils are described by NRCS as having high runoff potential when thoroughly wetted and consist chiefly of shallow clayey soils with less than 50 percent sand, and have clayey textures. HSG “D” soils have a high rate of water transmission in the range of 0.00 to 0.05 inches per hour. An area of 24 square miles of the basin or 21% is comprised of HSG “C” soils. These soils are described by NRCS as having moderately high runoff potential when thoroughly wetted. The initial abstraction and uniform loss rate parameters are very low for soils of these types and would represent wet antecedent conditions in the watershed. Table 7.2-1 summarizes the range of soil infiltration rates as a function of hydrologic soil group (NRCS, 1986). Sub-basin averaged infiltration rates based on the HSG infiltration rates are summarized in Table 7.2-2. The Gross Dam watershed average and maximum uniform infiltration loss rate by using the values shown in Table 7.2-2, are 0.073 inches/hour and 0.107 inches per hour respectively.
Table 7.2-1 Watershed Uniform Loss Rates
Hydrologic Soil Uniform Loss Group Rate (in/hr) A 0.30-0.45 B 0.15-0.30 C 0.05-0.15 D 0.00-0.05
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Table 7.2-2. Watershed Loss Rates
Primary Min. Uniform Avg. Uniform Max. Uniform Sub-basin Hydrologic Losses Losses Losses Name Soil Group (in/hr) (in/hr) (in/hr) B1 C 0.094 0.140 0.185 B2a C 0.061 0.109 0.158 B2b C 0.069 0.114 0.159 B3 D 0.035 0.074 0.114 B4 D 0.045 0.086 0.127 B5a D 0.021 0.053 0.084 B5b D 0.006 0.034 0.062 B6 D 0.000 0.025 0.050 B7 D 0.138 0.187 0.237 B8 D 0.013 0.041 0.069
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Figure 7.2-1. Hydrologic Soil Groups
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8. COINCIDENT HYDROMETEOROLOGICAL AND HYDROLOGICAL CONDITIONS FOR THE PROBABLE MAXIMUM FLOOD
A common definition for the PMF is the flood that may be expected from the most severe combination of critical meteorological and hydrologic conditions that are reasonably possible in the drainage basin under study (FERC 2001). A distinction is drawn between the PMF and the “maximum possible flood” which would result from simultaneously maximizing every possible flood producing factor. The maximum possible flood is not in current use as an inflow design flood in the USA. This chapter addresses conditions coincident to the PMP designed to avoid compounding of conservatism and to provide a reasonable PMF hydrograph given the limitations of basic hydrologic and meteorological data.
8.1 Reservoir Level
The raised Gross Dam is expected to have an uncontrolled spillway crest. The assumed initial reservoir water level for all PMF routings was at the raised dam uncontrolled spillway crest level at El 7406.
8.2 Baseflow
Baseflow is the streamflow rate occurring prior to the storm rainfall-runoff event. Baseflow recession occurs during the storm event. For simulation of PMF hydrographs, baseflow is a small to negligible factor. Baseflow can typically be estimated from the average monthly flow coincident with the PMP or as recorded prior to historic maximum floods. The baseflow used in the current study is conservatively based on the calculated maximum daily average unregulated inflow to Gross Dam of 870 cfs. Moffat Tunnel flows have historically been substantially reduced during flood periods.
8.3 Snowpack
Snowmelt is an important component of the spring PMF that contributes both to runoff volume and peak flow. This section summarizes the available snowpack data, the methodology to develop extreme snowpack data, and determines the required 100-year snowpack for South Boulder Creek tributary to Gross Dam. The 100-year snowpack and values were developed by Applied Weather Associates, with additional detail presented in Appendix B.
8.3.1 Available Historical Snowpack Data
Snowpack data is available at a number of stations in the general vicinity of the Gross Dam watershed but not within the basin. Four types of snow data stations are available. SNOTEL
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stations have daily measurements, with the closest SNOTEL station located at Lake Eldora, just north of the Gross Dam watershed boundary. Snow course data is available at several stations in the vicinity, but typically only four measurements per year are available for the snow courses, taken roughly around the first of the month from February 1 through May 1. Because the most critical combination of snowpack, temperature, and seasonal PMP could not be known in advance, PMF scenarios that included the 100-year snowpack were performed at four intervals: May 1, May 15, June 1, and June 15. Snow course data measurements are not available for May 15 through June, so snow course data was not used in the development of the 100-year snowpack. The third source of snow data is from the Global Historical Climatology Network (GHCN), which is an integrated database of climate summaries from land surface stations across the globe that have been subjected to a common suite of quality assurance reviews. Table 8.3-3 (following section) summarizes the location and elevation, of the point data stations used to develop the 100-year snowpack. In addition to the point SWE stations, as the fourth source of snow data, AWA utilized the National Operational Hydrologic Remote Sensing Center (NOHRSC) SNOw Data Assimilation System (SNODAS) gridded dataset. SNODAS integrates observed, remotely sensed, and modeled datasets into estimated snowpack variables.
Daily snow water equivalent data for the closest SNOTEL station to the Gross Dam watershed, Lake Eldora at elevation 9,700 feet, are plotted on Figure 8.3-1 and Figure 8.3-2. These plots depict the slow accumulation of SWE that typically peaks between April 1 and May 1, and then depletes rapidly until it is gone, typically between about May 1 and June 1. The peak SWE of 24.0 inches occurred on April 2-7, 1996. These dates and values would vary with elevation, but it is noted that only 26% of the watershed tributary to Gross Dam lies above elevation 10,000 feet.
Ballpark estimates of snowmelt runoff can be estimated from the Lake Eldora SNOTEL gage data. The ten years having periods with rapid SWE depletion were selected as presented on Table 8.3-1. Maximum day and average day SWE depletions were determined for the selected periods, with the maximum day losses tending to occur at lower SWE depths, probably due to a lower cold content of the remaining snowpack. Although the extent and SWE variability of the snowpack in Gross Dam watershed is not exactly known, various assumptions regarding loss rate and areal extent of the snowpack can be, from which an equivalent flow rate can be calculated, as presented on Table 8.3-2, assuming no losses from the snowmelt. An area of 26 square miles corresponds to the drainage area tributary to Gross Dam above El 10,000 feet, while 51 square miles is the area tributary to Gross Dam above El 9,000 feet. The ballpark flow results in Table 8.3-2 at the lower flow rates are similar to the maximum calculated historic daily inflows to Gross Reservoir, and at the higher flow rate end, the equivalent flow rates are similar to the 2,600 cfs peak of a previously determined Probable Maximum Snowmelt Flood (DMJM Phillips-Reister, 1977).
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25
1996 1997
20
Lake Eldora SNOTEL 2003 Elevation 9,700 feet
15
1995
10 Snow Equivalent Water (inches)
5
0 1-Oct 1-Nov 1-Dec 1-Jan 1-Feb 1-Mar 1-Apr 1-May 1-Jun 1-Jul 1-Aug 1-Sep
Figure 8.3-1. Lake Eldora SNOTEL Data – October 1978 to September 2005
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25
2014
20
Lake Eldora SNOTEL Elevation 9,700 feet
15 2011
10 Snow Equivalent Water (inches)
5
0 1-Oct 1-Nov 1-Dec 1-Jan 1-Feb 1-Mar 1-Apr 1-May 1-Jun 1-Jul 1-Aug 1-Sep
Figure 8.3-2. Lake Eldora SNOTEL Data – October 2005 to June 2016
Table 8.3-1. Lake Eldora SNOTEL – Rates of SWE Depletion
Lake Eldora SNOTEL Max. Day SWE Avg. Day SWE Period Loss (inches) Loss (inches) May 20 - June 2, 1980 2.0 1.24 May 9 - 24, 1984 2.3 1.26 May 4 - 12, 1994 1.7 1.14 May 31 - June 13, 1995 1.8 0.88 May 1 - May 15, 1996 2.5 1.38 May 3 - 22, 1997 2.2 1.05 April 28 - May 16, 1998 2.0 0.78 April 29 - May 13, 2009 1.8 1.06 May 22 - June 4, 2011 2.6 1.01 May 16 - May 25, 2014 1.6 1.01 Average 2.1 1.08 Maximum 2.6 1.38
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Table 8.3-2. Equivalent Flow Rates to SWE Loss Rates
Area SWE Loss Equivalent (sq.mi.) Rate (in.) Flow (cfs) 26 1.08 755 51 1.08 1,481 26 2.10 1,468 51 2.10 2,880 26 2.60 1,818 51 2.60 3,565
8.3.2 Methodology Used to Determine the Estimated PMF Snowpack
The seasonal 100-year snowpack coincident with the corresponding seasonal PMP is required by the FERC guidelines (2001, pg. 68) for developing candidate PMF inflow floods. AWA utilized daily gridded SNODAS data for May 1, May 15, June 1, and June 15 to calculate the 1% exceedance (100-year), mean, maximum, and minimum snowpack spatial variation. The daily gridded SNODAS climatologies were used to aid in the spatial interpolation of the station 1% exceedance snowpack (i.e. scaling the gridded spatial pattern to the observed station 1% exceedance values).
8.3.3 100-Year Snowpack Antecedent to the PMP
The 100-year snowpack SWE at the point stations is presented in Table 8.3-3. The 100-year snowpack snow water equivalent by sub-basin and for the four dates of the rain-on-snow PMF runs are presented on Table 8.3-4.
Table 8.3-3 100-Year Snowpack at Snow Stations (by AWA)
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Table 8.3-4. 100-Year Snowpack Snow Water Equivalent (by AWA)
Drainage Area May 1 100-Year May 15 100-Year June 1 100-Year June 15 100-Year Sub-basin (sq.mi.) SWE (inches) SWE (inches) SWE (inches) SWE (inches) B1 8.9 25.48 25.28 18.73 12.78 B2A 7.7 19.99 19.03 7.99 3.11 B2B 13.8 19.91 18.91 10.97 4.36 B3 12.6 12.57 11.50 3.26 0.05 B4 31.7 3.87 2.88 0.42 0.00 B5A 12.7 1.05 0.85 0.00 0.00 B5B 5.8 2.43 1.74 0.06 0.00 Total/Avg 93.0 10.35 9.55 4.66 2.13
To provide a sense of the temporal and spatial variation of the 100-year snowpack, Figure 8.3-3 and Figure 8.3-4 plot the gridded 100-year snowpack values for May 1 and June 15.
Figure 8.3-3. 100-Year Snow Water Equivalent for May 1 (by AWA)
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Figure 8.3-4. 100-Year Snow Water Equivalent for June 15 (by AWA)
8.4 Snowmelt
Temperature and initial snowpack snow water equivalent are the only meteorological data input for the determination of snowmelt for the temperature index method. The 100-year snowpack antecedent to the PMP is covered in Section 8.3.3. Example 120-hour (5 day) time-series of temperature coincident with the PMP time sequences are plotted on Figure 8.4-1 (by AWA). The temperature sequences vary with time of year and with elevation. The 5 day temperature sequence allows for hydrologic model simulations starting one day before and ending one day after the PMP. The temperature lapse rate was 2.7 degrees per 1,000-feet in elevation.
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Figure 8.4-1. Example Temperature at Four Sample Grid Cells for May 15
The rain on snow process and/or snowmelt was estimated using the Gridded Temperature Index Method within HEC-HMS. The gridded temperature index method dynamically computes the melt rate based on current atmospheric conditions and past conditions in the snowpack (USACE, 2015). The basic equation is:
Ms = Cm (Ta – Tb) (USACE 1998) Where:
Ms = snowmelt, in. per period
Cm = melt-rate coefficient that is often variable, in./(degree/period)
Ta = air temperature, (°F)
Tb = base temperature (°F)
Primary inputs for gridded temperature index snowmelt include temperature data, snowpack conditions, and melt rate coefficients. Gridded temperature, initial snowpack (as SWE) and rainfall data was developed and provided by AWA. Variation of the melt rate factor depends on site,
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meteorological and seasonal conditions (USACE 1998). Snowmelt input parameters used for the spring calibration events (i.e., rain-on-snow events) are shown in Table 8.4-1.
Table 8.4-1 Summary of Snowmelt Parameters
Parameter Description Gross Dam Rain-on-Snow and Snowmelt Parameters Snowmelt Method Gridded Temperature Index Method Melt Rate Coefficient - Gridded 0.04-0.08 inches/degree F-day (USACE, 1998) See ATI- Temperature Index Meltrate Function Rain-Snow Discrimination 32°F (USACE, 2015) Temperature, PX Base Temperature 32°F (USACE, 2015) Wet Meltrate 0.06 inches/degree F-day (NRCS, 2004) 0.04-0.08 inches/degree F-day (USACE, 1998)
ATI-Meltrate Function ATI (DEG F-DAY) Meltrate (IN/DEG F-DAY) 0 0.04 50 0.08 999 0.08 ATI-Meltrate Coefficient 0.98 (USACE, 2015) ATI-Coldrate Coefficient 0.50 (USACE, 2015) Groundmelt 0.02 inches/day (USACE, 2015) Initial Liquid Water Content 2% of Snowpack (USACE, 1998) Snowpack as SWE By AWA
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9. PMF HYDROGRAPHS
Under FERC guidelines, evaluation of two PMF scenarios is required where a seasonal snowpack can be present over at least part of the watershed, including (a) a PMP on a 100-year snowpack, and (b) an all-season PMP. This section also includes additional PMF runs that determine (1) the critical temporal distribution of the PMP, (2) the critical spatial distribution of the PMP, (3) the critical seasonal PMF in combination with seasonal PMPs and seasonal snowpack conditions, and (4) PMF sensitivity runs that determine the potential effects of both more conservative and less conservative values for key parameters. A determination of the critical PMF inflow hydrograph was made using a preliminary spillway size from among the PMF runs. A final section of this chapter compares results of the current studies with results of previous Gross Dam PMF studies.
9.1 PMF Inflow and Outflow Hydrographs
Table 9.1-1 presents a series of candidate PMF scenario runs that are intended to find the critical conditions from among the following parameters:
• PMP spatial distribution – Default (Scenarios 1-3, 7, 9-12), SPAS 1302 (Scenarios 4-6), and SPAS 1231 (Scenario 8). The Default spatial pattern is derived using the precipitation frequency data. Specific SPAS storms were selected by AWA to provide a range of storm distribution patterns (see Appendix B for detailed SPAS storm information).
• PMP temporal distribution – Gross General (Scenarios 1, 4), Gross Local (Scenarios 7, 8), SPAS 1253 (Scenarios 2, 5, and 9-12), or SPAS 1614 (Scenarios 3, 6).
• PMP storm type – general storm (Scenarios 1-6, 9-12) or local storm (Scenarios 7-8).
• PMP seasonality – Summer (Scenarios 1-8) or Spring with snowpack (Scenarios 9-12)
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Table 9.1-1. Candidate PMF Base Runs
PMF Description Scenario 1 Default Spatial and Gross General Temporal 2 Default Spatial and SPAS 1253 Temporal (General PMP) 3 Default Spatial and SPAS 1614 Temporal (General PMP) 4 SPAS 1302 Spatial and Gross General Temporal 5 SPAS 1302 Spatial and SPAS 1253 Temporal (General PMP) 6 SPAS 1302 Spatial and SPAS 1614 Temporal (General PMP) 7 Default Spatial and Gross Local Temporal 8 SPAS 1231 and Gross Local Temporal 9 Default Spatial and SPAS 1253 Temporal Adjusted to May 1 10 Default Spatial and SPAS 1253 Temporal Adjusted to May 15 11 Default Spatial and SPAS 1253 Temporal Adjusted to June 1 12 Default Spatial and SPAS 1253 Temporal Adjusted to June 15
Table 9.1-2 provides a results summary for all of the PMF scenarios. Note that the critical parameter is the peak water level rise above the spillway crest, not the peak inflow. All PMF runs were performed with HEC-HMS running on a 15-minute time increment. All PMF peak results represent 15-minute peak values.
All of the PMF routings assume a 322-ft wide spillway with an ogee shaped crest. The 322-ft wide spillway is not a recommended width, it was selected as a maximum width for PMF scenario comparison purposes only based on the fact that this spillway configuration would pass the entire range of PMF discharges so that a direct comparison could be made. The actual spillway width, type, and configuration will be determined in the Final Design phase of the proposed raising of Gross Dam.
A review of the results presented in Table 9.1-2 shows that:
• The Default spatial distribution is critical – compare Scenarios 1 vs 4, 2 vs 5, and 3 vs 6.
• The General Storm type is critical – compare Scenarios 1 vs 7 and 1 vs 8 using the peak water level rise as the comparison parameter. Although the local storm peak inflow is higher than for PMF Scenario 1, the greater inflow volume of the general storm causes the maximum reservoir level to be higher.
• The Summer General Storm season is critical – compare Scenario 1 vs any of Scenarios 9, 10, 11, 12.
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Table 9.1-2. Candidate PMF Routing Results at Gross Dam
PMF Scenario Description Gross Dam Candidate PMF Results (1)(2)(3) PMF Spatial Temporal Scenario Storm Inflow Outflow Peak Peak Water 3-day Inflow Distribution Distribution (cfs) (cfs) Elevation (ft) Rise (ft) Volume (ac-ft) 1 Summer General PMP Default Gross General 40,398 36,711 7,415.6 9.6 61,173 2 Summer General PMP Default SPAS 1253 23,570 21,625 7,412.8 6.8 63,045 3 Summer General PMP Default SPAS 1614 30,222 28,546 7,414.2 8.2 69,147 4 Summer General PMP SPAS 1302 Gross General 40,294 36,694 7,415.6 9.6 62,732 5 Summer General PMP SPAS 1302 SPAS 1253 23,570 21,697 7,412.8 6.8 64,233 6 Summer General PMP SPAS 1302 SPAS 1614 30,123 28,587 7,414.2 8.2 69,825 7 Summer Local PMP Default Gross Local 48,118 32,056 7,414.8 8.8 26,924 8 Summer Local PMP SPAS 1231 Gross Local 42,830 29,176 7,414.3 8.3 25,738 9 Spring General PMP Default SPAS 1253 May 01 16,435 15,024 7,411.3 5.3 36,207 10 Spring General PMP Default SPAS 1253 May 15 19,740 17,983 7,412.0 6.0 52,189 11 Spring General PMP Default SPAS 1253 June 01 20,829 19,058 7,412.2 6.2 55,420 12 Spring General PMP Default SPAS 1253 June 15 21,866 20,031 7,412.4 6.4 58,521 Notes: (1) Gross Dam outflow based on 322 foot wide spillway (2) Gross Dam reservoir initial water level was set to the spillway invert elevation of 7,406 feet and the raised dam crest elevation is 7,421 (3) R/(Tc+R) ratio = 0.6; ratio used in all runs.
That the local storm with a higher peak inflow does not form the critical condition compared to a general storm with a lower peak inflow but a higher volume is also an indicator that smaller (by area) storms that have lower areal reduction factors will not be the controlling case because of their smaller inflow volumes. The areal reduction factor for the 24-hour general storm for the 93 square mile Gross Dam watershed would be reduced by only by about 5% to 6% for a 50 square mile storm (see ARF in Appendix B). For a 50 square mile storm, the rainfall depths would be only slightly increased, but the runoff volume would be substantially decreased due to the much smaller area of application, such that the smaller area storm would not be the controlling condition due to lack of volume to raise the reservoir level enough to be controlling.
In Table 9.1-2, Scenario 3 and Scenario 6 have noticeably higher 3-day volumes than the other general storm scenarios. This is because of the interaction between the constant loss rates (see Table 7.2-2) and the number of hours with relatively low precipitation intensity. For example, the SPAS 1614 temporal that was used in Scenario 3 and Scenario 6 has 18 hours where the precipitation intensity was at about 0.02 inches per hour, and a total of 28 hours where the precipitation intensity was less than about 0.12 inches per hour. This means that the losses cannot be fully satisfied during 18 hours for effectively the entire watershed when using SPAS 1614 as the temporal distribution. For comparison, the Gross General temporal distribution provides at least about 0.12 inches per hour precipitation in every hour, so losses up to about 0.12 inches per hour can be satisfied at all times. When loss rates are not satisfied in many hours, it means there will be more excess precipitation in other hours because the 72-hour total precipitation is the same among the various temporal distributions. Therefore, the relatively low precipitation intensity in many hours of SPAS 1614 results in lower total losses and higher total runoff volume compared to the other temporal distributions.
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The summer general storm PMP/PMF resulted in a higher reservoir water surface elevation than any of the spring general storm PMP/PMF Scenarios 9 through 12 with snowmelt, primarily because of the difference in PMP temporal distributions, but also because a seasonal reduction factor is applied to the spring PMP values. Modeled snowmelt from the PMF Scenarios 9 through 12 resulted in a maximum SWE loss of 1.50 inches per day, which reasonably matches historic snowmelt data presented in Table 8.3-1.
The snowmelt contribution in Scenarios 9 through 12 is not great enough to make one of the scenarios with snowmelt result in the highest reservoir water surface elevation. Table 9.1-3 provides estimates of the snowmelt contributions to runoff for Scenarios 9 through 12. In the “Rain without Snowpack” runs, the initial snowpack was changed to zero at all elevations and locations while keeping all other parameters the same. The results clearly show that runoff from rainfall dominates the rain-on-snowpack PMF scenarios.
Table 9.1-3. PMF Scenario Snowmelt Summary
Rain-on-Snowpack Rain without Snowpack Peak 3-Day Peak 3-Day Date Inflow Volume Inflow Volume (cfs) (ac-ft) (cfs) (ac-ft) May 1 16,435 36,207 16,125 34,011 May 15 19,740 52,189 18,420 46,592 June 1 20,829 55,420 19,630 51,272 June 15 21,866 58,521 20,906 55,054
For comparison purposes, Figure 9.1-1 plots all 12 of the candidate PMF inflow hydrograph scenarios. Note that some hydrographs are so closely shaped that they plot under other hydrographs and are not visible. In a similar manner, Figure 9.1-2 plots all 12 of the candidate PMF outflow hydrographs.
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60,000
Scenario 01 - Inflow
Scenario 02 - Inflow
50,000 Scenario 03 - Inflow Scenario 04 - Inflow
Scenario 05 - Inflow
Scenario 06 - Inflow 40,000 Scenario 07 - Inflow
Scenario 08 - Inflow
Scenario 09 - Inflow
30,000 Scenario 10 - Inflow
Scenario 11 - Inflow Discharge (cfs) Discharge Scenario 12 - Inflow
20,000
10,000
0 1-Sep 2-Sep 3-Sep 4-Sep 5-Sep 6-Sep
Figure 9.1-1. Candidate PMF Inflow Hydrographs
50,000
Scenario 01 - Inflow
45,000 Scenario 02 - Inflow
Scenario 03 - Inflow 40,000 Scenario 04 - Inflow
Scenario 05 - Inflow 35,000 Scenario 06 - Inflow
Scenario 07 - Inflow 30,000 Scenario 08 - Inflow
Scenario 09 - Inflow 25,000 Scenario 10 - Inflow
Discharge (cfs) Discharge Scenario 11 - Inflow 20,000 Scenario 12 - Inflow
15,000
10,000
5,000
0 1-Sep 2-Sep 3-Sep 4-Sep 5-Sep 6-Sep
Figure 9.1-2. Candidate PMF Outflow Hydrographs
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9.2 Sensitivity Analysis
FERC PMF guidelines indicate that the first computed inflow PMF hydrograph should be considered as preliminary pending review of the assumptions considered to have a significant effect on the PMF and a determination of the sensitivity of individual parameters on the magnitude of the PMF. A sensitivity analysis is made to determine the degree the PMF is affected by key parameters, even if conservative values for those parameters were initially assumed.
Table 9.2-1 summarizes the results of the sensitivity analysis. The PMF sensitivity inflow hydrographs are plotted on Figure 9.2-1 and the outflow hydrographs are plotted on Figure 9.2-2. The individual PMF Sensitivity Scenarios are briefly discussed in the following paragraphs.
All of the PMF Sensitivity Scenarios used PMF Scenario 1 as the base case, then made changes in a key model parameter or input data group. Scenario S1a was included to demonstrate the effects of setting the loss rate at essentially a minimum value of 0.02 inches per hour. This scenario is not supported by higher infiltration rates found in the calibration. Loss rates as low as 0.02 inches per hour are also not characteristic of mostly forested areas (see also Section 9.3), which means that PMF Scenario S1a would represent unreasonably conservative parameters.
PMF Sensitivity Scenarios S1b, S1c, and S1d focused on the shape of the unit hydrograph as represented by the ratio R/(Tc + R). For each modeled sub-basin, the values obtained originally (directly w/equations by SEO) showed ratios ranging from 0.29 to 0.39, as included in PMF Scenario S1b. As discussed in Section 9.3, SEO guidelines on Tc and R parameter values were developed for smaller basins and may not be applicable to the Gross Dam watershed, thereby allowing adjustments. The Tc and R values from the original SEO equations produced calibration run peak flows that were noticeably much greater than historic values. In summary, Tc and R values from the original SEO equations are not applicable to the Gross Dam watershed.
The calibration runs consistently overestimated the peak flows, which could be reduced by increasing the time of concentration (PMF Scenario S1c) or increasing the ratio R/(Tc + R) to 0.8 (PMF Scenario S1d). Neither PMF Scenario S1c nor S1d resulted in maximum reservoir water surface elevations higher than Scenario 1.
Scenario S2, the Critically Stacked temporal distribution, includes all of the PMP maximum depth- duration values in a single storm. Scenario S2 represents a PMP scenario that was determined to be overly conservative because there are no recorded storms in the transposable region that have this type of temporal distribution over 72 hours. The meteorological characteristics of the actual storm driving the 1-hour PMP value (the short-duration-high-intensity Big Thompson storm), would not occur within the same event as the temporally diffuse storms driving the 24-hour and 72-hour PMP values, particularly the Big Elk Meadow storm, that are controlling at durations of
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more than 24 hours. It is over-conservative to assume that the Big Thompson storm’s intensity would occur inside the same event as the Big Elk Meadow storm’s 72-hour depth, which is the Critically Stacked assumption and is therefore rejected. See Section 6.2 for additional discussion of the Critically Stacked temporal distribution.
Scenario S3 is similar to PMF Scenario 9, except that the full summer PMP was applied without the seasonal reduction that was applied in PMF Scenario 9. Scenario S3 resulted in maximum reservoir water surface elevations that were significantly lower than Scenario 1.
Table 9.2-1. PMF Routing Sensitivity Analysis Results
Scenario Description Gross Dam PMF Results (322 Foot Spillway)(1)(2) PMF ModClark Time of Peak Peak 3-day Inflow Temporal Constant Infiltration Inflow Outflow Scenario UHG Ratio Concentration Elevation Water Volume Distribution Loss Rate (in/hr) (cfs) (cfs) R/(Tc+R) (hrs) (ft) Rise (ft) (ac-ft) 1 Gross General Average NRCS Losses 0.6 SEO (2008) 40,398 36,711 7,415.6 9.6 61,173 S1a Gross General 0.02 in/hr 0.6 SEO (2008) 44,165 40,700 7,416.3 10.3 80,041 S1b Gross General Average NRCS Losses ~0.3 SEO (2008) 48,762 43,607 7,416.8 10.8 61,678 S1c Gross General Average NRCS Losses 0.6 30% Tc increase 37,176 34,087 7,415.2 9.2 60,827 S1d Gross General Average NRCS Losses 0.8 SEO (2008) 28,123 26,281 7,413.7 7.7 58,717 S2 Critically Stacked Average NRCS Losses 0.6 SEO (2008) 53,711 42,039 7,416.6 10.6 61,245 S3 SPAS 1253 All-Season (3) Average NRCS Losses 0.6 SEO (2008) 23,748 22,079 7,412.9 6.9 55,377 Notes: (1) Gross Dam outflow based on 322 foot wide spillway (2) Gross Dam reservoir initial water level was set to the spillway invert elevation of 7,406 feet and the raised dam crest elevation is 7,421 feet (3) All season PMP with SPAS 1253 temporal distribution, and May 1 snowpack.
60,000 1: Avg Loss; R/(Tc+R)=0.6; Tc
S1a: 0.02 in/hr; R/(Tc+R)=0.6; Tc 50,000 S1b: Avg Loss; R/(Tc+R)~0.3 (SEO); Tc
S1c: Avg Loss; R/(Tc+R)=0.6 (SEO); Tc+30%
40,000 S1d: Avg Loss; R/(Tc+R)=0.8 ; Tc+30% S2: GEN PMP Crit Stacked; Avg Loss; R/(Tc+R)=0.6; Tc
S3: GEN PMP Rain-on-Snow; Avg Loss; R/(Tc+R)=0.8; Tc 30,000 Discharge (cfs)
20,000
10,000
0 1-Sep 2-Sep 3-Sep 4-Sep 5-Sep 6-Sep Figure 9.2-1. PMF Sensitivity Analysis Inflow Hydrographs
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50,000 1: Avg Loss; R/(Tc+R)=0.6; Tc 45,000 S1a: 0.02 in/hr; R/(Tc+R)=0.6; Tc S1b: Avg Loss; R/(Tc+R)~0.3 (SEO); Tc 40,000 S1c: Avg Loss; R/(Tc+R)=0.6 (SEO); Tc+30%
35,000 S1d: Avg Loss; R/(Tc+R)=0.8 ; Tc+30% S2: GEN PMP Crit Stacked; Avg Loss; R/(Tc+R)=0.6; Tc 30,000 S3: GEN PMP Rain-on-Snow; Avg Loss; R/(Tc+R)=0.8; Tc
25,000
20,000 Discharge (cfs) Discharge
15,000
10,000
5,000
0 1-Sep 2-Sep 3-Sep 4-Sep 5-Sep 6-Sep Figure 9.2-2. PMF Sensitivity Analysis Outflow Hydrographs
Table 9.2-2 provides candidate PMP depth-duration data for each of the candidate PMF scenarios. For completeness in a single table, sensitivity runs are also included in Table 9.2-2.
Table 9.2-2. Candidate PMP Depth-Duration Summary
Depth- Gross SPAS SPAS Critically SPAS 1253 Local Duration Duration General 1253 1614 Stacked May 1 May 15 June 1 June 15 Storm (hours) (inches) (inches) (inches) (inches) (inches) (inches) (inches) (inches) (inches) (inches) 1 2.88 0.93 0.88 0.77 2.88 0.66 0.72 0.76 0.80 2.70 6 5.38 4.86 3.22 3.69 5.38 2.40 2.62 2.77 2.92 5.40 24 11.69 11.63 9.62 11.25 11.69 7.18 7.83 8.27 8.72 N/A 72 17.54 17.56 17.57 17.62 17.56 13.11 14.30 15.11 15.92 N/A PMF 1, 4, N/A 2, 5, S3 3, 6 S2 9 10 11 12 7, 8 Scenarios S1a-S1d
9.3 Additional PMF Hydrograph Considerations Summary
There are several additional considerations that generally indicate that the PMF hydrographs are reasonable or may actually be on the conservative (high required spillway discharge capacity) side. Some of the items listed in this section were previously presented, but are repeated here for emphasis. When evaluating the reasonableness of the PMF, the following additional factors should also be considered.
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1. The Clark unit hydrograph method is not recommended for watersheds larger than 10 square miles in the SEO (2008) guidelines not because of any limitations in applicability of the general Clark method (or ModClark) to large watersheds, but only because the Clark Tc and R equations in the SEO guidelines were derived for much smaller watersheds (i.e., <5 sq. mi., George Sabol, personal communication). Tc and R values derived for much smaller watersheds might tend to develop a more peaky unit hydrograph than would be applicable to a larger, mostly forested watershed.
2. In the hydrologic model calibration runs, simulated peak flows consistently overestimated the recorded peak flows, even after the R (attenuation) values were increased from SEO guideline values. Hydrograph volumes were generally overestimated near the peak day, but some simulated hydrograph volumes were similar to observed volumes when considering a longer period of several days. This gives one indication of reasonable precipitation values, but a hydrograph that is too peaky, but does not eliminate rainfall data as a potential error source.
3. The USACE (1994) has indicated that overland flow rarely occurs in forested soils. Forested areas exhibit the greatest surface losses because of their well-developed canopies and significant surface storage provided by surface litter. In forested areas, lateral movement of the infiltrated water occurs along the lower conductivity layer as either saturated or unsaturated flow until it seeps out to the surface nearer the bottom of the hillslope. Observations have shown that the subsurface movement of water down the hillslope combined with overland flow from the source areas is the flood mechanism in forested areas. In some respects, the apparent rainfall excess in a flood hydrograph in a forested area is a combination of interflow, subsurface flow, and overland flow. Compared to less forested or more urban areas, the forested hydrographs would be longer and less peaky. Consideration for the effects of forested areas on flood hydrographs is one potential explanation of why the lag in the observed hydrographs is generally greater than the simulated hydrographs of the calibration events.
4. The USACE Hydrologic Engineering Center has developed hydrologic and hydraulic models of the Sacramento and San Joaquin Rivers for the purpose of developing comprehensive flood control plans (USACE HEC, 2001). The Sacramento and San Joaquin Rivers comprise nearly 60,000 square miles and were simulated in 33 individual models. This comprehensive flood study used the HEC-HMS model with gridded precipitation. The ModClark rainfall to runoff transformation was used to compute the subbasin hydrographs. Clark unit hydrograph parameter optimization was performed for 30 subbasins, ranging in size from 11.6 square miles to 613.8 square miles with longest flow path slopes ranging from 1.9% to 12.8% (similar slopes to Gross Dam watershed subbasins). The R/(Tc + R) ratio for the 30 parameter optimization subbasins ranged from 0.50 to 0.91, with 25 of 30 falling in the range of 0.59 to 0.86. The smallest subbasin (11.6 sq. mi.) had an R/(Tc + R) value of 0.84, and the largest subbasin (613.8 sq. mi.) had an R/(Tc + R) ratio of 0.77. Using the principle that the R/(Tc + R) ratio
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is constant for hydrologically similar areas, a value of either 0.6 or 0.8 was assigned to subbasins depending on the particular river. Considering the scope of this study that includes the western slope of the Sierra Nevada Mountains in much of California and the narrow range of R/(Tc + R) values used, it supports the R/(Tc + R) value of 0.6 for the Gross Dam watershed as being reasonable to somewhat conservative and confirms the applicability of the ModClark and gridded precipitation approach.
5. Table 9.3-1 summarizes selected MWH developed PMF experience values. The watershed areas were selected to be similar to the Gross Dam watershed in factors such as drainage area, mountainous terrain, mostly forested cover, and seasonal snowpacks. Historical flood calibration results for these mountainous, forested watersheds were generally good to excellent. MWH experience values for the R/(Tc + R) ratio are around 0.6 or slightly higher. Results show that although the recommended Gross Dam PMF had a somewhat lower PMF peak unit runoff than most of the other watersheds, it had the lowest 24-hour PMP. Gross Dam was also the only watershed cited in the table for which snowmelt did not contribute to the PMF runoff. This is an indication that the Gross Dam PMF peak flow is roughly comparable, or at least not too low, compared to MWH PMF experience in other forested, mountainous watersheds.
Table 9.3-1. MWH PMF Experience Values
Location or Drainage 24-Hour PMF Run PMF Run Peak Flow Ratio River or State Location or Gage Elev. Area R Tc R/(Tc+R) Forested PMP Peak Unit Q of Record Years of PMF to Creek Gage (feet) (sq.mi.) (hours) Percent (inches) (cfs) (cfs/sq.mi.) (cfs) Record Rec. Flood South Boulder CO Gross Dam 7,280 93 Note (1) Note (1) 0.60 76% 11.63 40,398 434 N/A (3) N/A (3) N/A (3) Cowlitz WA 14226500 1,048 287 14.36 7.07 0.67 53% 19.72 147,218 513 42,100 95 3.50 Cispus WA 14232500 1,222 321 30.75 15.14 0.67 66% 14.55 85,989 268 31,600 68 2.72 Nisqually WA 12082500 1,450 132 9.0 4.5 0.67 65% 23.25 96,603 732 21,800 73 4.43 Mineral WA 12083000 1,340 75.2 16.0 8.0 0.67 80% 18.71 37,867 504 9,740 72 3.89 Highland CA 11294000 6,340 45.4 6.8 3.2 0.68 NR (2) 19.90 28,471 627 9,860 64 2.89 NF Stanislaus CA 11294300 5,420 111 Note (1) Note (1) 0.68 NR (2) 17.86 65,576 591 21,000 6 3.12 Eleanor CA 11278000 4,500 78.4 Note (1) Note (1) 0.55 NR (2) 21.16 70,659 901 19,500 101 3.62 Notes: (1) The watershed is composed of several sub-basins, all having the same R/(Tc+R) ratio values. (2) NR - Data not included in PMF report. (3) N/A - No recorded hourly peak flow data is available.
9.4 Recommended PMF
The recommended PMF for spillway design is PMF Scenario 1, which has inflow, outflow and reservoir level plotted on Figure 9.4-1. The peak PMF inflow is 40,398 cfs, the peak outflow is 36,711 cfs, and with the maximum reservoir water surface level at elevation 7,415.6 feet. Because the final design of the spillway and total future outflow capability of the raised Gross Dam is unknown at this time, additional PMF routing runs will be necessary during the Final Design phase for development of a final spillway design.
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45,000 7422
40,000 7420
35,000 7418 Inflow (cfs)
30,000 Outflow (cfs) 7416 Reservoir Elevation (feet)
25,000 7414
Flow (cfs) 20,000 7412
15,000 7410 (feet) Elevation Reservoir
10,000 7408
5,000 7406
0 7404 0 12 24 36 48 60 72 84 96 Hours Figure 9.4-1. Gross Dam PMF Inflow, Outflow, and Reservoir Elevation
Because a site-specific PMP study was performed for Gross Dam, and the Gross General temporal distribution is based on a combination of actual recorded maximum storms, it is considered to represent a realistic temporal distribution that best represents actual storms in the transposable vicinity. Furthermore, the PMF Scenario 1 (Gross General) includes the 24-hour and 72-hour depth duration values. The 1-hour and 6-hour depth-duration values were tested as part of the local storm. Therefore, the “critical” loading is covered by the Gross General and local storm scenarios.
The current HEC-HMS PMF model runs described in Section 9 are attached as a zip file.
9.5 Comparison with Previous Gross Dam PMF Studies
A comparison of the PMP totals and peak PMF inflows for the watershed tributary to the Gross Dam from among the three available PMF studies are summarized in Table 9.5-1. The 6-hour PMP values for the DMJM and Goodson studies were 89% and 114% greater than the 6-hour PMP in the current study. The PMF peak inflow values for the DMJM and Goodson studies were 101% and 124% greater than the current study. Because the PMP values were significantly greater in the DMJM and Goodson PMF studies, the PMP is likely the primary cause of the difference between peak PMF inflows. This also indicates that the flood hydrology aspects appear to be more
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similar among the three studies shown on Table 9.5-1 and not a lot less conservative in the current study.
Snowmelt was included in the current MWH study using the temperature index method in HEC- HMS, but the spring candidate PMF runs with snowmelt did not form the critical PMF. The previous DMJM and Goodson studies did not discuss snowmelt. Additionally, because only an all- season PMP was used, there was assumed to be no snowmelt.
Table 9.5-1. PMP and PMF Inflow Comparison
PMF Drainage General Storm PMP PMF Peak PMF Modeled Prepared Report Location or Area 1-Hour 6-Hour 24-Hour Inflow Unit Q Snowmelt By Date Gage (sq.mi.) (inches) (inches) (inches) (cfs) (cfs/sq.mi.) Method DMJM 1982 Gross Dam 93 4.00 9.10 18.70 80,450 865 None Goodson 1987 Gross Dam 93 4.20 10.30 20.70 89,960 967 None MWH 2016 Gross Dam 93 0.93 4.86 11.63 40,398 434 Temperature Index
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10. REFERENCES
American Society of Civil Engineers, 1997. Flood-Runoff Analysis, Technical Engineering and Design Guides as Adapted from the U.S. Army Corps of Engineers, No. 19.
Committee on Safety Criteria for Dams, 1985. Safety of Dams, Flood and Earthquake Criteria, Water Science and Technology Board, Commission on Engineering and Technical Systems, National Research Council, published by National Academy Press.
Cudworth, Jr., Arthur G., 1989. Flood Hydrology Manual, A Water Resources Technical Publication, U.S. Department of the Interior, Bureau of Reclamation.
DMJM Phillips-Reister, 1977. Gross Dam and Reservoir, Safety Inspection Report, for Denver Board of Water Commissioners.
DMJM Phillips-Reister-Haley, 1982. Gross Dam and Reservoir, 1982 Safety Inspection Report, for Denver Board of Water Commissioners.
Federal Energy Regulatory Commission, September 2001. Engineering Guidelines for the Evaluation of Hydroelectric Projects, Chapter VIII, “Determination of the Probable Maximum Flood”.
GIS Elevation and elevation related data and derivatives (Contours, Contour Areas, etc.): U.S. Geological Survey, 2015-07-17, USGS NED 1/3 arc-second n41w106 1 x 1 degree IMG 2015.
GIS Land cover data (Watershed Cover): NLCD 2011 Land Cover (2011 Edition, amended 2014), National Geospatial Data Asset (NGDA) Land Use Land Cover, 3 x 3 Degree: NLCD2011_LC_N39W105.
GIS SURGO Soil Data (Hydrologic Soil Group): SDA-NRCS-NCGC, 09-DEC-15 FY16, Soil Survey Geographic (SSURGO II), Source_Scale_Denominator: 12,000 - 63,360.
Goodson & Associates, Inc., October 1987. Safety Inspection Report for Gross Dam, Reservoir and Appurtenant Structures, prepared for Denver Water Department.
Hansen, E.M, Fenn, D.D., Schreiner, L.C., Stodt, R.W., and J.F., Miller, 1988. Probable Maximum Precipitation Estimates, United States between the Continental Divide and the 103rd Meridian, Hydrometeorological Report Number 55A, National weather Service, National Oceanic and Atmospheric Association, U.S. Dept of Commerce, Silver Spring, MD.
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Jarrett, Robert R., 1984. “Hydraulics of High-Gradient Streams”, Journal of Hydraulic Engineering, Vol. 110, No. 11, pg. 1519-1539, ASCE.
Jarrett, Robert R., 1993. “Flood Elevation Limits in the Rocky Mountains”, Engineering Hydrology, proceedings of the Symposium sponsored by the Hydraulics Division/ASCE, July 25- 30, 1993, San Francisco, CA.
Jarrett, Robert D., and John E. Costa, 1988. “Evaluation of the Hydrology in the Colorado Front Range Using Precipitation, and Paleoflood Data for the Big Thompson River Basin”, U.S. Geological Service, Water Resources Investigation Report, 87-4117.
Jarrett, Robert D., and John E. Costa, 2006. “1976 Big Thompson Flood, Colorado – Thirty Years Later”, U.S. Geological Service, Fact Sheet FS-2006–3095.
Kohn, M.S., Stevens, M.R., Harden, T.M., Godaire, J.E., Klinger, R.E., and Mommandi, Amanullah, 2016, Paleoflood investigations to improve peak-streamflow regional-regression equations for natural streamflow in eastern Colorado, 2015: U.S. Geological Survey Scientific Investigations Report 2016–5099, 58 p., http://dx.doi.org/10.3133/sir20165099.
Natural Resources Conservation Service (NRCS), 1986. Urban hydrology for small watersheds, Technical Release 55 (TR-55), USDA, Springfield, VA.
Natural Resources Conservation Service (NRCS), 2004. National Engineering Handbook, Hydrology, Chapter 11 Snowmelt, USDA, Springfield, VA.
Sabol, George V, PhD, PE, December 2016. Personal communication. Note that George Sabol is the author of the SEO Hydrologic Basin Response Parameter Estimation Guidelines.
State of Colorado Office of the State Engineer, Dam Safety Branch (SEO), 2007. “Rules and Regulations for Dam Safety and Dam Construction”. January 1, 2007.
State of Colorado Office of the State Engineer, Dam Safety Branch (SEO), 2008. Hydrologic Basin Response Parameter Estimation Guidelines. Prepared by George V. Sabol, PhD, PE, Tierra Grande International, Inc., May 2008.
State of Colorado Office of the State Engineer, Dam Safety Branch (SEO), 2016. Project Review Guide. February 17, 2016.
U.S. Army Corps of Engineers (USACE), Hydrologic Engineering Center (HEC), August 2001. HEC-HMS for the Sacramento and San Joaquin River Basins Comprehensive Study, PR-46.
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U.S. Army Corps of Engineers (USACE), Hydrologic Engineering Center (HEC), July 2015. Hydrologic Modeling System HEC-HMS, User’s Manual, Version 4.1.
U.S. Army Corps of Engineers (USACE), 1994. Flood-Runoff Analysis, EM 1110-2-1417, August 31.
U.S. Army Corps of Engineers (USACE), 1998. Runoff from Snowmelt, EM 1110-2-1406, March 31.
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Appendix A Maximum Flood and PMF Data Background Information
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Maximum Flood and PMF Data Background Information
At the Gross Dam PMF Board of Consultants (BOC) meeting #1, the unusually large difference was noted between the Gross Dam PMF peak flows, as estimated in previous studies, and the historic recorded maximum flows on South Boulder Creek. Additional information is presented herein and in the referenced professional papers that is intended to help to better understand the large difference between the historic floods and the previous PMF estimates. This document provides background information on historic recorded maximum flows as well as MWH PMF experience data, but was not intended to provide direct input to the Gross Dam PMF modeling. The referencing of the professional papers written by others does not imply agreement by Denver Water or MWH with all material presented in the referenced papers.
A search for high elevation extreme floods in the Colorado Frontal Range was conducted by MWH, but this search was fruitless. In a telephone communication with Bob Jarrett, he indicated that this was the expected result and that none would be found. In documents authored or co- authored by Bob Jarrett (Jarrett, 1993; Jarrett and Costa, 1988), the lack of rainfall dominated extreme floods is documented for elevations above about 7,500 feet for the Colorado Front Range. Snowmelt dominated maximum flows do not have the high peak flows of extreme rainfall dominated floods, which contributes to the relatively low historic South Boulder Creek flows in comparison to the previously calculated PMF values. As stated by Jarrett and Costa (1988), “Precipitation, streamflow, and geomorphic evidence indicates that there is a distinct decrease in floods above about 7,500 feet in the foothills of northern Colorado”. This information generally supports a conclusion that the perceived low recorded South Boulder Creek maximum flows above Gross Dam are typical of Colorado Front Range watersheds at a similar elevation.
In a telephone communication, Bob Jarrett also indicated that the “biggest of the big” flood events for the Colorado Front Range was the Big Thompson Flood of 1976. Information for the Big Thompson Flood was summarized by Jarrett and Costa (2006) and is summarized for comparison with South Boulder Creek maximum flows in Table 1 below. It is noted that recorded flows at Pinecliffe and Rollinsville include Moffat Tunnel flows that should be removed to obtain natural flows.
For ballpark estimates of the PMP, a commonly used range has been 2 to 6 times the 100-year rainfall. Since the PMF is normally dominated by the PMP value, by extension a ballpark estimate of the PMF peak flow can be roughly approximated as 2 to 6 times the 100-year flood peak. MWH PMF experience values for mountainous areas with heavy snowpacks and drainage areas of roughly similar size to Gross Dam are presented in Table 2 below, which shows that the MWH calculated PMF values generally fall into the expected range in relation to recorded maximum
1 CEII – DO NOT RELEASE Gross Dam PMF BOC Meeting #2 August 4, 2016 flows. It is noted that if the recorded maximum peak flow at Gross Dam were estimated to be 2,000 cfs, the previously determined PMF to recorded peak flow ratio would be about 45. If the recorded maximum peak flow at Gross Dam were estimated to be 3,000 cfs, the previously determined PMF to recorded peak flow ration would be about 30, still an extraordinary value. It is also noted that a recent PMF study for Lake Eleanor using the temperature index method for snowmelt in HEC-HMS produced a relatively high PMF unit discharge. This is one indication that use of the temperature index method in mountainous areas with deep snowpacks does not automatically lead to low PMF peak values. Table 2 presents background information provided for comparison purposes only and does not limit the Gross Dam PMF study.
Jarrett (1993) has developed a comprehensive summary of peak flow data for the Rocky Mountains, noting the substantial variation in peak flow with elevation. Jarrett’s analysis included 935 gaging stations and 18,238 station-years of flow data for the Rocky Mountains in Colorado, while separating peak flow data for elevations above and below 2,300 m (about 7,500 feet). For stations above 7,500 feet, the maximum peak flow unit discharge was 1.1 cms/sq.km. (101 cfs/sq.mi.), which would correspond to a flow of about 9,400 cfs for a drainage area the size of the Gross Dam watershed. In contrast, Jarrett (1993) shows that for the gaging stations below 7,500 feet, the maximum peak flow unit discharge was 38 cms/sq.km. (3,475 cfs/sq.mi.), which would correspond to about 323,000 cfs if it occurred over a 93 square mile watershed.
A recent paper by Kampf and Lefsky (2016) [not attached] that includes information on the 2013 flood also references work by Jarrett and others and provides some information on the distribution of snow persistence with elevation and the dominant peak flow source transition from snowmelt to rainfall in the Colorado Front Range. Snow persistence (SP) was defined as the mean annual fraction of time with snow cover between January 1 and July 3. It was concluded that in the 30 year period from 1982 to 2013, annual peak flows have been dominated in watershed in the low snow zone (SP < 0.3; mean elevations < 2,000 m or 6,560 feet) and snowmelt dominated in most watersheds in the persistent snow zone (SP > 0.7; mean elevations > 3,100 m or 10,170 feet).
References Jarrett, Robert D., and John E. Costa, 1988. “Evaluation of the Hydrology in the Colorado Front Range Using Precipitation, and Paleoflood Data for the Big Thompson River Basin”, U.S. Geological Service, Water Resources Investigation Report, 87-4117. Jarrett, Robert D., 1993. “Flood Elevation Limits in the Rocky Mountains”, Engineering Hydrology, Proceedings of the Symposium, July 25-30, 1993, San Francisco, CA. Jarrett, Robert D., and John E. Costa, 2006. “1976 Big Thompson Flood, Colorado – Thirty Years Later”, U.S. Geological Service, Fact Sheet FS-2006–3095. Kampf, Stephanie K, and Michael A. Lefsky, 2016. “Transition of dominant peak flow source from snowmelt to rainfall along the Colorado Front Range: Historical patterns, trends, and lessons from the 2013 Colorado Front Range floods”, Water Resources Research, 52, 407–422.
2 CEII – DO NOT RELEASE Gross Dam PMF BOC Meeting #2 August 4, 2016
Table 1: Maximum Flows, South Boulder Creek and Big Thompson (values are peak flows unless noted as daily flows)
Drainage Site Years Flow Moffat Deregulated Deregulated Gage Number Flow Unit Flow Gage or Location Name Area Datum of Begin Date End Date Data Tunnel Flow Est. Flow Est. Unit Flow Date Notes or Method (cfs) (cfs/sq.mi.) (sq.mi) (feet) Record Source (cfs) (cfs) (cfs/sq.mi.) PMF South Boulder Creek DMJM 80,450 865.1 ------1982 PMF Study Negligible snowmelt 93 7,000 ------Studies at Gross Dam Goodson 89,960 967.3 ------1987 PMF Study Negligible snowmelt Calculated Big Thompson River 6,200 Contributing drainage area 189 ------Note (1) 28,200 149.2 ------July 31, 1976 Flow above Drake (approx.) noted as 34 sq. mi. Big Thompson River USGS and 5,500 40.1 ------July 15, 1982 Maximum of record (3) 06733000 137 7,493 52 6/21/1947 6/4/1998 at Estes Park Colo. DWR 1,870 13.6 ------June 18, 1995 2nd maximum of record North Fork Big Thompson USGS and 8,710 102.4 ------July 31, 1976 Maximum of record 06736000 85 6,170 30 6/21/1947 7/31/1976 River at Drake Note (1) 1,290 15.2 ------June 16, 1965 2nd maximum of record Big Thompson River at USGS and 31,200 102.3 ------July 31, 1976 150 sq.mi. contributing area 06738000 305 5,305 84 6-18-1888 5/29/2007 canyon mouth near Drake Note (1) 8,000 26.2 ------1919 2nd maximum of record South Boulder Creek 7,390 66.6 ------Sept. 2, 1938 Maximum of record (2) 06729500 111 6,080 96 1-1-1896 9/30/1995 USGS near Eldorado Springs 2,370 21.4 ------June 18, 1951 2nd maximum of record South Boulder Creek 718 16.8 Unavailable ------June 12, 1949 Maximum of record 06729000 (6) 42.7 8,380 11 10/1/1911 9/30/1949 USGS near Rollinsville 622 14.6 Unavailable ------June 21, 1947 2nd maximum of record South Boulder Creek 1,150 15.8 565 585 8.0 June 7, 1979 Maximum of record 06729300 (6) 72.7 7,930 2 5/18/1979 10/3/1980 USGS at Pinecliffe 914 12.6 446 468 6.4 June 20, 1980 2nd maximum of record BOCPINCO South Boulder Creek Colo. Div. of 1,370 18.8 705 665 9.1 June 6, 2011 Maximum of record (4) 72.7 ----- 18 1/1/1998 7/23/2016 (6) at Pinecliffe Water Res. 1,360 18.7 554 806 11.1 May 29, 2003 2nd maximum of record Calculated South Boulder Creek Denver 869 9.3 ------June 17, 1995 Daily max. of record (5) 93.0 7,000 59 3/1/1958 3/31/2016 Inflow (7) at Boulder Dam Water 838 9.0 ------June 7, 1997 Daily 2nd max. of record
Notes (1) USGS Fact Sheet 2006-3095, July 2006, "1976 Big Thompson Flood, Colorado - Thirty Years Later" (2) Peak flow during September 12-13, 2013 estimated as 2,100 cfs by Bob Jarrett (USGS, retired). (3) Peak flow on July 31, 1976 was 457 cfs. (4) Peak flow on September 12, 2013 was 780 cfs and Moffat Tunnel flow was 134 cfs, which means the natural peak flow was approximately 646 cfs. (5) Deregulated daily flow on September 12, 2013 was 770 cfs. (6) Includes Moffat Tunnel flows. (7) Daily flows have been deregulated to represent natural inflows at Gross Dam.
3 CEII – DO NOT RELEASE Gross Dam PMF BOC Meeting #2 August 4, 2016
Table 2: Probable Maximum Flood Estimates for Gross Dam and MWH PMF Experience Values
PMF Location or Drainage 24-Hour PMF Run PMF Run Modeled Rainfall Peak Flow Ratio Prepared River or State Location or Gage Elev. Area PMP (1) Peak Unit Q PMF Snowmelt Runoff of Record Years of PMF to By Creek Gage (feet) (sq.mi.) (inches) (cfs) (cfs/sq.mi.) Snowpack Method Model (cfs) Record Rec. Flood DMJM SF Boulder CO Gross Dam 7,000 93 18.79 80,450 865 None Incl. in base flow Uncalibrated Note (2) 59 ----- Goodson SF Boulder CO Gross Dam 7,000 93 23.90 89,960 967 None Incl. in base flow Uncalibrated Note (2) 59 ----- MWH Cowlitz WA 14226500 1,048 287 19.72 147,218 513 High Energy Budget Calibrated 42,100 95 3.50 MWH Cispus WA 14232500 1,222 321 14.55 85,989 268 Moderate Energy Budget Calibrated 31,600 68 2.72 MWH Nisqually WA 12082500 1,450 132 23.25 96,603 732 High Energy Budget Calibrated 21,800 73 4.43 MWH Mineral WA 12083000 1,340 75.2 18.71 37,867 504 High Energy Budget Calibrated 9,740 72 3.89 MWH Highland CA 11294000 6,340 45.4 19.90 28,471 627 High Energy Budget Calibrated 9,860 64 2.89 MWH NF Stanislaus CA 11294300 5,420 111 17.86 65,576 591 High Energy Budget Calibrated 21,000 6 3.12 MWH Eleanor CA 11278000 4,500 78.4 21.16 70,659 901 High Temperature Index Calibrated 19,500 101 3.62
Notes: (1) Includes areal reduction where part of a larger basin PMF study. (2) Instantaneous peak flow data are unavailable. Maximum calculated daily reservoir inflow of record is 869 cfs.
4 CEII – DO NOT RELEASE GROSS RESERVOIR HYDROELECTRIC PROJECT PROBABLE MAXIMUM FLOOD STUDY
Appendix B Site-Specific Probable Maximum Precipitation Study for Gross Reservoir by Applied Weather Associates