St.Vrain Watershed (Boulder County), Colorado. Hydrologic Data Development Technical Support Data Notebook

Transcription

1 Hydrologic Data Development Technical Support Data Notebook St.Vrain Watershed (Boulder County), Colorado FEMA Grant EMD-2011-GR-1182 MIP Case#: S March 17, 2016 Federal Emergency Management Agency Department of Homeland Security Region VIII Denver Federal Center, Building 710 P.O. Box Denver, CO 80255

2 Hydrologic Analysis TSDN for St. Vrain Creek, Colorado March 17, 2016 TECHNICAL SUPPORT DATA NOTEBOOK (TSDN) for St. Vrain Watershed (Boulder County), Colorado HYDROLOGIC ANALYSIS TSDN SUBMITTED ON BEHALF OF: The Colorado Water Conservation Board (CWCB) 1313 Sherman Street, Suite 721 Denver, CO BY: AECOM (formerly URS) 6200 S. Quebec St. Greenwood Village, CO DATE SUBMITTED: March 17, 2016

3 Table of Contents March 17, 2016 TABLE OF CONTENTS 1.0 Task Summary Project Overview Task Scope Develop Hydrologic Data Background Project Area Description Historic Flooding Previous Hydrologic Studies Methodology Summary of Approach Additional Analyses Model Junction Additions %+ Calculation Results Summary of Discharges Discharge Comparison Exceptions to Guidance and Standards/Special Situations MIP Submittal File Structure Conclusions References...16 TABLES Table 1 Summary of Discharges for Enhanced Study Streams in St. Vrain Creek Watershed. 10 Table 2 Comparison between Effective and Restudy Discharges FIGURES Figure 1 Project Study Area and Effective Flows... 2 Figure 2 - Discharge Values and Percent Change between Applied and Effective Flows... 9 APPENDICES Appendix A Appendix B Appendix C TSDN Documents Special Response Memorandums 1%+ Methodology i

4 List of Acronyms March 17, 2016 CDOT Colorado Department of Transportation cfs cubic feet per second CWCB Colorado Water Conservation Board DARF Depth-area reduction factor DCS Data Capture Standards EPA Environmental Protection Agency FEMA Federal Emergency Management Agency FIRM Flood Insurance Rate Map FIS Flood Insurance Study HEC-HMS Hydrologic Engineering Center Hydrologic Modeling System HUC Hydrologic Unit Code MAP Mapping, Assessment, and Planning MIP Mapping Information Platform NFIP National Flood Insurance Program NOAA National Oceanic and Atmospheric Administration NSVC North St. Vrain Creek Q 1% RAMPP SCS SSVC SVC SWMM TSDN USACE WSEL 1-percent annual chance peak discharge Risk Analysis, Mapping, and Planning Partners Soil Conservation Service South St. Vrain Creek St. Vrain Creek Storm Water Management Model Technical Support Data Notebook U.S. Army Corp of Engineers Water Surface Elevation ii

5 Hydrologic Analysis TSDN for St. Vrain Creek, Colorado March 17, TASK SUMMARY This Technical Support Data Notebook (TSDN) is being submitted for the St. Vrain Risk Mapping Assessment and Planning (MAP) study, Hydrology task. Data presented in this TSDN documents the hydrologic analysis that was performed for the study. This section presents a summary of the overall project, details the scope for the hydrology task, and provides background information pertaining to prior flooding events and previous hydrologic studies. 1.1 PROJECT OVERVIEW Following the extensive flooding in September 2013 along the Colorado Front Range, numerous flood recovery efforts were initiated. This particular project, the St. Vrain Watershed Risk MAP study, is funded jointly by Federal Emergency Management Agency (FEMA) and the Colorado Water Conservation Board (CWCB) to develop and/or support flood hazard data and program related activities by conducting technical risk analysis and mapping activities for portions of the St. Vrain Creek Watershed in Boulder County, Colorado. Tracking of this funding occurs in FEMA s Mapping Information Platform (MIP) assigned to Case Number S pertaining to FEMA Grant EMD-2011-GR Severe flooding significantly affected streams and floodplains within the St. Vrain Watershed, altering the geometry and flooding characteristics of streams. This project is purposed to revise FEMA Flood Insurance Rate Maps (FIRMs) for portions of North St. Vrain Creek (NSVC) and South St. Vrain Creek (SSVC) upstream of Lyons and for St. Vrain Creek (SVC) from Lyons to Longmont, as shown in Figure 1. Revised FIRMs will provide more accurate data that can be used for planning, permitting, and design decisions along with emergency response, infrastructure rebuilding, and private development and reconstruction. Concurrent with this effort, CWCB is also conducting a comparable study effort on portions of SVC beyond this project s study limits. The work is being completed under the Colorado Hazard Mapping Program (CHAMP), which was initiated following the approval of Senate Bill by Colorado Legislature. Both this Risk MAP project and the Colorado Hazard Mapping Program are being managed by CWCB and their contractor AECOM (formerly URS) to conduct the following tasks: field reconnaissance, channel and overbank surveys; creating terrain models from updated topographic data; evaluating hydrology; hydraulic modeling; and flood hazard delineations. Although two separate studies are being conducted, they complement each other and will result in seamless results following the same standards and criteria. 1

6 Hydrologic Analysis TSDN for St. Vrain Creek, Colorado March 17, 2016 Figure 1 Project Study Area and Effective Flows 2

7 Hydrologic Analysis TSDN for St. Vrain Creek, Colorado March 17, TASK SCOPE DEVELOP HYDROLOGIC DATA This report summarizes the task of developing hydrologic data for the following stream segments within the study limits (Figure 1): SVC from the Weld County line upstream to its confluence in Lyons for a length of approximately 13.9 miles; NSVC from its confluence in Lyons upstream to Longmont Dam Road for a length of approximately 5.4 miles; and SSVC from its confluence in Lyons upstream approximately 1.0 mile. The following is an excerpt from the scope provided to CWCB from URS in the proposal dated June 8, 2015: Scope: For this study URS shall leverage existing hydrologic data published in the August 2014 report titled: Hydrologic Evaluation of the St. Vrain Watershed, Post September 2013 Flood Event, CDOT Region 4 Flood Recovery Office. URS will compile and submit the relevant hydrologic information for the study streams described in this [scope of work] SOW. It is assumed that the Hydrology meets FEMA guidelines. No new hydrologic studies will be conducted as part of this SOW. Deliverables: URS shall make the following products available to FEMA by uploading the digital data to the MIP in accordance with the schedule outlined in Section 6 - Schedule. TSDN Metadata file; Digital Summary of Discharges Tables presenting discharge data for the flooding sources for which hydrologic analyses were performed Digital versions of draft text for inclusion in the FIS report; Format Hydrology Database or Data Delivery consistent with the DCS and FEMA standards for all return periods (see draft DCS language and coordinate with the Region regarding its appropriate use); The original scope assumed no new hydrologic studies were required for this Project. However, minor adjustments were made to existing studies/models to extract information from additional locations and to calculate the 1% plus (1%+) annual chance recurrence interval. The initial scope also included the use of the St. Vrain Watershed 2014 report (Jacobs 2014a). Three additional hydrologic studies and models conducted for the Colorado Department of Transportation (CDOT) and CWCB were also evaluated including a study of the Lower St. Vrain Watershed (Jacobs 2015), the inflow hydrographs from the Left Hand Creek Watershed study (Jacobs 2014b) and Boulder Creek Watershed study (CH2M Hill 2015). The details of the hydrologic analysis performed for this Project are explained further in Section 2.0. Regarding the scoped deliverables, the metadata, tabulated discharges, and 3

8 Hydrologic Analysis TSDN for St. Vrain Creek, Colorado March 17, 2016 draft text for the Flood Insurance Study (FIS) report have been posted to the MIP accompanying this TSDN according to the file structure described in Section BACKGROUND This section provides a brief summary of the project area geographically, accounts of historic flooding, and previous hydrologic studies pertaining to the St. Vrain Watershed. Background on the St. Vrain Watershed is provided in greater detail in the Lower St. Vrain Watershed Hydrologic Evaluation (Jacobs 2015) Project Area Description The SVC headwaters originate on the eastern slope of the Continental Divide and include numerous tributaries entering the forks of the North, Middle, and South St. Vrain Creeks until they converge into SVC just upstream of 2 nd Avenue in Lyons, Colorado. From its confluence, SVC flows approximately 35 miles before emptying into the South Platte River, initially flowing to the southeast through Longmont and then turning to the northeast until it joins the South Platte. The larger St. Vrain Watershed drains approximately 978 square miles with approximately 9,500 feet in vertical relief, and receives considerable inflows from Left Hand Creek and Boulder Creek, each representing watershed drainage areas of approximately 85 and 447 square miles, respectively. The study limits for this Project are shown on Figure 1 and include small portions of NSVC and SSVC upstream of Lyons and then SVC from Lyons to the Weld County Line. The studied portion of NSVC begins at the intersection of US-36 and Longmont Dam Road and then parallels US-36 through sparsely developed properties for a couple miles, after which the land becomes increasingly developed with residences as the creek nears Lyons. The studied portion of SSVC begins approximately 1 mile upstream from its confluence with NSVC, after it has exited the canyon, and it flows through partiallydeveloped residential properties as it enters Lyons near Fifth Avenue. From their confluence in Lyons, the studied portion of SVC extends approximately 13.9 miles downstream to the boundary between Boulder and Weld County in Longmont, approximately 1.9 miles downstream from its confluence with Left Hand Creek, but still upstream of its confluence with Dry Creek No. 2 and Boulder Creek. Between Lyons and Longmont, SVC flows east-southeast through mostly agricultural land with numerous gravel pits for approximately 8.5 miles before encountering developments associated with the City of Longmont Historic Flooding Most historical floods in the region have been caused by snowmelt coupled with heavy rainfall or by cloudbursts. This history is catalogued in the Boulder County FIS report (FEMA 2012a), but the years of documented significant flooding include: 1864, 1876, 1894, 1919, 1941, 1949, 1951, 1957, and The most recent documented flood event resulted from heavy rainfall spanning September 9 th to September 18 th, The extended storm duration escalated flooding 4

9 Hydrologic Analysis TSDN for St. Vrain Creek, Colorado March 17, 2016 as soils became saturated, thereby, increasing runoff potential. The flooding resulted in considerable changes to channel geometry and alignment, damage to property infrastructure, and was the cause of 10 fatalities Previous Hydrologic Studies Two primary sources of information were utilized for this task, including the Boulder County FIS report (FEMA 2012a) and four post-flood hydrologic evaluation reports prepared for CDOT and CWCB. The effective discharges presented in the Boulder County FIS for the restudied portions of SVC, NSVC, and SSVC through the City of Longmont and Town of Lyons were initially based on gage analysis results from Flood Plain Information Reports published by the U.S. Army Corps of Engineers (USACE) in The Flood Plain Information reports used a Log-Pearson Type II analysis on peak annual discharges from the SVC gages near Lyons and Platteville. In the Town of Lyons, the results from the Flood Plain Information Reports were used in 1977 to produce an updated statistical analysis for the SVC stream gage at Lyons. Based on the updated analysis, synthetic unit hydrographs were developed and used to route flow through NSVC, SSVC, and SVC (Jacobs 2014a) and develop the current regulatory discharges. For the City of Longmont, the 1972 USACE Flood Plain Reports were updated by USACE in 1981 using the Environmental Protection Agencies (EPA) Storm Water Management Model (SWMM). The USACE used 6-hour storm durations to produce discharges downstream of Lyons to the South Platte River confluence (Jacobs 2015). The effective discharges for the unincorporated areas of Boulder County within the restudied area were initially derived from hydrologic analyses performed by the SCS in August 1974, but were updated by the 1981 USACE SWMM model described above. The four post-flood hydrologic evaluations leveraged to compile and develop the restudied discharges for this evaluation included studies on the upper and lower portions of the St Vrain Watershed (Jacobs 2014a and 2015), the Left Hand Creek Watershed (Jacobs 2014b), and the lower portion of the Boulder Creek Watershed (CH2M Hill 2015). Each post-flood evaluation utilized a Hydrologic Engineering Center Hydrologic Modeling System (HEC-HMS) model calibrated to September 2013 flood to determine flow rates for the 10, 4, 2, 1, and 0.2% annual chance flood events. The four studies leveraged were part of the CDOT and CWCB response to the September 2013 floods. CDOT partnered with CWCB to aid longer-term rebuilding efforts through evaluating and updating hydrology in key affected watersheds including the Big and Little Thompson Rivers, SVC, Left Hand Creek, Boulder Creek, and Coal Creek. These studies were performed in two phases and documented in 10 reports, with Phase 1 reports published in August 2014 addressing the mountainous regions within the upper portion of the watersheds and Phase 2 reports published in June/July 2015 focusing on the plains in the lower portions of the watersheds. These studies are understood to be the best available data and were approved as such by FEMA as documented in the memorandums 5

10 Hydrologic Analysis TSDN for St. Vrain Creek, Colorado March 17, 2016 from FEMA on November 6, 2014 (FEMA 2014) and July 21, 2015 (FEMA 2015a), included as Appendix B. 2.0 METHODOLOGY This section provides an overview of the hydrologic analyses performed, describes the approach used, and summarizes calculations and modifications made to the post-flood models. 2.1 SUMMARY OF APPROACH When conducting a flood study, the hydrologic analysis is used to evaluate the volume of water that falls in a project area or watershed; calculates corresponding runoff rates, stream flows, and timing; and determines the associated flood discharge-frequency relations. For this study, the 10, 4, 2, 1, and 0.2% annual chance peak discharges were adopted from existing post-flood CDOT/CWCB hydrologic evaluations, which used HEC-HMS models to simulate rainfall-runoff in the upper and lower SVC Watersheds. The post-flood models were additionally used to determine the 1%+ discharges per FEMA requirements. The 1% annual chance event (referred to as the base flood ) is defined as the storm that has a 1% chance of being equaled or exceeded in any given year and is the basis of the flood extents shown on the FIRMs that determine the need for Flood Insurance purchase in the National Flood Insurance Program (NFIP). The flow rates determined in the hydrologic analysis are critical to the hydraulic analysis, which ultimately determine flood elevations and flood extents that are depicted on the FIRM panels. During this task, AECOM reviewed available hydrologic studies and sources within the study area. Within the greater St. Vrain watershed, there were multiple studies with overlapping extents; however, within the study area, only the CDOT/CWCB post-flood evaluations and the FIS reports were obtained. The post-flood studies published in 2014 and 2015 were utilized as the best available data having been developed from recent precipitation datasets (National Oceanic and Atmospheric Administration [NOAA] Atlas 14), calibrated against the September 2013 floods, and approved by FEMA (Appendix B). The previous work was reviewed with respect to basin delineations and whether supplemental junctions would be beneficial. Conversations were held with CDOT, CWCB, and their consultants concerning the post-flood studies to verify the proper application of their results and use of their models. The results for each reported junction were utilized as-is from the appendices within the post-flood hydrology reports (Jacobs 2014a and 2015). The post-flood models were then modified to extract additional information for supplemental locations and to calculate the 1%+ annual chance event. 2.2 ADDITIONAL ANALYSES As described in Section 2.1, AECOM adopted the reported post-flood hydrology values within the study area from the St. Vrain Phase 1 and Phase 2 reports (Jacobs 2014a and 2015, respectively). In order to enhance the hydrologic dataset within the study area, 6

11 Hydrologic Analysis TSDN for St. Vrain Creek, Colorado March 17, 2016 AECOM conducted the following additional analyses beyond adopting the post-flood results: Adding 3 and 2 supplemental junctions to the Phase 1 and Phase 2 St. Vrain postflood hydrologic models, respectively, to provide flow change locations; and Calculating the 1%+ annual peak discharges, as required by FEMA. These items are discussed in further detail in the subsections below Model Junction Additions Junction elements are used in HEC-HMS to combine the inflow from one or more elements such as basins, tributaries, or upstream reaches into one outflow value. Five supplemental junction elements were added within the study area of the post-flood hydrologic models to obtain flows from reaches prior to confluences of multiple drainages. In instances where the flows from multiple elements (basins or reaches) had been combined into one HEC-HMS junction element in the original model, supplemental model junctions were added without routing to isolate various elements. This enabled calculations of flows prior to confluences to avoid producing an overly conservative estimate of the peak discharge when applied upstream throughout a stream reach. Because routing was not applied between the supplemental junctions and the consecutive downstream model junction, the junction additions did not alter the peak discharges of any model elements. For the five supplemental junctions added, depth-area reduction factors (DARF) were used to determine the discharges for the six events studied. DARFs are used in order to offset the effects of storm size on large watersheds. The appropriate DARF for each junction was determined in accordance with the DARF selection criteria outlined in the Phase 1 and Phase 2 St. Vrain post-flood hydrology reports (Jacobs 2014a, Jacobs 2015). The final DARFs selected for each supplemental junction are documented in the Excel file titled NOAA_Atlas14_Percip_JuncitonAdditions.xlsx found in the Simulations folder of Appendix A %+ Calculation The FEMA Program Standard 84 requires calculating the 1%+ annual peak discharge in riverine engineering analyses to reflect the uncertainty in the 1% annual chance peak discharge value by considering the 84% upper confidence limit. The 1%+ value was initially determined in accordance with FEMA s Guidelines and Standards for Flood Risk Analysis and Mapping, Appendix N: Flood Risk Data Development (FEMA 2012b) using a statistical procedure prepared by the Risk Analysis, Mapping, and Planning Partners (RAMPP) that was approved for use on this project by FEMA Region VIII (Appendix C). However, upon review of the statistical procedure, it was determined that several key assumptions to the statistical method were violated in the St. Vrain RiskMAP study area due to the presence of diversions, storage capacity in the floodplain, and diverse basin characteristic within the watershed. As a result, the 1%+ discharge values were calculated using an alternative method where the rainfall-runoff derived values were 7

12 Hydrologic Analysis TSDN for St. Vrain Creek, Colorado March 17, 2016 adjusted by gage analysis error. The gage analysis method, along with the procedure used to select the gage analysis method as an appropriate alternative, is outlined in a technical memorandum included in Appendix C. 3.0 RESULTS This section presents the peak discharge values calculated for the various storm events at 21 node locations along the study streams. The node locations are displayed on Figure 2 and their spatial information is included in the TSDN spatial files. 3.1 SUMMARY OF DISCHARGES The 10, 4, 2, 1, and 0.2% annual chance peak discharges were determined for each node location, along with the 1%+ annual chance peak discharges, and are presented in Table 1 below. Of the 21 nodes, 6 nodes had drainage areas less than 125 square miles, being located along SSVC and NSVC, with the remaining nodes along SVC having drainage areas between 215 and 371 square miles. The 10, 4, 2, 1, and 0.2% annual chance discharges presented in Table 1 were taken directly from the post-flood hydrology reports (Jacobs 2014a and 2015) for the 15 original nodes, and calculated using the models for the 5 supplemental junctions. 8

13 Hydrologic Analysis TSDN for St. Vrain Creek, Colorado March 17, 2016 Figure 2 - Discharge Values and Percent Change between Applied and Effective Flows 9

14 Hydrologic Analysis TSDN for St. Vrain Creek, Colorado March 17, 2016 Table 1 Summary of Discharges for Enhanced Study Streams in St. Vrain Creek Watershed Node ID J14;SVP2 J24;SVP2 J35;SVP2 J21;SVP2 J215;SVP2 J22;SVP2 J29;SVP2 J29.1;SVP2 J49;SVP2 J49.1;SVP2 J77;SVP2 J78;SVP2 J255;SVP1 J255.1;SVP1 J258;SVP1 Location St. Vrain Creek upstream of confluence of Dry Creek No. 2 St. Vrain Creek downstream of confluence of Lefthand Creek St. Vrain Creek downstream of Martin Street above Lefthand Creek confluence St. Vrain Creek below U.S. Highway 287 St. Vrain Creek at BNSF Railroad (Downstream of Dry Creek confluence) St. Vrain Creek above confluence of Dry Creek #1 St. Vrain Creek at Golden Ponds (Downstream of Lykins Ditch confluence) St. Vrain Creek at Golden Ponds (upstream of Lykins Ditch confluence) St. Vrain Creek 1900 feet upstream of 75th Street St. Vrain Creek 1900 feet upstream of 75th Street (excluding side drainage) St. Vrain Creek downstream of Union Road St. Vrain Creek at U.S. Highway 36 St. Vrain Creek at confluence with Stone Canyon St. Vrain Creek upstream of confluence of Stone Canyon St. Vrain Creek just downstream of the confluence of North St. Vrain Creek and South St. Vrain Creek Drainage Area (mi 2 ) Percent Annual Chance Discharge (cfs) 10% 4% 2% 1% 1%+ 0.2% Used in Hydraulic Analysis 1? 371 4,869 7,380 11,953 17,410 24,722 40,205 Yes 368 4,740 7,367 11,935 17,399 24,707 40,122 No 296 5,147 6,719 10,377 16,332 23,191 36,163 Yes 276 3,588 5,988 9,722 15,244 21,646 33,676 Yes 275 3,523 6,016 9,726 15,248 21,652 33,647 Yes 262 2,701 5,746 9,292 14,480 20,562 31,942 Yes 259 2,688 5,760 9,267 14,422 20,479 31,739 Yes 247 2,465 5,502 8,888 13,729 19,495 30,188 Yes 237 2,364 5,289 8,581 13,182 18,718 28,989 Yes 231 2,307 5,138 8,370 12,801 18,177 28,140 Yes 222 2,212 4,912 8,073 12,268 17,421 26,984 Yes 218 2,202 4,860 7,949 12,089 17,166 26,599 Yes 218 2,202 4,857 7,943 12,080 17,154 26,581 Yes 216 2,183 4,784 7,831 11,930 16,941 26,261 Yes 215 2,178 4,786 7,828 11,910 13,935 26,222 No 10

15 Hydrologic Analysis TSDN for St. Vrain Creek, Colorado March 17, 2016 Node ID J260;SVP1 J286;SVP1 J286.1;SVP1 J286A;SVP1 J234;SVP1 J234.1;SVP1 Location North St. Vrain Creek at confluence with St. Vrain Creek and South St. Vrain Creek North St. Vrain Creek 1790 feet upstream of Apple Valley Road (downstream of drainage) North St. Vrain Creek 1790 feet upstream of Apple Valley Road (upstream of drainage) North St. Vrain Creek 2190 feet downstream of Longmont Dam Road South St. Vrain Creek at confluence with St. Vrain Creek and North St. Vrain Creek South St. Vrain Creek near intersection of Red Hill Gulch Road and Old St. Vrain Road Drainage Area (mi 2 ) Percent Annual Chance Discharge (cfs) 10% 4% 2% 1% 1%+ 0.2% Used in Hydraulic Analysis 1? 124 1,123 2,502 4,160 6,386 7,472 14,329 Yes 118 1,081 2,377 3,938 6,052 7,081 13,599 No 113 1,064 2,317 3,840 5,901 6,904 13,237 Yes 112 1,056 2,299 3,804 5,842 6,835 13,100 Yes 91 1,605 3,168 4,933 7,234 8,464 14,748 Yes 89 1,554 3,085 4,789 7,036 8,232 14,328 Yes 1. Junction locations noted as No for hydraulic analysis were excluded as supplemental junctions were placed immediately upstream that did not reflect input from side drainages. These values are still reported here for informational purposes, but are not proposed for use in hydraulics modeling. Discharges shown in italics were calculated at supplemental junction locations as noted in Section All other discharges were published in the St. Vrain Phase 1 (Jacobs 2014a) and Phase 2 (Jacobs 2015) post flood hydrology reports with the exception of the 1%-plus-annual-change discharges. 3.2 DISCHARGE COMPARISON The estimated 10, 2, 1, and 0.2% annual chance peak discharge values at each location were compared to their corresponding effective discharges from the nearest downstream location presented in the FIS report. The studied locations were compared to the nearest downstream effective location, as opposed to the nearest effective location, to reflect the manner in which discharge values will likely be applied upstream for future hydraulic modeling efforts. The tabulated results of the discharge comparison are presented in Table 2. The estimated 1% annual chance are presented on Figure 2, along with a visual representation of the percent change from the effective discharge for the corresponding stream reaches associated with each modeled junction. The 1% annual-chance peak discharge values presented have been approved by FEMA (Appendix B) and are compared herein to clarify the values that will be used, as well as to provide communities with information regarding potential changes from the FIS report that has not yet been updated with the FEMA-approved flows. The proposed text for the FIS update is included in Appendix A. 11

16 Hydrologic Analysis TSDN for St. Vrain Creek, Colorado March 17, 2016 In general, the restudied discharge values were notably higher than the effective discharge values to which they were compared against. Restudied 1% and 0.2% annual chance discharges were consistently higher than effective values, having maximum increases of 48% and 59%, respectively, and averaging increases of 34% and 36%, respectively. The restudied 2% annual chance discharges were higher than effective values along each reach except one, resulting in an average increase of 24%. The restudied 10% annual chance discharges were higher than effective values along NSVC and SSVC, as well as at their confluence, lower than effective values from Lyons to the downstream study limit along the SVC, with an average decrease of 9%. As described in the Lower St. Vrain Watershed Hydrologic Evaluation (Jacobs 2015), the increase in peak discharges (specifically the Q 1% ) is attributed to previously overestimating Button Rock Dam s storage ability when the effective discharges were calculated. This over-estimation led to a misrepresentation of the contribution of NSVC during flooding events, such as the September 2013 event (Jacobs 2015). This then explains why the maximum increase between restudied and effective Q 1% values occurred along NSVC just before its confluence with SSVC. Table 2 Comparison between Effective and Restudy Discharges Location St. Vrain Creek upstream of confluence of Dry Creek No. 2 St. Vrain Creek downstream of confluence of Lefthand Creek St. Vrain Creek downstream of Martin Street above Lefthand Creek confluence St. Vrain Creek below U.S. Highway 287 St. Vrain Creek at BNSF Railroad (Downstream of Dry Creek confluence) St. Vrain Creek above confluence of Dry Creek #1 St. Vrain Creek at Golden Ponds (Downstream of Lykins Ditch confluence) St. Vrain Creek at Golden Ponds (upstream of Lykins Ditch confluence) Effective Drainage Area Q 10% Q 2% Q 1% Q 0.2% (sq. mi.) 1 Drainage Area (sq. mi.) Restudy Q 10% Q 2% Q 1% Q 0.2% 351 5,520 10,950 14,850 28, ,869 11,953 17,410 40, ,520 10,950 14,850 28, ,740 11,935 17,399 40, ,520 10,950 14,850 28, ,147 10,377 16,332 36, ,110 8,240 10,580 21, ,588 9,722 15,244 33, ,110 8,240 10,580 21, ,523 9,726 15,248 33, ,110 8,240 10,580 21, ,701 9,292 14,480 31, ,690 7,610 10,160 20, ,688 9,267 14,422 31, ,690 7,610 10,160 20, ,465 8,888 13,729 30,188 12

17 Hydrologic Analysis TSDN for St. Vrain Creek, Colorado March 17, 2016 Location St. Vrain Creek 1900 feet upstream of 75th Street St. Vrain Creek 1900 feet upstream of 75th Street (excluding side drainage) St. Vrain Creek downstream of Union Road St. Vrain Creek at U.S. Highway 36 St. Vrain Creek at confluence with Stone Canyon St. Vrain Creek upstream of confluence of Stone Canyon St. Vrain Creek just downstream of the confluence of North St. Vrain Creek and South St. Vrain Creek North St. Vrain Creek at confluence with St. Vrain Creek and South St. Vrain Creek North St. Vrain Creek 1790 feet upstream of Apple Valley Road (downstream of drainage) North St. Vrain Creek 1790 feet upstream of Apple Valley Road (upstream of drainage) North St. Vrain Creek 2190 feet downstream of Longmont Dam Road South St. Vrain Creek at confluence with St. Vrain Creek and North St. Vrain Creek South St. Vrain Creek near intersection of Red Hill Gulch Road and Old St. Vrain Road Effective Drainage Area Q 10% Q 2% Q 1% Q 0.2% (sq. mi.) 1 Drainage Area (sq. mi.) Restudy Q 10% Q 2% Q 1% Q 0.2% - 3,050 6,850 9,750 21, ,364 8,581 13,182 28,989-3,050 6,850 9,750 21, ,307 8,370 12,801 28,140-2,480 6,060 8,970 20, ,212 8,073 12,268 26,984-2,480 6,060 8,970 20, ,202 7,949 12,089 26,599-2,480 6,060 8,970 20, ,202 7,943 12,080 26,581-2,480 6,060 8,970 20, ,183 7,831 11,930 26, ,040 6,670 8,880 20, ,178 7,828 11,910 26, ,000 2,850 4,310 10, ,123 4,160 6,386 14, ,000 2,850 4,310 10, ,081 3,938 6,052 13, ,000 2,850 4,310 10, ,064 3,840 5,901 13, ,000 2,850 4,310 10, ,056 3,804 5,842 13, ,400 3,750 5,430 11, ,605 4,933 7,234 14, ,400 3,750 5,430 11, ,554 4,789 7,036 14,328 1.Effective values in italics were not identified in the FIS report (FEMA 2012a), but taken from Table 2 of the upper St. Vrain report (Jacobs 2014a) and Table 1 and Appendix D.5 of the lower St. Vrain report (Jacobs 2015) as stated effective flows. 13

18 Hydrologic Analysis TSDN for St. Vrain Creek, Colorado March 17, 2016 Discharges shown in italics were calculated at supplemental junction locations as noted in Section All other restudy discharges were published in the St. Vrain Phase 1 (Jacobs 2014a) and Phase 2 (Jacobs 2015) post flood hydrology reports. 4.0 EXCEPTIONS TO GUIDANCE AND STANDARDS/SPECIAL SITUATIONS The S_Submittal_Info spatial file includes a field EFF_DATE to designate when the discharges are to become effective. For the purposes of this report, an effective date of December 31, 2019 was selected; however, this date is approximate and subject to considerable change pending the review process and comment period. As mentioned in the footnote to Table 2, there were several flow change locations that were referenced as effective values in the upper and lower St. Vrain reports (Jacobs 2014a and 2015), but were not found in the 2012 Boulder County FIS report (FEMA 2012a). Therefore, for the purposes of this study and comparison, these values were treated as effective values. According to FEMA s Technical Reference, a FIS reference is required to update section 5.1. However, the FIS for the County contains hydrology information in section 3.1 Submitted data is named to match the effective FIS. 5.0 MIP SUBMITTAL FILE STRUCTURE All hydrologic data development TSDN files have been submitted digitally along with this TSDN as Appendix A. The contents found under the parent Hydrologic Unit Code (HUC) folder (\ ) have been structured according to the Data Capture Standards (DCS) Technical Reference (FEMA 2015b) as follows: \General St_Vrain_CO_Hydrology_TSDN.docx hydrology report with appendices St_Vrain_CO_Hydrology_TSDN.pdf hydrology report in pdf form with appendices St_Vrain_Creek_Draft FIS Section 3.1.doc proposed draft text for the next FIS report 08103C_Hydrology_metadata.xml metadata for this hydrology submittal Project Completion Certification \Correspondence s and signed concurrence project study memos \St_Vrain_Creek Simulations \HMS_Models o SV_Phase1 Upper St. Vrain Phase 1 hydrologic model o SV_Phase2 Lower St. Vrain Phase 2 hydrologic model \Q1PP 14

19 Hydrologic Analysis TSDN for St. Vrain Creek, Colorado March 17, 2016 o \Bulletin17bResults Output files from the HEC-SSP analyses o \layouts HEC-SSP input format files o \Output Files HEC-SSP outputs in text file format o GageAnalysis_1PP.xlsx Final 1%+ discharge ratios and input data records for each discharge gage o Other Files assortment of HEC-SSP output files NOAA_Atlas14_Precip_JunctionAdditions.xlsx precipitation data used in the HMS models as well as the HEC-HMS results for the supplemental model junctions Supplemental data \FIS Locations Spatial file of the locations of effective discharges used for comparison purposes SV_Phase2_InflowHydrographs.xlsx inflow hydrographs used in the St. Vrain Phase 2 HEC-HMS model DARF_Master.xlsx depth-area reduction factor selected for each model element Spatial_Files Contains the following files as required per FEMA FIRM Database Technical Reference (FEMA 2015c): L_Source_Cit, L_Summary_Discharges, S_Gage, S_Nodes, S_Hydro_Reach, S_Subbasins, and S_Submittal_Info. These are described in the Metadata file. 6.0 CONCLUSIONS The post-flood rainfall-runoff models developed revised 50, 10, 4, 2, 1, and 0.2% annual chance peak discharges. Model outputs were used to calculate the 1%+ discharges and 21 flow change locations along the study streams within the St. Vrain Watershed. The restudy discharges were consistently higher than previous effective discharges, with the exception of the 10% annual chance peak discharges, resulting from increased precipitation predictions and decreased storage capacity at Button Rock Dam. This TSDN documents the discharge values to be applied to the hydraulic modeling in the studied reaches of NSVC, SSVC, and SVC. The FIRMs within the St. Vrain Watershed of Boulder County will contain floodplain mapping for the 1 and 0.2% annual chance flood for the enhanced level streams based on the Water Surface Elevation (WSEL) determined from the hydrologic computations and analyses presented herein. All hydrologic calculations and modeling meet FEMA specifications and will be used for FIRM production. A complete set of spatial files, model input and output files, where applicable, were developed in accordance with FEMA s FIRM Database Technical Reference (FEMA 2015c) and DCS Technical Reference (FEMA 2015b). This set of data was provided along with a digital copy of this report, as described in Section

20 Hydrologic Analysis TSDN for St. Vrain Creek, Colorado March 17, REFERENCES CH2M Hill Boulder Creek Hydrologic Analysis, Phase 2: Boulder Creek above St. Vrain Creek. Prepared for CDOT. June 10. Can be accessed at: Federal Emergency Management Agency (FEMA). 2012a. Flood Insurance Study #08013C, Boulder County, Colorado. Effective December 18. FEMA. 2012b. Guidelines and Standards for Flood Risk Analysis and Mapping. Appendix N: Flood Risk Data Development. January FEMA Memorandum from Roger Jones, Public Assistance Branch Director DR 4145, Ryan Pietramali, Risk Analysis Branch Chief, and Portia Ross, EHP Advisor DR November 6. FEMA. 2015a. Memorandum from Tom Bush, Public Assistance Branch Chief, and Ryan Pietramali, Risk Analysis Branch Chief. July 21. FEMA. 2015b. Data Capture Technical Reference. November. FEMA. 2015c. Flood Insurance Rate Map (FIRM) Database Technical Reference, Preparing Flood Insurance Rate Map Databases. November. Jacobs. 2014a. Hydrologic Evaluation of the St. Vrain Watershed, Post September 2013 Flood Event. Prepared for CDOT Region 4 Flood Recovery Office. August 29. Can be accessed at: Jacobs. 2014b. Hydrologic Evaluation of the Lefthand Creek Watershed, Post September 2013 Flood Event. Prepared for CDOT Region 4 Flood Recovery Office. Issued August 29 and revised in December Can be accessed at: Jacobs Lower St. Vrain Watershed Hydrologic Evaluation, Post September 2013 Flood Event. Prepared for CDOT Region 4 Flood Recovery Office. July 1. Can be accessed at: 16

21 Hydrologic Analysis TSDN for St. Vrain Creek, Colorado March 17, 2016 APPENDIX A TSDN Documents

22 Community name: Boulder County Technical Study Data Notebook (TSDN) Inventory Checklist Community number (CID): 08013C MIP/FEMA Case Number(s): S Effective Date: 12/31/2019 Submitted by (Company Name) and Date: Colorado Water Conservation Board Submittal Date: TBD Company Contact Name, Phone Number, & Thuy Patton, ext. 3230, There are two ways of submitting data to FEMA: MIP data is uploaded to the MIP through workflow steps where data upload is required. If this data is revised throughout the study process, the revised data is also submitted to the MIP either to replace the original file or to supplement it. Remaining study data, for which there are no defined submittal points in the workflow, are submitted to the MIP via Tools & Links at the end of the project. FEMA Engineering Library data is uploaded to the MIP through workflow steps where data upload is required and a complete TSDN is submitted (to the library) at the end of the project, including data already submitted to the MIP. TSDN Inventory Checklist 1 March 2009

23 Technical Study Data Notebook (TSDN) Inventory Checklist CERTIFICATION I HEREBY CERTIFY THAT ALL FINAL VERSIONS OF DATA GENERATED FOR THIS STUDY ARE SUBMITTED TO FEMA AS INDICATED IN THE ATTACHED CHECKLIST. NAME/COMPANY OR AGENCY: SIGNATURE: TSDN Inventory Checklist 2 March 2009

24 DATA TYPE Is data Required? YES/NO Was data submitted? Submittal Date Special Problem Reports Index Special Problem Reports Contact Reports Index Contact Reports Meeting Minutes/Reports Index Meeting Minutes/Reports Correspondence with/from FEMA, such as SOMAs, Revalidation letters etc. Correspondence with/from Contractor Correspondence with/from Community(ies) Flood Elevation Determination Docket (FEDD) as described in 44CFR Other General Correspondence, such as BFE notices, Affidavits etc. Scoping Assessment Documents Project Scoping Initiation Documents Scoping Meeting Documents Post-Scoping Documents Hydrologic Analyses Hydrologic Analyses Index Y Y Summary Report of Hydrologic Analyses Y Y Computer Models, Calculations, and Execution Y Y Summary Report for Independent QA/QC N N Hydraulic Analyses Hydraulic Analyses Index Cross Section Information Cross-Section Plots Floodway Analyses TSDN Inventory Checklist 3 March 2009

25 Levee Analyses Key to Cross-Section Labeling DATA TYPE Computer Models, Calculations, and Execution Summary Report for Independent QA/QC Shallow Flooding Models, Calculations and Summary Report for Independent QA/QC Ice-Jam Flooding Models, Calculations and Summary Report for Independent QA/QC Under Stage 1: Geologic maps Field Reconnaissance reports Topographic maps Aerial photos used to identify the landform Under Stage 2: Historic Records of flooding Photographs Time-Sequence Aerial photography Geomorphic information Under Stage 3: Model input and output Floodplain mapping Georeferenced stream channel network used for floodplain mapping Georeferenced line dataset Technical report that includes description of active and inactive alluvial fans Is data Required? YES/NO Was data submitted? Submittal Date TSDN Inventory Checklist 4 March 2009

26 Narrative DATA TYPE Atlantic and Gulf Coast Studies (Do not include Draft Intermediate Submittals) Final Approved Intermediate Submission No. 1 Scoping and Data Review Final Approved Intermediate Submission No. 2 Storm Surge Model Calibration and Storm Selection Final Approved Intermediate Submission No. 3 Storm Surge Runs and Flood- Frequency Analysis Final Approved Intermediate Submission No. 4 Nearshore Hydraulics Final Approved Intermediate Submission No. 5 Draft Flood Hazard Mapping Pacific Coast Studies (Do not include Draft Intermediate Submittals) Final Approved Intermediate Submission No. 1 Scoping and Data Review Final Approved Intermediate Submission No. 2 Offshore Water Levels and Waves Final Approved Intermediate Submission No. 3 Nearshore Hydraulics Final Approved Intermediate Submission No. 4 Draft Flood Hazard Mapping All Studies (Do not resubmit data already included in Intermediate Submissions) Key To Transect Labeling Transect and Surge Data Wave Height Information Computer Models, Calculations, and Execution Summary Report for Independent QA/QC Storm Climatology and Meteorological event selection Stillwater Elevations Offshore Wave Characteristics Is data Required? YES/NO Was data submitted? Submittal Date TSDN Inventory Checklist 5 March 2009

27 Nearshore Hydraulics Spatial Files Narrative FIS Report Narrative (Complete) DATA TYPE FIS Report Narrative (Revisions Summary) Summary of Discharges Table Floodway Data Table Summary of Elevations Table (for Lacustrine, Riverine, and Coastal Flooding) Transect Locations Table Transect Data Table Surge Elevations Table Flood Profiles Floodplain Boundary Standards self-certification documentation Certification of Compliance for Work Other Relevant Data Mapping Information Index Topographic Mapping (Hardcopy Version) Topographic Mapping (Digital Version) Summary Report for Independent QA/QC Work Maps (Hardcopy Version) Work Maps (Digital Version) Work Map Delineation Summary Preliminary DFIRM (Hardcopy Version) CD-ROM With DFIRM Data Digital Orthophotos Is data Required? YES/NO Was data submitted? Submittal Date TSDN Inventory Checklist 6 March 2009

28 Soil and Vegetation Maps USGS Topographic Quadrangle Maps Flood Hazard Boundary Maps Redelineation files Digital Conversion Files Community Maps All Other Maps DFIRM Database Data (Standard) DFIRM Database Data (Enhanced) Digital Data Submission Checklist Photogrammetric Survey Documentation Road and Railroad Centerline data Political Boundaries GPS Survey Documentation DATA TYPE LiDAR data (two files: Bare earth and one for all returns Bathymetry Digital Elevation Model (DEMs) Hydro-corrected DEMs Triangulated Irregular Networks (TINs) Hydro-corrected TINs Narrative Field Survey Notes/Notebook Digital Photographs Digital Sketches Narrative of Survey Data Is data Required? YES/NO Was data submitted? Submittal Date TSDN Inventory Checklist 7 March 2009

29 DATA TYPE SCS/NRCS Flood Hazard Analyses Report(s) USGS Floodplain Information Report(s) USACE Feasibility Study Report(s) Watershed Studies Site Visit Photographs Community Population and Demographic Tax Base Reports Legal references Is data Required? YES/NO Was data submitted? Submittal Date TSDN Inventory Checklist 8 March 2009

30 Hydrologic Analysis TSDN for St. Vrain Creek, Colorado March 17, 2016 APPENDIX B Special Response Memorandums Guidance for use of Colorado Front Range Hydrologic Evaluations (Phase I and II) in complying with 44 CFR Part 9 and Executive Order (Floodplain Management) for areas seeing an update.

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36 Hydrologic Analysis TSDN for St. Vrain Creek, Colorado March 17, 2016 APPENDIX C 1%+ Calculation Memorandum: Alternative Methods for Calculating the 1-Percent-Plus Flood Discharge FEMA Approval Alternatives Methods for Calculating the 1-Percent-Plus Flood Discharge Memorandum: Calculating the 1-Percent-Plus Flood Discharge FEMA Approval Calculating the 1-Percent-Plus Flood Discharge

37 Technical Memorandum To: Stephanie DiBetitto, CWCB Hazard Mapping Specialist Thuy Patton, CWCB Floodplain Mapping Coordinator From: Rigel Rucker, Deputy Project Manager Remmet degroot, AECOM Project Manager Date: February 5, 2016 Project Title: Colorado Hazard Mapping Program (CHAMP) Phase I Project Number: Subject: Alternate methods for calculating the 1-percent-plus flood discharge Overview of Recommended Finding/Solution AECOM has completed a 1-percent-plus (1%+) flood discharge analysis for all basins within the Colorado Hazard Mapping Program (CHAMP) project area using several different methods and has developed a recommended approach. The 1%+ flood discharge values initially calculated using a Federal Emergency Management Agency (FEMA)-approved statistical approach were highly varied and not defendable when compared to discharges from other recurrence intervals. Upon review of the basin models and the statistical procedure, it was determined that several key assumptions to the statistical method were violated in the study area due to the presence of diversions, storage capacity in the floodplain, structures that regulated flow, influence of snow melt, calibrated rainfall models, and diverse basin characteristic within the watersheds. Upon evaluating four alternate approaches, AECOM recommends that the 1%+ discharge values be calculated using a hybrid procedure that adjusts rainfall-derived flows by gage analysis-derived error factors. The gage analysis method utilities historic flow records that reflect drainage area specific conditions, such as varied basin characteristics, snow melt, diversions, and flow regulations. Because the post-flood hydrology models were calibrated against gage data, AECOM believes that the gage analysis method yields a more representative predictive error that can be used to estimate the uncertainty in the 1-percent-annual-chance flood event (1%). The following memorandum describes the recommended approach, as well as the alternative methods that were considered. Background & Summary of 1-Percent-Plus Issues FEMA Program Standard ID (SID) 84 requires including the 1%+ discharge in riverine engineering analyses to reflect the uncertainty in the 1%- percent- annual -chance peak discharge value. Although there are no regulatory requirements associated with the 1%+ discharge value, its elevation must be shown on the Flood Profile in the FIS Report to communicate the uncertainty of the 1% flood elevation. In anticipation of CHAMP hydrology being used for future FEMA flood studies, AECOM is including the 1%+ flood discharge values for scoped streams. Per FEMA Standard 84: The 1% plus flood elevation is defined as a flood elevation derived by using discharges that include the average predictive error for the regression equation discharge calculation for the Flood Risk Project. This error is then added to the 1% annual chance discharge to calculate the new 1% plus discharge. The upper 84-percent confidence limit is calculated for Gage and rainfall-runoff models for the 1% annual chance event. The majority of the hydrologic analysis in the CHAMP study area is based on rainfall-runoff modeling. FEMA s Guidelines and Standards for Flood Risk Analysis and Mapping, Appendix N: Flood Risk Data Development includes a recommendation for calculating the 1%+ discharge for rainfall-runoff models (hereafter referred to as the curve-fitting approach through this document). It follows a lengthy statistical procedure based on Bulletin 17B by the Interagency Advisory Committee on Water Data entitled Guidelines for Determining Flood Flow Frequency, as well as several other documents. The methodology to execute the curve-fitting approach is summarized in Technical Memorandum Calculating the 1-Percent-Plus Flood Discharge, which was approved by FEMA Region VIII, and relies upon a particular set of conditions to be meet in order to properly fit the Log- Pearson Type III curve. Applying the curve-fitting approach to the post-flood rainfall-runoff results resulted in numerous implausible instances where the 1%+ discharge was either lower than the 1% discharge value or was several times greater than the 1% discharge value, neither of which reasonably represent the uncertainty of the 1% flood elevation Technical Memorandum 1

38 calculation. The following issues contributed to the variable results: 1. The 50-percent-annual-chance discharge (50%) is an influential input in the curve-fitting approach. The 50% discharge calculated from rainfall alone (i.e., the NOAA Atlas 14 precipitation data) in the postflood hydrologic models was zero in a portion of the individual basin elements. Whether a zero-flow 50% discharge value was calculated, varied, depending on basin characteristics and applied precipitation. A check was conducted and verified that a non-zero value was calculated for every 10% annual chance event, suggesting that the 50% annual chance precipitation values were too small to generate a flow, or were being attenuated. This underestimation may be attributed to estimating flow from rainfall alone without the influence from snowmelt, which is presumed to be a major contributor to the 50% discharge event. Snowmelt was not accounted for as it would skew the results of the curve-fitting procedure rooted in analyzing data distribution from rainfall alone. 2. Flow diversions, storage capacity in the floodplain, and structures that regulate flow exist within many of the studied drainage basins. These factors create statistically unpredictable differences in the discharge versus frequency interval dataset, such that the data does not fit a Log-Pearson Type III curve. This is a violation of the key assumption to the curve-fitting approach. 3. The 1%+ discharge values at downstream junctions were calculated from a mixed group of upstream basins having varied characteristics (e.g., curve numbers, lag times, peaking coefficients) and discharge versus frequency curves. The curve-fitting approach assumes a homogenous dataset among the distribution. Since these factors limited the applicability of the curve-fitting approach, four alternative procedures were developed and attempted. Summary of Alternative Methods Method 1 (*Recommended Approach*): Rainfall-Runoff Derived Values Adjusted by Gage Analysis Error Procedure: The flow frequency analysis procedures outlined in Bulletin 17B are used to determine the 84% upper confidence limit of the 1% discharge at each discharge gage within the study area. There are 20 U.S. Geological Survey (USGS) and Colorado Division of Water Resources (CDWR) gages distributed along the scoped streams for CHAMP. The gages can be used as inputs to HEC-SSP 2.0 to determine the 84% upper confidence limit (UCL) for each gage. Using the 84% UCL (effectively the 1%+ discharge at each study gauge), the ratio of the 1%+ to 1% discharge is determined at each gage as an estimate of localized relative error. This error ratio is then multiplied to the 1% discharge at rainfall junction elements located at or upstream of each gage along the scoped reach until the stream ends or the next gage location is reached. For streams where no discharge gages are present, engineering judgment is used to select the most appropriate ratio based on similar basin properties and geographic regions. Figure 1 presents the 1%+/1% ratios at each gage location analyzed and the stream reaches along which the ratio is applied. The 1%+ discharge values selected for hydraulic modeling are calculated by multiplying the relative error (1%+/1% discharge ratio) derived from the gage analysis to the 1% discharge value at each flow change location derived from rainfall-runoff modeling along the respective stretch for which the relative error was calculated (Figure 1). Applying the relative error ratios to upstream reaches is used in lieu of the gage transfer method outlined in Bulletin 17B because it is not applicable for the large variation in basin sizes found in this study. Pros: This method produces fairly consistent relative error values (1%+/1% discharge ratio) for nearly all 20 stream gages analyzed, with ratios ranging from 1.07 to This method yields 1%+ discharge values greater than the corresponding 1% discharge value for each model element in the Study Area. The relatively high density and even spread of the discharge gages throughout the CHAMP study area provides a better estimate of the 1%+ discharge than the curve-fitting approach. Each region has a gage that is representative of localized flow conditions. Gage analyses were used as the basis for calibrating the rainfall-runoff models used in the CHAMP hydrologic analysis; therefore, it is reasonable to estimate uncertainty from gage data Alternate 1-Percent-Plus Calculation Methods 2

39 Cons: This method does not require calculation of the 50% discharge, which has been shown to be unreliable throughout the study area when calculated with the existing rainfall-runoff models. Furthermore, the gage analysis does take snowmelt into account when calculating flows. This approach mixes methods by applying an error derived from gage data to values derived from rainfall-runoff models. Figure 1: Relative Errors from Gage Analysis along CHAMP Study Streams Alternate 1-Percent-Plus Calculation Methods 3

40 Method 2: Rainfall-Runoff Modeling with the Upper Confidence Interval from NOAA Atlas 14 Procedure: The 90% UCL of the NOAA Atlas hr precipitation data is applied to basin elements and simulated in the rainfall-runoff models. Pros: Cons: The UCL precipitation method avoids calculating 1%+ discharge values that are smaller than their corresponding 1% discharge values. The values are calculated in the rainfall-runoff model, so all reported discharge values use the same methodology. The 1%+ values calculated for a trial run of the Big Thompson Phase 1 HEC-HMS model were on average two times greater than the 1% discharge. In some instances, the 1%+ discharge values were comparable with the 0.2-percent-annual chance discharge (0.2%). The 90% UCL of the precipitation does not directly correlate to the 90% UCL of discharge, and the statistical error in a precipitation dataset is not a representation of error in a flow dataset. Method 3: Regional Precipitation Scalars based on the Curve-Fitting Approach Procedure: The curve-fitting approach is applied to all drainage basin elements within the rainfall-runoff models to determine a 1%+/1% discharge ratio. The drainage basins are then divided into several geographically distinct areas chosen using engineering judgement including the Upper and Lower Plains Regions, the Foothills Region, and the Upper and Lower Mountain Regions, as shown in Figure 2. Within each region, the 1%+/1% ratio is averaged to produce a region wide discharge scalar after removing outliers. Next, the 24-hr precipitation for the 1% chance event is iteratively adjusted such that the ratio of the basin discharge between the adjusted and unadjusted 1% discharge values matches the region-wide discharge scalar ratio determined from the curve fitting approach. This method requires an iterative process to determine the final precipitation values used in basin elements, upon which modeled discharges can be used for all flow junctions. Pros: Cons: This method partly uses the curve-fitting approach for calculating the 1%+ discharge approved by FEMA. Rainfall-runoff modeling is used to determine the final 1%+ discharge, avoiding the use of multiple methods. The selection of the regional divisions and appropriate precipitation scalars is subjective and open to interpretation. The same key assumptions to the Log-Pearson Type III curve are violated (diversions, regulations, storage, snow melt, etc.) and call into question the validity of using the statistical approach in the CHAMP study area. There are numerous basins that are considered outliers, with scalars (1%+/1% discharge ratios) greater than 2 or equal to zero (due to the 50% discharge equaling zero). Alternate 1-Percent-Plus Calculation Methods 4

41 Figure 2: Regional Division of Rainfall-Runoff Basins (from left to right the basin regions are: Upper Mountains, Lower Mountains, Foothills, Upper Plains, and Lower Plains) Method 4: Regression Analysis Applied at Flow Change Locations Procedure: The average predictive error is calculated using the USGS Regional Regression Equations for Estimation of Natural Streamflow Statistics at each junction or basin element used in the CHAMP hydrologic analysis. The average predictive error is then multiplied by the 1% discharge to determine the 1%+ discharge. Pros: Using the predictive error from a regression equation produces a Q1%+/Q1% ratio greater than 1, eliminating the potential for Q1%+ values being lower than their corresponding Q1% values. Cons: A 1%+ discharge is calculated at each junction or basin outlet used in the hydrologic analysis. This approach mixes methods by applying an error derived from regression analysis to values derived from rainfall-runoff models. Regression is not used in the rainfall runoff analysis, so the percent error is unrelated. AECOM respectfully requests feedback from CWCB and FEMA on the identified issue. The recommended approach will be employed unless an alternative approach is identified or additional coordination is deemed necessary within one week of this memo. Please contact Remmet degroot at (303) or Rigel Rucker at (303) if additional discussion or information is necessary. Thank you. Items below line to be completed by approving agency(ies) Additional Notes: Concurrence: Name Signature Name Signature Alternate 1-Percent-Plus Calculation Methods 5