EstesParkLoop-FinalHydraulicsReport 2022 0711.pdf

Submitted to:

US Department of Transportation Federal Highway Administration Central Federal Lands Highway Division 1200 West Dakota Avenue Lakewood, Colorado 80228

Final Hydraulics Report Downtown Estes Park Loop

CO FLAP US36(1)

AECOM Project Number 60332740 July 2022

FLOODPLAIN ANALYSIS CERTIFICATION

I hereby affirm that the Floodplain Analysis and mapping documented in this report were prepared under my direct supervision, in accordance with applicable, viable and pertinent criteria for the Town of Estes Park, the Colorado Water Conservation Board, and the Federal Emergency Management Agency and are correct to the best of my knowledge and belief.

_____________________________________________ 08/05/2022 Kimberley Pirri, PE, CFM, State of Colorado, No. 40453 Date

CHANNEL STABILITY & BRIDGE HYDRAULIC DESIGN CERTIFICATION

The attached channel stability and bridge report were prepared under my direction and supervision and are correct to the best of my knowledge and belief. Said drainage report has been prepared according to the criteria established by the Federal Highway Administration and was designed with the provisions thereof.

_________________________________________ 08/05/2022 Michael Scurlock, PE State of Colorado, No. 51759 Date

ROADWAY DRAINAGE CERTIFICATION

The attached drainage plan and report were prepared under my direction and supervision and are correct to the best of my knowledge and belief. Said drainage report has been prepared according to the criteria established by the Federal Highway Administration and was designed with the provisions thereof.

_____________________________________ 08/05/2022 Will Carrier, PE, State of Colorado, No. 37119 Date

AECOM Final Hydraulics Report Downtown Estes Park Loop

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Table of Contents 1 Introduction .......................................................................................................................................... 1-1

1.1 Project Background and Objectives 1-1

1.2 Field Reviews 1-2

1.3 Relevant Studies 1-2

1.4 Design Criteria 1-3

1.5 Project Data 1-3

1.6 Project Area Description 1-3

2 Hydrologic Analysis ............................................................................................................................. 2-1

2.1 Precipitation 2-1

2.2 Soils 2-1

2.3 Peak Flows 2-1

2.4 Time of Concentration 2-5

3 Floodplain Hydraulics .......................................................................................................................... 3-1

3.1 FEMA Regulatory Floodplains 3-1

3.2 Best Available Model: CHAMP 3-1

3.3 Existing Conditions Model 3-2

3.3.1 Model Domain and Boundary Conditions ............................................................ 3-2

3.3.2 Terrain Development (Channel Modifications) ..................................................... 3-3

3.3.3 Mesh Generation ................................................................................................ 3-4

3.3.4 Manning’s “n” Values .......................................................................................... 3-4

3.3.5 Structures & Boundary Conditions ...................................................................... 3-4

3.3.6 Model Control Parameters .................................................................................. 3-5

3.3.7 SMS File Organization ........................................................................................ 3-5

3.4 Proposed Conditions Model 3-5

3.5 Comparison of Models 3-8

4 Stream Morphology Considerations ................................................................................................... 4-1 5 Bridge Hydraulics ................................................................................................................................ 5-1

5.1 Hydraulic Modeling Data Extraction 5-1

5.2 Bridge Scour Analyses 5-8

5.2.1 Freeboard........................................................................................................... 5-8

5.2.2 Long-term Degradation ....................................................................................... 5-9

5.2.3 Live-bed/Clear Water .......................................................................................... 5-9

5.2.4 Contraction ......................................................................................................... 5-9

5.2.5 Pier .................................................................................................................. 5-10

5.2.6 Pressure Flow (Vertical).................................................................................... 5-11

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5.2.7 Abutment .......................................................................................................... 5-11

5.2.8 Scour Results and Summary ............................................................................ 5-12

5.3 Scour Countermeasures 5-13

6 Roadway Hydraulics ............................................................................................................................ 6-1

6.1 Pavement Drainage 6-1

6.1.1 Design Criteria.................................................................................................... 6-1

6.1.2 Design Discussion .............................................................................................. 6-1

6.2 Water Quality 6-4

7 Summary of Recommendations .......................................................................................................... 7-1

7.1 Variances 7-1

8 References ........................................................................................................................................... 8-1

Appendices Appendix A: Resources

Appendix B: Hydrology

Appendix C: Hydraulics

Appendix D: Construction Plans

Appendix E: Electronic Files: SMS electronic files only

Table of Figures Figure 1-1: Project Location .......................................................................................................................................... 1-2 Figure 3-1: Extent of Trimmed Model Domain Used for EPL Hydraulic Analysis Compared to CHAMP Model Domain ... 3-3 Figure 3-2: Overview of Proposed Design Features ....................................................................................................... 3-7 Figure 3-3: SMS Phase 1 Improvements Observation Lines (100 year) ....................................................................... 3-10 Figure 3-4: Comparison of Proposed vs. Existing Water Surface Elevations (left) and Velocity Magnitudes (right) for 100-year Event .................................................................................................................................................................. 3-11 Figure 3-5: Comparison of Proposed vs. Existing Water Surface Elevations (left) and Velocity Magnitudes (right) for 50-year Event. ................................................................................................................................................................. 3-11 Figure 3-6: Comparison of Proposed vs. Existing Water Surface Elevations (left) and Velocity Magnitudes (right) for 25-year Event. ................................................................................................................................................................. 3-12 Figure 3-7: Comparison of Proposed vs. Existing Water Surface Elevations (left) and Velocity Magnitudes (right) for 10-year Event .................................................................................................................................................................. 3-13 Figure 4-1. Perspective towards Ivy Street upstream of project reach ............................................................................ 4-3

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Figure 4-2. Boulder grade-control structures downstream of Rockwell Street ................................................................. 4-4 Figure 4-3. Boulder grade-control structures downstream of Riverside Avenue .............................................................. 4-5 Figure 5-1. Bridge scour data extraction locations ......................................................................................................... 5-2 Figure 5-2. Existing 2D hydraulic model velocity magnitude and flow distributions ......................................................... 5-3 Figure 5-3. Proposed 2D hydraulic model velocity magnitude and flow distributions ....................................................... 5-4 Figure 5-4. Water-surface elevation profiles – 50-year flood event ................................................................................. 5-5 Figure 5-5. Water-surface elevation profiles – 100-year flood event ............................................................................... 5-6 Figure 5-6. Water-surface Elevation Profiles – 200-year Flood Event ............................................................................. 5-7

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List of Tables Table 2-1: NOAA Point Rainfall Depths ......................................................................................................................... 2-1 Table 2-2: WWE Study Peak Discharges....................................................................................................................... 2-2 Table 2-3: Local Drainage Peak Discharges and Time of Concentration ........................................................................ 2-3 Table 2-4: Time of Concentration .................................................................................................................................. 2-5 Table 3-1: Outlet WSEL Boundary Conditions for Existing and Proposed Conditions Models ......................................... 3-2 Table 3-2: Manning’s n Summary .................................................................................................................................. 3-4 Table 3-3: Model Control Parameters ............................................................................................................................ 3-5 Table 3-4: 100-year Flow Conveyance (cfs) Summary for Figure 3-2 Phase 1 Observation Lines .................................. 3-9 Table 3-5: 100-year Average WSEL Summary for Figure 3-2 Phase 1 Observation Lines .............................................. 3-9 Table 5-1: Scour evaluations ......................................................................................................................................... 5-1 Table 5-2: Ivy Street Bridge Hydraulic Data Summary Table (1) .................................................................................... 5-6 Table 5-3: Ivy Street Bridge Hydraulic Data Summary Table (2) .................................................................................... 5-7 Table 5-4: Rockwell Street Bridge Hydraulic Data Summary Table (1) ........................................................................... 5-7 Table 5-5: Rockwell Street Bridge Hydraulic Data Summary Table (2) ........................................................................... 5-8 Table 5-6: Project Bridge Parameters ............................................................................................................................ 5-8 Table 5-7: Freeboard Determination .............................................................................................................................. 5-8 Table 5-8: Scour Results – Ivy Street Bridge ............................................................................................................... 5-12 Table 5-9: Scour Analysis Results – Rockwell Street Bridge ........................................................................................ 5-12 Table 6-1 Summary of Tailwater Conditions .................................................................................................................. 6-2 Table 6-2 Summary of Water Quality Sumps .…………………………………………………………………………………… 6-4

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This Final Hydraulics Report presents a summary of the key elements and the hydrologic and hydraulic (H&H) analysis to a 95% design level for the Estes Park Downtown Loop Project (Loop Project). The intent is to provide information to:

Determine floodplain impacts based on best available data.

Determine local roadway H&H

This report updates the design presented in the Draft Hydraulics Report completed to the 70% design level.

1.1 Project Background and Objectives

The scope of work for this project is to perform environmental, engineering, hydraulic, geotechnical, right-of-way, surveying, mapping, and project management services toward delivery of 100% Plans, Specifications & Estimates (PS&E) for the Federal Highway Administration (FHWA), Central Federal Lands Highway Division (CFLHD) for proposed improvements for the one-way couplet in the Town of Estes Park, Colorado.

In July 2013, CFLHD, Colorado Department of Transportation (CDOT), and the Town met to review the Town’s Federal Lands Access Program Application (FLAP), and discuss proposed elements and issues associated with the project. This produced a Scoping Report, which is the basis of the project agreement between the partner agencies and set the initial scope of work.

The project traverses the Town of Estes Park via Elkhorn Avenue (US34/36), Moraine Avenue (US 36) and West and East Riverside Drive. The proposed project would reconfigure the circulation system through Estes Park by realigning and reconstructing West and East Riverside Drives into a continuous one-way (eastbound) roadway and reconfiguring Elkhorn Avenue/Moraine Avenue to a one-way configuration (westbound) creating one-way couplets through Estes Park that would be US Highway 36. The reconfiguration would also include upgrades to Rockwell Street to accommodate additional traffic from the change of Elkhorn Avenue to one-way in the westbound direction.

The project also includes the reconstruction of the Ivy Street Bridge across the Big Thompson River. This new structure will be on a skew and will be raised to provide better clearance above the river. The application did not intend for the Rockwell Street or East Riverside Drive Bridges to be replaced.

The project area is located within the city limits of Estes Park, Colorado in Larimer County. The project location is Northwest Colorado, Latitude 40.376079 North and Longitude 105.523610 West. The project area provides access to Rocky Mountain National Park (RMNP) and multiple National Wilderness Areas. Figure 1-1 shows the routes that will be improved with project improvements.

The goal of these improvements is to reduce downtown congestion, improve connectivity to the RMNP, provide pedestrian safety on the Riverwalk Trail, evaluate the existing hydraulic conditions, and design a new bridge with greater capacity and more structural stability.

1 Introduction

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Figure 1-1: Project Location

1.2 Field Reviews

In July 2013, AECOM and CFL conducting a scoping field review. This review evaluated the existing conditions of the project to produce a Scoping Report. Photos from the field review are included in Appendix A. Field reviews were also conducted in May and December 2015.

1.3 Relevant Studies

The following presents a summary timeline of the events, studies, and future regulatory changes resulting from the 2013 floods.

1. September 2013 Floods (results in breaches of Big Thompson and Fall Rivers and flooding throughout downtown and surrounding areas)

2. Colorado Water Conservation Board (CWCB) Hydrologic Evaluation of the Big Thompson River Watershed: Post September 2013 Flood Event (August 2014)

3. Town of Estes Park adopts Ordinance 16-14, which allows the Town to regulate floodplains using CWCB Best Available Flows (October 2014)

4. Estes Downtown Loop Project (this project) preliminary hydraulic analysis and development of channel/floodplain improvement concepts (Spring 2016)

5. Town of Estes Park Hydrologic Analysis of Fall River, Upper Big Thompson River, Black Canyon Creek & Dry Gulch, Estes Park, Colorado by Wright Water Engineers, Incorporated (WWE) (January 2017) – Establishes peak discharges for the Big Thompson River through Estes Park

6. CWCB Draft Hydraulic Analysis of Downtown Estes Park as part of the Colorado Hazard Mapping Program (CHAMP) (Spring 2018)

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7. Town of Estes Park, Moraine Avenue Bridge Replacement Project

8. Federal Emergency Management Agency (FEMA) completion of preliminary Flood Insurance Study (FIS) and associated revised Flood Insurance Rate Maps (FIRMs) based on results of the CHAMP study (January 2021)

9. Issuance of effective FIS/FIRMs based on CHAMP study (anticipated early 2022)

1.4 Design Criteria

Hydrologic and hydraulic analysis and design work associated with the proposed improvements are in accordance with the methods, guidelines, and criteria set forth within the following:

Federal Lands Highway Project Development and Design Manual (PDDM)

Colorado Department of Transportation Drainage Design Manual

Larimer County Stormwater Design Standards

FEMA Standards for Flood Risk Analysis and Mapping

A summary of the applicable criteria can be found in the Hydrologic and Hydraulic Criteria and Computation Methods Technical Memorandum, in Appendix A.

1.5 Project Data

Detailed mapping information of the project corridor was surveyed by AECOM’s project team. The survey included both a ground survey near the Ivy, Rockwell and Riverside Bridge and aerial LiDAR flown in early 2015. The mapping products provided were referenced to the North American Datum of 1983 (NAD83) horizontal datum and the North American Vertical Datum of 1988 (NAVD88). Also, CDOT’s post flood LiDAR data (2013) was used. The area of the most relevance, as defined in Section 1.1, has been surveyed.

1.6 Project Area Description

The general scopes of improvements are programmed as pavement rehabilitation, reconstruction of the Ivy Street Bridge, and enhancement of the roadway system management in downtown Estes Park. Most of the proposed roadway improvements are parallel to the Big Thompson River (BTR), which flows south to north through the town before receiving confluence from the Fall River (FR) flowing from west to east. From the confluence, the BTR continues to the east before eventually draining into Lake Estes on the edge of the Town.

Prior to the confluence of Fall River, the BTR 100-year recurrence interval floodplain is roughly 400 feet wide with an average slope of 0.95 percent. The main channel is 40 feet wide with medium cobble to sandy bed material (d50 = 4mm to 10mm), while the overbanks consist of gravel sandbags or grass.

The roadway at the existing Ivy Street Bridge crosses the BTR at an 85-degree angle and constricts the flow through the existing 35-foot, single span bridge opening. The existing bridge deck elevation is a few feet higher and there is no freeboard with the WWE 100-year event within the bridge opening. This bridge is to be replaced per the Federal Lands Access Program (FLAP) application and the Scoping Report.

Approximately 250 feet downstream of the existing Ivy Street Bridge is the existing Rockwell Street Bridge, which is at a 95-degree angle and constricts the flow through the existing 50-foot, single span bridge opening.

The bridge deck is 3.3 feet deep. There is no freeboard with the WWE 100-year event within the bridge opening.

Another 200 feet downstream of the Rockwell Street Bridge is the FR and BTR confluence, followed by the Riverside Drive Bridge. This bridge is skewed at a 45-degree angle and constricts the flow through the existing 75-foot, two span bridge opening. The bridge deck is 3.6 feet deep. There is no freeboard with the WWE 100-year storm event within the bridge opening.

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In September 2013, Estes Park and the surrounding areas experienced one of the worst flood disasters in Colorado’s history. Relevant to this project, the FR and BTR breached their banks and flooded the Downtown area resulting in property damage and road closures.

2.1 Precipitation

An intensity-duration-frequency (IDF) curve using the National Oceanic and Atmospheric Administration (NOAA) Point Precipitation Frequency Estimates from NOAA’s online data server (September 2017) and Loveland’s Storm Drainage Criteria Manual produced the following one-point rainfall depths that are summarized in Table 2-1. The Recommendations from the NOAA online data server were used because they are more conservative estimates.

Table 2-1: NOAA Point Rainfall Depths

Recurrence Interval (Year) Rainfall Depth (inches)

2 0.63

5 0.79

10 0.99

25 1.35

50 1.71

100 2.13

2.2 Soils

Soil types for the project area have been determined using the Natural Resources Conservation Service (NRCS) web-based soil survey. The soils located in the project area consist of predominantly hydrologic soil Groups B and D, exhibiting a slow infiltration rate. For the rational analysis Type D was used.

2.3 Peak Flows

Peak discharges for fluvial analysis and drainage portions of this project were based on the following methods and studies:

Rational Method: To be used for contributing drainage areas less than 200 acres, mostly local drainage

Estes Park WWE Hydrology Study: The Town commissioned WWE to study FR, upper BTR, Black Canyon Creek and Dry Gulch. The Town has requested that these flows be used for analysis.

Fluvial Analysis

The restudied peak flows developed as part of the WWE hydrologic analysis resulted in peak flows that increased significantly from the effective discharges presented in the Larimer County FIS report. In fact, peak 100-year flows for FR through the Town increased by greater than 100 percent and peak flows for BTR above the confluence

2 Hydrologic Analysis

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2-2 with FR increased by 25-50 percent. Restudied flows are believed to provide a more accurate estimation of peak flood potential, ultimately producing a better estimate of potential flood risk through the Town. In addition, peak flows for FR, BTR, and Black Canyon Creek (BCC) determined by WWE were recently approved by FEMA as part of the Hydrologic Analysis Technical Support Data Notebook for Estes Park, Colorado completed as part of the Colorado Hazard Mapping Program (CHAMP) run by the CWCB. Table 2.2 provides a summary of the FEMA approved WWE flows.

Table 2-2: WWE Study Peak Discharges

Discharge Recurrence (yr.)

Fall River (cfs)

Big Thompson River (cfs) – Upstream of Fall River

Confluence Black Canyon

Creek (cfs) Big Thompson River – Estes

Park Visitors’ Center (cfs)

10 665 1020 165 1680

25 1020 1210 255 2020

50 1370 1400 280 2600

100 1860 2170 344 3100

500 3370 3700 910 4040

The WWE study only included determining the peak flows of the BTR above the confluence of FR, FR above its confluence with BTR, and BCC above its confluence with BTR. To study the peaks at the confluence was out of the scope of services, therefore, the WWE Study did not provide peak discharges at the FR and BTR confluence and at the BCC and BTR confluence. That said, the WWE Study did provide recommendations for peak discharges at the Visitors’ Center just downstream of the US 36 crossing. Additionally, WWE provided recommendations on the probabilistic timing of the BTR, FR, and BCC peaks relative to the various recurrence intervals that were studied. Using this information, the BTR, FR, and BCC hydrographs were calibrated to match each recommended discharge at the Visitors’ Center. Further discussion of how peak flows and hydrographs were used in the fluvial modeling is included in Section 3 below.

The WWE study was completed after the preliminary analysis documented in the 30% and 50% design Draft Hydraulics Report for this project. Hydraulic modeling for the analysis documented in this report; however, was based on revised fluvial analysis which used the updated WWE study flows. The revised fluvial analysis was used as the basis for evaluating water surface elevation changes produced by the proposed improvements.

Local Drainage

Peak discharges for local drainage were based on Rational Method calculations. Roadway hydraulics considered the following for design and analysis of local storm drainage:

1. Minor Event: Street capacity and storm drainage hydraulics for the Local 5-year storm event

a. Water quality treatment for the 2-year storm event.

b. 50-year low point analysis.

2. Major Event: Street capacity only

a. Local 100-year storm event with BTR 10-year peak

b. BTR 100-year peak for street capacity

Table 2-3 below summarizes local drainage peak discharges and time of concentrations calculated. Rational calculations and supporting data are included in Appendix B.

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Table 2-3: Local Drainage Peak Discharges and Time of Concentration

Basin Id Area (acres) 5 year (cfs)

100 year (cfs)

Off02.1 2.97 4.68 16.06

Off02.2 0.13 0.26 0.79

Off02.3 0.27 0.46 1.55

Off03.1 2.17 3.80 12.32

Off03.2 0.92 1.50 5.07

Off03.3 0.21 0.37 1.21

Off04 0.18 0.40 1.15

Off05 3.18 4.12 16.27

Off06 0.11 0.25 0.71

Off07 2.92 3.77 13.35

Off08 10.06 9.32 39.61

A01 0.95 1.84 5.65

A02-A 0.02 0.04 0.10

A02-B 0.02 0.06 0.16

A03 0.10 0.22 0.61

A04 0.14 0.32 0.90

A05 0.07 0.15 0.42

A06 0.47 1.07 3.03

A07 0.07 0.16 0.46

A08 0.22 0.41 1.29

A09 0.16 0.37 1.04

OFFB1 0.15 0.28 0.90

B01 0.30 0.69 0.54

B02 0.38 0.77 2.30

D01 0.54 1.15 3.37

D02 0.11 0.25 0.71

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Basin Id Area (acres) 5 year (cfs)

100 year (cfs)

D03 0.06 0.12 0.36

D04 0.45 0.96 2.81

E01 0.11 0.24 0.68

E02 0.25 0.53 1.52

E03 0.10 0.14 0.47

E04 1.38 2.02 6.45

F01 0.03 0.08 0.21

F02 0.02 0.05 0.13

O-F3 0.18 0.40 1.14

EX-IN1 0.05 0.12 0.35

OFF-G1 0.19 0.43 1.21

G01 0.45 1.02 2.90

G02 0.75 1.22 3.67

H01 0.74 1.62 4.60

H03 0.37 0.84 2.39

J01 0.37 0.84 2.39

K01 0.57 1.09 3.38

K02 0.22 0.51 1.44

L01 0.74 1.40 4.38

L02 0.09 0.21 0.58

M01 0.08 0.18 0.55

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2.4 Time of Concentration

The range for the time of concentration for the local drainage 5 year and 100 year is 5 minutes to 10 minutes. The BTR peaks for the 10 year and 100 year are summarized in Table 2-4 below.

Table 2-4: Time of Concentration

Basin Id Time (hours) Flow (cfs) Comments

5-year (Local) 0.08 Varies See Appendix B-2

100-year (Local) 0.08 Varies See Appendix B-2

10-year (BTR) Peak 17 1020 CHAMP Hydrograph

100-year (BTR) Peak 20 2170 CHAMP Hydrograph

10-year (BTR) 6 111 CHAMP Hydrograph

100-year (BTR) 5 88 CHAMP Hydrograph

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Preliminary hydraulic models were developed using the US Army Corp of Engineers (USACE) Hydrologic Engineering Center River Analysis Software (HEC-RAS) Version 4.1. Following the 30% Design Submittal, a two-dimensional (2D) model, the U.S. Bureau of Reclamation’s Sediment and River Hydraulics 2D (SRH-2D) Version 3.2, coupled with Aquaveo’ s Surface-water Modeling System (SMS) Version 12.2.11 for the 70% design report and Version 13.1.15 for the 95% design for pre- and post-processing, was adopted to advance the Loop Project hydraulic design to the 50%,70%, and now 95% design levels.

A Conditional Letter of Map (CLOMR) Application was submitted based on the updated hydrology and revised existing conditions fluvial modeling in April 2021. With this, the 95% Design details are described in Section 3.1 through Section 3.5 with summary of results in Section 3.6.

3.1 FEMA Regulatory Floodplains

The flood hazard analysis for the BTR was studied by detailed methods within the Town of Estes Park as reported on the effective FEMA FIRMs, resulting in a Zone AE designation. Zone AE floodplain designations include base flood elevations (BFE) which are determined as part of the fluvial analysis and reported on the FIRM panels, as opposed to a Zone A floodplain (developed through a base level study) which is based on approximate methods with no BFEs determined. The BTR detailed floodplain also includes a regulatory floodway. By definition (Code of Federal Regulations (CFR) Chapter 44, Section 59.1) the regulatory floodway is “the channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the based flood without cumulatively increasing the water surface elevation more than a designated height.” The CWCB has adopted a more stringent floodway standard than outlined in FEMA’s Standards which permits a BFE increase of 0.5’ above the non-floodway BFE. The floodway BFE increase is typically developed by a floodplain encroachment method permitted under FEMA guidance for riverine projects in the regulatory floodplain.

The proposed project extents overlap the regulatory floodplain for the Big Thompson River (Preliminary FEMA FIRM panel numbers 08069C1094G and 08069C1282G). The local community administers the effects of impacts from such improvements on the regulatory floodplain through the community Flood Insurance Study (FIS) developed by FEMA, while FEMA regulates the 100-year floodplain (Zone A or AE) for flood insurance purposes under the National Flood Insurance Program (NFIP). A Conditional Letter of Map Revision (CLOMR) is required for this project before proposed improvements can be implemented because improvements cross the floodway zone and cannot be constructed without any rise to the floodplain elevations. In compliance with the CFR Chapter 44, Section 65.12 impacts of the proposed project were designed to ensure no-rise to the regulatory flood elevations at existing insurable structures.

3.2 Best Available Model: CHAMP

The FEMA regulatory floodplains through the Town of Estes Park are in the process of being revised as part of CWCB’s CHAMP. After the Colorado 2013 Floods, CWCB led efforts to establish CHAMP in order to provide a comprehensive state hazard mapping program. This effort is helping communities to become more resilient through comprehensive mapping of floodplains and other natural disasters. This multi-year and multi-phase program is also providing a mitigation and land use framework in areas likely to be affected by future flooding, erosion, and debris flow events.

Commissioned by CWCB, the CHAMP floodplain hazard analysis project included completion of detailed hydraulic analyses for several streams within the Big Thompson Hydrologic Unit Code (HUC)-8 Watershed, including BTR, FR, and BCC through the Town of Estes Park. All flood hazard mapping products created as part of CHAMP are currently being used to updated regulatory flood risk products in Estes Park, with Preliminary FIS and FIRMs being issued in January of 2021. Given that the project area for the Loop Project was contained within the footprint of the CHAMP, the decision was made to combine efforts for the CHAMP and EPL projects and use

3 Floodplain Hydraulics

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3-2 the SRH-2D model developed for the downtown Estes Park area as part of CHAMP to serve the dual purpose of the future regulatory model as well as the existing conditions model for the Loop Project analyses. The decision to use the CHAMP model as the basis for the EPL hydraulic analysis was also made in coordination with FEMA Region 8 in advance of submitting the CLOMR application to ensure appropriate tie-in with the future regulatory mapping. Based on timing of the CHAMP regulatory update at the time of this 95% design report, it is anticipated that the updated flood risk information from CHAMP will be effective prior to the LOMR being completed for this project.

3.3 Existing Conditions Model

The hydraulic model developed for the 95% design package is the same used for the CLOMR submission and therefore includes three mesh geometries consistent with a typically CLOMR application: Duplicate Effective (DE), Corrected Effective (CE), and Proposed Conditions (PR). The DE mesh and associated files are the same as those used in the CHAMP regulatory model. The CE mesh serves as the existing conditions model, reflecting site conditions prior to any construction from the EPL project. Finally, the Proposed Conditions model reflects the proposed improvements. Development of the DE mesh is described in the CHAMP Big Thompson Year 2 Hydraulics Analysis TSDN. Development of the CE or existing conditions model based off of the CHAMP model and used for Loop Projects is detailed in subsections below.

3.3.1 Model Domain and Boundary Conditions

The Corrected Effective 2D model domain was reduced from the original CHAMP model to capture only the reach of the Big Thompson River impacted by the project limits (see Figure 3-1). This was done to simplify the model and removed the computational requirements to evaluate the other two streams included in the CHAMP model.

To accommodate the reduction in model extent the boundary conditions for the existing conditions model were revised from the CHAMP model. At the upstream boundary, the inflow arc was repositioned to match the extent of the model. The peak discharge was kept the same as the CHAMP model. Peak flow rates are based off results from the Hydrologic Analysis of Fall River, Upper Big Thompson River, Black Canyon Creek & Dry Gulch, Estes Park, CO study completed by WWE. These release rates were approved and accepted by FEMA and are documented in CHAMP Hydrologic Analysis TSDN for Estes Park, CO.

At the downstream end, the known WSEL boundary arcs were repositioned at the extent of the model domain.

The WSELs selected for the outlet boundaries were extracted from the results of the CHAMP model. The values set for each recurrence interval are shown in Table 3-1 below. For the 100-year and 500-year events, the outlet boundary was broken into arcs to capture the variation in the WSEL horizontally at the mesh boundary. Boundary conditions for existing conditions and proposed conditions model simulations were set such that the differences in water surface between those plans and the CHAMP information does not exceed 0.5 foot at any location consistent with tie-in requirements for the CLOMR application.

Table 3-1: Outlet WSEL Boundary Conditions for Existing and Proposed Conditions Models Annual Chance of

Exceedance Overbank WSEL

(ft. NAVD 88) Channel WSEL

(ft. NAVD 88)

10% ---* 7524.2

4% ---* 7525.4

2% ---* 7526.9

1% 7529.1 7528.0

1%+ 7529.6 7528.9

0.2% 7530.1 7529.6

1% FW 7529.0 7528.0

*Second outlet boundary was not required based on size of floodplain

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Figure 3-1: Extent of Trimmed Model Domain Used for EPL Hydraulic Analysis Compared to CHAMP Model Domain

3.3.2 Terrain Development (Channel Modifications)

Base Terrain

The existing conditions model utilized a combination of the Loop Projects LiDAR data, the CHAMP’s Digital Elevation Model (DEM) in areas outside of the Loop Project LiDAR extent, and both the Loop Project’s and CHAMP’s ground survey data. The Loop Project’s ground survey was completed and received in fall of 2015, with some additional channel survey points collected in the spring of 2018. The vertical datum for all survey elevations is NAVD88. This survey information was included in the existing conditions model. Data conversion was not necessary between models because no prior models were used for comparison and all models generated subsequently used elevation data from the NAVD88 datum.

The CHAMP DEM was approved by FEMA as part of the CHAMP Topographic Data Development TSDN completed by AECOM on behalf of CWCB in January 2017. The DEM was developed using existing LiDAR in the watershed which included:

The United States Geologic Survey (USGS) procured, funded and/or collected high-resolution (0.6-meter) LiDAR data across Central and Northern Colorado Counties during 2013 (USGS 2014). The datasets include classified point cloud, contours, breakline, and bare-earth DEM data, along with appropriate metadata (XML, project tile indexes, and area completion reports).

In October of 2014, the USACE Omaha District procured approximately 458 square miles of high-resolution (0.7-meter) LiDAR for the post-2013 flood-impacted Colorado counties (USACE 2015). The datasets include a classified point cloud, bare earth elevation models, contours, breakline, Triangulated Irregular Network (TIN), along with appropriate metadata.

Channel Modifications

The underlying terrain data used to create the SRH-2D hydraulic model was modified to account for additional channel conveyance not captured in the LiDAR datasets. The terrain modifications were made by using the Loop Project and CHAMP survey data to develop an interpolated surface representing the channel bottom and then mosaicking the channel with the unmodified DEM. The process of generating the channel grid included:

1. Five to seven consistent channel points were identified at each surveyed cross section. The minimum five points included:

a. Top of Bank Left and Right (TOBL, TOBR, respectively)

b. Toe of Slope Left and Right (TOSL, TOSR, respectively)

c. Channel Centerline (CHCL)

Additional points defining the channel (CH) were used where available in the survey data.

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2. Polylines were drawn between consistent points on each channel cross sections. This included one polyline to connect all TOBL points, one polyline to connect all TOSL points, etc. Placement of the polylines between the surveyed locations was based on a combination of the CHAMP contour datasets, the Loop Project survey data, and aerial imagery.

3. Both TOBL and TOBR polylines drawn in Step 2 were assigned a depth of zero (in other words given an elevation equal to the terrain data) to set the extents of the channel depth grid. The TOSL, TOSR, CHCL, and CH polylines were subdivided and assigned elevation based on the survey dataset at surveyed locations.

The polylines attributed with elevation were used to interpolate a surface which was then clipped to the extents of the channel. The grid was the mosaicked with the unmodified DEM to create the DEM used in the SRH-2D model.

3.3.3 Mesh Generation

The computational mesh was generated in SMS using a mesh generator coverage. Meshes included a combination of both triangular and quadrilateral elements, which quadrilateral elements predominantly for the definition of the stream channel and road decks, particularly where overtopping was expected to occur. The placement of mesh breaklines and the division of breakline arcs to nodes to create mesh size was adjusted as appropriate to ensure adequate detail of topographic features such as streams banks, as well as to define hydraulically significant areas including but not limited to the area surrounding bridges and culverts, inline structures such as drop structures and weirs, and locations where overtopping occurred, and the road deck was functioning as a weir.

3.3.4 Manning’s “n” Values

Manning’s roughness coefficient (n) values used for modeling purposes are consistent with the CHAMP model.

AECOM completed a field inspection and are in agreement with the same values and engineering judgment used in the CHAMP model. Table 3-2 summarizes the Manning’s n within the Town. The values reported in Table 3-2 were used in both the existing and proposed conditions models and are captured in the materials coverages within the hydraulic modeling files.

Table 3-2: Manning’s n Summary Number Description n

1 Road - Asphalt 0.016 2 Urban - Low Density 0.03 3 Urban - Medium Density 0.04 4 Urban - High Density 0.05 5 Forested - Low Density 0.08 6 Forested - Medium Density 0.1 7 Forested - High Density 0.12 8 Channel - Gravel/Cobble 0.05 9 Channel - Gravel/Cobble with Concrete Sides 0.035

10 Channel - Gravel 0.04 11 Pasture - Short Grass 0.03 12 Open Space 0.06 13 Open Water 0.02 14 Buildings 1 15 Channel - Gravel/Cobble with Boulders 0.06 16 Channel - Gravel/Cobble Low Flow 0.045 17 Channel - Gravel/Cobble Banks 0.09 18 Overbank w/ Building 0.13

3.3.5 Structures & Boundary Conditions

Internal Boundary Conditions

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Hydraulic structures were represented in the SRH-2D models with pressure flow boundary arcs to simulates pressure flow based off of bridge ceiling elevations at the upstream and downstream faces of the structure. The weir height, length, and material for each pressure flow boundary pair were set based on CHAMP and Loop Project survey data. The final inputs for all pressure, weir, and obstruction arcs used in the 2D models referenced in this TSDN are presented in Appendix C-2.

3.3.6 Model Control Parameters

The SRH-2D model control parameters for both the existing and proposed simulations were set as noted in Table 3-3 below.

Table 3-3: Model Control Parameters Parameter Value

Time Step (seconds) 1.0

Run Time (hours) 1

Initial Condition Automatic

Turbulence Model Parabolic

Parabolic Turbulence 0.7

Sensitivity analyses were completed for both the existing and proposed conditions simulations to validate the selection of a 1.0 second time step as minimum element size varied. Switch from 1.0 to a 0.5 second time step produced minor changes in water surface elevation (on the order of 0.05’) that did not impact areas of interest for the Loop Project. Additionally, monitor lines included in simulation boundary conditions showed that continuity was achieved with the 1.0 second time-step. A run time of 1 hour was deemed sufficient to achieve steady-state results based on output results in monitor line files. Finally, an initial condition of “Automatic” was selected to avoid starting the model from a dry condition and reduce model runtimes.

3.3.7 SMS File Organization

The SMS file names included in the 95% hydraulic model are document in Appendix C.

3.4 Proposed Conditions Model

The FLAP application, prior to the September 2013 Floods, proposed that the Ivy Street Bridge be reconstructed on a skew and will need to be lower to minimize impacts to local roads and to tie into Riverside Drive without impacting local properties. In addition, the channel between Ivy Street Bridge and Rockwell Street Bridge will be widened to provide more conveyance capacity and provide pedestrian connectivity to the BTR.

The proposed improvements based on the 95% design are shown on certified plans included in Appendix D with features of significant hydraulic impact summarized on Figure 3-2. Callouts on Figure 3-2 are described below.

CAD data for the proposed grading was incorporated into the LiDAR terrain files for use in the Proposed Conditions hydraulic modeling.

Channel Grading: As included in the original FLAP application, the 95% design includes widening of the Big Thompson River channel between Ivy and Rockwell Bridges to accommodate additional channel capacity and the proposed two-span structure replacing the existing Ivy Street bridge. The proposed channel has also been regraded between Ivy Street and Rockwell Streets in combination with the drop structure introduced upstream of Ivy (see number 4 below) which drops the invert through structure, also providing additional capacity. Inclusion of both the channel widening and drop structure was necessary to

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3-6 offset the impact that the skewed bridge layout had on existing flood flow paths along the east overbank of the river.

Flood Bench: Several structures on the east bank of the river were removed. The area has been regraded as a flood bench to assist with flood mitigation and create a dense riparian overbank. No formal decision has been made on structure replacements at Rockwell Street or Riverside Drive or additional channel corridor improvements to reduce flood risk, but the flood bench is in line with options that have been considered by the Town.

Two-Span Bridge Replacement: The existing Ivy Street bridge will be removed and replaced with a two-span bridge set at a skew to the existing structure. The channel invert through both spans of the proposed bridge is equal, allowing maximum conveyance through the structure. Additional details on the bridges are included in Section 5.

Drop Structure: A drop structure is proposed upstream of the proposed two-span bridge to increase capacity of the proposed bridge and mitigate flood impacts in the east and west overbanks. The channel grading associated with the drop structure continues to the upstream face of Rockwell bridge.

Bypass Culvert: A bypass culvert is proposed to reduce flooding to the south of the proposed roundabout. The bypass culvert was necessary to resolve increases in the regulatory water surface elevations through Piccadilly Square on the east side of Crags Drive. Increases in this area were caused by encroachment of the floodplain to accommodate the roundabout footprint. The bypass culvert was designed to divert 150 cfs during major flooding events (1% annual-chance and above). In doing so, it reduces the amount of water overtopping Crags Drive and making its way into the Piccadilly Square parking lot. Water diverted through the bypass culvert is outlet to the stream in Baldwin Park.

Roundabout: A roundabout will replace the existing intersection between Crags Drive and West Riverside.

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Figure 3-2: Overview of Proposed Design Features The proposed conditions model built on the existing conditions model with the following modifications:

Node Elevations: The DEM used to determine mesh node elevations was revised based on proposed grading and improvements. The proposed surface was developed in Bentley MicroStation, exported to a LandXML file and then converted to a DEM in ESRI ArcMap for use in the SMS model builder.

Boundary Conditions: The existing pressure arcs for the Ivy Street bridge were replaced with a pair of arcs to capture the east and west openings. Elevations for the pressure arcs were based on the average

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3-8 elevation of the bridge across the river as a best representation of the sloping deck included in the 95% design. A pair of arcs corresponding to the inlet and outlet of the bypass culvert were added as a “link” type boundary in the simulation. Hydraulics for the bypass culvert were completed outside of SRH-2D to ensure the culvert could pass the required 150 cfs needed during the 100-year flood event. The link boundary was set using a discharge of 150 cfs for the flooding events where the culvert was engaged.

Mesh Generator Updates: Only elements that were necessary to revise to capture the hydraulic influence of the proposed features were modified in the proposed mesh. All remaining elements were kept consistent with the existing conditions geometry to best capture differences between the two geometries.

Materials: Modified to consider channel realignment and other proposed design features

The monitor lines remained in the same place as the existing conditions model.

3.5 Comparison of Models

This section compares the existing conditions and proposed conditions models discussed in Sections 3.3 and

3.4. The 70% design report documented the comparison of four models including: CHAMP, Existing Conditions, Proposed: Phase 1, and Proposed: Full Build. In this report only the existing conditions and proposed conditions are compared. Differences between the CHAMP and Existing Conditions for the Loop Project simulations are discussed in the CLOMR application. The “Proposed: Full Build” option discussed in the 70% report is not being advanced as part of this stage of the project and therefore is not referenced in sections below.

100-Year

Results from the 100-year existing and proposed runs were compared on four observation lines across the Loop Project footprint. Observation lines can be seen on Figure 3-2 with 100-year average flow conveyance and water surface elevation (WSEL) summary in Table 3-4 and Table 3-5, respectively. Results in Table 3-4 shows that both existing and proposed conditions conserved mass well relative to the 2,170 cfs target for this reach of the Big Thompson River. Results in Table 3-5 indicate that the proposed condition generally leads to lower water surface elevations upstream of the Ivy Bridge replacement followed by slightly higher water surfaces downstream of the proposed bridge. This is also observed on Figure 3-4 showing a comparison of the water surface grids between the existing and proposed conditions. Under existing conditions, backup at the Ivy Street causes a significant overbank flowpath to the west of the structure opening, ultimately flow through the parking lot near the Estes Park Post Office. Under proposed conditions, with the capacity through the bridge increased that flow path is almost entirely disconnected, which is evident in the decrease in velocity magnitude shown on Figure 3-4.

Near the proposed roundabout feature the expected change in the floodplain extents due to encroachment from the required fill is observed in Figure 3-4. That encroachment results in more flow overtopping Crags Drive and flowing into the Piccadilly Square parking lot. This can be seen on Figure 3-4 where the water surface immediately to the west of the Crags Drive Road crown has increased and is also observed in the velocity differences through this area.

Overall, the Proposed improvements result in a general decrease in the water surface elevations throughout the 100-year floodplain which translated to a decreased in the floodplain extents in a number of areas. Figure 3-4 shows flooding around several structures immediately east of the proposed bridge has decreased and a similar impact is seen near the Post Office to the west of the proposed Ivy structure.

With the introduction of the drop structure upstream of the proposed Ivy Street bridge Figure 3-4 shows an increase in the velocity magnitudes just upstream of the bridge. There are also some increased in velocity just downstream of the Ivy Street bridge due to the introduction of the second span and widening of the channel.

50-Year

Figure 3-5 displaying differences between the existing and proposed conditions water surface elevations and velocity magnitudes shows some similar patterns as seen in the 100-year results around the proposed Ivy structure. Water surface elevations upstream of the bridge in the proposed conditions are reduced by over 1 foot due to the increased capacity through the structure. Upstream of Ivy there are very few differences between the

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3-9 existing and proposed conditions, largely because…