HomeMy WebLinkAboutIItem 4A - Floodplain Development Permit Call Up - FLD2019-00088 - 950 Regent DriveM E M O R A N D U M
TO: Planning Board
FROM: Christin Shepherd, Senior Civil Engineer
DATE: August 26, 2019
SUBJECT: Call Up Item: 950 Regent Dr
Floodplain Development Permit (FLD2019-00088)
Replacement of 19th Street Pedestrian Bridge over Boulder Creek
This decision may be called up before Planning Board on or before September 11, 2019
The project is a proposed pedestrian bridge along Boulder Creek. The original pedestrian
bridge was located just south of the intersection of 19th Street and Grandview Avenue and
was severely damaged and ultimately removed following the September 2013 flooding
event. The proposed replacement bridge is located approximately 800 feet downstream of
the original bridge. The project also includes trail improvements and ADA-compliant
bridge approaches. See Attachment A for a Vicinity Map.
The proposed work results in an increase to the 100-year floodplain water surface
elevation and will result in a post-project Letter of Map Revision (LOMR).
The increase in water surface elevation does not add any new structures to the floodplain
but has negligible increases on impacted structures. The water surface elevation change
impacts the applicant’s (CU) property, City of Boulder right-of-way and 23 structures on
six separate properties. All property owners have been notified. See table 7-1 in the
Floodplain Development Report in Attachment B.
The applicant has demonstrated compliance with the City’s floodplain and regulations
and a Floodplain Development Permit was approved by Planning and Development staff
on August 28, 2019. The decision may be called up before Planning Board on or before
September 11, 2019. There is one Planning Board meeting within the 14 day call up
period on September 5, 2018. A copy of the floodplain development permit and
application materials is attached for reference.
Questions regarding this project should be directed to Christin Shepherd in Planning and
Development Services at 303-441-3425 or ShepherdC2@bouldercolorado.gov.
Attachments:
A. Vicinity Map
B. Floodplain Development Permit & Report
Agenda Item 4A Page 1 of 52
ATTACHEMNT AAgenda Item 4A Page 2 of 52
UNIVERSITY OF COLORADO AT BOULDER
CP251277 SITE – DRCOG 19th STREET BRIDGE
FEDERAL AID PROJET NO. TAP M110-109-21737
BOULDER, COLORADO
FLOODPLAIN DEVELOPMENT PERMIT APPLICATION
Prepared for:
University of Colorado at Boulder
RM/FLR/STE CP187489
1540 30th St
Research Lab No. 2
Boulder, CO 80303
Prepared by:
ICON Engineering, Inc.
7000 South Yosemite Street, Suite 120
Centennial, CO 80112
303.221.0802
(Job No. 18-011-NSB-415)
July 26th, 2019
ATTACHMENT B
Agenda Item 4A Page 3 of 52
TABLE OF CONTENTS
1.0 Introduction ................................................................................................................... 1
1.1. Purpose ................................................................................................................... 1
1.2 Background and Previous Projects .......................................................................... 1
1.3 Site Location............................................................................................................ 1
1.4 Site Description ....................................................................................................... 3
1.5 Floodplains .............................................................................................................. 3
2.0 Analysis Criteria ........................................................................................................... 4
2.1 Regulations ............................................................................................................. 4
2.2 Drainage Studies, Master Plans, Site Constraints ................................................... 4
3.0 Topographic Mapping .................................................................................................. 5
3.1 Existing Topography ................................................................................................ 5
3.2 Proposed Topography ............................................................................................. 5
4.0 Hydrology ...................................................................................................................... 6
4.1 Peak Flows .............................................................................................................. 6
5.0 Hydraulics ..................................................................................................................... 7
5.1 Hydraulic Analysis ................................................................................................... 7
5.2 Input Parameters ..................................................................................................... 8
5.3 Results .................................................................................................................... 9
5.4 Tie-Ins ..................................................................................................................... 9
6.0. Floodplain Mapping .................................................................................................... 11
6.1 Floodplain Work Map ............................................................................................. 11
7.0. Conclusions ................................................................................................................ 11
7.1 Compliance with Standards ................................................................................... 11
7.2 Impacts to Adjacent Properties .............................................................................. 11
Tables
Table 4.1: Summary of Effective Discharges for Boulder Creek ...................................... 6
Table 5.1: Water Surface Elevation Comparisons ......................................................... 10
Table 7.1: Structure Impacts ......................................................................................... 12
Figures
Figure 1 Vicinity Map ............................................................................................... 2
Appendices
A – Floodplain Development Permit Application and Review Comment Response
Letters
B – Responses to Boulder Revised Code Section 9-3-6 (d) 15 Factors
C – Effective Floodplain Information
D – Hydraulic Computations
E – Topographic Work Maps and Profile
F – Proposed 19TH Street Bridge Plans
G –Property Owner Notification Letters
H - Digital Data
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1.0 INTRODUCTION
1.1 Purpose
The purpose of this report and supporting documentation is to request a Floodplain
Development Permit (FDP) from the City of Boulder for the 19TH Street Bridge project. On
behalf of the University of Colorado at Boulder (CU) we are requesting that the City of Boulder
review the proposed pedestrian bridge construction at 19th Street over Boulder Creek.
1.2 Background and Previous Projects
The effective FIRM and Flood Insurance Study (FIS) for the City of Boulder were issued
December 7, 2017. Excerpts from the effective flood information are included in Appendix C.
The effective information for this reach of Boulder Creek is based on a 2013 existing conditions
analysis prepared by Anderson Consulting Engineers, Inc. (Anderson).
Located approximately 800 feet downstream of the 19th Street Bridge location, the 23rd Street
Bridge project has recently been completed. This project removed the 21st Street and Stadium
Bridges and replaced them with a new 23rd Street Bridge structure. Substantial completion of
this project occurred in April 2019. The project was completed under Floodplain Development
Permit LUR2018-00020 that was issued on June 18, 2018.
The original 19th Street Bridge deck was severely damaged and ultimately removed following
the September 2013 flooding event on Boulder Creek. The 19th Street Bridge abutments remain
in place, as does the adjacent steam line and associated support structures.
This FDP application is based on construction of a new bridge and associated trail
improvements along Boulder Creek at and just downstream from 19th Street (extended). CU
plans to install a new 19th Street Bridge with a more elevated structure in order to improve
floodplain conveyance along Boulder Creek. This project is proposed to be funded jointly by CU
and a Transportation Improvement Grant (TIP) from Denver Regional Council of Governments
(DRCOG). The new 19th Street Bridge will also comply with the Americans with Disabilities Act
(ADA). Additionally, a break-away bridge structure will be installed at the original 19th Street
Bridge location in order to provide maintenance access to the south bank of Boulder Creek.
This maintenance bridge will also include a reconstructed crossing of the overhead steam line
adjacent to the bridge.
1.3 Site Location
The project area is located entirely within the City of Boulder, Colorado. The project area is
generally bound by the 23rd Street pedestrian bridge on the east, Grandview Avenue on the
north, the CU campus on the south, and 19th Street (extended) on the west. The following
vicinity map illustrates the project location. The project is located entirely on land owned by the
University of Colorado.
Agenda Item 4A Page 5 of 52
Grandview Ave.
Boulder Creek
Boulder High School
CU Athletic Fields
Naropa University
Recently completed 23rd Streetpedestrian bridge (LUR2018-00020)
Proposed 19th Streetpedestrian bridge
Proposed 19th Streetmaintenance bridge
CU Recreation Center17th StArapahoe Av
19th St20th StAthens St
Marine St18th St21st StHillside Rd
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1.4 Site Description
Pre-Project
The existing 19th Street Bridge deck was removed by the September 2013 flooding event along
Boulder Creek. This original crossing, if it were still in place, would be partially inundated by-
and cause a significant flow obstruction during the 1% Annual Chance (A.C) flood event.
Proposed
CU is utilizing a TIP grant to replace the failed 19th Street Bridge in order to improve access and
public safety for this crossing. The original 19th Street Bridge will be replaced by a new structure
with a different alignment that is located just downstream. The south abutment of the proposed
19th Street Bridge will tie into the CU campus well above the Boulder Creek floodplain. The
proposed bridge deck will be elevated above the 1% A.C. water surface elevation, though the
piers and trail improvements will be located within the 1% A.C. floodplain. The bridge approach
slopes will be ADA-compliant.
A second portion of the proposed project will be the installation of a maintenance access bridge
at the original 19th Street Bridge location. This structure will facilitate maintenance access to the
south side of Boulder Creek and will allow for modifications to the overhead steam line that
crosses Boulder Creek and is currently aligned directly overhead of the original 19th Street
Bridge location. The steam line will be moved slightly downstream to accommodate the
proposed 19th Street Maintenance Bridge.
1.5 Floodplains
Boulder Creek is defined as an area of Zone AE 1% A.C. floodplain with base flood elevations
(BFEs) and regulatory floodway. Areas of 0.2% A.C. flooding are symbolized by X (shaded)
zone designations. The effective floodplain delineations are illustrated in Figure E.1 in
Appendix E.
The Zone AE 1% A.C. floodplain, the regulatory floodway, and the high hazard zone (HHZ) will
be impacted by the proposed project. The delineations for these floodplain zones are illustrated
on Figures E.3 through E.6 in the Appendix E.
Study Limits
The upstream revision limit along Boulder Creek is cross section 28193 at the downstream face
of the existing 17th Street Bridge. The downstream study limit is cross section 26867 (FEMA
cross section AR).
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2.0 ANALYSIS CRITERIA
2.1 Regulations
A Floodplain Development Permit is requested from the City of Boulder, based on the proposed
project resulting in no adverse impact compared to the Pre-Project conditions BFEs,
Conveyance Zone (CZ), or High Hazard Zone (HHZ).
Additional regulatory considerations are addressed in Appendix B, Responses to Boulder
Revised Code Section 9-3-6 (d) “15 Factors.”
2.2 Drainage Studies, Master Plans, Site Constraints
The construction of the 19th Street Bridge and 19th Street Maintenance Bridge is consistent with
the site constraints. The pending North of Boulder Creek master plan also indicates the need
for an improved crossing and floodplain improvements to facilitate the planned redevelopment
of the North of Boulder Creek area. The proposed project will preserve the current multi-use
Boulder Creek path the runs through the project area.
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3.0 TOPOGRAPHIC MAPPING
3.1 Pre-Project Topography
Topographic mapping for the project area was taken from 2013 City of Boulder LiDAR aerial
mapping acquired on April 26th, 2013. Topographic mapping was prepared with one-foot contour
intervals on the NAVD 1988 vertical datum, using the horizontal datum and mapping projection
described below:
Projected Coordinate System: NAD_1983_StatePlane_Colorado_North_FIPS_0501_Feet
Projection: Lambert_Conformal_Conic
False_Easting: 3000000.00031608
False_Northing: 999999.99999600
Central_Meridian: -105.50000000
Standard_Parallel_1: 39.71666667
Standard_Parallel_2: 40.78333333
Latitude_Of_Origin: 39.33333333
Linear Unit: Foot_US
Geographic Coordinate System: GCS_North_American_1983
Datum: D_North_American_1983
Prime Meridian: Greenwich
Angular Unit: Degree
Additional field survey was completed by King Surveyors on November 30th, 2018.
3.2 Proposed Topography
Proposed 1-foot contour grading is presented on the topographic workmaps in Figure E.3
through E.7 in Appendix E. The proposed grading is on the same horizontal and vertical datum
and mapping projection as the pre-project topography. For reference, all data shown on the
topographic workmaps is included in electronic format in Appendix H.
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4.0 HYDROLOGY
4.1 Peak Flows
No changes to the effective peak discharges along Boulder Creek are proposed as part of this
request. Table 4-1 below summarizes the effective discharges through the proposed revision
area.
Table 4.1: Summary of Effective Discharges for Boulder Creek
Effective ICON FDP 10% A.C. 2% A.C. 1% A.C. 0.2% A.C.
Cross
Section
Cross
Section Discharge Discharge Discharge Discharge
(cfs) (cfs) (cfs) (cfs)
27520 27520 2,200 7,863 11,129 21,400
28193 28638 2,200 7,841 11,085 21,400
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5.0 HYDRAULICS
5.1 Hydraulic Analysis
Effective Model
The effective hydraulic model for this reach of Boulder Creek comes from the 2013 Anderson
study. This model was prepared using HEC-RAS version 4.0.0. It contained separate steady
flow plans for each of the flood frequencies and the regulatory floodway. It should be noted that
the effective model did not include the steam line support structures at the original 19th Street
Bridge location. This approach has been carried forward throughout this study as the impact of
these support structures is considered to be negligible.
Duplicate Effective
The effective hydraulic models were obtained from the City of Boulder, truncated to the project
area, and re-run in HEC-RAS version 4.1.0. The downstream boundary condition makes use of
known water surfaces from the effective hydraulic model. Two cross sections indicated a +0.01
foot difference compared to the effective model.
The truncated Duplicate Effective model was then run in the current version of HEC-RAS
(5.0.6). This run had several cross sections that indicated a +/- 0.01 foot and a single cross
section with a +0.02 foot change relative to the 4.1.0 version run.
Corrected Effective
The Duplicate Effective hydraulic model was the starting point for the Corrected Effective model
prepared for this FDP Application. The Corrected Effective model adjusted the downstream
starting water surface elevation to match the 23rd Street Bridge water surface elevation at cross
section 26867 per the LUR2018-00020 floodplain development permit. Additionally, the original
(pre-September 2013 flood) 19th Street Bridge deck was removed to reflect the current effective
conditions per the direction of the City of Boulder.
Pre-Project Conditions
An inspection of field conditions within the project area found no indication of man-made
changes to Boulder Creek since the time of the effective analysis, with the exception to the 23rd
Street Bridge construction (addressed in starting water surface elevation) and the removal of the
original 19th Street Bridge deck. The Pre-Project model was the then completed using the City
of Boulder’s 2013 LiDAR data for increased surface detail relative to the effective model.
Additionally, cross sections (27400, 27445, 27590) were added to the Pre-Project conditions
model in order to more accurately reflect the floodplain conditions in anticipation of the proposed
improvements.
Proposed Conditions (Main 19th Street Bridge ONLY)
The proposed conditions hydraulic model (main bridge ONLY) reflects the proposed grading,
bridge and trail improvements within the project construction area for the main 19th Street
Bridge only. It does not reflect the proposed 19th Street Maintenance Bridge.
Based on the height of the proposed 19th Street Bridge deck being above the 1%-annual-
chance water surface elevation (minimum freeboard of ~2 feet), the bridge deck has not been
included in the HEC-RAS model. The piers and associated north abutment have been included
in the model to reflect their impact to the floodplain. A debris blockage factor of approximately
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5% has been applied to the crossing to remain consistent with previous modeling efforts along
Boulder Creek. This debris blockage has been accomplished by adding ineffective flows at pier
locations within the floodplain and at the north abutment.
For comparison purposes, a HEC-RAS model was completed that included the proposed 19th
Street bridge deck. This approach resulted in rises to the proposed floodplain on the order of
2.0 feet over the pier and abutment only approach to the bridge crossing. Given that the deck
has freeboard above the water surface elevation, this increase in water surface elevation is
considered a modeling abnormality and is not considered reasonable when compared to the
modeling approach that utilizes ground form for the abutment and blocked obstructions for the
piers. As noted in the April 4th, 2018 meeting with the City of Boulder (for the previous 23rd
Street Bridge), the bridge deck model has been included for comparison purposes, but has not
been used as the proposed conditions hydraulic model. This same modeling issue and
approach were noted and taken for the previously approved 23rd Street Bridge floodplain
development permit (LUR2018-00020)
Outside the channel banks and areas disturbed by construction, the cross-sections reflect the
pre-project topography. The HEC-RAS proposed hydraulic cross-sections were cut and/or
manually coded from a composite of surfaces making use of the proposed grading in the project
area, and the pre-project topography surface outside of that area. Electronic HEC RAS files for
the proposed model are included in Appendix H.
Proposed Conditions (with 19th Street Maintenance Bridge)
The proposed conditions hydraulic model (with 19th Street Maintenance Bridge) reflects the
proposed grading, bridge and trail improvements within the project construction area for the
main 19th Street Bridge, in addition to the proposed 19th Street Maintenance Bridge
improvements. The deck of the 19th Street Maintenance Bridge has been designed as a break-
away structure. Accordingly, the bridge deck is not included in the hydraulic modeling, but the
proposed abutments, debris blockage, and associated grading changes adjacent to the channel
have been included.
Effective, Duplicate Effective, Corrected Effective, Pre-Project, and Proposed Conditions BFEs
are compared in Table 5-1. All elevations presented in the BFE Comparison Table are in
vertical datum NAVD 1988.
5.2 Input Parameters
The Boulder Creek downstream hydraulic model boundary condition was based on known water
surface elevations from the 23rd Street Bridge Floodplain Development Permit. As noted
previously, this project has been substantially completed as of April 2019. A Duplicate effective
hydraulic cross-section (Boulder Creek section 28638) was used at the upstream tie-in location.
At the downstream and upstream tie-in locations, the proposed floodway encroachment stations
match those of the effective floodway.
Estimates of channel and overbank roughness were made from field observations, aerial
imagery, and the project seeding plans.
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Blocked obstructions were used to represent bridge piers. Because the proposed 19th Street
Bridge deck is elevated above the proposed conditions BFE, the bridge deck is not modeled as
a crossing structure. Instead, the bridge piers are modeled using the aforementioned blocked
obstructions and the north abutment is modeled using ground elevations and areas of
ineffective flow as applicable.
Contraction and expansion coefficients utilized upstream and downstream of the bridge
locations are the recommended values of 0.3 and 0.5, respectively. Typical channel sections
use the recommended contraction coefficient values of 0.1 and expansion coefficient values of
0.3.
5.3 Results
Floodplain and floodway models and their output are included in Appendix D. Table 5-1
provides a comparison of the Effective, Duplicate Effective Corrected Effective, Pre-Project, and
Proposed Conditions BFEs. No insurable structures are adversely impacted due to the
proposed Boulder Creek revisions compared to the Pre-Project conditions. The model results
demonstrate that there is no adverse impact to the BFEs compared to the Pre-Project
information. There are increases in BFE compared to the effective BFEs, but the results show
that these increases are entirely due to the updated Corrected Effective and Pre-Project
analysis. There are also minor increases in the conveyance zone and High Hazard Zone
compared to the effective conveyance and High Hazard Zones. These increases in the
conveyance and High Hazard Zones are attributed to both the proposed project, additional
cross sections, and more detailed analysis in the case of the HHZ. The effective and proposed
condition floodplains are illustrated in Figures E.1 through E.7 in Appendix E.
5.4 Tie-Ins
The downstream Boulder Creek tie-in occurs at cross section 26867. A duplicate effective
hydraulic cross-section, known starting water surface elevations (from the 23rd Street Bridge
floodplain development permit), and the effective floodway encroachment stations have been
used at this location to ensure exact tie-ins. The upstream tie-in occurs at the Duplicate
Effective cross-section 28193. The BFE, floodway encroachment stations, and 1% A.C.
floodplain width all match within typical tolerances at the upstream tie-in location.
Agenda Item 4A Page 13 of 52
Table 5.1: Water Surface Elevation Comparisons - Boulder Creek 19th Street Bridge Replacement
ICON Engineering Inc.
Location Description
Effective
Cross
Section
Project Cross
Section
Discharge
(cfs)
WSEL
(NAVD 88)
Discharge
(cfs)
WSEL
(NAVD 88)
Δ WSEL
vs. Effective
Discharge
(cfs)
WSEL
(NAVD 88)
Δ WSEL
vs. Effective
WSEL
(NAVD 88)
Δ WSEL
vs. Effective
WSEL
(NAVD 88)
Δ WSEL
vs. Effective
Δ WSEL
vs. Corr.
Effective4 WSEL
(NAVD 88)
Δ WSEL
vs. Effective
Δ WSEL
vs. Corr.
Effective4
Δ WSEL
vs. Pre-
Project5 WSEL
(NAVD 88)
Δ WSEL
vs. Effective
Δ WSEL
vs. Corr.
Effective4
Δ WSEL
vs. Pre-
Project5
d/s model limit / FEMA section AR 26867 26867 11128.96 5310.58 11128.96 5310.58 0.00 11128.96 5310.59 0.01 5310.71 0.13 5310.71 0.13 0.00 5310.71 0.13 0.00 0.00 5310.71 0.13 0.00 0.00
27046 27046 11128.96 5312.48 11128.96 5312.48 0.00 11128.96 5312.47 -0.01 5312.40 -0.08 5311.99 -0.49 -0.41 5311.99 -0.49 -0.41 0.00 5311.99 -0.49 -0.41 0.00
FEMA section AS 27347 27347 11128.96 5315.42 11128.96 5315.42 0.00 11128.96 5315.43 0.01 5315.58 0.16 5315.28 -0.14 -0.30 5315.28 -0.14 -0.30 0.00 5315.28 -0.14 -0.30 0.00
--27400 --5316.13 --5316.13 0.00 --5316.13 0.00 5316.33 0.20 5316.10 -0.03 -0.23 5316.01 -0.12 -0.32 -0.09 5316.01 -0.12 -0.32 -0.09
--27445 --5316.73 --5316.73 0.00 --5316.73 0.00 5316.96 0.23 5316.75 0.02 -0.21 5316.35 -0.38 -0.61 -0.40 5316.35 -0.38 -0.61 -0.40
d/s face existing 19th Street Bridge 27499 27498 11128.96 5317.45 11128.96 5317.45 0.00 11128.96 5317.44 -0.01 5317.71 0.26 5317.65 0.20 -0.06 5317.62 0.17 -0.09 -0.03 5317.49 0.04 -0.22 -0.16
u/s face existing 19th Street Bridge / FEMA section AT 27520 27520 11128.96 5319.25 11128.96 5319.26 0.01 11128.96 5319.25 0.00 5318.83 -0.42 5318.75 -0.50 -0.08 5318.68 -0.57 -0.15 -0.07 5318.49 -0.76 -0.34 -0.26
--27590 --5319.55 --5319.56 0.01 --5319.55 0.00 5319.10 -0.45 5318.99 -0.56 -0.11 5318.94 -0.61 -0.16 -0.05 5318.81 -0.74 -0.29 -0.18
FEMA section AU 27903 27903 11084.57 5320.89 11084.57 5320.89 0.00 11084.57 5320.89 0.00 5320.33 -0.56 5321.04 0.15 0.71 5321.04 0.15 0.71 0.00 5321.04 0.15 0.71 0.00
d/s face 17th Street Bridge 28193 28193 11084.57 5326.62 11084.57 5326.62 0.00 11084.57 5326.64 0.02 5326.64 0.02 5326.64 0.02 0.00 5326.64 0.02 0.00 0.00 5326.64 0.02 0.00 0.00
53XX.YY indicates an interpolated value
1 Effective Model determined to be the "Boulder Creek FP Model_Sept 2013\COBLDR02, Plan: 100-yr w/o Roach Sept 2013" HEC-RAS 4.0
2 Duplicate Effective Model shortened to project reach. (HEC-RAS 4.1.0)
3 Duplicate Effective Model shortened to project reach. (HEC-RAS 5.0.6)
4 Corrected Effective Model revises downstream starting WSEL (xs 26867) to match 23rd Street Bridge FDP model, 19th Street Bridge removed per 2013 flooding (HEC-RAS 5.0.6)
5 Additional cross sections added for additonal detail; utilizes City of Boulder 2013 LiDAR data (except for cross section 28193)
6 Proposed 19th Street main bridge only; orignial 19th Street abutments in place
7 Proposed 19th Street main bridge and maintenance bridge
Proposed 19th Street Main Bridge ONLY6
(Main Bridge ONLY, original 19th Street abutments in place
including debris blockage at abutments)
Proposed 19th Street Bridges7
(Main Bridge AND Maintenance Bridge
including debris blockage at abutments)
Effective Model 1
(100-yr w/o Roach - Sept
2013)
Duplicate Effective2
HEC-RAS 4.1.0
Duplicate Effective3
HEC-RAS 5.0.6
Corrected Effective4
(PMR Topography,
existing 19th removed)
Pre-Project5
(2013 COB LiDAR, additional XS)
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6.0 FLOODPLAIN MAPPING
6.1 Floodplain Work Map
The effective 1%- and 0.2%-A.C. floodplains and floodway for Boulder Creek are shown on the
FIRM included in Appendix C and Figure E.1 in Appendix E. The datum and mapping
projection are as described in Section 3.1 and are listed on the workmap. All elevations shown
on the workmaps are presented on vertical datum NAVD 1988.
The effective and proposed condition floodplains are illustrated in Figures E.1 through E.7 in
Appendix E. Electronic data for all information presented on the topographic workmaps is
included in Appendix H for reference.
7.0 CONCLUSIONS
7.1 Compliance with Standards
City of Boulder FDP Criteria
This analysis demonstrates that the proposed project will not increase the Boulder Creek BFEs,
regulatory floodway, 1%-A.C. floodplain, Conveyance Zone, or High Hazard Zone compared to
the Pre-Project conditions. The responses included in Appendix B to the 15 factors listed in
Section 9-3-6 (d) for consideration in the review of Floodplain Development Permit applications
further demonstrate that the proposed project does not cause any other adverse impacts.
7.2 Impacts to Adjacent Properties
The Corrected Effective and Pre-Project models result in increases to the 1% A.C. water
surface elevations compared to the effective conditions. However, the proposed project does
not result in an increase in 1% A.C. water surface elevations compared to the Pre-Project
conditions. The increases in water surface elevation as a result of the Corrected Effective and
Pre-Project models impact the applicant’s (CU) property, City of Boulder right-of-way, and 23
structures on 6 separate property owners. These impacted structures and properties are
illustrated in Figure E.7 in Appendix E. Table 7.1 includes applicable information on the
impacted structures. The impacted structure owners and property owners have been notified of
the Corrected Effective and Pre-Project impacts to their structures and property. Copies of
these notifications are included in Appendix G.
Agenda Item 4A Page 15 of 52
Table 7.1: Structure Impacts - 19th Street Pedestrian Bridge
Effective
Structure
ID Structure Description Propoerty Owner Flood Proofing
Effective
Zone
Proposed
Zone
WSEL
(NAVD 88)
WSEL
(NAVD 88)
Δ WSEL
vs. Effective
WSEL
(NAVD 88)
Δ WSEL
vs. Effective
Δ WSEL
vs. Pre-Project
1 Athens Ct B3 - 2050 Athens Street State of Colorado (CU)none (non compliant)Zone AE Zone AE 5310.90 5310.90 0.00 5310.90 0.00 0.00
2 Athens Ct C1 - 2030 Athens Street State of Colorado (CU)none (non compliant)Zone AE Zone AE 5312.66 5311.98 -0.68 5311.98 -0.68 0.00
3 Athens Ct C2 - 2031 Grandview Ave.State of Colorado (CU)none (non compliant)Zone AE Zone AE 5312.66 5312.22 -0.44 5312.22 -0.44 0.00
4 Athens Ct B2 - 2010 Athens Street State of Colorado (CU)none (non compliant)Zone AE Zone AE 5313.17 5312.78 -0.39 5312.78 -0.39 0.00
5 Athens Ct A2 - 1950 Athens Street State of Colorado (CU)none (non compliant)Zone AO Zone AO 5313.76 n/a n/a n/a n/a n/a
6 Athens Ct A4 - 1951 Grandview Ave.State of Colorado (CU)none (non compliant)Zone AE Zone AE 5314.66 5314.41 -0.25 5314.40 -0.26 -0.01
7 Athens Ct A1 - 1930 Athens Street State of Colorado (CU)none (non compliant)Zone AO Zone AO n/a n/a n/a n/a n/a n/a
8 Athens Ct A3 - 1391 Grandview Ave.State of Colorado (CU)none (non compliant)Zone AE Zone AE 5316.57 5316.66 0.09 5316.34 -0.23 -0.32
9 Athens Ct B1 - 1910 Athens Street State of Colorado (CU)none (non compliant)Zone AO Zone AE 5317.33 5317.56 0.23 5317.33 0.00 -0.23
10 Steam Conversion Shed - 1201 19th Street State of Colorado (CU)none (non compliant)Zone AE Zone AE 5317.28 5317.51 0.23 5317.25 -0.03 -0.26
11 Athens North Ct - 1855 Athens Street State of Colorado (CU)none (non compliant)Zone AE Zone AE 5319.77 5319.16 -0.61 5318.92 -0.85 -0.24
12 Faculty - Staff Ct 8 - 1850 Athens Street State of Colorado (CU)none (non compliant)Zone AE Zone AE 5319.66 5319.28 -0.38 5319.08 -0.58 -0.20
13 Faculty - Staff Ct 6/7 - 1750 Athens Street State of Colorado (CU)none (non compliant)Zone AE Zone AE 5320.37 5320.30 -0.07 5320.22 -0.15 -0.08
14 Faculty - Staff Ct 9 - 1860 Athens Street State of Colorado (CU)none (non compliant)Zone AE Zone AE 5320.01 5319.76 -0.25 5319.61 -0.40 -0.15
15 Faculty - Staff Ct 5 - 1740 Athens Street State of Colorado (CU)none (non compliant)Zone AE Zone AE 5320.48 5320.48 0.00 5320.41 -0.07 -0.07
16 Faculty - Staff Ct 4 - 1255 19th Street State of Colorado (CU)none (non compliant)Zone AE Zone AE 5321.53 5321.70 0.17 5321.66 0.13 -0.04
17 Faculty - Staff Ct 2/3 - 1250 18th Street State of Colorado (CU)none (non compliant)Zone AE Zone AE 5323.89 5324.07 0.18 5324.07 0.18 0.00
18 Faculty - Staff Ct 1 - 1245 19th Street State of Colorado (CU)none (non compliant)Zone AE Zone AE 5324.24 5324.42 0.18 5324.42 0.18 0.00
19 1810 Hilside Road Burgin none (non compliant)Zone AE Zone AE 5319.62 5319.14 -0.48 5318.93 -0.69 -0.21
20 1810 Hilside Road Burgin none (non compliant)Zone AE Zone AE 5320.27 5320.07 -0.20 5319.97 -0.30 -0.10
21 1709 Hillside Road Vincent none (non compliant)Zone AE Zone AE 5320.88 5321.02 0.14 5321.01 0.13 -0.01
22 1709 Hillside Road Vincent none (non compliant)Zone AE Zone AE 5322.30 5322.41 0.11 5322.34 0.04 -0.07
23 Utility Housing State of Colorado (CU)none (non compliant)Zone AE Zone AE 5313.39 5313.01 -0.38 5313.00 -0.39 -0.01
Proposed 19th Street BridgesPre-Project
ICON Engineering Inc.
P:\P\18011NSB\10_Reports\Floodplain_Developement_Permit_Application\working\NSB_FDP_Table_7_1_Structure_Impacts.xlsx 2019-05-17
Agenda Item 4A Page 16 of 52
ICON
19th Street Bridge – Floodplain Development Permit Application
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Appendix A-
Floodplain Development Permit Application
and Review Comment Response Letter
Agenda Item 4A Page 17 of 52
Agenda Item 4A Page 18 of 52
Agenda Item 4A Page 19 of 52
ICON
7000 S. Yosemite Street, Suite 120, Centennial, CO 80112
303.221.0802 | www.iconeng.com
Planning | Design | Management
July 26th, 2019
Ms. Christin Shepherd P.E., CFM
Flood and Wetland Administrator
City of Boulder – Planning and Development Services
1739 Broadway, Third Floor
Boulder, CO 80306-0791
RE: FLD2019-00088 – Floodplain Development Permit for Reconstruction of the 19th
Street pedestrian bridge over Boulder Creek on CU Main Campus – DRAFT FOR
REFERENCE
Dear Ms. Shepherd,
This letter provides responses to the Floodplain comments provided by the City of Boulder –
Planning and Development Services on July 9th, 2019, as part of the FLD2019-00088 review. A
revised Floodplain Development Permit Application report is provided under separate cover.
Floodplain
1. Please provide a site plan showing the location of where construction materials will
be stored during construction. This area must be located outside of the conveyance
and high hazard zones.
The construction drawings have been revised to indicate a location for construction material
storage that is outside of the conveyance and high hazard zones. Revised drawings are
included in Appendix F.
2. At Cross Section 27590 there is a building in the right overbank, Building 19 in Figure
E.4, that is not represented in the model, please add a block obstruction to the pre-
project and proposed plans.
A blocked obstruction has been added to cross section 27590 in the pre- and post-project
models in order to reflect the building.
3. Please label proposed bridge cross sections in the cross section descriptions.
Proposed bridges have been labeled in the cross section descriptions.
4. In Section 5.1 Hydraulic Analysis, it is stated that “A debris blockage factor of
approximately 5% has been applied to the crossing to remain consistent with
previous modeling efforts”, this is not accurate, for the effective analysis a debris
blockage factor of 20% to 25% was used through the bridges in the vicinity of 19th
Street. Please provide justification and calculations of the debris blockage factors
utilized for the proposed condition.
Agenda Item 4A Page 20 of 52
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FLD2019-00088 Comment Response 2019-07-26
The reference to “previous modeling efforts” was directed at the approach taken for the
similarly configured 23rd Street bridge as addressed in a previous Floodplain Development
Permit Application process (LUR2018-00020). Per a 2018-04-04 meeting to discuss the
debris blockage approach for 23rd Street, the project team and the City of Boulder
determined that moving forward with a ~5% blockage factor (based on top width) would be
reasonable for the crossing based in part due to the freeboard between both the 100-year
and 500-year water surface elevations and the low chord of the deck. Based on the similar
bridge configuration of the proposed 19th Street Bridge, this same approach has been
applied. The 20% to 25% debris blockage values come from effective modeling for bridges
where the low chord of the bridge deck is below or very close to the base flood elevation – a
condition that is not applicable to the proposed 19th Street Bridge.
For clarity, cross section 27400 has a 100-year top width of 389 feet resulting in a target
debris blockage of 19.5 feet. This cross section has 7 piers that were modeled with 1 foot of
debris blockage on each side of the pier for a total of 14 feet of blockage. This totals 3.6%
of the top width. However, debris blockage was also applied to the north abutment that is
modeled in cross section 27445. The total 100-year top width blocked adjacent to the north
abutment is 19.8 feet, which is approximately 5.3% of the top width of cross section 27445.
If the debris blockage at both cross sections is viewed as a singular element, we have a
total of 33.8 feet of debris blockage for an average top width of 382 feet resulting in a debris
blockage of 8.8%. This exceeds the target of 5%, but does error in the side of being
conservative.
Per discussions at the 2019-07-16 review meeting, the project team and the City of Boulder
determined that a similar debris blockage approach as used on the 23rd Street Bridge would
be applicable to the 19th Street Bridge. The debris blockage approach for the 19th Street
Bridge has not been revised.
5. In Appendix B factor number 4, it is stated that the split flow path entering the main
channel will not be adversely impacted, however, Cross Section 27590 is cut across
the downstream end of the split flow path thus affecting the junction length from the
effective model. Please add the “15th Street-Arapahoe Split Flow Path” to the model
to confirm that there are not adverse impacts.
Cross section 27590 is actually located upstream of the junction and the portion near the
split flow is ineffective flow. For clarity, cross section 27590 has been truncated and re-
aligned such that the appearance of conflict with the split flow is eliminated. This approach
was discussed at the 2019-07-16 review meeting and it was determined that modeling of the
“15th Street-Arapahoe Split Flow Path” would not be necessary based on the previous and
revised layout of cross section 27590.
6. Please provide pre-project BFE’s, CZ, and HHZ line work
The requested information has been provided in Appendix H of the revised Floodplain
Development Permit Application Report.
Agenda Item 4A Page 21 of 52
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FLD2019-00088 Comment Response 2019-07-26
7. Pre-Project conveyance zone analysis is not included in the model, please provide
this analysis.
The pre-project conveyance zone analysis has been completed in the hydraulic modeling
and is included in Appendix H of the revised Floodplain Development Permit Application
Report. Additionally, the pre-project conveyance zone is now illustrated in Figures E.4 and
E.5.
8. Cross Section 27499 is not included in either the pre-project or proposed models, this
is an effective cross section, Cross Section 27498 is the modeled pre-project and
proposed cross section, please change the model or the map (whichever is correct) in
order to be consistent between mapping and modeling.
The workmaps have been revised to reflect the 27498 label for the pre-project and post-
project modeling.
9. In Section 5.0 Hydraulics, please provide additional documentation for the starting
water surface. Additionally, please explain why the effective water surface elevation
was not utilized, given that the 23rd Street Bridge model is not effective.
Use of the resulting water surface elevation from the 23rd Street project was done per the
pre-submittal discussions with Alysha Geiger on May 21st, 2019. Construction of the 23rd
Street Bridge is now complete and a letter of map revision submittal to FEMA is in progress.
The starting water surface elevation used in this FDP application comes directly out of the
LUR2018-00020 floodplain development permit as highlighted in the table below:
10. Please explain why the contraction and expansion coefficients at Cross Section
26867 are 0.1 and 0.3 in the pre-project and proposed condition plans as opposed to
expansion and contractions coefficients of 0.3 and 0.5 used in the effective and
corrected effective plans.
The expansion and contraction coefficients have been revised to 0.3 and 0.5 in the pre-
project and post-project models in order to be consistent with the effective and corrected
effective plans.
Agenda Item 4A Page 22 of 52
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FLD2019-00088 Comment Response 2019-07-26
11. Please remove Not for Construction on the construction plans and provide plans
signed and stamped by a Colorado P.E.
As, discussed at the 2019-07-16 review meeting, signed and stamped drawings will be
provided at a later date.
12. Cross Sections 27903 and 28193 are not contained in the left overbank, this is
consistent with the effective model; however, because it is adjacent with the 15th
Street-Arapahoe SFP that split flow path should be modeled to promote clear
understanding of the hydraulics in this area.
It should be noted that the split flow BFEs are higher than main Boulder Creek channel
BFEs and the effective modeling approach does not have lateral structures between the two
flow paths to address this difference. Given the relatively minor changes in water surface
elevations in the main Boulder Creek channel (still remaining lower than the split flow path)
and the effective modeling approach to this area, additional modeling of the split flow path
was determined to be unnecessary based on discussions with the City of Boulder during the
2019-07-16 review meeting.
13. In the Section 4.0 Hydrology, in Table 4.1 Cross Section 28638 is not included in the
Pre-Project or Proposed hydraulic model, please adjust table accordingly.
Additionally, please include Boulder Creek 15th Street -Arapahoe Split Flow Path flow
data in Table 4.1
Table 4.1 has been revised to reflect the upstream most cross section of 28193 . Based on
the response to comment #12, the Boulder Creek 15th Street – Arapahoe Split Flow Path
flow data has not been included in Table 4.1.
14. Section 5.4 Tie-Ins, although not necessary, given the complicated nature of the
Boulder Creek Hydraulic model, to ensure that upstream flow splits are not affected
by the 19th Street Bridge project, it may be beneficial to match effective water surface
elevations exactly at the upstream tie-in. This could possibly be accomplished by
matching the effective cross section geometry exactly as well as running the model in
HEC-RAS version 4.0.0.
As noted in the response to comment #18, cross section 28193 at the upstream tie-in
location has been revised to reflect the effective geometry. This results in a 0.02 foot water
surface elevation difference at the upstream tie-in. It is anticipated that this minor difference
in water surface elevation is negligible when it comes to the upstream split flow which is
located at Broadway (approximately 2,200 feet upstream). Leaving the 0.02 foot elevation
difference as-is was discussed at the 2019-07-16 review meeting and determined to be
reasonable. No additional revisions where completed and the HEC-RAS model remains in
version 5.0.6.
15. In Appendix F please provide force calculations for the breakaway bridge as well as
the proposed location of the bridge if it should breakaway.
Based on the 2019-07-16 review meeting, it was determined that the breakaway apparatus
is part of the prefabricated bridge package. The prefabricated bridge will not be designed
until a contractor has been selected. The plan notes currently require the contractor to
submit calculations pertaining to the breakaway apparatus. The plans in Appendix F have
been revised to include the hydraulic force imposed upon the bridge during the 100-year
Agenda Item 4A Page 23 of 52
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FLD2019-00088 Comment Response 2019-07-26
event, and the notes revised to require the contractor’s submittal to demonstrate that the
bridge will bread away under this force.
16. As previously mentioned, Cross Section 27590 is cut across Split Flow Path 15th
Street-Arapahoe, please update model to reflect the effects of the added cross
section on the split flow path.
As noted in the response to comment #5, 27590 is entirely ineffective near the split flow path
and has no impact on the split flow. Cross section 27590 has been re-aligned to provide
additional clarity to this condition.
17. The new chainlink fence shall be securely anchored to resist damage and washing
away as debris during flooding events. Please provide fence details including an
elevation view and anchoring details and certification that the fence will not wash
away during a flood event.
Fence details are provided on Sheet 6 of the FOR drawings in Appendix F. Per this detail,
the fence is lightly connected at the bottom but firmly connected at the top. If it performs as
intended, it will break away at the bottom and swing open during a flood event. We can
certify that this system has been designed to function as such during a flood event. But
understanding the unpredictable nature of floods and debris, the variable alignment of the
fence with respect to the creek flow, and the relatively low strength of the fence and post
system, we cannot certify that the system will perform as intended.
18. As previously mentioned, please adjust blocked obstructions due to debris to be
consistent with effective analysis. Because the blocked obstructions due to debris
are calculated using the bridge opening area, and this is a tie-in cross section, the
geometry at Cross Section 28193 (downstream face of 17th Street) should be
unchanged from the effective model.
The geometry of cross section 28193 has been revised to reflect the effective model in all
hydraulic models for this study (i.e. no longer reflects 2013 City of Boulder LiDAR elevation
data).
19. In Cross Section 27400 please be consistent with the modeling of piers, either use
blocked obstructions like the two right-most piers or use only one ineffective flow for
each pier (as opposed to two). Please use the same modeling approach for the pier
modeled in Cross Section 27347.
All piers in cross sections 27400 and 27347 have been revised to reflect a single ineffective
flow for debris blockage at each pier.
20. To provide a clear comparison of the effects to the flood hazards due to the project
work maps comparing the effective vs pre-project, pre-project vs proposed and the
effective vs proposed flood hazards are recommended to be included in the report.
The figures in Appendix E have been revised to provide the requested comparisons.
Agenda Item 4A Page 24 of 52
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FLD2019-00088 Comment Response 2019-07-26
21. In Section 3.0 please document the date and source of the topography. Additionally,
please provide the certified stamped and signed survey from King Surveyors.
Section 3.0 has been updated to reflect the date and source of the topography. Certified
stamped and signed survey from King Surveyors is provided in Appendix E.
22. Please include the following on the flood hazard work maps (as required by FEMA’s
MT-2 Form);
a. Study Limit Extents
b. Provide more topography labels, especially in the vicinity of the project
c. Stream centerlines
d. Tie-in labels (tie ins should occur at the study limit cross sections)
e. Signed professional engineers stamp
f. Source of topography
g. Source of aerial imagery
The figures in Appendix E have been revised to provide the requested information. Tie-in
information is graphical as presented in the figures at the study limits as noted. The tie -in to
the split flow is as shown on the figures
23. In Section 5.3 Results, there is mention of increases in the widths of the CZ and HHZ,
where and what are these increases? Please include a comparison table of the
effective, pre-project and proposed CZ and HHZ widths. Also, please provide HHZ
width calculation documentation.
The increases in widths of the CZ and HHZ are illustrated in Figures E.5 and E.6 in
Appendix E. Given that most width changes are not at specific cross sections, a
comparison table has not been provided as discussed at the 2019-07-16 review meeting.
The HHZ width (by general delineation) calculation is done directly in HEC-RAS mapper
based on depth and velocity grids as produced by the model. No specific width calculations
were completed and no other documentation is available. Depth, velocity, and HHZ
information can be accessed through the HEC-RAS Mapper window.
24. Please extend pre-project and proposed line work to study limits.
Pre-project and Post-project line work has been extended to the study limits.
25. The model description is not accurate, the current description is from the effective
model. Please update the model description and plan descriptions, please include;
date, version of HEC-RAS, topography information, and hydrology source.
The effective model description has been removed. The model date and version are
included in the HEC-RAS output files. Topography information and hydrology source are
included in the FDP Application report.
26. The legend indicates that the corrected effective floodplain is mapped, is this
correct? The floodplain is mapped according to the pre-project model. Please
address accordingly
Yes, the corrected effective floodplain has been mapped. Workmaps in Appendix E have
been revised to provide additional clarity.
Agenda Item 4A Page 25 of 52
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FLD2019-00088 Comment Response 2019-07-26
27. As previously mentioned, it may be beneficial to run all of the plans in HEC-RAS
version 4.0.0 to be consistent with the effective model as well as maintaining a
constant modeling platform for all of the modeled plans.
As discussed in Section 5.0 of the report and illustrated in Table 5.1, the move to HEC -RAS
5.0.6 has a negligible change from the effective information. As it is desirable to stay with a
more current version of HEC-RAS, the modeling for this FDP will remain in version 5.0.6.
28. In the corrected effective and pre-project plans there are ineffective flow areas in the
channel at Cross Sections 27498 and 27520, please explain why. The ineffective flow
areas at Cross Sections 27498 and 27520 were in the effective model to represent
debris blockages. Due to the removal of the 19th Street Bridge deck the ineffective
flow areas in the channel should be removed or reduced in the corrected effective
and pre-project plans because the bridge deck is no longer in place to cause the
majority of the debris accumulations. Since, the 19th Street Bridge abutments are still
in place it is reasonable to assume some level of debris accumulation at Cross
Section 27520, however, likely not the 20% blockage utilized in the effective model.
The ineffective flow at cross sections 27498 and 27520 (in the corrected effective, pre-
project, and post-project main only models) have been revised to reflect only the abutment
locations (per the effective model bridge deck information) and not any debris blockage.
Please let me know if there is any additional information needed to clarify our responses to the
above review comments.
Sincerely,
ICON Engineering, Inc.
Brian LeDoux, P.E., CFM
Project Manager
Agenda Item 4A Page 26 of 52
ICON
19th Street Bridge – Floodplain Development Permit Application
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Appendix B-
Responses to Boulder Revised Code Section 9-3-6 (d) 15 Factors
Agenda Item 4A Page 27 of 52
Responses to Boulder Revised Code 9-3-6 (d) - 15 Factors
Boulder Revised Code Section 9-3-6 (d)
When reviewing an application for a permit, the city manager shall determine which portions of
the floodplain are affected by the particular development request and shall then apply the
provisions of Sections 9-3-2, "Floodplains," 9-3-3, "Regulations Governing the One Hundred-
Year Floodplain," 9-3-4, "Regulations Governing the Conveyance Zone," and 9-3-5,
"Regulations Governing the High Hazard Zone," B.R.C. 1981, as applicable. The manager also
shall determine whether the application meets the intent of this chapter prescribed by Subsection
9-3-2(a), B.R.C. 1981, after considering the following factors:
(1) The effects upon the efficiency or capacity of the conveyance zone and high hazard zone;
The proposed project will not decrease the efficiency or capacity of the conveyance zone or
the high hazard zone. The minor areas of fill along the north bank do not cause a rise in
water surface elevation and do not reduce the main channel conveyance capacity. The 19th
Street Bridge deck is elevated above the 1%-annual-chance water surface. Additionally,
the replacement of the existing 19th Street Bridge abutments will increase capacity within
the project reach and the 19th Street Maintenance Bridge has been designed as a
breakaway structure that will not adversely impact the floodplain.
(2) The effects upon lands upstream, downstream, and in the immediate vicinity;
The proposed project will have no adverse effects upon lands upstream, downstream, or in
the immediate vicinity. The increases in BFE compared to the Effective flood hazard
information are due to updated modeling software and pre-project conditions. The
affected property owners will be notified regarding the change in floodplain as a result of
the pre-project conditions (additional cross sections, updated mapping).
(3) The effects upon the one hundred-year flood profile;
The proposed conditions 1% A.C. flood profile as less-than or equal-to the Pre-Project
conditions profile at all locations. There are increases compared to the Effective profile, but
these are due entirely to the updated Corrected Effective conditions and Pre-Project
analysis. Profile comparisons in both tabular and graphical formats are included with this
submittal.
(4) The effects upon any tributaries to the main stream, drainage ditches, and any other drainage
facilities or systems;
The project is localized in its impact and has no adverse impact upon any tributaries to
Boulder Creek, drainage ditches, or any other drainage facilities or systems. The split flow
from Boulder Creek that re-joins the main channel at 19th Street is not adversely impacted
as the proposed water surface elevations immediately upstream of 19th Street decrease
compared to the Effective, Corrected Effective, and Pre-Project conditions. Additionally,
Agenda Item 4A Page 28 of 52
the downstream most cross section on this split flow is located on Marine Street
approximately 850 feet upstream of the Boulder Creek channel.
(5) Whether additional public expenditures for flood protection or prevention will be required;
No additional public expenditures for flood protection or prevention will be required. The
project will result in an improvement in flood safety and a reduction in the need for future
flood protection/prevention.
(6) Whether the proposed use is for human occupancy;
The proposed use of the project is not for human occupancy.
(7) The potential danger to persons upstream, downstream, and in the immediate vicinity;
As demonstrated by this analysis, the proposed project will not adversely affect the Boulder
Creek BFEs, Conveyance Zone, or High Hazard Zone. The potential danger to persons in
the immediate vicinity will decrease because the proposed bridge deck will be elevated
above the proposed conditions BFE.
(8) Whether any proposed changes in a watercourse will have an adverse environmental effect
on the watercourse, including, without limitation, stream banks and streamside trees and
vegetation;
The proposed bridge and trail project will not change the Boulder Creek watercourse and
will not have any adverse environmental effect.
(9) Whether any proposed water supply and sanitation systems and other utility systems can
prevent disease, contamination, and unsanitary or hazardous conditions during a flood;
The proposed project does not include any water supply or sanitation system components.
The elevated steam line that services the CU campus will be restructured at its crossing of
Boulder Creek. The improvements to the overhead steam line will increase the resilience of
this utility crossing over Boulder Creek.
(10) Whether any proposed facility and its contents will be susceptible to flood damage and the
effect of such damage;
No facilities are proposed as part of the 23rd Street bridge and trail project. The proposed
main bridge itself will be less susceptible to flood damage than the existing 19th Street
Bridge that was damaged and ultimately removed following the September 2013 flooding.
The 19th Street Maintenance Bridge will be designed as a break-away structure and will
include revised abutments that will increase the main Boulder Creek channel capacity.
Agenda Item 4A Page 29 of 52
(11) The relationship of the proposed development to the Boulder Valley Comprehensive Plan
and any applicable floodplain management programs;
The proposed 19th Street Bridge and trail project will have no adverse impact on the
Boulder Creek BFEs, 1% A.C. floodplain, regulatory floodway/Conveyance Zone, or High
Hazard Zone. The project goals are aligned with those of the Boulder Valley
Comprehensive Plan. Specific examples include:
• A coherent and recognizable structure of paths, edges, etc.
• An integrated multimodal transportation system with affordable, accessible and
pleasant ways to get around on foot, by bike, etc.
• Opportunities for people to connect to nature and each other
• Daily needs met within easy access from home, work, school, services or recreation
without driving a car
• Comfortable, safe and attractive places to live, work, learn and recreate that have a
distinct, memorable character and high-quality design and that promote healthy,
active living
• Buildings, streets, utilities and other infrastructure that protect natural systems,
minimize environmental impacts, and support clean energy generation
• Boulder Creek, Tributaries & Ditches as Important Urban Design Features
• Commitment to a Walkable & Accessible City
• Trail Corridors/Linkages
• Physical Design for People
• Environmentally Sensitive Urban Design
• Protecting Native Ecosystems
• & Biodiversity;
• Enhancing Urban Environmental Quality;
• Protecting Geologic Resources & Reducing Risks from Natural Hazards;
• Sustaining & Improving Water & Air Quality.
• Wetland & Riparian Protection
• Floodplain Management
• Larger Flooding Events
• Support for the University of Colorado
• Transportation Safety
(12) Whether safe access is available to the property in times of flood for ordinary and
emergency vehicles;
The existing and proposed bridges are not intended for vehicular use. All properties
affected by the bridge replacement and any resulting flood hazard impacts have safe
vehicular access via roadways.
Agenda Item 4A Page 30 of 52
(13) Whether the applicant will provide flood warning systems to notify floodplain occupants of
impending floods;
The City of Boulder already has a flood warning system in place; no changes to the flood
warning system are proposed.
(14) Whether the cumulative effect of the proposed development with other existing and
anticipated uses will increase flood heights; and
The submitted analyses account for all known existing uses as well as the proposed trail
and bridge project. The only increase in flood elevations is due entirely to the updated
Corrected Effective and Pre-Project analysis. Anticipated development along the north
bank of Boulder Creek is still in the conceptual planning stage and is being designed to
avoid causing any increase in flood heights compared to the corrected effective or pre-
project conditions at all phases of construction.
(15) Whether the expected heights, velocities, duration, rate of rise, and sediment transport of
the floodwaters expected at the site will adversely affect the development or surrounding
property.
The proposed 19th Street Bridge and trail improvements will not affect the heights,
velocities, duration, rate of rise, or sediment transport of the floodwaters expected at the
site in any manner that would adversely impact the development or surrounding property.
Agenda Item 4A Page 31 of 52
ICON
19th Street Bridge – Floodplain Development Permit Application
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Appendix C-
Effective Floodplain Information
Agenda Item 4A Page 32 of 52
BOULDER COUNTY,
COLORADO
AND INCORPORATED AREAS
Community Community
Name Number
BOULDER, CITY OF 080024
BOULDER COUNTY
(UNINCORPORATED AREAS) 080023
ERIE, TOWN OF 080181
JAMESTOWN, TOWN OF 080216
LAFAYETTE, CITY OF 080026
LONGMONT, CITY OF 080027
LOUSIVILLE, CITY OF 085076
LYONS, TOWN OF 080029
NEDERLAND, TOWN OF 080255
SUPERIOR, TOWN OF 080203
*WARD, TOWN OF 080292
* No Special Flood Hazard Areas Identified
VOLUME 1 OF 7
Revised: December 7, 2017
FLOOD INSURANCE STUDY NUMBER
08013CV001C
Boulder County
Agenda Item 4A Page 33 of 52
37
Table 4 – Summary of Discharges
Flooding Source and Location
Drainage Area
(Square Miles)
Peak Discharges (cfs)
10-Percent
Annual Chance
2-Percent
Annual Chance
1-Percent
Annual Chance
0.2-Percent
Annual Chance
Arapahoe Avenue Spill Flow
Approximately 800 feet downstream of the
divergence from Gregory Canyon Creek --1 323 975 1,209 2,149
Balarat Creek
At Upstream Limit of Detailed Study 0.5 30 150 270 760
Bear Canyon Creek
At Confluence with Boulder Creek 8.24 2,050 3,762 4,880 7,500
At Confluence of Skunk Creek 5.35 1,170 2,360 3,070 5,100
At Baseline Road 4.96 1,110 2,352 2,930 5,000
At U.S. Highway 36 4.34 820 1,780 2,210 3,850
At Broadway 4.08 680 1,512 1,930 3,400
At Upstream Limit of Detailed Study 3.71 480 1,190 1,600 3,000
Boulder Creek
At Confluence with Fourmile Canyon Creek --1 3,650 10,100 14,400 29,600
At Valmont Drive --1 3,450 9,200 13,000 23,000
At 28th Street --1 2,200 7,800 8,000 20,600
At County Road 54 --1 350 1,560 2,340 4,770
Boulder Creek- 13th Street Split --1 --1 275.13 1,084.78 --1
Boulder Creek- 14th Street Split --1 --1 331.50 954.07 --1
Boulder Creek- 15th St/Arapahoe Ave/Marine
St
At Grove St.--1 --1 172.65 437.79 --1
At Confluence with Boulder Creek --1 --1 331.50 954.07 --1
Boulder Creek- 16th Street Split --1 --1 18.58 112.79 --1
Boulder Creek- 17th Street Split --1 --1 17.96 126.17 --1
1 Data Not Available
Agenda Item 4A Page 34 of 52
6
FLOODING SOURCE FLOODWAY 1-PERCENT-ANNUAL CHANCE FLOOD
WATER-SURFACE ELEVATION
CROSS SECTION DISTANCE
1 WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
(FEET NAVD)
WITH
FLOODWAY
(FEET NAVD)
INCREASE
(FEET)
BOULDER CREEK
(cont.)
X 106,247 58 198 6.9 5,230.6 5,230.6 5,231.0 0.4
Y 106,885 700 2,220 5.3 5,234.7 5,234.7 5,234.9 0.2
Z 107,160 750 2,040 5.8 5,236.0 5,236.0 5,236.3 0.3
AA 107,666 880 2,013 5.9 5,239.1 5,239.1 5,239.2 0.1
AB 108,419 782 2,040 3.9 5,245.3 5,245.3 5,245.3 0.0
AC 108,883 166 645 9.7 5,248.7 5,248.7 5,248.7 0.0
AD 109,338 325 1,377 5.2 5,255.3 5,255.3 5,255.3 0.0
AE 109,688 301 929 9.2 5,256.4 5,256.4 5,256.4 0.0
AF 110,563 365 1,809 5.7 5,262.5 5,262.5 5,262.8 0.3
AG 111,076 505 1,439 7.3 5,264.7 5,264.7 5,264.8 0.1
AH 111,530 814 1,746 6.0 5,268.3 5,268.3 5,268.5 0.2
AI 111,797 591 1,421 7.9 5,271.6 5,271.6 5,271.6 0.0
AJ 112,279 1248 3,170 4.8 5,275.7 5,275.7 5,275.7 0.0
AK 112,767 106 575 13.7 5,277.2 5,277.2 5,277.2 0.0
AL 113,250 217 796 9.8 5,283.1 5,283.1 5,283.1 0.0
AM 113,786 810 3,279 3.3 5,291.3 5,291.3 5,291.3 0.0
AN 113,904 314 1,377 5.7 5,291.3 5,291.3 5,291.3 0.0
AO 114,440 320 990 7.9 5,294.4 5,294.4 5,294.6 0.2
AP 115,244 435 2,184 5.1 5,304.4 5,304.4 5,304.4 0.0
AQ 115,827 728 2,217 5.0 5,309.6 5,309.6 5,309.6 0.0
AR 116,267 209 1,061 10.5 5,310.6 5,310.6 5,311.1 0.5
AS 116,747 269 1,258 8.8 5,315.4 5,315.4 5,315.5 0.1
AT 116,920 333 1,830 6.5 5,319.3 5,319.3 5,319.4 0.1
1 Feet above mouth
TABLE 6
FEDERAL EMERGENCY MANAGEMENT AGENCY
BOULDER COUNTY, CO
AND INCORPORATED AREAS
FLOODWAY DATA
BOULDER CREEK
Agenda Item 4A Page 35 of 52
7
FLOODING SOURCE FLOODWAY 1-PERCENT-ANNUAL CHANCE FLOOD
WATER-SURFACE ELEVATION
CROSS SECTION DISTANCE
1 WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
(FEET NAVD)
WITH
FLOODWAY
(FEET NAVD)
INCREASE
(FEET)
BOULDER CREEK
(cont.)
AU 117,303 196 1,070 10.4 5,320.9 5,320.9 5,321.0 0.1
AV 118,038 184 1,440 7.7 5,330.0 5,330.0 5,330.4 0.4
AW 118,757 220 1,488 7.5 5,336.0 5,336.0 5,336.5 0.5
AX 119,108 450 1,338 7.6 5,338.8 5,338.8 5,338.8 0.0
AY 119,820 420 2,026 5.6 5,349.7 5,349.7 5,350.2 0.5
AZ 120,159 775 1,763 6.9 5,352.3 5,352.3 5,352.3 0.0
BA 120,532 366 1,942 6.3 5,356.3 5,356.3 5,356.6 0.3
BB 121,227 340 1,206 10.1 5,362.1 5,362.1 5,362.1 0.0
BC 121,836 465 1,353 9.0 5,368.3 5,368.3 5,368.3 0.0
BD 122,300 209 1,940 6.3 5,380.1 5,380.1 5,380.4 0.3
BE 122,807 151 905 13.2 5,382.1 5,382.1 5,382.2 0.1
BF 123,440 99 954 12.5 5,391.7 5,391.7 5,392.2 0.5
BG 124,185 179 988 12.1 5,406.3 5,406.3 5,406.6 0.3
BH 124,717 187 1,000 11.9 5,416.8 5,416.8 5,416.9 0.1
BI 125,374 93 1,007 11.6 5,437.4 5,437.4 5,437.4 0.0
BJ 125,713 98 886 13.1 5,440.1 5,440.1 5,440.1 0.0
BK 126,177 148 858 13.6 5,447.4 5,447.4 5,447.4 0.0
BL 126,403 154 871 13.4 5,454.6 5,454.6 5,454.6 0.0
1 Feet above mouth
TABLE 6
FEDERAL EMERGENCY MANAGEMENT AGENCY
BOULDER COUNTY, CO
AND INCORPORATED AREAS
FLOODWAY DATA
BOULDER CREEK
Agenda Item 4A Page 36 of 52
ELEVATION IN FEET (NAVD 88)40PAgenda Item 4A Page 37 of 52
ELEVATION IN FEET (NAVD 88)41PAgenda Item 4A Page 38 of 52
Agenda Item 4A Page 39 of 52
ICON
19th Street Bridge – Floodplain Development Permit Application
P:\P\18011NSB\10_Reports\Floodplain_Developement_Permit_Application\working\NSB_FDP_Application_Report.doc
Appendix D-
Hydraulic Computations
Agenda Item 4A Page 40 of 52
HEC-RAS HEC-RAS 5.0.6 November 2018
U.S. Army Corps of Engineers
Hydrologic Engineering Center
609 Second Street
Davis, California
X X XXXXXX XXXX XXXX XX XXXX
X X X X X X X X X X
X X X X X X X X X
XXXXXXX XXXX X XXX XXXX XXXXXX XXXX
X X X X X X X X X
X X X X X X X X X X
X X XXXXXX XXXX X X X X XXXXX
PROJECT DATA
Project Title: 19th_Steet_Bridge
Project File : 19th_Street_Bridge.prj
Run Date and Time: 2019-07-10 11:31:09 AM
Project in English units
PLAN DATA
Plan Title: 52_Duplicate_Effective
Plan File : p:\P\18011NSB\09_Hydraulics\HecRas\19th_Street_Bridge.p54
Geometry Title: 52_Duplicate_Effective
Geometry File : p:\P\18011NSB\09_Hydraulics\HecRas\19th_Street_Bridge.g54
Flow Title : 52_Duplicate_Effective
Flow File : p:\P\18011NSB\09_Hydraulics\HecRas\19th_Street_Bridge.f11
Plan Summary Information:
Number of: Cross Sections = 8 Multiple Openings = 0
Culverts = 0 Inline Structures = 0
Bridges = 1 Lateral Structures = 0
Computational Information
Water surface calculation tolerance = 0.01
Critical depth calculation tolerance = 0.01
Maximum number of iterations = 40
Maximum difference tolerance = 0.3
Flow tolerance factor = 0.001
Computation Options
Critical depth computed at all cross sections
Conveyance Calculation Method: At breaks in n values only
Friction Slope Method: Average Conveyance
Computational Flow Regime: Subcritical Flow
FLOW DATA
Flow Title: 52_Duplicate_Effective
Flow File : p:\P\18011NSB\09_Hydraulics\HecRas\19th_Street_Bridge.f11
Flow Data (cfs)
River Reach RS 100-Year
Boulder Creek 14th-19th 28193 11084.57
Boulder Creek 14th-19th 27520. 11128.96
Boundary Conditions
River Reach Profile Upstream Downstream
Boulder Creek 14th-19th 100-Year Known WS = 5309.93
Duplicate Effective Conditions (52)
1 of 14 Agenda Item 4A Page 41 of 52
ICON
19th Street Bridge – Floodplain Development Permit Application
P:\P\18011NSB\10_Reports\Floodplain_Developement_Permit_Application\working\NSB_FDP_Application_Report.doc
Appendix E-
Topographic Work Maps and Profile
Agenda Item 4A Page 42 of 52
Grandview Ave.
Boulder Creek
Boulder High School
CU Athletic Fields
Naropa University
Recently completed 23rd Streetpedestrian bridge (LUR2018-00020)
Proposed 19th Streetpedestrian bridge
Proposed 19th Streetmaintenance bridge
CU Recreation Center
Datum: NAVD 88Projection: NAD_1983_HARN_StatePlane_Colorado_North_FIPS_0501_FeetAerial Imagery: Esri 201917th StArapahoe Av
19th St20th StAthens St
Marine St18th St21st StHillside Rd
Marine St
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Grandview Ave.
Boulder Creek
Boulder High School
CU Athletic Fields
Naropa University
Recently completed 23rd Streetpedestrian bridge (LUR2018-00020)
Proposed 19th Streetpedestrian bridge
Proposed 19th Streetmaintenance bridge
CU Recreation Center
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Upstream limit of study Downstream limit of study
17th StArapahoe Av
19th St20th StAthens St
Marine St18th St21st StHillside Rd
Marine St
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Grandview Ave.
Boulder Creek
CU Athletic Fields
Proposed 19th Streetpedestrian bridge
Proposed 19th Streetmaintenance bridge
CU Recreation Center
Datum: NAVD 88Projection: NAD_1983_HARN_StatePlane_Colorado_North_FIPS_0501_FeetTopography: 2013 City of Boulder LiDAR / King Surveying 2018Aerial Image: Esri 2019
Upstream limitof study
Downstream limit of study
53155315
53185314
5315
5316
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Grandview Ave.
Boulder Creek
CU Athletic Fields
Proposed 19th Streetpedestrian bridge
Proposed 19th Streetmaintenance bridge
CU Recreation Center
Datum: NAVD 88Projection: NAD_1983_HARN_StatePlane_Colorado_North_FIPS_0501_FeetTopography: 2013 City of Boulder LiDAR / King Surveying 2018Aerial Image: Esri 2019
Upstream limitof study
Downstream limit of study
53155315
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5316
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Grandview Ave.
Boulder Creek
CU Athletic Fields
Proposed 19th Streetpedestrian bridge
Proposed 19th Streetmaintenance bridge
CU Recreation Center
Datum: NAVD 88Projection: NAD_1983_HARN_StatePlane_Colorado_North_FIPS_0501_FeetTopography: 2013 City of Boulder LiDAR / King Surveying 2018Aerial Image: Esri 2019
Upstream limitof study
Downstream limit of study
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53185314
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5316
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Grandview Ave.
Boulder Creek
CU Athletic Fields
Proposed 19th Streetpedestrian bridge
Proposed 19th Streetmaintenance bridge
CU Recreation Center
Datum: NAVD 88Projection: NAD_1983_HARN_StatePlane_Colorado_North_FIPS_0501_FeetTopography: 2013 City of Boulder LiDAR / King Surveying 2018Aerial Image: Esri 2019
Upstream limitof study
Downstream limit of study
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5316
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Grandview Ave.
Boulder Creek
CU Athletic Fields
Proposed 19th Streetpedestrian bridge
Proposed 19th Streetmaintenance bridge
CU Recreation Center
See Table 7.1 forstructure information
1810 Hillside Road1710 Hillside Road
1220 17th Street
1709 Hillside Road
State of CO
State of COState of CO
State of COState of CO
City of Boulder
City of Boulder
Datum: NAVD 88Projection: NAD_1983_HARN_StatePlane_Colorado_North_FIPS_0501_FeetAerial Image: Esri 2019
Upstream limitof study
Downstream limit of study2749827520Athens St 19th StHillside Rd 20th St5310532053265322531253245314531653235321532453185319532053225321532653235324532553175316531553135314531211
2
3
49 1
7
8
5
6
1317
15 12
14
18
10
20
16
22
19
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2680027000272002740027600278002800028200530053055310531553205325533019th_Steet_Bridge Plan: 1) 53_Corrected_Effective_Truncated_v3 2019-05-17 2) 54_Pre_Project_Truncated_v3 2019-05-17 3) 55_Proposed_2019_04_24_v01_Truncated_v3 2019-05-17 4) 56_Proposed_2019_04_25_v01_Truncated_v3 2019-05-17 Main Channel Distance (ft)Elevation (ft)LegendWS 100-Year - 53_Corrected_Effective_Truncated_v3WS 100-Year - 54_Pre_Project_Truncated_v3WS 100-Year - 55_Proposed_2019_04_24_v01_Truncated_v3WS 100-Year - 56_Proposed_2019_04_25_v01_Truncated_v3GroundBoulder Creek 14th-19thAgenda Item 4A Page 50 of 52
Agenda Item 4A Page 51 of 52
Agenda Item 4A Page 52 of 52