4 - Call Up: Table Mesa and Lehigh Intersection Floodplain Development Permit (LUR2009-00024)
MEMORANDUM
TO: Planning Board
FROM: Katie Knapp, Civil Engineer, Floodplain and Wetlands Penmitting
DATE: April 3, 2009
SUBJECT: Call Up Item: Table Mesa and Lehigh Intersection
Floodplain Development Pen-nit (LUR2009-00024)
This decision may be called up before Planning Board on or before
April 16, 2009
A Floodplain Development Permit was approved by Planning and Development staff on
April 2, 2009. The project involves intersection improvements at Table Mesa and Lehigh
Drive.
The proposed project includes the construction of a raised pedestrian crosswalk,
reconfiguration of the medians, installation of drainage structures and grading. The
proposed work will be within the flood conveyance zone of Bear Canyon Creek. The
applicant has demonstrated that the project will not adversely impact the floodplain or
cause a rise in the floodwater elevation during the 100-year flood event. A copy of the
floodplain development permit and the application materials are attached.
This floodplain development permit was approved by Planning and Development
Services staff on April 2, 2009, and the decision may be called up before Planning Board
on or before April 16, 2009. There is one Planning Board meeting within the required
14-day call-up period on April 16, 2009. Questions regarding this floodplain
development permit should be directed to Katie Knapp in Plazuiing and Development
Services at 303-441-3273 or knappk a bouldercolorado.gov.
Attachments:
A. Floodplain Development Permit
B. Application Materials
Attachment A
CITY OF BOULDER
A .rf Planning and Development Services
-ell "j 1739 Broadway, Third Floor • P.O. Box 791, Boulder, CO 80306-0791
phone 303-441-1880 • fax 303-441-3241 • web boulderplandevelop net
Land Use Review Floodplain Development Permit
Date Issued: April 2, 2009 Expiration Date: April 2, 2012
(Pursuant to Subsection 9-3-6(e), B.R.C, 1981)
Permit Number: LUR2009-00024
Contact Information
PEGGY SILCOX
PO BOX 791
BOULDER, CO 80306
x 3857
Project Information
Location: 0 LEHIGH ST
Legal Description: OUTLOT B BLK 4 TABLE MESA 4 PR OPERTY ADDRESS: 000000 LEHIGH
ST BOULDER
Description of Work: Floodplain Development permit for the intersection of Table Mesa Dr & Lehigh
St. modification of propsed pedestrian crossing & median modification.
Type of Floodplain Permit: Floodplain Review W/ Analysis
Creek Name: Bear Canyon
Flood Protection Elevation:
Conditions of Approval
The proposed project/activity is approved on the basis that it satisfies applicable requirements of Chapter
9-3-3, "Floodplain Regulations," Boulder Revised Code 1981. Other floodplain requirements as set forth in
Chapter 9-3-3 which are not specifically outlined in the conditions of approval below remain applicable to this
project/activity.
Improvements shall be constructed in accordance with the plans submitted as part of the floodplain
development permit application.
The applicant shall confirm in writing that all improvements have been completed in conformance with this
Floodplain Development Permit.
The applicant shall obtain a site inspection and approval from the City of Boulder Floodplain and Wetlands
Coordinator upon completion of the project.
inspections
To schedule an inspection, call 303-441-3280 and refer to your permit number (LUR2009-00024).
Final Floodplain Inspection
Attachment B
FLOODPLAIN DEVELOPMENT PERMIT APPLICATION -
PROPOSED PEDESTRIAN CROSSING
AND MEDIAN IMPROVMENTS
AT LEHIGH AND TABLE MESA
Prepared For:
CITY OF BOULDER
TRANSPORTATION DIVISION
P.O Box 791
Boulder, Colorado 80306
Prepared By:
BELT COLLINS WEST (dba LOVE & ASSOCIATES, INC.)
800 Jefferson Street, Suite B
Louisville, CO 80027
•
MARCH 17, 2009
3
March 17, 2009
Katie Knapp
Development Review Coordinator
City of Boulder
P.O. Box 791
Boulder, Colorado 80306
RE: 0906A - FLOODPLAIN DEVELOPMENT PERMIT FOR PEDESTRIAN
Crossing and Median Improvements at Lehigh and Table Mesa
Dear Katie:
The City of Boulder Transportation Division is submitting this Floodplain Development
Permit Application to widen the pedestrian refuge island on the southeast corner of the
intersection of Table Mesa Drive and Lehigh Street, to relocate the pedestrian sidewalk
through the island, and construct a 6-in raised pedestrian walk across the eastbound
Lehigh turn lane. The project will also include construction of a pedestrian refuge island
between the northbound and southbound lanes of Lehigh just south of Table Mesa. The
project area is located within the Bear Canyon Creek floodplain, floodway, and high
hazard zone. Figure 1 shows the location of the project.
The floodplain impacts of the planned intersection improvements were hydraulically
analyzed to ensure no adverse impact to flood conditions. The basis for the hydraulic
analysis was the regulatory study approved by the Federal Emergency Management
Agency (FEMA) for Bear Canyon Creek and developed by Love & Associates, Inc. in
2003.
While this intersection is located in the floodway and high hazard zone, adverse impacts
to flood conditions are avoided by depressing the majority of the interior area of the
median to increase conveyance capacity and lowering the turn lane roadway elevation. A
grated inlet will be placed in the interior of the median to drain into the existing box
culvert beneath Lehigh. Two new CDOT Type R inlets will capture roadway drainage
and outlet via a storm sewer into the existing box culvert. The construction plans
showing the proposed improvements are included in the Appendix.
The following report includes a description of the hydraulic analysis, discussion of the
floodplain criteria items required for the Floodplain Development Pennit, and the
Floodplain Development Permit Application.
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LANDSCAPE ARCHITECTURE
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Belt Collins West (formerly Lave & Associates, Inc.)
B00 Jefferson Avenue. Suite B O SOD COQ FDP - BEAR CREEK LEHIGH & TABLE MESA
Louisville, Colorado 80027-1873 `J CITY OF BOULDER
t: 303-673-9795 f:303-873-9796
nlove@bellcolllnswestcom NORTH SCALE IN FEET TRANSPORTATION DEPARTMENT
3/17/09
BELTCOLLINS
Ms. Katie Knapp
March 17, 2009
Page 3
HYDRAULIC ANALYSIS
The regulatory study completed by Love & Associates, Inc. in support of the Letter of
Map Revision (LOMR) granted on October 10, 2003 was used as the basis of this study.
The regulatory hydraulic analysis was completed using HEC-2. The vertical datum of the
regulatory study is NGVD 29. The regulatory 100-yr flow through the subject reach is
1600 efs.
A segment of the regulatory model, US 100YR.dat, from the top of a drop structure
downstream of the Lehigh Street culvert (XS 515.2) to the top of the drop structure
upstream of the Lehigh Street culvert (XS 517.4) was converted from HEC-2 to HEC-
RAS version 4.0.0 to create a Duplicate Effective Model. The datum of the Duplicate
Effective Model was shifted from NGVD 29 to NAVD 88 using a Corpscon conversion
factor of 3.34-ft. The conversion equation is shown below.
NGVD 29 + 3.34-ft = NAVD 88
The culvert at Lehigh is overtopped during the 100-year flood event. Bear Canyon Creek
flows mostly at critical depth through the subject reach. The Effective and Duplicate
Effective water surface elevations (WSEL) are shown in Table 1.
Table 1. Effective and Duplicate Effective WSEL
Cross Effective 100-yr Duplicate Effective 100-yr
Section WSEL (ft)* WSEL (ft)*
515.2 5527.15 5527.15
515.3 5529.47 5529.44
515.4 5530.04 5530.04
515.8 5531.87 5531.84
516 5533.66 5533.70
517 5539.76 5539.51
517.3 553995 5539.99
517.4 5541.45 5541.43
* Note: Elevations given in NAVD 88 vertical datum.
A Corrected Effective Model was developed which incorporates updated survey data at
the upstream and downstream sections of the culvert at Lehigh. The updated survey
consists of spot elevations obtained by Boulder Land Consultants in January 2009 as well
as the City's 1-ft contour interval mapping flown in 2003 by Merrick & Company. The
existing culvert at Lehigh Street, a double 4-ft H by 8-ft W concrete box, was modeled
with 50% blockage which is consistent with the Effective Model. The culvert inverts and
the high chord of the roadway were adjusted per the updated survey data. The Corrected
Ms. Katie Knapp
March 11, 2009
Page 4
Effective Model represents the existing conditions along Bear Canyon Creek in the
vicinity of the project area. The Duplicate Effective and Corrected Effective WSEL are
shown in Table 2.
The project is located between the Lehigh culverts upstream and downstream cross
sections. In order to assess the impact of proposed changes at the Lehigh/Table Mesa
intersection above the Lehigh culvert, an existing and proposed model was created for the
overflow reach through the intersection.
Table 2. Duplicate Effective and Corrected Effective WSEL
Cross Duplicate Effective Corrected Effective Corrected Effective -
Section 100- r WSEL (ft) 100-yr WSEL (ft) Duplicate Effective (ft)
515.2 5527.15 5527.15 0.00
515.3 5529.44 5529.44 0.00
515.4 5530.04 5530.04 0.00
515.8 5531.84 5531.84 0.00
516 5533.70 5533.91 0.21
517 5539.51 5539.68 0.17
517.3 5539.99 5539.99 0.00
517.4 5541.43 5541.43 0.00
The existing culvert 100-yr capacity is 350 efs (taken from the Corrected Effective
Model). The flow through the intersection is equal to the total flow (1600 efs) minus the
culvert flow, or 1250 efs.
Five cross sections spanning the intersection were field surveyed and added to the
Existing Overflow Model. The overflow reach cross sections are shown in Figure 2. The
Existing Overflow Model includes the reach from the downstream face of the Lehigh
culvert (XS 516) to the contraction section just upstream of Lehigh (XS 517.3). Cross
section 517 at the upstream face of Lehigh was modeled from the high chord of the
roadway (without the main channel) and the culvert flow was subtracted at this cross
section and through the intersection reach (for a Qove,tl,,w = 1250 efs).
A Manning's `n' of 0.05 was assumed for the median area which will be vegetated with
grasses and shrubs. For the asphalt street, the Manning's `n' is 0.016 and for other
grassed areas it is 0.03, which is consistent with the Effective Model.
The Corrected Effective WSEL at cross section 516 was used as the starting water
surface elevation. The Existing Overflow Model ties to the Corrected Effective Model at
cross section 516 on the downstream end and cross section 517.3 on the upstream end.
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Ms. Katie Knapp
March 17, 2009
Page 6
The Corrected Effective and Overflow WSEL are shown in Table 3.
A Proposed Overflow Model was developed which reflects the proposed improvements
to the intersection at Lehigh and Table Mesa. Conveyance capacity is maintained by
matching the top of the raised crossing to* the existing grade and by depressing the
median interior for additional conveyance. Because the top of the raised crossing
matches existing grades, the turn lane roadway is lowered east and west of the crossing.
The highest elevation of the upstream nose of the median is 5535.74-ft. This was used as
a control elevation in the model at cross section 516.6 as it is the highest median
elevation. Similarly, the highest elevation of the downstream nose of the median,
5534.05-ft, was used as a hydraulic control for the effective flow area of the depressed
portion of the median. Ineffective flow was assumed below this elevation. This
ineffective flow area was not included in the flow area of hydraulic cross section 516.2.
Two 1.5-ft diameter bollards will be placed west of the pedestrian crossing on the north
and south ends of the crossing. The bollards were modeled as blocked obstructions.
Table 3. Corrected Effective and Existing Overflow Reach WSEL
Cross Corrected Effective Existing Overflow Existing Overflow -
Section 100-yr WSEL (ft) 100-yr WSEL (ft) Corrected Effective (ft)
516 5533.91 5533.91 0.00
516.2 N/A 5536.33 N/A
516.4 N/A 5537.33 N/A
516.5 N/A 5537.50 N/A
516.6 N/A 5537.80 N/A
516.8 N/A 5538.60 N/A
517 5539.68 5539.62 -0.06
517.3 5539.99 5539.99 0.00
Table 4 compares the Existing Overflow and Proposed Overflow water surface
elevations. The Proposed Overflow water surface elevations tie to the Existing Overflow
water surface elevations at cross section 516 downstream and cross section 517.3
upstream. The hydraulic models described herein are included in the Appendix.
q
Ms. Katie Knapp
March 17, 2009
Page 7
Table 4. Existing Overflow and Proposed Overflow WSEL
Cross Existing Overflow Proposed Overflow Prop Overflow - Ex
Section 100- r WSEL (ft) 100- r WSEL (ft) Overflow WSEL (ft)
516 5533.91 5533.91 0.00
516.2 5536.37 5536.20 -0.17
516.4 5537.33 5537.25 -0.08
516.5 5537.58 5537.49 -0.09
516.6 5537.89 5537.66 -0.23
516.8 5538.54 5538.50 -0.04
517 5539.62 5539.62 0.00
517.3 5539.99 5539.99 0.00
DESIGN
The new pedestrian crosswalk through the Lehigh turn lane will be raised approximately
6-in. The top of the raised crossing roughly matches the existing roadway grade. The
roadway is lowered by up to 0.5-ft just west of the raised crossing and up to 0.4-ft east of
the raised crossing. This design prevents the raised crossing from obstructing the existing
flow path.
The median interior will be depressed and will drain to a grate inlet with an invert
elevation of 5533.75 which will outlet into the existing Lehigh box culvert. The median
is depressed by up to 2-ft. The median depression provides additional conveyance
capacity and prevents an adverse impact on flood conditions.
A pedestrian island will be constructed on Lehigh just south of Table Mesa and will
connect the west and east sides of the roadway. A 6-in curb and gutter will surround the
pedestrian refuge and the gutter will roughly match existing grades (see the plan sheets in
the Appendix). The pedestrian island is located on the outer edge of the effective flow
area and will not adversely impact water surface elevations due to the additional
conveyance capacity provided by depressing the median and lowering the roadway.
The existing storm sewer inlet located on the south side of the Lehigh turn lane and the
12-in storm sewer extending from the inlet into the Lehigh box culvert will be removed.
A new CDOT Type R (with a 5-ft inlet length) inlet will be placed along the south side of
the Lehigh turn lane just west of the raised pedestrian crossing. The drainage area
tributary to this inlet is 0.83 acres. The upstream limits of the drainage area are located at
the intersection of Darley and Lehigh. The Urban Drainage and Flood Control District's
(UDFCD) UD-Inlet Version 2.14c was used to size the inlet. The inlet was sized for the
5-yr peak flow of 1.45 cfs. The inlet sizing calculations are included in the Appendix.
I l)
Ms. Katie Knapp
March 17, 2009
Page S
A 12-in reinforced concrete pipe (RCP) will extend from the turn lane inlet to a second
inlet to be located on the west curb of the median.
The second inlet along the west curb of the median is required to drain a localized low
area along this curb created by lowering the roadway. A CDOT Type R inlet will also be
used here. An 18-in RCP will connect this inlet to the existing Lehigh box culvert. A
profile of the inlet configuration is included in the plans located in the Appendix.
DISCUSSION OF FLOODPLAIN CRITERIA ITEMS
Section 9-3-6(c) of the Boulder Revised Code requires a discussion of several floodplain
criteria items for a Floodplain Development Permit. These items are discussed below.
The Floodplain Development Permit Application form can be found in the Appendix.
1. The effects of the efficiency or The capacity of the conveyance and high
capacity of the conveyance zone and hazard zones is increased by this project.
high hazard zone. Earthwork associated with this project
results in a net cut due to creation of the
depressed median area and lowering of the
turn lane roadway surface.
2. The effects upon lands upstream, This project will not impact lands upstream
downstream, and in the immediate and downstream of the project site. The
vicinity. median and roadway areas within the
project site will be re-graded. Flood
elevations on the residence at 1010 Lehigh
located adjacent to and south of the project
site will be slightly lowered (see Table 4).
All improvements are located within
public right-of-way. This project will not
disturb the property at 1010 Lehigh.
3. The effects upon the 100-year flood The hydraulic analysis shows the 100-year
profile. flood profile will remain the same or be
lowered as a result of this project. See
Table 4 and the description of the
hydraulic analysis.
4. The effects upon any tributaries to the The existing inlet on the south side of the
main stream, drainage ditches, and Lehigh turn lane and the 12-in RCP from
any other drainage facilities or this inlet to the existing box culvert will be
systems. removed. A new CDOT Type R inlet will
be installed on the south side of the turn
lane just west of the pedestrian crossing.
Under existing conditions, flows on the
north side of the turn lane, north of the
11
Ms. Katie Knapp
March 17, 2009
Page 9
existing crown, do not drain to the existing
inlet. This project will result in
elimination of the existing crown and the
turn lane will be graded to drain entirely to
the south curb. As a result of the new
roadway design, the new inlet will be more
effective at capturing roadside drainage
and reduce carry-over flow bypassing the
inlet.
The second inlet along the west curb of the
median will improve drainage along the
northbound lane of Lehigh. Under existing
conditions, there are two low spots along
this curb and no inlets so water will pool.
The proposed inlet will eliminate standing
water along this curb.
No tributaries or drainage ditches are
im acted by the project.
5. Whether additional public No.
expenditures for flood protection or
prevention will be required.
6. Whether the proposed use is for No.
human occupancy.
7. The potential danger to persons There is a slight decrease in flood risk in
downstream and in the immediate immediate project vicinity within the
vicinity. public right-of-way and at 1010 Lehigh
due to reduction in flood elevations. The
raised crossing and pedestrian island will
increase pedestrian safety, particularly for
the high volume of school children who
use this crossing.
8. Whether any proposed changes in a Thee are no proposed changes to the
watercourse will have an adverse watercourse. The project area is not within
environmental effect on the the City's new wetland boundary and it is
watercourse, including without not within the main channel of the stream.
limitation, streambanks, and
streamside trees and vegetation.
9. Whether the proposed water supply No change.
and sanitation system and other
utility systems can prevent disease,
contamination, and unsanitary or
hazardous conditions during a flood.
A
Ms. Katie Knapp
March 17, 2009
Page 10
10. Whether the proposed facilities will There is no change in the susceptibility of
be susceptible to flood damage and the crossing to flooding. The Lehigh
the effects of periodic flooding. culvert will be overtopped and the crossing
will be inundated during the base flood
under existing and proposed conditions.
11. The relationship of the proposed This project fulfills a key goal of the
development to the Boulder Valley Comprehensive Plan in that it will increase
Comprehensive Plan and any pedestrian safety at the turn lane crossing.
applicable floodplain management The turn lane will be narrowed to reduce
programs. vehicular speeds. Additionally, the
pedestrian island on Lehigh will create a
safe refuge for pedestrians crossing when
crossing Lehi h.
12. Whether safe access is available to No change. Emergency vehicles may
the property in times of flood for access the site from either Lehigh or Table
ordinary and emergency vehicles. Mesa.
13. Whether the applicant will provide No.
flood warning systems to notify
floodplain occupants of impending
floods.
14. Whether the cumulative effect of the The project results in no change or a
proposed development with other decrease in flood elevations when
existing and anticipated uses will considering both existing and anticipated
increase flood levels. uses.
15. Whether the expected heights, This project does not adversely affect
velocities, duration, rate of rise, and heights, velocities, durations, rate of rise,
sediment transport of the floodwaters and sediment transport of floodwaters at
expected at the site will adversely the site or in the vicinity of the project.
affect the development or The average velocity of the 100-year flood
surrounding property. through the intersection is 7 ft/s for both
existing and proposed conditions.
l3
Ms. Katie Knapp
March 17, 2009
Page 11
We thank you for the opportunity to submit this Floodplain Development Permit
Application and we look forward to your issuance of a flood permit for the proposed
intersection modifications. Please contact us if you have any comments or questions.
Sincerely,
BELT COLLINS WEST (formerly Love &
Associates, Inc.)
f U-~ ~ ivdM6
Prepared By: Brianna L. Wallace, E.I.
'~)"j I AC
Reviewed By: David/. Love, P.E.
Appendices
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