HomeMy WebLinkAboutFile Documents.701 Gibson Ave.0086-2021-BRES (52)
1517 Blake avenue, suite 101
Glenwood Springs, CO 81601
970-945-8676 phone
970-945-2555 fax
www.hceng.com
An Employee-Owned Company
MEMORANDUM
To: PJ Murray – City of Aspen Engineering Dept.
From: Roger Neal, PE
Date: July 8, 2022
Project: 701 Gibson Avenue
Bldg Permit: #0086-2021-BRES
Subject: Storm Pipe Design Variance Request
Please accept this application of a request for variance from the URMP and Engineering Standards in the
design of a proposed storm pipe within city right-of-way to tie to the City of Aspen stormwater system
associated with proposed site and public improvements for stormwater management. This request
includes the following variances:
Pipe invert to have no manhole drop opposed to the standard drop of 0.2 feet minimum.
Pipe material to be C900 D-14 instead of reinforced concrete
Reducing minimum pipe diameter from 15-inches to 12 inches
Pipe slope to be at 1 percent opposed to the 2 percent minimum
Horizontal separation from storm inlet basin and water main to be less than 10 feet
Vertical separation from gas main to be less than 2 feet
The following are provided for support of this request:
Request for Variance and Code Response
Civil Engineering – Hydraflow Channel Capacity Report Output
Civil Engineering – Storm sewer plan and profile sheet
Code Response
Sec. 29.01.060. – Variances
(a) Authority. The City Engineer may grant variances to the design standards. Variances are deviations
from the terms of this Chapter which would not be contrary to the public interest.
(b) Standards. In order to authorize a variance from the design standards the following standards shall be
met:
(1) There exist practical difficulties and unnecessary hardships inconsistent with the purpose and
intent of the design standards.
A constraint for the storm sewer design of this site is the limited vertical height and
horizontal location available to connect a curb inlet and storm sewer pipe to an existing
Civil engineering Land surveying
07/12/2022
Reviewed by Engineering
07/25/2022 10:54:41 AM
"It should be known that this review shall not
relieve the applicant of their responsibility to
comply with the requirements of the City of
Aspen. The review and approval by the City is
offered only to assist the applicant's
understanding of the applicable Engineering
requirements." The issuance of a permit based
on construction documents and other data shall
not prevent the City of Aspen from requiring the
correction of errors in the construction
documents and other data.
1517 Blake avenue, suite 101
Glenwood Springs, CO 81601
970-945-8676 phone
970-945-2555 fax
www.hceng.com
city storm sewer manhole. Vertical height is constrained by numerous utility crossings
and the shallow depth to the existing storm manhole and pipes. Horizontal location is
constrained by the location of the storm manhole connection and the location of a water
main. The public stormwater improvements cannot meet the design standards due to
these constraints and conditions. The requested variances are viewed as the most viable
solution for constructability and function of the proposed storm system improvements
(2) The issuance of a variance shall not compromise the safety of a site or structure.
A hydraulic analysis has been performed for the pipe in which a variance is requested.
During the 100-yr event, the pipe has adequate capacity to convey the required flowrate
and will not adversely affect the site or structure. Furthermore, the location of the curb
inlet will improve the conditions of a neighboring structure that is currently receiving
runoff from Gibson Avenue.
(c) Application. Requests for variances from the standards, policies or submittal requirements of this
document shall be submitted in writing with appropriate documentation and justification to the City
Engineer. An application for a variance from the design standards shall include, at a minimum, the
following information:
(1) The criteria from which the applicant seeks a variance.
URMP 4.8.2. - Storm sewer pipes within the right-of-way shall be Reinforced Concrete
Pipe (RCP) with a minimum diameter of 15 inches, and a manhole shall have a minimum
0.20 foot drop.
City of Aspen: Engineering Standards 3.21 – Table 2 – Storm lines shall have a minimum
10 feet of horizontal separation from parallel water lines.
City of Aspen: Engineering Standards 3.21 – Table 3 – Storm lines shall have a minimum
2 feet of vertical separation when crossing a gas line.
(2) A written justification outlining why the criterion cannot be met.
The variance requests are a result of the following factors. The location of the storm inlet
is at the feasible location for improving the public stormwater conditions and connecting
to the City of Aspen stormwater system. The proposed curb inlet is on the south side of
Gibson Avenue, which happens to be where a majority of utilities in the city right-of-way
are located. The existing city manhole and storm pipes are approximately 4.7 feet beneath
the road surface. In order to cross 10 utility conduits and pipes while still connecting to
the shallow manhole, the mentioned design criteria cannot be met.
(3) An alternate criteria or standard that is proposed to comply with the intent of the criteria from
which the variance is sought.
The proposed pipe enters the manhole at the same invert elevation of the existing outlet
pipe.
Using C900 DR-14 pipe instead of reinforced concrete pipe (RCP). A 15 inch RCP has a
wall thickness of approximately 2.25 inches compared to the C900 wall thickness of
approximately 1 inch.
A diameter reduction of 15-inches to 12-inches.
07/12/2022
1517 Blake avenue, suite 101
Glenwood Springs, CO 81601
970-945-8676 phone
970-945-2555 fax
www.hceng.com
Reducing the minimum pipe slope of 2 percent to 1 percent.
The bottom of the curb inlet catch basin will have a maximum layback of 1:1 to
accommodate horizontal separation, as requested by the City of Aspen Water
Department. The proposed design has a layback of approximately 1.5:1.
4” of blueboard insulation will surround the proposed storm pipe and catch basin to aid
vertical/horizontal separation from the water main and gas main.
(4) Supporting documentation, including necessary calculations, etc.
Attached is a Hydraflow channel report output for the propped 12-inch diameter pipe.
The pipe is approximately 33 percent full during the 100-yr storm event. Also attached
for reference is storm sewer plan and profile sheet C6.0.
(5) An explanation of any potential adverse impacts of the proposed variance to adjacent
landowners.
The proposed variance is located within street right-of-way. Currently the stormwater
that will be captured by the proposed curb inlet is draining into the garage of the adjacent
landowner, so the proposed variances needed for the stormwater infrastructure will have
a favorable impact to the adjacent landowner. There are no other adjacent landowners
that would be impacted before runoff reaches the Roaring Fork River.
(6) An analysis of the variance request must be signed and stamped by a Professional Engineer
licensed in the State of Colorado.
Attached is an engineering analysis signed and sealed by myself, Roger D. Neal, P.E.
(CO PE #29975).
Please feel free to contact us with any questions you may have.
Thank you,
Roger D. Neal, P.E.
High Country Engineering, Inc.
Enc.
-Hydraflow output – 12 Inch Pipe 10yr and 100yr capacities
-Plan and profile sheet C2.001
07/12/2022
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Friday, Jul 8 2022
12IN C900 FROM GUTTER INLET 10YR FLOW
Circular
Diameter (ft) = 1.00
Invert Elev (ft) = 7907.98
Slope (%) = 1.00
N-Value = 0.011
Calculations
Compute by: Known Q
Known Q (cfs) = 0.58
Highlighted
Depth (ft) = 0.25
Q (cfs) = 0.580
Area (sqft) = 0.15
Velocity (ft/s) = 3.75
Wetted Perim (ft) = 1.05
Crit Depth, Yc (ft) = 0.32
Top Width (ft) = 0.87
EGL (ft) = 0.47
0 1 2
Elev (ft)Section
7907.00
7907.50
7908.00
7908.50
7909.00
Reach (ft)
07/12/2022
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Friday, Jul 8 2022
12IN C900 FROM GUTTER INLET 100YR FLOW
Circular
Diameter (ft) = 1.00
Invert Elev (ft) = 7907.98
Slope (%) = 1.00
N-Value = 0.011
Calculations
Compute by: Known Q
Known Q (cfs) = 0.97
Highlighted
Depth (ft) = 0.33
Q (cfs) = 0.970
Area (sqft) = 0.23
Velocity (ft/s) = 4.28
Wetted Perim (ft) = 1.22
Crit Depth, Yc (ft) = 0.42
Top Width (ft) = 0.94
EGL (ft) = 0.62
0 1 2
Elev (ft)Section
7907.00
7907.50
7908.00
7908.50
7909.00
Reach (ft)
07/12/2022
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