HomeMy WebLinkAboutDrainage Reports - 05/12/2017COLORADO CIVIL GROUP, INC.
fngineenng Consultants
February 5, 2017
City of FortC I 'ns Appr ed Plans
Ms. Heather McDowell Approved by:
City of Fort Collins 5
}. 1 Jam_
281 N. College Ave. - —
Fort Collins, CO 80522
Re: Mason Street North Railroad Easement Grading & Drainage Improvements
Ms. McDowell,
The purpose of this letter is to present to the City of Fort Collins planned grading and drainage
improvements associated with a tract of land that was formerly a Burlington Northern railroad
easement. Landscaping improvements are also planned for this tract of land, and have been
coordinated with the grading and drainage design to ensure proper functioning of the system.
The tract of land, owned by the Mason Street North Condo Association, is located on the north
side of Cherry Street, west of Mason Court, and within a former Burlington Northern Railroad
easement. A vicinity map with the site shaded in yellow is provided below for reference.
The land is part of Mason Street North, a Replat of Lot 12 and Tract A, Martinez P.U.D. and a
Tract of Land Being a Part of Block 34 of the Original City Plat. With the vacation of the railroad
easement, ownership of the land is now with the Mason Street North Condo Association.
COLORADO CIVIL GROUP, INC. • 5110 Granite St., Unit D. Loveland, Colorado 80538 • 970-278-0029
Proiect Background
The proposed improvements have been requested by the Mason Street North Condo
Association with a desire to improve the grading, drainage, and landscaping within the former
railroad easement. Additionally, the area has experienced significant flooding, so the
Association wishes to improve the overall function of the drainage system, and minimize the
potential and extent of future flooding.
The original drainage design and report for Mason Street North was prepared by Shear
Engineering Corporation, and dated July 2004. On July 20, 2009 a rainfall and hail event
occurred at the site which caused significant flooding. Shear Engineering Corporation prepared
a Drainage Damage Observations, Assessment and Recommendations report discussing the
rainfall and hail event and the observed flooding. Observations presented within this report
were utilized in preparing the proposed grading and drainage design discussed in this letter.
There were two key issues identified in the damage report, the first was a 60-inch by 60-inch
area inlet at the west end of the site that was plugged by debris, and eventually contributed to
flooding within a nearby underground parking structure. The second issue identified was
flooding due to runoff from Cherry Street. There are a series of Type `R' curb inlets within
Cherry Street that are designed to intercept all runoff produced from a 100-year event.
However, at the time of the July 20, 2009 rainfall and hail event, the railway line was still in
place along with the corresponding curb cut. This curb cut allowed a significant amount of
runoff within Cherry Street to discharge into the former railroad easement prior to reaching the
series of Type 'R' curb inlets. However, the railroad tracks have since been removed, and the
curb cuts have been replaced with standard vertical curb and gutter.
The Mason Street North Condo Association is motivated to improve the grading and drainage
within the former railway easement, as well as improve the aesthetic look of the corridor with
landscaping improvements.
Proposed Improvements
The improvements planned within the former railroad easement are intended to reduce possible
debris that can clog the drainage system, and improve the effectiveness of the existing drainage
system. The site can effectively be divided into two separate areas, the first area is located
west of the existing concrete sidewalk that also incorporates a small section of the old railway,
and the second area is located east of the concrete sidewalk and old railway section. The
concrete sidewalk and old railway section form a high -point within the site and force runoff either
west or east. The exhibit below shows the two areas.
WEST
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COLORADO CIVIL GROUP, INC. " 5110 Granite St., Unit D" Loveland, Colorado 80538 9 970-278-0029
West Improvements
The proposed west improvements, shown in the exhibit below, include removing all existing
landscaping mulch and debris that can contribute to clogging the existing 60-inch by 60-inch
area inlet (labeled "GRATE") located at the west end of the small parking lot. This area will be
improved with new planting and mulch that cannot float and clog the area inlet. Additionally, two
small drainage swales are proposed to intercept runoff from the site and old Burlington Northern
railroad corridor, and carry it to the existing area inlet. The construction of the swales will divide
the area contributing to the existing area inlet into smaller sections, helping the drainage
infrastructure to more effectively convey runoff.
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One other proposed improvement was suggested to the Owner, but eventually it was decided to
not pursue this improvement, because it was viewed as a marginal improvement. This
suggested improvement would increase the collection capacity of the area inlet "GRATE" by
elevating the two grates 6 inches and increasing the open area by which runoff can enter the
stormwater system. A detail is provided below showing the grates elevated an additional 6-
inches using 24 - #4 rebar, which increases the open area of the grate from 16 square feet
(based on Neenah Catalog for 2 — R-4977-5 grates) to 25.5 square feet. Raising the grates 6
inches will allow any residual debris to collect along the sides of the area inlet, and maximize
the collection capacity at the top of the grate. Capacity calculations of the existing area inlet
and proposed area inlet are shown below. Calculations were performed using Neenah
calculators found on the manufacturer's website. This design information provided below has
been included in this letter for future reference, in case the Owner decides to purse this
improvement at a later date.
COLORADO CIVIL GROUP, INC. 9 5110 Granite St., Unit D• Loveland, Colorado 80538 • 970-278-0029
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Existing Capacity
Catalog Number and Grate Type:
R4977 5: B •
Feet perimeter (P): Head in feet (k): Free open area in sq. ft. (A):
5 1.3 6 —
Weir capacity in cfs: Transitional flow in cfs: Orifice capacity in cfs:
I 1-7F— Al
The calculator shows an orifice capacity of 43.9 cfs, which can be multiplied by 2 for the two
existing grates, totaling 87.8 cfs. The head used for the calculation (1.3 feet) is based on a
recent site survey with the invert elevation of the area inlet at 4979.72, and the low point in the
curb and gutter near the parking garage at an elevation of 4981.02. According to the original
Mason Street North final drainage report, the 100-year discharge to the area inlet is 110.6 cfs,
which is greater than the existing capacity of the inlet.
Proposed Capacity of Two Grates
Catalog Number and Grate Type:
R-4977-5: $
Feet perimeter (Pj: Head in feet (h): Free open area in sq. ft. (A):
15 0.8 8
Weir capacity in cfs: Transitional flow in cfs: Orifice capacity in cfs:
COLORADO CIVIL GROUP, INC. 9 5110 Granite St., Unit Da Loveland, Colorado 80538 a 970-278-0029
The calculator shows a flow capacity of 28 cfs, which can be multiplied by 2 for the two existing
grates, totaling 56 cfs. This is based on only 0.8 feet of head which accounts for raising the
grates 6 inches.
Next the capacity of the side grates created from raising the existing grates 6 inches is shown
below.
Catalog Number and Grate Type:
LseleSelecttdurnberand Grate •
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Feet perimeter (P):
11
Head in feet (h):
1.14
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Free open area in sq. ft. (A):
1 2.375
Weir capacity in cfs: Transitional flow in cfs: orifice capacity in cfs:
12.2
The calculator shows a flow capacity of 12.2 cfs, which can be multiplied by 4 for the four sides
of the raised grate, totaling 48.8 cfs. The head of 1.14 feet is based on the centroid of a
rectangular shape. Adding the top and side grate capacities yields a capacity of 104.8 cfs,
greater than the existing capacity of 87.8 cfs. The calculations show an increase in the capacity
of the grates, and by raising the grates 6 inches, it is believed debris will be less likely to clog
the inlet.
East Improvements
The proposed east improvements, shown in the exhibit below, include removing all existing
landscaping mulch and debris that can contribute to clogging of the two 24 inch area inlets
constructed above the existing Howes Street outfall box culverts. The two existing area inlets
were designed to collect surface runoff from the former railway easement and between the
buildings. This area will also be improved with new planting and mulch that cannot float and
clog the area inlets. Additionally, a drainage swale will be constructed within the former railway
easement to convey runoff to the southern area inlet. Currently, grading in this area is poorly
defined resulting in many areas with depressions that do not properly drain.
COLORADO CIVIL GROUP, INC. • 5110 Granite St., Unit D• Loveland, Colorado 80538 • 970-278-0029
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Additional improvements include lowering the southern area inlet and adjusting the grading to
create an overflow spill location between buildings. The southern area inlet ring and cover
currently sits on an 8 inch concrete grade ring, which will be removed with the proposed
improvements, which will lower the invert of the area inlet by 8 inches. This allows for improved
slopes (1% and greater) along the proposed drainage swale flowline. This modification also
improves drainage away from the existing buildings, where pooling runoff was observed after
the July 20, 2009 rainfall and hail event. The final improvement includes a 25 foot long overflow
spillway. The overflow spillway forces the low point between the existing buildings and over the
box culvert, where currently the low point is near the existing building face. Although the
overflow spillway has not been design to convey a specific discharge rate, using the Urban
Drainage Weir Equation S0716, the capacity of the proposed spillway is estimated at
approximately 25 cfs. The Mason Street North drainage report indicates 100-year discharge
rates to the two existing area inlets totaling approximately 5 cfs (design points 1d and 2).
Since it is being proposed to lower the invert of the southern 24 inch area inlet, a check of the
hydraulic grade line for the Howes Street box culverts was performed. From the Howes Street
Outfall Project As -Built Hydraulic Evaluation, prepared by Anderson Consulting Engineers, Inc.
and dated January 12, 2001, the Hydraulic and Energy Grade lines at the North Inlets in Cherry
Street are as follows (based on NGVD 1929 datum):
Hydraulic Grade line Energy Grade Line
Out In Out In
4972.2 4975.2 4974.9 4976.8
The Hydraulic and Energy Grade lines at the North Inlets in Cherry Street based on the NAVD
1988 datum are as follows (applies a positive shift of 2.95 feet according to NOAKs Vertcon
Website):
Hydraulic Grade Line Energy Grade Line
Out In Out In
4975.15 4978.15 4977.85 4979.75
The recent site survey shows the south area inlet invert elevation at 4979.58 (NAVD 1988).
After dropping the area inlet ring and cover 8 inches by removing the 8 inch concrete .grade
adjustment ring, the proposed invert elevation is 4978.91 (NAVD 1988). Based on this
information, it is possible that by lowering the invert elevation of the southern area inlet, it will be
located below the energy grade line within the Howes Street Outfall box culverts. However, the
hydraulic and energy grade lines in the box culvert correspond to a peak discharge that.occurs
much later than the peak discharge associated with on -site runoff from Mason Street North. For
this reason, lowering the invert elevation of the southern area inlet should not have an adverse
effect on the operational hydraulics of the box culverts or the surface runoff associated with
Mason Street North.
COLORADO CIVIL GROUP, INC. :• 5110 Granite St., Unit D. Loveland, Colorado 80538 4 970-27&0029
Conclusion
The proposed improvements presented within this letter are intended to improve the
effectiveness of the drainage system at the 'Mason Street North development, but not
necessarily solve all drainage issues. The proposed changes along with the previous removal
of the railway lines and the previous construction of vertical curb and gutter, are likely to reduce
the frequency and extent of future flooding. Debris from landscaping that can lead to clogging
will be eliminated, which will also improve the effectiveness of the system. The proposed
changes to the surface are minimal with respect to overall site imperviousness, and should not
significantly change site runoff rates. Additionally, since the proposed surface material and
landscaping is expected to be rock mulch and irrigated grass, less debris is expected to enter
the storm sewer system, and should improve water quality. A general grading plan and detailed
grading plan are included with the letter for reference.
Sincerely,
Colorado Civil Group, Inc. CAP
40727
SS�CNAL.��G
David A. Huwa, P.E.
COLORADO CIVIL GROUP, INC. :• 5110 Granite 5t., Unit D. Loveland, Colorado 80538 • 970-278-0029
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NOTES
I. A DETALED GRADING PLAN NTH SPOT ELEVATIONS IS FOUND ON
SHEET Cs
2 CONTRACTOR SHALL VERIFY HORIZONTAL AND VERTICAL LOCATION
OF ALL UTILITIES PRIOR TO WORK
T CONTRACTOR SHALL INSTAL. EROSION CONTROL DMP'S IDENTIFIED
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ON THESE PLANS PRIOR TO THE START OF CONSTRUCTION
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ACTIHTIES. DETAILS ARE FO.HD ION SHEET C6.
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CONTRACTOR SHALL INSTALL ANY EROSION CONTROL BMP
NECESSARY TO PREVENT SEDIMENT TRANSPORT CFF OF S7E
WHETHER IT IS SHOWN ON THESE PLANS OR NOT.
5 CONTRACTOR SHALL REMOVE UP TO ♦- OF TOPSOIL AND
STOCKPILE FOR RE-VSE.
6 ALL AREAS WITHIN THE APPROXIMATE WORK LIMITS SHOWN SHALL
BE SURFACED WITH DIRER CONCRETE, MULCH, SOO. OR NATIVE
GRASS AS SHOWN ON THE LANDSCAPE PLAN- ANY AREA
-
DISTURBEO AND NOT RECEIVING SURFACING. OESCRBED ABOVE
SHALL RECEIVE NAIVE $ED IN ACCORDANCE WITH CITY OF FORT
COLLINS SPECInCAT10NS.
/. CONTRACTOR SHALL ENSURE THAT ALL ROOF DRAMS AND
ADJACENT GRADING SHOW POSITIVE FLOW AWAY MOM EXISTING
BUILDINGS AND STRUCTURES AND INTO PROPOSED DRAINAGE
SWALES
6 CONTRACTOR SMALL ENSURE THAT GRADING AND LANDSCAPING 15
INSTALLED AT AN ELEVATION THAT DOES NOT PREVENT RUNOFF
FROM REACHING THE PROPOSED DRAINAGE SWALES.
9 SPOT ELEVATIONS SHOWN ARE GRADE BREAKS AND EACH MUST
BE STAKED BY A SURVEYOR TO ENSURE PROPER GRADING AND
DRAINAGE IS ACHIEVED.
10. THE PROPOSED DRAINAGE SWALES WITH SLOPES LESS THAN 7%
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SMALL FOLLOW THE SOFT SWALE DETAIL FOUND ON SHEET C6
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1 A GENERAL GRADING PLAN IS FOUND ON SLEET CL.
2 CONTRACTOR SHALL VERIFY HORIZONTAL NTAL AND VERTICAL LOCATION
OF ALL UTUTES PRIOR TO WORK.
J CONTRACTOR SHALL INSTALL EROSION CONTROL BMP'S IDENTIFIED
ON THESE PLANS PRIOR TO THE START OF CONSTRUCTION
ACTIVITIES. DETAILS ARE FOUND ON SHEET C6
A CONTRACTOR SHALL INSTALL ANY EROSICH CONTROL OUP
NECESSARY TO PRETEST SEDIMENT TRANSPORT OFF CIF SITE.
WHETHER 1T IS SHOWN ON THESE PUNS OR NOT.
5 CONTRACTOR SNAIL REMOVE UP TO A' OF TOPSOIL AND
STOCKPILE FOR RE -USE.
6 ALL AREAS WITHIN THE APPROMMATE WORK LIMITS SHOWN SHALL
OF SURFACED WITH EITHER CONCRETE, MULCH. SOD. OR NATIVE
GRASS AS SHOWN ON THE LANDSCAPE PLAN. ANY AREA
DISTURBED AND NOT RECEIVING SURFACING DESCRIBED ABOVE
SHALL RECEIVE NATIVE SEED N ACCORDANCE WITH CITY OF FORT
COLLINS SPECBICAPONS.
CONTRACTOR SHALL ENSURE THAT ALL ROOF DRAINS AND
ADJACENT GRADING SHOW POSITIVE FLOW AWAY FROM EXISTING
BUILDINGS AND STRUCTURES AND INTO PROPOSED DRAINAGE
SWAIEs.
8 CONTRACTOR SHALL ENSURE THAT GRADNG AND LANDSCAPING IS
INSTILLED AT AN ELEVATION THAT DOES NOT PREVENT RUNOFF
FROM REACHING THE PROPOSED DRAINAGE SWALES.
9 SPOT ELEVATIONS SHOWN ARE GRADE BREAKS AND EACH MUST
BE STAKED BY A SURVEYOR TO ENSURE PROPER GRADING AND
DRAINAGE IS ACHIEVED
10 ME PROPOSED DRAINAGE SWALES WITH SLOPES LESS MAN ]x
SHALL FOLLOW THE SOFT SWALE DETAI FOUND ON SHEET CIS
LEGEND
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