FINAL DRAINAGE REPORT Archer Park Subdivision Archer Park Subdivision Filing No. 1 10 El ...

82 Pages·2017·6.47 MB·English

by  John Olson

FINAL DRAINAGE REPORT

Archer Park Subdivision
Archer Park Subdivision Filing No. 1
10 El Encanto Drive
CITY OF COLORADO SPRINGS
EL PASO COUNTY, COLORADO

Preparation Date:
September 24, 2016
Revised: September 11, 2017

Developer:

Rick DeLesk
The Newport Company
620 Southpoint Ct. #280
Colorado Springs, CO 80906
[phone]
Denver Office: Colorado Springs Office:
3461 Ringsby Court, #125 2727 N. Cascade Avenue, #160
Denver, Colorado 80216 Colorado Springs, CO 80907
720.413.9691 719.231.3959
AltitudeLandCo.com
SMALL SUBDIVISION DRAINAGE REPORT

Archer Park Subdivision
Archer Park Subdivision Filing No. 1
10 El Encanto Drive
City of Colorado Springs,
El Paso County, Colorado

JOB NUMBER 16-121

Archer Park Subdivision Filing No. 1, Section 36, Township 14 South,
Range 67 West of 6th P.M.
City of Colorado Springs, County of El Paso, State of Colorado

PREPARED BY:
Mr. Eric T.Burtzlaff, P.E.
Altitude Land Consultants, Inc.
3461 Ringsby Court, Suite 125
Denver, CO 80216

Denver Office: Colorado Springs Office:
3461 Ringsby Court, #125 2727 N. Cascade #160
Denver, Colorado 80216 Colorado Springs, CO 80907
720.413.9691 719.231.3959
AltitudeLandCo.com
TABLE OF CONTENTS:

1.0 – ENGINEERING SIGNATURE PAGE

2.0 – EXISTING AND PROPOSED DRAINAGE
2.1 - Location and Description of Development
2.2 - Soils
2.3 - Existing Drainage Characteristics
2.4 – Previous Studies conducted on site
2.5 – Floodplain Statement
2.6 – Proposed On-Site and Off-Site Drainage
2.6.1 – Proposed Facilities
2.6.2 – Routing
2.6.3 – Detention
2.7 - Compliance with Master and Development Plans
2.8 – Adequacy of existing outfall facility
2.9 – Public or Private maintenance of facilities
2.10 – Stable Natural Channels
2.11 – Stormwater Quality

3.0 – DRAINAGE DESIGN CRITERIA
3.1 – Regulations
3.2 – Development Criteria Reference and Constraints
3.3 – Hydrological Criteria
3.4 – Waivers from Criteria
3.5 - Conclusions

4.0 – Financials
4.1 – Drainage Fees
4.2 – Bridge Fees
4.3 – Construction Cost Opinion

5.0 – SPECIAL AGREEMENTS
5.1 - Private Maintenance Agreement
5.2 – Acceptance of Downstream Owners

6.0 - REFERENCES

7.0 - APPENDICIES

Denver Office: Colorado Springs Office:
3461 Ringsby Court, #125 2727 N. Cascade #160
Denver, Colorado 80216 Colorado Springs, CO 80907
720.413.9691 719.231.3959
AltitudeLandCo.com
1.0 - STATEMENT PAGE:
Engineer’s Statement
This report and plan for the drainage design of Archer Park Subdivision was prepared by me (or under my
direct supervision) and is correct to the best of my knowledge and belief. Said report and plan has been
prepared in accordance with the City of Colorado Springs Drainage Criteria Manual and is in conformity
with the master plan of the drainage basin. I understand that the City of Colorado Springs does not and will
not assume liability for drainage facilities designed by others. I accept responsibility for any liability caused
by any negligent acts, errors or omissions on my part in preparing this report.

______________________________________________ Seal:
Eric T. Burtzlaff, P.E.
Colorado P.E #50061___

Developer’s Statement:

Rick DeLesk hereby certifies that the drainage facilities for Archer Park Subdivision shall be constructed
according to the design presented in this report. I understand that the City of Colorado Springs does not
and will not assume liability for the drainage facilities designed and/or certified by my engineer and that
are submitted to the City of Colorado Springs pursuant of section 7.7.906 of the City Code; and cannot, on
behalf of Archer Park Subdivision, guarantee that the final drainage design review will absolve Archer Park
Subdivision and/or their successors and/or assigns of future liability for improper design. I further understand
that approval of the final plat does not imply approval of my engineer’s drainage design.

___________________________________________
Name of Developer

___________________________________________
Authorized Signature Date

___________________________________________
Printed Name

___________________________________________
Title

___________________________________________
Address:

City of Colorado Springs:

Filed in accordance with Section 7.7.906 of the Code of the City of Colorado Springs, 2001, as amended.

______________________________________________________________________________________
For the City Engineer Date
Denver Office: Colorado Springs Office:
3461 Ringsby Court, #125 2727 N. Cascade #160
Denver, Colorado 80216 Colorado Springs, CO 80907
720.413.9691 719.231.3959
AltitudeLandCo.com
2.0 – EXISTING AND PROPOSED DRAINAGE:

2.1 – Location and Description of Development:

The purpose of this drainage report for the Archer Park Subdivision is to establish that
proposed development will not negatively impact adjacent properties during storm
events. It is also intended to show that existing storm infrastructure has capacity to
convey the additional flows generated by the proposed development.

The subject property of this report is addressed as 10 El Encanto Drive, Colorado Springs,
El Paso County, CO and is located south of Mayfield Lane where Mayfield Lane meets
El Encanto Drive. The site is bordered by unplatted land to the South, and single family
residences on all other sides. The names of the surrounding plats are as follows:
Broadmoor Northstar Lots 1-4 are to the North, Replat of Lot 9, Block 1, Sierra Vista
Estates to the West, and Count Pourtables Addition Broadmoor Block 7, Lots 5 & 6 to the
East. The subject property is located in Section 36, Township 14 South, and Range 67
West of the 6th Principal Meridian.

The subject property is accessed via the intersection of Mayfield Lane and El Encanto
Drive per the private access drive on the North.

Part of the site, 1.196 acres, is currently referred to as Sierra Vista Estates Replat, Lot 3,
Block 1. The rest of the site, 3.505 acres, referred to herein as “Unplatted Parcel”, is
currently unplatted. A Vicinity Map is included in this report in the (Appendix A).

The subject property is 4.701 acres.

Most of the site (3.505 acres) currently exists as an unplatted parcel covered with
medium and low growing grass. The unplatted parcel flows northeast towards the
intersection of Mayfield Lane and El Encanto Drive at an average grade of roughly 4%.
The existing drainage pattern for the 1.19 acre parcel is to the north towards the same
intersection of Mayfield Land and El Encanto Drive at the same grade, roughly 4%.
Please refer to the historic drainage plan in (Appendix E) for more details.

Seven single family residential lots are proposed onsite with three tracts of land to be
used for parking, drainage, and landscaped areas.

This report is required as a submittal item of the City of Colorado Springs Development
Plan process.

2.2 – Soils:

A soil survey of the area (Appendix B) indicated soil type ‘12’ (Bresser Sandy Loam)
which exhibit Hydrologic Soil Group C characteristics, respectively. Bresser Sandy Loam
is classified as “well drained”. Please see (Appendix B) for full report. Additionally, a
Denver Office: Colorado Springs Office:
3461 Ringsby Court, #125 2727 N. Cascade #160
Denver, Colorado 80216 Colorado Springs, CO 80907
720.413.9691 719.231.3959
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Geologic Hazard Investigation was performed for the subject site by Entech
Engineering, Inc., on December 29th, 2016.

2.3 – Existing Drainage Characteristics

The subject property lies to the south of the existing Broadmoor Northstar Subdivision
and to the west of existing Sierra Vista Estates Replat, Block 1, Lots 1 & 2. The parcel also
lies to the north of another 5.59 acres unplatted parcel of land.

All flows from the Broadmoor Northstar parcel are conveyed onto El Encanto Drive
which is then conveyed to existing storm infrastructure located at the intersection of El
Encanto Drive & Sierra Vista Drive. Flows generated on the parcels to the east are
conveyed to the northeast onto El Encanto Drive. Off-site flows come onto the subject
site from the parcel to the south of the subject site. These flows were partially
accounted for in the drainage report completed for the Broadmoor Northstar
Subdivision prepared by JR Engineering, LTD., dated May 1987, and are conveyed to El
Encanto Drive and the existing infrastructure at the intersection of El Encanto Drive &
Sierra View Drive. This infrastructure ultimately leads to City of Colorado Springs Regional
Detention Facility known as Spring Run Reservoir No. 2.

The site is located within the Stratton Drainage Basin.

There is a large offsite basin that enters the site from the South determined to be 34.6
acres. These offsite flows enter the site and are conveyed in an existing grass-lined
swale that runs southwest to northeast on the subject site.

2.4 – Previous Studies conducted on site

To the best knowledge of this engineer, there are no masterplans prepared or studies
conducted on the Archer Park Subdivision site.

2.5 – Floodplain Statement

Per FEMA flood firmette 08041C0737F, dated March 17, 1997, Archer Park Subdivision is
not located within any FEMA floodplain. See (Appendix C).

2.6 – Proposed On-Site and Off-Site Drainage

2.6.1 – Proposed Facilities

As part of the design, we have obtained and reviewed the Existing Drainage Report for
the adjacent parcel, Broadmoor Northstar Subdivision, prepared by JR Engineering,
LTD., dated May 1987 (Appendix F).

Denver Office: Colorado Springs Office:
3461 Ringsby Court, #125 2727 N. Cascade #160
Denver, Colorado 80216 Colorado Springs, CO 80907
720.413.9691 719.231.3959
AltitudeLandCo.com
After review of this report, Altitude Land Consultants, Inc. proposes to convey all on-site
developed stormwater to a new detention facility which shall release to El Encanto
Drive and be conveyed to existing storm infrastructure to the north. Existing storm
infrastructure in Sierra View Drive includes 2 - 20’ curb inlets and 2 – 6’ curb inlets which
have a combined capacity of 126.6 cfs per the approved and recorded drainage
report completed by JR Engineering, LTD. for the Broadmoor Northstar Subdivision
dated May 1987. Additionally, historic offsite flows shall be routed around the subject
property via grass lined swales and a concrete elliptical pipe to its historical discharge
point, located at the Northeast corner of the site.

Altitude Land Consultants, Inc. is proposing a single, private full spectrum pond located
within Tract A near the entrance of the subject site. The pond is designed and sized for
the 100-year storm event per the standards provided in the Colorado Springs Drainage
Criteria Manual (CSDCM) for a Full Spectrum Detention (FSD) Pond. Calculations were
also performed to determine the required volume for Excess Urban Runoff Volume
(EURV) and the 5-year storm event. The calculations can be found in Appendix D. Per
CSDCM Vol. 1, Ch 13, Section 4.1. The proposed FSD facility was designed to provide
water quality for the EURV. The pond is designed to drain in 72 hours (per CSDCM Vol. 1,
Ch 13, Section 4.2). A 1.75 foot deep swale, located on the south side of the proposed
20’ private drive known as “Broadmoor Meadows View” conveys flows generated
onsite to the intersection of Broadmoor Meadows View and Waterfield View, the
proposed private driveway located along the Easterly, to a 24 inch reinforced concrete
pipe and headwall. The swale was designed with AutoCAD Hydraflow Extension and
has 1.0’ of freeboard. Hydraflow express software’s culvert tool was used to determine
the size of the reinforced concrete pipe required to convey the flow (Appendix D). The
concrete pipe will convey flows to the full spectrum pond located within Tract A which
contains a forebay, micro-pool, and trickle channel. The forebay will discharge through
a 3.7 inch notch onto a 3 foot concrete pan which will act as the trickle channel and
convey all flows to the 5 foot by 9 foot concrete outfall structure. Flows are then
conveyed via an 18 inch PVC pipe to a headwall and rip rap lined swale oriented
towards the intersection of El Encanto Drive and Mayfield Lane. A table listing all design
information for the pond, forebay, and outfall structure can be found with the drainage
calculations in Appendix D.

In an overflow event, the full spectrum pond runoff shall be conveyed through a rip rap
armored spillway to the grass-lined swale located to the Northeast of the pond. The
pond also provides a 1 foot freeboard to ensure the pond overtops at the spillway per
CSDCM Standards.

Please see drainage calculations (Appendix D) for more details.

2.6.2 – Routing

As mentioned above, developed flows shall be routed to the detention pond with a 72-
hour drain time. The flows will be conveyed via roof drains, underdrains, and overland
Denver Office: Colorado Springs Office:
3461 Ringsby Court, #125 2727 N. Cascade #160
Denver, Colorado 80216 Colorado Springs, CO 80907
720.413.9691 719.231.3959
AltitudeLandCo.com
flows. The detention pond will ultimately outfall to El Encanto Drive. The calculations for
the street capacity of El Encanto Drive can be found in appendix of the drainage
report completed for Broadmoor Northstar Subdivision. Per the above mentioned
drainage report, El Encanto Drive currently has a street capacity of 59.83 cfs. Altitude
Land Consultants has conducted field work to confirm capacity as indicated in the
aforementioned drainage report is accurate in the field. It was determined by Altitude
Land Consultants that historically the street capacity demand is 81.04 cfs (12.53 cfs from
the subject site and 68.51 cfs from the offsite basin). This gives an excess demand of
21.21 cfs. The proposed full spectrum detention pond will have a maximum release rate
of 2.35 cfs, per Table 13-2 in CSDCM Vol. 1 Ch. 13 and given the site contains Type’C’
soils. Along with the runoff generated by Basin D2 & D3, this reduces the demand in the
street to 72.97 cfs, which is yields an excess of 13.14 cfs. Through on-site design, demand
on El Encanto Drive is reduced by 8.07 cfs from historic conditions. Please refer to
(Appendix D) for calculations.

Offsite flows will be routed through the site in a 2.25’ deep grass lined swale to be
located on the southern edge of the subject property. This swale includes 1.00’ of
freeboard and conveys flows to a rip rap enforced sump which will contain a headwall
and 42 inch elliptical concrete pipe. This concrete pipe will convey flows underneath
“Broadmoor Meadows View” to an additional headwall and grass lined swale. Another
set of boulders will reduce the velocity of the flows before discharging into the
intersection of Mayfield Lane & El Encanto Drive, as they historically discharge.

2.6.3 – Detention

Per Section 6.4 of the CSDCM Volume I, detention is required for new developments
larger than 1 acre. Detention has been provided for the entire subject site in the form of
a full spectrum pond located in Tract A.

Offsite flows will be conveyed to the historical discharge point at the intersection of
Mayfield Lane & EL Encanto Drive and were not incorporated into the detention pond
design, per CSDSM offsite flow bypass standards.

2.7 – Compliance with Master and Development Plans

To our knowledge, the proposed development does not conflict with any existing
master plans or development plans of parcels surrounding the proposed development.

2.8 – Adequacy of Outfall Facility

Per our calculations, located in (Appendix D) and shown on the Drainage plan
(Appendix E), Altitude Land Consultants, Inc. has found that the developed runoff
generated by our site will increase from 12.53 cfs to 18.16 cfs for the 100-year storm
event. This is a total increase of 5.63 cfs.

Denver Office: Colorado Springs Office:
3461 Ringsby Court, #125 2727 N. Cascade #160
Denver, Colorado 80216 Colorado Springs, CO 80907
720.413.9691 719.231.3959
AltitudeLandCo.com
Following the 4 Step Process outlined CSDCM Vol. 2., Step 1 was addressed by using the
low impact development (LID) practice of using grass lined swales instead of a curb
and gutter system. A porous paver system was also incorporated into the design in
effort to reduce the amount of asphalt for a majority of the private North-South road to
be known as “Waterfield View”.

Step 1 is followed by the proposed full spectrum pond, which will provide detention as
well as water quality treatment for the majority of the site, also satisfying Step 2 of the 4
Step Process. The grass lined swale on the south side of “Broadmoor Meadows View”
which conveys flows to this facility will allow infiltration providing additional water quality
treatment. The porous paver system will provide water quality and storage to flows not
captured within the swale or pond (Basin D3). A grass lined swale located on the
Southern property line will be used to convey offsite flows through the site which will
provide minor water quality in the form of infiltration.

Step 2 will be followed by Step 3 which includes protecting existing drainage ways and
reinforcing areas of high flow. There were no established drainage ways located on the
subject site previously. The proposed drainage swale located on the South side of
“Broadmoor Meadows View” will convey flows generated on-site, while the proposed
drainage swale on the south edge of the subject property will convey offsite flows.
Riprap and concrete headwalls will be placed at all inlets and outfalls of the drainage
infrastructure for stabilization and erosion control. The offsite swale to the North of the
intersection of “Broadmoor Meadows View & Waterfield View” will have boulders to
provide additional protection for high flows. The outfall for the pond will also have a
concrete headwall and riprap rundown. In the event the pond is overtopped, the
spillway is reinforced with rip rap to prevent erosion.

The final step of the 4 Step Process will be addressed through the maintenance of the
proposed full spectrum pond, porous paver system, and grass lined swales by the
Broadmoor Meadows HOA. Standards and equations within the Colorado Springs
Drainage Criteria Manual (CSDCM) were used to design the pond and features
including the forebay, trickle channel, and outfall structure. The outfall structure located
at the Northeast corner of the full spectrum pond will control on-site flows onto El
Encanto Drive. Offsite flows are conveyed to the Northeast corner of the site as well via
concrete flumes and a concrete elliptical pipe. The outfall structure includes a micro
pool with an orifice plate which has been designed to treat for the required EURV.

Based on the drainage report for Broadmoor Northgate, the existing storm inlets in Sierra
Vista Drive have a combined capacity to carry 98.6 cfs. Two additional inlets were
installed in the aforementioned subdivision, bringing the total capacity for inlets in El
Encanto Drive to 126.6 cfs. The historic demand on these inlets was determined using
the historic offsite basin determined within this report, as well as the approved drainage
report completed by JR Engineering, LTD. for the Broadmoor Northstar Subdivision
dated May 1987, using values found on page 25. The SCS hydraulic calculation
method was used by JR Engineering, LTD. for their drainage calculations and report.
Denver Office: Colorado Springs Office:
3461 Ringsby Court, #125 2727 N. Cascade #160
Denver, Colorado 80216 Colorado Springs, CO 80907
720.413.9691 719.231.3959
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Accounting for the additional demand generated from the proposed development it
was determined that the total demand on these inlets would be 108.97 cfs. Therefore,
existing storm infrastructure in El Encanto has adequate capacity to handle the
increase in runoff. Calculations showing the existing storm infrastructure has adequate
capacity to handle the additional flows can be found in (Appendix D) of this report.

The full spectrum detention pond has a maximum release rate of 2.35 cfs per Table 13-2
in CSDCM Vol. 1 Ch 13 given the site contains Type ’C’ soils. Together with the flows
generated from Basins D2 & D3, the total release rate for this proposed development
will be 8.07 cfs less than in historic conditions.

2.9 – Public and Private Maintenance of Facilities

All responsibility for maintenance of private facilities located on Archer Park Subdivision
shall be the responsibilityof Broadmoor Meadows HOA. This includes but is not limited to;
rock swales, storm sewer, grass swales, and the detention pond.

There are no public facilities proposed for this development.

3.0 – DRAINAGE BASINS AND SUB-BASINS:

3.1 – Regulations:

The proposed project is being designed to conform to the design criteria as discussed in
the City of Colorado Springs Drainage Criteria Manual. The proposed development will
increase the amount of storm runoff from the subject property by 4.89 cfs for the 100-
year storm event. Developed on-site flows will be conveyed to the proposed facilities
and released at a maximum rate of 2.35 cfs per Table 13-2 in CSDCM Vol. 1 Ch. 13.
Together with the flows generated from Basins D2 & D3, the total release rate for this
proposed development will be 4.46 cfs. Which is 8.07 cfs less than in historic conditions.
Flows will discharge into the intersection of Mayfield Lane & El Encanto Drive and are
conveyed to existing storm infrastructure via the historical route for the site. Which was
determined to have capacity for the additional flows. Altitude Land Consultants
confirmed through calculations that the existing storm infrastructure has adequate
capacity to capture the developed flows. See the Drainage Calculations (Appendix D)
for the outfall capacity calculations. Altitude Land Consultants did find that El Encanto
Drive historically does not have capacity to convey runoff within the street. Therefore,
since this an existing condition and that this is a public roadway, Altitude Land
Consultants does not propose any public improvements but through on-site design,
demand on El Encanto Drive will be reduced from the historic conditions.

3.2 – Development Criteria Reference and Constraints:

The drainage report for adjacent parcel, Broadmoor Northstar prepared by JR
Engineering, LTD., dated May 1987 establishes that historically all storm runoff for the
Denver Office: Colorado Springs Office:
3461 Ringsby Court, #125 2727 N. Cascade #160
Denver, Colorado 80216 Colorado Springs, CO 80907
720.413.9691 719.231.3959
AltitudeLandCo.com