HomeMy WebLinkAboutPLANETSCALE LIVING - PDP200015 - SUBMITTAL DOCUMENTS - ROUND 2 - DRAINAGE REPORT
January 15, 2021
City of Fort Collins
Public Works and Development (Stormwater Department)
201 Laporte Ave,
Fort Collins, CO 80521
RE: Drainage Letter – Low Impact Development
Project: 2020-006 (TT) Serdar Badem, 906 E Stuart St, Fort Collins, CO 80525
To Whom it may Concern:
This Drainage Letter is for analysis of on-site drainage for Low Impact Development for Project 2020-006
(TT) Serdar Badem, 906 E Stuart St, Fort Collins, CO 80525. The project is in Fort Collins, Colorado and will
entail converting (remodeling) an existing childcare center into an affordable housing 4-Plex. The site
(parcel #9724106924) is located 850 feet west of S. Lemay Avenue and 1,200 feet south of E. Prospect
Road. Access is taken from E. Stuart Street directly to the south. The property is within the Low Density
Mixed-Use Neighborhood District (L-M-N) zone district and is subject to Administrative (Type 1) Review.
The existing drainage on site has no drainage structures and minimal surface flow (on-site or off-site).
Most of the existing site (Figure 1) consists of Concrete access and driveway; most of the southern portion
of the site, the concrete footprint of building, concrete patio and the remaining consists of
grass/dirt/gravel mixture with trees. The existing site is flat with minimal to zero slopes throughout the
site. To the east there is an HOA Easement that parallels the site and Spring Meadows Court. The HOA
will not allow any construction and/or stormwater work within this area. Because of this limitation, the
project will be implementing Low Impact Development drainage solutions using City of Fort Collins
Stormwater Criteria Manual – Chapter 7, Section 6.0 and Appendix C – LID Implementation Manual.
Because of the inability to go of site with any of the on-site run-offs; the project will design LID solutions
that will improve the entire site.
Table 1 Below shows that around forty-two percent of the existing site is impervious (Areas 2 & 4) and the
remaining pervious areas (1 & 3) are limited because of no positive slope and random native material
mixed with dirt, grass, gravel and rocks. The proposed solution will be to implement Pavers and
Landscape Beds/Areas throughout most of the site, which will improve and be below the 1000 s.f.
requirement for LID water quality requirement (Table 1). No LID water quality requirement will be needed
for this site.
Table 1
Figures 1 (existing) and Figure 2 (Proposed) [correspond with Table 1]
Figure 1 - Existing
Figure 2 – Proposed
MAJOR DRAINAGE BASINS & SUB-BASINS
A. The site is not within a Floodplain; the site does not lie within the Regulatory Floodway Zone. (All
new construction is outside the floodway boundaries). There are no storm drainage structures
located in the property. All flows inflow and outflow are surface. The main site is made up of
existing building, parking and access road south front which is concrete. The northern/western
portion of the existing site is native grass / dirt / gravel. The existing southern access drive and
eastern concrete areas are standing water and minimal surface flows to eastern HOA easement
towards Spring Meadows Court. This project will not be allowed to construct any drainage
structures or grading within the HOA Easement.
B. Drainage analysis – Mile High Flood District: Rational Method v.2 was used (spreadsheet-based
calculations for surface, %I runoff).
C. The proposed site is with an Urban Drainage Area. The site is LID and all drainage and runoff with
be onsite controlled with Pavers and landscaped control areas. The proposed site will match
exiting grades and historic flows.
D. Basins & Sub-Basins:
BASIN 1 – (0.25 Acres) Existing Site with existing structure; full area.
Sub-basin 1a - (0.06 Acres, %I 0.10) This area will remain with no change to grade or
surface. Existing area with no construction.
Sub-basin 1b - (0.10 Acres, %I .40) New parking area with permeable pavers. No
change in grade or historic.
Sub-basin 1r - (0.07 Acres, %I .95) Proposed building and boundary. Foundation
remains. No change in FF or grades around building. Matching grades and flows
away from structure.
Sub-basin 1c - (0.02 Acres, %I 0.10) Proposed area in front of building. No change in
grade (match existing from building). Removed and re-aligned new sidewalk.
Landscape area.
Sub-basin 1d - (0.03 Acres, %I .10) New proposed detached sidewalk (adjusted from
existing front area. No change in grade. Proposed access moved to eastern edge.
FINISH FLOOR ELEVATIONS
The Finish Floors have been established to set at 4980.75. Proposed site will match flow patterns
and drain as per existing conditions.
FLOOD PLAINS
The site is not within a Floodplain; There is no documented history of flooding.
MAINTENANCE PLAN:
Owner will complete a monthly maintenance inspection once a month to clean and remove any
trash, debris and vegetation that is restricting or not allowing pavers and/or landscape areas to
drain. This will include any mowing or trimming of landscaped areas. Any repairs to the grading
because of washout and/or sedimentation will be removed.
CONCLUSIONS
Proposed construction will have minimal to no impact to the existing flows onsite / offsite. The
design drainage will provide measures that protect public health, safety, and the general welfare
of the proposed site and surrounding areas; plus have no impacts on public rights-of-way or
offsite properties.
The proposed project includes 816.48 sf (<1000 s.f.) of added/modified impervious area. The project will
treat 48.30% of this area (5854.14 sf) using permeable pavers and run-on to the pavers. The proposed
project provides 7.86% of new impervious area as permeable pavers (1520 sf) located in parking areas
within the site. The project will treat new impervious area using a combination of LID treatments (both
permeable pavers and landscape beds/areas). A summary LID Compliance Table is provided below.
Summary
In summary, the proposed project incorporates permeable pavers as the primary component of
stormwater LID treatment. The proposed solution will be to implement Pavers and Landscape Beds/Areas
throughout most of the site, which will improve and be below the 1000 s.f. requirement for LID water
quality requirement (Table 1). No LID water quality requirement will be needed for this site.
This project and memo are in compliance with the City of Fort Collins Stormwater Criteria Manual
(December 2018) and associated references to the latest version of the Urban Drainage and Flood Control
District Manual.
If you have any questions regarding this submittal, please do not hesitate to contact me by email at
apowell@pec-inc.co or by phone at (720) 409-2454. Thank you.
Adam A. Powell, PE
PEC Inc
(From Fort Collins Stormwater Criteria Manual - 6.0 Low Impact Development)
This Section of this Chapter presents information that is specific to the City of Fort Collins and may be a
significant deviation from the information presented in the UDFCD Manuals. Utilizing UDFCD
methodologies for Low Impact Development (LID) designs may not be accepted by FCU.
In February 2013, Fort Collins City Council adopted Ordinance No. 152, 2012, to incorporate provisions
implementing LID principles; with the goal to declare that the purpose of the City Stormwater Utility is
to provide an integrated, sustainable stormwater management program that reflects the community’s
values of protecting and restoring the City’s watersheds. This was subsequently modified and updated in
January 2016 with Ordinance No. 007, 2016 to allow for some added flexibility in the implementation of
the LID policy.
Reference: Both the initial LID ordinance, Ordinance No. 152, 2012, and the subsequent
ordinance, Ordinance No. 007, 2016, can be found on the City of Fort Collins website.
LID is simply defined as an integrated, sustainable stormwater management program that requires a
distributed, closer to the source stormwater runoff control that simulates natural processes and relies
mainly on filtration and infiltration to locally treat and manage stormwater runoff.
Integration of LID systems into the drainage design is required for all development projects in order to
comply with the City’s policies on LID, the requirements of this Manual, the City Code and the Land Use
Code. LID systems provide a higher degree of stormwater quality treatment than that provided with
standard water quality design. The implementation of LID systems requires one of the following two
options:
1) 50% of the newly added or modified impervious area must be treated by LID techniques and
25% of new paved (vehicle use) areas must be pervious.
2) 75% of all newly added or modified impervious area must be treated by LID techniques.
Impervious surfaces are defined as hardscape surfaces that do not allow stormwater to infiltrate into
the ground. Impervious surfaces include asphalt and concrete surfaces, concrete curbs, gutters,
sidewalks, patios and rooftops. (Impervious surface areas must be assumed for single family residential
lots when overall impervious areas are being determined for residential developments. The assumed
areas must then be included in LID calculations.) “Added” impervious area stated in the two options
above is further defined as existing vegetation (or pervious) areas becoming hardscape (or impervious)
areas. “Modified” impervious area stated in the two options above is further defined as existing
impervious areas on an existing site being removed and replaced with other impervious surfaces through
a redevelopment process (i.e. existing asphalt surface becoming a rooftop surface). Mill and overlay of
asphalt areas is not considered a “modified” impervious area. “Paved” areas, as stated in option 1 above
are generally considered to be private vehicle use areas only.
Reference: Refer to the City of Fort Collins LID Implementation Manual in Appendix C for
detailed information and requirements on LID systems.
(From Fort Collins Stormwater Criteria Manual - 6.2 Permeable Pavement)
The term “permeable pavement” is a general term to describe any one of several pavement systems that
allow infiltration of water into the layers below the pavement through openings within the pavement
surface. Use of permeable pavements is an accepted Low Impact Development (LID) practice in Fort
Collins and is often used in combination with other BMPs to provide full treatment and slow release of the
WQCV. In addition, there are some installations in Fort Collins that have also been designed with an outlet
control and increased depth of aggregate material in order to provide quantity detention in excess of the
water quality (80th percentile) storm event. Design considerations for permeable pavement systems are
presented in the LID Implementation Manual, included in Appendix C.
Included here are some general design requirements applicable for all types of LID system designs in
Fort Collins.
• Overall added or modified impervious areas that amount to less than 1000 square feet (<
1000 sf) on a site will not require LID system treatment for water quality.
• For development sites that are adding or modifying 1000 square feet of imperviousness or
more (≥ 1000 sf) are required to implement LID system treatment at the site. The LID system
is allowed to treat existing imperviousness in exchange for the newly added imperviousness
if the surface character is similar (e.g. existing pavement may be treated in lieu of newly
added rooftop)
• For single-family residential developments, LID must be placed in tracts or common areas
for ownership and maintenance by the HOA. LID systems installed as part of the
development requirement shall not be placed on single-family lots.
• LID is not required for private, single-family residential improvement projects that are not a
part of a larger subdivision project. (i.e. an existing lot in an older part of Fort Collins that is
being re-built)
APPENDIX A
(VICINITY MAP)
APPENDIX B
(N.O.A.A. ATLAS 14)
APPENDIX C
APPENDIX D
GRADING AND DRAINAGE PLAN