HomeMy WebLinkAboutVILLAS ON MAIN - FDP240006 - SUBMITTAL DOCUMENTS - ROUND 1 - Drainage Related DocumentPreliminary Drainage & Sediment / Erosion Control Report
Single Family Residential Project
Fort Collins, Colorado
VILLAS ON MAIN
Prepared for:
Robert Long
PO BOX 1936
Fort Collins, CO 80522
970-227-1633
Rlong@pro-value.net
Prepared By:
Fetzer Engineering, LLC
Attention: Shawn Fetzer, P.E.
308 Jewel Ct
Fort Collins, CO 80525
970.702.4884
Sfetzerfetzerengineering.com
Prepared: January 21, 2024
Revised:
Revised:
Project no.: ALTAVISTA
Simplified
Drainage report
January 21, 2024
Table of Contents:
1. Cover Letter and Certification
2. General Location and Existing Site Information
a. Location
b. Description of Property
3. Master Drainage Basin Description
4. Floodplain Information
5. Project Description
6. Proposed Drainage Facilities
a. Major Basin Description
b. Sub-basin Description
7. Drainage Design Criteria
a. Regulations
b. Development Criteria Reference and Constraints
c. Hydrological Criteria
d. Street Capacity
e. Hydraulic Criteria
8. Variance Requests
9. Erosion Control
10. Conclusion
11. Appendices
Appendix A – Location Map
Appendix B – Soils Report
Appendix C – Hydrologic & Hydraulic Calculation
Appendix D – Northfield Final Drainage Report
Appendix E – FEMA Floodplain Map
Appendix F – Drainage Plan
Appendix G – Sediment & Erosion Control Plan
January 21, 2024
Fort Collins Utilities
700 Wood Street
Fort Collins, CO 80522
RE: Final Drainage Report for Alta Vista Residential Development
Dear Bob,
I am pleased to submit for your review, the Final Drainage Report for the Villas on Main
subdivision (parcel 9701400032). This report describes the drainage design intent to be in
accordance with the criteria in the City of Fort Collins Storm Drainage Manual.
I appreciate your time and consideration in reviewing this submittal. Please call if you have
any questions.
Sincerely,
Shawn Fetzer, P.E.
Engineering Services Certifications
The report for the drainage design of Single Family Residencial Lots at Parcel 9701400032 was
prepared by me (or under my supervision) for the owners thereof and meet or exceed the
criteria in the Fort Collins Stormwater Design Standards.
.
Signature:_____________________
Shawn Fetzer
Registered Professional Engineer
State Of Colorado No. 49791
1. General Location and Description
a. Location
Parcel No. 9701400032 is located in the Southeast Quarter of Section 1, Township 7 North, Range 67
West of the Sixth Principal Meridian, City of Fort Collins, County of Larimer, within the State of
Colorado. More specifically, is located north and east of the Alta Vista Subdivision, west of
N. Lemay Avenue, and southeast of the Lake Canal irrigation ditch. A vicinity map is included in Appendix
A.
b. Description of Property
The project site is approximately 0.65 acres and is currently undeveloped grassland. The site appears to
be vegetated with grasses overgrowing on the site. The site generally slopes from the west to the east at
about 0 - 0.5% slope. There are no existing surface improvements currently on the property. This site
consists of Loveland clay loam and Nunn clay loam and are classified as Type C hydrologic group. More
details are provided in Appendix B.
2. Master Plan Drainage Basin
a. This site is located within the City’s Dry Creek Drainage Basin. In addition to the City of Fort
Collins Stormwater Design Standards, drainage requirements are also described in the “North
East College Corridor Outfall (NECCO) Design Report”, prepared by Ayres Associates, dated
August 2009. The improvements associated with Northfield are located within basin
114 as delineated in the above report. This report has been referred to in the Northfield Drainage Report
and has been included in Appendix D.
3. Floodplain Information
a. The site is within “Flood Insurance Rate Map (FIRM) – Panel No. 08069C0977G” by the Federal
Emergency Management Agency, dated June 17, 2008. The FIRM shows that all the site is outside the
100yr floodplain but inside the 500 yr floodplain. This floodplain information is included in Appendix E.
4. Project Description
a. Project includes subdividing this lot into 4 separate lots. A single-family residential structure is planned
for each lot. The site will be raised so that flows from the lots can be conveyed to the north to Northfield
Pond D and to the south to an existing inlet in Main St which also flows via concrete pipe north to the
Northfield Pond D. See the Drainage Plan in Appendix F for more information.
5. Proposed Drainage Facilities
a. Drainage Basin Description
This site is located within the Dry Creek Drainage basin.
Sub-basin Description – Historically, the entire site basin drains to the east towards Lindenmeier Rd with
existing shallow slopes of less than 0.5%.
i. Basin E1 – Entire Site (Area = 0.61 AC)
- The existing site is currently vacant with native grasses and plants covering most of the
site. Existing slopes are 0-1% with no clear outfall. In larger storm events the site will
outfall to the east and south to Main St. Existing basin 2-yr C-coefficient is 0.04 and
generates 0.04 cfs of stormwater. Existing basin 100yr C-coefficient is 0.19 and produces
0.64 cfs of stormwater.
A more detailed breakdown of the runoff flows and calculations is available in Appendix C
b. Offsite Flows: There are no offsite flow enter the site
6. Drainage Design Criteria
This final report was prepared to meet or exceed the City of Fort Collins storm water criteria.
The City of Fort Collin’s Storm Drainage Design Criteria and amendments to the Urban Drainage
Flood Control District’s (UDFCD) Drainage Criteria Manual (USDCM) Volumes 1, 2 and 3 were
referenced as guidelines for this design.
a. Hydrologic Criteria
The rational method was performed to calculate the peak runoff rates for each basin. Percent
imperviousness and runoff coefficients were calculated for each basin using the surface characteristics of
each basin. The time of concentration was calculated using City of Fort Collins initial time of
concentration and the intensity was calculated using the corresponding storm rainfall depth and the City
of Fort Collins’ Intensity-Duration-Frequency Curve.
b. Hydraulic Criteria
This residential site storm drainage system has been designed to convey the minor and major storm
events through the combination of streets, inlets, storm sewer pipes, and pans. Per the
requirements provided by the City of Fort Collins Storm Drainage Design Criteria, all inlets and
storm pipes have been designed such that, during the minor initial storm event, the 2-yr storm, the flows
for the Local Street do not top the curb and may only rise to the street crown elevation, and for an
Arterial (with Median) do not top the curb and the flow spread will leave at least one 12 ft lane width
free of water in each direction. During the major storm event, the 100-yr storm, the water depth for the
Local Street is held to a maximum depth of 6” at the street crown, 12” at the gutter flow line, and/or flow
must be contained within right-of-way or easements paralleling the right-of-way. The Arterial (with
Median) is held to a maximum depth that will not exceed the median gutter elevation, 12” at the gutter
flowline, and/or flow must be contained within right-of-way or easements paralleling the right-of-way.
7. Drainage Facility Design
a. General Concept – As mentioned, the proposed site work includes subdividing this single lot into 4
separate lots with one residential structure on each site. In addition to the residential structure, each lot
will have a concrete drive. Main St will be reconstructed and upgraded to include curb, gutter and
sidewalk with an inverted crown and a pan in the middle of the road flowing to an existing inlet. The site
on which each proposed structure sits will be raised to above the elevation of the sidewalk and the
adjacent detention pond. The site will then drain either towards Main St or towards the Northfield Pond
D. general site grading does not significantly change with these proposed improvements and drainage
patterns remain the unchanged.
b. Sub-basin Description -
i. Basin P1 – Northern Area of the site (Area = 0.48 AC)
- This proposed basin includes the northern portion of the proposed lots. The site is
raised at a level that aids in routing storm flows towards Northfield Pond D. Swales are
proposed between the lots to help route flows from the high point and then to the
north. Proposed basin 2-yr C-coefficient is 0.38 and generates 0.53 cfs of stormwater.
Existing basin 100yr C-coefficient is 0.50 and produces 2.39 cfs of stormwater.
ii. Basin P2 – Southern Area of the site (Area = 0.13 AC)
- This proposed basin includes the southern portion of the proposed lots. The site is
raised at a level that aids in routing storm flows towards Main Street. Swales are
proposed between the lots to help route flows from the high point and then to the
south. Proposed basin 2-yr C-coefficient is 0.07 and generates 0.03 cfs of stormwater.
Existing basin 100yr C-coefficient is 0.10 and produces 0.13 cfs of stormwater.
iii. Site Detention
- With an authorized release rate equal to the existing site 100 yr flow rate and with the
increased imperviousness of the proposed site the require detention is 3571 cf. Pond D
of the Northfield subdivision has 1.46’ of additional volume available according to the
final drainage report for Northfield. With the given area of the pond, the additional
height of detained stormflows from this proposed development equals ¾”. By
subtracting this from the overall additional volume the pond, there will be 1.40’ of
additional volume still available in the Northfield Pond D.
8. Stormwater Quality and Sediment/Erosion Control
a. Stormwater Quality – Larimer County is an operator of a Phase II Regulated Small Municipal Separate
Storm Sewer System (MS4), Larimer County is required by the State of Colorado to be certified to
discharge stormwater under the general permit (COR090000). As part of the certification, control
measures need to be implemented within the County's MS4 permit coverage area. Since this site mainly
flows to the detention area to the north, the required water quality volume will be met. In addition,
Impervious areas will be disconnected as much as possible to help facilitate infiltration prior to entering
the pond.
b. Sediment/Erosion – Several erosion control methods will be utilized during and after construction. Initial
setup will require the installation of silt fence along the perimeter of the site, and the placement of
vehicle tracking pad at construction entrance. After completion of over lot grading, temporary seeding
will be placed on the ground in areas that will be exposed for over 30 days. See Sedimentation and
Erosion Control Plans in Appendix G of this report as well as the construction plans for more information.
c. Inspection Schedule: An assigned Erosion Control Inspector shall inspect with the Superintendent and the
Engineer (or their designated representatives) the stormwater management system at least every seven
calendar days. Post-storm event inspections shall be conducted within 24 hours after the end of any
precipitation or snow melt event that may cause surface erosion. If no construction activities will occur
following a storm event, post-storm event inspections shall be conducted prior to commencing
construction activities, but no later than 72 hours following the storm event.
9. Drainage Conclusions - The drainage design for project site is in general compliance with the Fort Collins
Stormwater Design Standards as well as the Urban Drainage and Flood Control District Criteria Manual.
Appendix A
Location Map
Location Map
1000 ft
N
➤➤
N
Appendix B
Soils Report
United States
Department of
Agriculture
A product of the National
Cooperative Soil Survey,
a joint effort of the United
States Department of
Agriculture and other
Federal agencies, State
agencies including the
Agricultural Experiment
Stations, and local
participants
Custom Soil Resource
Report for
Larimer County
Area, Colorado
Alta Vista Residential
Natural
Resources
Conservation
Service
October 9, 2023
Preface
Soil surveys contain information that affects land use planning in survey areas.
They highlight soil limitations that affect various land uses and provide information
about the properties of the soils in the survey areas. Soil surveys are designed for
many different users, including farmers, ranchers, foresters, agronomists, urban
planners, community officials, engineers, developers, builders, and home buyers.
Also, conservationists, teachers, students, and specialists in recreation, waste
disposal, and pollution control can use the surveys to help them understand,
protect, or enhance the environment.
Various land use regulations of Federal, State, and local governments may impose
special restrictions on land use or land treatment. Soil surveys identify soil
properties that are used in making various land use or land treatment decisions.
The information is intended to help the land users identify and reduce the effects of
soil limitations on various land uses. The landowner or user is responsible for
identifying and complying with existing laws and regulations.
Although soil survey information can be used for general farm, local, and wider area
planning, onsite investigation is needed to supplement this information in some
cases. Examples include soil quality assessments (http://www.nrcs.usda.gov/wps/
portal/nrcs/main/soils/health/) and certain conservation and engineering
applications. For more detailed information, contact your local USDA Service Center
(https://offices.sc.egov.usda.gov/locator/app?agency=nrcs) or your NRCS State Soil
Scientist (http://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/contactus/?
cid=nrcs142p2_053951).
Great differences in soil properties can occur within short distances. Some soils are
seasonally wet or subject to flooding. Some are too unstable to be used as a
foundation for buildings or roads. Clayey or wet soils are poorly suited to use as
septic tank absorption fields. A high water table makes a soil poorly suited to
basements or underground installations.
The National Cooperative Soil Survey is a joint effort of the United States
Department of Agriculture and other Federal agencies, State agencies including the
Agricultural Experiment Stations, and local agencies. The Natural Resources
Conservation Service (NRCS) has leadership for the Federal part of the National
Cooperative Soil Survey.
Information about soils is updated periodically. Updated information is available
through the NRCS Web Soil Survey, the site for official soil survey information.
The U.S. Department of Agriculture (USDA) prohibits discrimination in all its
programs and activities on the basis of race, color, national origin, age, disability,
and where applicable, sex, marital status, familial status, parental status, religion,
sexual orientation, genetic information, political beliefs, reprisal, or because all or a
part of an individual's income is derived from any public assistance program. (Not
all prohibited bases apply to all programs.) Persons with disabilities who require
2
alternative means for communication of program information (Braille, large print,
audiotape, etc.) should contact USDA's TARGET Center at (202) 720-2600 (voice
and TDD). To file a complaint of discrimination, write to USDA, Director, Office of
Civil Rights, 1400 Independence Avenue, S.W., Washington, D.C. 20250-9410 or
call (800) 795-3272 (voice) or (202) 720-6382 (TDD). USDA is an equal opportunity
provider and employer.
3
Contents
Preface....................................................................................................................2
Soil Map..................................................................................................................5
Soil Map................................................................................................................6
Legend..................................................................................................................7
Map Unit Legend..................................................................................................8
Map Unit Descriptions..........................................................................................8
Larimer County Area, Colorado......................................................................10
76—Nunn clay loam, wet, 1 to 3 percent slopes.........................................10
References............................................................................................................12
4
Soil Map
The soil map section includes the soil map for the defined area of interest, a list of
soil map units on the map and extent of each map unit, and cartographic symbols
displayed on the map. Also presented are various metadata about data used to
produce the map, and a description of each soil map unit.
5
6
Custom Soil Resource Report
Soil Map
44
9
4
1
1
0
44
9
4
1
2
0
44
9
4
1
3
0
44
9
4
1
4
0
44
9
4
1
5
0
44
9
4
1
6
0
44
9
4
1
7
0
44
9
4
1
8
0
44
9
4
1
1
0
44
9
4
1
2
0
44
9
4
1
3
0
44
9
4
1
4
0
44
9
4
1
5
0
44
9
4
1
6
0
44
9
4
1
7
0
44
9
4
1
8
0
495020 495030 495040 495050 495060 495070 495080 495090 495100 495110 495120
495020 495030 495040 495050 495060 495070 495080 495090 495100 495110 495120
40° 35' 54'' N
10
5
°
3
'
3
2
'
'
W
40° 35' 54'' N
10
5
°
3
'
2
7
'
'
W
40° 35' 51'' N
10
5
°
3
'
3
2
'
'
W
40° 35' 51'' N
10
5
°
3
'
2
7
'
'
W
N
Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 13N WGS84
0 20 40 80 120Feet
0 5 10 20 30Meters
Map Scale: 1:508 if printed on A landscape (11" x 8.5") sheet.
Soil Map may not be valid at this scale.
MAP LEGEND MAP INFORMATION
Area of Interest (AOI)
Area of Interest (AOI)
Soils
Soil Map Unit Polygons
Soil Map Unit Lines
Soil Map Unit Points
Special Point Features
Blowout
Borrow Pit
Clay Spot
Closed Depression
Gravel Pit
Gravelly Spot
Landfill
Lava Flow
Marsh or swamp
Mine or Quarry
Miscellaneous Water
Perennial Water
Rock Outcrop
Saline Spot
Sandy Spot
Severely Eroded Spot
Sinkhole
Slide or Slip
Sodic Spot
Spoil Area
Stony Spot
Very Stony Spot
Wet Spot
Other
Special Line Features
Water Features
Streams and Canals
Transportation
Rails
Interstate Highways
US Routes
Major Roads
Local Roads
Background
Aerial Photography
The soil surveys that comprise your AOI were mapped at
1:24,000.
Warning: Soil Map may not be valid at this scale.
Enlargement of maps beyond the scale of mapping can cause
misunderstanding of the detail of mapping and accuracy of soil
line placement. The maps do not show the small areas of
contrasting soils that could have been shown at a more detailed
scale.
Please rely on the bar scale on each map sheet for map
measurements.
Source of Map: Natural Resources Conservation Service
Web Soil Survey URL:
Coordinate System: Web Mercator (EPSG:3857)
Maps from the Web Soil Survey are based on the Web Mercator
projection, which preserves direction and shape but distorts
distance and area. A projection that preserves area, such as the
Albers equal-area conic projection, should be used if more
accurate calculations of distance or area are required.
This product is generated from the USDA-NRCS certified data as
of the version date(s) listed below.
Soil Survey Area: Larimer County Area, Colorado
Survey Area Data: Version 18, Aug 24, 2023
Soil map units are labeled (as space allows) for map scales
1:50,000 or larger.
Date(s) aerial images were photographed: Jul 2, 2021—Aug 25,
2021
The orthophoto or other base map on which the soil lines were
compiled and digitized probably differs from the background
imagery displayed on these maps. As a result, some minor
shifting of map unit boundaries may be evident.
Custom Soil Resource Report
7
Map Unit Legend
Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI
76 Nunn clay loam, wet, 1 to 3
percent slopes
0.8 100.0%
Totals for Area of Interest 0.8 100.0%
Map Unit Descriptions
The map units delineated on the detailed soil maps in a soil survey represent the
soils or miscellaneous areas in the survey area. The map unit descriptions, along
with the maps, can be used to determine the composition and properties of a unit.
A map unit delineation on a soil map represents an area dominated by one or more
major kinds of soil or miscellaneous areas. A map unit is identified and named
according to the taxonomic classification of the dominant soils. Within a taxonomic
class there are precisely defined limits for the properties of the soils. On the
landscape, however, the soils are natural phenomena, and they have the
characteristic variability of all natural phenomena. Thus, the range of some
observed properties may extend beyond the limits defined for a taxonomic class.
Areas of soils of a single taxonomic class rarely, if ever, can be mapped without
including areas of other taxonomic classes. Consequently, every map unit is made
up of the soils or miscellaneous areas for which it is named and some minor
components that belong to taxonomic classes other than those of the major soils.
Most minor soils have properties similar to those of the dominant soil or soils in the
map unit, and thus they do not affect use and management. These are called
noncontrasting, or similar, components. They may or may not be mentioned in a
particular map unit description. Other minor components, however, have properties
and behavioral characteristics divergent enough to affect use or to require different
management. These are called contrasting, or dissimilar, components. They
generally are in small areas and could not be mapped separately because of the
scale used. Some small areas of strongly contrasting soils or miscellaneous areas
are identified by a special symbol on the maps. If included in the database for a
given area, the contrasting minor components are identified in the map unit
descriptions along with some characteristics of each. A few areas of minor
components may not have been observed, and consequently they are not
mentioned in the descriptions, especially where the pattern was so complex that it
was impractical to make enough observations to identify all the soils and
miscellaneous areas on the landscape.
The presence of minor components in a map unit in no way diminishes the
usefulness or accuracy of the data. The objective of mapping is not to delineate
pure taxonomic classes but rather to separate the landscape into landforms or
landform segments that have similar use and management requirements. The
delineation of such segments on the map provides sufficient information for the
development of resource plans. If intensive use of small areas is planned, however,
onsite investigation is needed to define and locate the soils and miscellaneous
areas.
Custom Soil Resource Report
8
An identifying symbol precedes the map unit name in the map unit descriptions.
Each description includes general facts about the unit and gives important soil
properties and qualities.
Soils that have profiles that are almost alike make up a soil series. Except for
differences in texture of the surface layer, all the soils of a series have major
horizons that are similar in composition, thickness, and arrangement.
Soils of one series can differ in texture of the surface layer, slope, stoniness,
salinity, degree of erosion, and other characteristics that affect their use. On the
basis of such differences, a soil series is divided into soil phases. Most of the areas
shown on the detailed soil maps are phases of soil series. The name of a soil phase
commonly indicates a feature that affects use or management. For example, Alpha
silt loam, 0 to 2 percent slopes, is a phase of the Alpha series.
Some map units are made up of two or more major soils or miscellaneous areas.
These map units are complexes, associations, or undifferentiated groups.
A complex consists of two or more soils or miscellaneous areas in such an intricate
pattern or in such small areas that they cannot be shown separately on the maps.
The pattern and proportion of the soils or miscellaneous areas are somewhat similar
in all areas. Alpha-Beta complex, 0 to 6 percent slopes, is an example.
An association is made up of two or more geographically associated soils or
miscellaneous areas that are shown as one unit on the maps. Because of present
or anticipated uses of the map units in the survey area, it was not considered
practical or necessary to map the soils or miscellaneous areas separately. The
pattern and relative proportion of the soils or miscellaneous areas are somewhat
similar. Alpha-Beta association, 0 to 2 percent slopes, is an example.
An undifferentiated group is made up of two or more soils or miscellaneous areas
that could be mapped individually but are mapped as one unit because similar
interpretations can be made for use and management. The pattern and proportion
of the soils or miscellaneous areas in a mapped area are not uniform. An area can
be made up of only one of the major soils or miscellaneous areas, or it can be made
up of all of them. Alpha and Beta soils, 0 to 2 percent slopes, is an example.
Some surveys include miscellaneous areas. Such areas have little or no soil
material and support little or no vegetation. Rock outcrop is an example.
Custom Soil Resource Report
9
Larimer County Area, Colorado
76—Nunn clay loam, wet, 1 to 3 percent slopes
Map Unit Setting
National map unit symbol: jpxq
Elevation: 4,800 to 5,600 feet
Mean annual precipitation: 13 to 15 inches
Mean annual air temperature: 48 to 50 degrees F
Frost-free period: 135 to 150 days
Farmland classification: Prime farmland if irrigated
Map Unit Composition
Nunn, wet, and similar soils:90 percent
Minor components:10 percent
Estimates are based on observations, descriptions, and transects of the mapunit.
Description of Nunn, Wet
Setting
Landform:Alluvial fans, stream terraces
Landform position (three-dimensional):Base slope, tread
Down-slope shape:Linear
Across-slope shape:Linear
Parent material:Alluvium
Typical profile
H1 - 0 to 10 inches: clay loam
H2 - 10 to 47 inches: clay
H3 - 47 to 60 inches: gravelly loam
Properties and qualities
Slope:1 to 3 percent
Depth to restrictive feature:More than 80 inches
Drainage class:Somewhat poorly drained
Runoff class: Medium
Capacity of the most limiting layer to transmit water (Ksat):Moderately low to
moderately high (0.06 to 0.60 in/hr)
Depth to water table:About 24 to 36 inches
Frequency of flooding:RareNone
Frequency of ponding:None
Calcium carbonate, maximum content:10 percent
Maximum salinity:Nonsaline to very slightly saline (0.0 to 2.0 mmhos/cm)
Available water supply, 0 to 60 inches: High (about 9.9 inches)
Interpretive groups
Land capability classification (irrigated): 2w
Land capability classification (nonirrigated): 3s
Hydrologic Soil Group: C
Ecological site: R067BY038CO - Wet Meadow
Hydric soil rating: No
Minor Components
Heldt
Percent of map unit:6 percent
Custom Soil Resource Report
10
Ecological site:R067BY042CO - Clayey Plains
Hydric soil rating: No
Dacono
Percent of map unit:3 percent
Ecological site:R067BY042CO - Clayey Plains
Hydric soil rating: No
Mollic halaquepts
Percent of map unit:1 percent
Landform:Swales
Hydric soil rating: Yes
Custom Soil Resource Report
11
References
American Association of State Highway and Transportation Officials (AASHTO).
2004. Standard specifications for transportation materials and methods of sampling
and testing. 24th edition.
American Society for Testing and Materials (ASTM). 2005. Standard classification of
soils for engineering purposes. ASTM Standard D2487-00.
Cowardin, L.M., V. Carter, F.C. Golet, and E.T. LaRoe. 1979. Classification of
wetlands and deep-water habitats of the United States. U.S. Fish and Wildlife
Service FWS/OBS-79/31.
Federal Register. July 13, 1994. Changes in hydric soils of the United States.
Federal Register. September 18, 2002. Hydric soils of the United States.
Hurt, G.W., and L.M. Vasilas, editors. Version 6.0, 2006. Field indicators of hydric
soils in the United States.
National Research Council. 1995. Wetlands: Characteristics and boundaries.
Soil Survey Division Staff. 1993. Soil survey manual. Soil Conservation Service.
U.S. Department of Agriculture Handbook 18. http://www.nrcs.usda.gov/wps/portal/
nrcs/detail/national/soils/?cid=nrcs142p2_054262
Soil Survey Staff. 1999. Soil taxonomy: A basic system of soil classification for
making and interpreting soil surveys. 2nd edition. Natural Resources Conservation
Service, U.S. Department of Agriculture Handbook 436. http://
www.nrcs.usda.gov/wps/portal/nrcs/detail/national/soils/?cid=nrcs142p2_053577
Soil Survey Staff. 2010. Keys to soil taxonomy. 11th edition. U.S. Department of
Agriculture, Natural Resources Conservation Service. http://
www.nrcs.usda.gov/wps/portal/nrcs/detail/national/soils/?cid=nrcs142p2_053580
Tiner, R.W., Jr. 1985. Wetlands of Delaware. U.S. Fish and Wildlife Service and
Delaware Department of Natural Resources and Environmental Control, Wetlands
Section.
United States Army Corps of Engineers, Environmental Laboratory. 1987. Corps of
Engineers wetlands delineation manual. Waterways Experiment Station Technical
Report Y-87-1.
United States Department of Agriculture, Natural Resources Conservation Service.
National forestry manual. http://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/
home/?cid=nrcs142p2_053374
United States Department of Agriculture, Natural Resources Conservation Service.
National range and pasture handbook. http://www.nrcs.usda.gov/wps/portal/nrcs/
detail/national/landuse/rangepasture/?cid=stelprdb1043084
12
United States Department of Agriculture, Natural Resources Conservation Service.
National soil survey handbook, title 430-VI. http://www.nrcs.usda.gov/wps/portal/
nrcs/detail/soils/scientists/?cid=nrcs142p2_054242
United States Department of Agriculture, Natural Resources Conservation Service.
2006. Land resource regions and major land resource areas of the United States,
the Caribbean, and the Pacific Basin. U.S. Department of Agriculture Handbook
296. http://www.nrcs.usda.gov/wps/portal/nrcs/detail/national/soils/?
cid=nrcs142p2_053624
United States Department of Agriculture, Soil Conservation Service. 1961. Land
capability classification. U.S. Department of Agriculture Handbook 210. http://
www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_052290.pdf
Custom Soil Resource Report
13
Appendix C
Hydrologic & Hydraulic Calculations
Basin -Existing (E1)
Area of Analyzed Lot (acre) =0.61
Soil Type
(A,B,C/D)Area (SF) Area (AC) % of Site Lot Equivalent % Imperviousness C2 Weighted C2 C100 Weighted C100
Weighted %
Impervious
A 0 0.00 0.00%0.00 0.9 0.73 0.00 0.81 0.00 0.00%
A 0 0.00 0.00%0.00 1 0.84 0.00 0.89 0.00 0.00%
A 0 0.00 0.00%0.00 0.4 0.25 0.00 0.42 0.00 0.00%
A 26650 0.61 100.00%0.61 0.1 0.04 0.04 0.19 0.19 10.00%
26650 0.61 100.00%0.61 0.04 0.19 10.00%
Basin - Proposed (P1)
Area of Analyzed Lot (acre) =0.48
Soil Type
(A,B,C/D)Area (SF) Area (AC) % of Site
Lot Equivalent % Imperviousness C2 Weighted C2 C100 Weighted C100
Weighted %
Impervious
A 10400 0.24 49.35%0.24 0.9 0.73 0.36 0.81 0.40 44.41%
A 0 0.00 0.00%0.00 1 0.84 0.00 0.89 0.00 0.00%
A 0 0.00 0.00%0.00 0.4 0.25 0.00 0.42 0.00 0.00%
A 10675 0.25 50.65%0.25 0.1 0.04 0.02 0.19 0.10 5.07%
21075 0.48 100.00%0.48 0.38 0.50 49.48%
Basin - Proposed (P2)
Area of Analyzed Lot (acre) =0.13
Soil Type
(A,B,C/D)Area (SF) Area (AC) % of Site
Lot Equivalent % Imperviousness C2 Weighted C2 C100 Weighted C100
Weighted %
Impervious
A 0 0.00 0.00%0.00 0.9 0.73 0.00 0.81 0.00 0.00%
A 1580 0.04 7.50%0.04 1 0.84 0.06 0.89 0.07 7.50%
A 0 0.00 0.00%0.00 0.4 0.25 0.00 0.42 0.00 0.00%
A 3995 0.09 18.96%0.09 0.1 0.04 0.01 0.19 0.04 1.90%
5575 0.13 26.45%0.13 0.07 0.10 9.39%
Total Area Analyzed (acre) =0.61
Area (SF) Area (AC) % of Site
Lot Equivalent % Imperviousness C2 Weighted C2 C100 Weighted C100
Weighted %
Impervious
21075 0.48 79.08%0.48 49.48%0.38 0.30 0.69 0.54 39.13%
5575 0.13 20.92%0.13 9.39%0.06 0.01 0.52 0.11 1.96%
Total =26650 0.61 100.00%0.61 0.31 0.65 41.09%
Total =
Basin P1
Basin P2
Buildings
Asphalt/Concrete
Gravel
Open Area
Impervious Area / C-coefficient - Existing Basin
Buildings
Asphalt/Concrete
Gravel
Gravel
Open Area
Total =
Open Area
Total =
Impervious Area / C-coefficient - Proposed Basin
Buildings
Asphalt/Concrete
Basin No Basin No Basin No Basin No
Avg I 10.00%Avg I 49.48%Avg I 9.39%Avg I #REF!
C5 0.05 Table 6-4 C5 0.35 Table 6-4 C5 0.04 Table 6-4 C5 --- Table 6-4
Li 200 ft
Li 36 ft
Li 51 ft
Li --- ft
So 0.02 ft/ft So 0.25 ft/ft So 0.03 ft/ft So --- ft/ft
Ti 21.42 min
Ti 2.81 min
Ti 9.49 min
Ti --- min
Lt 80 ft Lt 47 ft Lt 255 ft Lt --- ft
So 0.05 ft/ft
So 0.05 ft/ft
So 0.02 ft/ft
So --- ft/ft
Vt 2.24 ft/sec Vt 2.24 ft/sec Vt 1.41421356 ft/sec Vt --- ft/sec
K 10 Table 6-2 K 10 Table 6-2 K 10 Table 6-2 K ---Table 6-2
Tt 0.60 min Tt 0.35 min Tt 3.01 min Tt --- min
Tc 22.02 mins Tc 3.16 mins Tc 12.50 mins Tc ---mins
i 10.00%i 49.48%i 9.39%i ---
St 0.005 St 0.005 St 3 St ---
Lt 350 Lt 350 Lt 306 Lt ---
Tc 32.23 Tc 22.77 Tc 24.69 Tc ---
Time of Concentration
E1 P1 P2 ---
Existing Site Proposed Site
Initial or Overland Flow Time Initial or Overland Flow Time Initial or Overland Flow Time Initial or Overland Flow Time
Channelized Flow Time Channelized Flow Time Channelized Flow Time Channelized Flow Time
Time of Concentration Time of Concentration Time of Concentration Time of Concentration
Urban Catchment Check Urban Catchment Check Urban Catchment Check Urban Catchment Check
Q = CIA (2 yr) =0.04 cfs Q = CIA (2 yr) =0.53 cfs
Q = CIA (100 yr) =0.64 cfs Q = CIA (100 yr) =2.39 cfs
C2 =0.04 C2 =0.38
C100 =0.19 C100 =0.50
I2 = 1.53 in/hr I2 = 2.85 in/hr
I100 =5.60 I100 =9.95
A=0.61 Acres A=0.48 Acres
Time of Concentration (storm duration)=22.02 mins Time of Concentration (storm duration)=3.16 mins
I2=1.53 in/hr I2=2.85 in/hr
I100=5.60 in/hr I100=9.95 in/hr
Q = CIA (2 yr) =cfs Q = CIA (2 yr) =0.03 cfs
Q = CIA (100 yr) =cfs Q = CIA (100 yr) =0.13 cfs
C2 =C2 =0.07
C100 =C100 =0.10
I2 =in/hr I2 =2.85 in/hr
I100 =I100 =9.95
A=Acres A=0.13 Acres
Time of Concentration (storm duration)=mins Time of Concentration (storm duration)=12.50 mins
I2=in/hr I2=2.85 in/hr
I100=in/hr I100=9.95 in/hr
Proposed Basin P2Existing Basin
Intensity obtained from Tabel 3.4-1 of the Fort Collins Stormwater Criteria Manual
Rational Method Calculations
Existing Basin E1 Proposed Basin P1
For Spread
n=0.04 Table 8-5
Bottom Width = 0 ft
Depth = 0.22 ft - Varable based on Q
Side Slope (Lt) = 0.1 ft/ft
Side Length = 2.21 ft
Side Slope (Rt) = 0.1 ft/ft
Side Length = 2.21 ft
Top Width = 4.40 ft
A 0.49 ft/ft
P 4.42 ft/ft
R 0.11
S=0.0221 ft/ft
Q=0.62 cfs
V=0.79 fps
Fr=0.295536
Basin P2 Trapazoidal Channel (Between Buildings) Analysis
a =1
I =41.09%decimal percent
A =0.61 acres
WQCV = 0.183 watershed-inches
V = 0.009288 Acre-Feet
404.60 CF
Site Water Quality Analysis
Cexisting**I* A (ac) Q (cfs) V (cf)
Cproposed**I* A (ac) Q (cfc) V (cf)
5 0.19 7.24 0.61 0.830 249.085 0.65 7.24 0.61 2.871 861.198
10 0.19 5.51 0.61 0.632 379.132 0.65 5.51 0.61 2.185 1310.829
15 0.19 4.55 0.61 0.522 469.615 0.65 4.55 0.61 1.804 1623.668
20 0.19 5.60 0.61 0.642 770.650 0.65 5.60 0.61 2.220 2664.480
25 0.19 4.98 0.61 0.571 856.660 0.65 4.98 0.61 1.975 2961.855
30 0.19 3.06 0.61 0.351 631.657 0.65 3.06 0.61 1.213 2183.922
35 0.19 4.08 0.61 0.468 982.578 0.65 4.08 0.61 1.618 3397.212
40 0.19 3.74 0.61 0.429 1029.368 0.65 3.74 0.61 1.483 3558.984
45 0.19 3.46 0.61 0.397 1071.341 0.65 3.46 0.61 1.372 3704.103
50 0.19 3.23 0.61 0.370 1111.249 0.65 3.23 0.61 1.281 3842.085
55 0.19 3.03 0.61 0.347 1146.685 0.65 3.03 0.61 1.201 3964.604
60 0.19 2.86 0.61 0.328 1180.745 0.65 2.86 0.61 1.134 4082.364
65 0.19 2.71 0.61 0.311 1212.053 0.65 2.71 0.61 1.075 4190.609
70 0.19 2.59 0.61 0.297 1247.489 0.65 2.59 0.61 1.027 4313.127
75 0.19 2.48 0.61 0.284 1279.829 0.65 2.48 0.61 0.983 4424.940
80 0.19 2.38 0.61 0.273 1310.104 0.65 2.38 0.61 0.944 4529.616
85 0.19 2.29 0.61 0.263 1339.348 0.65 2.29 0.61 0.908 4630.724
90 0.19 2.21 0.61 0.253 1368.591 0.65 2.21 0.61 0.876 4731.831
95 0.19 2.13 0.61 0.244 1392.330 0.65 2.13 0.61 0.845 4813.907
100 0.19 2.06 0.61 0.236 1417.445 0.65 2.06 0.61 0.817 4900.740
105 0.19 2.00 0.61 0.229 1444.968 0.65 2.00 0.61 0.793 4995.900
110 0.19 1.94 0.61 0.222 1468.363 0.65 1.94 0.61 0.769 5076.786
115 0.19 1.88 0.61 0.216 1487.629 0.65 1.88 0.61 0.745 5143.398
120 0.19 1.84 0.61 0.211 1519.281 0.65 1.84 0.61 0.730 5252.832
Site Total 3733.551Required Storage (CF)=
Modified FAA Storage Calculations (Entire Lot)
Existing Proposed
Time
Duration
100 Year 100 Year
Appendix D
Northfield Final Drainage Report
FINAL DRAINAGE REPORT
FOR
Northfield
Prepared by:
Highland Development Services
6355 Fairgrounds Ave, Suite 100
Windsor, Colorado 80550
Phone: 970.674.7550
Prepared for:
Landmark Real Estate Holdings, LLC
6341 Fairgrounds Ave, Suite 100
Windsor, Colorado 80550
Office: 970.460.0567
February 26, 2020
Job Number 18-1000-00
6355 FAIRGROUNDS AVE, SUITE 100, WINDSOR, COLORADO 80550 | PHONE 970.674.7550
February 26, 2020
Mr. Shane Boyle
Fort Collins Utilities
700 Wood Street
Fort Collins, CO 80522
RE: Final Drainage Report for Northfield
Dear Shane,
We are pleased to submit for your review, the Final Drainage Report for the Northfield
subdivision. This report describes the drainage design intent to be in accordance with the
criteria in the City of Fort Collins Storm Drainage Manual.
I appreciate your time and consideration in reviewing this submittal. Please call if you have any
questions.
Sincerely,
Jason T. Claeys, P.E., LEED AP
Highland Development Services
Page i
TABLE OF CONTENTS
Table of Contents ............................................................................................................................. i
Engineer’s Certification Block ..........................................................................................................ii
General Description and Location .................................................................................................. 1
Site Description and Location ...................................................................................................... 1
Site Soils ....................................................................................................................................... 2
Storm Drainage Criteria .................................................................................................................. 4
Hydrologic Criteria ....................................................................................................................... 4
Hydraulic Criteria ......................................................................................................................... 4
Drainage Basins ............................................................................................................................... 6
Drainage Facility Design ................................................................................................................ 13
Drainage Conveyance Design .................................................................................................... 13
Detention/Water Quality Pond Design ..................................................................................... 13
Low Impact Development ............................................................................................................. 18
Stormwater Pollution Prevention ................................................................................................. 21
Conclusions ................................................................................................................................... 22
References .................................................................................................................................... 23
Appendix ....................................................................................................................................... 24
Appendix A – Site Descriptions, Characteristics, & References .................................................. A
Appendix B – SWMM Input/Output ............................................................................................ B
Appendix C – Rational Calculations ............................................................................................. C
Appendix D – Detention Pond Calculations ................................................................................ D
Appendix E – Street Capacities & Inlet Calculations ................................................................... E
Appendix F – Low Impact Development Calculations ..................................................................F
Page ii
ENGINEER’S CERTIFICATION BLOCK
I hereby certify that this Final Drainage Report for Northfield was prepared by me (or under my
direct supervision) for the owners thereof and meets or exceeds the criteria of the City of Fort
Collins Stormwater Design Standards.
________________________________________
Jason T. Claeys, PE
Registered Professional Engineer
State of Colorado No. 42122
2/26/2020
Page 1
GENERAL DESCRIPTION AND LOCATION
SITE DESCRIPTION AND LOCATION
Northfield is located in the Southeast Quarter of Section 1, Township 7 North, Range 67 West of
the Sixth Principal Meridian, City of Fort Collins, County of Larimer, within the State of
Colorado. More specifically, Northfield is located north of the ALTA Vista Subdivision, west of
N. Lemay Avenue, and southeast of the Lake Canal irrigation ditch.
The project site is approximately 55.26 acres and is currently undeveloped agricultural land.
The site appears to be vegetated with grasses harvested for livestock feed. The site generally
slopes from the northwest to the southeast at about 0.5% slope. There are no existing surface
improvements currently on the property.
The property is proposed to be subdivided into multiple single-family lots and condominium
tracts. Northfield will consist of 139 single family attached (fee simple) units and 301
condominium units, a mixed-use building with a combination of retail and apartments, and a
community clubhouse.
Northfield is located within the City’s Dry Creek Drainage Basin. In addition to the City of Fort
Collins Stormwater Design Standards, drainage requirements are also described in the “North
East College Corridor Outfall (NECCO) Design Report”, prepared by Ayres Associates, dated
August 2009. The improvements associated with Northfield are located within basins 115, 116,
117, 414, 950, 951, 960, 961, 970, 971, 980, and 981 as delineated in the above report.
A portion of the Northfield improvements are located within the FEMA regulatory, Dry Creek
100-year Floodplain as shown on FEMA FIRM map numbers 08069C0977G (revised June 17,
2008), and 08069C0981G (revised June 17, 2008). All improvements are outside of the 100-yr
floodway, and a portion of the improvements are within the 100-yr floodplain. The first phase
of the NECCO improvements were completed early in 2017 with the remaining improvements
completed in the spring of 2018. It is anticipated that the LOMR associated with these
improvements will be completed by the late spring/early summer of 2020. Once the LOMR is
complete, the 100-yr floodplain is anticipated to be removed from the site. If circumstances
occur that the LOMR is not complete at the time of construction for the Northfield
improvements, a Floodplain Use Permit will be required. It is anticipated that the floodplain
limits will be adjusted prior to the construction of the residential units. If buildings are
constructed prior to the map revision, the buildings will be elevated above the Regulatory Base
Floodplain Elevation and adhering to Fort Collins Municipal Code Chapter 10 requirements. A
portion of the site is located within the Cache la Poudre River 500-yr floodplain. Critical facilities
Page 2
are prohibited within the 100-yr and/or 500-yr floodplains. Critical facilities include facilities for
at-risk populations (daycares, schools, nursing homes, etc) and emergency service facilities
(urgent care, hospitals, fire stations, police stations, etc).
SITE SOILS
The Northfield site consists of Loveland clay loam and Nunn clay loam and are classified as Type
C hydrologic group. According to USDA/Natural Resource Conservation Group, this soil has a
slow infiltration rate when thoroughly wet. These soils consist chiefly of soils having a layer
that impedes the downward movement of water or soils of moderately fine texture or fine
texture. These soils have a slow rate of water transmission.
Per the “Preliminary Subsurface Exploration and Groundwater Report – Schlagel Property”,
prepared be Earth Engineering Consultants (EEC), dated August 16, 2017:
Vegetation growth and topsoil was encountered at the surface of each boring. Underlying the
topsoil/vegetation layer was a zone of cohesive subsoils classified lean to fat clay with varying
amounts of sand and clayey sand to approximate depths of 2 to 5-1/2 feet. Underlying the
cohesive subsoils was a granular layer of poorly to well graded sand with silt and gravel
extending to the depths explored at approximately 14 to 21-1/2 feet or to underlying bedrock.
It appears that below the lean to fat clay, there is a granular materials layer that would exhibit
more moderate infiltration rates than the surface materials.
Groundwater was also observed during the subsurface soil exploration:
Groundwater and/or the presence of a piezometric water surface, was generally observed at
depths ranging from approximately 3 to 7 feet below ground surface as indicated on the
enclosed boring logs.
EEC monitored the groundwater level depths over a 12 month period after the original borings.
Due to the observed groundwater elevations and in consideration of the building foundations,
the site will include a permanent dewatering system that will be installed shortly after
construction activities commence. The dewatering system will consist of 8” perforated
underdrain pipes down around the east, north and west perimeters of the site. These drains
will outfall to the existing NECCO regional stormwater system. Miller Groundwater Engineering
modeled this proposed dewatering system in their groundwater study for the site. Their
analysis of the monitoring data shows that the inflow rate of groundwater to the underdrains
ranges from 200 to 500 gpm during seasonally high summer water conditions. The drains will
take several weeks to draw the groundwater table down to the elevation of the underdrain
Page 3
pipes, but once established, the flow rate from the underdrains into the stormwater system will
decrease.
Supporting figures can be found in Appendix A.
Page 4
STORM DRAINAGE CRITERIA
This final report was prepared to meet or exceed the City of Fort Collins storm water criteria.
The City of Fort Collin’s Storm Drainage Design Criteria and amendments to the Urban Drainage
Flood Control District’s (UDFCD) Drainage Criteria Manual (USDCM) Volumes 1, 2 and 3 were
referenced as guidelines for this design.
HYDROLOGIC CRITERIA
The rational method was performed to calculate the peak runoff rates for each basin. Percent
imperviousness and runoff coefficients were calculated for each basin using the surface
characteristics of each basin. The time of concentration was calculated using City of Fort Collins
initial time of concentration and the intensity was calculated using the corresponding storm
rainfall depth and the City of Fort Collins’ Intensity-Duration-Frequency Curve.
HYDRAULIC CRITERIA
The final Northfield storm drainage system has been designed to convey the minor and major
storm events through the combination of streets, inlets, storm sewer pipes, and pans. Per the
requirements provided by the City of Fort Collins Storm Drainage Design Criteria, all inlets and
storm pipes have been designed such that, during the minor initial storm event, the 2-yr storm,
the flows for the Local Street do not top the curb and may only rise to the street crown
elevation, and for an Arterial (with Median) do not top the curb and the flow spread will leave
at least one 12 ft lane width free of water in each direction. During the major storm event, the
100-yr storm, the water depth for the Local Street is held to a maximum depth of 6” at the
street crown, 12” at the gutter flow line, and/or flow must be contained within right-of-way or
easements paralleling the right-of-way. The Arterial (with Median) is held to a maximum depth
that will not exceed the median gutter elevation, 12” at the gutter flowline, and/or flow must
be contained within right-of-way or easements paralleling the right-of-way.
The peak basin runoff flows from the rational calculations were then used to determine the
street capacities, and on-grade inlet locations. The on-grade and sump inlet capacities have
been calculated using UDFCD’s spreadsheet UD-Inlet_v4.05.xlsm. The inlet capacities have
been analyzed and the necessary adjustments applied to the street runoff flows based on the
amount of runoff collected by on-grade inlets. The sump inlets have been sized using the
previous mentioned street capacity allowances with overtopping if needed.
The captured flows have been analyzed using EPA’s SWMM 5.0 program to design pipe size and
slopes. UD-Inlet applies a clogging factor while sizing the inlets, but while sizing the storm
Page 5
pipes, it is assumed that the inlets are free of clogging debris and capturing the maximum
amount of street runoff. The program accounts for head losses within manholes and bends and
head losses associated with pipe friction. Hydraulic grade line rating curves were provided by
Ayres to represent the outfall conditions along the NECCO system.
Hydraflow has been used to check the capacities of the private drive capacities throughout the
site. Freeboard is provided by confirming that the 100-yr flows with an additional 30% do not
overtop the private drive curb.
All pipe outlets are protected with buried riprap or suitable erosion protection methods. Storm
sewer pipe outlets are protected using the requirements set by the USDCM for the protection
downstream of culverts.
All supporting calculations for runoff rates, street sections, and inlets are located in the
Appendix.
Page 6
DRAINAGE BASINS
The Northfield site is divided into 5 main drainage systems, which all outlet into the existing
North East College Corridor Outfall (NECCO) storm system. These basins are broken into
smaller sub-basins based on selected design points. The design points were placed in locations
where the street capacities were analyzed and in sump locations. The peak 2-yr and 100-yr
runoff values were determined for these design points using the rational method and used to
size all onsite storm pipes and inlets. The five NECCO north/south flowing laterals to be
constructed with this project, identified from west to east as lateral A5 (nearest Redwood St),
A3 (near Lake Canal), A7 (west of Landmark Way), A8 (east of Landmark Way), & A4 (east of
Lemay Ave) are sized per Ayres’ design report. Inlet structures and Snout hoods on these
lateral lines have been updated from the original plans to reflect the proposed Northfield
design conditions. Laterals A3 and A4 will only be partially constructed with this project, with
future connections made as development occurs. All calculations can be found in the
Appendix.
Historical drainage analysis was completed for the areas of Northfield located within the NECCO
basin map that allows the runoff to be released at historic rates. The remaining areas of
Northfield did not calculate the historic runoff rates since the developed runoff is restricted to
0.2 cfs/acre release rate.
Seven interconnected extended detention basins located throughout the site were analyzed
using EPA SWMM 5.0 as a dynamic routing method. Outlet structures with control orifices
were sized in the SWMM model to maximize detention volumes and restrict flows to the
allowable release rate. Due to the limited grade across the site, utility conflicts, and the depth
of the NECCO outfall, the drainage system is designed to surcharge runoff into the detention
basins.
The sub-basin descriptions have been summarized per inlet and are described as follows (refer
to the Drainage & Erosion Control Plan located in the Appendix):
Drainage System A:
Drainage System A consists of 18.10 acres of improvements along the east side of the
project site. Runoff from basins in System A drain to two interconnected extended
detention basins, Ponds A1 and A2.
Sub-basins A1 thru A2 consist of 0.85 acres of single-family attached residential lots and
private drives. Flows from these basins are conveyed via curb and gutter and are collected
by 2’ wide concrete channels with sidewalk chases and directed to roadways that ultimately
Page 7
drain to detention Pond A2. All of sub-basins A1 & A2 are treated by a LID infiltration
gallery in Pond A2.
Sub-basins A3 thru A4 consist of 2.41 acres of single-family attached and condominium
residential lots, public roadways and private drives. These basins include portions of
Schlagel Street and Pioneer Trail Road. Runoff from these basins (along with the upstream
flows from sub-basins A1 & A2) is conveyed along roadways and is captured by two 10’ Type
R sump curb inlets and directed to detention Pond A2. All of sub-basins A3 & A4 are treated
by a LID infiltration gallery in Pond A2.
Sub-basins A5 thru A6 consist of 1.49 acres of single-family attached and condominium
residential lots and private drives. Runoff from these basins is conveyed along private
drives and is captured by a double Type 13 valley sump inlet and directed to detention Pond
A2. All of sub-basins A5 & A6 are treated by a LID infiltration gallery in Pond A2.
Sub-basins A7 thru A9 consist of 1.99 acres of single-family attached and condominium
residential lots, a portion of the community clubhouse lot, and public roadways. These
basins include portions of Landmark Way, Collamer Drive, and Pioneer Trail Road. Runoff
from these basins is conveyed along roadways and discharged directly into detention Pond
A2 via a sump CoFC sidewalk culvert. All of sub-basins A7 thru A9 are treated by a LID
infiltration gallery in Pond A2.
Sub-basins A10 thru A11 consist of 1.82 acres of single-family attached and condominium
residential lots, and private drives. Runoff from these basins is conveyed along private
drives and is captured by a sump 10’ Type R curb inlet and directed to detention Pond A2.
All of sub-basins A10 & A11 are treated by a LID infiltration gallery in Pond A2.
Sub-basin A12 consists of 1.25 acres of condominium residential lots and the open space
encompassing detention Pond A2 and Infiltration Gallery A2. Flows within this basin are
conveyed via roof downspouts and concrete pans. Runoff is conveyed from Pond A2 via
storm pipes into Pond A1. All of sub-basin A12 is treated by the Infiltration Gallery A2.
Sub-basins A13 thru A14 consist of 1.08 acres of single-family attached and condominium
residential lots and private drives. Runoff from these basins is conveyed along private
drives and is captured by a double Type 13 valley sump inlet and directed to detention Pond
A1 via storm pipes. Conflicts with water & sewer utilities as well as grading constraints
prevent these basins from discharging to the surface of Pond A1. Flows are discharged
directly into Outlet Structure A02 before the flow control plate, therefore detained in Pond
A1. These basins are unable to be treated in either a LID facility or standard water quality
treatment. These basins represent approximately 2% of the total onsite area of the
Northfield project.
Page 8
Sub-basin A15 consists of 2.48 acres of single-family attached and condominium residential
lots and public roadways. This basin includes the western half street section of N. Lemay
Avenue and a portion of Schlagel Street. Runoff from this basin is conveyed along roadways
and is captured by a sump 5’ Type R curb inlet and directed to detention Pond A1 via storm
pipes. Grading constraints prevent these basins from discharging to the surface of Pond A1.
Flows are discharged directly into Water Quality Outlet Structure A01, behind the WQ plate
and before the flow control plate, therefore detained in Pond A1. Water quality treatment
is provided by a Snout hood inside the inlet prior to discharging into the pond.
Sub-basin A16 consists of 0.80 acres of single-family attached residential lots and public
roadways. This basin includes the eastern half street section of N. Lemay Avenue and a
portion of Schlagel Street. Runoff from this basin is conveyed along roadways to a 4’ wide
curb cut on the east side of Lemay Avenue which directs runoff to a Type C area inlet. This
inlet is to be constructed as part of the A4 NECCO lateral. Due to this basin being on the
east side of Lemay, away from Northfield, runoff is unable to be directed to a water quality
enhancement facility. This basin is unable to be directed to a detention pond.
Sub-basin A17 consists of 0.90 acres of landscaped open space to the north. Runoff from
this basin is conveyed over landscaping to a concrete pan near the north property line, then
into a flared end section and piped directly into the A4 NECCO lateral. This landscaped basin
is unable to be directed to a detention pond.
Sub-basin A18 consists of 3.02 acres of single-family attached residential lots and the open
space encompassing detention Pond A1. Flows within this basin are conveyed via roof
downspouts and concrete pans. A total of three outlet structures in Pond A1 have been
designed to limit discharge from the Northfield site into the NECCO storm system and
provide the water quality capture volume to treat areas untreated by LID features. Water
Quality Outlet Structure A01, located at the east end of Pond A1, includes both a water
quality plate and a 100-yr flow control plate, and discharges into the existing NECCO 3’x10’
reinforced concrete box culvert (RCBC). Outlet Structure A02, located near the center of
Pond A1, is a modified Type C inlet with a 100-yr flow control plate added over the outlet
pipe, and discharges into the existing NECCO 4’x7’ RCBC. Water Quality Outlet Structure
A03, located at the west end of Pond A1, includes a water quality plate to treat surface
runoff but does not include a 100-yr flow control plate since flows discharge directly into
downstream Outlet Structure A02, before its flow control plate.
Page 9
Drainage System B:
Drainage System B consists of 19.48 acres of improvements in the central portion of the
project site. Runoff from basins in System B drain to three interconnected extended
detention basins, Ponds B1, B2, & B3.
Sub-basin B1 consists of 2.13 acres of condominium and multifamily residential lots and
open space. Runoff is conveyed via roof downspouts and a concrete pan and is captured by
a Type 13 inlet. All of basin B1 will be treated by LID Infiltration Gallery B3 in Pond B3.
Sub-basins B2 thru B3 consist of 2.37 acres of single-family attached, condominium, and
multifamily residential lots, public roadways and private drives. This basin includes portions
of Schlagel Street. Runoff from these basins is conveyed along roadways and is captured by
two 10’ Type R sump curb inlets. All of sub-basins B2 & B3 will be treated by LID Infiltration
Gallery B3 in Pond B3.
Sub-basin B4 consists of 1.32 acres of single-family attached residential lots and private
drives. Runoff from this basin is conveyed along private drives and is captured by a double
Type 13 valley sump inlet. All of sub-basin B4 will be treated by LID Infiltration Gallery B3 in
Pond B3.
Sub-basins B5 thru B6 consist of 1.02 acres of single-family attached and condominium
residential lots and private drives. Runoff from these basins is conveyed along private
drives and is captured by a double Type 13 valley sump inlet. All of sub-basins B5 & B6 will
be treated by a LID Infiltration Gallery B3 in Pond B3.
Sub-basin B7 consists of 0.27 acres of single-family attached and condominium residential
lots and private drives. Flows from this basin are conveyed via drain pan and are collected
by a 2’ wide concrete channel with sidewalk chase and directed to detention Pond B3. All of
sub-basin B7 is treated by a LID Infiltration Gallery B3 in Pond B3.
Sub-basin B8 consists of 0.40 acres of single-family attached residential lots and private
drives. Runoff from this basin is conveyed along private drives and is captured by a single
Type 13 valley sump inlet and directed to detention Pond B3. All of sub-basin B8 will be
treated by a LID Infiltration Gallery in Pond B3.
Sub-basin B9 consists of 2.29 acres of single-family attached and condominium residential
lots, a portion of the community clubhouse lot, and the open space encompassing
detention Pond B3 and Infiltration Gallery B3. Flows within this basin are conveyed via roof
downspouts and concrete pans. All of sub-basin B9 is treated by the LID Infiltration Gallery
B3. An outlet structure in Pond B3 includes an orifice plate sized to restrict the release rate
in order to maximize detention volume in Pond B3. Runoff is conveyed from Pond B3 via
storm pipes into Pond B1.
Page 10
Sub-basins B10 thru B11 consist of 2.95 acres of single-family attached, condominium, and
multifamily residential lots, public roadways and private drives. This basin includes portions
of Schlagel Street and Collamer Drive. Runoff from these basins is conveyed along
roadways and is captured by two 5’ Type R sump curb inlets. All of sub-basins B10 & B11
will be treated by LID Infiltration Gallery B2 in Pond B2.
Sub-basin B12 consists of 0.41 acres of open space encompassing detention Pond B2 and
Infiltration Gallery B2. Flows within this basin are conveyed via roof downspouts and
concrete pans. All of sub-basin B12 is treated by the LID Infiltration Gallery B2. An outlet
structure in Pond B2 includes an orifice plate sized to restrict the release rate in order to
maximize detention volume in Pond B2. Runoff is conveyed from Pond B2 via storm pipes
into Pond B1.
Sub-basins B13 thru B17 consist of 2.53 acres of single-family attached and condominium
residential lots and private drives. Runoff from these basins is conveyed along private
drives and is captured by a quad Type 13 valley sump inlet and discharged to LID facility,
Rain Garden B1.
Sub-basins B18 thru B19 consist of 0.86 acres of condominium residential lots and public
roadways. This basin includes portions of Landmark Way. Runoff from these basins is
conveyed along roadways and is captured by two on-grade 5’ Type R curb inlets and
discharged onto the surface of Pond B1 and treated for standard water quality.
Sub-basin B20 consists of 2.94 acres of single-family attached and condominium residential
lots and the open space encompassing detention Pond B1. Flows within this basin are
conveyed via roof downspouts and concrete pans. An outlet structure in Pond B3 includes
an orifice plate sized to restrict the release rate in order to maximize detention volume in
Pond B3 and a water quality plate to treat the WQCV. Runoff is conveyed from Pond B1 via
storm pipes into Pond A1.
Drainage System C:
Drainage System C consists of 3.67 acres of improvements along the western side of the
project site. Runoff from basins in the C system drain to an extended detention basin, Pond
C.
Sub-basin C1 consists of 1.30 acres of multifamily residential buildings and open space.
Runoff is conveyed via concrete pan and into a culvert crossing under Steeley Drive. Basin
C1 is treated for standard water quality within Pond C.
Sub-basin C2 consists of 0.14 acres of open space. Runoff is conveyed via overland flow
into a Type C area inlet and discharged to the surface of Pond C and is treated for standard
water quality.
Page 11
Sub-basins C3 thru C4 consist of 1.08 acres of condominium residential lots, a portion of the
commercial lot, and public roadways. This basin includes portions of Steeley Drive and
Schlagel Street. Runoff from these basins is conveyed along roadways and is captured by a
5’ and 10’ Type R sump curb inlets and discharged to the surface of Pond C and is treated
for standard water quality.
Sub-basin C5 consists of 1.15 acres of commercial lots and the open space encompassing
Pond C. Flows within this basin are conveyed via sheet flow, roof downspouts and concrete
pans. An outlet structure in Pond C includes an orifice plate sized to restrict the release rate
in order to maximize detention volume in Pond C as well as a water quality plate to treat
the WQCV. Runoff is conveyed from Pond C via storm pipes into Pond B1.
Drainage System D:
Drainage System D consists of 9.21 acres of improvements within the area south of Suniga
Road. Runoff from basins in the D system drain to an extended detention basin, Pond D.
Sub-basins D1 thru D4 consist of 2.70 acres of multifamily residential lots, private drives,
and open space. Runoff is conveyed via roof downspouts, roadways, and concrete pans,
and is captured by Type 13 and area inlets, ultimately discharged to the surface of Pond D
and treated for standard water quality.
Sub-basin D5 consists of 0.43 acres of multifamily residential lots and open space. Runoff is
conveyed via roof downspouts and concrete pans and is captured by a single Type 13 inlet,
ultimately discharged to the surface of Pond D and treated for standard water quality.
Sub-basins D6 thru D7 consist of 0.19 acres of multifamily residential lots, and public
roadways. This basin includes portions of Steeley Drive. Runoff from these basins is
conveyed along roadways and is captured by two on-grade 5’ Type R curb inlets, ultimately
discharging to the surface of Pond D and treated for standard water quality.
Sub-basins D8 thru D13 consist of 3.40 acres of multifamily residential lots, private drives,
and open space. Runoff from these basins is conveyed via roof downspouts, roadways,
concrete pans and storm pipes, ultimately discharging to LID facility, Rain Garden D1.
Sub-basin D14 consists of 2.50 acres of open space encompassing detention Pond D. This
basin’s detention requirements allows a release rate equal to the current/historic runoff
rates and is accounted for in the Pond D detention volume calculations. An outlet structure
in Pond D has been designed to limit discharge into the NECCO storm system as well as treat
for standard water quality enhancement.
Sub-basin D15 consists of 0.44 acres of public roadway along the western half of N. Lemay
Avenue. Runoff is conveyed along Lemay Ave and collected by a sump 5’ Type R curb inlet.
Page 12
Runoff is then conveyed by storm drain pipe into the existing 3’x10’ RCBC NECCO storm
drain. Water quality treatment is provided by a Snout hood in the inlet. Due to grade
constraints, this sub-basin cannot be routed to Pond D; the difference in 100-yr runoff rate
for the developed condition is reduced from the allowable release rate. Refer to the
tabulated release rate summary on page 17.
Drainage System E:
Drainage System E consists of 8.23 acres of public roadway improvements along Suniga
Road. Runoff from basins within the System E are conveyed to the existing NECCO storm
drain system. Detention for these basins is not required and water quality enhancement is
provided by Snout hoods within the curb inlets, per the NECCO report. Refer to Exhibit 4.2
of the NECCO report, included in the Appendix.
Sub-basins E1 thru E2 consist of 1.14 acres of public roadway along Suniga Rd. Runoff is
conveyed along Suniga Rd and collected by a double combination curb inlet on the south
and a 10’ Type R inlet on the north. The north (downstream) inlet includes a Snout hood to
filter debris before runoff enters the existing 54” RCP NECCO storm drain.
Sub-basins E3 thru E4 consist of 3.51 acres of public roadway along Suniga Rd. Runoff is
conveyed along Suniga Rd and collected by a double combination curb inlet on the south
and a 10’ Type R inlet on the north. The north (downstream) inlet includes a Snout hood to
filter debris before runoff enters the existing 4’x7’ RCBC NECCO storm drain.
Sub-basins E5 thru E8 consist of 2.50 acres of public roadway along Suniga Rd. Runoff is
conveyed along Suniga Rd and collected by a double combination curb inlet on the south
and a sump 10’ Type R inlet on the north. The north (downstream) inlet includes a Snout
hood to filter debris before runoff enters the existing 4’x7’ RCBC NECCO storm drain.
Sub-basins E9 thru E10 consist of 0.64 acres of public roadway along Suniga Rd and the
eastern half-street of N. Lemay Avenue. Runoff is conveyed along Suniga Rd and Lemay Ave
and collected by a sump 5’ Type R curb inlet on the west side of Lemay, and a curb cut with
run down to a Type C inlet on the east side of Lemay. Runoff is then conveyed by storm
drain pipe into the existing 3’x10’ RCBC NECCO storm drain.
Page 13
DRAINAGE FACILITY DESIGN
DRAINAGE CONVEYANCE DESIGN
Storm infrastructure to convey runoff will include concrete trickle pans, inlets, storm sewer and
culverts. Stormwater detention and water quality enhancement will be achieved through the
use of multiple interconnected extended detention basins, rain gardens, and infiltration
galleries. Storm inlets, storm sewers and the roadway culverts have been appropriately sized to
convey the minor and major storms.
DETENTION/WATER QUALITY POND DESIGN
EPA SWMM 5.0 has been utilized for the final drainage design. UDFCD is referenced for the
standard water quality capture volume (WQCV) with a 40-hr drain time and the LID rain
gardens and infiltration galleries WQCV with a 12-hr drain time. Due to the limited grades on
the site, multiple ponds are proposed throughout the site. These ponds are interconnected via
storm pipes and control structures that are designed with orifice plates to restrict flow and
maximize pond volume while providing adequate freeboard. Overflow paths from each of the
ponds have been provided in the event that the outlet structures clog and overtop. Below is a
brief summary of the proposed design for each pond:
Pond A1:
Furthest downstream extended detention basin for all areas north of Suniga Road, with
standard water quality treatment provided via two structures at the east and west ends (Water
Quality Outlet Structures A01 and A03, respectively). WQ Outlet Structure A01 along with
Outlet Structure A02 (located near the center of the pond) discharge directly into the existing
NECCO system via 11” and 9” 100-yr orifice plates to maximize volume and restrict release
rates. The 100-yr water surface elevation is 4950.36 feet, with a maximum depth in the pond
of 2.36 feet, with another 1.64 feet of additional volume available. The 6” dewatering
underdrain in Pond A1 will maintain a permanent groundwater surface elevation at
approximately 3.8’ below the bottom of the pond. Should the pond outlets become clogged,
overtopping will occur simultaneously at two locations; near the NECCO A7 lateral in Suniga
Road, flows will spill over sidewalk and into public ROW across a length of 125 feet at a depth of
0.09 feet (spill elevation = 4952.36); near design point A15 in Lemay Ave, flows will spill over
sidewalk and into the public ROW across a length of 161 feet at a depth of 0.36 feet (spill
elevation = 4952.36). All calculations are provided in the Appendix. See plans for locations of
spillways.
Page 14
Pond A2:
Detention basin with LID Infiltration Gallery A2 for water quality treatment. A Modified Type D
outlet structure with a baffle wall separating the inlet and outlet compartments will direct
incoming flows into the gallery. Flows will then spread throughout and fill the gallery
chambers, settling out sediments and infiltrating through the gravel sub-base, where an 8”
perforated underdrain set 12” below the bottom of the gallery will collect treated runoff and
convey it to the outlet side of the modified Type D structure. A 24” 100-yr orifice plate
maximizes volume in the pond and restricts flow to downstream Pond A1. During the major
storm event, pipe flow will surcharge into the pond and be detained. The 100-yr water surface
elevation is 4952.39 feet, with a maximum depth in the pond of 2.39 feet, with another 2.11
feet of additional volume available. The 8” dewatering underdrain in Pond A2 will maintain a
permanent groundwater surface elevation at approximately 5.7’ below the bottom of the pond.
Overtopping will occur simultaneously at two locations should the pond outlet become
clogged; near the southeast corner, flows will spill over a depressed sidewalk across a length of
38 feet at a depth of 0.48 feet and down toward design point A10/A11; near the northeast
corner, flows will spill into the 24’ wide private drive at a depth of 0.48 feet (water surface will
not inundate finished floors at either spillway). The northern spill water surface elevation is
approximately 4955.72 with the lowest adjacent top of foundation at 4957.43, providing 1.71
feet of freeboard. Flows will continue south and then overtop sidewalk into Pond A1 across a
length of 98 feet at a depth of 0.36 feet. The southern spill water surface elevation is
approximately 4954.54 with the lowest adjacent top of foundation at 4956.65, providing 2.11
feet of freeboard. All calculations are provided in the Appendix.
Pond B1:
Extended detention basin with LID rain garden and standard water quality treatment via the
outlet structure located at the east end of the pond. An 11 ½” 100-yr orifice plate maximizes
volume in the pond and restricts flow to downstream Pond A1. The rain garden includes a
concrete forebay sized for 1% of the surface area for the WQCV with a rock embankment to
dissipate flows before entering the media material bed. Large storm events in excess of the
rain garden capacity will spill over a turf mat reinforced berm and down into Pond B1. The 100-
yr water surface elevation is 4951.68 feet, with a maximum depth in the pond of 3.68 feet, with
another 1.32 feet of additional volume available. The 6” dewatering underdrain in Pond B1 will
maintain a permanent groundwater surface elevation at approximately 3.4’ below the bottom
of the pond. Should the pond outlet become clogged, overtopping will occur at the southeast
corner near the intersection of Landmark Way and Suniga Road, spilling over the sidewalk and
into public ROW across a length of 93 feet at a depth of 0.35 feet (spill elevation = 4953.83). All
calculations are provided in the Appendix.
Page 15
Pond B2:
Detention basin with LID infiltration gallery for water quality treatment. Modified Type D outlet
structure with a baffle wall separating the inlet and outlet compartments will direct incoming
flows into the gallery. Flows will then spread throughout and fill the gallery chambers, settling
out sediments and infiltrating through the gravel sub-base, where an 8” perforated underdrain
set 12” below the bottom of the galleries will collect treated runoff and convey it to the outlet
side of the modified Type D structure. A 7” 100-yr orifice plate maximizes volume in the pond
and restricts flow to downstream Pond B1. During the major storm event, pipe flow will
surcharge into the pond and be detained. The 100-yr water surface elevation is 4955.94 feet,
with a maximum depth in the pond of 2.44 feet, with another 1.06 feet of additional volume
available. The 8” dewatering underdrain in Pond B2 will maintain a permanent groundwater
surface elevation at approximately 5.7’ below the bottom of the pond. Should the pond outlet
become clogged, overtopping will occur at the east corner near design point B11, spilling over
the sidewalk into Collamer Drive ROW across a length of 54 feet at a depth of 0.22 feet (spill
elevation = 4957.55). Flows will then continue over the street high point to the east
(approximately 6” depth) and continuing down Landmark Way toward Suniga Road ROW.
Pond B3:
Detention basin with LID infiltration gallery for water quality treatment. Modified Type D outlet
structure with a baffle wall separating the inlet and outlet compartments will direct incoming
flows into the gallery. Flows will then spread throughout and fill the gallery chambers, settling
out sediments and infiltrating through the gravel sub-base, where an 8” perforated underdrain
set 12” below the bottom of the galleries will collect treated runoff and convey it to the outlet
side of the modified Type D structure. A 7” 100-yr orifice plate maximizes volume in the pond
and restricts flow to downstream Pond B2. During the major storm event, pipe flow will
surcharge into the pond and be detained. The 100-yr water surface elevation is 4954.96 feet,
with a maximum depth in the pond of 3.46 feet, with another 1.04 feet of additional volume
available. The 8” dewatering underdrain in Pond B3 will maintain a permanent groundwater
surface elevation at approximately 5.7’ below the bottom of the pond. Should the pond outlet
become clogged, overtopping will occur at the northeast corner of the pond near design point
B7, spilling into the 26‘ wide private drive and then splitting, with some flows running east
toward Pioneer Trail ROW at a depth of 0.62 feet, and the rest to the south in the private drive
(Coonts Lane) at a depth of 0.39 feet. The northern spill water surface elevation is
approximately 4956.32 with the lowest adjacent top of foundation at 4958.17, providing 1.35
feet of freeboard. Flows will split again at the south end of the private drive, with some flows
running east toward Pioneer Trail Road ROW at a depth of 0.34 feet, and the rest to the
southeast into Collamer Dr ROW at a depth of 0.44 feet. The southern spill water surface
elevation is approximately 4956.82 with the lowest adjacent top of foundation at 4958.00,
Page 16
providing 1.68 feet of freeboard. Flows ultimately converge at the low point in Collamer
Drive/Pioneer Trail and overtop the sidewalk into Pond A2. All calculations are provided in the
Appendix.
Pond C:
Extended detention basin with standard water quality treatment provided via the outlet
structure located at the east end of the pond. A 5” 100-yr orifice plate maximizes volume in
the pond and restricts flow to downstream Pond B1. The 100-yr water surface elevation is
4955.52 feet, with a maximum depth in the pond of 3.32 feet, with another 2.98 feet of
additional volume available. Should the pond outlet become clogged, overtopping will occur at
the southeast corner, spilling over the sidewalk across a length of 52 feet at a depth of 0.35 feet
(spill elevation = 4959.01), into Steeley Drive and Suniga Road public ROW.
Pond D:
Extended detention basin for all area south of Suniga Road, with LID rain garden and standard
water quality treatment via the outlet structure located at the north center of the pond. An 4-
3/4” 100-yr orifice plate maximizes volume in the pond and restricts release rate into the
NECCO system. The rain garden includes a concrete forebay sized for 1% of the surface area
for WQCV with a rock embankment to dissipate flows before entering the media material.
Large storm events in excess of the rain garden capacity will spill over a turf mat reinforced
berm and down into Pond D. The 100-yr water surface elevation is 4949.54 feet, with a
maximum depth in the pond of 3.92 feet, with another 1.46 feet of additional volume available.
Should the pond outlet become clogged, overtopping will occur along the east side near design
point D15, spilling over the sidewalk across a length of 120 feet at a depth of 0.32 feet (spill
elevation = 4951.75), into Lemay Ave public ROW.
As stated, the ultimate outfall for the Northfield site is the existing NECCO storm drain system.
The area north of Suniga Road is required to reduce the 100-yr developed runoff to a rate of 0.2
cfs/acre. A portion of the area south of Suniga Road (delineated as sub-basins D13-D15) is
allowed to release at a rate equal to current undeveloped/historic rates with the remaining
area (delineated as sub-basins D1-D12) restricted to the 0.2 cfs/acre rate. Sub-basin D15,
representing the western half-street section of Lemay Avenue adjacent to the development,
cannot be routed to Pond D due to grade constraints; the difference in 100-yr runoff rate for
the developed condition has been subtracted from the calculated allowable release rate to
account for the undetained flows. Refer to the tabulated release rate summary below.
There are three proposed outlet structures that will discharge runoff from the Northfield site
into the NECCO system: one located at the east end of Pond A1 (Outlet A01), one near the
Page 17
center of Pond A1 (Outlet A02), and one in Pond D (Outlet D01). The combined allowable
release rate for the Northfield site of 13.87 cfs has been spread across these three outlet
structures in order to achieve maximum detention volumes in all the interconnected ponds.
Below is a summary of allowable and provided release rates:
Allowable Release Rate:
47.69 ac = Total area north of Suniga & portion of area south of Suniga with max
allowable release rate of 0.20 cfs/ac:
47.69 ac x 0.20 cfs/ac = 9.54 cfs
3.21 ac = Portion of area south of Suniga that can be released at historic rate of 5.51 cfs
Flow from undetained Lemay Ave (sub-basin D15) (∆ historic - developed) is -1.18 cfs
Total combined allowable release rate from Northfield site = 13.87 cfs
Provided orifice-restricted release rates from each proposed three outlet structures:
Outlet A01 4.55 cfs
Outlet A02 7.02 cfs
Outlet D01 1.22 cfs
Total provided release rate = 12.79 cfs
Total provided release rate of 12.79 cfs < Total allowable release rate of 13.87 cfs
All supporting calculations are located in the Appendix.
Page 18
LOW IMPACT DEVELOPMENT
The City of Fort Collins updated the Low Impact Development ordinance in 2016 (Ordinance No.
007, 2016) to require:
· Treat at least 75% of any newly developed or redeveloped impervious area using one or
a combination of LID techniques, or
· Treat at least 50% of any newly developed or redeveloped impervious area using one or
a combination of LID techniques when 25% of private driveable surfaces are permeable.
To satisfy the required implementation of Low Impact Development (LID) techniques,
Northfield will utilize below grade infiltration galleries (such as ADS StormTech chamber
system) and bioretention ponds/rain gardens. Details and specifications for these facilities are
provided in the Appendix. Other LID techniques were explored, but due to the limited grade
available and the amount of detention volume required, shallow infiltration galleries assisted in
maintaining storm drain grades and detention volumes. Pavers within the private drives did not
treat enough contributing area to justify their use. Infiltration galleries promote infiltration
while capturing fine sediment that drains off the impervious areas. The proposed galleries are
configured entirely as ‘Isolator rows’ which are surrounded by filter fabric and include an inlet
manifold for access, removal of sediment and maintenance, along with an inspection port on
each row. Standard water quality treatment is also provided within the extended detention
ponds and has been sized for those tributary basins not already treated by the LID facilities. For
basins A15 & D15 that are not able to be routed to the extended detention basins for standard
water quality treatment or a LID facility, Snout hoods have been utilized at the design point
inlet to filter debris and sediment from runoff prior to discharging into the storm system. A
Standard Operations Procedure will be included as part of the Development Agreement to
assist in ensuring that these BMPs will adequately perform over time.
Impervious calculations for the tributary areas to each of the LID infiltration galleries, rain
gardens, and standard water quality facilities along with a colored coded exhibit showing all
areas treated are provided in the Appendix. LID treatment requirements have been exceeded,
achieving approximately 107% of the required impervious area treatment in the rain gardens
and infiltration galleries. As previously noted in the drainage basin descriptions, grading
constraints and tight/shallow utility conflicts prevented a fraction of the site from being routed
through a water quality treatment facility before being discharged into the storm system. Two
interior basins A13 & A14, along with the small portion of basin A16 lying on the Northfield site
(not including the eastern half street section of Lemay), and the northeastern-most landscape
basin A17 together represent approximately 3.9% of the total site area not able to be treated
Page 19
for water quality. Consideration is requested that the overtreatment provided within the LID
facilities be a compromise for the areas unable to be directed to a water quality or LID facility.
We believe this meets the intent of the water quality treatment criteria to the maximum extent
possible.
Below is a description of the 4-step process for selecting structural BMPs:
Urban Drainage and Flood Control District (UDFCD) recommends a Four Step Process for
receiving water protection that focuses on reducing runoff volumes, treating the water quality
capture volume (WQCV), stabilizing drainageways and implementing long-term source controls.
The Four Step Process applies to the management of smaller, frequently occurring events.
Step 1: Employ Runoff Reduction Practices
To reduce runoff peaks, volumes, and pollutant loads from urbanizing areas, implement Low
Impact Development (LID) strategies, including Minimizing Directly Connected Impervious
Areas (MDCIA).
Captured runoff from strategic areas are routed through below grade infiltration galleries and
bioretention pond/rain gardens. Infiltration galleries and rain gardens will slow runoff, promote
infiltration, and filter runoff prior to being released into the adjacent storm drain system.
Step 2: Implement BMPs that Provide a Water Quality Capture Volume with Slow Release
The infiltration galleries and rain gardens are designed to provide water quality capture volume
per Urban Drainage’s recommendations and calculations. The captured runoff is designed for a
12-hr drain time.
Step 3: Stabilize Drainageways
Natural Drainageways are subject to bed and bank erosion due to increases in frequency,
duration, rate and volume of runoff during and following development. Because the site will
drain to an existing storm system, bank stabilization is unnecessary with this project. Riprap
and turf reinforcement mats are utilized throughout for onsite permanent stabilization.
Step 4: Implement Site Specific and Other Source Control BMPs
The proposed project will improve upon site specific source controls compared to historic
conditions by providing localized trash enclosures within the development for disposal of solid
waste; Snouts, infiltration galleries, and rain gardens for water treatment prior to flows
Page 20
entering storm systems; and additional water quality measures to protect and prolong the
design life of BMPS delineated in Step 1.
Page 21
STORMWATER POLLUTION PREVENTION
Erosion and sedimentation will be controlled on-site by use of sediment control logs, silt
fencing, inlet protection, gravel construction entrances, temporary sediment basins, seeding,
mulch, and turf. The measures are designed to limit the overall sediment yield increase due to
construction as required by the City of Fort Collins. During overlot and final grading the soil will
be roughened and furrowed perpendicular to the prevailing winds.
During the performance of the work required by these specifications or any operations
appurtenant thereto, whether on right-of-way provided by the City or elsewhere, the
contractor shall furnish all labor, equipment, materials, and means required. The Contractor
shall carry out proper efficient measures wherever and as necessary to reduce dust nuisance,
and to prevent dust nuisance that has originated from his operations from damaging crops,
orchards, cultivated fields, and dwellings, or causing naissance to persons. The Contractor will
be held liable for any damage resulting from dust originating from his operations under these
specifications on right-of-way or elsewhere.
It is unlawful to track or cause to be tracked mud or other debris onto city streets or rights-of-
way. Wherever construction vehicles access routes or intersect paved public roads, previsions
must be made to minimize the transport of sediment by runoff or vehicles tracking onto the
paved surface. Stabilized construction entrances are required with base material consisting of
6” coarse aggregate. The contractor will be responsible for clearing mud tracked onto city
streets on a daily basis.
All temporary and permanent erosion and sediment control practices must be maintained and
repaired as needed to assure continued performance of their intended function. Silt fence and
sediment control logs will require periodic replacement. Maintenance is the responsibility of
the contractor.
All disturbed areas must be seeded and mulched within 30 days of project start. Vegetation
shall not be considered established until a ground cover is achieved which is demonstrated to
be mature enough to control soil erosion to the satisfaction of the City Inspector and to survive
severe weather condition.
Page 22
CONCLUSIONS
This Final Drainage Report for Northfield has been prepared to comply with the stormwater
criteria set by the City of Fort Collins. The proposed drainage system presented in this report is
designed to convey the developed peak storm water runoff through the site to the existing
storm drain system and to the development’s LID features, detention and water quality
facilities. Storm drains have been sized to provide the required roadway relief in both the 2-yr
and 100-yr storm events, and to adequately convey the released runoff from the detention
ponds disbursed throughout the site. Overland relief is provided at all sump locations. The
calculated 100-yr peak flows released from Northfield adhere to the allowed rates as
established in the NECCO drainage studies. This final drainage report anticipates the
implementation of best management practices for erosion control, temporary and permanent,
and on-site construction facilities.
It can therefore be concluded that development of Northfield complies with the storm water
jurisdictional criteria and will not adversely affect the adjacent properties, streets, storm drain
system and/or detention/water quality facilities. Controlling the developed runoff from these
improvements will improve the situations currently existing on the site.
Page 23
REFERENCES
1. Urban Storm Drainage Criteria Manual (Volumes 1, 2, and 3), Urban Drainage and Flood
Control District, June 2001, Revised November 2010.
2. Storm Drainage Criteria Manual and Construction Standards, City of Fort Collins Storm
Water Utility, City of Fort Collins, Colorado, Updated January, 1997 with 2012
amendments.
3. Fort Collins Stormwater Criterial Manual, Fort Collins Utilities, City of Fort Collins,
Colorado, Dated December 2018, Adopted January 2019.
4. “Hydrologic Group Rating for Larimer County Area, Colorado”, USDA-Natural Resources
Conservation Service, National Cooperative Soil Survey. Web Soil Survey URL:
http://websoilsurvey.nrcs.usda.gov. [12/06/2017]
5. “Preliminary Subsurface Exploration and Groundwater Report – Schlagel Property”,
prepared by Earth Engineering Consultants, LLC, Dated August 16, 2017
6. “Public Improvement Plans, East Vine Drive Relocation”, prepared by Ayres Associates,
Dated February 2009, obtained from City of Fort Collins.
7. “Utility Plans for NECCO Ponds and Backbone” – Record Drawings, prepared by Ayres
Associates, Dated June 2017, obtained from the City of Fort Collins.
8. “Flood Insurance Rate Map, Larimer County, Colorado – Map Number 08069C0977G”,
Federal Emergency Management Agency, Map Revised June 17, 2008.
9. “Flood Insurance Rate Map, Larimer County, Colorado – Map Number 08069C0981G”,
Federal Emergency Management Agency, Map Revised June 17, 2008.
10. “Northfield Development, Fort Collins, Colorado – Groundwater Evaluation”, prepared
by Miller Groundwater Engineering, LLC, Dated June 25, 2019.
Appendix E
FEMA Floodplain Map
Appendix F
Drainage Plan
Know what's below.before you dig.Call
R
Fe
t
z
e
r
E
n
g
i
n
e
e
r
i
n
g
,
L
L
C
Fo
r
t
C
o
l
l
i
n
s
,
C
O
8
0
5
2
5
97
0
-
7
0
2
-
4
8
8
4
Sf
e
t
z
e
r
@
f
e
t
z
e
r
e
n
g
i
n
e
e
r
i
n
g
.
c
o
m
Feet
0 30 60
Appendix G
Sediment & Erosion Control Plan
Know what's below.before you dig.Call
R
Fe
t
z
e
r
E
n
g
i
n
e
e
r
i
n
g
,
L
L
C
Fo
r
t
C
o
l
l
i
n
s
,
C
O
8
0
5
2
5
97
0
-
7
0
2
-
4
8
8
4
Sf
e
t
z
e
r
@
f
e
t
z
e
r
e
n
g
i
n
e
e
r
i
n
g
.
c
o
m
Feet
0 30 60