HomeMy WebLinkAbout010 Stormwater Design ReportSS62
This report and drainage plan for the Sundance Springs commercial lot 2 was prepared by me (or under
my direct supervision) in accordance with the provisions of the City of Bozeman “Design Standards and
Specifications Policy”.
Signature:
State of Montana No. 20459 10-1-2025
STORM DRAINAGE REPORT
Sundance Springs Comm Lot 2
Project Number 25020
Synergy Engineering and Konsulting
3731 Equestrian Lane, STE 102, Bozeman, MT 59718/ 406-624-6137 marlene@seakmt.com
Sundance Springs
COMM LOT 2 REVISED September 30, 2025
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Table of Contents
Introduction .................................................................................................................................................. 3
1. DESCRIPTION OF PROPERTY.............................................................................................................. 3
A. LOCATION ...................................................................................................................................... 3
B. AREA .............................................................................................................................................. 4
C. GROUND COVER............................................................................................................................ 4
D. EXISTING LAND USE ...................................................................................................................... 4
E. TOPOGRAPHIC FEATURES AND CHARACTERIZATION ................................................................... 4
F. GEOLOGIC FEATURES AND CHARACTERIZATION .......................................................................... 4
G. EXISTING DRAINAGE CONDITIONS AND FACILITIES ...................................................................... 4
H. NATURAL WATERCOURSES ........................................................................................................... 4
I. WETLANDS .................................................................................................................................... 5
J. FLOOD HAZARD ZONES ................................................................................................................. 5
1. PREVIOUS DRAINAGE STUDIES ......................................................................................................... 6
2. STATE OR FEDERAL REGULATIONS.................................................................................................... 6
HYDROLOGY AND HYDROGEOLOGY ............................................................................................................. 6
1. DESIGN STORM RAINFALL ................................................................................................................. 6
2. GEOTECHNICAL EVALUATION SUMMARY ......................................................................................... 6
EXISTING STORMWATER DRAINAGE CONDITIONS ....................................................................................... 6
1. EXISTING DRAINAGE CONDITIONS .................................................................................................... 6
2. HYDROLOGIC AND HYDRAULIC CALCULATIONS ............................................................................... 7
A. HYDRAULIC METHODS AND MODELS USED ................................................................................. 7
B. HYDRAULIC CAPACITY OF EXISTING DRAINAGE FEATURES .......................................................... 7
PROPOSED STORMWATER DRAINAGE SYSTEM ............................................................................................ 8
1. APPLICABLE DESIGN STANDARDS ..................................................................................................... 8
2. DRAINAGE SYSTEM DESIGN .............................................................................................................. 8
A. POST PROJECT RUNOFF CHARACTERISTICS. ................................................................................. 8
B. PLANNED DRAINAGE FEATURES ................................................................................................... 9
3. HYDROLOGIC AND HYDRAULIC CALCULATIONS AND MODELING .................................................... 9
A. HYDRAULIC AND HYDROLOGIC METHODS. .................................................................................. 9
B. POST PROJECT RUNOFF RATES AND VOLUMES AS COMPARED TO EXISTING. ........................... 10
C. HYDRAULIC DESIGN OF STORMWATER DRAINAGE SYSTEM ELEMENTS .................................... 11
D. HYDRAULIC DESIGN OF RIP RAP ENERGY DISSAPATOR .............................................................. 12
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E. DESIGN HYDRAULICS COMPARED TO DESIGN CRITERIA ............................................................ 16
F. WATER QUALITY TREATMENT DESIGN. ...................................................................................... 16
EVALUATION OF MAJOR STORM FLOOD RISKS .......................................................................................... 17
1. COMPARISON OF POST-PROJECT FLOOD HAZARDS DURING THE MAJOR STORM TO EXISTING
CONDITIONS ............................................................................................................................................ 17
2. POTENTIAL FLOOD HAZARDS AND MITIGATION MEASURES .......................................................... 17
3. IMPACTS TO FEMA FLOODPLAINS .................................................................................................. 17
OPERATION, INSPECTION, AND MAINTENACE CONSIDERATIONS ............................................................. 17
REFERENCES ................................................................................................................................................ 17
APPENDICES ................................................................................................................................................ 17
A. FEMA FIRMette 30031C0818E ........................................................................................................ 17
B. Basin Area and Parameter Calculations .......................................................................................... 17
C. SSA Flow and Drawdown Graph ..................................................................................................... 17
D. SSA SWMM Model .......................................................................................................................... 17
Figure 1 Sundance Springs Comm Lot 2 Location Map ................................................................................. 3
Figure 2 Wetlands Delineation ..................................................................................................................... 5
Figure 3 Sundance Springs Comm Lot 2 existing conditions. The longest drainage path is shown in blue. . 7
Table 1 Pre-development flows .................................................................................................................... 7
Figure 4 Onsite Basins with Longest Flow Paths ........................................................................................... 9
Table 2 Pre and Post Development Flows .................................................................................................. 10
Figure 5 100yr-24hr Model ......................................................................................................................... 11
Table 3 RTV and RTF .................................................................................................................................... 11
Figure 6 East Outlet Detail .......................................................................................................................... 12
Table 4 Inputs for Riprap Design ................................................................................................................. 13
Table 5 Normal Depth Calculations ............................................................................................................ 14
Table 6 Sizing Calculations .......................................................................................................................... 15
Table 7 Calculated Geometry and Quantities ............................................................................................. 16
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Introduction
1. DESCRIPTION OF PROPERTY
This Storm Drainage Plan is prepared for the 2-building commercial project proposed on COMM
Lot 2 of Sundance Springs Subdivision Phase 1B, Plat J-257. Total acreage is 1.31 acres. The
project includes paved access and parking, gravel path connecting to neighborhood trail
systems, two buildings with patios and landscaped areas.
A. LOCATION
Sundance Springs Comm Lot 2 is located near the southern boundary of the City of Bozeman
limits. The closest intersection to the site is South 3rd Ave and Little Horse Drive. The site is
bordered on the north, east, and west by platted open space, with Little Horse Drive on the
south. The site is currently undeveloped.
1. Owner/Developer: 406 Coal Blowers LLC
2. B-1 Zoning
3. Township 2S, Range 5E, Section 25
4. Access provided by Little Horse Drive
5. Address: 675 Little Horse Drive
Figure 1 Sundance Springs Comm Lot 2 Location Map
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B. AREA
1.31 acres.
C. GROUND COVER
Short herbaceous grasses.
D. EXISTING LAND USE
Undeveloped.
E. TOPOGRAPHIC FEATURES AND CHARACTERIZATION
The site is generally level, with a slight slope from the northwest corner towards the south
eastern side adjacent to middle creek ditch.
F. GEOLOGIC FEATURES AND CHARACTERIZATION
Groundwater elevation: A monitoring pipe was installed when test pits were excavated by
Allied Engineering in 2019 for geotechnical assessment of the soils pertaining to building
foundation designs. The monitoring well is approximately nine feet deep. Groundwater depth
was recorded at 6.5 feet below ground surface on August 21, 2019 and 4.7 feet below ground
surface on September 24, 2019. The monitoring pipe was checked for ground water in the
Spring of 2020 and again in the Spring of 2021 and Spring and Summer of 2022 after an
unusually wet weather cycle. No groundwater was found in the monitoring well during any of
these site visits. Depth to ground water is estimated at between 6 and greater than 9 feet.
Seasonal high groundwater between 4.5-6 feet is possible and most likely due to localized
irrigation effects. Interpolated groundwater elevation at the site is approximately 5020’ (Slagle
Contours, Gallatin County GIS).
Because high groundwater conditions are common throughout the Gallatin Valley, precautions
will be taken to ensure ductile iron water mains will be protected against corrosion effects.
G. EXISTING DRAINAGE CONDITIONS AND FACILITIES
The drainage in the general area flows in a north-northeasterly direction. Surveyed ground
elevation at the site is between 5032’-5040’. The site is adjacent to the Middle Creek Ditch and
the topography of the site slopes gently toward the ditch in an easterly direction.
H. NATURAL WATERCOURSES
The site is adjacent to Middle Creek Ditch. No natural watercourses flow through the site. A 35 foot
watercourse setback was used for the stormwater design.
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I. WETLANDS
A total of two wetlands totaling 0.15 acres were delineated along Middle Creek Ditch, outside of the
project boundary. The delineated wetlands are outside the property lines, directly adjacent to Middle
Creek Ditch and will not be impacted by the proposed site plan.
Figure 2 Wetlands Delineation
J. FLOOD HAZARD ZONES
Per attached FEMA FIRMette 30031C0818E effective 4/21/2021 the site is located in an area of minimal
flood hazard Zone X.
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1. PREVIOUS DRAINAGE STUDIES
Sundance Springs PUD/Sundance Springs Major Subdivision drainage studies.
2. STATE OR FEDERAL REGULATIONS
This site is subject to the City of Bozeman’s design and construction standards as required by the
Municipal Separate Storm Sewer System (MS4) permit issued to the city by the State of Montana. The
site is subject to the requirements for the Montana Standards for Subdivision Storm Water Drainage
outlined in Circular DEQ-8
HYDROLOGY AND HYDROGEOLOGY
1. DESIGN STORM RAINFALL
Drainage design is based on the requirements in the City of Bozeman Design and Constructions
Standards published October 2024 (DCS). DCS requires that detention ponds be sized based on a 100-
year, 24-hour storm intensity. Rainfall totals from DCS Table 6.5.1 were used with an SCS type II
distribution to model flows.
2. GEOTECHNICAL EVALUATION SUMMARY
Retention ponds are not recommended. Detention ponds with a controlled outlet are the preferred
option.
EXISTING STORMWATER DRAINAGE CONDITIONS
1. EXISTING DRAINAGE CONDITIONS
Pre-development, the entire site drains as one basin.
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Figure 3 Sundance Springs Comm Lot 2 existing conditions. The longest drainage path is shown in blue.
2. HYDROLOGIC AND HYDRAULIC CALCULATIONS
A. HYDRAULIC METHODS AND MODELS USED
EPA SWMM was used with Autodesk Storm and Sanitary Analysis (SSA) to model hydrologic conditions
for the site. Per attached NRCS web soil survey the soil at the site is in hydrologic soil group C. Per DCS
table 6.6.2 the curve number for the site is 74 ( Herbaceous mixture of grass, weeds, and low growing
brush, with the minor element >70% groundcover). As the basin is small, relatively flat, and
undeveloped, all flow was assumed to be shallow overland flow.
B. HYDRAULIC CAPACITY OF EXISTING DRAINAGE FEATURES
Table 1 Pre-development flows
Time of concentration 9.92 min
Recurrence interval Runoff (EPA SWMM) CFS TR-55 Curve # Runoff (Rational) CFS
0.5in storm 0.00 0.03 --
2yr-24hr 0.01 0.12 0.01
10yr-24hr 0.19 0.41 0.02
50yr-24hr 0.50 0.73 0.02
100yr-24hr 0.66 0.88 0.03
The rational method gave runoff flows that seemed unusually low when compared to EPA SWMM and
SCS TR-55. As such, EPA SWMM was selected for side modeling. EPA SWMM Flows were used for
storage sizing and other design calculations. These numbers were checked against TR-55 for
reasonableness.
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PROPOSED STORMWATER DRAINAGE SYSTEM
1. APPLICABLE DESIGN STANDARDS
City of Bozeman, MT-DEQ Circular 8
2. DRAINAGE SYSTEM DESIGN
A. POST PROJECT RUNOFF CHARACTERISTICS.
Pre-development, the entire site drains as one basin. Proposed grading and development break the lot
into four separate drainage basins. The parking lot and building areas are divided into two basins that
terminate in proposed detention basins. Smaller basins on the east and west site perimeters drain
offsite. Basin 3 is entirely within the watercourse setback and will remain undeveloped. Basin 4 includes
an existing pedestrian trail and a proposed landscape berm that serves as a barrier between the
pedestrian’s and proposed parking lot draining towards the existing roadside ditch along S 3rd Ave post
development
Proposed Conditions:
• Asphalt access drive and parking, sidewalks, paths, and patios, 0.45 acres
• Two commercial buildings, 0.14 acres
• Permeable interlocking concrete pavers (PICP) with partial infiltration, 0.15 acres
• Landscape and natural scape areas 0.58 acres
• Two detention ponds to capture and treat runoff.
Stormwater detention ponds have been designed to a maximum depth of 4.5 feet below
ground surface including freeboard to ensure that groundwater is not encountered. Offsite
flows are not expected to contribute to any of the onsite basins. Onsite basin 1 is bordered by a
road and ditch blocking flow, and the existing grade is away from onsite basin 2. Onsite basin 3
& 4 are contained by the other basins with slopes down to existing waterways.
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B. PLANNED DRAINAGE FEATURES
Figure 4 Onsite Basins with Longest Flow Paths
Onsite Basin 3 will not undergo any development; flows will remain unchanged.
Onsite Basin 4 will be landscaped with grass which will not increase post development flows.
Onsite Basin 1 and 2 will contain the parking lot and buildings. Flow will be directed into two detention
basins with outlet control devices and micro pools for sediment treatment. A combination of orifices
and weirs will reduce runoff flow to predevelopment levels.
3. HYDROLOGIC AND HYDRAULIC CALCULATIONS AND MODELING
A. HYDRAULIC AND HYDROLOGIC METHODS.
Design was completed in accordance with procedures outlined in chapter 10 of the HEC-22 manual,
using Autodesk SSA and an EPA SWMM model for calculations. Four Curve numbers were used in
calculations, taken from DCS table 6.6.2. Soil type was set as C per NRCS web soil survey. For PICP, the
curve number was selected based on partial infiltration of rainfall.
• Undeveloped land – 74, Herbaceous - mixture of grass, weeds and low growing brush, with
brush the minor element (>70% ground cover)
• Lawn – 74, Fully developed lawn in Good condition; grass cover on 75% or more of the area
• Parking lot and building – 98, Parking lots, roofs, driveways, etc.
• Permeable pavers - 88
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To ensure each pond could handle the change in flow of each subbasin, pre and post development
models were created in Autodesk Storm and Sanitary Analysis (SSA). Flows from each subbasin were
compared in SSA. Stage storage and orifice flows were modeled, as well as time to drain after a storm
event to ensure the ponds will dry within 72hrs post storm event. Ponds were designed for a 3.5’ max
depth with an additional one foot of freeboard. The proposed detention ponds can comfortably handle
the 100yr storm even with additional margin for safety to prevent flooding. Pond outlet size was limited
to the smallest reasonable size to reduce flows. For other basin specific parameters such as percent
impervious and equivalent width used in SWMM modeling please see Basin Area attachment.
B. POST PROJECT RUNOFF RATES AND VOLUMES AS COMPARED TO EXISTING.
Table 2 Pre and Post Development Flows
Before Control After Control
Peak Runoff
(CFS)
Peak
Runoff
W. Pond
Max Depth
E. Pond
Max Depth
W. Pond
Dry
E. Pond
Dry
Controlled Area
(Basins 1&2) Pre Post CFS FT FT HRS HRS
0.5in Storm 0.00 0.35 0.08 0.53 0.74 24 23
2yr-24hr 0.01 0.91 0.13 1.28 1.71 28 25
10yr-24hr 0.10 1.57 0.17 2.12 2.52 29 25
50yr-24hr 0.32 2.21 0.30 2.81 2.89 29 26
100yr-24hr 0.46 2.49 0.34 3.06 3.08 30 26
Uncontrolled Area
(Basins 3&4) Pre Post
0.5in Storm 0.00 0.00 2yr-24hr 0.00 0.00 10yr-24hr 0.09 0.09 50yr-24hr 0.18 0.18 100yr-24hr 0.20 0.20
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Figure 5 100yr-24hr Model
Table 3 RTV and RTF
Storage Sizing Basin 1 Basin 2 Total
Pond Capacity at 3.5ft water depth 2471 1498 3969
Montana BMP Method Basin 1 Basin 2 Total
RTV (CF) 592.24 468.70 1060.94
RTF (CFS) 0.115 0.151 0.27
EPA SWMM Method Model Check Basin 1 Basin 2 Total
RTV (CF) 525 424 949.00
RTF (CFS) 0.18 0.17 0.35
C. HYDRAULIC DESIGN OF STORMWATER DRAINAGE SYSTEM ELEMENTS
All sidewalks and patios are proposed to be constructed of PICP set up for partial infiltration as detailed
on sheet C-507 of the plan set.
Both detention ponds will be constructed with 2:1 slopes lined with riprap. Pond bottoms will contain
unmowed native vegetation per guidelines in DCS 6.8.2. The West detention pond shall have a bottom
elevation 4.5ft below ground level, with a capacity of 2471 CF while maintaining 1ft of freeboard. The
East detention pond shall have a bottom elevation of 4.5ft below ground level, with a capacity of 1498
CF. Both shall have multistage outlet risers as detailed on sheet C-507 of the plan set. 10ft setbacks
from building foundations shall be maintained.
Generally, the system is designed to allow more runoff from the smaller, east pond under most
circumstances. The west pond is sized to contain most flows and discharge them over a longer period.
The west pond will have a 1ft deep concrete lined micro pool next to the outlet. A 1.5” diameter orifice
set behind a trash rack below the bottom of the pond will handle most flows, up to and including a 100-
year storm event. Set 3.5ft above the bottom of the pond (above the anticipated water surface level for
a 100 year storm) a grate on top of the outlet to prevent overflows. The east pond will have a similar low
flow outlet, with a 1.5” orifice level set in a 1ft deep concrete lined micro pool. It will also include a 3”
orifice set 2.5’ above the pond bottom to handle 50 and 100yr storms. Finally, an emergency overflow
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grate set 3.5’ above the bottom of the pond. Both will channel flow into 8” PVC pipes and discharge
from a low point at the edge of the property.
Figure 6 East Outlet Detail
D. HYDRAULIC DESIGN OF RIP RAP ENERGY DISSAPATOR
A riprap apron will be constructed at the stormwater channel discharge point. The purpose of the apron
is to dissipate flow energy and provide a stable transition to the natural drainageway. Riprap design and
classification follow the City of Bozeman Design and Construction Standards, Section 6.F.II.c, which
requires that riprap used for erosion protection or energy dissipation meet the Montana Department of
Transportation Standard Specifications for Construction (MDTSSC). The project will incorporate random
riprap as defined in MDTSSC Section 613.03.1. Stones will be uniformly distributed below the pipe
openings and placed by hand or machine to create a uniform surface and stable mass.
The riprap sizing calculations were completed in accordance with the U.S. Department of
Transportation’s Hydraulic Engineering Circular No. 14 (HEC-14), Hydraulic Design of Energy Dissipators
for Culverts and Channels, and the gradation requirements outlined in MDT Standard Specifications for
Construction (MDTSSC) Tables 701-21 and 701-22 for random riprap. The key design elements of the
apron include the median stone size (D₅₀), as well as the apron’s length, width, and thickness. A
corresponding detail sheet (C-508) is included in the Civil Plan set, which illustrates the riprap apron
layout and dimensions derived from these calculations.
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Inputs
Table 4 below shows the following parameters that are required for designing the size of the rip rap
apron.
Table 4 Inputs for Riprap Design
Normal Depth
HEC-14 specifies that, under certain conditions, an “effective culvert rise” (D′) must be used in place of
the full culvert diameter (D) when sizing outlet protection. This adjustment applies when the culvert is
not flowing full and the flow regime is supercritical. In these cases, the water depth in the pipel is less
than the physical rise of the pipe, and therefore using the full diameter would underestimate flow
velocity and lead to undersized riprap. Instead, the normal depth of flow (yₙ) is substituted for D in the
equations.
The normal depth yₙ was determined using Manning’s equation for partially full circular conduits, which
accounts for the wetted area and hydraulic radius at different flow depths. This ensures that the
calculated depth reflects actual hydraulic conditions within the culvert for the design discharge. Table 5
below presents the normal depth calculation results used to define the effective culvert rise (D′) for this
design.
Input Value Notes
Design discharge, Q (ft^3/s )0.34 SWMM Calculations: 100yr-24hr Peak Runoff. See Appendix B in the Storm
Drainage Report
Pipe Diameter, D (ft)0.666666667 8 inch, Schedule 40 PVC.
If supercritical: normal depth y_n (ft)0.228644124 Used in D′ when supercritical (Eq 10.5)
Adjusted Culvert Size D'(ft)0.447655395 Adjusted Culvert Size per Eq. 10.5 HEC-14
Tailwater depth, TW (ft)0.179062158 Per HEC-14 use TW = 0.4D
Flow regime in culvert (normal/supercritical)supercritical 'normal' or 'supercritical'
Riprap specific gravity, Sr 2.65 2.65 as assumed in HEC-14
Channel bottom width downstream, b_ch [ft or m] (optional)0.016 Pipe invert slope for normal depth. See Appendix D in Storm Drainage Report
Manning roughness n 0.01 Material roughness, PVC ~0.009–0.011; Engineering Toolbox
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Table 5 Normal Depth Calculations
Riprap Sizing
The median stone size (D₅₀) was determined using the outlet protection equations provided in HEC-14
(Eq. 10.4 and Eq. 10.5). These equations relate stone size to the culvert rise (effective diameter D′),
tailwater depth (TW), and unit discharge (Q). For reference, the equations are shown below:
Eq. 10.4: 𝐷𝐷50 =0.2𝐷𝐷′(𝑄𝑄√𝑔𝑔𝐷𝐷2.5)4 3�(𝐷𝐷′𝑇𝑇𝑇𝑇)
Eq. 10.5: 𝐷𝐷′= 𝐷𝐷+𝑦𝑦𝑛𝑛2
Equation 10.5 defines the effective culvert rise (D′) as the average of the physical culvert rise (D) and the
normal depth (yₙ). This substitution is required when the culvert does not flow full (supercritical
conditions), ensuring the equations reflect actual hydraulic conditions inside the pipe.
Equation 10.4 was developed on the basis of a rock specific gravity (Sᵣ) of 2.65, which is typical of dense
materials such as quartzite and granite. As noted in HEC-14, if the actual riprap source has a
substantially different specific gravity, the computed median stone size (D₅₀) should be adjusted
accordingly. The MDT Standard Specifications (Table 701-21) require a minimum specific gravity of 2.40,
which is reasonably close to the 2.65 reference value. For this project, the conservative value of 2.65
was applied in order to simplify calculations and provide an added margin of stability.
Mannings Normal Depth Mannings: Partially Full Pipe:
Known Parameters
Design discharge, Q (cfs)0.34
Diameter, D (ft)0.666666667
Slope, S_o (ft/ft)0.016
Manning n 0.01
Theta (θ)2.502643794
Cross Sectional Area of Flow (A)0.1059051
Wetted perimeter (P)0.834214598
Right Side 1.589948065
Left Side 1.597666556
y_n 0.228644124
Solve for Normal Depth (y_n)
Normal Depth Equation
Mannings Partially Full Pipe Equation
Equations
Normal Depth for a Partially-Full Circular Pipe
https://www.youtube.com/watch?v=gyGixsNdLGo
Calculate the left side, then Goal Seek θ to match
the right side.
Normal Depth For Partially Full Pipe
Calculated Parameters
Solve for Theta
Notes
SWMM Calculations: 100yr-24hr Peak Runoff. See
Appendix B in the Storm Drainage Report.
8 inch, Schedule 40 PVC.
Pipe invert slope for normal depth. See Appendix D
in Storm Drainage Report
PVC ~0.009–0.011; Engineering Toolbox
Goal Seak
Normal Depth for a Partially-Full Circular Pipe
https://www.youtube.com/watch?v=gyGixsNdLGo
𝑄=1.486𝑛𝐴𝑅2 3�𝑆1 2�𝑦𝑛=𝐷𝐷2 (1 −cos 𝜃2 )(𝜃−sin 𝜃)5 3�𝜃2 3�=20.16 𝑛𝑄𝐷𝐷8 3�∗𝑆
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This procedure ensures that the apron dimensions and riprap class meet both HEC-14 design guidance
and the MDTSSC construction standards for erosion protection and outlet stability. The resulting riprap
sizing calculations are summarized in Table 6, which presents the design inputs, computed D₅₀, selected
riprap class, and the corresponding apron dimensions derived using the procedure described above.
Table 6 Sizing Calculations
Geometry and Quantities
Once the median stone size (D₅₀) was calculated, it was compared to the available riprap classes defined
in MDT Standard Specifications for Construction (MDTSSC) Table 701-22 and cross-referenced with the
classification system presented in HEC-14. The selected riprap class establishes the apron geometry
through standard multipliers prescribed by HEC-14:
• Apron length (L): 4 × D′ (effective culvert rise)
• Apron thickness (T): 3.5 × D₅₀
• Apron width (W): 3D at the culvert face, flaring to the downstream width determined by a 1:3
side flare, as illustrated in HEC-14 Figure 10.4.
After determining the geometry, quantities were computed by multiplying the apron length, average
width, and thickness to obtain the total riprap volume. This volume was then converted to weight using
an assumed bulk unit weight of 2.0 tons/yd³. This assumption corresponds to a rock specific gravity of
2.65 and is consistent with MDTSSC Table 701-22, which specifies the gradation and minimum density
requirements for riprap.
The results of this procedure—including the calculated apron dimensions, material quantities, and the
associated excavation and fill volumes—are presented in Table 7 below.
Calc Item Formula / Reference US Units Notes
Units selected US US
g (gravity)HEC-14 Eq 10.4 32.2
Specific gravity S_r HEC-14 note under Eq 10.4 2.65 Assumed rock specific gravity HEC-14
Design discharge Q 0.34 SWMM Calculations: 100yr-24hr Peak Runoff. See
Appendix B in the Storm Drainage Report
Culvert rise/diameter D (ft)0.447655395 Adjusted Culvert Size per Eq. 10.5 HEC-14
Tailwater TW (raw)(ft)0.179062158 Per HEC-14 use TW = 0.4D
Normal depth y_n (if supercritical)(ft)HEC-14 Eq 10.5 0.228644124 See Normal Depth Calculation
Adjusted culvert rise D'(ft)HEC-14 Eq 10.5 0.447655395 Use D' in Eq 10.4 when supercritical
Median size D50 (ft)HEC-14 Eq 10.4 0.07647199 Calculated Value
D50 (in)Unit conversion 0.91766388
Selected Riprap Class Frome Table 701-22, riprap class 1 (D50 = 3 in)Class 1
Class D50 (display)From table 701-22 3 D50 in inch
Apron length multiplier (×D)From Table 10.1 4 L = mult × D' (use adjusted rise)
Apron depth multiplier (×D50)From Table 10.1 3.5 Depth = mult × required D50
Since calculations show D50 as approximately 1 inch,
Class 1 with D50 = 3 inches is required.
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Table 7 Calculated Geometry and Quantities
E. DESIGN HYDRAULICS COMPARED TO DESIGN CRITERIA
For 50-yr and 100-yr storms, this design limits flows to below preconstruction levels. For 10-year storm
events, this design limits flows to levels comparable to preconstruction conditions. 2yr and 0.5in storm
event runoff is contained, treated, and slowed. Per the geotechnical report for this area, stormwater
should not be retained and infiltrated. Limiting runoff to 0.01 CFS without infiltrating all runoff was not
possible while also draining ponds in less than 72hrs and without unreasonably small exit orifices. Outlet
orifice sizing was selected with the aim of allowing continued operation of the system without clogs. The
Montana Post-Construction Storm Water BMP design Guidance Manual recommends a minimum outlet
orifice size of 3”. The smallest orifice on both ponds’ outlet device was sized at 1.5.” This should allow a
balance between limiting flows and preventing clogging of the orfices. While smaller than the
recommended, this reduces flows such that they are similar overall to the 10-year storm flow. With a
trash rack and regular maintenance, the 1.5” orifices should still be large enough to remain clear of clogs
due to the small overall size of the system. Furthermore, this orifice size limits outflow to significantly
below the runoff treatment flow for storms below 10 years.
F. WATER QUALITY TREATMENT DESIGN.
Sidewalk chases from the parking lot will act as sediment forebays, with easy access for cleaning.
1ft deep concrete lined micro pools set before the outlet structure will facilitate the removal of TSS from
the system. This combined with the detention basin’s ability to contain 100% of the RTV volume, and the
volume of runoff from larger storms will effectively remove TSS from the system.
Geometry & Quantities Formula / Reference US Units Notes
Adjusted culvert rise D'From Sizing 0.447655395
Apron length, L Table 10.1: L = 4× D'1.790621582 ft
Required D50 (length units)From Sizing (adjusted)3 Median rock size (in)
Apron depth (thickness)Depth = 3.5 × D50 (Table 10.1)0.875 Placed riprap thickness (ft)
Width at culvert, W0 Adopt 3×D' or channel width if larger 1.342966186 3xD' (ft)
Width at apron end, W_end W_end = 3D' + (2/3)L 2.536713907 1:3 flare per HEC-14 example
Plan area of apron Trapezoid: A = (W0 + W_end)/2 × L 3.473519453 Horizontal footprint (ft^2)
Riprap volume (apron only)V = Area × Depth 3.039329521
Excavation for riprap thickness
(ignore subgrade
irregularities)(ft^3)
Material Quantities
Bulk unit weight (riprap)Assumptions 2 US ton/yd^3
Riprap mass (US tons)Mass = V × γ_bulk 0.22513552 US ton
Excavation & Fill
Total excavation volume (riprap + filter)V_excav = V_riprap 3.039329521 yd^3
Fill volume (riprap only)Equals riprap volume if subgrade is removed to full depth 3.039329521 yd^3
Sundance Springs
COMM LOT 2 REVISED September 30, 2025
17
EVALUATION OF MAJOR STORM FLOOD RISKS
1. COMPARISON OF POST-PROJECT FLOOD HAZARDS DURING THE MAJOR STORM TO
EXISTING CONDITIONS
This stormwater system is designed to handle 100-year storms. In the event of a larger storm, the 1ft of
freeboard and emergency drains in the detention ponds should reduce flood risk to below existing
conditions.
2. POTENTIAL FLOOD HAZARDS AND MITIGATION MEASURES
The outlet control structure must be adequately maintained to allow the ponds to drain during storm
conditions.
3. IMPACTS TO FEMA FLOODPLAINS
No impacts to FEMA floodplains have been identified.
OPERATION, INSPECTION, AND MAINTENACE CONSIDERATIONS
The retention area should be regularly inspected for signs of erosion, seepage, and structural damage.
Sediment should be removed from the micro pool and the pool restored to its original dimensions when
the sediment has accumulated to one-half the design depth of the pool. Sediment shall be removed and
the pond re-graded when accumulated sediment volume exceeds 10 percent of the main treatment cell
volume. Any signs of erosion and/or structural damage to the detention basin noted during an
inspection should be repaired immediately. Basin should be inspected once every 14 calendar days and
within 24 hours of the end of a storm event of 0.5 inches or greater.
REFERENCES
USDA TR-55 Urban Hydrology for Small Watersheds 1984
FHA HEC-22 Urban Drainage Design 2004
Montana Post-Construction Storm Water BMP Design Guidance Manual 2017
MDEQ Circular-8
City of Bozeman Design and Construction Standards 2024
EPA Storm Water Management Model User’s Manual Version 5.1 2015
Baiamonte, G. SCS Curve Number and Green-Ampt Infiltration Models. J. Hydrol. Eng. 2019, 24,
USDT HEC_14 Hydraulic Design of Energy Dissipators for Culverts and Channels 3rd Edition 2006
MDT Standard Specifications for Road and Bridge Construction V5.1 2020
APPENDICES
A. FEMA FIRMette 30031C0818E
B. Basin Area and Parameter Calculations
C. SSA Flow and Drawdown Graph
D. SSA SWMM Model
Synergy Engineering and Konsulting · 3731 Equestrian Lane STE 102, Bozeman, MT 59718 · 406-624-6137
APPENDIX A
FEMA FIRMETTE 30031C0818E
Synergy Engineering and Konsulting · 3731 Equestrian Lane STE 102, Bozeman, MT 59718 · 406-624-6137
APPENDIX B
BASIN AREA AND PARAMETER CALCULATIONS
Sundance Springs EPA SWMM Calculations 7/10/2025
Location Curve Numbers
Subdivision Sundance Springs Subdivision Soil Group C
EQ#00RGG41698 Herbaceous Range 74
County Gallatin County Asphalt Drive/Parking 98
Location South 3rd & Little horse drive Commercial Buildings 98
Lot/Area no.Common lot 2 Lawn, Landscape 74
Permeable Pavers 88
City of Bozeman table 6.5.1 w/ SCS Type II rainfall
Rainfall total In Pre (min)Post (min)
100yr-24hr 2.34 Onsite Basin 1 9.92 9.92
50yr-24hr 2.15 Onsite Basin 2 3.22 3.22
10yr-24hr 1.7 Onsite Basin 3 0.07 0.07
2yr-24hr 1.18 Onsite Basin 4 0.03 0.06
Longest Tc 9.92 9.92
Final Summary Table for Report
Peak Runoff W. Pond Max E. Pond Max W Pond Dry E Pond Dry
Controlled Area Pre Post CFS FT FT HRS HRS
0.5in Storm 0.00 0.35 0.08 0.53 0.74 24 23
2yr-24hr 0.01 0.91 0.13 1.28 1.71 28 25
10yr-24hr 0.10 1.57 0.17 2.12 2.52 29 25
50yr-24hr 0.32 2.21 0.30 2.81 2.89 29 26
100yr-24hr 0.46 2.49 0.34 3.06 3.08 30 26
Uncontrolled Area Pre Post
0.5in Storm 0.00 0.00
2yr-24hr 0.00 0.00
10yr-24hr 0.09 0.09
50yr-24hr 0.18 0.18
100yr-24hr 0.20 0.20
Time of Concentration
Before Control
Peak Runoff (CFS)
TC (HEC 22)
After Control
𝑇 =𝐿
𝑉=𝐿
3.28 ∗𝑘∗𝑆.ହ
Sundance Springs EPA SWMM Calculations 7/10/2025
Predevelopment Post Development
Basin Parameters AC CN Basin Parameters AC CN
Total Area 1.31 74.0 Total Area 1.31 76.1
Area Undeveloped 1.315 74 Area Undeveloped 0.136 74
Area Building 0.000 98 Area Building 0.134 98
Area Permeable 0.000 88 Area Permeable 0.151 88
Area Asphalt 0.000 98 Area Asphalt 0.450 98
Area Lawn 0.000 74 Area Lawn 0.443 74
Total Controlled Total Controlled
Basin Parameters AC CN Basin Parameters AC CN
Total Area 1.18 74.00 Total Area 1.18 87.7
Area Undeveloped 1.179 74 Area Undeveloped 0.000 74
Area Building 0.000 98 Area Building 0.134 98
Area Permeable 0.000 88 Area Permeable 0.151 88
Area Asphalt 0.000 98 Area Asphalt 0.450 98
Area Lawn 0.000 74 Area Lawn 0.443 74
EPA SWMM Parameters EPA SWMM Parameters
Length Longest Path 214.00 ft
Equivalent Width 240 ft
Average Slope 0.96%
Percent Impervious 0.00%
Post Development: Total Basin
Predevelopment: Total Basin Post Development: Total Basin
Predevelopment: Total Basin
Sundance Springs EPA SWMM Calculations 7/10/2025
West Pond West Pond
Basin Parameters AC CN Basin Parameters AC CN
Total Area 0.720 74.0 Total Area 0.720 86.1
Area Undeveloped 0.720 74 Area Undeveloped 0.000 74
Area Building 0.000 98 Area Building 0.077 98
Area Permeable 0.000 88 Area Permeable 0.068 88
Area Asphalt 0.000 98 Area Asphalt 0.246 98
Area Lawn 0.000 74 Area Lawn 0.329 74
EPA SWMM Parameters EPA SWMM Parameters
Length Longest Path 267.00 ft Length Longest Path 267.00 ft
Equivalent Width 117 ft Equivalent Width 117 ft
Average Slope 0.41%Average Slope 0.41%
Percent Impervious 0.00%Percent Impervious 44.81%
East Pond East Pond
Basin Parameters AC CN Basin Parameters AC 0
Total Area 0.459 74.0 Total Area 0.459 90.2
Area Undeveloped 0.459 74 Area Undeveloped 0.000 74
Area Building 0.000 98 Area Building 0.057 98
Area Permeable 0.000 88 Area Permeable 0.083 88
Area Asphalt 0.000 98 Area Asphalt 0.204 98
Area Lawn 0.000 74 Area Lawn 0.114 74
EPA SWMM Parameters EPA SWMM Parameters
Length Longest Path 172.00 ft Length Longest Path 172.00 ft
Equivalent Width 116 ft Equivalent Width 116 ft
Average Slope 1.63%Average Slope 1.63%
Percent Impervious 0.00%Percent Impervious 56.99%
Onsite Basin 1 Onsite Basin 1
Onsite Basin 2 Onsite Basin 2
Sundance Springs EPA SWMM Calculations 7/10/2025
Undeveloped Undeveloped
AC CN AC CN
Total Area 0.061 74.0 Total Area 0.061 74.0
Area Undeveloped 0.061 74 Area Undeveloped 0.061 74
Area Building 0.000 98 Area Building 0.000 98
Area Permeable 0.000 88 Area Permeable 0.000 88
Area Asphalt 0.000 98 Area Asphalt 0.000 98
Area Lawn 0.000 74 Area Lawn 0.000 74
EPA SWMM Parameters EPA SWMM Parameters
Length Longest Path 14.00 ft Length Longest Path 14.00 ft
Equivalent Width 190 ft Equivalent Width 190 ft
Average Slope 23.57%Average Slope 23.57%
Percent Impervious 0.00%Percent Impervious 0.00%
AC CN AC CN
Total Area 0.075 74.0 Total Area 0.075 74.0
Area Undeveloped 0.075 74 Area Undeveloped 0.075 74
Area Building 0.000 98 Area Building 0.000 98
Area Permeable 0.000 88 Area Permeable 0.000 88
Area Asphalt 0.000 98 Area Asphalt 0.000 98
Area Lawn 0.000 74 Area Lawn 0.000 74
EPA SWMM Parameters EPA SWMM Parameters
Length Longest Path 8.00 ft Length Longest Path 13.00 ft
Equivalent Width 408 ft Equivalent Width 251 ft
Average Slope 41.25%Average Slope 25.38%
Percent Impervious 0.00%Percent Impervious 0.00%
Onsite Basin 4 Onsite Basin 4
Onsite Basin 3 Onsite Basin 3
Sundance Springs SCS TR-55 Calculations 7/10/2025
Location Curve Numbers
Subdivision Sundance Springs Subdivision Soil Group C
EQ#00RGG41698 Herbaceous Range 74
County Gallatin County Asphalt Drive/Parking 98
Location South 3rd & Little horse drive Commercial Buildings 98
Lot/Area no.Common lot 2 Lawn, Landscape 74
Permeable Pavers 88
City of Bozeman table 6.5.1 w/ SCS Type II rainfall
Rainfall total In Pre (min)Post (min)
100yr-24hr 2.34 Onsite Basin 1 9.92 9.92
50yr-24hr 2.15 Onsite Basin 2 3.22 3.22
10yr-24hr 1.7 Onsite Basin 3 0.07 0.07
2yr-24hr 1.18 Onsite Basin 4 0.03 0.06
0.5-in event 0.5 Longest Tc 9.92 9.92
Controlled Area Pre Post
0.5in Storm 0.03 0.06
2yr-24hr 0.12 0.55
10yr-24hr 0.41 1.02
50yr-24hr 0.73 2.21
100yr-24hr 0.88 2.49
TR-55 Summary
Peak Runoff (CFS)
Time of Concentration
TC (HEC 22)
𝑇 =𝐿
𝑉=𝐿
3.28 ∗𝑘∗𝑆.ହ
Sundance Springs SCS TR-55 Calculations 7/10/2025
Predevelopment Post Development
Basin Parameters AC CN Basin Parameters AC CN
Total Area 1.31 74.0 Total Area 1.31 76.1
Area Undeveloped 1.315 74 Area Undeveloped 0.136 74
Area Building 0.000 98 Area Building 0.134 98
Area Permeable 0.000 88 Area Permeable 0.151 88
Area Asphalt 0.000 98 Area Asphalt 0.450 98
Area Lawn 0.000 74 Area Lawn 0.443 74
Total Controlled Total Controlled
Basin Parameters AC CN Basin Parameters AC CN
Total Area 1.18 74.00 Total Area 1.18 87.7
Area Undeveloped 1.179 74 Area Undeveloped 0.000 74
Area Building 0.000 98 Area Building 0.134 98
Area Permeable 0.000 88 Area Permeable 0.151 88
Area Asphalt 0.000 98 Area Asphalt 0.450 98
Area Lawn 0.000 74 Area Lawn 0.443 74
TR-55 Flows TR-55 Flows
S 3.51 S 1.40
Q 100-yr 0.88 CFS Q 100-yr 1.62 CFS
Q 50-yr 0.73 CFS Q 50-yr 1.44 CFS
Q 10-yr 0.41 CFS Q 10-yr 1.02 CFS
Q 2-yr 0.12 CFS Q 2-yr 0.55 CFS
Q 0.5in 0.03 CFS Q 0.5in 0.06 CFS
Predevelopment: Total Basin Post Development: Total Basin
Predevelopment: Total Basin Post Development: Total Basin
Sundance Springs SCS TR-55 Calculations 7/10/2025
West Pond West Pond
Basin Parameters AC CN Basin Parameters AC CN
Total Area 0.720 74.0 Total Area 0.720 86.1
Area Undeveloped 0.720 74 Area Undeveloped 0.000 74
Area Building 0.000 98 Area Building 0.077 98
Area Permeable 0.000 88 Area Permeable 0.068 88
Area Asphalt 0.000 98 Area Asphalt 0.246 98
Area Lawn 0.000 74 Area Lawn 0.329 74
TR-55 Flows TR-55 Flows
S 3.51 S 1.62
Q 100-yr 0.88 CFS Q 100-yr 1.53 CFS
Q 50-yr 0.73 CFS Q 50-yr 1.35 CFS
Q 10-yr 0.41 CFS Q 10-yr 0.94 CFS
Q 2-yr 0.12 CFS Q 2-yr 0.49 CFS
Q 0.5in 0.03 CFS Q 0.5in 0.04 CFS
East Pond East Pond
Basin Parameters AC CN Basin Parameters AC 0
Total Area 0.459 74.0 Total Area 0.459 90.2
Area Undeveloped 0.459 74 Area Undeveloped 0.000 74
Area Building 0.000 98 Area Building 0.057 98
Area Permeable 0.000 88 Area Permeable 0.083 88
Area Asphalt 0.000 98 Area Asphalt 0.204 98
Area Lawn 0.000 74 Area Lawn 0.114 74
TR-55 Flows TR-55 Flows
S 3.51 S 1.08
Q 100-yr 0.88 CFS Q 100-yr 1.76 CFS
Q 50-yr 0.73 CFS Q 50-yr 1.58 CFS
Q 10-yr 0.41 CFS Q 10-yr 1.15 CFS
Q 2-yr 0.12 CFS Q 2-yr 0.66 CFS
Q 0.5in 0.03 CFS Q 0.5in 0.11 CFS
Onsite Basin 1 Onsite Basin 1
Onsite Basin 2 Onsite Basin 2
Sundance Springs SCS TR-55 Calculations 7/10/2025
Undeveloped Undeveloped
AC CN AC CN
Total Area 0.061 74.0 Total Area 0.061 74.0
Area Undeveloped 0.061 74 Area Undeveloped 0.061 74
Area Building 0.000 98 Area Building 0.000 98
Area Permeable 0.000 88 Area Permeable 0.000 88
Area Asphalt 0.000 98 Area Asphalt 0.000 98
Area Lawn 0.000 74 Area Lawn 0.000 74
TR-55 Flows TR-55 Flows
S 3.51 S 3.51
Q 100-yr 0.88 CFS Q 100-yr 0.88 CFS
Q 50-yr 0.73 CFS Q 50-yr 0.73 CFS
Q 10-yr 0.41 CFS Q 10-yr 0.41 CFS
Q 2-yr 0.12 CFS Q 2-yr 0.12 CFS
Q 0.5in 0.03 CFS Q 0.5in 0.03 CFS
AC CN AC CN
Total Area 0.075 74.0 Total Area 0.075 74.0
Area Undeveloped 0.075 74 Area Undeveloped 0.075 74
Area Building 0.000 98 Area Building 0.000 98
Area Permeable 0.000 88 Area Permeable 0.000 88
Area Asphalt 0.000 98 Area Asphalt 0.000 98
Area Lawn 0.000 74 Area Lawn 0.000 74
TR-55 Flows TR-55 Flows
S 3.51 S 3.51
Q 100-yr 0.88 CFS Q 100-yr 0.88 CFS
Q 50-yr 0.73 CFS Q 50-yr 0.73 CFS
Q 10-yr 0.41 CFS Q 10-yr 0.41 CFS
Q 2-yr 0.12 CFS Q 2-yr 0.12 CFS
Q 0.5in 0.03 CFS Q 0.5in 0.03 CFS
Onsite Basin 4 Onsite Basin 4
Onsite Basin 3 Onsite Basin 3
Sundance Springs Ration Method Calculations 7/10/2025
Rainfall intensity table 6.5.2
Unimproved Area 0.20 Intensity In/hr
Asphalt Drive/Parking 0.95 100yr-24hr 0.1225 1.25 factor of safety
Commercial Buildings 0.95 50yr-24hr 0.09
Lawn, Landscape 0.15 10yr-24hr 0.071
Permeable Pavers 0.30 2yr-24hr 0.049
Predevelopment Post Development
Basin Parameters AC C Basin Parameters AC C
Total Area 1.18 0.20 Total Area 1.18 0.53
Area Undeveloped 1.179 0.2 Area Undeveloped 0.000 0.2
Area Building 0.000 0.95 Area Building 0.134 0.95
Area Permeable 0.000 0.30 Area Permeable 0.151 0.30
Area Asphalt 0.000 0.95 Area Asphalt 0.450 0.95
Area Lawn 0.000 0.15 Area Lawn 0.443 0.15
Rational Calcs Rational Calcs
100yr-24hr 0.029 CFS 100yr-24hr 0.076 CFS
50yr-24hr 0.021 CFS 50yr-24hr 0.056 CFS
10yr-24hr 0.017 CFS 10yr-24hr 0.044 CFS
2yr-24hr 0.012 CFS 2yr-24hr 0.030 CFS
Post Development: Controlled AreaPredevelopment: Controlled Area
Runoff Coefficients C
Sundance Springs Ration Method Calculations 7/10/2025
West Pond West Pond
Onsite Basin 1 Onsite Basin 1
Basin Parameters AC CN Basin Parameters AC CN
Total Area 0.720 0.2 Total Area 0.720 0.5
Area Undeveloped 0.720 0.2 Area Undeveloped 0.000 0.2
Area Building 0.000 0.95 Area Building 0.077 0.95
Area Permeable 0.000 0.30 Area Permeable 0.068 0.30
Area Asphalt 0.000 0.95 Area Asphalt 0.246 0.95
Area Lawn 0.000 0.15 Area Lawn 0.329 0.15
Rational Calcs Rational Calcs
100yr-24hr 0.018 CFS 100yr-24hr 0.046 CFS
50yr-24hr 0.013 CFS 50yr-24hr 0.034 CFS
10yr-24hr 0.010 CFS 10yr-24hr 0.027 CFS
2yr-24hr 0.007 CFS 2yr-24hr 0.018 CFS
East Pond East Pond
Basin Parameters AC C Basin Parameters AC C
Total Area 0.459 0.2 Total Area 0.459 0.6
Area Undeveloped 0.459 0.2 Area Undeveloped 0.000 0.2
Area Building 0.000 0.95 Area Building 0.057 0.95
Area Permeable 0.000 0.30 Area Permeable 0.083 0.30
Area Asphalt 0.000 0.95 Area Asphalt 0.204 0.95
Area Lawn 0.000 0.15 Area Lawn 0.114 0.15
Rational Calcs Rational Calcs
100yr-24hr 0.011 CFS 100yr-24hr 0.036 CFS
50yr-24hr 0.008 CFS 50yr-24hr 0.026 CFS
10yr-24hr 0.007 CFS 10yr-24hr 0.021 CFS
2yr-24hr 0.004 CFS 2yr-24hr 0.014 CFS
Onsite Basin 2 Onsite Basin 2
Sundance Springs Ration Method Calculations 7/10/2025
Undeveloped Undeveloped
AC CN AC CN
Total Area 0.061 0.2 Total Area 0.061 0.2
Area Undeveloped 0.061 0.2 Area Undeveloped 0.061 0.2
Area Building 0.000 0.95 Area Building 0.000 0.95
Area Permeable 0.000 0.30 Area Permeable 0.000 0.30
Area Asphalt 0.000 0.95 Area Asphalt 0.000 0.95
Area Lawn 0.000 0.15 Area Lawn 0.000 0.15
Rational Calcs Rational Calcs
100yr-24hr 0.0015 CFS 100yr-24hr 0.0015 CFS
50yr-24hr 0.0011 CFS 50yr-24hr 0.0011 CFS
10yr-24hr 0.0009 CFS 10yr-24hr 0.0009 CFS
2yr-24hr 0.0006 CFS 2yr-24hr 0.0006 CFS
AC CN AC CN
Total Area 0.075 0.2 Total Area 0.075 0.2
Area Undeveloped 0.075 0.2 Area Undeveloped 0.075 0.2
Area Building 0.000 0.95 Area Building 0.000 0.95
Area Permeable 0.000 0.30 Area Permeable 0.000 0.30
Area Asphalt 0.000 0.95 Area Asphalt 0.000 0.95
Area Lawn 0.000 0.15 Area Lawn 0.000 0.15
Rational Calcs Rational Calcs
100yr-24hr 0.0018 CFS 100yr-24hr 0.0018 CFS
50yr-24hr 0.0013 CFS 50yr-24hr 0.0013 CFS
10yr-24hr 0.0011 CFS 10yr-24hr 0.0011 CFS
2yr-24hr 0.0007 CFS 2yr-24hr 0.0007 CFS
Onsite Basin 4 Onsite Basin 4
Onsite Basin 3 Onsite Basin 3
Sundance Springs RRV and RTF Calculations 7/10/2025
Final Summary Table
Storage Sizing Basin 1 Basin 2 Total
Pond Capacity at 3.5ft water depth2471 1498 3969
Montana BMP (Rational) MethodBasin 1 Basin 2 Total
RTV (CF)592.24 468.70 1060.94
RTF (CFS)0.115 0.151 0.27
EPA SWMM flows Basin 1 Basin 2 Total
RTV (CF)525 424 949.00
RTF (CFS)0.18 0.17 0.35
RTV eqn
Sundance Springs RRV and RTF Calculations 7/10/2025
P=0.5 in
Onsite Basin 1 Onsite Basin 2
Basin Parameters AC CN Basin Parameters AC CN
Total Area 0.720 86.1 Total Area 0.459 90.2
Area Undeveloped 0.000 74 Area Undeveloped 0.000 74
Area Building 0.077 98 Area Building 0.057 98
Area Permeable 0.068 88 Area Permeable 0.083 88
Area Asphalt 0.246 98 Area Asphalt 0.204 98
Area Lawn 0.329 74 Area Lawn 0.114 74
Impervious 45%Impervious 57%
Rv 0.45 Rv 0.56
RTV 0.01 acre-ft RTV 0.01 acre-ft
RTV 592.24 CF RTV 468.70 CF
RTF Calculation RTF Calculation
Q 0.227 in Q 0.281 in
CN 86.1 CN 90.2
T_c 0.149 hrs Tc 0.100 hrs
I_a 0.323 in I_a 0.217 in
I_a/P 0.647 in I_a/P 0.434 in
q_u 450 csm/in q_u 750 csm/in
RTF 0.115 CFS RTF 0.151 CF
Synergy Engineering and Konsulting · 3731 Equestrian Lane STE 102, Bozeman, MT 59718 · 406-624-6137
APPENDIX C
SSA FLOW AND DRAWDOWN GRAPH
Controlled Area 0.5in Storm
Controlled Area 2-yr Storm
Controlled Area 10-yr Storm
Controlled Area 50-yr Storm
Controlled Area 100-yr Storm
Outlet Pipe Size Flow Check for 100-yr Storm
Uncontrolled Area 2-yr Storm
Uncontrolled Area 10-yr Storm
Uncontrolled Area 50-yr Storm
Uncontrolled Area 100-yr Storm
Synergy Engineering and Konsulting · 3731 Equestrian Lane STE 102, Bozeman, MT 59718 · 406-624-6137
APPENDIX D
SSA SWMM MODEL
Project Description
All in one SWMM 100yr24hrV3.1.SPF
Project Options
CFS
Elevation
EPA SWMM
SCS Curve NumberHydrodynamic
YES
NO
Analysis Options
Mar 23, 2022 01:00:00
Mar 27, 2022 02:00:00
Mar 23, 2022 01:00:000days0 01:00:00 days hh:mm:ss
0 00:05:00 days hh:mm:ss
0 00:05:00 days hh:mm:ss
30 seconds
Number of Elements
Qty
2
37
3
2
0
0
28
1
2
0
3
200
0
Rainfall Details
SN Rain Gage Data Data Source Rainfall Rain State County Return Rainfall Rainfall
ID Source ID Type Units Period Depth Distribution
(years)(inches)
1 Time Series 100yr24hr Cumulative inches Montana Gallatin 100 2.34 SCS Type II 24-hr
Outlets ...............................................
Pollutants ...................................................
Land Uses ..................................................
Links............................................................
Channels ...........................................
Pipes .................................................
Pumps ...............................................
Orifices ..............................................
Weirs .................................................
Nodes..........................................................
Junctions ...........................................
Outfalls ..............................................
Flow Diversions .................................
Inlets ..................................................
Storage Nodes ..................................
Runoff (Dry Weather) Time Step ...............
Runoff (Wet Weather) Time Step ..............
Reporting Time Step ..................................
Routing Time Step .....................................
Rain Gages ................................................
Subbasins...................................................
Enable Overflow Ponding at Nodes ...........
Skip Steady State Analysis Time Periods ..
Start Analysis On .......................................
End Analysis On ........................................
Start Reporting On .....................................Antecedent Dry Days .................................
File Name ...................................................
Flow Units ..................................................
Elevation Type ...........................................
Hydrology Method ......................................
EPA SWMM Infiltration Method ..................Link Routing Method ..................................
Subbasin Summary
SN Subbasin Area Impervious Weighted Average Equivalent Impervious Pervious Total Total Total Total Peak Time ofIDAreaCurveSlopeWidthAreaAreaRainfallInfiltrationRunoffRunoffRunoffConcentration
Number Manning's Manning's Volume
Roughness Roughness(ac)(%)(%)(ft)(in)(in)(in)(ac-in)(cfs)(days hh:mm:ss)1 Pre-Basin 1.18 0.00 74.00 0.9600 239.00 0.0150 0.1000 2.34 1.5770 0.71 0.84 0.49 0 01:00:29
2 SubBasin1 0.72 45.00 86.10 0.4100 117.00 0.0150 0.1000 2.34 0.6190 1.67 1.20 1.27 0 01:02:22
3 SubBasin2 0.46 57.00 90.20 1.6300 116.00 0.0150 0.1000 2.34 0.3870 1.90 0.87 1.22 0 00:27:17
Node Summary
SN Element Element Invert Ground/Rim Initial Surcharge Ponded Peak Max HGL Max Min Time of Total Total Time
ID Type Elevation (Max)Water Elevation Area Inflow Elevation Surcharge Freeboard Peak Flooded FloodedElevationElevationAttainedDepthAttainedFloodingVolumeAttainedOccurrence(ft)(ft)(ft)(ft)(ft²)(cfs)(ft)(ft)(ft)(days hh:mm)(ac-in)(min)
1 EastJunc Junction 5030.50 5037.00 5030.50 0.00 0.00 0.25 5030.75 0.00 6.25 0 00:00 0.00 0.00
2 JuncAll Junction 5030.10 5035.00 5030.10 0.00 0.00 0.34 5030.32 0.00 4.68 0 00:00 0.00 0.003WestJuncJunction5033.50 5038.00 5033.50 0.00 0.00 0.10 5033.61 0.00 4.39 0 00:00 0.00 0.006EastPondStorage Node 5030.50 5038.00 5030.50 0.00
7 WestPond Storage Node 5033.50 5039.00 5033.50 0.00
Link Summary
SN Element Element From To (Outlet)Length Inlet Outlet Average Diameter or Manning's Peak Design Flow Peak Flow/Peak Flow Peak Flow Peak Flow Total Time Reported
ID Type (Inlet)Node Invert Invert Slope Height Roughness Flow Capacity Design Flow Velocity Depth Depth/Surcharged ConditionNodeElevationElevationRatioTotal Depth
Ratio
(ft)(ft)(ft)(%)(ft)(cfs)(cfs)(ft/sec)(ft)(min)7 East-Big Weir EastPond EastJunc 1.00 5030.50 5030.50 0.0000 0.250 0.0130
8 West-Big Weir WestPond WestJunc 25.00 5033.50 5033.50 1.6000 0.120 0.01506West-SmaOrifice WestPond WestJunc 5033.50 5033.50 0.120
Subbasin Hydrology
Subbasin : Pre-Basin
Input Data
Area (ac) ...................................................1.18Impervious Area (%) ..................................0.00
Weighted Curve Number ...........................74.00Conductivity (in/hr) ....................................0.1500Drying Time (days) ....................................7.00
Average Slope (%) ....................................0.9600
Equivalent Width (ft) ..................................239.00Impervious Area
Manning's Roughness ..........................0.0150
Pervious Area Manning's Roughness ..........................0.1000Curb & Gutter Length (ft) ...........................0.00Rain Gage ID ............................................Bozman100-24hr
Composite Curve Number
Area Soil Curve
Soil/Surface Description (acres)Group Number
-1.18 -74.00Composite Area & Weighted CN 1.18 74.00
Subbasin Runoff Results
Total Rainfall (in) .......................................2.34Total Runon (in) .........................................0.00
Total Evaporation (in) ................................0.0000Total Infiltration (in) ....................................1.5770Total Runoff (in) ........................................0.71
Peak Runoff (cfs) .......................................0.49
Weighted Curve Number ...........................74.00Time of Concentration (days hh:mm:ss) ....0 01:00:29
Subbasin : Pre-Basin
Subbasin : SubBasin1
Input Data
Area (ac) ...................................................0.72Impervious Area (%) ..................................45.00
Weighted Curve Number ...........................86.10
Conductivity (in/hr) ....................................0.1500Drying Time (days) ....................................7.00Average Slope (%) ....................................0.4100
Equivalent Width (ft) ..................................117.00Impervious Area Manning's Roughness ..........................0.0150Pervious Area
Manning's Roughness ..........................0.1000Curb & Gutter Length (ft) ...........................0.00Rain Gage ID ............................................Bozman100-24hr
Composite Curve Number
Area Soil CurveSoil/Surface Description (acres)Group Number
-0.72 -86.10Composite Area & Weighted CN 0.72 86.10
Subbasin Runoff Results
Total Rainfall (in) .......................................2.34Total Runon (in) .........................................0.00
Total Evaporation (in) ................................0.0000
Total Infiltration (in) ....................................0.6190Total Runoff (in) ........................................1.67Peak Runoff (cfs) .......................................1.27
Weighted Curve Number ...........................86.10Time of Concentration (days hh:mm:ss) ....0 01:02:22
Subbasin : SubBasin1
Subbasin : SubBasin2
Input Data
Area (ac) ...................................................0.46
Impervious Area (%) ..................................57.00Weighted Curve Number ...........................90.20Conductivity (in/hr) ....................................0.1500Drying Time (days) ....................................7.00
Average Slope (%) ....................................1.6300Equivalent Width (ft) ..................................116.00Impervious Area
Manning's Roughness ..........................0.0150Pervious Area
Manning's Roughness ..........................0.1000Curb & Gutter Length (ft) ...........................0.00Rain Gage ID ............................................Bozman100-24hr
Composite Curve Number
Area Soil CurveSoil/Surface Description (acres)Group Number
-0.46 -90.20
Composite Area & Weighted CN 0.46 90.20
Subbasin Runoff Results
Total Rainfall (in) .......................................2.34Total Runon (in) .........................................0.00Total Evaporation (in) ................................0.0000
Total Infiltration (in) ....................................0.3870Total Runoff (in) ........................................1.90Peak Runoff (cfs) .......................................1.22
Weighted Curve Number ...........................90.20
Time of Concentration (days hh:mm:ss) ....0 00:27:17
Subbasin : SubBasin2
Junction Input
SN Element Invert Ground/Rim Ground/Rim Initial Initial Surcharge Surcharge Ponded Minimum
ID Elevation (Max)(Max)Water Water Elevation Depth Area Pipe
Elevation Offset Elevation Depth Cover
(ft)(ft)(ft)(ft)(ft)(ft)(ft)(ft²)(ft)
1 EastJunc 5030.50 5037.00 6.50 5030.50 0.00 0.00 -5037.00 0.00 0.00
2 JuncAll 5030.10 5035.00 4.90 5030.10 0.00 0.00 -5035.00 0.00 0.00
3 WestJunc 5033.50 5038.00 4.50 5033.50 0.00 0.00 -5038.00 0.00 0.00
Junction Results
SN Element Peak Peak Max HGL Max HGL Max Min Average HGL Average HGL Time of Time of Total Total TimeIDInflowLateralElevationDepthSurchargeFreeboardElevationDepthMax HGL Peak Flooded FloodedInflowAttainedAttainedDepthAttainedAttainedAttainedOccurrenceFloodingVolumeAttainedOccurrence(cfs)(cfs)(ft)(ft)(ft)(ft)(ft)(ft)(days hh:mm)(days hh:mm)(ac-in)(min)
1 EastJunc 0.25 0.00 5030.75 0.25 0.00 6.25 5030.55 0.05 0 12:16 0 00:00 0.00 0.00
2 JuncAll 0.34 0.00 5030.32 0.22 0.00 4.68 5030.16 0.06 0 12:18 0 00:00 0.00 0.00
3 WestJunc 0.10 0.00 5033.61 0.11 0.00 4.39 5033.55 0.05 0 13:34 0 00:00 0.00 0.00
Channel Input
SN Element Length Inlet Inlet Outlet Outlet Total Average Shape Height Width Manning's Entrance Exit/Bend Additional Initial FlapIDInvertInvertInvertInvertDropSlopeRoughnessLossesLossesLossesFlowGateElevationOffsetElevationOffset
(ft)(ft)(ft)(ft)(ft)(ft)(%)(ft)(ft)(cfs)
1 Link-04 1.00 5030.10 0.00 5030.10 5030.10 0.00 0.0000 Trapezoidal
Channel Results
SN Element Peak Time of Design Flow Peak Flow/Peak Flow Travel Peak Flow Peak Flow Total Time Froude ReportedIDFlowPeak Flow Capacity Design Flow Velocity Time Depth Depth/Surcharged Number ConditionOccurrenceRatioTotal Depth
Ratio
(cfs)(days hh:mm)(cfs)(ft/sec)(min)(ft)(min)
1 Link-04
Pipe Input
SN Element Length Inlet Inlet Outlet Outlet Total Average Pipe Pipe Pipe Manning's Entrance Exit/Bend Additional Initial Flap No. ofIDInvertInvertInvertInvertDropSlopeShapeDiameter or Width Roughness Losses Losses Losses Flow Gate BarrelsElevationOffsetElevationOffsetHeight
(ft)(ft)(ft)(ft)(ft)(ft)(%)(ft)(ft)(cfs)1 Link-01 150.00 5033.50 0.00 5030.10 0.00 3.40 2.2700 CIRCULAR2Link-03 25.00 5030.50 0.00 5030.10 0.00 0.40 1.6000 CIRCULAR
Pipe Results
SN Element Peak Time of Design Flow Peak Flow/Peak Flow Travel Peak Flow Peak Flow Total Time Froude ReportedIDFlowPeak Flow Capacity Design Flow Velocity Time Depth Depth/Surcharged Number ConditionOccurrenceRatioTotal Depth
Ratio
(cfs)(days hh:mm)(cfs)(ft/sec)(min)(ft)(min)
1 Link-01
2 Link-03
Storage Nodes
Storage Node : EastPond
Input Data
5030.505038.007.50
5030.50
0.00
0.00
0.00
Storage Area Volume Curves
Storage Curve : East Pond
Stage Storage Storage
Area Volume
(ft)(ft²)(ft³)0 93.29 0.0000.5 210.21 75.881.5 365.80 363.892.5 557.89 825.743.5 786.48 1497.93
4.5 786.48 2284.41
Evaporation Loss ...............................................................
Invert Elevation (ft) .............................................................Max (Rim) Elevation (ft) .....................................................Max (Rim) Offset (ft) ..........................................................
Initial Water Elevation (ft) ...................................................
Initial Water Depth (ft) ........................................................
Ponded Area (ft²) ...............................................................
Storage Node : EastPond (continued)
Outflow Weirs
SN Element Weir Flap Crest Crest Length Weir Total DischargeIDTypeGateElevationOffsetHeightCoefficient(ft)(ft)(ft)(ft)
1 East-Big Rectangular No 5034.80 4.30 1.00 1.00 3.33
Outflow Orifices
SN Element Orifice Orifice Flap Circular Rectangular Rectangular Orifice OrificeIDTypeShapeGateOrificeOrificeOrificeInvertCoefficientDiameterHeightWidthElevation(ft)(ft)(ft)(ft)
1 East-Med Side CIRCULAR No 0.25 5033.00 0.61
2 East-Small Side CIRCULAR No 0.12 5030.50 0.61
Output Summary Results
Storage Node : WestPond
Input Data
5033.505039.005.50
5033.50
0.00
0.000.00
Storage Area Volume Curves
Storage Curve : West Pond
Stage Storage Storage
Area Volume
(ft)(ft²)(ft³)0 331 0.000
Ponded Area (ft²) ...............................................................Evaporation Loss ...............................................................
Invert Elevation (ft) .............................................................Max (Rim) Elevation (ft) .....................................................Max (Rim) Offset (ft) ..........................................................
Initial Water Elevation (ft) ...................................................
Initial Water Depth (ft) ........................................................
0.5 419.56 187.641.5 628.71 711.782.5 871.81 1462.043.5 1146.13 2471.01
4.5 1451.68 3769.92
Storage Node : WestPond (continued)
Outflow Weirs
SN Element Weir Flap Crest Crest Length Weir Total DischargeIDTypeGateElevationOffsetHeightCoefficient(ft)(ft)(ft)(ft)
1 West-Big Rectangular No 5036.80 3.30 1.00 1.00 3.33
Outflow Orifices
SN Element Orifice Orifice Flap Circular Rectangular Rectangular Orifice OrificeIDTypeShapeGateOrificeOrificeOrificeInvertCoefficientDiameterHeightWidthElevation(ft)(ft)(ft)(ft)
1 West-Small Side CIRCULAR No 0.12 5033.50 0.61
Output Summary Results