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P:15063_01_Flood_Hazard_Ev_111716_SC 2 (11/17/16) SN/jil
which include the main drainage area of Cattail Creek upstream of the model reach, the Sectionline
Ditch that discharges into Cattail Creek at the southwest corner of the proposed development, and
additional drainage areas within the model reach. Due to the three watersheds within the model and
the addition of the Sectionline ditch to Cattail Creek, there are several flow change locations
throughout the model. Table 1 outlines the contributing drainage areas and estimated peak flows for
the regression equation method.
The TR-55 SCS method was the second methodology used for estimating the 100-year peak flows to
Cattail Creek. The 100-year peak flows were calculated for the watershed to Cattail Creek at the
upstream end of the model using the SCS method as shown on Table 1. Similar to the approach
taken with the regression equations, the peak flows at the flow change locations were estimated by
updating the watershed area, curve number and time of concentration to represent the total
watershed to Cattail Creek at the specific flow change locations. The calculations for the peak flows
estimated from the SCS method that are shown in Table 1 are located in Appendix B. The HEC-
RAS model reflects the flows estimated using the SCS method, which are more conservative.
Table 1.
Regression Equation: Q=189*A0.695 *(E 6000 +1) -0.199
Watershed Cattail Sectionline
Ditch
A B C
Area (sq.mi.) 1.119 0.444 0.11 0.012 0.181
Percentage of Area
above 6,000-FT
0 0 0 0 0
Mean Basin Elevation
divided by 1000
4740 4740 4740 4740 4740
Regression (cfs) 204.4 257.8 270.7 271.6 291.6
SCS (cfs) 375.4 386.0 407.4 408.2 443.0
HYDRAULIC ANALYSIS
The base flood elevations and floodplain boundary were estimated by using the USACE HEC-RAS
Version 5.0.1 river analysis modeling system. The model was built in AutoCAD and exported to
HEC-RAS for analysis. A topographic survey of the proposed development was completed as part
of this analysis. The entire Lot 2A was surveyed as well as 44 cross sections. All elevations are
referenced to the North American Vertical Datum (NAVD) 88. The surveyed cross sections
included the cross sections directly upstream and downstream of structures within the analysis area.
The cross sections near the structures included flowline and invert elevation as well as size and
material of the structure. The Lot 2A survey and the surveyed cross sections do not cover the
estimated floodplain area; therefore, LiDAR data provided by the City of Bozeman was used to
supplement the survey data. A topographic surface was created by merging the cross section data,
Lot 2A survey data and the LiDAR data in that order of priority.
P:15063_01_Flood_Hazard_Ev_111716_SC 3 (11/17/16) SN/jil
1. Cross Sections
The Hec-Ras model includes 62 cross sections cut from the topographic surface. The cross
sections were cut in order to represent the structures, ground breaks, land cover changes,
natural constrictions and meanders. The channel of the cross sections was assumed to have a
Manning’s roughness coefficient of 0.05 and the overbanks were assumed to have a
Manning’s roughness coefficient of 0.06 to represent the tall grass and shrub conditions of
the floodplain. Each cross section crosses the entire floodplain and into the proposed
subdivision in order to analyze the impact of the 100-year peak flows on the proposed
development.
The model begins south of the proposed development, south of Catamount Street in the
Cattail Creek Subdivision, East of Blackbird Drive and West of Warbler Way and ends north
of Frontage Road as show on Exhibit B.
2. Bridges and Culverts
There were 12 bridges and culverts modeled as part of this evaluation, however only 6
bridges were determined to affect the base flood elevations of the proposed development.
Figures 3A-F show the cross sections of the bridges and/or culverts with their respective
invert elevations and pipe diameter. The contraction and expansion coefficients were
changed from 0.1 and 0.3 to 0.3 and 0.5 respectively at the entrances and exits of the culverts
to account for the change in flow conditions. Ineffective flow areas were also added to the
cross sections, upstream and downstream of the culverts and bridges in order to account for
the restricted flow area at these locations.
RESULTS
The estimated 100-year floodplain extents for Cattail Creek adjacent to the proposed development
are shown on Exhibit C in Appendix A. At the upstream end of the model, Cattail Creek flows
under Catamount Street through three 36-inch equivalent elliptical culverts. The peak runoff from
the 100-year storm event escapes the channel upstream of Catamount Street and overtops the low
point of the roadway. The flow re-enters Cattail Creek just north of Catamount Street and is
contained in the channel until cross-section 5951.22 where the base flood elevation overtops the
channel bank into the adjacent pond through cross section 4984.39. Cattail Creek continues north
past the pond located southwest of the property and starts to flow parallel to Davis Lane, and then
leaves the property boundary. At the southeast corner of Davis Lane and Westlake Road, Cattail
Creek flows through a small pond located on the adjacent property. At this pond, cross section
3493.34 overflows the pond to the west over Davis Lane and overtops Westlake Road. The flow
that overtops Westlake Road reenters Cattail Creek after overtopping Westlake Road and flows
through an old culvert and then continues north, parallel to Davis Lane. Cattail Creek makes a 90-
degree bend on the east side of the intersection of Davis Lane and East Valley Center Road and is
culverted under Davis Lane. At this intersection the flow from Cattail Creek leaves the channel and
overtops Davis Lane. It appears that the area north of Westlake Road and south of East Valley
Center Road experiences flooding as a result of the 36-inch culvert under Davis Lane which restricts
P:15063_01_Flood_Hazard_Ev_111716_SC 4 (11/17/16) SN/jil
flow and causes backup in this area. After Cattail Creek flows through the culvert under Davis Lane,
the HEC-RAS model continues to model Cattail Creek west and parallel to East Valley Center Road
and then north under East Valley Center Road, Interstate 90, the railroad and Frontage Road;
however this portion of Cattail Creek does not have any backwater effect on Cattail Creek east and
south of Davis Lane.
It should be noted that where flow overtops Davis Lane or the existing pond west of Cattail Creek,
this flow loss was not modeled and the extents of the floodplain on the west side of Cattail Creek
and Davis Lane was not mapped. This results in a more conservative analysis of the estimated
floodplain boundary adjacent to the proposed development on the east side of Cattail Creek.
CONCULSIONS
The estimated 100-year floodplain boundary of Cattail Creek remains within the east bank of Cattail
Creek adjacent to Lot 2A of Minor Sub 221E as shown on Exhibit C. However, there are four cross
sections, 4984.39, 5012.24, 5054.73 and 5067.05 that surround the narrow dirt access drive through
the site and constricts flow through Cattail Creek. Sanderson Stewart recommends widening the
channel between the cross sections mentioned above to match the channel cross sections upstream
and downstream to create a more uniform channel. The widening of the channel would rehabilitate
Cattail Creek into a more natural and pre-development condition. This would require obtaining the
necessary stream permits.
Along the east and northwest boundary of the site there is a depression that appears to act as a
conveyance swale towards a 60-inch culvert that runs under East Valley Center Drive. The proposed
development should maintain this drainage pattern and provide a replacement conveyance swale if
the existing swale is filled in for the development. Flow from Cattail Creek does not appear to enter
this swale and it is unclear what watershed area besides that of the site the swale services. Even
though the estimated 100-year floodplain boundary of Cattail Creek appears to not impact the
proposed development, it is recommended that buildings on Lot 2A of Minor Sub 221E be
constructed with a minimum finished floor elevation of two feet above the base flood elevation of
Cattail Creek.
This flood study is specific to Lot 2A of Minor Sub 221E and the proposed development and
should not be used to determine the base flood elevation or floodplain boundary for any of the
surrounding lots or subdivision.
As part of this development there are offsite improvements in order to serve the future
development of this area. The offsite improvements include a proposed lift station, and as a result of
this flood hazard study it is recommended to place the lift station outside the floodplain boundary
limits.
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TABLE OF CONTENTS
Appendix A
Exhibit A: Watershed Map
Exhibit B: Model Layout Map
Exhibit C: Estimated Floodplain Boundary Map
Figure 3A-F: Bridge Cross Sections
Appendix B
SCS Method Calculations
Appendix C
HEC-RAS Files
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Appendix A
Figure 3A:Upstream Invert: 4656.49'Downstream Invert: 4655.34'Type: 3 Elliptical ConcreteMaterial: ConcreteSize: 36" equivalent
Figure 3B:Upstream Invert: 4634.91'Downstream Invert: 4633.99'Type: 1 CircularMaterial: Corrugated MetalSize: 18" Diameter
Figure 3C:Upstream Invert: 4629.65'Downstream Invert: 4625.19'Type: 1 CircularMaterial: ConcreteSize: 24" Diameter
Figure 3D:Upstream Invert: 4624.55'Downstream Invert: 4623.89'Type: 1 CircularMaterial: Corrugated MetalSize: 42" Diameter
Figure 3E:Upstream Invert: 4623.56'Downstream Invert: 4622.71'Type: 1 CircularMaterial: ConcreteSize: 36" Diameter
Figure 3F:Upstream Invert: 4614.57'Downstream Invert: 4614.18'Type: 1 CircularMaterial: Corrugated MetalSize: 36" Diameter
P:15063_01_Flood_Hazard_Ev_111716_SC 7 (11/17/16) SN/jil
Appendix B
Project: Cattail Creek Flood Hazard Study
Project No.: 15063.01
Date: 11/2016
Watershed Curve Number
Land Use Description Soil Group Curve Number Area
(acres)
C 85 716
Z 0 0
Z 0 0
Z 0 0
Z 0 0
Z 0 0
Composite Curve Number / Total 85.0 716
Direct Runoff
Design Storm Post-Developed
(inches)Runoff (ft3)
P2 = 0 0
P10 = 0 0
P25 = 0 0
P50 = 0 0
P100 = 2.8 3,695,255
Table 2-7 of Storm Water Management Manual
Office of the City Engineer
City of Billings
May 2015
A B C D
Lawns, open space, etc. Good Condition 75% or more cover 39 61 74 80
Lawns, open space, etc. Fair Condition 50-75% cover 49 69 79 84
Paved parking lots, roofs, driveways, etc. 98 98 98 98
Streets and roads; paved with curbs and storm sewers 98 98 98 98
Streets and roads; gravel 76 85 89 91
Streets and roads; dirt 72 82 87 89
Streets and roads; paved with open ditches 83 89 92 93
Commercial and business areas (85% impervious) 89 92 94 95
Industrial districts (72% impervious)81 88 91 93
Residential - (65% impervious)77 85 90 92
Residential - (38% impervious)61 75 83 87
Residential - (30% impervious)57 72 81 86
Residential - (25% impervious)54 70 80 85
Newly graded area 77 86 91 94
Herbaceous - mixture of grass, weeds and low growing bursh (30-70% cover) 71 81 89
Herbaceous - mixture of grass, weeds and low growing bursh (>70% cover) 62 74 85
Sagebrush with grass understory 35 47 55
Pasture, grassland or range - continuous forage for grazing 39 61 74 80
Meadow - continuous grass, protected from grazing, mowed for hay 30 58 71 78
Row crops 67 78 85 89
Close-seeded or broadcast legumes or rotation meadow 58 72 81 85
Small grain crops 63 75 83 87
Hydrologic Soil GroupLand Use Description
SCS CURVE NUMBER AND DIRECT RUNOFF ANALYSES
CATTAIL CREEK WATERSHED
Pasture, grassland or range - continuous forage for grazing
Residential - (65% impervious)
Project: Cattail Creek Flood Hazard Study
Project No.: 15063.01
Date: 11/2016
100-Year, 24-Hour Design Storm
From Curve Number and Direct Runoff Analyses:
Q (in) = 1.42
A (mi2) = 1.11875
Tc (hr) = 1.7
Discharge Rate (cfs) = 0.00
Design Theoretical Cumulative Cumulative Actual Required
Time Multiplier Storm Outflow Storm Outflow Outflow Storage
(hrs)Qp R*A (cfs)(cfs)(CF)(CF)(cfs)(CF)
0.0 0 1.5906 0.00 0.00 0 0 0 0
11.0 10 1.5906 15.91 0.00 629873 0 0.00 629873
11.5 18 1.5906 28.63 0.00 681408 0 0.00 681408
11.7 31 1.5906 49.31 0.00 716910 0 0.00 716910
11.8 42 1.5906 66.80 0.00 740960 0 0.00 740960
11.9 57 1.5906 90.66 0.00 773598 0 0.00 773598
12.0 81 1.5906 128.84 0.00 819980 0 0.00 819980
12.1 105 1.5906 167.01 0.00 880104 0 0.00 880104
12.2 133 1.5906 211.55 0.00 956262 0 0.00 956262
12.3 164 1.5906 260.86 0.00 1050170 0 0.00 1050170
12.4 192 1.5906 305.39 0.00 1160111 0 0.00 1160111
12.5 209 1.5906 332.43 0.00 1279787 0 0.00 1279787
12.6 227 1.5906 361.06 0.00 1409770 0 0.00 1409770
12.7 235 1.5906 373.79 0.00 1544334 0 0.00 1544334
12.8 236 1.5906 375.38 0.00 1679470 0 0.00 1679470
12.9 236 1.5906 375.38 0.00 1814607 0 0.00 1814607
13.0 225 1.5906 357.88 0.00 1943444 0 0.00 1943444
13.2 201 1.5906 319.71 0.00 2173634 0 0.00 2173634
13.5 153 1.5906 243.36 0.00 2436463 0 0.00 2436463
14.0 99 1.5906 157.47 0.00 2719906 0 0.00 2719906
14.5 68 1.5906 108.16 0.00 2914594 0 0.00 2914594
15.0 50 1.5906 79.53 0.00 3057747 0 0.00 3057747
16.0 32 1.5906 50.90 0.00 3240983 0 0.00 3240983
18.0 20 1.5906 31.81 0.00 3470027 0 0.00 3470027
20.0 16 1.5906 25.45 0.00 3653263 0 0.00 3653263
Peak Discharge Prior to Routing =375.4 cfs
Storage Volume Required = 3,653,263 cf
SCS HYDROGRAPH ANALYSIS
TYPE II UNIT HYDROGRAPH
CATTAIL CREEK WATERSHED
0.00
50.00
100.00
150.00
200.00
250.00
300.00
350.00
400.00
0.0 5.0 10.0 15.0 20.0 25.0
Dis
c
h
a
r
g
e
(
c
f
s
)
Time (hrs)
Post-Development Hydrograph
Design Storm
Routing
Project: Cattail Creek Flood Hazard Study
Project No.: 15063.01
Date: 11/2016
.FLOW FLOW VELOCITY TIME OF
FLOW TYPE LENGTH (FT)SLOPE (FT/FT)n (FT/S)CONC. (MIN)
SHEET FLOW 300 0.0150 0.15 --28.3
SHALLOW pav 244 0.0150 --2.49 1.63
PIPE 1230 0.0050 1.44 14.26
CREEK 6896 0.0422 1.90 60.49
Total Time of Concentration=104.7
CATTAIL CREEK WATERSHED
TIME OF CONCENTRATION WORKSHEET (TR-55)
Project: Cattail Creek Flood Hazard Study
Project No.: 15063.01
Date: 11/2016
Cumulative Watershed Curve Number
Land Use Description Soil Group Curve Number Area
(acres)
C 85 716
C 78 284
Z 0 0
Z 0 0
Z 0 0
Z 0 0
Composite Curve Number / Total 83.0 1000
Direct Runoff
Design Storm Post-Developed
(inches)Runoff (ft3)
P2 = 0 0
P10 = 0 0
P25 = 0 0
P50 = 0 0
P100 = 2.8 4,675,792
Table 2-7 of Storm Water Management Manual
Office of the City Engineer
City of Billings
May 2015
A B C D
Lawns, open space, etc. Good Condition 75% or more cover 39 61 74 80
Lawns, open space, etc. Fair Condition 50-75% cover 49 69 79 84
Paved parking lots, roofs, driveways, etc. 98 98 98 98
Streets and roads; paved with curbs and storm sewers 98 98 98 98
Streets and roads; gravel 76 85 89 91
Streets and roads; dirt 72 82 87 89
Streets and roads; paved with open ditches 83 89 92 93
Commercial and business areas (85% impervious) 89 92 94 95
Industrial districts (72% impervious)81 88 91 93
Residential - (65% impervious)77 85 90 92
Residential - (38% impervious)61 75 83 87
Residential - (30% impervious)57 72 81 86
Residential - (25% impervious)54 70 80 85
Newly graded area 77 86 91 94
Herbaceous - mixture of grass, weeds and low growing bursh (30-70% cover) 71 81 89
Herbaceous - mixture of grass, weeds and low growing bursh (>70% cover) 62 74 85
Sagebrush with grass understory 35 47 55
Pasture, grassland or range - continuous forage for grazing 39 61 74 80
Meadow - continuous grass, protected from grazing, mowed for hay 30 58 71 78
Row crops 67 78 85 89
Close-seeded or broadcast legumes or rotation meadow 58 72 81 85
Small grain crops 63 75 83 87
Hydrologic Soil GroupLand Use Description
SCS CURVE NUMBER AND DIRECT RUNOFF ANALYSES
CUMULATIVE WATERSHED AFTER SIDELINE DITCH
Pasture, grassland or range - continuous forage for grazing
Cattail Creek
Sideline Ditch
Project: Cattail Creek Flood Hazard Study
Project No.: 15063.01
Date: 11/2016
100-Year, 24-Hour Design Storm
From Curve Number and Direct Runoff Analyses:
Q (in) = 1.29
A (mi2) = 1.5625
Tc (hr) = 2.4
Discharge Rate (cfs) = 0.00
Design Theoretical Cumulative Cumulative Actual Required
Time Multiplier Storm Outflow Storm Outflow Outflow Storage
(hrs)Qp R*A (cfs)(cfs)(CF)(CF)(cfs)(CF)
0.0 0 2.0127 0.00 0.00 0 0 0 0
11.0 7 2.0127 14.09 0.00 557907 0 0.00 557907
11.5 14 2.0127 28.18 0.00 608626 0 0.00 608626
11.7 22 2.0127 44.28 0.00 640506 0 0.00 640506
11.8 30 2.0127 60.38 0.00 662243 0 0.00 662243
11.9 38 2.0127 76.48 0.00 689776 0 0.00 689776
12.0 49 2.0127 98.62 0.00 725279 0 0.00 725279
12.1 64 2.0127 128.81 0.00 771651 0 0.00 771651
12.2 80 2.0127 161.01 0.00 829615 0 0.00 829615
12.3 95 2.0127 191.20 0.00 898448 0 0.00 898448
12.4 114 2.0127 229.44 0.00 981047 0 0.00 981047
12.5 133 2.0127 267.68 0.00 1077413 0 0.00 1077413
12.6 152 2.0127 305.92 0.00 1187545 0 0.00 1187545
12.7 165 2.0127 332.09 0.00 1307096 0 0.00 1307096
12.8 175 2.0127 352.21 0.00 1433893 0 0.00 1433893
12.9 184 2.0127 370.33 0.00 1567211 0 0.00 1567211
13.0 192 2.0127 386.43 0.00 1706326 0 0.00 1706326
13.2 190 2.0127 382.40 0.00 1981657 0 0.00 1981657
13.5 176 2.0127 354.23 0.00 2364221 0 0.00 2364221
14.0 129 2.0127 259.63 0.00 2831559 0 0.00 2831559
14.5 93 2.0127 187.18 0.00 3168477 0 0.00 3168477
15.0 68 2.0127 136.86 0.00 3414825 0 0.00 3414825
16.0 41 2.0127 82.52 0.00 3711893 0 0.00 3711893
18.0 23 2.0127 46.29 0.00 4045188 0 0.00 4045188
20.0 17 2.0127 34.22 0.00 4291536 0 0.00 4291536
Peak Discharge Prior to Routing =386.4 cfs
Storage Volume Required = 4,291,536 cf
SCS HYDROGRAPH ANALYSIS
TYPE II UNIT HYDROGRAPH
CUMULATIVE WATERSHED AFTER SIDELINE DITCH
0.00
50.00
100.00
150.00
200.00
250.00
300.00
350.00
400.00
450.00
0.0 5.0 10.0 15.0 20.0 25.0
Dis
c
h
a
r
g
e
(
c
f
s
)
Time (hrs)
Post-Development Hydrograph
Design Storm
Routing
Project Name:Project: Cattail Creek Flood Hazard Study
Project Number:Project No.: 15063.01
Date:Date: 11/2016
Designer:
.FLOW FLOW VELOCITY TIME OF
FLOW TYPE LENGTH (FT)SLOPE (FT/FT)n (FT/S)CONC. (MIN)
SHEET FLOW 300 0.0150 0.15 --28.3
SHALLOW pav 50 0.0150 --2.49 0.33
PIPE 890 0.0050 1.44 10.32
CREEK 16352 0.0422 3.31 82.23
BEG- DITCH 2157 0.0080 1.44 24.91
Total Time of Concentration=146.1
CUMULATIVE WATERSHED AFTER SIDELINE DITCH
TIME OF CONCENTRATION WORKSHEET (TR-55)
Project: Cattail Creek Flood Hazard Study
Project No.: 15063.01
Date: 11/2016
Cumulative Watershed Curve Number
Land Use Description Soil Group Curve Number Area
(acres)
C 85 716
C 78 284
C 78.1 71
Z 0 0
Z 0 0
Z 0 0
Composite Curve Number / Total 82.7 1071
Direct Runoff
Design Storm Post-Developed
(inches)Runoff (ft3)
P2 = 0 0
P10 = 0 0
P25 = 0 0
P50 = 0 0
P100 = 2.8 4,929,375
Table 2-7 of Storm Water Management Manual
Office of the City Engineer
City of Billings
May 2015
A B C D
Lawns, open space, etc. Good Condition 75% or more cover 39 61 74 80
Lawns, open space, etc. Fair Condition 50-75% cover 49 69 79 84
Paved parking lots, roofs, driveways, etc. 98 98 98 98
Streets and roads; paved with curbs and storm sewers 98 98 98 98
Streets and roads; gravel 76 85 89 91
Streets and roads; dirt 72 82 87 89
Streets and roads; paved with open ditches 83 89 92 93
Commercial and business areas (85% impervious) 89 92 94 95
Industrial districts (72% impervious)81 88 91 93
Residential - (65% impervious)77 85 90 92
Residential - (38% impervious)61 75 83 87
Residential - (30% impervious)57 72 81 86
Residential - (25% impervious)54 70 80 85
Newly graded area 77 86 91 94
Herbaceous - mixture of grass, weeds and low growing bursh (30-70% cover) 71 81 89
Herbaceous - mixture of grass, weeds and low growing bursh (>70% cover) 62 74 85
Sagebrush with grass understory 35 47 55
Pasture, grassland or range - continuous forage for grazing 39 61 74 80
Meadow - continuous grass, protected from grazing, mowed for hay 30 58 71 78
Row crops 67 78 85 89
Close-seeded or broadcast legumes or rotation meadow 58 72 81 85
Small grain crops 63 75 83 87
Hydrologic Soil GroupLand Use Description
SCS CURVE NUMBER AND DIRECT RUNOFF ANALYSES
CUMULATIVE WATERSHED AFTER WATERSHED A
Pasture, grassland or range - continuous forage for grazing
Cattail Creek
Sideiine Ditch
Watershed A
Project: Cattail Creek Flood Hazard Study
Project No.: 15063.01
Date: 11/2016
100-Year, 24-Hour Design Storm
From Curve Number and Direct Runoff Analyses:
Q (in) = 1.27
A (mi2) = 1.6734375
Tc (hr) = 2.6
Discharge Rate (cfs) = 0.00
Design Theoretical Cumulative Cumulative Actual Required
Time Multiplier Storm Outflow Storm Outflow Outflow Storage
(hrs)Qp R*A (cfs)(cfs)(CF)(CF)(cfs)(CF)
0.0 0 2.1218 0.00 0.00 0 0 0 0
11.0 7 2.1218 14.85 0.00 588164 0 0.00 588164
11.5 14 2.1218 29.71 0.00 641634 0 0.00 641634
11.7 22 2.1218 46.68 0.00 675243 0 0.00 675243
11.8 30 2.1218 63.65 0.00 698158 0 0.00 698158
11.9 38 2.1218 80.63 0.00 727185 0 0.00 727185
12.0 49 2.1218 103.97 0.00 764613 0 0.00 764613
12.1 64 2.1218 135.80 0.00 813500 0 0.00 813500
12.2 80 2.1218 169.74 0.00 874608 0 0.00 874608
12.3 95 2.1218 201.57 0.00 947173 0 0.00 947173
12.4 114 2.1218 241.89 0.00 1034252 0 0.00 1034252
12.5 133 2.1218 282.20 0.00 1135844 0 0.00 1135844
12.6 152 2.1218 322.51 0.00 1251949 0 0.00 1251949
12.7 165 2.1218 350.10 0.00 1377984 0 0.00 1377984
12.8 175 2.1218 371.32 0.00 1511658 0 0.00 1511658
12.9 184 2.1218 390.41 0.00 1652206 0 0.00 1652206
13.0 192 2.1218 407.39 0.00 1798866 0 0.00 1798866
13.2 190 2.1218 403.14 0.00 2089128 0 0.00 2089128
13.5 176 2.1218 373.44 0.00 2492441 0 0.00 2492441
14.0 129 2.1218 273.71 0.00 2985124 0 0.00 2985124
14.5 93 2.1218 197.33 0.00 3340314 0 0.00 3340314
15.0 68 2.1218 144.28 0.00 3600023 0 0.00 3600023
16.0 41 2.1218 86.99 0.00 3913201 0 0.00 3913201
18.0 23 2.1218 48.80 0.00 4264572 0 0.00 4264572
20.0 17 2.1218 36.07 0.00 4524280 0 0.00 4524280
Peak Discharge Prior to Routing =407.4 cfs
Storage Volume Required = 4,524,280 cf
SCS HYDROGRAPH ANALYSIS
TYPE II UNIT HYDROGRAPH
CUMULATIVE WATERSHED AFTER WATERSHED A
0.00
50.00
100.00
150.00
200.00
250.00
300.00
350.00
400.00
450.00
0.0 5.0 10.0 15.0 20.0 25.0
Dis
c
h
a
r
g
e
(
c
f
s
)
Time (hrs)
Post-Development Hydrograph
Design Storm
Routing
Project Name:Project: Cattail Creek Flood Hazard Study
Project Number:Project No.: 15063.01
Date:Date: 11/2016
.FLOW FLOW VELOCITY TIME OF
FLOW TYPE LENGTH (FT)SLOPE (FT/FT)n (FT/S)CONC. (MIN)
SHEET FLOW 300 0.0150 0.15 --28.3
SHALLOW pav 50 0.0150 --2.49 0.33
PIPE 890 0.0050 1.44 10.32
CREEK 16352 0.0422 3.31 82.23
BEG-SIDE DITCH 2157 0.0080 1.44 24.91
SIDE DITCH-WAT A 1067 0.0080 1.44 12.32
Total Time of Concentration=158.4
CUMULATIVE WATERSHED AFTER WATERSHED A
TIME OF CONCENTRATION WORKSHEET (TR-55)
Project: Cattail Creek Flood Hazard Study
Project No.: 15063.01
Date: 11/2016
Cumulative Watershed Curve Number
Land Use Description Soil Group Curve Number Area
(acres)
C 85 716
C 78 284
C 78.1 71
C 72.8 7.6
Z 0 0
Z 0 0
Composite Curve Number / Total 82.6 1078.6
Direct Runoff
Design Storm Post-Developed
(inches)Runoff (ft3)
P2 = 0 0
P10 = 0 0
P25 = 0 0
P50 = 0 0
P100 = 2.8 4,943,419
Table 2-7 of Storm Water Management Manual
Office of the City Engineer
City of Billings
May 2015
A B C D
Lawns, open space, etc. Good Condition 75% or more cover 39 61 74 80
Lawns, open space, etc. Fair Condition 50-75% cover 49 69 79 84
Paved parking lots, roofs, driveways, etc. 98 98 98 98
Streets and roads; paved with curbs and storm sewers 98 98 98 98
Streets and roads; gravel 76 85 89 91
Streets and roads; dirt 72 82 87 89
Streets and roads; paved with open ditches 83 89 92 93
Commercial and business areas (85% impervious) 89 92 94 95
Industrial districts (72% impervious)81 88 91 93
Residential - (65% impervious)77 85 90 92
Residential - (38% impervious)61 75 83 87
Residential - (30% impervious)57 72 81 86
Residential - (25% impervious)54 70 80 85
Newly graded area 77 86 91 94
Herbaceous - mixture of grass, weeds and low growing bursh (30-70% cover) 71 81 89
Herbaceous - mixture of grass, weeds and low growing bursh (>70% cover) 62 74 85
Sagebrush with grass understory 35 47 55
Pasture, grassland or range - continuous forage for grazing 39 61 74 80
Meadow - continuous grass, protected from grazing, mowed for hay 30 58 71 78
Row crops 67 78 85 89
Close-seeded or broadcast legumes or rotation meadow 58 72 81 85
Small grain crops 63 75 83 87
Hydrologic Soil GroupLand Use Description
SCS CURVE NUMBER AND DIRECT RUNOFF ANALYSES
CUMULATIVE WATERSHED AFTER WATERSHED B
Pasture, grassland or range - continuous forage for grazing
Cattail Creek
SIdeline Ditch
Watershed A
Watershed B
Project: Cattail Creek Flood Hazard Study
Project No.: 15063.01
Date: 11/2016
100-Year, 24-Hour Design Storm
From Curve Number and Direct Runoff Analyses:
Q (in) = 1.26
A (mi2) = 1.6853125
Tc (hr) = 2.8
Discharge Rate (cfs) = 0.00
Design Theoretical Cumulative Cumulative Actual Required
Time Multiplier Storm Outflow Storm Outflow Outflow Storage
(hrs)Qp R*A (cfs)(cfs)(CF)(CF)(cfs)(CF)
0.0 0 2.1278 0.00 0.00 0 0 0 0
11.0 7 2.1278 14.89 0.00 589840 0 0.00 589840
11.5 14 2.1278 29.79 0.00 643462 0 0.00 643462
11.7 22 2.1278 46.81 0.00 677167 0 0.00 677167
11.8 30 2.1278 63.84 0.00 700147 0 0.00 700147
11.9 38 2.1278 80.86 0.00 729256 0 0.00 729256
12.0 49 2.1278 104.26 0.00 766792 0 0.00 766792
12.1 64 2.1278 136.18 0.00 815817 0 0.00 815817
12.2 80 2.1278 170.23 0.00 877099 0 0.00 877099
12.3 95 2.1278 202.15 0.00 949872 0 0.00 949872
12.4 114 2.1278 242.57 0.00 1037199 0 0.00 1037199
12.5 133 2.1278 283.00 0.00 1139080 0 0.00 1139080
12.6 152 2.1278 323.43 0.00 1255516 0 0.00 1255516
12.7 165 2.1278 351.10 0.00 1381910 0 0.00 1381910
12.8 175 2.1278 372.37 0.00 1515965 0 0.00 1515965
12.9 184 2.1278 391.52 0.00 1656913 0 0.00 1656913
13.0 192 2.1278 408.55 0.00 1803990 0 0.00 1803990
13.2 190 2.1278 404.29 0.00 2095080 0 0.00 2095080
13.5 176 2.1278 374.50 0.00 2499542 0 0.00 2499542
14.0 129 2.1278 274.49 0.00 2993628 0 0.00 2993628
14.5 93 2.1278 197.89 0.00 3349830 0 0.00 3349830
15.0 68 2.1278 144.69 0.00 3610279 0 0.00 3610279
16.0 41 2.1278 87.24 0.00 3924349 0 0.00 3924349
18.0 23 2.1278 48.94 0.00 4276721 0 0.00 4276721
20.0 17 2.1278 36.17 0.00 4537170 0 0.00 4537170
Peak Discharge Prior to Routing =408.5 cfs
Storage Volume Required = 4,537,170 cf
SCS HYDROGRAPH ANALYSIS
TYPE II UNIT HYDROGRAPH
CUMULATIVE WATERSHED AFTER WATERSHED B
0.00
50.00
100.00
150.00
200.00
250.00
300.00
350.00
400.00
450.00
0.0 5.0 10.0 15.0 20.0 25.0
Dis
c
h
a
r
g
e
(
c
f
s
)
Time (hrs)
Post-Development Hydrograph
Design Storm
Routing
Project Name:Project: Cattail Creek Flood Hazard Study
Project Number:Project No.: 15063.01
Date:Date: 11/2016
Designer:
.FLOW FLOW VELOCITY TIME OF
FLOW TYPE LENGTH (FT)SLOPE (FT/FT)n (FT/S)CONC. (MIN)
SHEET FLOW 300 0.0150 0.15 --28.3
SHALLOW pav 50 0.0150 --2.49 0.33
PIPE 890 0.0050 1.44 10.32
CREEK 16352 0.0422 3.31 82.23
BEG-SIDE DITCH 2157 0.0080 1.44 24.91
SIDE DITCH-WAT A 1067 0.0080 1.44 12.32
WAT A-WAT B 865 0.0080 1.44 9.99
Total Time of Concentration=168.4
CUMULATIVE WATERSHED AFTER WATERSHED B
TIME OF CONCENTRATION WORKSHEET (TR-55)
Project: Cattail Creek Flood Hazard Study
Project No.: 15063.01
Date: 11/2016
Cumulative Watershed Curve Number
Land Use Description Soil Group Curve Number Area
(acres)
C 85 716
C 78 284
C 78.1 71
C 72.8 7.6
C 78.8 116
Z 0 0
Composite Curve Number / Total 82.2 1194.6
Direct Runoff
Design Storm Post-Developed
(inches)Runoff (ft3)
P2 = 0 0
P10 = 0 0
P25 = 0 0
P50 = 0 0
P100 = 2.8 5,360,042
Table 2-7 of Storm Water Management Manual
Office of the City Engineer
City of Billings
May 2015
A B C D
Lawns, open space, etc. Good Condition 75% or more cover 39 61 74 80
Lawns, open space, etc. Fair Condition 50-75% cover 49 69 79 84
Paved parking lots, roofs, driveways, etc. 98 98 98 98
Streets and roads; paved with curbs and storm sewers 98 98 98 98
Streets and roads; gravel 76 85 89 91
Streets and roads; dirt 72 82 87 89
Streets and roads; paved with open ditches 83 89 92 93
Commercial and business areas (85% impervious) 89 92 94 95
Industrial districts (72% impervious)81 88 91 93
Residential - (65% impervious)77 85 90 92
Residential - (38% impervious)61 75 83 87
Residential - (30% impervious)57 72 81 86
Residential - (25% impervious)54 70 80 85
Newly graded area 77 86 91 94
Herbaceous - mixture of grass, weeds and low growing bursh (30-70% cover) 71 81 89
Herbaceous - mixture of grass, weeds and low growing bursh (>70% cover) 62 74 85
Sagebrush with grass understory 35 47 55
Pasture, grassland or range - continuous forage for grazing 39 61 74 80
Meadow - continuous grass, protected from grazing, mowed for hay 30 58 71 78
Row crops 67 78 85 89
Close-seeded or broadcast legumes or rotation meadow 58 72 81 85
Small grain crops 63 75 83 87
Hydrologic Soil GroupLand Use Description
SCS CURVE NUMBER AND DIRECT RUNOFF ANALYSES
CUMULATIVE WATERSHED AFTER WATERSHED C
Pasture, grassland or range - continuous forage for grazing
Cattail Creek
SIdeline Ditch
Watershed A
Watershed B
Watershed C
Project: Cattail Creek Flood Hazard Study
Project No.: 15063.01
Date: 11/2016
100-Year, 24-Hour Design Storm
From Curve Number and Direct Runoff Analyses:
Q (in) = 1.24
A (mi2) = 1.8665625
Tc (hr) = 3.0
Discharge Rate (cfs) = 0.00
Design Theoretical Cumulative Cumulative Actual Required
Time Multiplier Storm Outflow Storm Outflow Outflow Storage
(hrs)Qp R*A (cfs)(cfs)(CF)(CF)(cfs)(CF)
0.0 0 2.3072 0.00 0.00 0 0 0 0
11.0 7 2.3072 16.15 0.00 639550 0 0.00 639550
11.5 14 2.3072 32.30 0.00 697691 0 0.00 697691
11.7 22 2.3072 50.76 0.00 734237 0 0.00 734237
11.8 30 2.3072 69.22 0.00 759155 0 0.00 759155
11.9 38 2.3072 87.67 0.00 790717 0 0.00 790717
12.0 49 2.3072 113.05 0.00 831416 0 0.00 831416
12.1 64 2.3072 147.66 0.00 884573 0 0.00 884573
12.2 80 2.3072 184.57 0.00 951020 0 0.00 951020
12.3 95 2.3072 219.18 0.00 1029925 0 0.00 1029925
12.4 114 2.3072 263.02 0.00 1124612 0 0.00 1124612
12.5 133 2.3072 306.86 0.00 1235080 0 0.00 1235080
12.6 152 2.3072 350.69 0.00 1361329 0 0.00 1361329
12.7 165 2.3072 380.68 0.00 1498375 0 0.00 1498375
12.8 175 2.3072 403.76 0.00 1643728 0 0.00 1643728
12.9 184 2.3072 424.52 0.00 1796555 0 0.00 1796555
13.0 192 2.3072 442.98 0.00 1956028 0 0.00 1956028
13.2 190 2.3072 438.36 0.00 2271650 0 0.00 2271650
13.5 176 2.3072 406.06 0.00 2710199 0 0.00 2710199
14.0 129 2.3072 297.63 0.00 3245926 0 0.00 3245926
14.5 93 2.3072 214.57 0.00 3632148 0 0.00 3632148
15.0 68 2.3072 156.89 0.00 3914547 0 0.00 3914547
16.0 41 2.3072 94.59 0.00 4255087 0 0.00 4255087
18.0 23 2.3072 53.07 0.00 4637156 0 0.00 4637156
20.0 17 2.3072 39.22 0.00 4919555 0 0.00 4919555
Peak Discharge Prior to Routing =443.0 cfs
Storage Volume Required = 4,919,555 cf
SCS HYDROGRAPH ANALYSIS
TYPE II UNIT HYDROGRAPH
CUMULATIVE WATERSHED AFTER WATERSHED C
0.00
50.00
100.00
150.00
200.00
250.00
300.00
350.00
400.00
450.00
500.00
0.0 5.0 10.0 15.0 20.0 25.0
Dis
c
h
a
r
g
e
(
c
f
s
)
Time (hrs)
Post-Development Hydrograph
Design Storm
Routing
Project Name:Project: Cattail Creek Flood Hazard Study
Project Number:Project No.: 15063.01
Date:Date: 11/2016
Designer:
. FLOW FLOW VELOCITY TIME OF
FLOW TYPE LENGTH (FT)SLOPE (FT/FT)n (FT/S)CONC. (MIN)
SHEET FLOW 300 0.0150 0.15 --28.3
SHALLOW pav 50 0.0150 --2.49 0.33
PIPE 890 0.0050 1.44 10.32
CREEK 16352 0.0422 3.31 82.23
BEG-SIDE DITCH 2157 0.0080 1.44 24.91
SIDE DITCH-WAT A 1067 0.0080 1.44 12.32
WAT A-WAT B 865 0.0080 1.44 9.99
WAT B-WAT C 1026 0.0080 1.44 11.85
Total Time of Concentration=180.3
CUMULATIVE WATERSHED AFTER WATERSHED C
TIME OF CONCENTRATION WORKSHEET (TR-55)
P:15063_01_Flood_Hazard_Ev_111716_SC 8 (11/17/16) SN/jil
Appendix C
See attached flash drive