HomeMy WebLinkAbout015 Appendix N - Flood Hazard Evaluation
FLOOD HAZARD EVALUATION
AAKER PHASE 1 SUBDIVISION
MINOR SUB 191B, S14, T02 S, R05 E, LOT 2A, ACRES 95.639,
Bozeman, Montana
September 2022
Prepared for:
West University, LLC
109 E Oak St, Ste 2B
Bozeman, MT 59715
Prepared by:
PO Box 1133
Bozeman, MT 59771-1133
(406) 585-9500
Aaker Phase 1 Subdivision – Flood Hazard Evaluation
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TABLE OF CONTENTS
1. INTRODUCTION ........................................................................................ 1
2. HYDROLOGY ............................................................................................ 1
3. GEOMETRY ............................................................................................... 2
4. HYDRAULICS ............................................................................................ 2
5. RESULTS ................................................................................................... 3
6. MITIGATION .............................................................................................. 3
FIGURE 1 – Drainage Areas
FIGURE 2 – Inundation Areas
FIGURE 3 – Ditch Map
Appendix A - HEC-HMS Reports
Cover: Google Earth, June 2021
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1. INTRODUCTION
This flood hazard evaluation is provided in association with the preliminary plat application for
the subject property. The subject property is located west of S. 19th Ave. and north of Stucky
Rd. in Bozeman, MT. There are three channels that flow from south to north across the
property, the East Fork Catron Creek (EFCC) and West Fork Catron Creek (WFCC), as well as
an irrigation ditch from the Middle Creek Ditch (MCD).
The channels have drainage areas less than 25 square miles and are not mapped with a
regulated flood hazard area. This report is prepared in accordance with section 38.600.150 of
the City of Bozeman Unified Development Code.
2. HYDROLOGY
Drainage basins for the east and west water courses across the property were delineated
between the Middle Creek Ditch and West Lincoln Street using USGS topographic maps (Figure
1). The hydrology of the water courses is more dependent on storm events than on snowmelt
runoff due to the small size and low elevation of the drainage basin. Therefore, rainfall runoff
calculations using the U.S. Army Corps of Engineers HEC-HMS program were used for this
project site. HEC-HMS uses a selected unit hydrograph method to estimate the runoff from a
given storm event for a drainage area of specified characteristics. The input variables for the
HEC-HMS model using the SCS Curve Number Method are provided in Table 1.
Table 1. HEC-HMS input variables.
Parameter Current/Planned Conditions Potential Future Conditions
Water Course West East West East
SCS Storm Type Type I Type I Type I Type I
Basin Size (sq mi) 1.48 0.32 1.48 0.32
Lag Time (min) 224 174 211 158
Curve Number 70.9 75.9 73.0 79.1
Initial Loss (in) 0.82 0.63 0.74 0.53
Impervious (%) 7.7 17.2 10.3 21.0
The SCS has developed 24-hour rainfall distributions for the entire country and Montana is
located in the region where the Type II rainfall distribution is applicable. However, a review of
the ratio of the 6-hour rainfall to the 24-hour rainfall revealed that the Type I rainfall distribution
is better suited for the Bozeman area. The curve number was developed based on the soil type
and land use. The lag time was estimated based on the curve number, the slope of the basin,
and the length of the channel within the basin. The initial loss (I) is the estimated amount of
water adsorbed by the ground before runoff begins. The initial loss is based on the curve
number (CN) and the equation I = 0.2 * [(1000/CN)-10].
Two different conditions were evaluated to determine a curve number and percent impervious:
the current/planned conditions using the existing and proposed land use; and the potential
future conditions which assumes full development based on area zoning. The 24-hour rainfall
for the 2-, 25-, and 100-year return intervals for the Bozeman Airport from the MDT Hydraulics
Manual (2022), Chapter 9 Hydrology, Appendix 9B are shown in Table 2. The results of the
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calculations are also provided in Table 2. Result reports from HEC-HMS are included in
Appendix A.
Table 2. Hydrology for east and west water courses across Aaker Subdivision property.
WEST EAST
2-year 25-
year
100-
year
2-year 25-
year
100-
year
Precipitation
(in) 1.18 1.96 2.34 1.18 1.96 2.34
Current/Planned Discharge (cfs) 8.1 25.3 39.5 4.7 11.8 16.6
Potential Future Discharge (cfs) 11.3 34.0 50.7 6.5 16.1 22.1
3. GEOMETRY
The basis for the terrain data is the 2018 LiDAR data flown for the City of Bozeman. Two
existing culverts were included in the existing conditions model for the EFCC; one at Kagy Blvd.
and one at Remington Way. These culverts are 30” equivalent RCPA culverts with flared ends.
For the proposed conditions model, the proposed development includes extending Kagy Blvd to
the west. and expanding the road to the south. The proposed work on Kagy Blvd. will impact all
three water courses. On the EFCC, it is proposed to extend the existing 30” equivalent RCPA
to the new inlet location. On the WFCC, it is proposed to install a 6 ft wide box culvert.
Additionally, there is a trail crossing proposed over the WFCC. The irrigation ditch will be piped
through the subject property and along the proposed South 23rd Ave.
Changes to the existing terrain and geometry for the proposed conditions included:
Adding elevated blocks to represent the road fill for Kagy Blvd. on either side of WFCC
with a 6 ft opening to represent the proposed box culvert.
Adding elevated blocks to represent the trail fill on either side of WFCC with a 26 ft
opening to represent the proposed pedestrian bridge.
Adding an elevated block to represent the road fill on the south side of Kagy Blvd.
extending into EFCC.
Creating a small inlet depression (6 ft square) for the proposed new culvert inlet for
EFCC and extending the existing culvert to the proposed inlet.
4. HYDRAULICS
To evaluate potential flood hazards on the subject property, a 2D hydraulic model was
developed using the U.S. Army Corps of Engineers Hydrologic Engineering Center – River
Analysis System (HEC-RAS) ver. 6.2. A 2D grid with a 20 ft cell size was used to model the
area. Refinement regions along the channel corridors reduced the cell size to 10 ft within the
subject property and breaklines along the channel centerline further reduced the cell size to 5 ft.
Roughness coefficients (Manning’s n-values) for the 2D model were delineated using a 2019
aerial photo. The Manning’s n-value for the undeveloped areas was 0.05 and 0.10 was used for
the developed areas. An n-value of 0.7 was used for the more rural development west of the
property.
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The downstream boundary conditions for both the east and west channels were defined as
normal depth. The west channel slope is 0.0123 ft/ft and east channel slope is 0.0144 ft/ft.
These slopes are based on the terrain downstream of West Lincoln St.
The computation time step was set to 1 second and the Shallow Water Equations – Eulerian-
Lagrangian Method (SWE-ELM) equation set was used.
5. RESULTS
The existing conditions model results show that the 100-year inundation area is mostly
contained within the area that was also delineated as wetland and is within the 50’ watercourse
setback area.
The results of the model for the proposed conditions show that there is some backwater created
by the new road crossings for Kagy Blvd., but that the additional flooded area is within the
watercourse setback and on the subject property. A comparison of the existing conditions and
proposed conditions results are shown on Figure 2.
6. MITIGATION
Additionally, how flood flows enter onto the subject property was reviewed in order to mitigate
potential flood hazards. There are several culverts under Stucky Road, and flow paths have
been altered both due to development upstream in the drainages and land use downstream. A
ditch map from the 1961 reprint of a Water Resource Survey for Gallatin County by the State
Engineer’s Office shows the ditch flow paths as well as the WFCC and EFCC channels (Figure
3).
Development south of Stucky Rd. appears to have cut off the upper drainage area for the
WFCC and combined it with the irrigation ditch flows. As a results, there is currently very little
flow through the culvert that aligns with the channel at Stucky Rd. Perennial and flood flows for
the WFCC currently flow through the culvert under Stucky Rd. that discharges into the ditch.
Prior to annexation into the City, the subject property was used for agriculture. The ditch across
the property was used to irrigate the area and as such, the ditch is located to facilitate that
function. The ditch is located on a slight rise across the middle of the property to allow for flood
irrigation of the fields on either side.
While appropriate for the previous land use, the open channel for the ditch poses a flood risk to
proposed development. Not only is the ditch slightly perched, but it is receiving both irrigation
water diversions and WFCC flows. The ditch is not sized to handle a flood event. To mitigate
the potential flood hazard, it is proposed to separate the WFCC flows from the irrigation flows.
A junction box is proposed at the culvert outlet with a headgate to allow downstream water
users to divert water into the proposed ditch pipe and the remainder of the water will be piped to
the WFCC channel. The WFCC channel has sufficient capacity for both perennial flows and
estimated flood flows.
The irrigation ditch is proposed to be piped from the junction box, along the proposed South 23rd
Ave., to a point just south of West Lincoln St. The pipe will be sized to allow conveyance of the
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water rights downstream of the property. The WFCC flows are proposed to be piped from the
junction box back to the original channel. A large diameter pipe will be sized to convey not only
the perennial flows, but the estimated flood flows for the WFCC. Piping the irrigation ditch will
allow for continued delivery of irrigation water to downstream water users while eliminating the
potential flood hazard from the open ditch channel on the property to be developed. Also, by
returning the perennial and flood flows back to the WFCC channel, flood hazards will be
contained within the water course setback and natural channel functions will be returned to this
section of channel.
For the EFCC, there are two culverts under Stucky Rd. The first culvert has its inlet located in a
barrow ditch along the south side of the road and its outlet lines up with an old channel
alignment of EFCC. The second culvert currently carries the perennial channel flow. Flood
flows are expected to pass primarily through this culvert with potential overflows passing
through the first culvert. On the north side of the road, the second culvert outlets to a point that
was excavated to split flows back to the EFCC channel and send flows to the east to irrigate the
pasture. Again, while appropriate for the previous land use, the current channel alignment
presents a potential flood risk to proposed development. To mitigate the hazard, a new channel
alignment will be created from the culvert outlet which currently carries the EFCC flows and
routes the flow back to the EFCC channel. The channel will be designed to accommodate the
perennial and flood flows. This re-alignment will allow for the flood hazard to be contained
within the water course setback and reduce hazards on the east of the subject property.
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Appendix A
HEC-HMS Reports
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