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NORTHWEST CROSSING SUBDIVISION
FLOOD HAZARD ANALYSIS FOR
BAXTER CREEK
&
BAXTER DITCH
BOZEMAN, MONTANA
SEPTEMBER, 2019
PREPARED BY: Morrison-Maierle, Inc. 2880 Technology Boulevard West Bozeman, MT 59771
N:\5659\005 NWX Phase 1 Major Sub\04 Design\Flood Hazard Analysis\Table of Contents.docx
TABLE OF CONTENTS
1 FLOOD HAZARD ANALYSIS
2 FLOOD HAZARD EXHIBIT
3 APPENDICIES
3.1 HYDROLOGY CALCULATIONS
3.2 BAXTER CREEK CROSS SECTIONS
3.3 BAXTER DTICH CROSS SECTIONS
3.4 ALLIED ENGINEERING- LAUREL GLEN FLOOD HAZARD ASSESMENT- 2002
3.5 VALLEY WEST SUBDIVISION- PUD DRAINAGE PLAN
Page 1 of 2
1.1 INTRODUCTION
This report provides an analysis of the flood hazard for the proposed Northwest Crossing
Subdivision. The 160 acre development has two waterways that run south to north through the property. Baxter Creek runs along the western boundary. Baxter Ditch runs through the center of the property. Both water ways are classified as stream/ditch as they are fed by natural drainage and irrigation. Both are bordered by wetland plants and meander through cultivated pasture. Two previous studies were used to help develop flowrates for these stretches. First, Morrison-Maierle analyzed both waterways in the Drainage Plan provided in the Valley West PUD application in 2001 (See Appendix 3.5) Second, Allied Engineering submitted two reports for the Laurel Glen subdivision in 2002 ( See Appendix 3.4- Flood Hazard Assessment-Baxter
Creek and Phase 1 Storm Water Facilities Design Report).
1.2 METHOD
Morrison-Maierle surveyed the 160 acre property, including the water ways with GPS survey equiptment. Cross sections were developed approximately every 100 ft along each water way in AutoCAD. Bentley Flowmaster V8i was used to analyze each cross section to determine the water surface elevation for the given flow rate. The water surface elevation was then mapped in
AutoCAD to show the extents of flooding in the current condition and areas that will require grading to contain flood waters.
1.3 HYDROLOGY: BAXTER CREEK
The 2002 report from Allied identifies the 100-year flowrate of Baxter Creek at Durston Road to be 169 cfs. This includes potential overflow from Farmers Canal south of Huffine Lane.
Approximately 90 acres of residential land in Laurel Glen will contribute to Baxter Creek between Durston Road and W. Oak St., the south boundary of Northwest Crossing. The 100-year runoff rate for this area is estimated to be 31 cfs. It is likely that the peak flow from Laurel Glen runoff would pass through the study area in this report long before the peak of 169 cfs originating south of Huffine Road and overflow from Famers Canal, but it is conservative to add the two together. For this report, cross sections of Baxter Creek were analyzed with a 100-year flowrate of 200 cfs. Currently, there is head gate at approximately station 15+00 that diverts water to a channel that runs to the west of the main channel. This side channel is proposed to be removed as it runs into the proposed Laurel Parkway right-of-way. For this analysis, we conservatively assumed all flow will go through the main channel.
1.4 HYDROLOGY: BAXTER DITCH
The 1999 Drainage Plan for Valley West prepared by Morrison-Maierle identified the 100-year flowrate of Baxter Ditch at Durston Road to be 74.8 cfs. About 120 acres of residential land between Durston and Oak will runoff into Baxter Ditch before reaching the proposed Northwest
Crossing area. This additional runoff is estimated to be 45 cfs, see calculations in Appendix 3.1. For this study, cross sections of Baxter Creek were analyzed with a 100-year flowrate of 120 cfs. A roughness coefficient of 0.050 was used for all cross sections.
1.5 RECOMMENDATIONS: BAXTER CREEK.
Our analysis shows that all flood waters will be contained within the existing banks of Baxter Creek. We recommend all structures in the vicinity of the creek maintain a minimum floor
elevation at least two feet above the identified flood water surface elevation.
Page 2 of 2
1.6 RECOMMENDATIONS: BAXTER DITCH
The majority of the flood water will be contained within the existing banks of the waterways.
Exhibit 1- “Baxter Creek and Baxter Ditch Cross Sections and Flood Extents” identifies a few areas where flooding will overtop the banks and flow out beyond the 50’ wetland setback. In these cases, we recommend filling areas along the outer 20’ of the wetland setback. This may be accomplished with a stabilized and landscaped berm or trail that will run the length of the creek within the open space. In cases that the open spaces is adjacent to proposed right-of-way, set grades at the back of the sidewalk to be at least one foot above the projected flood water surface elevations. Additionally, we recommend all structures in the vicinity of the ditch maintain a minimum floor elevation at least two feet above the identified flood water surface elevation.