HomeMy WebLinkAbout027 Track C Culvert Revision Memo
MEMO
32 Discovery Drive Bozeman, MT 59718 (406) 582-0221 Fax (406) 582-5770
To: City of Bozeman
From: Rory Romey, PE, Andrew Graham, PE, CFM, Eric Foss, PE
Project Name: Track C Rail Extension Site Plan
Project Number: 14-167.10
Date: 7/18/2025
Re:
Track C Culvert Revisions and Effects
Introduction
The purpose of this memo is to outline the effects of the revised culvert configuration proposed for the
Track C crossing at Mandeville Creek. The crossing was originally modeled as a 144-inch equivalent
CMP Arch culvert; the current configuration consists of a 138-inch CMP.
Background
The update to the 2016 Flood Hazard Evaluation (FHE) for Mandeville Creek (dated 2/14/2025) was
accepted by the City of Bozeman as part of the Master Site Plan Modification for the North Park
Development. This FHE included modeling the impacts of the proposed Track C crossing at
Mandeville Creek. The structure for this crossing was modeled as a 144-inch equivalent CMPA
culvert (171”x110”) with a 1-foot embedment depth. The model shows water surface elevations
during the 100-year event equalizing at an elevation of 4674.5 on both sides of the proposed CMPA
culvert. This is a result of the existing 36-inch culvert at the main rail line being undersized and unable
to pass the 100-year peak flow rate of 517-cfs. A large area of ponded backwater occurs in the vicinity
of the Track C rail crossing and the overtopping flow follows the existing overflow path to the
northwest as described and shown in the FHE.
Revised Culvert
The structure for this crossing is now proposed as a 138-inch diameter CMP with approximately 2.3-
feet of embedment depth. Constraints stemming from the railroad’s E80 loading criteria and
manufacturing limitations have pushed the design towards a slightly smaller pipe with a different
shape. The round culvert will be hydraulically less efficient than the arch culvert. However, the
resulting water surface elevations within the Track C Condition and subsequent scenarios of the
accepted FHE remain unaffected. This revised culvert is still large enough that water surface
elevations will equalize on each side during the peak flow from 100-year event. This results in an
identical floodplain boundary to what is shown in the accepted FHE.