Bozeman Creek LOMR

Allied Engineering Services, Inc. (AESI)

Jennifer Johnson, PE, CFMPaul Sanford, PE, CFM

Presentation Outline

Project Background & Objectives

Modeling Approach

2D Modeling

1D Modeling

Results

Questions and Answers

Project Background

Large-scale floodplain mapping was last completed for the Bozeman area in 1988 with a digital conversion of the data occurring in 2011.

Bozeman Creek & Tributaries Floodplain Study, Phase 2 Completed by FEMA, DNRC, Contractors Draft hydraulic model and floodplain mapping – 2015 Completed for Incorporation into the Gallatin County

and Incorporated Areas DFIRM and FIS Supersedes 2011 FIS

Image Credit: DNRC

Project Background

Project Background• Draft floodplain mapping for

Bozeman Creek was completed around the time HEC-RAS 2D was emerging as a modeling tool

• The City of Bozeman was interested in exploring 2D modeling in the downtown area

Image Credit: Google Earth

Project Background

1D Modeling Challenges Urban environment with complex flow splits Pervasive shallow flooding Long hydraulic structures with limited conveyance capacity

Olive-Rouse: ~230 feet; 12 ft x 5 ft opening Babcock Street: ~185 feet; 16 ft x 5.3 ft opening Main Street: ~320 feet; 12.6 ft x 6 ft opening Overtopping of streets at Q < 10% Annual Chance Event

Project Objectives

Update the floodplain mapping in the downtown area using HEC-RAS 2D

It is important to be able to use the best modeling tools available!

HEC-RAS 2D was a relatively new tool Limited guidance available for 2D studies

Modeling Approach

Challenges associated with using a 2D model Hydraulic structures

Floodways in 2D environments Considered 2D floodway configurations including looking at (1)

individual cell surcharges and (2) average WSEs at preselected cross sections

Modeling Approach

After discussions with the City of Bozeman, FEMA, and DNRC, we settled on a 1D model informed by a 2D model

Benefits of this approach: Better conceptualized 1D model More confidence in split flows We can still model a floodway per FEMA regulations

2D Modeling

General Setup The 2D area encompassed downtown Bozeman and the

area to the northeast Rating curves and normal flow boundary conditions

were used for outflows 1% Q = 1,007 cfs at upstream end of 2D model

2D Modeling

General Setup HEC-RAS 5.0.3 Full Momentum Cell size 20 x 20 ft generally, 5 x 5 or 10 x 10 across main

conveyance paths Eddy viscosity coefficient of 1.5 Modeled buildings with a high Manning’s n Added buildings as breaklines to accurately model

Manning’s n values Simulations required a small time step

2D Modeling

How to model the hydraulic structures?• Modeled as bridges in 1D draft hydraulic

model to account for geometry changes along each culvert and an open bottom

• HEC-RAS 5.0.3 – no bridge modeling in 2D• Wormhole Culverts

• Introduced by a user named Con Katsoulason the RAS Solution forum

• Wormhole culverts became obsolete with HEC-RAS Version 5.0.4

2D Modeling

Calibration We did not have gage data or other measurements to calibrate to Structure modeling is not as advanced in the HEC-RAS 2D

environment as in 1D To justify the use of wormhole culverts, we compared their

performance to the bridges in the draft hydraulic model

2D Modeling

Complex split flows complicated the comparison between 1D and 2D structures

Used Hydraulic Property Table (HPT) values to compare 1D structures to 2D structures

2D Modeling

Used HW, TW, and flow from 2D model culverts to pull bridge HW from 1D model HPT

Very similar results for low flow (impending road overtopping) comparison since no complex flow splits

Complex flow splits affect HW/TW relationship for the 1% event. More variance in comparison but still reasonable agreement.

2D Modeling

2D Modeling

Results

• The model showed Rouse as a significant conveyance path (~190 cfs to 350 cfs)

• ~70 cfs of flow was leaving the Bozeman Creek/Rouse system via Main Street

• More widespread shallow flooding

1D Modeling

General Model Setup• Modeled from upstream of Olive to the

East Gallatin Confluence• Used the 2D model to establish reaches

and flow exit/entrance points

1D Modeling

General Model Setup• Added a reach down Rouse and a reach at Main Street• Tied flow into the I90 split reach

1D Modeling

Split Flow Determinations• Used profile lines in 2D • Challenging accounting given the complexity of flow

paths

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Results

Flow Split comparison between Draft and LOMR

WSE comparison between Draft and LOMR WSEs generally dropped ~0.5 feet along Bozeman Creek in

downtown area

Floodway comparison between Draft and LOMR Reduced FW along Bozeman Creek Added ~440 feet of FW along Rouse Avenue Added administrative FW along ~780 feet of Main Street

2015 Draft Model

LOMR Model

Location Flooding Source Q (cfs) Q (cfs)Bogert Park Bozeman Creek 1007 1007

US of East Olive Street Bozeman Creek 839 768US of Main Street Bozeman Creek 839 540

US of E Mendenhall Street Bozeman Creek 839 540US of E Lamme Street Bozeman Creek 853 777

US end of Church Ave Split Bozeman Creek 169 239US end of Wallace Ave Split Bozeman Creek 35 113Rouse Ave Downtown Split Bozeman Creek No Split 355 to 189

Main Street Split Bozeman Creek No Split 71

Results

Results

Questions and Answers