Lowell Creek Flood Diversion Project - Industry Day JUL 2024.pdf

LOWELL CREEK FLOOD

DIVERSION

Industry Day Presentation

Presenters:

Carly Lynch, PM Coleman Chalup P.E., Lead Engineer

Seward, AK

JUL 2024

The U.S. Army Corps of Engineers, Alaska District, is conducting an Industry Day to improve Industry's understanding of USACE's Lowell Creek Flood Diversion project, specifically for construction of the inlet structure, 18ft diameter tunnel and outlet flume located in Seward, Alaska, answer questions on the project and gain an understanding of how the contractor community can support this project.

Presentation Agenda (0900-1030)

• Welcome

• Introductions

• Project Overview

Site Visit (1030-1200)

One-on-ones (1200-1700)

• Sign-up sheet

PURPOSE & AGENDA

• The existing Lowell Creek Flood Diversion System authorized by the Flood Control Act of 1936 (PL 74-738) with an authorized project purpose of flood risk management.

• The flood diversion system reroutes Lowell Creek through Bear Mountain and diverts flows to Resurrection Bay prior to those flows entering Seward. Completed by USACE in 1940.

• Present structure has diversion dam, emergency spillway, and 10ft tunnel that is around 2000ft in length estimated to divert 2,800cfs

• Spillway is 800ft upstream of critical infrastructure including hospital and senior living center

PROJECT BACKGROUND

• System was designed to pass the largest event at that time of 2,800cfs which is now considered to have greater than 2% annual chance of exceedance.

• Landslides are commonplace in the basin and can contribute to an outburst surge of 2.5 times larger than existing stream flow.

• Basin above diversion is in steep, rugged, mountainous terrain with near endless supply of sediment.

• Sediment is transported through the tunnel and accumulates at the outfall, which in high water events cuts off Lowell Point road access and city uses heavy equipment to flood fight/manage debris at bridge.

• Accumulation of debris can also damage critical infrastructure at outfall including City’s sewage treatment facility, shellfish hatchery and Alaska SeaLife Center.

PROJECT BACKGROUND CONTINUED

• Identified feasible solution that provides safe, reliable, and efficient flood diversion of the waters from Lowell Creek during precipitation and surge events.

• This project reduces risk to:

life safety economic damages flood fighting activities reactionary debris management costs

• Project justified by reduction in risk to life safety

FEASIBILITY STUDY

EXISTING PROJECT FEATURES

N

S

EW

CURRENT DIVERSION FOR REPAIRS

2003 Repair

2017/18 Repair

The Recommended Plan as stated in the Director’s Memo is Alternative 4A: Construct New Flood Diversion System

RECOMMENDED PLAN

Components of this alternative include:

Diversion dam

18-ft-diameter tunnel capable of conveying ~8,500 cfs flow

Extending the outfall 150ft to take flow and debris over the Lowell Point Road

Protecting the tunnel inlet from landslide with a canopy

Non-structural components include select tree removal

Refurbishing the existing tunnel to provide backup for new diversion system and to accept diverted flow during inspection and maintenance

PROJECT OVERVIEW

AREA GEOLOGY

GEOPHYSICAL INTERPRETED BEDROCK

OUTFALL BORINGS AND GEOPHYSICS

A

BA

D

B

C

E

E CD

INLET BORINGS AND GEOPHYSICS

F

G

K J I

H

H

I

J

K

G F

CURRENT DESIGN – INLET & BYPASS

CURRENT DESIGN – INLET CHANNEL

6 ft

Short Curve

CURRENT DESIGN - OUTLET

TERRACING CONCEPT

• Terracing will allow any debris to flow away from tunnel inlet similar to canopy

• 12 - 15ft sloped benches

LAYDOWN/STAGING AREAS

Two areas identified as potential laydown/staging areas

Potential for utilizing Barge to assist with staging

Disposal area – Alaska Railroad new terminal area

~ 10 Miles from project site to laydown and staging area

Project Site

Snow Removal Area

Alaska Railroad Disposal

Laydown and Staging Area

NS

E W

SITE ACCESS

SITE VISIT

Outfall Meeting Location

Inlet Meeting Location

Library

QUESTIONS?