Westpark Stormwater Bypass Reconstruction Project · 2019. 6. 11. · Existing Storm Drain Structure ... Dimond Detention 1,600 LF New Parallel Ditch (12' Wide and 3' Deep) 750 LF

Westpark Stormwater Bypass Reconstruction ProjectStakeholder Working Group Meeting # 2

April 3, 2019

Project Purpose

To identify, design, and construct an infrastructure project that will disconnect the existing stormwater overflow to the south pond while not causing or worsening flooding in the Westpark Subdivision.

System Overview

Project Schedule

Current Project Schedule

Project Kickoff: Oct 2018 (completed)

Data Collection: Oct ‐Nov 2018 (completed)

Pipe Inspection: Oct 2018 (completed)

Stormwater model of basin: Nov ‐Dec 2018 (completed)

Develop & evaluate alternatives: Dec 2018 ‐March 2019

Underway –This is the focus of today’s meeting

Concept Design of Preferred Alternative: March – May 2019

Draft DSR: July 2019

Final DSR: August 2019

Tentative Future Project Schedule

Survey and geotech for selected alternative: Fall 2019

Design selected alternative: Fall 2019 – Spring 2020

Project Bidding: Late Spring 2020

Project Construction – Summer 2020

Recent Activities

Recent Community and Coordination Activities

SWG Meeting #1: November 2018

SLCC Presentations : October 2018 and January 2019

Written update to SLCC: February 2019

Meeting with local developer (The Peterson Group): March 6, 2019, regarding development planned this year and in coming years.

Meeting with Anchorage School District: March 28, 2019

Project Phases

1. Drainage Study Report (current)

2. Detailed Design (Plans, Specs, and Estimate)

3. Construction

Phase 1 Goals and Objectives

1. To identify alternatives to disconnect the existing overflow to the pond.

2. To select a preferred alternative that meets the technical and community objectives. E.g. Do not worsen flooding compared to existing conditions, and do not allow stormwater into the pond.

3. To complete concept design of the preferred alternative.

Planning for Risk

Q. What is a 10‐year rainfall event?

A. It has a statistical likelihood of occurring once every 10 years.

In other words, it has a 10% likelihood of occurring in any given year.

Q. What is a 100‐year rainfall event?

A. It has a statistical likelihood of occurring once every 100 years.

In other words, it has a 1% likelihood of occurring in any given year.

Planning for New Rainfall Data

Design Storm Old Depth (in) New Depth (in) Percent

Increase

10‐year, 24‐hour 1.77 2.28 29%

100‐year, 24‐hour 2.48 3.59 45%

Baseline Modeling

Existing Conditions Model

Shows how the drainage system is performing today, with the pond overflow in place.

Looked at performance during a 10‐year, 24‐hour event and a 100‐year 24‐hour event.

The model simulates rainfall as it hits the ground surface, flows toward the drainage system, and is routed to either the pond or toward Dimond.

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Tarsus Drive

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80th Avenue

Kenai Fjords Loop

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73rd Avenue

Skyhills Drive

Kidro

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Yukon Charlie Loop

82nd Avenue

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Wandering Drive

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Legend!( Existing Storm Drain Structure

Existing Storm Drain Pipe

³

Pond

DimondBore

EmergencyOverflow

Westpark Sag

Westpark Stormwater Bypass ReconstructionExisting Conditions

CurrentDevelopment

CurrentDevelopment

CurrentDevelopment

School Site

Station (ft)4+804+604+404+204+003+803+603+403+203+002+802+602+402+202+001+801+601+401+201+00

Eleva

tion (

ft)

141.5141

140.5140

139.5139

138.5138

137.5137

136.5136

135.5135

134.5134

133.5133

132.5132

131.5131

130.5130

129.5129

128.5

_ __ _

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J-M-WP

D-23-C

B

Link ID L-M-WPD-22-P

J-M-WP

D-22-C

B


J-M-WP

D-21-M

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J-M-WP

D-20-C

B


J-M-WP

D-19-C

B

Sample of Existing Conditions Model - Westpark Drive 10-Year Storm

100-Year Storm

High Water Level

Manhole

Pipe

Ground Surface

Manhole

Pipe

Pipe

Ground SurfaceHigh Water Level

Baseline Modeling

Full Build Conditions Model

Shows how today’s drainage system would perform once the basin development is complete.

Future basin development includes: Sonoma Glen Phases 2, 3, and 4 The school site The northwest property

This was used as a baseline for “don’t make flooding worse.”

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Tarsus Drive

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iew St

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80th Avenue

Kenai Fjords Loop

Grand Teton Loop

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Legend!( Future Storm Drain Structure

Future Storm Drain Pipe!( Existing Storm Drain Structure


³

Pond

DimondBore

EmergencyOverflow

Westpark Sag

Westpark Stormwater Bypass ReconstructionFull Build Conditions

CurrentDevelopment

CurrentDevelopment

Current & FutureDevelopment

School SiteFutureDevelopment

FutureDevelopment

Alternatives Modeling and Results

Alternatives Analysis

Evaluated many different configurations

In each case, the priorities were: No stormwater to the pond. Do not make flooding/ponding worse.

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Westpark Stormwater Bypass Reconstruction

Legend!( Summer 2019 Storm Drain Structure

Summer 2019 Storm Drain Pipe!( Proposed Storm Drain Structure

Proposed Storm Drain Pipe!( Existing Storm Drain Structure


³

Emergency Overflow Removed

Pond

DimondBore

Bypass + School Detention + Dimond Detention

School Detention

Dimond Detention

750 LF New ParallelBypass Pipe (30" to 42")

750 LF Upsized Pipe (42")

1,800 LF New ParallelBypass Pipe (30")

CurrentDevelopment

CurrentDevelopment



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Drive

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liff St

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iew St

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Kenai Fjords Loop

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Yukon Charlie Loop

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Proposed Storm Drain Pipe!( Existing Storm Drain Structure


³


Pond

DimondBore

Extended Bypass + Dimond Detention

Dimond Detention

2,300 LF New ParallelBypass Pipe (30" to 42")

750 LF Lowered andUpsized Pipe (48")


CurrentDevelopment

CurrentDevelopment



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liff St

reet

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iew St

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Kenai Fjords Loop

Grand

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Gate

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k Driv

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73rd Avenue

Skyhills Drive

Kidro

n Street

Yukon Charlie Loop

82nd Avenue

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Wandering Drive

Somm

ers Pl

ace

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Unnamed Road

Park WestCircle

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Proposed DitchProposed Storm Drain Pipe

!( Existing Storm Drain StructureExisting Storm Drain Pipe

³


Pond

DimondBore

Extended Bypass with Westpark Ditch + Dimond Detention

Dimond Detention

1,600 LF New Parallel Ditch(12' Wide and 3' Deep)



CurrentDevelopment

CurrentDevelopment


School Site


FutureDevelopment

FutureDevelopment

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Seac

liff St

reet

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iew St

reet

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80th Avenue

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Grand

Teton Loop

Gate

Cree

k Driv

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Cobbler Drive

73rd Avenue

Skyhills Drive

Kidro

n Street

Yukon Charlie Loop

82nd Avenue

Dry C reek Loop

Quiet Circle

Wandering Drive

Somm

ers Pl

ace

Lost Circle

Unnamed Road

Park WestCircle

Violet

Plac

e

Sky M

ountain

Circle

Skylark Avenue

Heather Wood Circle

Flagstone CircleSk

y Mou

ntain Lane

80th Avenue

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Wand

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Driv

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Proposed Pipe or DitchProposed Storm Drain Pipe

!( Existing Storm Drain StructureExisting Storm Drain Pipe

³


Pond

DimondBore

Extended Bypass + Small Dimond Detention + Second Dimond Bore

Small DimondDetention

1,600 LF New ParallelBypass Pipe OR Ditch



CurrentDevelopment

CurrentDevelopment


School Site


Second Dimond Bore (28" O.D.)

FutureDevelopment

FutureDevelopment


Bypass Pipe vs Ditch Along Westpark Drive

Pipe expected to be more expensive.

Pathway conflict—would have to move the pathway.

General aesthetic differences.


Bypass Pipe vs Ditch Along Westpark Drive

Existing Westpark Drive

Example Open Ditch in Urban Area


Westpark Stormwater Bypass ReconstructionAlternative Evaluation

Bypass(various sizes &

alignments evaluated)

Bypass + School

Detention

Bypass + School Detention + Dimond

Detention

Extended Bypass + Dimond Detention

Extended Bypass w/ Westpark Ditch + Dimond Detention

Extended Bypass +Small Dimond Detention +

Second Dimond Bore

Dimond Bore Area X X

Westpark Drive X

Kenai Fjords Loop

Grand Teton X

Big Bend Loop X

West Gate Area X

Dimond Bore Area X X

Westpark Drive X

Kenai Fjords Loop

Grand Teton

Big Bend Loop X

West Gate Area

Key

X = Worsens Drainage Issues

= Does Not Worsen or Improves Drainage Issues

Gray Text = Other Fatal Flaw (e.g. right‐of‐way constraint)

Even

t

Location

Alternatives

10‐year

100‐year

AWR Engineering Page 1 of 1 4/1/2019

Project Budget Status

Westpark Stormwater Bypass ReconstuctionBudget Summary

Item Amount ($) Notes

2018 Bond $2,140,000 Bond Approved in April 2018Available Project Amount $2,030,000 5% of the Bond goes to MOA Central Services and Bond CostCurrent Project Budget $2,030,000

Design $470,000Utilities $100,000ROW $35,000Construction $903,000 Expected to be underfunded based on draft alternatives15% Contingency $225,000Overhead $300,000

Amount Spent to Date $113,000 Design and OverheadAmount Remaining $1,917,000

AWR is only contracted through final DSR. Amount needed for final design, ROW, and utilities not known until alternative selected.

Closing Questions or Comments?

Documents

Westpark Stormwater Bypass Reconstruction Project · 2019. 6. 11. · Existing Storm Drain Structure ... Dimond Detention 1,600 LF New Parallel Ditch (12' Wide and 3' Deep) 750 LF