HomeMy WebLinkAbout2013-09-26-MemoSeptember 26, 2013
Page 2
Level of Protection for Dubuque Street
Staff recommends protecting Dubuque Street to the 2008 + 1’ level. While this recommendation
offers less protection than the option outlined in the Environmental Assessment (EA), staff feels
that this is a reasonable level of protection and provides a good balance of performance while
limiting impacts. The table below outlines the estimated number of days each option would
have been closed over the past 20 year period and compares the relative elevations of each
option to the recommended option.
Level of Protection Option Estimated number of days
closed over the past 20 years
due to Iowa River flooding
Relative Comparison:
inches above/below the
2008 + 1’ protection level
Existing 150 NA
100 y ear + 1’ 7* -39”
200 year + 1’ 5* -11”
2008 flood + 1’ (Recommended) 0 0”
500 year + 1’ (EA Preferred Alt) 0 +19”
*Including one day for cleanup, inspection and repair after inundation
Backwater Reduction Goals and Bridge Type
Three bridge options are outlined below. The first provides the maximum level of protection and
backwater reduction and is the option that is represented in the Environmental Assessment
(EA). The next two options represent staff’s recommendation for the optimization of backwater
reduction, bridge deck elevation and cost. Two recommendations are made because of the
unique properties of each bridge type. The factors used for this recommendation are outlined
on attached Figure 1, Bridge Type and Level of Protection Comparison. Although both
recommendations sacrifice some backwater reduction when compared to the EA’s Preferred
Alternative, they offer substantially less elevation of the intersection of Dubuque Street and Park
Road. This translates to reduced impact on the wooded bluffs.
Bridge Options
Option Bridge
Type
Low
Steel
Elevation
Elevation
at the
Dubuque
St/Park Rd
Intersection
Backwater
Reduction
at
Idyllwild
and Taft
Speedway
Dubuque
Street
Protection
Level
Construction
Cost
Estimate of
Bridge and
Road
Maximum
Protection and
Backwater
Reduction
(EA Preferred
Alternative)
Deck
Girder
500yr + 1’ 665.03 7” 500yr + 1’ $36.65 M
Recommended
Arch
Through
Arch
200yr + 1’ 659.02 4.9” 2008 + 1’ $38.31 M
Recommended
Girder
Deck
Girder
200yr + 1’ 662.52 6.1 ” 2008 + 1’ $35.01 M
To assist in evaluating the staff recommendations, comparisons of each option to the
Environmental Assessment’s preferred alternative are outlined below as well as a comparison to
each other.
September 26, 2013
Page 3
Comparison of Recommended Arch to EA Preferred Alternative
Bridge Type Backwater
Reduction
Deck Elevation Cost
Through Arch @ 200yr +1’ Advantage by 6’
EA Preferred Alternative Advantage by 2.1 ” Advantage by $ 1.66 M
Comparison of Recommended Girder to EA Preferred Alternative
Br idge Type Backwater
Reduction
Deck Elevation Cost
Deck Girder @ 200yr +1’ Advantage by 2.5 ’ Advantage by $1.64M
EA Preferred Alternative Advantage by 0.9 ”
Comparison of Recommended Arch to Recommended Girder
Bridge Type Backwater
Reduction
Deck E levation Cost
Through Arch @ 200yr +1’ Advantage by 3.5’
Deck Girder @ 200yr +1” Advantage by 1.2” Advantage by $3.3M
Staff recommends that we proceed with either the Through Arch Bridge with a low steel
elevation of 200yr + 1’ or the Deck Girder Bridge with a low steel elevation of 200yr + 1’. The
final decision will need to weigh backwater reduction and cost against deck elevation.
Aesthetics will also be an important factor.
Through Arch
Deck Girder
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