HomeMy WebLinkAbout8 - Fourmile Canyon and Wonderland Creek Major Drainageway flood mitigation planning update
CITYOFBOULDER
WATER RESOURCES ADVISORY BOARD
AGENDA ITEM
MEETING DATE: August 17, 2009
AGENDA TITLE:
Fourmile Canyon and Wonderland Creek Major Drainageway flood mitigation planning
update
PRESENTER/S:
Kurt Bauer, Engineering Project Manager
Bob Harberg, Utilities Planning and Project Management Coordinator
EXECUTIVE SUMMARY:
The purpose of this agenda item is to provide an update on the Fourmile Canyon Creek
and Wonderland Creek flood mitigation planning effort including current staff
recommendations.
The Major Drainageway Phase A plan, prepared by Love and Associates in June 2007,
developed, evaluated, and recommended flood mitigation conceptual-level alternatives
along both Wonderland and Fourmile Canyon Creeks. The work was jointly sponsored
by the Urban Drainage and Flood Control District and the City. The report
recommendations have been reviewed through an extensive public involvement process.
Based on feedback from the public involvement process, staff believes there is general
concurrence on the flood mitigation approach for all stream areas with the exception of
Wonderland Creek between 26th St. and Foothills Parkway. In this area there are
questions regarding the following two approaches:
1. Containment of the entire 100-year flood flows at public expense, thus
mitigating all potential property damage due to the 100-year storm event
2. Containment of the high hazard flood zone at public expense, coupled with
voluntary private financing of floodproofing private property owners'
structures.
Staff has evaluated these approaches as summarized in this agenda item, including staff's
preliminary recommendations, and is seeking WRAB input and feedback.
AGENDA ITEM # 8 PAGE 1
BACKGROUND:
The Phase A study area includes the Fourmile Canyon Creek floodplain from the mouth
of the canyon to its confluence with Boulder Creek and the Wonderland Creek floodplain
from downstream of Wonderland Lake to its confluence with Goose Creek (Figure 1).
The study identified reaches of Fourmile Canyon Creek west of 19`h Street where flow
spills to Wonderland Creek during large events. It was determined to be unfeasible to
eliminate the spills and as a result, all future mitigation measures along Wonderland will
be sized to convey this increased flow caused by the spill from Fourmile Canyon Creek.
A broad range of alternatives were evaluated along specific reaches of both Fourmile
Canyon Creek and Wonderland Creek. The alternatives included: doing nothing,
construction of detention facilities, trans-basin diversions, acquisition of flood prone
properties, high hazard zone containment and floodproofing of structures, channel
modifications to provide 50-year conveyance capacity, and channel modifications to
provide 100-year conveyance capacity. Providing flood water detention and constructing
trans-basin diversions were eliminated from further consideration early on because of the
cost and legal liabilities associated with these alternatives. For each of the remaining
alternatives, estimates of probable construction cost, maintenance requirements, residual
flood damage estimates, and a benefit/cost analysis was developed.
The report has gone through an extensive public involvement process including the
following most recent meetings:
■ Greenways Advisory Committee (September 19, 2007)
■ Public Open House (September 27, 2007)
■ Water Resources Advisory Board (October 15, 2007, December 17, 2007, and
January 28, 2008)
■ Planning Board (February 21 and March 20, 2008)
■ Public Hearing with City Council (November 10, 2008)
■ City Council Special Study Session (April 28, 2009)
In January 2008 WRAB passed a motion with a 4-0 vote to recommend approval of the
Phase A plan as modified by staff with the following recommendations and guiding
principles as this project continues to future phases and more detailed concept design:
1. Protect life safety by addressing structures in the high hazard zone through:
a) Acquiring properties from willing sellers
b) Constructing flood improvements at time of redevelopment of properties
along Fourmile Canyon Creek west of Broadway and Wonderland Creek
near 30th.
c) Constructing high hazard zone containment and other improvements as
funding is available, including coordinating with the county on expediting
improvements located jointly in the city and county.
2. The intent of the overall approach is to minimize disruption to private property
and riparian areas. This implies that flooding during 100-year events will not be
contained in a channel minimizing impacts to downstream properties. Many
AGENDA ITEM # 8 PAGE 2
3. During the next phase all potentially impacted properties and persons including
students and parents should be notified of proposed approach and tradeoffs of
minimizing property impacts versus the potential for flood damages.
4. Public education of flood risks should be emphasized including signage and flood
markers and response plans for impacted schools.
5. Opportunities for facilitating and encouraging private flood proofing should be
explored.
6. Continue to maintain high level of public involvement and feedback.
7. This non-structural approach requires active regulatory flood plain management
in order to preserve flood conveyance areas.
On April 28, 2009 staff presented information to City Council during a Study Session to
address issues raised during the November 2008 public hearing. Council members
generally expressed support for the approach to flood mitigation planning and felt that
existing policies were appropriate, with the following comments related to the Phase A
report:
■ The current approach to flood mitigation should continue and is mostly in the
right direction.
■ Consider doing the least amount of work necessary with the structural
improvement approach to mitigate flood hazards.
■ Flood mitigation work proposed along Fourmile Canyon Creek east of 281h should
be reconsidered and possibly scaled back.
■ Alternatives that leave drainageways in their natural state should be a priority.
■ Mitigation measures should be kept as "green" as possible, i.e. minimize use of
asphalt and concrete.
■ The need to disturb natural areas for the benefit of a few homes was questioned.
■ Removal of structures from the 100-year floodplain was questioned if the
removal/mitigation was only to reduce property damage. Focus removal efforts
on structures in the high hazard and conveyance zone.
■ City council members requested that proposed mitigation costs be presented to
distinguish the cost of containing high hazard flood flows versus containing all
100-year flood flows.
ANALYSIS:
Council's comments can be summarized through the following key issue - should the
City implement high hazard containment and floodproofing or construction of a 100-year
channel system? This key issue relates to three remaining stream reaches, all located
along Wonderland Creek.
The reaches still under evaluation are located between 26th Street and Foothills Parkway
as shown in Figure 1. The existing floodplain is extensive in this segment of the creek
with 111 structures (417 individually owned dwelling units) located in the 100-year
floodplain and 24 structures (181 individually owned dwelling units) located in the High
AGENDA ITEM # 8 PAGE 3
Hazard Zone. Annual premiums are just over $154,000 to insure structures within this
reach. Estimated potential flood damages averaged on an annual basis exceed $2.6
million. Figure 2 graphically presents existing conditions.
Figure 3 graphically summarizes the HHZ Containment and Floodproofing alternative
for this stream segment. This alternative includes:
■ Construction of an overflow underpass and associated channel at 281h Street
■ Channel modifications to reduce the high hazard zone from Kalmia Street to the
Diagonal and upstream from Foothills Parkway
■ Four new culvert crossings
■ Separation of Wonderland Creek from the Boulder and Whiterock Ditch
This alternative would remove all 24 structures currently in the high hazard zone at an
estimated public cost of just over $9.5 million and a private cost of $6.8 million. The
private costs reflect the cost of property owners to floodproof their structures.
Figure 4 graphically summarizes the 100-year Channel alternative for this stream
segment. The alternative includes:
■ Channel modifications along the entire reach to provide adequate conveyance for
the 100-year flows
■ Eight culvert upgrades or replacements
■ Construction of a bridge underpass and overflow channel
■ Separation of Wonderland Creek from the Boulder and Whiterock Ditch
This alternative would remove all 111 structures from the 100-year flood hazard at an
estimated public cost of just over $19.5 million.
The following presents a comparison between the two alternatives.
100-Year Containment HHZ Containment and
Floodproofing
Cost $19.5 million public $9.5 million public
$0 rivate $6.8 million private
Damage Reduction $3.8 million (HHZ only)
(present worth) $66 million $54 million (voluntary
flood roofin
Removes all structures (111) from Removes all structures (24) from
Structures 100-year flood hazard HHZ Zone. 61 structures remain in
100-year flood lain
Maintenance Costs $1 million $1.7 million
Insurance Premiums $154,000 annual savings $69,000 annual savings
0.4 (HHZ only)
Benefit / Cost Ratio 3.4 3.6 (HHZ and voluntary
floodproofing)
Staff conducted a more detailed evaluation of the alternatives to identify the cost of
proposed flood mitigation measures in relationship to the specific benefits of structures
and dwelling units removed. Attachment 1 and associated Figure 5 present this analysis
AGENDA ITEM # 8 PAGE 4
for the High Hazard Containment alternative. Attachment 2 and associated Figure 6
present this analysis for the 100-year Containment alternative.
Staff conducted an open house on Wednesday, August 5 to solicit public input on
whether to recommend high hazard containment and floodproofing or 100-year
containment for Wonderland Creek reaches 5, 4, and 3. The meeting agenda is as
follows:
■ 6:30 - 7 p.m. open house
■ 7 - 7:30 p.m. presentation
■ 7:30 - 8 p.m. questions
■ 8 - 8:30 p.m. open house
The open house included large-format figures graphically presenting existing conditions,
HHZ Containment alternative, 100-year Containment alternative, and a comparative
listing of differences between the alternatives. Staff were available to discuss the
alternatives, answer questions, and document comments. Following the initial open
house session, staff used a PowerPoint presentation to provide a summary of the planning
effort, present the alternatives, and provide a summary comparison. A question and
answer period followed the presentation. A final open house session concluded the
meeting. Citizens were encouraged to provide comments either to staff or on comment
sheets. Staff will provide an update on the outcome of the public open house that was
held on Wednesday, August 5.
PRELIMINARY STAFF RECOMMENDATIONS:
The staff preliminary recommendation is a modified 100-year Containment alternative
for Wonderland Creek between 26th Street and Foothills Parkway. The following
describes the recommended modification.
The 100-year Containment Alternative includes a new bridge and channel modifications
between the downstream side of the Diagonal Highway and the upstream side of Iris
Avenue. These features have an estimated cost of just over $3.3 million. These proposed
features would remove one commercial structure from the 100-year floodplain. The High
Hazard Containment and Floodproofing alternative includes a new culvert and lesser
channel modifications for this same reach of stream. The estimated cost for these
features is approximately $1.5 million. Under this alternative, the one commercial
structure would be removed from the high hazard zone but would remain within the 100-
year floodplain. The estimated cost difference between the alternatives in this section of
the stream is approximately $1.8 million. Staff believes that the cost difference to
remove one structure from the 100-year floodplain justifies recommending consideration
of the High Hazard Containment alternative for Wonderland Creek between the
downstream side of the Diagonal Highway and the upstream side of Iris Avenue.
Staff recommends the 100-year Containment Alternative for the remaining reaches of
Wonderland Creek between 26th Street and Foothills Parkway for the following reasons.
Although at a higher public cost than the high hazard containment alternative, the 100-
AGENDA ITEM # 8 PAGE 5
year Containment alternative would remove all 110 structures currently located in the
100-year floodplain. Removing structures from the 100-year floodplain will result in
annual savings of approximately $152,000 on flood insurance premiums and avoid the
potential of over $2 million in average annual flood damages ($66 million present worth
cost at 3% interest and 50 years.) In addition, the 100-year Containment alternative does
not rely on voluntary actions from the public to implement floodproofing of individual
structures. The cost of voluntary floodproofing of individual structures is significant
($25-$50K per property) and will not eliminate mortgage required flood insurance. As a
result, staff believes that most homeowners will not choose to implement floodproofing
measures.
Figures 5 and 6 and Attachments 1 and 2 show the location and itemized costs
associated with the two alternatives for this segment of Wonderland Creek. The
following presents a comparison between the three alternatives.
100-Year HHZ Containment Modified 100-Year
Containment and Floodproofing Containment
$19.5 million public $9.5 million public $17.6 million public
Cost $0 private $6.8 million private $0 private'
Damage Reduction $3.8 million (HHZ only) z
(present worth) $66 million $54 million (voluntary $65.5 million
flood roofin
Removes all structures Removes all structures Removes 110 structures
(111) from 100-year (24) from HHZ Zone, 61 from 100-year floodplain
Structures flood hazard structures remain in 100- (one remains), removes
year floodplain all structures (24) from
HHZ zone
Maintenance Costs $1 million $1.7 million $1.1 million'
Insurance $154,000 annual savings $69,000 annual savings $152,000 annual savings'
Premiums
Benefit / Cost 0.4 (HHZ only)
3.4 3.6 (HHZ and voluntary 3.7
Ratio floodproofing)
One commercial property would require floodproofing
z Estimated based on ratio values
In addition, the original intent was that once the Phase A report was accepted by Council,
staff would work with the consultant to develop Phase B of the flood mitigation study.
This second phase of the project would develop preliminary design of recommended
alternatives. The Phase B report would be reviewed as part of the city's Community and
Environmental Assessment Process (LEAP). The Phase A recommended improvements,
however, total greater than $32,000,000 in public expenditures and therefore will be
implemented in phases over many years. The Phase B preliminary design and the
associated CEAP will likely no longer be relevant when many of the recommended
improvements would be implemented, many years from now. As a result, staff will not
prepare a Phase B preliminary design report and associated CEAP. Specific projects will
instead be developed using the Phase A report as a planning guide. Under this approach,
design and CEAP would be developed for individual mitigation projects located along
Wonderland and Fourmile Canyon Creeks. An example of this approach is the
AGENDA ITEM # 8 PAGE 6
Wonderland Creek Improvements Project that has begun for the reach of Wonderland
Creek from Foothills Parkway upstream to 34th Street.
NEXT STEPS:
■ Public hearing with the WRAB on September 21, 2009 and request for a final
recommendation to Planning Board and Council.
■ Public hearing with the Planning Board - October 2009
■ Public hearing with the City Council -late 2009 or early 2010
ATTACHMENTS:
Figure 1: Location of Evaluated Reaches
Figure 2: Existing Conditions
Figure 3: High Hazard Containment and Floodproofing Alternative
Figure 4: 100-year Containment Alternative
Figure 5: HHZ Containment Itemized Cost Key
Attachment 1: HHZ Containment Itemized Costs
Figure 6: 100-Year Containment Itemized Cost Key
Attachment 2: 100-Year Containment Itemized Costs
AGENDA ITEM # 8 PAGE 7
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177
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- C L'i -y~•! 7Cr-100-Year Floodplain High Hazard Flood Zone , +
Structures in Building Located in "-`s 1~ : 4rlL "
Stream Reaches
r, Flcodpiain High Hazard Zone
'Y' ` I ` r I1 nth! t~ r '
f1'•w •g Y ~T, t/t. t y~ ti y •4}Q ~r~ '~.a h'~,' • p 'y(Tj 1 'J
Figure 3: HHZ Containment and ,
Floodproofing Alternative
Llmr
yz sue} : h I _ :A structures remain i 1, 00 y e f 11110,0 :!:t~
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..<syr/:-',
jq. f( stnu ture~Y ;24) p_ ,-~r~~!ec from High Hazard
y 4M
~ °4
m' it 4 6`~i ~G xS`n psi ~ r y r $6.8 million '"(vate c
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ref' r~ ~.b _ _ - Dtagona! Highwa mom, vr- : ,
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F 100 Year Floodplain High Hazard Zone' *R- Iris
f ~ New Crossing Stream Reaches
1• • F^ T
z
Structures Removed from HHZ Zone
XStructures Removed from 100-Year Floodplain '
.
~ ~--~~s ! a~~iUT ..sa i.. l Sti'1i~L~
F'i ure 4: 104- Year Containment
AI ternative
ll.. t 44Y' r, .ter I l:~ a, % d p y S,,.I P c S o s lri I. •1 ■ r~ ~ E ~ 1 0.A L~ h~9 ~ 1 I~ ~ Y 1 1 '~.J~ ~ ~ ~ C-'V I i i .b ~J i,/
- ~ ~ ~a, i l ~i ~ ;~.9 ~ ~ ~ 7 3
z y r-cl ee_ , ` (111) and High Hazard Zone (241
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` , ! V - t i iiyn/ "-sue r T I4
100-Year Floodplain and High Hazard Zone ar• x~ 1 ~~V!!//+ 91'q' r 'r
riF+.~ ~:'s~ys:t..arw ~.a3t' =~rIS~~;+. '..j--'. ~
Stream Reaches - t
Structures Removed from High Hazard Zone a F (?,,.~'-'Q I "'w` t , T, ~('+''t~`~~ rK4t~' ~1
_ as d . ,v - -'I , J ' f °".l
- Structures Removed from 100-Year Floodplain = i,~ _F,, a.T~ '
~y~ _V1/ C.Llll~~i`~ ~ ( r _ ~1 ail
New Crossings ?y~ # .~a. ;p `~3t ' ' 1' t
- - .•1...11.- y ~ liJ r.~rr~~~.~)IS~~~ y 16
F 0
iure 5: Wonderland Creek
HHZ Containment Cost Key
1
Norwood Ave
Numbers refer to specific 1.
L - 7T
in o
Proposed system upgrades
Itemized on Attachment 1
e_
2,3
C1. ; O
Kalmia Avenue -J
cok
- - 4 I ~
13-15
Diagonal Highway
c
. Y
9
a
o
I
fS
1
100-Y
ear Flood lain Hi h Hazard Zone Ir
P 9
k
New Grossin
9
Stream Reaches
- Structures Removed from HHZ Zone IN ! 1-2 ~ }a! r, i- • ,et e, ~T
Structures Removed from 100-Year Floodplain d T 1E
+~r
rmt~7p 41M
- - - _ I 111 _ + a ! 7
Figure 6: Wonderland Creek .r
100-Year Containment Cost Key
r ~
1 Norwood Ave +
.
iR ' 1.
I
Numbers refer to specific
4)5
Proposed
U x. system upgrades
- -
Itemized on Attachment 2
N
_ r •r
.9
0
- mI
.N Kalmia Avenue
- - _ _ 12-15 Y
> a
6-w
Pii
TIl, t7-7 '-1-;I Diagonal Highway /r NET[- FT
T 7
-2325 0
W /
21
-
_ 100-Year Floodplain and High Hazard Zane L 221 I f
Iris;
Stream Reaches
L it
- Structures Removed from High Hazard Zone
`i7
- structures Removed from 100-Year Floodplain y
S-4~ -
New Crossings - - - I'
( [pG1
26 _ 27
% ,28
Wonderland Creek Reaches 5, 4, 3 Attachment 1
Comparison of Alternatives
100-Year Channel Alternative
Reach Item Item Raw Cost Contingency Total Removed from 100-year Flood lain
Location Number Item Location Crossing Channel Wetlands Drops ROW Subtotal (40%) Cost Structures Dwelling Units
26th to downstream side of 28th 1 26th $747,600 $747,600
2 Winding Trail $560,700 $560,700
3 26th to Winding Trail $532,586 $532,586
4 Winding Trail to 28th $105,172 $105,172
5 Winding Trail to 28th $110,000 $110,000
6 26th to 28th $126,740 $126,740
7 26th to 28th $509,800 $509,800
8 28th $407,400 $407,400
1$3,099,998 $1,239,999 $4,340,000 51 146
Downstream side of 28th to 9 28th to Kalmia $465,528 $465,528
downstream side of Kalmia 10 28th to Kalmia $294,650 $294,650
11 Kalmia $77,700 $77,700
1 $837,878 $335,151 $1,173,000 0 0
Downstream side of Kalmia to 12 Kalmia to Diagonal $2,340,000 $2,340,000
Diagonal Highway 13 Kalmia to Diagonal $100,000 $100,000
14 Kalmia to Diagonal $168,290 $168,290
15 Kalmia to Diagonal $294,650 $294,650
16 Modify Ped Bridge $150,000 $150,000
17 Fire Access $541,800 $541,800
1$3,594,740 $1,437,896 $5,033,000 17 97
Diagonal Hwy to downstream side 18 Diagonal Hwy to Iris $307,331 $307,331
of Iris 19 Diagonal Hwy to Iris $70,000 $70,000
20 Diagonal Hwy to Iris $371,200 $371,200
21 Bank Bridge $1,633,800 $1,633,800
22 34th and Iris $2,520,000 $2,520,000
1$4,902,331 $1,96D,932 $6,863,000 10 47
Downstream side of Iris to 23 Iris to Foothills Parkway $507,451 $507,451
Foothills Parkway 24 Iris to Foothills Parkway $82,660 $82,660
25 Iris to Foothills Parkway $371,200 $371,200
26 Spring Creek $94,500 $94,500
27 Railroad $396,900 $396,900
28 Ditch crossing $25,000 $25,000
$1,477,711 $591,084 $2,069,000 33 97
Grand Total $19,478,000 111 387
Attachment 2
Wonderland Creek Reaches 5, 4, 3
Comparison of Alternatives
HHZ Containment I Floodproofing Alternative
Reach Item Item Raw Cost Contingency Total Removed from 100 year Floodplain
Location Number Item Location Crossing Channel Wetlands Drops ROW Subtotal (40%) Cost Structures Dwelling Units
26th to downstream side of 28th $0
$0 $0 $0 6 47
Downstream side of 28th to 1 28th $407,400 $407,400
downstream side of Kalmia 2 28th to Kalmia $237,696 $237,696
3 28th to Kalmia $161,000 $161,000
1 $806,096 $322,438 $1,129,000 0 0
Downstream side of Kalmia to 4 Modify ped bridge $150,000 $150,000
Diagonal Highway 5 Modify fire access $541,800 $541,800
6 Kalmia to Diagonal $240,000 $240,000
7 Kalmia to Diagonal $83,400 $83,400
8 Kalmia to Diagonal $161,000 $161,000
11 $1,176,200 $470,480 $1,647,000 9 15
Diagonal Hwy to downstream side 9 Bank culvert $655,200 $655,200
of Iris 10 34th and Iris $2,520,000 $2,520,000
11 Diagonal to Iris $115,752 $115,752
12 Diagonal to Iris $280,000 $280,000
1$3,570,952 $1,428,381 $4,999,000 4 46 Grand Total
Downstream side of Iris to 13 Iris to Foothills $70,500 $70,500 Public Cost $9,501,000
Foothills Parkway 14 Iris to Foothills $86,800 $86,800
15 Iris to Foothills $280,000 $280,000 Grand Total
16 Spring Creek $373,800 $373,800 Private Cost $6,815,004
17 Railroad $396,900 $396,900
18 Ditch Crossing $25,000 $25,000 Total all
1 $1,233,000 $493,200 $1,726,000 31 97 Costs $16,316,000
50 205