HomeMy WebLinkAboutComplete WRAB Packet 9-15-14
WATER RESOURCES ADVISORY BOARD MEETING
MEETING DATE: Monday, 15 September2014
MEETING TIME: 7:00 p.m.
MEETING LOCATION: City Council Chambers, 1777 Broadway
Agenda Highlights:
1.Call to Order (7:00 p.m.)
2.Approval of 18 August Meeting Minutes (7:01 p.m.)
3.*Public Comment (7:05 p.m.)
4.*Public Hearing and Consideration of a Recommendation to City Council Regarding Floodplain
Mapping Updates for Skunk Creek, Bluebell Canyon Creek, and King’s Gulch (7:20 p.m.)
5.Matters (8:20 p.m)
-From the Board
-From Staff
6.Future Schedule (8:40 p.m.)
7.Adjournment (8:45 p.m.)
* Public Comment Item
Agenda item times are approximate.
Information:
Please contact the WRAB Secretary email group at:
WRABSecretary@bouldercolorado.gov.
Packets are available on-line at: http://www.bouldercolorado.gov – A to Z,
Water
Resources Advisory Board (WRAB), Next Water Rescources Advisory Board Meeting
CITY OF BOULDER, COLORADO
BOARDS AND COMMISSIONS MEETING MINUTES
Name of Board / Commission:
Water Resources Advisory Board
Date of Meeting:
18August2014
Contact Information of Person Preparing Minutes:
Kaaren Davis303.441.3203
Board Members Present:
Vicki Scharnhorst,Dan Johnson, Mark Squillace, Lesley Smith, Ed Clancy
Board Members Absent:
None
Staff Present:Jeff Arthur, Director of Public Works for Utilities
Bob Harberg, Principal Engineer -Utilities
Katie Knapp, Engineering Project Manager
Kurt Bauer, Engineering Project Manager-Utilities
Bret Linenfelser, Water Quality and Environmental Services Manager
Don D’Amico,Ecological Systems Supervisor -OSMP
Kaaren Davis, Board Secretary
Cooperating Agencies Present:
Ken McKenzie, Master Planning Manager for the Urban Drainage and Flood Control
District
Alan Turner, CH2M Hill, Primary Consultant for Project
Meeting Type:
Regular
Agenda Item 1 –Call to Order [7:00p.m.]
Agenda Item 2–Approval of the21 July2014Meeting Minutes:
[7:01pm]
21JulyminutesJohnson
: Motion to approve minutes from July 21stas presented. Movedby:
Smith
Seconded by:
Vote: 4:1 (Clancy Opposed)
Agenda Item 3–Public Participation and Comment [7:02p.m.]
Public Comment:
Steve Pomerance
: Former Council member who was directly involved in the original flood
work process during the 80’s.Spent much time with staff. Suggestswe go back to this culture
so that board is fully aware of what is going on. Mapping process is getting frustrating. It is
unclear how the analysis is going to occur, especially with regard to cost/benefit analysis
around the specific neighborhoods. WRAB is the first line of defense, and it would be
appreciated if the board would be active in the process.
Board follow up:
None
Agenda Item 4 – [7:05p.m.]
Public Hearing and Consideration of a Recommendation to City Council Regarding the South
Boulder Creek Major Drainageway Planand Mitigation
Presenters:
Jeff Arthur, Director of Public Works for Utilities
Bob Harberg, Principal Engineer -Utilities
Annie Noble, Flood and Greenways Engineering Coordinator
Don D’Amico, Ecological Systems Supervisor, Open Space and Mountain Parks
Kurt Bauer, Engineering Project Manager
Cooperating Agencies Present:
Ken McKenzie, Master Planning Manager for the Urban Drainage and Flood Control District
Alan Turner, CH2M Hill,Primary Consultant for Project
WRAB Minutes
18 August 2014
Page No.1
Executive Summary from the Packet Materials:
The purpose of this memorandum is to provide a summary of the South Boulder Creek major
drainageway flood mitigation planning study including the study recommendation. This mitigation plan
was initiated in 2010 after the floodplain mapping was updated in 2007. The focus of the study is on
developing and evaluating alternatives designed to mitigate flood hazards affecting structures and areas
along South Boulder Creek and the West Valley within the current incorporated city limits.
A Risk Assessment completed in 2009 estimates that a 100-year storm event would result in
approximately $215 million in damages in the South Boulder Creek drainage basin. The September
2013 flood resulted in overtopping of US36 and corresponding flooding through the West Valley with
reported damages of $38 million. The 2013 flood is estimated to have resulted in flows above a 50-year
event but below the 100-year flow used in the Risk Assessment to estimate damages.
Since the major drainageway mitigation study was initiated, multiple flood mitigation alternatives have
been evaluated to address flooding associated with South Boulder Creek. The alternatives were
screened from fifteen concepts down to four via the planning process. Consultants for the study
recommend an alternative that would eliminate the overtopping of US36 during a 100-year event and
provide flood protection for 362 structures (893 dwelling units) at a cost of $46 million. The $46
million alternative would include a 560acre-foot regional flood detention facility, three smaller
stormwater detention features at various locations ranging from 9 to 58 acre feet and piping a segment of
Dry Creek No. 2 Ditch. The alternative could be constructed in phases ranging from $12 to$23 million.
Construction of the project would require numerous federal, state and local permits, agreements with the
University of Colorado and Boulder Valley School District, and disposal of Open Space and Mountain
Park land. The regional detention facility portion of the alternative would be regulated by the State as a
high hazard dam.Construction of the regional detention facility at US36 would result in significant
impacts to federal and city regulated wetlands, habitat for federally threatened plantand animal species,
agricultural resources, the South Boulder Creek State Natural Area (SBCSNA) and other environmental
and aesthetic resources.
Staff is recommendingthat the “West Valley Improvements” and “Arapahoe Detention” components of
the “recommended alternative” proceed to the Community Environmental AssessmentProcess (CEAP)
separate from the “Regional Detention at U.S. 36” component. While these measures are not as
comprehensive as the full recommended alternative, it is anticipated that they could be designed,
permitted, funded, and implemented in the relatively near future. Staff recommendsseeking additional
direction from City Council regarding the “Regional Detention at U.S. 36” component. If City Council
determines that flood mitigation benefits outweigh associated environmental impacts after considering
input from the WRAB, the Open Space Board of Trustees, city staff, and the public, the “Regional
Detention at U.S. 36,”theproject could proceed on a separate time table.
Public Comment:
rd
Rick Mahan: generation in his house. Understands that there are environmental impacts
3
involved with mapping process, but there are ways to mitigate the impact to wildlife to allow
the options discussed.Frustrated that South Boulder area seems to have many great plans that
are never followed through with.Lost irreplaceable family history during the flood.
Kathie Joyner:
Glad to see staff’s recommendations for US 36 with downstream mitigation
for removing many structures from the high hazard zones. Concerned about the option of
removing the detention facility from the whole package as a separate phase. Delay could
negatively impact about half of the structures in the flood hazard zone. Potential for significant
environmental impacts is valid, but the degree of impact and ability to mitigate can’t be known
unless theEnvironmental Impact Statement (EIS)is conducted. South Boulder Creek sustained
the most damage. Any delays in the analysis of impacts and multi-agency evaluations will
further delay the removal of over 200 structures from the flood hazard zone. Encourage that
impact analysis and multi-agency consultation move forward as fast as possible.
David McGuire:
Had about $70,000in flood damagesto his house and about 9’ of water over
two days. Aware that there are sensitive species in the area, but it is important to analyze the
actual impacts of the flood to those species versus the impacts of the US 36 detention facility.
Species impact mitigations are possible.
WRAB Minutes
18 August 2014
Page No.2
Liliane Stacishin:
Home was flooded. Have not heard how the future development of the area
around the proposed projects will affect the plan as put forth. New structures are being built all
the time.
Shannon Chinatti:
President of Pawnee Meadows HOA. Their neighborhood was ground
zero. No sirens, no reverse 911, no notifications of highway closures. Contests the statement
that the flooding last year was not flash flooding. Her neighborhood experienced very rapid
flooding. Frazier Meadows facility severely damaged. A berm would be a welcomedsight to
the affected neighbors. Flooding will happen again. Mitigation measures discussed should be
put in place as soon as possible. Thanks for working on our behalf and dealing with FEMA,
which is difficult. City is responsive to storm drain cleaning and this is appreciated.
Laura Tyler:
Shared a photo of her front yard after the flood event. Water rose very quickly
during the event. Shared an account of the flood on her property.Read an article from the Daily
Camera about CU Boulder that boasted how well the South Campus faredin comparison to
other areas affected by the flood. The berm around that property is probably responsible for that
protection but also the possible cause forsome of the water being diverted into their
neighborhood. Questions about cost/benefit analysis.
Jeff McWhirter:
President of SE Boulder Neighborhood Association. Neighborhoods are built
on the old floodplains. Buildingcontinuesin the area, which is not smart. Thatsaid, this is what
we are stuckwith and something must be done. If this event was not quite a 100-year event,
what would a 200-year event look like? Don’t know whether this plan is the best plan or not,
but we need to be able to look at perhaps some other alternatives.
Ben Binder:
Presented handout to board. Board and city are being shortchanged because
optimal alternative was never investigated—detention pond on south end of the CU property.
No environmental impacts. Lower cost, but high effectiveness. The CU berm is illegal and built
counter to city and county policies. Counter to FEMA requirements. The old gravel pit can be
used to cut peak flows.CU says they have other intentions for the property, but that could be
combined with athletic fields. CU is being callous if they will not cooperate with the city on
creating this detention facility. Suggests that city reject current alternatives and work with CU
instead.
Kathleen Motylenski:
First level of home flooded during theflood event. She carried herdog
half a mile away to a neighbor who was dry. Neighbors doing all they can but are frightened of
what an actual 100 year or greater flood event would look like.
Jeff Rifkin:
Benefit/cost ratio just looks at cost associated with flood events,but there are costs
associated from flood impacts between events. Concerned about pipeline option and Manhattan
Middle School detention facility. Also concerned about storing water at Manhattan Middle’s
fields andplans to put in three different ponds, but nothing is known about how high the water
table is. What are the impacts and the alternative plans if the proposed area cannot be used as
detention facilities?
WRAB Discussion Included:
Cost/benefit study –reasons for discrepancy between the 2010 predicted cost of a flood event
and the actual costs we are seeing after the 2013 flood event. Evaluation is being done using
FEMA’s HAZUStool.
Environmental values seem to get lost in the cost/benefit evaluation discussions. Would like to
see it clearly included.
Costs and Benefits of just going the first two steps and never pursuing the US 36 Detention
structure option? Are the cost/benefits of the first two steps adequate to pursue without the
detention structure?Phases do have positive benefit to cost ratios by themselves.
Chances of building US 36 Detention Facility seem not very high,given likely impacts on
threatened and endangered species in the area.
Changes in calculating cost/benefit ratios since the 2010 evaluations of the original 15
alternatives?
Community and Environmental Assessment Process Report (CEAP)-Will that be public
input? CEAP is very much an extensive public process.
Wetlands serve important functions (species habitat, flood mitigation). Would we need
permission from Fish & Wildlife for endangered species, as well as Open Space?That would
be a very drawn-out process. If we go ahead with the US36 Detention Facility,we could get
bogged down for a long time.
WRAB Minutes
18 August 2014
Page No.3
2010 Bear Creek Canyon pipelinewas eliminated. Back then there was no discussion of the
wildlife and habitat impacts. Are there second thoughts about adding it back into the mix now
that there is a better understanding of the eco-impacts? Staffresponse: have not yet looked at
utility conflicts related to maintaining large, closed conduits that do not serve any ecological
benefits, which does createconcern. Large banks of inlets required. The amount of debris that
could clog the inlets is huge. Confined space issues. Urban Drainage Flood Control District
does not support large pipelines. Installation and maintenance costs are large.
Any set of alternatives recommended will likely require preparation ofan EIS and we will have
to bring forward 4-5 alternatives anyway. How many options do we leave open? Consider
adding the pipeline back in to open up one more alternative.
Some of the options have a marginal cost/benefit ratio at this time. What happens if during the
design phase we discover that something has a cost/benefit ratio of less than one, and is
thereforea less readily fundable option?(Lower rating could affect grant possibilities.)
If could use more CU land for detention, could the berm be less high, less impactfuland less
costly? Have not surveyed for endangered species on CU property.
How big an impact would inundation cause as opposed to the berm impacts for the threatened
species?Flooding would have less impact as this type of flooding is more normal to thistype of
area.
Alternatives considered to minimize the footprint and impact of the proposed berm.
Elevation fill to elevate property for the berm. Could that not come from putting a pond in the
proposed area?
Goats a possibility for weed control on the berm? Yes, reasonable success with some species of
weeds. Size of the berm would require a lot of goats.
Can eminent domain be used against CU? No -Wedo not have the authority to condemn a
state or federal property.
CU should be persuaded to give up control over some of its land if it opens up better options.
There are political ways to deal with CU to compel cooperation, such as the Board of Regents.
Eachoption should be studied regardless of apparent feasibility.
If WRAB does not send staff forward with a recommendation on the current path, how long
would it be before something new came back to the board?
Motion to recommend that City Council accept the “West Valley Improvements and Arapahoe
Detention Phases” of the South Boulder Creek Major Drainageway Plan and the “Regional
Detention at U.S. 36 with Downstream Improvements” as the recommended alternative to
mitigate flood risks associated with South Boulder Creek.
The WRAB recommends proceeding with the “Regional Detention at US 36”component of the
alternative only after looking at alternatives which may have lesser potential for environmental
impacts, and which may provide faster and less costly opportunities for equivalent mitigation in
that area, such as use of CU propertydetention, private property detention, and eminent domain
options.
Motion by: Johnson; Seconded: Clancy
Vote: 5-0, Motion Passes
Agenda Item 5– [9:20p.m.]
Information Item -Skunk Creek, Bluebell Creek and King’s Gulch Floodplain Mapping Update
Presenters:
Jeff Arthur, Director of Public Works for Utilities
Bob Harberg, Principal Engineer -Utilities
Katie Knapp, Engineering Project Manager
Executive Summary from the Packet Materials:
The purpose of this memorandum is to provide a general summary of the history and results of the
Skunk Creek Floodplain Mapping Update.
WRAB Minutes
18 August 2014
Page No.4
Floodplain mapping provides the basis for flood management by identifying the areas subject to the
greatest risk of flooding. This information is essential for determining areas where life safety is
threatened and property damage is likely and is the basis for floodplain regulations and the National
Flood Insurance Program (NFIP). The city’s floodplain maps need to be periodically updated to reflect
changes in the floodplain resulting from land development, flood mitigation improvements, new
topographic mapping information and new mapping study technologies.
The Skunk Creek Floodplain Mapping Update includes the King’s Gulch, Skunk and Bluebell Canyon
Creek floodplains between the city limits to east of Foothills Parkway where Skunk Creek confluences
into Bear Canyon Creek as shown in red below.
Engineering consultants provided hydraulic modeling to update the existing Federal Emergency
Management Agency (FEMA) Flood Insurance Rate Maps(FIRM) and City of Boulder floodplains,
water surface elevations, conveyance and high hazard zones.
The proposed mapping of the Skunk Creek Floodplain would result in a net:
Increase of 49 structures identified in the 100-year floodplain;
Decrease of 25 structures identified in the conveyance zone and;
Decrease of 14 structures identified in the high hazard zone.
The WRAB review of the floodplain mapping update does not require board members to verify the
analysis and calculations, but accepts the overall mapping study process and that results are reasonable
and acceptable.
Following input from the August WRAB meeting, any information requested about the mapping study
will be presented at the September WRAB meeting. A request for a motion will also be made at the
September WRAB meeting.
WRAB Discussion Included:
Reasons for the increased number of structures in the 100-year flood plain in spite of mitigation
done in the area in recent years.
What the 2013 eventwas rated at (between 50-100-year event). With LiDAR (Light Detection
and Ranging), can individual properties be evaluated and perhaps be removed from the high
hazard zone (HHZ)?There is no FEMA process by which property owners can get their
properties removed from the HHZ.
Did the flood models and LiDAR compare well to the mapping of the actual boundaries of the
event?
How much contact has staff had with thosewho have been added to the 100-year floodplain
and what guidance have they been given? Many appear to be eligible for removal from the
floodplain through the LOMA (Letterof Map Amendment) process.
WRAB Minutes
18 August 2014
Page No.5
No Board action was requested at this time.
Agenda Item 6–Matters [9:43p.m.]
From the Board:
Board member Johnsonbrought up the below matter(s):
Impressed with city maintenance crews during the last rain storm. They were out looking at
trash racks.
Email from Council about recommendation with regard to proposed rate increases and City
Council thought we might be light on sanitary sewer increase.
People who were affected seemed pleasedwith rate increase, and wondering if looking at
setting up a local district to put money into for upgrades might be a possibility.
Board Member Smithbrought up the below matter(s):
Had a conversation with Steve Pomerance. Should we push more to correct some of these
major flooding problems? How do we decide which things get tackled now as opposed to later?
Board Member Clancy brought up the below matter(s):
Raised concerns about the capacity of the WWTF and compliance with related design
standards.
From Staff: [9:47p.m.]
CIP Outreach to Businesses:
Had a meeting with the Boulder Chamber to get their thoughts
on how to proceed and on key peopleto target. Have been doing one-on-onemeetings with
some of the big users to discuss rate impacts.Generally,people have understoodthe need for
rate increases.
CIP Agenda Item at CouncilPrevious Week:
Most of the discussion was around the
Utilities CIP and specifically the Wastewater section. Discussion went well. A majority of
Council looked interested in an option to expedite the wastewater collection system
th.Board
improvements. Council asked about WRAB input.Study Session on September 9
cannottake action before thatdue to meeting schedule, but if individual board members have
input about parameters, comparisons to other communities, priorities for investments, etc.,
please let staff know on an individual basis.
Agenda Item 7–Future Schedule [10:15
p.m.]
Skunk CreekFloodplain Mapping
TwomileCanyonCreek Floodplain Mapping
Adjournment [10:18
p.m.]
There being no furtherbusiness to come before the Board at this time, by motion regularly adopted, the
meeting was adjourned at 10:18p.m.
Motion to adjourn by: Smith;Seconded by: Squillace
Motion Passes 5:0
Date, Time, and Location of Next Meeting:
The next WRAB meeting will beMonday, 15September2014at 7:00 p.m., in the Council Chambers,
1777Broadway, 80302.
APPROVED BY:ATTESTED BY:
____________________________________________________________________
Board ChairBoard Secretary
____________________________________________________________________
DateDate
An audiorecording of the full meeting for which these minutes are a summary,is available on the Water
Resources Advisory Board web page.
WRAB Minutes
18 August 2014
Page No.6
C I T Y OF B O U L D E R
WATER RESOURCES ADVISORY BOARD
AGENDA ITEM
MEETING DATE:September 15, 2014
AGENDA TITLE:
Public hearing and consideration of a recommendation to City
Council to adopt the Skunk Creek, Bluebell Creek and King’s Gulch Floodplain Mapping
Update
PRESENTER/S:
Jeff Arthur, Director of Public Works for Utilities
Bob Harberg, Principal Engineer - Utilities
Annie Noble, Flood and Greenways Engineering Coordinator
Katie Knapp, Engineering Project Manager
EXECUTIVE SUMMARY
Floodplain mapping provides the basis for flood management by identifying the areas
subject to the greatest risk of flooding. This information is essential for determining
areas where life safety is threatened and property damage is likely and is the basis for
floodplain regulations and the National Flood Insurance Program (NFIP).The city’s
floodplain maps need to be periodically updated to reflect changes in the floodplain
resulting from land development, flood mitigation improvements, new topographic
mapping information and new mapping study technologies.
The Skunk Creek Floodplain Mapping Update includes the King’s Gulch, Skunk and
Bluebell Canyon Creek floodplains between the city limits to east of Foothills Parkway
where Skunk Creek confluences into Bear Canyon Creekas shown in redbelow.
AGENDA ITEM # IV PAGE 1
Engineering consultants provided hydraulic modeling to update the existing Federal
Emergency Management Agency (FEMA) Flood Insurance Rate Map (FIRM) and City
of Boulder floodplains, water surface elevations, conveyance and high hazard zones.
The proposed mapping of the Skunk Creek Floodplain would result in a net:
Increaseof 44structures identified in the 100-year floodplain;
Decreaseof 27structures identified in the conveyance zone and;
Decreaseof 17 structures identified in the high hazard zone.
STAFF RECOMMENDATION:
Staff requests Water Resources Advisory Board consideration of this matter and action in
the form of the following motion:
Motion to recommend that City Council adopt the Skunk Creek floodplain
mapping update including potential additional refinements made prior to
Council’s consideration.
COUNCIL FILTER IMPACTS:
Economic: Flood insurance is required for properties located in the 100-year
floodplain if they are financed by a federally-backed mortgage. Flood insurance
rates are set by FEMA based on the flood risk as shown on the flood insurance
rate maps.Accurate floodplain mapping helps facilitate accurate flood insurance
rates. The average annual rate for flood insurance within the city in 2013 was
$760 (3,830 policies). Flood protection land use regulations also create costs for
the property owners in the form of permit fees, increased costs of remodeling and
restrictions on development. Flood insurance and land use regulations do,
however, provide protection from potentially catastrophic losses due to floods.
Environmental: Flood events can result in damage or destruction to buildings and
corresponding release of man-made contaminants. Flood waters canalso cause
erosion and damage to areas of the natural environment that are not capable of
conveying high-velocity stormwater. The updated mapping will more accurately
identify the areas with the greatest flooding risks.
Social: Floodplain mapping provides the basis for flood management by
identifying the areas subject to flooding. This information is essential for
determining areas where life safety is threatened and property damage is likely.
Land use regulations help reduce risks to people and property in these high flood-
risk areas. Accurate mapping of flood risks also helps implement effective flood
preparedness and response programs, thereby increasing the safety of people
living, working or visiting the City of Boulder.
OTHER IMPACTS:
Fiscal: Funding for this study is included in the Department of Public Works
Utilities Division budget.
Staff Time: Time for completing the study is included in existing work plans.
AGENDA ITEM # IV PAGE 2
BOARD AND COMMISSION FEEDBACK
The Skunk Creek floodplain mapping was presented to the WRAB as an information
item on August 18, 2014. The board requested that staff continue to work with the public
to inform them about the proposed floodplain mapping and address comments and
concerns. It was also requested that information about FEMA’s Letter of Map
Amendment (LOMA)process be made availableon the city’s website.Staff is
continuing to work with the public and will provide additional notification letters prior to
Council consideration of the mapping study. Information about FEMA’s LOMA process
has also been included on the project website.
AfterWRAB considersthe mapping update, it will be provided to the Planning Board as
an informational item and then presented to City Council for their consideration.
PUBLIC FEEDBACK
Public notification post cards about the mapping update have been sent to all property
owners in the study area and a project web site has been developed to provide
information (https://bouldercolorado.gov/water/skunk-creek-floodplain-mapping-update).
An open house was held on August 18, 2018 immediately prior to the WRAB meeting to
inform the public about the mapping update and hear comments and concerns about the
study. Staff has also met with residents in person and responded to phone calls and
emails. In general, most of the comments and questions have been about impacts to
specific properties and requests for more detailed information such as proposed base
flood water elevations. There were also concerns about the high hazard zone delineations
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and the distribution of the Bluebell Canyon Creek split flow paths downstream of 15 St.
In response to the public feedback, the high hazard zone delineations have been re-
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evaluated and refined. The flow distribution at 15Street has also been reviewed. At this
time, changes have not been made in this area, but staff is working with the engineering
consultant to re-evaluate the flow distribution and revisions may be forthcoming. A
Attachment A
summary of the public feedback is provided in .
BACKGROUND
The risk of flash flooding is an important issue for the City of Boulder primarily due to
its location at the mouth of Boulder Canyon and other canyon creeks. Approximately 13
percent of the city is located within the 100-year floodplains of Boulder Creek and its 14
tributaries. Additional information about the city’s floodplain management program,
Floodplain Management
floodplain regulations and flood insurance can be found at:
Overview.
The city delineates four flood zones:
500-year floodplain: The 500-year floodplain delineates the flood limits resulting
from a storm that has a 0.2 percent chance of occurring in any given year.
AGENDA ITEM # IV PAGE 3
100-year floodplain: The 100-year floodplain delineates the flood limits resulting
from a storm that has a one percent chance of occurring in any given year (26
percent chance over a 30-year mortgage).
Conveyance zone: The conveyance zone is defined as the areas in the floodplain
that are reserved for the main passage of the entire 100-year flood flow when the
100-year floodplain is artificially narrowed until a maximum six-inch increase in
flood water depth is created. This zone is delineated to allow development to
occur up to the narrowed floodplain and still provide passage of 100-year storm
flows.
High hazard zone: The high hazard zone defines the area of the floodplain where
water depth and velocity pose a threat to life and safety. This area is delineated for
areas in the floodplain where water depths are four feet or greater or where the
water velocity multiplied by water depth equals or exceeds the number four.
Skunk Creek, Bluebell Canyon Creek, and Kings Gulch were first studied in 1987 by the
consulting firm Greenhorne & O'Mara and the resulting Flood Hazard Area Delineation
(FHAD) report included the delineation of the 100-year floodplain along these creeks.
The Flood Insurance Study (FIS) and Flood Insurance Rate Map (FIRM) approved for
these creeks were originally based on the 1987 FHAD and included a federally-regulated
one foot rise floodway. Since that time, both the City of Boulder and the State of
Colorado have adopted a ½ foot rise floodway, which the City refers to as the
Conveyance Zone.
In 1989, Love and Associates delineated the High Hazard Zone and City of Boulder
Conveyance Zone (½ foot rise floodway).The delineations were based on the hydraulic
models used in the 1987 FHAD.
On May 6, 1991, FEMA issued a Letter of Map Revision (LOMR) for Skunk Creek to
incorporate the results of a channel improvement project.The limit of the LOMR was in
the University of Colorado’s Research Park, downstream of Colorado Avenue to just
upstream of the confluence of Boulder Creek.
Several road-crossing structures for Skunk Creek have been improved since the
regulatory floodplain was adopted in 1991. Culverts at Broadway and at 27th Way,
crossings at Anderson Ditch and the cemetery maintenance road, and the low water
crossing upstream of 27th Way were not included in the 1991 regulatory model, but were
incorporated into the current mapping study.
The City initially contracted with Belt Collins to develop the updated floodplain maps but
they closed their Boulder office in 2013. ICON Engineering provided a peer review of
Belt Collin’s 2011initial study and was selected tocomplete the project.
In 2013, the city acquired state-of-the-art Light Detection and Ranging (LiDAR)
technology to produce high-resolution topographic mapping. The new LiDAR mapping
was compared to the 2003 topographic base mapping and areas showing substantial
differences were updated in the hydraulic models.
AGENDA ITEM # IV PAGE 4
ANALYSIS
This mapping study updates the hydraulic models and flood hazard mapping for the 100-
year floodplain, Conveyance and High Hazard Zones for the entire reach of Skunk Creek,
including the King’s Gulch, and Bluebell Canyon Creek tributaries.
A 2-dimensional hydraulic model was developed for the creek system to determine
primary flow paths and split flow areas. Information from the 2-dimensional model was
used as a “roadmap” to develop the conventional 1-dimensional hydraulic model used for
the analysis.
The existing 100-year floodplain for Skunk Creek, King’s Gulch and Bluebell Canyon
Creek isprimarily along the creek corridors and roadway areas with some spillage into
surrounding properties. The proposed 100-year floodplain is more extensive than the
existing mapping in most areas and bears resemblance to the September 2013 flood
extents. The September 2013 flood extents were not used to delineate the floodplains but
were used to check assumptions on flow paths. For Skunk Creek, King’s Gulch and
Bluebell Canyon Creek, the September 2013 flood extents are similar to the proposed
floodplain mapping.
The existing Conveyance and High Hazard Zone mapping for Bluebell Canyon Creek
and King’s Gulch did not include a significant neighborhood area that has ahistory of
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flooding east of 15 Street.The proposed mapping extends the Conveyance and High
Hazard Zones through thisresidential area to their confluence with Skunk Creek along
Broadway. The proposed mapping also extends the Conveyance and High Hazard Zones
for Skunk Creek north of Broadway to include more roadways, split flows and other
areas not previously mapped.
The revised mapping indicates that the flood risk impacts more structures in the Skunk
Creek Drainage Basin than was shown in the previous mapping. A majority of the
th
structures newly identified as being at risk are located within the bounds of 15Street to
the east, Broadway to the west, Baseline to the north and King Avenue to the south. This
area experienced significant damage during the September 2013 flood.
Attachments B through D
present figures showing a comparison between existing and
proposed floodplain mapping. A summary of how these changes impact existing
Attachment E
structures is included in .
NEXT STEPS:
Following input from the WRAB, the floodplain mapping study will be finalized and then
considered by City Council. If City Council approves the map revisions, the city will
submit a request to FEMA for review. During the FEMA review and approval process it
is recommended that the new mapping be used for regulatory purposes by regulating to
the more restrictive of the existing and proposed mapping. This would mean that
development within the newly identified flood zones would be subject to the city
floodplain regulations. In order to comply with FEMA requirements, development
AGENDA ITEM # IV PAGE 5
within the areas that are being removed from the floodplain would still be subject to the
city’s floodplain regulations until FEMA officially adopts the new floodplain mapping.
Following formal adoption by FEMA, the city would regulate solely based on the new
mapping.
ATTACHMENTS
A.Public Comments
B.Existing and Proposed 100-Year Floodplain
C.Existing and Proposed Conveyance Zone
D.Existing and Proposed High Hazard Zone
E.Summary of Impacts to Existing Structures
AGENDA ITEM # IV PAGE 6
ATTACHMENT A: PUBLIC COMMENT SUMMARY
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Open House Date:
Aug. 18, 2014
Open House Meeting Location:
Municipal Building Lobby
Number of attendees that signed-in:
23
Staff in Attendance:
Robert Harberg Katie Knapp Kristin Dean
Laurel Olsen-Horen Douglas Sullivan
Public Comments:
1.Location: 2042 Baseline
Commenter: Property owner (Ben Chancellor; Christina Jurgens)
Comment: Did not see flooding in September 2013 and do not feel that the high hazard
designation iswarranted; question split values for Mariposa vs. Columbine
Response: The high hazard zone delineations have been refined based on a review of
adjacent grades. Adjacent to the structure at 2042 baseline, the delineation was revised
such that the structure sits just outside of thehigh hazard zone. Split flow values for
Mariposa and Columbine are being re-evaluated using 2Dmodeling to see if the September
flood event can be more closely replicated in the modeling. It should be noted that the
September 2013 flooding reflected a lower intensity and longer duration storm compared
to the regulatory 100-year design storm that is a significantly higher intensity but shorter
duration storm. This difference in storms can result in significant differences between the
regulatory 100-year floodplain mapping and what was experienced in the September flood
event.
2.Location: Area south of Baseline Road between 20th and Broadway
Commenter: Several property owners
Comment: Flooding in September 2013 was confined to streets; no flow behind homes;
water did not appear to be originating from Bluebell Canyon Creek proper.
Response: Split flow values for Mariposa and Columbine are being re-evaluated using
2Dmodeling to see if the September flood event can be more closely replicated in the
modeling. It should be noted that the September 2013 flooding reflected a lower intensity
and longer duration storm compared to the regulatory 100-year design storm that is a
significantly higher intensity but shorter duration storm. This difference in storms can result
ATTACHMENT A: PUBLIC COMMENT SUMMARY
in significant differences between the regulatory 100-year floodplain mapping and what
was experienced in the September flood event.
3.Location: 22nd and Mariposa Avenue
Commenter: Several property owners
Comment: Flows traveling east on Mariposa turned north on 22nd Street and continued to
Columbine Avenue; this is not shown as 100-year flooding.
Response: This flow path has been added to the documentation of the September flood
event. The portion of 22nd Street between Mariposa and Columbine is shown as shallow
flooding (Zone X) for the proposed floodplain. The proposed floodplain mapping is this area
is being re-evaluated.
4.Location: 19th and Mariposa Avenue
Commenter: Property owner
Comment: structure at south east corner is shown in the 100-year floodplain but did not
experiencedamage during the September 2013 event; please review assumptions here.
Response: Split flow values for Mariposa and Columbine are being re-evaluated using
2Dmodeling to see if the September flood event can be more closely replicated in the
modeling. It should be noted that the September 2013 flooding reflected a lower intensity
and longer duration storm compared to the regulatory 100-year design storm that is a
significantly higher intensity but shorter duration storm. This difference in storms can result
in significant differences between the regulatory 100-year floodplain mapping and what
was experienced in the September flood event.
5.Location: 955 Quinn Street
Commenter: Property owner (Lee Payne)
Comment: Structure does not show as impacted on floodplain maps (tree cover issue?);
how was floodplain delineated at corner of Denton Avenue and Quinn Street.
Resp
onse: Due to the large amount of tree cover, the structure was inadvertently excluded
from the proposed floodplain map exhibit. The maps have been corrected to show the
-yea
principal structure located outside of the proposed 100rfloodplain.
6.Loca
tion: 3130 Aurora
Commenter: Property Owner
Comment: It seems like the HHZ could be the result of a small depression that we may not
want to include
in the mapping.
Response:
7.Location: 1700 Bluebell
Commenter: Property Owner (Bill Mooz)
Comment: Structure is shown as in proposed floodplain but was not impactedby
September 2013 event;wants to know why actual data was disregarded.
ATTACHMENT A: PUBLIC COMMENT SUMMARY
Response: The September 2013 flooding reflected a lower intensity and longer duration
storm compared to the regulatory 100-year design storm that is a significantly higher
intensity but shorter duration storm. This difference in storms can result in significant
differences between the regulatory 100-year floodplain mapping and what was experienced
in the September flood event. Split flow values for Mariposa and Columbine are being re-
evaluated using 2Dmodeling to see if the September flood event can be more closely
replicated in the modeling.
8.Location: 1849 Mariposa Ave,
Commenter: Property Owner (Steve Brown, Guen Simons)
Comment: Water from Bluebell creek did not flow to Mariposa. It flowed down the Bluebell
drainage but primarily to the north along 19th Street and down Columbine.
Response: Split flow values for Mariposa and Columbine are being re-evaluated using
2Dmodeling to see if the September flood event can be more closely replicated in the
modeling. It should be noted that the September 2013 flooding reflected a lower intensity
and longer duration storm compared to the regulatory 100-year design storm that is a
significantly higher intensity but shorter duration storm. This difference in storms can result
in significant differences between the regulatory 100-year floodplain mapping and what
was experienced in the September flood event.
9.Location: 2100 Baseline
Commenter: Property Owner (Jamie Karpohl)
Comment: a) There were no eastbound flows observed on Columbine west of 20th Street.
b) The flooding at 20th and Columbine originated from the Anderson ditch on the north side
of Columbine. This waterflowed through properties to the north-east and down the
Columbine North alley towards 21st. At 21st the flows split - continuing down the alley and
heading north towards Baseline. c) During the flood, there wasno flow observed coming
down Columbine west of 20th. The only flows observed in Columbine were from Anderson
ditch on the north side of the street. When I visited the location of Bluebell Canyon Creek at
th
St. on the morning of September 14th, I observed all of the flow heading down
15
Mariposa. I did not observe any man-made diversions at this location.
Response: a) The city has received conflicting information about the flooding observed
thth
and 20 Streets. At this time the flood extent
along Columbine between 19
th
documentation shows this area as having flows that came north from Mariposa along 19
Street and then continuing east on Columbine. The documentation of the September 2013
flood extents will continue to be refined as additional information is received.
b) Split flow values for Mariposa and Columbine are being re-evaluated using 2Dmodeling
to see if the September flood event can be more closely replicated in the modeling. It
should be noted that the September 2013 flooding reflected a lower intensity and longer
duration storm compared to the regulatory 100-year design storm that is a significantly
higher intensity but shorter duration storm. This difference in storms can result in
significant differences between the regulatory 100-year floodplain mapping and what was
experienced in the September flood event.
28th St
28th St
22nd St
Legend
Legend
28th St
28th St
22nd St
Legend
Legend
28th St
28th St
22nd St
Legend
Legend
Attachment E
Skunk Creek, Bluebell Canyon Creek and Kings Gulch
Floodplain Mapping Revisions
The tables below present a summary of how the proposed floodplain mapping revisions impact
existing structures.
Summary of Proposed Changes
Skunk Creek
Number of Structures 100-Year Floodplain Conveyance Zone High Hazard Zone
Existing Floodplain 178 82 30
Proposed Floodplain 222 55 13
Change
No Longer Affected 61 55 23
Newly Affected 110* 28 6
No Change 117** 27 7
*There are (3) LOMRs which previously excluded structures from the 100-year floodplain that are newly affected by
the proposed 100-year floodplain extents
**There are (5) LOMRs which previously excludedstructures from the 100-year floodplainthat are nolonger
affected by the proposed100-year floodplain extents(117(no change structures)+ 110(newly affected structures)–
(5)LOMRs = 222 structures in proposed floodplain)