HomeMy WebLinkAbout5 - Public Handouts - Boulder Creek Floodplain Mapping Study
Comments on City of Boulder Floodplain Remapping page 1 of 25
David Leserman, 3/28/2011
Comments on City of Boulder Floodplain Remapping
Prepared by David Leserman
2806 Cordry Court, Boulder CO
for presentation to the City of Boulder Water Advisory Board
3/28/2011
Executive Summary .................................................................................................................................2
Thanks to City staff ................................................................................................................................2
My Request of WRAB ..............................................................................................................................3
Issues ......................................................................................................................................................3
Issue 1. Consequences of High Hazard Zone regulations ............................................................................3
Illustration ..............................................................................................................................................3
Mitigating factors? .................................................................................................................................4
Issue 2. Favoring private property ..............................................................................................................5
Examples ................................................................................................................................................5
Cordry Court ..........................................................................................................................................6
WRC Engineering demonstrated proof of concept ................................................................................6
Issue 3. Conveyance Zone that leaps a barrier in a single bound ...............................................................7
Issue 4. Gold Run development .................................................................................................................10
Background .........................................................................10
WRC Engineering Analysis ...................................................................................................................10
Effect on HHZ and CZ calculations .......................................................................................................10
Correlation of data with an actual real-life 25-year event ...................................................................11
So what? ..............................................................................................................................................11
Conclusion ............................................................................................................................................11
Rebuttal To Specific Items In the Staff Packet ........................................................................................12
Attachment A .................12
Item 1: Section-by-section comparison of studies ...............................................................................12
Item 2: Are model differences due to floodway modifications or modeling methodology .................12
Item 3: WSEL before-and-after Gold Run Comparisons ......................................................................12
Item 4: Anderson Hydrology Verification Report questionable ...........................................................12
Item 5: There is no evidence that HEC-RAS is an improvement over HEC-2 .......................................12
Item 6: Correction of mapping errors later via LOMA .........................................................................13
Item 7: Reduction of property use and value ......................................................................................13
Item 8: Neighborhood effects on floodplain mapping .........................................................................13
Item 9: Determination of fair market value after flood damage .........................................................13
Issues that were raised but are not addressed byAttochmentA .............................................................14
Water profiles prior to Gold Run development ...................................................................................14
Margins of error ...................................................................................................................................14
Abbreviations used in this document ....................................................................................................14
Attachment A, WRC Boulder Creek Floodplain Analysis Review .............................................................16
Attachment B, WRC Boulder Creek floodplain analysis - Gold Run development ...................................18
Attachment C, Possible Conveyance Zone Elevation Errors ....................................................................19
Attachment D, Memo, Koch to Coleman, 1/18/11, re Comments by Leserman ......................................21
Attachment E, Email regarding the 1969 25-year event .........................................................................22
Attachment F, Barrie McRoberts statement re 1969 flood .....................................................................24
Attachment G, Boulder Creek Exceedance Frequency Chart .................................................................25
Comments on City of Boulder Floodplain Remapping page 2 of 25
David Leserman, 3/28/2011
Executive Summary
My home at 2806 Cordry Court has been in the 100-year flood plain for the entire 32
years that I've lived here and I accept that, albeit with a bit of anxiety every flood
season. In the 1980s Boulder added a High Hazard Zone. Because my home was not
in the boundaries of that zone, I did not object to the regulatory change. Now, in
2010 and 2011, the new Floodplain Study balloons the boundaries of the High
Hazard and Conveyance Zones to include my home. This change limits my use and
reduces the value of my property and my home.
Based on new information provided by WRC Engineering, I request that, rather than
approve the current Boulder Creek Floodplain Mapping Study Update, WRAB
request that the staff perform further analysis. In this document I describe the
effects of reasonable minor revisions to the model input variables on floodwater
surface elevations that would take my home, and possibly all the residential
structures in the neighborhood, out of the High Hazard Zone and maybe completely
out of the Conveyance Zone as well.
In 1979, the City allowed development of the Gold Run Condominiums on the bank
of the Boulder Creek directly across from my neighborhood. Herein I describe the
detrimental effect of that development. The engineering analysis shows that without
the development, 100-year water surface elevations would be less than half of what
they are predicted to be with the development in place.
In allowing the Gold Run development, the City was a participant in creating a
potentially hazardous flood situation on my property. This is indicated by the High
Hazard Zoning designation for my home in the current Anderson Study. By including
my home in High Hazard Zone, the City is placing a regulatory burden on me and my
property for a situation created by the Gold Run developer and by the City itself
when it allowed that development. I look to the City to relieve me of this regulatory
burden by making reasonable and minor revisions to the model input, as has been
done elsewhere in town, with the goal of improving model accuracy.
Thanks to City staff
Before launching into another diatribe about what's wrong with the current
floodplain study and whining that it harms me and that it's not fair, I want to thank
the City staff and Anderson Consulting for all the work they have done to-date to
answer my questions and address issues that I've raised. The Anderson Study is an
impressive and complex engineering feat. Staff has made considerable effort on my
behalf, including rerunning the HEC-RAS model with a split-flow configuration
between Folsom and 3011, and also re-surveying the neighborhood.
The results of the new survey in my yard shows ground levels about 1/2 foot higher
than had been previously used in the model, and hence a reduction in flood water
depths. That already affords some relief from the impacts of the regulatory zones.
The split-flow analysis resulted in more of a mixed bag, favoring some neighbors
and further distressing others. Unfortunately, my house did not escape the High
Comments on City of Boulder Floodplain Remapping page 3 of 25
David Leserman, 3/28/2011
Hazard Zone boundaries and is still clipped on the southwest corner by the
Conveyance Zone.
In any case, staff efforts have been considerable and are appreciated. Thank you.
My Request of WRAB
Now - can you believe I have the nerve? - I ask for more. I make the following
requests of the Water Resources Advisory Board.
a. Do not recommend approval of any version of the Boulder Creek Floodplain
Mapping Study Update. Rather, request additional information regarding the
Study. There are still open issues, which 1 describe in detail herein.
b. Reject the staff request to use the Study results in any regulatory manner during
any interim periods in which the Planning Board, the City Council, or FEMA is
reviewing the study results. Again, there are still open issues.
c. To verify results of my engineering study regarding Gold Run, request that staff
complete an analysis of the floodplain boundaries in the Cordry Court
neighborhood using elevation data prior to the Gold Run development.
d. Request that staff complete an analysis of the floodplain boundaries in the
Cordry Count neighborhood using minor and appropriate revisions to the model
input variables, such as those I discuss as being employed by WRC Engineering,
and that favor reduction in regulatory impact on private property, as has been
done in other areas in the study reach (which I describe below).
e. Request that staff survey and include model input elevations for the median strip
on 28th St. at Olson.
Issues
Issue 1. Consequences of High Hazard Zone regulations
The City of Boulder is the only non-coastal entity in the US to implement a High
Hazard Flood Zone. In this section I describe how the High Hazard Zone regulations
create a losing scenario for my neighbors and me.
Illustration
For purposes of illustration, I use my own home. Country Assessor records indicate
that the current actual value of the structure is $136,500 and the actual value of the
land is $266,000, for a total actual value of $402,500.
Consider this. A major flood comes and damages my house next summer to the
extent that it would cost about 60% of its value to repair it. As I understand it, I
would receive about 60% of $136,500, about $82,000, as compensation for the loss
from flood insurance and those funds can be used only to repair the damage or to
buy a new home. However, under the City High Hazard Zone regulations', I would
not be allowed to repair or rebuild the structure because doing so would cost more
Comments on City of Boulder Floodplain Remapping page 4 of 25
David Leserman, 3/28/2011
than SO% of its value. If I requested, the City might buy the property - but the City's
starting point in negotiations would be the value after the damage occurred, likely
only $10,000 or $20,0002 since, under the HHZ regulations, the lot is no longer
available for its only zoned use, that is, residential. Someone else might buy the lot
for a similar amount. So, after a major flood, the property may be worth $20,000
and, if I purchase another home, I might collect about $82,000 from FEMA Flood
insurance.
Under this scenario, my net loss is at least $300,000, about 3/4 of the value of the
property before the flood. That's a considerable loss for a person whose home is the
biggest single investment in his life savings. And, it's far from fair.
Mitigating factors?
High Hazard Property Acquisition Program
In Item 7 of Attachment A of your packet, staff states
In the event that this property does experience more than 50 percent
damage, the city's high hazard property acquisition program may be
available for his assistance.
After struggling to learn about the high hazard property acquisition program, I've
been told only that
At a general level the city does have a high hazard zone property
acquisition program, however due to funding limitations this program is
direct to the high hazard zone residential structures will the highest
levels of risk (product numbers). Applicability of this program for your
structure would depend of the final flood risk evaluation.3
It gives me no reassurance whatsoever to learn that this secretive program would
give me any relief after a flood when the stated purpose of the program, as I
understand it, is to purchase high-risk properties before a flood.
Unlikeliness of event
Also in Item 7 of Attachment A of your packet, staff points out that "anticipated
flooding" (I assume that means the 100-year event) will most likely not create
damage in excess of 50% of the value of the structure. Why, I ask, if such an event
would happen only less frequently than once every 100 years on average, does the
City see fit to prevent rebuilding on such a site? The hazard to life and safety exists,
but it is small - I expect smaller than the risk to life and health from driving a car or
crossing the street. I would think that's it's perfectly reasonable to rebuild such a
structure every hundred years or so. Are these stringent regulations surrounding
the High Hazard Zone appropriate in a riverine community such as ours?
Comments on City of Boulder Floodplain Remapping page 5 of 25
David Leserman, 3/28/2011
Issue 2. Favoring private property
On page 18, the Anderson Study4 states "In addition, at the request of the City of
Boulder, adjustments were made to the floodway where possible in an effort to
contain the floodway to City open space property and public right-of-way, thereby
minimizing impacts to private property."
I greatly appreciate the efforts of the City to minimize regulatory burdens on private
property owners. It seems that much of this effort revolves around making
favorable engineering judgments about the values of model input variables on a
very localized scale.
Examples
Examples of locations where favorable values have been chosen to reduce impacts
on private property include the following:
a. Example 1. Goss-Grove neighborhood:
The Goss-Grove neighborhood was used as an example of favorable treatment
for private property at the October 6, 2010, Open House. WL,
.
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The image above shows the HHZ from the Anderson Study output. The shaded
areas represent the existing regulatory zone and the outlined areas represent
the regulatory zone as defined by the Anderson Study. HHZ boundaries are, to a
great extent, constrained to roadways. Only about 10% of the residential
structures along Grove St (just north of Arapahoe) are touched by the HHZ. Eight
commercial structures are removed from the zone while only 2 are added.
b. Example 2• Folsom and 28th:
The images below show the CZ (left) and the HHZ (right) for the split-flow model
output. In both images, the shaded areas represent the existing regulatory zones
and the outlined areas represent the regulatory zones as defined by the
Anderson Study. Note that in both cases, the current zones cut diagonally across
the development at the southeast corner of Folsom and Arapahoe but that the
newly defined zones take a very clean 90-degree turn at that intersection,
removing five commercial buildings from the zones.
Comments on City of Boulder Floodplain Remapping page 6 of 25
David Leserman, 3/28/2011
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I pondered this, questioning Christie Coleman about a retaining wall that I took
to be a floodwall. Eventually she convinced me that no floodwall was there and I
asked what could possibly explain this neat 90-degree turn phenomenon.
Christie explained that, even though the crown of Arapahoe is higher than the
surroundings, Arapahoe was considered to be a conveyance zone for the
purposes of the Study.5 I make no claim to understand this and Christie did not
explain further. However, this is exactly the sort of favorable treatment based on
localized inputs that I'm talking about.
Cordry Court
I hope that WRAB will ask the staff to explore local optimizations to similar effect for
the Cordry Court neighborhood.
WRC Engineering demonstrated proof of concept
I asked my consulting engineers at WRC Engineering6 to analyze the possibility of
removing my home from the High Hazard Zone by making minor revisions to the
model. (See Attachment A.)
By increasing the number of flow/velocity distribution areas per cross section,
improving the accuracy of the modeled street width, and making small changes in N-
values (roughness coefficients), WRC was able to remove my house from the HHZ. In
the conclusion of their detailed report (Attachment A, WRC Boulder Creek
Floodplain Analysis Review), they state
Our review of the Boulder Creek HEC-RAS analysis revealed that with a
few minor revisions to the model input variables, all within a reasonable
tolerance, the velocity and resulting High Hazard Zone value can be
reduced at your property. It is our opinion that the HHZ delineation can
Comments on City of Boulder Floodplain Remapping page 7 of 25
David Leserman, 3/28/2011
be shown confined to the street and main channel, as has been done at
other locations.
I repeat: WRC was able to remove my house from the HHZ by making a few
reasonable and minor revisions to the model input variables.
Issue 3, Conveyance Zone that leaps a barrier in a single bound
The Cordry Court neighborhood is also affected by a Conveyance Zone, depicted
below, that begins as floodwaters flow out of the creek west of Folsom. The Zone
continues north (downhill) on Folsom, cuts down an alley between Jo-Ann Fabrics
and Harvest Manor Apartments, and then flows east between the Millennium Hotel
and the Safeway. As it crosses 28th Street it overtops a median strip that is about
one foot higher than the predicted 100-year WSEL, flows across commercial
properties on the 28th Street Frontage Road, and ultimately affects Cordry Court
residential structures.
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I am astounded by this flow, particularly how it overcomes a barrier a foot higher
than the water. It would not surprise me for some water to get past a barrier but it
does surprise me that the amount needed to represent a conveyance zone does. I
describe the overtopping elevations in more detail below.
Comments on City of Boulder Floodplain Remapping page 8 of 25
David Leserman, 3/28/2011
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In the close-up view above (located by the rectangular outline in the prior figure),
you can see conveyance zone (shaded green) and the outline of the median strip in
the middle of 28th at Olson. There is no study-related cross-section going down the
middle of 28th St, only one on each side. And the City has not shown the median on
their 1-foot contour map. So, apparently, the elevation of the median strip is not
modeled. But it's there and it's about a foot higher than the 5289' water surface
elevation of the 100-year event (shown as the blue line on the west side of 28th St.).
Note that the City's 1-foot contour at 5288 is shown as the ragged red line.
Below is a slightly wider view 100-year inundation area (outlined). Here you can see
that a tongue of dry land extends from the 28th St Bridge south to where it almost
touches the end of the raised part of the median strip. So, very little water can flow
around this median strip on the south. And the conveyance zone as depicted does
not include any significant area to the north of the median strip.
I reported possible elevation errors in this conveyance zone to the City in an email
to staff dated 1/6/2011 (Attachment Q. The City responded with a memo from Greg
Koch of Anderson Consulting to Christie Coleman and forwarded to me (Attachment
D) denying such errors.
Greg Koch writes' (excerpted from Attachment D),
Mr. Leserman claims that the 28th Street median is a barrier to flow and
this effect is not accounted for in the model. The 28th Street median in the
vicinity of the bridge over Boulder Creek is indeed a barrier to the
passage of the 100-year flow, but the median is fully represented in the
Comments on City of Boulder Floodplain Remapping page 9 of 25
David Leserman, 3/28/2011
hydraulic model. The City's 1-foot contours were compared to as-built
survey data provided by the City for the 28th Street median; the 1-foot
contours appeared to accurately represent the detailed as-built survey
data. In order to simplify the use of topographic data from various
sources for an already complex model, the 1-foot contours were used to
define overtopping elevations for 28th Street. As indicated on the flood
hazard work maps, 28th Street does not overtop near the bridge; rather,
overtopping of the street begins approximately 500 feet north of the
Boulder Creek Bridge and extends north to Arapahoe Avenue.
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The City's 1-foot contours completely miss the median as a feature. This is my point.
The issue I point out is not a problem in the Study; rather, it is that the City failed to
map a contour representing the median. It's too expensive to look at this level of
detail for the entire big picture, but not necessarily inappropriate to do so when an
issue is raised.
Here, then, is another example of where the City, with special consideration given to
localized model input variables, might reduce the impact of regulatory flood zones,
in this case, the Conveyance Zone, in the Cordry Court area. I appeal to the City to do
just that. Make a survey of the median strip and incorporate the elevations into the
model.
Comments on City of Boulder Floodplain Remapping page 10 of 25
David Leserman, 3/28/2011
Issue 4. Gold Run development
Background
The Gold Run development, along with its protective floodwall and berm, was built
on the bank of Boulder Creek, across the Creek from my home, beginning in 1979.
The project had received City approval during a short period in which floodwater
estimates were relatively low and regulation relatively lax. By the start of
construction, the estimates of flood flows had been increased and flood regulations
considerably tightened, but the development began and proceeded even as building
permits were about to expire. Meanwhile, the Governor of Colorado and the
Colorado Land Use Commission stepped in. That resulted in the requirement for a
floodwall to protect the development. Subsequently, the City of Boulder petitioned
the court and got a restraining order against the continued development; but then
the City backed away, allowing the development to proceed and eliminating several
acres from the floodplain directly across the creek from my home. I believe that
without this Gold Run development, my home would not be within the High Hazard
Zone or the Conveyance Zone boundaries that have been redefined by the 2010
Anderson Study.
The City played a role in permitting the Gold Run development that, I show in the
next three subsections below, created an increased flood hazard for the
neighborhood. By including my home in High Hazard Zone, the City is placing a
regulatory burden on me for a situation that it, itself, created.
WRC Engineering Analysis
I asked my consulting engineers at WRC Engineering to run an analysis using the
HEC-RAS model developed by Anderson with elevation data that existed prior to the
Gold Run development. In summary, WRC reports:
"The pre-development model resulted in a WSEL of 5,281.5 ft. The
current HEC-RAS analysis, prepared by Anderson, computed a WSEL of
5,282.3. Based on this analysis, the difference between pre-development
conditions and the construction of the Gold Run site is +0.8 ft."
In other words, Anderson HEC-RAS model output shows that, without the Gold Run
development, the WSELs at my house would be 0.8 ft lower. (The full WRC report is
an email attached as Attachment B, WRC Boulder Creek floodplain analysis - Gold
Run development.)
Effect on HHZ and CZ calculations
Item 7 of Attachment A of your WRAB packet reports the 100-year water depth
around my house as 1.3 ft. Thus, without Gold Run, the water depth would be only
0.5 ft (1.3 - 0.8), less than half as deep as predicted in the split-flow model. At the
flow velocities reported, the depth-times-velocity product would be in the range of
2.5 ft2/sec at the highest, well below the HHZ threshold of 4 ftz/sec.
Comments on City of Boulder Floodplain Remapping page 11 of 25
David Leserman, 3/28/2011
This very significant decrease in WSEL shown by WRC would most likely remove
the entire neighborhood from the High Hazard Zone and maybe the Conveyance
Zone as well. I did not opt to hire WRC for a complete zonal boundary analysis, but
Anderson can do that cost-effectively.
Correlation of data with an actual real-life 25-year event
An actual 25-year event is likely another indicator of the difference in flood
elevations created by the Gold Run development.
In my backyard, the 25-year flood event predicted by the 2010 HEC-RAS model
shows a water surface elevation (WSEL) that is approximately 1.85' higher than the
observed WSEL for an actual 25-year event that occurred in 1969.1 attribute this
difference to the model itself and to physical developments in the area, mostly the
creek side Gold Run buildings and protective floodwall.
City staff has not taken this suggestion seriously. The WRC Analysis of the pre-Gold-
Run-development is likely informative on this discrepancy. This could be verified by
running the 25-year event with the Anderson model using pre-development
elevations for Gold Run.
Details are provided in Attachment E, Email regarding the 1969 25-year event,
Attachment F, Barrie McRoberts statement re 1969 flood, and Attachment G,
Boulder Creek Exceedance Frequency Chart.
So what?
It's clear from the engineering analysis that the existence of Gold Run has a
detrimental effect on the flood elevations and hazards for the Cordry Court
neighborhood. Over thirty years ago I argued that construction should cease and the
partially completed creek side buildings be removed. It's a little too late for that
now. The City played a role in permitting that development and in creating the
resulting detrimental flood hazard. It's not fair or appropriate for the City to place a
regulatory burden on my property for a situation to which it, itself, contributed.
1 ask that now the City at least ameliorate that situation by doing everything
reasonable to create a favorable model output result and relieve the Cordry Court
neighborhood property owners of the undue regulatory burden. We're stuck with
the situation that brings whatever hazard now exists. We do not need to be stuck
with a regulatory burden on top of that.
Conclusion
I am suggesting that, rather than approve the current Boulder Creek Floodplain
Mapping Study Update, WRAB should request that the staff perform further analysis
directed toward finding and making minor and appropriate revisions to model input
variables in order to reduce regulatory impact on the Cordry Court neighborhood.
This has been done elsewhere in the Study and WRC Engineering has shown that it
is feasible at this location.
Comments on City of Boulder Floodplain Remapping page 12 of 25
David Leserman, 3/28/2011
The City has contributed to the flood hazard on Cordry by allowing the Gold Run
development. The City can at least minimize the regulatory burden of that hazard on
private property owners that will eventually suffer the result of increased flooding.
Rebuttal To Specific Items In the Staff Packet
Attachment A
Item 1: Section-by-section comparison of studies
While city staff represents that it would be difficult to compare current and previous
studies at a section-by-section level of detail, it is perfectly reasonable to run the
current study with historical data and then compare those model runs to determine
how changes on the ground affect flood elevation. Such an analysis would clearly
show which changes in results are due to the variation in simulation model and
which are due to changes on the ground.
Item 2: Are model differences due to floodway modifications or modeling
methodology
See Item 1.
Item 3: WSEL before-and-after Gold Run Comparisons
The WSELs given by staff are consistent with, but less revealing than, the
engineering analysis done by WRC to show the WSEL at my home without Gold Run
using the Anderson model. As discussed above that WRC analysis shows that the
Gold Run development accounts for about 9" of the 15" or 16" 100-year WSEL at my
house.
Item 4: Anderson Hydrology Verification Report questionable
I refer you to my 10/15/2010 written statement to WRAB, Issues Regarding the City
of Boulder Floodplain Remapping. The staff response to my challenges to the
Verification Report considers only one of the six issues raised. However, I will not
pursue my objections to the Hydrology Verification Report at this time.
Item 5: There is no evidence that HEC-RAS is an improvement over HEC-2
The staff response that I am incorrect in this "opinion" is incorrect. This is not my
opinion. The information is clearly stated on page 2-6 of the HEC-RAS Reference
Manual8, the same reference cited by staff for the improvements in HEC-RAS over
HEC-2. However, I qualify my previous statement that it is not known whether HEC-
RAS or HEC-2 is more accurate. The reference manual makes this statement only in
relation to conveyance comparisons. I apologize for not getting this right initially.
Comments on City of Boulder Floodplain Remapping page 13 of 25
David Leserman, 3/28/2011
Item 6: Correction of mapping errors later via LOMA
I add only that it's much more cost effective for the City to get mapping right in the
first place than for someone to file a Letter of Map Amendment with FEMA after the
mapping has been adopted.
Item 7: Reduction of property use and value
I appreciate this staff acknowledgement of the existence of the language of BRC 9-3-
8(b)l which states that a structure in the HHZ damaged 50x/0 or more by a flood
cannot be repaired or rebuilt (because it is a residential use in a residential zone).
I take to heart the point made by staff that "anticipated flooding" (I assume that
means the 100-year event) will most likely not create damage in excess of 50% of
the value of the structure.
However, I ask, if such an event would happen only less frequently than once every
100 years on average, why does the City see fit to prevent rebuilding on such a site?
The hazard to life and safety exists, but it is small here - probably smaller that the
risk of crossing the street. I would think that's it's perfectly reasonable to rebuild
such a structure every hundred years or so. These stringent regulations surrounding
the High Hazard Zone are inappropriate for a riverine community such as ours.
Item 8: Neighborhood effects on floodplain mapping
I understand and appreciate floodplain regulations for the purposes of protecting
and promoting public safety, preserving the character of neighborhoods and
conserving property values and supporting the goals of the BVCP.
But it is inappropriate to reduce land use and diminish property value for properties
that would so rarely be damaged by a flood. The regulatory restrictions are not
commensurate with the hazard.
Item 9: Determination of fair market value after flood damage
Clearly, if one is not allowed to rebuild a residence and residential use is the only
use permitted by zoning, the property becomes worth nothing more than parkland.
Such parkland would be great, right in the center of town and along the creek. If the
City wants the land for a park, they should make me an offer. Maybe I'd sell it. But to
regulate the current use out of existence is not fair, even in the event of a flood.
The staff response indicates that yes, the land would be close to worthless but the
City Council might agree to buy it and that the City Manager might agree to pay
more than it's worth after a flood. This is not in any way reassuring. To date, every
time (maybe six times) that I've asked staff to tell me about the high hazard
property acquisition program I've received only a vague response and the
information that it's underfunded. Why would I expect such a program to offer any
relief after a flood?
Is there some document that might explain this high hazard property acquisition
program? Please make such document available.
Comments on City of Boulder Floodplain Remapping page 14 of 25
David Leserman, 3/28/2011
issues that were raised but are not addressed by Attachment A
The following issues were raised by WRAB members at the 11/15/2010 meeting
but were either not captured by the minutes or the staff did not respond.
Water profiles prior to Gold Run development
The WRAB minutes for the 11/15/2010 meeting state:
The board would like to see a water profile model run with the ground
elevations below 28th street based on the contours existing prior to the
construction of the Gold Run apartment project.... Some board members
voiced their concern with asking staff to add more expense to the study. If
there are some public that wish to have that area reevaluated, then they
could go to City Council and ask to have that done.9
But, at the 12/14/2010 meeting, on the topic of the Board's 2011 Goals letter to City
Council, Chuck Howe suggested that an item be included in the letter:
What should the obligations of the City to those who lose property value
as a result of steps taken in the broad public interest? Can Council
consider cases like Cordrey Court for example?10
Ned Williams responded, convincing the Board to refrain from asking City Council to
study this issue, indicating that the Board should, before sending such a question to
the Council, wait to see the Staff response regarding the impacts of the Gold Run
development. But, staff has not reported such impacts. And now it's too late to bring
the question to the City Council agenda for 2011.
Margins of error
The 11/15/2010 meeting minutes report, "The board understands that when
dealing with computer models, there will be some margin of error." As I recall, the
board asked the staff to report back on the margins of error inherent in the results
of the various studies and reports including the Final Floodplain Study and the
Hydrology Verification Report.
Abbreviations used in this document
• CZ - Conveyance Zone
• HHZ - High Hazard Zone
• WSEL - Water Surface Elevation
Comments on City of Boulder Floodplain Remapping page 15 of 25
David Leserman, 3/28/2011
Notes
1 BRC 9-3-8 (b) "In the event of a flood, when a structure intended for human occupancy
located within a high hazard zone is damaged to an extent exceeding fifty percent of its
market value before the flood damage occurred, it may not be repaired or replaced, and use
of the structure for human occupancy shall cease. After written request of the property
owner within ninety days after the date on which the damage occurred, subject to
appropriation by the city council of sufficient funds therefore, the city manager shall agree
to contract or purchase the land upon which the structure was located at its fair market
value after the damage occurred."
Z The value estimates are based on a price of $0.88 to $2.00/square foot for my lot, which is
about 12,000 square feet. I got the numbers $0.88 to $2.00 from the discussion at the
12/14/2010 WRAB meeting about the cost of land for the South Boulder Creek drainage
way acquisition.
3 Email from Christie Coleman dated 1/26/2011, subject: FW: Floodplain study information
requests
4 Final Report, Boulder Creek Floodplain Mapping Study, Anderson Consulting Engineers,
9/22/2010
5 Phone conversation with Christie Coleman, 1/31/2011. For the study, they defined the
Arapahoe ROW as conveyance in order to move flow off of private property even though, for
the most part, Arapahoe is crowned above points north and south. This allows less water to
be conveyed into the alley behind Jo-Ann Fabric.
6 WRC Engineering, Inc., 950 South Cherry Street, Suite 404, Denver, CO 80246, 303-757-
8513, 303-758-3208 (fax). I've been working with Alan Leak, PE, President,
7 Memorandum dated 1/18/2011 from Greg Koch, Anderson Consulting Engineers, Inc., to
Christie Coleman, City of Boulder, regarding Responses to Comments Submitted via Email
by David Leserman on January 6, 2011 (Attachment D)
s HEC-RAS River Analysis System, Hydraulic Reference Manual, Version 4.1, January 2010.
In particular pages 2-6 and C-3. From page 2-6: "The results from the conveyance
comparisons do not show which method is more accurate, they only show differences. In
general, it is felt that the HEC-RAS default method is more commensurate with the Manning
equation and the concept of separate flow elements. Further research, with observed water
surface profiles, will be needed to make any conclusions about the accuracy of the two
methods."
9 WRAB meeting minutes of 11/15/2010.
10 WRAB meeting minutes of 12/14/2010.
Comments on City of Boulder Floodplain Remapping page 16 of 25
David Leserman, 3/28/2011
Attachment A, WRC Boulder Creek Floodplain Analysis Review
From: "Nathan Torrey" <nathant@wrceng.com>
Date: March 25, 2011 11:13:32 AM MDT
To: "'David Leserman"' <david@leserman.com>
Cc: "'Alan Leak'" <alanl@wrceng.com>
Subject: Boulder Creek Floodplain Analysis Review
David,
You requested that WRC Engineering, Inc. (WRC) review the Boulder Creek Floodplain
Mapping Study for reasonableness and provide recommendations for possible revisions.
The city staff is recommending the use of a revised HEC-RAS analysis incorporating a split
flow at Arapahoe Ave., therefore our work focused only on this model.
Our review of the Boulder Creek HEC-RAS analysis revealed that with a few minor revisions
to the model input variables, all within a reasonable tolerance, the velocity and resulting
High Hazard Zone value can be reduced at your property. The following is a summary of
our review and results.
The High Hazard Zone (HHZ) is defined as that area where the product of velocity and flow
depth is equal to or greater than 4 ft2/sec. The HEC-RAS model was used to determine the
velocity distribution for each cross section. HEC-RAS allows for a total of 45 flow/velocity
distribution areas per cross section and a specific velocity is calculated for each area. The
Boulder Creels floodplain analysis specified 9 distribution areas for the main channel and 18
for each overbank, for a total of 45.
The 100-year water surface elevation at your property, Cross Section 23699, is 5282.33 ft.
Based on a recent survey of the property, the lowest adjacent grade appears to be 5281.02
ft. As a result, the maximum depth of flooding at the house will be 1.31 ft, however the
average depth will be less. From the HEC-RAS model, the velocity distribution at the south
side of your house is 3.13 ft/s and is 4.59 ft/s at the north side of the house. This results in a
HHZ value of 4.1 ft2/sec at the south side of the house and 6.0 ft2/sec at the north side
(based on a depth of 1.31 ft.)
N-values (roughness coefficients) at Cross Section 23699 were reviewed for
reasonableness. Cordry Court was modeled with an n-value of 0.020, which is reasonable
for a street. However, the width of the 0.020 area is 58 feet, wider than the actual street.
From aerial photography, Cordry Court, including sidewalks, is approximately 35 feet. The
HEC-RAS model was revised to reflect this. The result was that the velocity on the north
side of the house decreased from 4.59 ft/s to 3.82 ft/s, the south side was unchanged. The
resulting HHZ value is 5.0 ft2/sec on the north side of the house.
In addition, the north side of your property is modeled with an n-value of 0.040. However,
this area more closely resembles the rest of the property which is modeled with a 0.050 n-
value. The HEC-RAS model was revised to reflect this, in combination with the n-value
adjustment for the street, described above, The result is the velocity on the north side of
your house decreased to 3.29 ft/s, the south side was unchanged. The resulting HHZ value
is 4.3 ft2/sec on the north side of the house.
Comments on City of Boulder Floodplain Remapping page 17 of 25
David Leserman, 3/28/2011
The HEC-RAS analysis developed by Anderson utilized 18 flow distribution locations to
define the velocity at specific locations in the overbanks and 9 in the main channel. At Cross
Section 23699, a right overbank does not exist. WRC increased the number of flow
distribution locations in the left overbank to 25 to analyze the impacts. The result is that
the velocity on the south side of your house is reduced to 2.65 ft/s and on the north side it is
reduced to 2.67 ft/s (in combination with the n-value adjustments described above). The
resulting HHZ value is 3.5 ft2/sec.
HHZ areas are directly related to velocity, and velocity is directly tied to n-value. Lower n-
values, particularly in streets, produce higher velocities and HHZ values. Based on this, it
may be reasonable to confine the HHZ area to the street and main channel, removing the
Cordry Court homes from the HHZ area, as has been done in other areas of the study. For
example, structures in the right overbank from Cross Section 24639 to Cross Section 25182
are shown outside of the HHZ in the split flow analysis. Upon review of the HEC-RAS model,
the velocity distributions and depths adjacent to these structures would indicate that they
could be shown in the HHZ. Several other examples exist in the study area.
Our review of the Boulder Creek HEC-RAS analysis revealed that with a few minor revisions
to the model input variables, all within a reasonable tolerance, the velocity and resulting
High Hazard Zone value can be reduced at your property. It is our opinion that the HHZ
delineation can be shown confined to the street and main channel, as has been done at other
locations.
Please let us know if you have any questions.
Thank you
Nathan R. Torrey, P.E.
WRC Engineering, Inc.
950 South Cherry Street, Suite 404
Denver, CO 80246
303-757-8513
303-758-3208 (fax)
Comments on City of Boulder Floodplain Remapping page 18 of 25
David Leserman, 3/28/2021
Attachment B,
WRC Boulder Creek floodplain analysis - Gold Run development
From: "Nathan Torrey" <nathant@wrceng.com>
Date: March 25, 201111:13:27 AM MDT
To: "'David Leserman'" <david@leserman.com>
Cc: "'Alan Leak"' <alanl@wrceng.com>
Subject: Boulder Creek floodplain analysis - Gold Run development
David,
You requested that WRC Engineering, Inc. (WRC) analyze the Gold Run development and
the potential impacts it's construction had on the 100-year water surface elevations (WSEL)
at your property. The Gold Run site is located in the right overbank of Boulder Creek, just
downstream of 28th Street, directly opposite your property located in the left overbank.
WRC used the HEC-RAS model, with the Arapahoe Ave. split flow, developed by Anderson
Consulting Engineers, as the base model for analysis. Topographic mapping prepared prior
to the construction of the Gold Run development was provided by you. The pre-
development topographic mapping was obtained from U.R.S. Co. Project 9048, City of
Boulder, Boulder Creek, Improved Crossings at 24th, 28th, and 30th Streets. The vertical
datum is assumed to be NGVD 29 and was converted to NAVD 88 using a conversion of +3.7
ft. WRC incorporated the pre-development mapping into the current HEC-RAS model at
Cross Sections 23367 and 23699. This also included removing the blocked obstructions
(used to model buildings) and ineffective flow areas in the right overbank. An n-value of
0.050 was used for pre-development conditions.
The pre-development model resulted in a WSEL of 5,281.5 ft at Cross Section 23699, your
property. The current HEC-RAS analysis, prepared by Anderson, computed a WSEL of
5,282.3 ft at Cross Section 23699. Based on this analysis, the difference between pre-
development conditions and the construction of the Gold Run site is +0.8 ft.
Please let us know if you have any questions.
Thank you
Nathan R. Torrey, P.E.
WRC Engineering, Inc.
950 South Cherry Street, Suite 404
Denver, CO 80246
303-757-8513
303-758-3208 (fax)
Comments on City of Boulder Floodplain Remapping page 19 of 25
David Leserman, 3/28/2011
Attachment C, Possible Conveyance Zone Elevation Errors
From: David Leserman <david@leserman.com>
Date: January 6, 20112:42:44 PM MST
To: Christie Coleman <coleinanc@bouldercolorado.gov>
Cc: William DeOreo <bill@aquacraft.com>, Charles W Howe <charles.howe@colorado-edu>,
Susan Iott <sei_rsp87@hotmail.com>, Kate Ryan <kadryan@gmail.com>, Vicki Scharnhorst
<vickischarnho rst@msn.co m>
Subject: Possible Conveyance Zone Elevation Errors
Hi, Christie.
It was good to talk with you yesterday. As I said, I've carefully compared various maps and I
believe I've found that elevation errors were input into the HEC-RAS model. Here are the
details:
As you know, the maps show that the Conveyance Zone waters in my neighborhood do not
come from the creek, but rather from flows northbound out of the creek at Folsom, then
toward the east down an alley/driveway/parking lot (with JoAnn Fabrics at the west end
and the Millennium at the east end) and east across 28th St, and then south along properties
on the east side of the Frontage Road back to the creels. See the CZ Map and
BC_Convey_Bldgs_92010-DL6-Tight-presentedToWRAB-sgz.pdf, attached.
WSELs shown on the Work Map along with a careful study of elevations at Boulder's
Interactive GIS Map shows two things. The HEC-RAS computer model inputs most likely a)
use incorrect elevations at the west end of the alley, showing a 5' or 6' drop instead of an
approximate 30 yard ramp, and b) fail to account for the medians in 28th St as a barrier to
the flow. The 28th St median is between 6" and 18" higher (see photos) than the water
surface elevation where flows supposedly cross the street. The water may flow across 28th
St, but seemingly not at the location shown on the maps.
Folsom St Detail:
The flood study seems to incorporate a 5' or 6' drop in elevation across the sidewalk on the
east side of Folsom at the alleyway just south of JoAnn Fabrics. Such a drop is incorrectly
shown on Boulder's Interactive GIS Map. There is such a drop nearby, but not at the
entrance to the alley (see photos). It makes sense that HEC-RAS would interpret such a drop
as a high hazard flow as is shown on the HHZ Map. If there indeed is high hazard flow at the
alley entrance, then, surveying the lay of the land, one would expect high hazard flows
within the shopping center near JoAnn Fabrics. But, in fact, the drop at the alley, seemingly
incorporated into the HEC-RAS input, does not exist.
28th St Detail:
Flood WSELs decline toward the east along 28th St between Arapahoe and Boulder
Creek. The Work Map shows flood WSELs at 5289' along the west side of 28th between
Arapahoe and Boulder Creek. Boulder's Interactive GIS Map shows the contour line running
primarily north-south just west of the 28th St centerline is 5289', the same elevation as the
flood WSEL 20 feet to the west (and the WSEL is presumably declining toward in the middle
of the street). The median strip of 28th St is higher than the center of the roadway. There
Comments on City of Boulder Floodplain Remapping page 20 of 25
David Leserman, 3/28/2011
are breaks in the median, but it would seem that the water cannot cross the 28th, especially
in Conveyance Zone volumes, where indicated on the CZ and Work Maps.
Please provide me with a written response that explains these inconsistencies and possible
errors or undertake a more accurate survey of the areas, rerun HEC-RAS with corrected
inputs, and provide the resulting map(s). I realize that completion of such may not allow
time for my careful review it before this item next comes to WRAB on 1/20/2011. This
might be a reason to have WRAB (the most qualified citizen body to evaluate technical
issues) review the study again at a later date. On the other hand, it would be great to get a
response with plenty of time to review, ask questions, and comprehend before the
1/20/2011 WRAB meeting.
Nate that I'm still studying the HHZ and I expect to have questions regarding that as well. A
conversation with Greg Coke will probably help immensely.
Thanks.
David
303.931.2475
Attached images:
1-HowDoesTheWaterCrossTheRoadFacingWest.jpg
2-HowDoesTheWaterCrossTheRoadFacingEast.jpg
3-FolsomAlleyFacingWest.jpg
4-FolsomAlleyFacingEast-Googl.png
BC_Convey_Bldgs_920 10-DL6-Tight-presentedToWRAB-sgz.pdf
References:
CZ Map:
littp://www.bouldercolorado.gov/files/Utilities/Projects/boulder-creek/BC-Convey_Bidgs-92010.pdf
HHZ Map:
http://www.bouldercolorado.gov/files/Utilities/Projects/boulder_creek/BC-H HZ_BIdgs_91710.pdf
Work Map:
http://www.bouldercolorado.gov/files/Utilities/Projects/boulder_creek/BC_FHWM_7.pdf
Boulder's Interactive GIS Map:
http://gisweb.ci.boulder.co.us/website/pds/pds_eMapLink/viewer.htm
Comments on City of Boulder Floodplain Remapping page 21 of 25
David Leserman, 3/28/2011
Attachment D, Memo, Koch to Coleman, 1/18/11, re Comments
by Leserman
MEMORANDUM
DATE: January 18, 2011 ACE PROJECT NO.: CCBLDR02
TO: Christie Coleman, City of Boulder
FROM: Greg Koch, Anderson Consulting Engineers, Inc.
SUBJECT: Responses to Comments Submitted via Email by David Leserman on January 6, 2011
Based on our review of Mr. Leserman's message date January 6, 2011, it appears that he raises three
issues; two of which he specifically requested that the City address, and a third which should be
addressed in order to clarify the conveyance zone mapping.
In his message, Mr. Leserman states, " As you know, the maps show that the Conveyance Zone waters in
my neighborhood do not come from the creek, but rather from flow northbound out of the creek at
Folsom... This appears to be a misinterpretation of the conveyance zone mapping in this area. The
flood hazard work maps indicate that the conveyance zone boundaries at Cross Sections 23367 and
23699 (both located within the Cordry Court neighborhood) are not influenced by the overflow of 28'h
Avenue north of the creek. A portion of the conveyance zone directly east of 28'h Avenue can be
attributed to street overtopping flows returning to the creek, but the majority of the conveyance zone in
this area is due to flows along Boulder Creek.
Mr. Leserman also raises a concern about the City's 1-foot contour mapping being inaccurate at the
west end of an alley running east from Folsom. The topography indicates an abrupt drop of about 6 feet
at the west end of the alley which is not represented in the field. However, the cross section included in
the hydraulic model along the east side of Folsom is located approximately 60 feet east of Folsom. Over
this 60 feet, it appears that approximately half of the 6-foot elevation difference is lost. Although the 1-
foot contours do not provide an exact representation of the ground in this area, it is our opinion that
this discrepancy does not significantly influence the overall hydraulic modeling results. Furthermore,
since the bridge flows and overtopping flows at Folsom join back together west of 28'h Street, the
topographic representation of the alley directly east of Folsom has no bearing on the hydraulic modeling
results in the Cordry Court neighborhood, east of 28th Street.
Finally, Mr. Leserman claims that the 28 h Street median is a barrier to flow and this effect is not
accounted for in the model. The 28m Street median in the vicinity of the bridge over Boulder Creek is
indeed a barrier to the passage of the 100-year flow, but the median is fully represented in the hydraulic
model. The City's 1-foot contours were compared to as-built survey data provided by the City for the
28i1, Street median; the 1-foot contours appeared to accurately represent the detailed as-built survey
data. In order to simplify the use of topographic data from various sources for an already complex
model, the 1-foot contours were used to define overtopping elevations for 28`' Street. As indicated on
the flood hazard work maps, 28th Street does not overtop near the bridge; rather, overtopping of the
street begins approximately 500 feet north of the Boulder Creek Bridge and extends north to Arapahoe
Avenue.
COBLDR02_leserman_2011A1.06_memo.doc Page 1 of 1 ANdouon ConsUIIN(i ErvgineERS, INC.
Comments on City of Boulder Floodplain Remapping page 22 of 25
David Leserman, 3/28/2011
Attachment E, Email regarding the 1969 25-year event
From: David Leserman <david@Ieserman.com>
Date: January 31, 2011 10:33:02 PM MST
To: Christie Coleman <colemanc@bouldercolorado.gov>
Cc: bill deoreo <bill@aquacraft.com>, Charles W Howe <charles.liowe@colorado.edu>,
Susan Iott <sei_rsp87@hotmail.com>, Kate Ryan <kadryan@gmail.com>, Vicki Scharnhorst
<vickischarnhorst@msn.com>, Ned Williams <williamsn@bouldercolorado.gov>, Greg Koch
<gjkoch@acewater.com>, Bob Harberg <HarbergB@bouldercolorado.gov>, Annie Noble
<Nob1eA@bouldercolorado.gov>
Subject: 1.85' rise in flood WSEL in current study vis-a-vis 1969 event
Hi, Christie.
Summary:
In my backyard, the 25-year flood event predicted by the 2010 HEC-RAS model shows a
water surface elevation (WSEL) that is approximately 1.85' higher than the observed WSEL
for an actual 25-year event that occurred in 1969. I attribute this difference to the model
itself and to physical developments in the area, mostly the creekside Gold Run buildings and
protective flood wall. I hope that the City will take this into account while appropriately
tweaking the HEC-RAS model and while analyzing the impact of the Gold Run development
on the neighborhood.
Details:
The 1969 flood on Boulder Creek is classified as a 25- to 30-year event. See Muller 1983
(Ref 1) and the Original Hydrologic Report (Ref 2).
In a statement dated 7/7/1979 (attached), eye witness Barrie McRoberts (owner of 2806
Cordry in 1969) states the "flood waters from Boulder Creek rose to a level of about one
foot below the top of the block retaining wall in the backyard of 2806 Cordry."
The November 2010 survey of 2806 Cordry by Drexel Barrell (which you now have), shows
the top of that retaining wall ranging from 5,279.6' at the west end to 5,280.1' at the east. (I
believe that the King Surveyors survey of 1/26/2011 will also report an elevation for the
top of that retaining wall - right on Cross Section 23699, which goes through my house.)
Subtracting 1' from Drexel Barrell elevations puts the 1969 WSEL reported by McRoberts
somewhere between 5,278.6' and 5,279.1'.
The current floodplain study shows that 25-year WSEL at my house (Cross Section 23699)
is 5,2807. See Anderson Final Report Appendix E (Ref 3).
Again by subtraction, the modeled HEC-RAS WSEL is between 1.6' and 2.1' higher than
the 1969 observation by Barrie McRoberts.
Conclusion:
The new study shows a 1.85' (+-.25') increase in WSEL over a comparable and fairly recent
actual event.
Comments on City of Boulder Floodplain Remapping page 23 of 25
David Leserman, 3/28/2011
I attribute this difference to some combination of the following: a) the 9/2010 HEC-RAS
model itself, b) inputs to the model, and c) changes on the ground (such as buildings and a
floodwall).
The difference is very close to the water depth of the 100-year event as predicted by the
Anderson study at the lowest adjacent grade to my house. In other words, possible
overstatements in the HEC-RAS model along with physical changes to ground in the area
may account for whether there would be any water at all at the foundation of my house in a
100-year event.
I hope that the City and Anderson Consulting will take this information into account as the
HEC-RAS model is rerun and as the impacts of the Gold Run Development are analyzed.
Thanks for your consideration.
Best wishes,
David Leserman
References:
Ref 1. [Muller 1983] Muller Engineering Company, Inc., "Boulder Creek Flood Hazard Area
Delineation," prepared for the City of Boulder and the Urban Drainage and Flood Control
District, January 1983. [Referenced in Anderson 9/2010] I received this document via email
as "Boulder Creek floodhazard 1983.pdf' from Christie Coleman on 1/5/11. (For reference,
I have renamed my copy to BoulderCreekFloodHazardMuller1983.pdf.) See "Figure 2,
Discharge Probability Curve" on p 6 (and also attached) and the section on the Flood
History on pp 2-4.
Ref 2: [Original Hydrologic Report] "Water and Related Land Resources Management Study:
Metropolitan Denver and South Platte River and Tributaries - Colorado, Wyoming, and
Nebraska," Sept. 1977, Vol. V, Supporting Technical Reports Appendices, Appendix H,
Hydrology. See Plate 60 (PDF p 130) and p 43 (PDF 54), Boulder Creek Discharge
Probability,
http://www.bouldercolorado.gov/files/Utilities/Projects/boulder_creek/BC_Original_Hydr
ologic_Report,pdf
Ref 3: [Anderson Final Report Appendix E] "Appendix E, Tabular and Graphical WS Profiles
for Boulder Creek,"
http ://www.bouldercolorado.gov/files/Utilities/Projects/boulder_creek/Appendix_E_final.
pdf. See Appendix E,1, Boulder Creek Tabular Water Surface Profiles and Conveyance Zone
Results, p E.1-3 (PDF p 5).
Attachments:
1. BarrieMcRobertsStatement1979.pdf alias
2. BC-Original-Hydrologic-Report RiskChartExceedanceFrequency p130.pdf
Comments on City of Boulder Floodplain Remapping page 24 of 25
David Leserman, 3/28/2011
Attachment F, Barrie McRoberts statement re 1969 flood
I hereby testify that during the flood of 4 ISay through 6 Hay
of 1969 flood waters from Boulder Creek rose to a level of about one
foot below the top of the block retaining wall in the backyard of
2806 Cordry, Boulder, Colorado. On that date I was owner of and resident
at 2806 Cordry. The possible danger to my home allows me to recall
this event. I willingly hand this testimony to David Fl. Lesorm n, current
owner/occupant of 2806 Cordry, for presentation to the Colorado Land
Use Commission.
-Y 0-x
~l ej 9
y,13
Comments on City of Boulder Floodplain Remapping page 25 of 25
David Leserman, 3/28/2011
Attachment G, Boulder Creek Exceedance Frequency Chart
0.0 0000 -
50000
DISCHARGE VALUES OBTAINED WITH SWnn
U
CURVE OFrVELOPFO Ff OM REGIONAL I8 ~
sooao CRITERIA PRESENTED IN TM # 1
w
Uj 6000 - 191¢
OTIM14TED YRLUES FOR THE
1894. 1914 NN0 199 EV NTS~'` 1969
v
M
w
-»---E YE FIT LINE
Q 10D0
U
try
C3 6D0 -
~7"-RECORDEO~ FLOWS 1889 TO 1908 WITH
1893. 1894, 1902. 11903 AND 1910'1 MISSING
100
99.9 9916 99 98 95 80 60 70 60 50 +0 3D 20 10 5 2 1 E .1
EXCEEOANCE FREQUENCY IN PERCENT
METROPOLITAN DENVER AND
SOUTH PLATTE RIVER AND TRIBUTARIES
COLORADO, WYOMING AND NEBRASKA
OOUI UER CREEK
018CMAROE PROBABILITY
BY TNREE METHODS
U .1. IRNT INa LLR Ot111M LNNNN
C[RIl O[ LN{II[CIf O.RNR.I[aR1.NR
aS• fall
VOL. V APPENOIX H PLATE 60
Attachment B Corrected, Comments on City of Boulder Floodplain Remapping page 1 of 2
David teserman, 3/28/2011
Attachment B, CORRECTED 3/28/20113:00 PM
WRC Boulder Creek floodplain analysis - Gold Run development
From: "Nathan Torrey" <nathant@wrceng.com>
Date: March 28, 20113:03:11 PM MDT
To: "'David Leserman"' <david@leserman.com>
Cc; "'Alan Leak"' <alanl@wrceng.com>
Subject: RE: Boulder Creek floodplain analysis - Gold Run development
David,
This email is written to correct our analysis summarized in the email below, dated March
25, 2011. As described below, the pre-Gold Run development topographic mapping was
converted from NGVD 29 to NAVD 88 using a conversion of +3.7 feet. This conversion rate
was given to you by the City of Boulder, which you supplied to us. The correct conversion is
+3.1 feet, as recently stated by the City of Boulder.
WRC revised the analysis outlined below using the correct conversion rate of +3.1 feet. The
pre-development model resulted in a WSEL of 5,280.9 ft at Cross Section 23699, your
property. The current HEC-RAS analysis, prepared by Anderson, computed a WSEL of
5,282.3 ft at Cross Section 23699. Based on this analysis, the difference between pre-
development conditions and the construction of the Gold Run site is +1.4 ft.
Please let us know if you have any questions.
Thank you
Nathan R. Torrey, P.E.
WRC Engineering, Inc.
950 South Cherry Street, Suite 404
Denver, CO 80246
303-757-8513
303-758-3208 (fax)
Attachment B Corrected, Comments on City of Boulder Floodplain Remapping page 2 of 2
David Leserman, 3/28/2011
Attachment B, ORIGINAL
WRC Boulder Creek floodplain analysis - Gold Run development
From: "Nathan Torrey" <nathant@wrceng.com>
Date: March 25, 201111:13:27 AM MDT
To: "'David Leserman"' <david@leserman.com>
Cc: "'Alan Leak"' <alanl@wrceng.com>
Subject: Boulder Creek floodplain analysis - Gold Run development
David,
You requested that WRC Engineering, Inc. (WRC) analyze the Gold Run development and
the potential impacts it's construction had on the 100-year water surface elevations (WSEL)
at your property. The Gold Run site is located in the right overbank of Boulder Creek, just
downstream of 28th Street, directly opposite your property located in the left overbank.
WRC used the HEC-RAS model, with the Arapahoe Ave. split flow, developed by Anderson
Consulting Engineers, as the base model for analysis. Topographic mapping prepared prior
to the construction of the Gold Run development was provided by you. The pre-
development topographic mapping was obtained from U.R.S. Co. Project 9048, City of
Boulder, Boulder Creek, Improved Crossings at 24th, 28th, and 30th Streets. The vertical
datum is assumed to be NGVD 29 and was converted to NAVD 88 using a conversion of +3.7
ft. WRC incorporated the pre-development mapping into the current HEC-RAS model at
Cross Sections 23367 and 23699. This also included removing the blocked obstructions
(used to model buildings) and ineffective flow areas in the right overbank. An n-value of
0.050 was used for pre-development conditions.
The pre-development model resulted in a WSEL of 5,281.5 ft at Cross Section 23699, your
property. The current HEC-RAS analysis, prepared by Anderson, computed a WSEL of
5,282.3 ft at Cross Section 23699. Based on this analysis, the difference between pre-
development conditions and the construction of the Gold Run site is +0.8 ft.
Please let us know if you have any questions.
Thank you
Nathan R. Torrey, P.E.
WRC Engineering, Inc.
950 South Cherry Street, Suite 404
Denver, CO 80246
303-757-8513
303-758-3208 (fax)
Addendum to Comments on the City of Boulder Floodplain Remapping page 1 of 6
David Leserman, 3/28/2011
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Pre-construction contours at the Gold Run site.
The red box is my home at 2806 Cordry Ct The blue The area within Gold Run that would be inundated (Contours from Meurer, Serafini and Meurer, Inc,
shaded area, where no flooding has yet occurred on by the 100-year event is the approximately 400 x Hydraulic Analysis, Sherrelwood Estates, Boulder
my lot, is at or below 5279.1'. The LAG at 2806 is 600 feet directly south of the the Cordry Court Colorado, Prepared for Fowler Real Estate, Overall Map,
5281.' According to the WRC report, the 100-year neighborhood (not counting the part south of Scott Exhibit 3, 6/18/79, with Elevation Contours Source: City of
WSEL is 5,281.5' without the development Carpenter Park). This is about 5 acres. Average flood Boulder, Boulder Creek, Improved Crossings at 24th, 28th
and 30th Streets, U.R.S. Co Project 9048. Note contours are
(Since this old drawing shows contours only every 2 depth across this area is about 5'. The area that is labeled using NGVD 1929. NAVD 88 data is 1929 plus 3.1')
feet, I can't identify a contour for the 100-year WSEL now built up would have stored about 25 acre-feet
of water.
at my house. Anderson Consulting could.)
Addendum to Comments on the City of Boulder Floodplain Remapping page 2 of 6
David Leserman, 3/28/2011
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Issue 2. Favoring private property
Example 1. Goss-Grove neighborhood
(GossGroveHHZ.jpg)
Addendum to Comments on the City of Boulder Floodplain Remapping page 3 of 6
David Leserman, 3/28/2011
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Example 2. Folsom and 28th
(CZ left, HHZ right)
(ArapSphtCZ.jpg &ArapSplitHHZ.jpg)
Addendum to Comments on the City of Boulder Floodplain Remapping page 4 of 6
David Leserman, 3/28/2011
rww
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Issue 3. Conveyance Zone that leaps a barrier in a single bound, Page 7
{BC_Con vey_Rlcigs_92010-DL3-showing2oomArea.jpg)
Addendum to Comments on the City of Boulder Floodplain Remapping page 5 of 6
David Leserman, 3/28/2011
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Issue 3. Conveyance Zone that leaps a barrier in a single bound, Page 8
(ConveyanceZoneln28thStElevations2011U326.ipg)
Addendum to Comments on the City of Boulder Floodplain Remapping page 6 of 6
David Leserman, 3/28/2011
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100-year floodplain at 28th
Issue 3. Conveyance Zone that leaps a barrier in a single bound, Page 8
(100yrF]oodplainAt28th+OlsonB.jpg)
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