At the January 19th 2012 Concept Plan review meeting
we presented a discussion of the impacts of the "Lower Basin Storm Center".
Among other sources, this information was gathered
from the
minutes
of the South Boulder Creek Mitigation Study team and the
December 2010
SOUTH BOULDER CREEK MAJOR DRAINAGEWAY PLAN- ALTERNATIVES ANALYSIS REPORT.
Below are excepts from those sources.
February 11th 2010 SBC Mitigation team meeting
The
minutes
of the February 11th 2010 meeting
of the South Boulder Creek Mitigation Study team (Kurt Bauer/COB, Bob Harberg/COB, Shea Thomas/UDFCD
Mark Glidden/CH2M HILL, Alan Turner/CH2M HILL, Aaron Cook/CH2M HILL) state:
Flood Threat from Lower Basin Storm
During the kick off meeting, there were questions as to whether the peak
flow and maximum flooding extents seen in the West Valley come from a
thunderstorm upstream over the mainstem of South Boulder Creek (South
Boulder Creek floodplain study) or from a similar thunderstorm placed
directly over the C-2 Basin (HDR XPSWMM Analysis). The results of the
cutoff model will be compared with the HDR XPSWMM Analysis to determine
the differences in the flooding extents in the West Valley between the
two approaches. CH2M HILL will also compare the hydrographs at the C-2
loading points from the two approaches.
February 23rd 2010 SBC Mitigation team meeting
The February 23rd 2010 meeting minutes state:
Review of Past Meetings Actions Items
...
CH2M HILL has reviewed the hydrology for the C2 basin loading between the SBC Study and the lower basin thunderstorm.
The minutes go on to state:
Review of Lower Basin Hydrology vs. SBC Hydrology for C2 Basin Loading
CH2M HILL presented a spreadsheet to the team showing the C2 sub-basin loadings from the MikeFlood Floodplain Study,
the MikeFlood Lower Storm Center Analysis, and the HDR XPSWMM Analysis. For the MikeFlood Analysis, the sub-basin
loading was extracted from the MikeFlood regulatory model. Only the peak flows in the C2 basin were available for
the Lower Storm Center Analysis and the XPSWMM Model. The peak flow from the C2 basin was apportioned using the
same ratio as in the MikeFlood Regulatory Model to determine the peak flow at each C2 sub-basin.
...
After looking at the spreadsheet, the team decided infrastructure in the
West Valley will be sized according to the peak flows from the
MikeFlood Lower Storm Center Analysis.
And they continue with a discussion of the the hazards the new hydrology presents:
All damage estimates from the floodplain study are based on the results
of the regulatory model so there may not be a direct comparison between
the damage estimates from the floodplain study and the damage estimates
for the flood mitigation study. The 500-year damage estimates from the
floodplain study may approximate the 100-year peak flows in the lower
storm center analysis.
The minutes close with the following action item:
Action Items
All infrastructure in the West Valley will be sized according to the results of the
MikeFlood Lower Storm Center Analysis (Qpeak = 1100 cfs).
March 17 2010 SBC Mitigation team meeting
Hydrology
The 10-year regulatory hydrology for the West Valley is being used
because there is no 10- year hydrology for the lower storm center
analysis and the stormwater master plan only evaluated the 2- and 5-year
events. The regulatory hydrology will also be used for the 100- year
event for alternatives with and without storage upstream of US-36. For
alternatives with storage upstream of US-36, the lower storm center
analysis with a peak flow of 1100 cfs will also be considered.
March 26 2010 SBC Mitigation team meeting
These four general alternatives will be assessed for two separate conditions:
1. Detention Upstream of US-36
2. No Detention Upstream of US-36
and for the most conservative flow conditions between the following:
1. 10-year to the 500-year regulatory flows
2. 5-year to 100-year Lower Storm Center Flow Analysis
One could conclude from these ranges that the 100 year Lower Storm Center Flow Analysis
is the equivalent to the 500 year regulatory flows.
Design Hydrology
A review of the memorandum of understanding regarding design flow rates. The discussion has lead
to a review of the flowrates and an update of the memorandum of understanding. The memorandum
will be revised and presented during the next progress meeting.
April 14 2010 SBC Mitigation team meeting
Report of Design Hydrology and Hydrology Memorandum
The hydrology Technical Memorandum has been finalized and includes peak flows at C2 loading points.
The peak flows that will be used to size infrastructure are a combination of flows loaded directly
at the C2 loading point plus the flows loaded at loading points upstream. This approach is similar
to a typical master planning approach where a one- dimensional model is used.
This memorandum of understanding has not been provided to the public.
April 28 2010 SBC Mitigation team meeting
Hydrology
Hydrology will be discussed with Kurt Bauer in greater detail outside of this progress meeting.
Below is a summary of flows that will be evaluated for improvements in the West Valley:
- Regulatory Model Flows - determined from the MIKE FLOOD model in which overtopping of US 36 occurs.
Large pipes through the West Valley are required to convey the 100-year Regulatory Model Flows.
- Lower Storm Center Flows - the 100-year peak flows from the Lower Storm Center analysis are greater
than the 10-, 25-, and 50-year peak flows from the Regulatory Model.
June 21 2010 WRAB meeting
At the June 21 2010 Water Resources Advisory Board meeting a presentation was given describing the Mitigation study.
From the audio recording (1:57)
of the meeting:
But what I want to point out here is that there is a good deal of flooding all through the west valley here before the
main stem even influences this area. So you'll see when we go through these alternatives some of the proposed alternative
infrastructure is to deal with what we're calling the local basin flooding.
They're describing flood impacts that are not coming from the main stem
thus one can assume these are coming from the lower basin.
December 12 2010 SBC Mitigation Report
In the
SOUTH BOULDER CREEK MAJOR DRAINAGEWAY PLAN- ALTERNATIVES ANALYSIS REPORT
the adopted hydrology is described:
III. Hydrologic Analysis
...
The focus of the hydrologic evaluation was on the flows in the mainstem of South Boulder Creek. The combination
of rainfall and basin assumptions used in the Mapping Study produced the most critical conditions and resulted
in the highest flows along the mainstem. However, that study, as well as the current Study, recognizes that
other rainfall events may result in higher flood flows and a higher consequent hazard to areas within the
tributary basin. This Study relies on other work to identify those other critical storms and the resulting
flows and to assess the potential effectiveness of proposed improvements to address the flood threat
associated with those flows. These other investigations include the flows reported in an HDR memorandum
(Lower Storm Center memorandum) developed during the Mapping Study that addressed the lower basin storms.
That section concludes with:
For the design of infrastructure improvements in the West Valley, the
City has indicated that infrastructure needs to be adequately sized to
convey the most conservative flow expected for the location. For the
West Valley, two conditions control flow.
- Overflows from South Boulder Creek into the West Valley over US-36 contribute up to approximately 1,600 cfs to the West Valley
- In absence of the overflows from South Boulder Creek, the
Stormwater Master Plan Design Storm that focuses a high intensity
thunderstorm over the West Valley causes the most conservative flow
rates.
Infrastructure Improvements in the West Valley Will be designed to the
regulatory MIKE FLOOD flow rates through the West Valley in absence of
storage or other improvements to eliminate the overflows of US-36. The
lower storm center flows will be used to design infrastructure through
the West Valley if storage or other infrastructure is recommended to
prevent overflows South Boulder Creek of US-36.
In the December 2010 report they describe the flooding that can occur along the Dry Creek #2 Ditch corridor:
Dry Creek No. 2 Ditch
This ditch runs south to north roughly parallel to 55th Street through
the West Valley. This channel is a remnant of the South Boulder Creek
alluvial fan channel system prior to development. Currently the ditch
diverts water near Eldorado Springs south of the project area, runs
north through Marshall and the South CU Campus. The ditch is conveyed
through a 6' x 4' Reinforced Concrete Box Culvert (RCBC) under US-36 and
then enters the West Valley. Currently this ditch has three remaining
shareholders north of US-36 that include Manhattan Middle School,
Flatirons Golf Course and a private owner in the Flatirons Industrial
Park.
The active decree for the ditch is 69 cfs.
This ditches' capacity is overwhelmed during the 25 year and 50 year
recurrence intervals due to local basin inflows including C2 basin
loadings at Manhattan Middle School and inflows from the Wellman Canal.
As they state above the ditch's capacity is overwhelmed in even a 25 or 50 year flood.
They go on to describe the impacts of the 100 year flood:
The 100-year overflow of US-36 by South Boulder Creek enters Dry Creek
No. 2 Ditch and overwhelms the entire capacity of the ditch for storms
greater than the 100-year storm.
They continue describing the flood behavior along the Dry Creek #2 Ditch (Reach DC2):
Reach DC2
The Dry Creek No. 2 Ditch corridor is the source of considerable
flooding below US-36. This conveyance is part of the primary outlet for
the ponded water above US-36. As the water from larger floods passes
under the highway through Dry Creek Ditch #2, the Anderson Extension and
Viele Channel, it exceeds the combined capacity of these systems at
South Boulder Road. Water overtops South Boulder Road and flows through
the neighborhoods west of Dry Creek Ditch #2. Water generally follows
the street corridors and causes some localized flooding when street and
drainage system capacity are undersized.
In Table 6-3, Design Flows by Sub-Reach, the report provides a summary of the design flow target for improvements along
each of the study reaches and forms the basis for the facility sizing:
Peak Flows
The peak flow along the Dry Creek Ditch #2 corridor is 695 CFS. This is 4
times larger than the 177 CFS that impacts the Hogan/Pancost
property from the FEMA 100 year storm.
Furthermore, in the
Alternative Development Figures report they show that there is 1170 CFS peak flow
just downstream of the Hogan/Pancost property:
FEMA - 100 & 500 Year Flooding
And this is the type of flooding we may see from the adopted hydrology of the mitigation study:
Adopted Mitigation Hydrology Flooding
This flood extent estimate is in agreement with the flood
animation
that was presented at the
June 2010
Water Resources Advisory Board (WRAB) meeting.
This inundation map was also presented at the
March 3rd 2010 public meeting.
Here is a close up from that flood map showing the impacts on the Hogan/Pancost property:
Area Inundation
Here is an animation of the above close up view of the flood inundation.
City Flood Regulations
The City has identified and documented an extreme flood hazard from the 100 year Lower Basin Storm.
This is the same likelihood of occurrence as the FEMA Regulatory Flood.
This documented flood hazard threatens the lives of the residents of South Boulder
and will bring catastrophic impacts to our neighborhoods.
The primary intent of the City of Boulder
flood plain regulations
is to:
CHAPTER 9-3-2: Floodplains
(a) LEGISLATIVE INTENT The purpose of this Section is to regulate
certain areas of the city subject to flooding in order to protect the
public health, safety, and welfare by:
(1) Restricting or prohibiting certain uses that are hazardous to life or property in time of flood;
More importantly, there is a strong legal, ethical and moral responsibility on the part of the City to
safeguard the lives and property of its citizens and in now way allow new development to increase
those risks to the adjacent and downstream properties.
Furthermore, the floodplain regulations specifically give the City Manager the power and the responsbility to:
CHAPTER 9-3-2: Floodplains
(13) Obtain, review, and reasonably utilize any base flood elevation and
floodway data available from federal, state, and other sources,
including data developed pursuant to Chapter 9-12, "Subdivisions,"
B.R.C. 1981, as criteria for requiring that all new development meet the
requirements of this Section.
The regulations also give the City Manager the authority to:
(f) Amend the boundaries of the high hazard zone and the conveyance zone pursuant to Subsection (f) of this Section;
This hazard exists and it must be taken into account when considering
annexation and development of the Hogan/Pancost property.
