HomeMy WebLinkAboutMemo - Mail Packet - 8/4/2020 - Memorandum From Theresa Connor, Ken Sampley, Mike Beckstead Re: Boxelder Creek Watershed Dams Update (Dams B-2, B-3, B-4, B-5 And B-6)Page 1 of 3
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700 Wood Street
PO Box 580
Fort Collins, CO 80522
970.221.6700
970.221.6619 – fax
970.224.6003 – TDD
utilities@fcgov.com
fcgov.com/utilities
M E M O R A N D U M
DATE: July 29, 2020
TO: Mayor Troxell and Councilmembers
THROUGH: Darin Atteberry, City Manager
FROM: Theresa Connor, P.E., Interim Executive Director, Utilities
Ken Sampley, P.E., Director, Stormwater Engineering and Dev Review
Mike Beckstead, Executive Advisor
RE: Boxelder Creek Watershed Dams Update (Dams B-2, B-3, B-4, B-5 and B-6)
Bottom Line:
The State Engineer’s Office (SEO) Division of Dam Safety recently updated its regulations
governing dams of this type to allow for a Risk and Consequence analysis approach. This
approach is different than prior prescriptive regulations and introduces the determination of
"potential failure modes" and characterizes the "likelihood" of the failure mode within a risk
context. This analysis has been completed for the Boxelder Creek Watershed Dams using
recently updated technical information in collaboration with stakeholders from the City of
Fort Collins (Fort Collins), Larimer County (Larimer), the Towns of Wellington and
Timnath, the North Poudre Irrigation Company (NPIC), the SEO Division of Dam Safety, the
Natural Resources Conservation Service (NRCS) and the consulting firm of Short Elliot
Hendrickson Inc (SEH).
It was concluded that no major reconstruction or rehabilitation of Dams B-2, B-3 and B-4 is
needed at this time (given the probability of a major event is exceptionally low) if the
following recommended improvements and actions are implemented for each of these dams:
1. Update the respective Emergency Action Plans (EAPs); and,
2. Install water level monitoring and advance warning systems.
Dams B-5 and B-6 were determined to meet current SEO regulatory requirements, however
routine maintenance activities such as removal of vegetative growth and other minor items is
needed at each facility.
Representatives from the stakeholders will be meeting over the next 2-3 months to further
develop and refine the scopes of service for this work and associated cost estimates. An IGA
will be established to allocate financial responsibilities for these improvements as well as
future operations and maintenance activities. Fort Collins Stormwater staff has submitted a
2021 BFO offer in the amount of $200,000 to fund its anticipated respective share of the
costs for the improvements listed above.
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Background:
A series of flood control dams were built in the 1970s and 1980s to protect agricultural lands from
flooding and erosion damage. At the time, the NPIC acted as the local partner with the Soil
Conservation Service (SCS), now the NRCS, and with the United States Department of Agricultural
(USDA) to build the dams. NPIC currently operates and maintains the dams.
In 2014, the SEO completed a routine review of the dams for certification. The land use
changes downstream of Dams B-2, B-3 and B-4 from agricultural use to suburban and urban
development required a change in the dam classification from “moderate” to “high hazard”.
This change in classification of the dams could require expensive rehabilitation and/or
reconstruction of the dam emergency spillways and embankments. NPIC and NRCS
initiated Supplemental Watershed Plan and Environmental Assessments (SWPEAs) for
Dams B-2 and B-3 in 2015 to evaluate alternatives to bring the facilities into compliance
with current SEO and Federal regulations and dam safety standards. B-4 was not included
due to funding limitations.
In accordance with SEO criteria at that time, if improvements are not made and the dams are
decertified, there would be a dramatic change to the downstream floodplain impacting over
1,000 residences and businesses in Wellington, Larimer County, Fort Collins, and Timnath.
These dams need to be in place and functional to provide 100-year flood protection for
downstream properties and to work in conjunction with the recently-completed Boxelder
Basin Regional Stormwater Authority (BBRSA) improvements. The Draft B-2 and B-3
SWPEAs were completed in 2017 and 2018. The assessments identified a range of potential
improvements to each of the dams that included spillway modifications, overtopping
protection installed on the dam embankments, or decommissioning. Conceptual alternative
cost estimates varied widely from $3 to $25 Million (depending on the dam under review).
In mid-2018, representatives from Fort Collins and Larimer County approached other local
governments to form a stakeholder group to further evaluate the Boxelder Creek Watershed
Dams and to determine actions and responsibilities to appropriately address flood protection
efforts. An IGA was established in April 2019 by the stakeholders to review and refine the
potential improvements and to develop potential cost-share approaches to capital and
operations and maintenance costs. SEH reviewed the draft SWPEAs and provide comments
that resulted in revisions to the hydrologic modeling, updated alternative evaluations, and
updates to the conceptual estimated costs.
Boxelder Creek Watershed Dams Risk and Consequence Analysis:
The SEO Dam Safety Division is the agency responsible for ensuring all dams within
Colorado meet standards to protect the general public. The Division updated its Rules and
Regulations for Dam Safety and Dam Construction on January 1, 2020 (Colorado Rules). As
a result, a new approach to assess dam safety has been incorporated into the rules. The
Comprehensive Dan Safety Evaluation (CDSE) tools and process include the completion of a
Potential Failure Mode Analysis (PFMA) to evaluate and reduce dam safety risk. This
CDSE process enables the determination of "potential failure modes" and then characterizes
the "likelihood" of the failure mode within a risk context. Confidence levels are associated
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with the characterization, along with information that can modify the characterization or
confidence levels. The result is a semi-quantitative risk assessment of the dam as opposed to
a qualitative report of observations, which provides the framework to determine if and what
actions are required to improve public safety.
The PFMA process was used to evaluate Dams B-2, B-3 and B-4 (Dams B-5 and B-6 were
not included since they currently meet SEO regulatory requirements). Figure 1 below
illustrates what the Risk Summary Charts are for Dams B-2, B-3 and B-4).
Figure 1 – Risk Summary Chart – Dams B-2, B-3, and B-4
SEH’s risk analysis indicates that the risks associated with the dams would be greatly
mitigated if adequate warning of flooding could be provided. Additionally, the current 2015
Emergency Action Plans (EAPs) for the dams do not explicitly describe the triggers for
emergency actions. The two recommendations from the SHE report are:
1. Update the EAPs for Dams B-2, B-3 and B-4; and,
2. Install water level monitoring and warning systems at Dams B-2, B-3 and B-4.
There are no external construction costs associated with updating the EAP’s. There would be
potential consulting costs (no estimate yet) to develop the updated EAP plans and documents.
The conceptual costs for the monitoring systems are expected to range from $225,000 to
$325,000. Implementing these recommendations would change the Consequence Levels
from Level 4 to Level 3 for each dam. The risk would then move from a yellow to green cell
as shown above. In accordance with SEO Dam Safety Division regulations, risks that fall in
the green cells do not compel requirements for spillway and/or dam rehabilitation.
The SEH report entitled “Executive Summary for Project Development for Boxelder
Watershed Dam Rehabilitation - Hydrologic Analyses of Boxelder Watershed Dams B-2, B-
3, B-4, B-5, and B-6” gives a good synopsis of the hydrologic modeling and results as well as
the PFMA results and is available for review in the Stormwater Engineering and
Development Review Division office.
DocuSign Envelope ID: 46440182-213E-42C1-96EA-D193874ABFDD