09.c1 Engineering Updates & Watershed Memo
City of Scandia
Engineering Updates 6/15/2021
Engineering italics = old information
· 2021 Street Improvement Project & Layton Ave N
City Council authorized Plans and Specifications for the 2021 Street Improvement Project at the
October 20th City Council Meeting and Approved Plans & Specifications and Authorized Ad for Bid at
the January 19th City Council Meeting.
Bid Opening was held on February 24, 2021 at 10:00am. March 16, 2021 City Council awarded the
project to North Valley, Inc. and the Preconstruction Meeting has been scheduled for April 20, 2021,
Contractor’s schedule will be discussed at that meeting and available for better updates at the City
Council meeting.
Contractor paved bituminous driveways the week of May 24th. Contractor paved wear
course on June 2nd and 3rd on Pomroy Avenue N/Peabody Trail N/220th Street N. These
roads will be shouldered and gravel driveways will be resurfaced the week of June 7th.
Layton Avenue will have the wear course paved the week of June 7th (a day has not yet
been provided by Contractor).
As a separate business item, Contractor’s Request for Payment No. 2 will be on the regular
agenda.
· Calcium Chloride Quotes
Quotes were received and will be discussed as a separate business item on the May regular agenda.
City Staff is coordinating the construction scheduling with the Contractor.
· Bliss Drainfield System
Both the 2020 permit application for this system and the “final report” for nitrogen mitigation have
been completed and submitted to the MPCA. The memo which serves as a final report doesn’t
present final solutions for the nitrogen issue, but instead outlines the issues that have been found
and solved in this treatment system, as well as some additional testing that will now be
recommended to get a handle on the nitrogen, BOD, and TSS entering and leaving the facility. The
hope is to gain us a little more time from the MPCA before they mandate action. We’ll use this time
to perform the aforementioned testing and evaluate options for removing nitrogen at Bliss.
City received on February 22, 2021 a Pre-Public Notice Review of Draft Permit - Bliss Collector WWTP
(#MN0054119) that also included a statement of basis outlining any changes or new requirements to
the draft permit. This 30 day pre-public notice review period starts on February 22, 2021 and ends on
March 24, 2021. Once your pre-public notice review is complete the draft SDS permit will be placed
on public notice for 60 days.
· First thing that needs to be done is complete a Facility Evaluation Report within 180 days of
issuance of the new permit they requested. Based on the report, MPCA will then ask the City
to follow either Track 1 or Track 2. Track one is the new monitoring well. Track two is a
facility upgrade. There are a few other changes to the permit, they include some additional
monitoring required over this permit. Also, MPCA is requesting flow monitoring at WS 004,
WS 005 and WS 006. These are to each of the infiltration trenches. We are in the pre-public
draft notice stage where just the City received these draft documents and can
comment. We can review and suggest changes or modifications prior to March 24th.
· Current updates: Under track 1, the requirement to abandon GW 008 is included because
using this well for monitoring is problematic, since we have no information on the well’s
construction nor well boring records. Based on the limited information we do have, it is the
opinion of MPCA hydrologist, Steven Stark, that this well is too deep to be used as a
monitoring well and may be sampling a confined aquifer. All monitoring wells should be
sampling the surficial aquifer and have well screens that bisect the watertable.
· The justification for proposed timeline is sufficient and the new timeline will be incorporated
into the draft permit. Shown below (last page of this report) is a revised compliance
schedule. Flow monitoring requirements at WS 004, WS 005, and WS 006 will become
effective upon installation of the flow monitoring equipment. The permit will continue to
the 60-day Public Notice period.
We are currently in the 60-day public comment period of the draft SDS permit, which
started on May 24, 2021.
· Sump Pump Inspection Program
2 homes/cabin remain to be inspected.
· Bliss Addition Stormwater Planning
Public Outreach meeting being held at City Hall with the adjacent property owners prior to the City
Council meeting, on April 16, 2019, from 5:30pm – 6:30pm.
Met with watershed district on 8/2 to discuss preliminary options for expanding regional water
storage, restoring infiltration capacity and improve water quality. The watershed made some great
recommendations for managing overall project costs and those options are currently being explored.
Second public meeting occurred on August 20th. Preliminary stormwater management practices
were presented to the residents and were well received. Some new recommendations were also
presented and considered in the plan revisions.
Final Draft will be presented to the City Council at the November 19, 2019 meeting.
Meeting with City Staff and Watershed District was held on January 16, 2020 to discuss prioritization
and implementation schedule for the items presented in the final stormwater study. Separate costs
estimates have been put together based on the prioritization which will be used for future funding
submittals.
Recent engagement with the Carnelian Marine St. Croix Watershed District resulted in the two bullet
points below:
· Cost responsibility - Who would take on what costs and how? Ideally the District would like
to participate with a Clean Water Fund Grant.
· Timeline - The District is working to complete the Subwatershed analysis for Big Marine
Lake. It will be complete by June 2021. Clean Water Fund Grants are awarded based on
how the score. Without a completed Subwatershed analysis for the entire lake and
application would score low. We planned to apply for a CWF grant in the fall of 2021 (after
the completion of the analysis). If the City would like to dedicate funding and move up the
timeline, they could request the CMSCWD Board of Managers consider funding a phase of
the project without a CWF grant. I do know that next year’s budget is tight with the state
required 10-year management plan update.
Temporary drainage improvements that included two options were presented at the August 5th City
Council Workshop that were intended to relieve drainage concerns at the property of 19123 Layton
Ave. Upon the discussion there seems to be separate opinions forming on both sides of the street of
what temporary solutions are favored but why the ultimate correction is not just completed in
conjunction with the storm sewer improvements identified in the neighborhood stormwater study.
On October 14, 2020 at the Watershed Board meeting, Board Managers approved $3,600 cost share
for option 1.
Watershed anticipates in 2021 they will apply for 2022-2025 Clean Water Funds to implement stage
1 and 2, but could potentially also apply for all three stages. It should also be noted it will be difficult
for the District to participate in just the directional pipe installation without the water quality BMPs
being installed in that area. City may want to consider adding water quality practices to this stage to
leverage more funds from District (with or without Clean Water Funds).
· Stage 1 – directional drilled pipe to re-route drainage to wetland
· Stage 2 – North Layton Ave bioinfiltration basin, concrete trench and enhanced ditch
improvements, west bioinfilration basins, and 189th culvert crossings and enhanced ditches.
Watershed has completed the evaluation of potential projects on Big Marine. The Subwatershed
Analysis is currently being prepared. Schedule still anticipates to be on track to apply for Clean Water
Funds in July 2021.
· CLFLWD Comprehensive Watershed Management Plan
60-Day Public Comment Period ended on April 30, 2021
Responses to comments were sent to review authorities on May 27, 2021. Visit:
https://www.clflwdplanupdate.org/documents
The Board will hold a public hearing for the WMP on June 10, 2021. Visit:
https://www.clflwd.org/6-10-21_regular_board_meeting.php
· CLFLWD & Scandia Staff Quarterly Meetings
Meeting held on October 19, 2020.
Meeting on February 9, 2021.
Meeting on May 11, 2021 covered the following topics:
· Lake Management District Updates
· Bone Lake projects status
· Greenway Corridor Planning & Parks
· Roadway projects
· CMSCWD Managment Plan
· Bone Lake H&H Modeling Update at 5/13 Board Meeting.
· MnDOT/Washington County – Hwy 97 & Oakhill/CR 52
First initial contact/conference meeting with MnDOT to discuss issues and concerns with intersection
safety on October 13, 2020.
· This work has been completed; Signing – MnDOT will upgrade the size of all the fluorescent
yellow-green signs and add fluorescent yellow green tape to the sign structure (the post). At
the crossing itself MnDOT will be placing the enlarged signs back to back as to warn
motorist on approach of the crossing on Hwy 97. These additions will enhance the visibility
of the intersection.
· Striping - The school Xing pavement markings were completed with the last pavement
maintenance project, if City staff observes that these are worn out, please send in a request
to MnDOT and they will submit it to our pavement makings crew.
County Road 52/Oakhill
· City Council Workshop on June 2, 2021; Scandia City Council heard concerns
residents who live along Oakhill Road, east of the business area, where they have
observed speeding traffic and parking issues. One resident who lives at the site of
the 30 mph zone remarked that the majority of drivers do not slow their speed at
all and race in and out of town, causing safety concerns for the many pedestrians
and bicyclists who share the road.
· Washington County has scheduled a meeting with City Staff on June 21, 2021 to
discuss the concerns brought forward to the City Council.
· Coincidentally the County has been communicating with the City Engineer to
discuss Oakhill, Highway 97, and Olinda Trail. We think this is a good time to have a
conversation about overall traffic flow, pedestrian connections, speeding, bike
routing, school issues, etc. in the “downtown” Scandia area and potentially start to
develop a joint strategic plan for this entire area – including engaging MnDOT
about improvements to highway 97.
· Floodplain Risk Assessment
Online Workshop held, January 5, 2021
The Brown’s Creek Watershed District (BCWD), Carnelian-Marine-St. Croix Watershed District (CMSCWD) and
the Comfort Lake – Forest Lake Watershed District (CLFLWD) are jointly evaluating approaches to conducting a
floodplain mapping/flood risk assessment with two goals in mind:
· The short-term goal is to outline potential approaches the watershed districts could take to map and
quantify future flood risk.
· The long-term goal for these watershed districts is to build a common understanding of risk and
prioritize projects and other resiliency actions.
Small group is meeting on February 3, 2021 to discuss the outcomes of the online workshop and next steps.
Consultants are still working on recommendations for an approach to define and agree on what should be the
intensity, duration, and frequency of design events under future climate conditions (e.g., 100-year, 24-hr rainfall
event) they will be sharing with District Administrators for their consideration.
CMSCWD has completed their Hydraulic and Hydrologic Model update and calibration. This model will be the
foundation for any future climate resiliency modeling we conduct. This information will be presented to their
Board at the April 10, 2021 CMSCWD Board meeting.
See attached preliminary draft of the Memo – Floodplain Mapping/Flood Risk Assessment.
· MnDOT – Hwy 95 Tree Removal Work
In March 2021, in preparation of a drainage culvert repair project in late summer, a MnDOT
contractor will be removing trees and brush at four drainage culvert locations along Hwy 95 between
Hwy 97 in Scandia and Pleasant Valley Road (County Road 26) in Franconia Township. All the
vegetation to be removed is on state right-of-way. Crews will use flaggers to control traffic during
the work, so there may be some short traffic delays near work areas. Work will be done during
daytime hours and is expected to take about one week to complete. Due to the Federal Bat
Protection Act, tree removals must be done prior March 31 annually.
Emmons & Olivier Resources, Inc.
1919 University Avenue West, Suite 300 St. Paul, MN 55104 T/ 651.770.8448 F/ 651.770.2552 www.eorinc.com page 1 of 14
technical memo
Project Name | Floodplain Mapping / Floodplain Risk Assessment Date | 4/7/2021
To / Contact info | BCWD, CMSCWD, and CLFLWD Administrators
Cc / Contact info | MSCWMO Administrator
From / Contact info | Camilla Correll, PE; Mike Talbot
Regarding | Floodplain Mapping / Flood Risk Assessment Recommendations
Background
On January 5, 2021 the Brown’s Creek Watershed District, Carnelian -Marine-St. Croix Watershed
District and the Comfort Lake–Forest Lake Watershed District held a joint workshop to share the
findings of the Floodplain Mapping/Risk Assessment. The goal of this workshop was to inform
participants of the various approaches that others have used to assess potential flooding impacts
under future climate scenarios in order to determine an acceptable approach for the watershed
districts. The details of what was discussed at this meeting can be found in the Floodplain
Vulnerability Assessment Meeting Summary (January 1, 2021). As the meeting summary indicates,
many of the workshop participants were new to the topic and information shared. While they
appreciated being invited to learn more about the watershed district’s concerns related to flooding
and how to evaluate the impacts of climate change, they generally felt the watershed districts should
provide guidance and technical information to the communities. This memorandum articulates EOR’s
recommendation for the approach the watershed districts should use to evaluate the impacts of a
changing climate on flooding to share with the local communities.
Recommendation
Climate risk is a function of hazards, exposure, and vulnerability.1 As a result, it is recommended that
the watershed districts conduct a flood risk assessment by evaluating the hazards, exposure, and
vulnerability to historical extremes as well as future climate change projections. The purpose of
conducting a flood risk assessment is to develop a better understanding of the current and future
climate risks facing the watershed districts and their member communities. This assessment will
ultimately inform the development of adaptation goals and strategies aimed at improving the
response to flooding and resiliency of the system. The Global Covenant of Mayors for Climate &
Energy2 defines these terms as follows:
Hazard: The potential occurrence of a natural or human-induced physical event or trend or
physical impact that may cause loss of life, injury or other health impacts, as well as damage
and loss to property, infrastructure, livelihoods, service provision, ecosystems, and
environmental resources.
Exposure: The presence of people, livelihoods, species or ecosystems, environmental
functions, services, resources, infrastructure or economic, social or cultural asse ts in places
and settings that could be adversely affected.
Vulnerability: The propensity or predisposition to being adversely affected. Vulnerability
encompasses a variety of concepts and elements including sensitivity or susceptibility to
harm and lack of capacity to cope and adapt.
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By evaluating each of these components of climate risk, the watershed districts can better prioritize
the need for additional information and the response to flooding.
While each of the watershed districts has a SWMM-based hydrologic and hydraulic model, these tools
have not been developed to the level of detail needed to inform all of the flood-related impacts that
may occur during an extreme precipitation event. For example, the average size of the subwatersheds
in the BCWD H/H model is approximately 50 acres (range is 0.1 acre to 725 acres) with the more
developed (urban) areas being more refined than the rural portions of the watershed. In order to
more cost-effectively assess the impacts of climate change on flood risk, we recommend taking a two-
step approach to assess flood risk and vulnerability:
1. GIS-Based Flood Hazard Assessment
2. H/H Floodplain and Infrastructure Assessment
One of the over-arching goals of this evaluation was to assess the benefits of conducting all or
portions of this floodplain mapping/floodplain risk assessment regionally (i.e., northern Washington
County or the area encompassed by the BCWD, CMSCWD, and CLFLWD) or separately (i.e.,
watershed-by-watershed). Since the workshop, it has become apparent that the need for this
information and the ability to conduct the work may vary between the three watershed districts. To
provide flexibility at this stage of the planning process we are recommending the same approach be
taken for all three watersheds, but the cost estimate to conduct the first step (GIS-Based Flood Hazard
Assessment) reflects the cost per watershed.
Task 1. GIS-Based Flood Hazard Assessment
Using both novel and established GIS techniques and workflows, an analysis of existing GIS data will
be performed both to identify previously unknown or underestimated issue areas and to target and
prioritize the resources allocated for more detailed hydrologic and hydraulic model development.
This task will consist of the following three sub-tasks:
• Develop Flood Hazard Layer
• Conduct Flood Risk Assessment
• Review Findings with Workshop Participants
• Pivot to Task 2
Task 1.1. Flood Hazard Layer
Performing the flood hazard assessment will involve the use of primarily topographic, land use, land
cover, and stormwater infrastructure datasets to develop a Flood Hazard Layer (FHL) for each
watershed. The FHL is developed by chaining together a combination of pre-built terrain analysis
tools and watershed-specific calculations in a GIS application. There are multiple variations on the
FHL workflow depending on the scale of interest and the nature and quality of the available data
within a watershed, but the backbone of the workflow that EOR has developed is based on research
by Rincón et al.3 The FHL is a relative metric, which means that it can help identify which locations in
the watershed are likely subject to a high degree of flood hazard (i.e., are flooded more frequently)
than other areas, but it does not provide an absolute indication of the frequency of that flooding (i.e.,
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whether those locations are flooded once every 1, 10 or 100 years). The FHL is used to identify
sources of both pluvial (flooding independent of an overflowing waterbody) and fluvial (when rivers
or streams exceed their capacity) flooding that do not directly result from capacity limitations
inherent in existing stormwater infrastructure and, as such, has limited usefulness in directly
analyzing and optimizing stormwater conveyance systems. However, because the FHL is derived
from high resolution topographic data (i.e., LiDAR), the analysis produces fine-scale outputs that do
not require the same level of data collection and other a priori knowledge that is required when
constructing a hydrologic and hydraulic model. The FHL can thus be used to strategically inform
where additional data collection and model refinement is needed.
Modeling results from existing SWMM models will be used to create a version of the FHL that is
“calibrated” to simulated high water levels in the watershed. A statistical analysis will be conducted
that correlates high water contours with FHL v alues, resulting in the identification of the maximum
FHL classification value that most closely reflects the extent of the floodplain (see Figure 1). These
maps should be used and interpreted with caution but can provide a useful point of reference in the
interim.
Completion of this task will involve:
• Hydrocorrecting the LiDAR-derived digital elevation model. The recommended approach
uses the most current geospatial (lidar) data, which needs to be hydrocorrected – a process
by which the presence of features such as culverts, bridges, and other conveyance features
can be “burned” into the digital elevation model so that lidar -derived flow paths are
hydrologically accurate. Hydrocorrection consists of significant manual processing that
involves the use of both high-resolution aerial photography and ground -truthing to
accurately locate these conveyance features. Extensive information about the locations and
the nature of conveyance infrastructure has already been collected over many years as part
of the development and maintenance of the Districts’ H/H models, and hydrocorrection of the
statewide lidar has already been performed across most of these watersheds. Therefore,
within Districts’ boundaries, only a small amount of field work (e.g., surveying) will be
required to ensure that this information is complete and up to date.
• Generating the FHL, which consists of the following intermediate layers:
o Topographic slope
o Height Above Nearest Drainage (HAND)
o Distance to Nearest Drainage (DND)
• Identifying issues areas, followed by validation (comparison with documented reports of
flood-related issues) and prioritization
• Generating a classified FHL using high water levels from District’s existing SWMM model
• Conducting a simple water balance to estimate the conditions required to fill a landlocked
depression if groundwater were eliminated from the equation
• Producing a set of static reference maps and/or an interactive web-based reference map
• Meetings with District Staff and member communities to review the first generation of the
FHL and to prioritize areas for further assessment
Emmons & Olivier Resources, Inc. - page 4 of 14
Figure 1. Example showing multiple uses and interpretations of the GIS-based Flood Hazard Layer.
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Task 1.2. Flood Risk Assessment
The FHL will be intersected with other spatial datasets as part of a holistic flood risk assessment. This
stacking of datasets will help with the prioritization and targeting of flooding issues and
opportunities by evaluating the intersections of flood hazard with multiple datasets including those
not traditionally considered in flood risk assessments such as information about social and economic
vulnerability. Spatial datasets to be considered for this exercise include:
• Existing stormwater BMPs
• Elevations of critical infrastructure (e.g., fire stations, pumping stations, schools, hospitals,
etc.)
• Public and private parcels
• Parks/Open Space/Natural Assets
• Existing and Future Impervious Coverage (e.g., buildings, parking)
• Groundwater recharge areas
• Social Equity layers (e.g., Center for Disease Control and Prevention’s Social Vulnerability
Index)
• Demographic data (e.g., from the Census Bureau)
• Historical/cultural features
• Contaminated sites
Task 1.3. Review Findings with Workshop Participants
Following the development of the Flood Hazard Layer and the Flood Risk Assessment, EOR and
Freshwater will share this information with District Staff, member communities, Washington County,
and state agency representatives to identify where additional refinement of the information is
needed. The goal of this workshop will be to address the following questions:
➢ Where is there potential flood hazard that the watershed district was not previously aware
of?
➢ Where is there flood risk that the watershed district and the communities want to better
define?
➢ What are the priorities for refining the District’s existing H/H model?
Completion of this task will involve:
• Review the Flood Hazard Layer with workshop participants. Identify climate hazards that
occurred in the past, the intensity and frequency of these hazards, and document their
associated impacts.
• Use the information generated under Task 1.2 Flood Risk Assessment to identify critical assets
and populations vulnerable to climate hazards. This initial vulnerability assessment will
consider the adaptive capacity of these assets and population groups to flooding due to
climate change.
• This information will be used to populate a risk scoring matrix (see Figure 2 for an example)
which can be used to inform refinement of the Districts ’ existing hydrologic & hydraulic
models (if needed).
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Figure 2. Example risk scoring matrix
Task 1.4. Pivot to Task 2
Following the workshop, EOR and Freshwater will meet with District Staff to discuss the results of
the workshop and the needs for the H/H Floodplain and Infrastructure Assessment. EOR and
Freshwater will develop a Scope of Services for Task 2 once there is better definition of the needs
and geographic extent of the assessment.
Task 2. H/H Floodplain and Infrastructure Assessment
As stated previously, the FHL is a relative metric, which means that it can help identify which
locations in the watershed are likely subject to a high degree of flood hazard (i.e., are flooded more
frequently) than other areas but it does not provide an absolute indication of the frequency of that
flooding (i.e., whether those locations are f looded once every 1, 10 or 100 years).
To gain a better understanding of the frequency and duration of flooding, accounting for existing
stormwater infrastructure, the Districts’ H/H models should be refined using the priorities identified
under Task 1 to assess vulnerabilities more accurately under existing and future climate scenarios.
While the scope of this effort cannot be determined until Task 1 has been completed, it is
recommended that the following steps be taken in refining and running the H/H mo del(s). A more
detailed and targeted scope to conduct Task 2 will be developed upon completion of Task 1.
Task 2.1. Hydrologic & Hydraulic Modeling Scenarios
Once the number, location, and priority issues/areas have been defined, the H/H model will be
refined using the approach described below. As this section describes, the H/H model will be used to
identify how the hazards identified under Task 1 are expected to change in the future and what the
impact of these changes means in terms of vulnerabilities in the system.
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Rainfall (Design) Events
The rainfall events to be modeled as part of this task, Floodplain and Infrastructure
Assessment, will be informed by the Flood Hazard Layer and Risk Assessment completed
under Task 1 as well as the areas of interest and issues identified by the watershed districts
and their member communities.
Table 1 includes the criteria currently used for the design and construction of infrastructure
such as roads, culverts, and buildings. Depending upon the findings of the Flood Hazard and
Flood Risk Assessment, we will evaluate which design events may be more relevant to predict
flood risk under existing and future climatic conditions.
Table 1. Summary of design events for local infrastructure
Infrastructure Design Criteria Source
Stormsewer
Infrastructure
Lateral Systems: 10-year rainfall
Trunk Systems: 100-year pond discharge and 10-year
design flow for directly tributary areas
Engineering
Design
Guidelines for
Stormwater
Management,
City of
Stillwater
Culvert Sizing
50-year design frequency for minor culverts (48” or less
in diameter)
MnDOT
Drainage
Manual, 2000
More conservative design frequency (e.g., 100-year
event) may be required if there is significant flood
potential upstream
Risk assessment shall be computed for all major culverts
greater than 48”. The 500-year flood or overtopping
flood shall be computed, whichever is less.
Road
Overtopping
MnDOT
Drainage
Manual, 2000
Lowest floor of a dwelling must be at or above the
regulatory flood protection level which refers to an
MNDNR
Floodplain
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Elevation
Requirements
elevation 1 foot (minimum) above the 100-year flood
plus any stage increase due to the designation of flood
fringe areas.
Elevation
Requirements
Floodplain management ordinances of local
communities require a structure's lowest floor to be
placed 1 foot (at a minimum) above the 100-year flood
level. Many communities have chosen to have 2 feet or
even 3 feet for their freeboard requirements.
Local regulations will also require the top of the access
road elevations to be within 2 feet of the flood protection
elevation.
Landlocked
Basins
10-day precipitation event with a 100-year return
frequency
BCWD,
CMSCWD
and CLFLWD
Rules
Modeling Approach (Sensitivity Analysis)
Based on the feedback obtained at the January 5th, 2021 workshop, it is recommended that
the watershed districts consider both historical and projected rainfall data in conducting a
floodplain mapping / floodplain risk assessment. Changes to historical information will be
used to evaluate current design practices (as illustrated in Table 2) while projected rainfall
data will be used to inform future planning and management decisions.
Projections of future rainfall data obtained from downscaled climate modeling can be
interpreted in different ways. Changes in the mean annual maximum precipitation event for
a future period (e.g., 2020-2039, 2040-2069, 2070-2099) relative to that for historic
conditions could be relatively small, but the distribution of the annual maxima as reflected by
statistics other than the mean (e.g., the standard deviation from the mean) could change
significantly – which is what climate scientists are predicting. For example, while the average
100-year rainfall event for some future period may only increase slightly, the magnitude of
the largest event or the number of large events predicted to occur in an individual year of the
simulation could increase dramatically. In other words, averaging large rainfall events over
20-year periods will likely serve to “wash out” the increase in the number and intensity of
extreme rainfall events that we are already observing as the climate changes.
As a result, modeling scenarios should include an evaluation of the average rainfall depths as
well as the rainfall depths associated with the upper bounds of the “confidence intervals” of
the IDF curves. The upper bounds can provide insight into both the changes predicted to the
distribution of rainfall events and the uncertainty inherent in those predictions. By including
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both the average and the higher confidence interval values in this Flood Risk Assessment, the
watershed districts would ensure that future conditions model runs aren’t underestimating
the potential impacts of climate change on precipitation . These results will help inform both
current design guidelines and future emergency preparedness.
Table 2. Recommended approach to modeling design events under existing and future conditions
Questions being answered Source of
Information
Rainfall Depth Scenarios
What does the most
current information
(design guidance) tell us
today?
NOAA-Atlas 14
- Average value from the existing
IDF curve
- Upper bound of the existing IDF
curve
What should we be
designing for, based on
the most current
precipitation record?
Updated IDF Curve
using the same
method as NOAA-
Atlas 14
- Average value from the existing
IDF curve
- Upper bound of the existing IDF
curve
What should we be
planning for if we want to
consider climate change
projections?
Projected IDF Curve
being developed by
UMN
- Average value from mid-century
and end-of-century projected
IDF curves
- Upper bound from mid-century
and end-of-century projected
IDF curves
In addition to evaluating how rainfall depths are projected to change in the future, it will be
important to consider changes in how this rainfall will be delivered (i.e., duration). Historical
climate evaluation and trend analysis demonstrates that we are experiencing more extreme
precipitation events and that the frequency of these events is increasing. As a result, the
watershed districts should consider evaluating a ran ge of storm durations, as shorter
duration storms can be more relevant to predicting flooding in highly urban catchments and
longer duration storms can be more important in volume driven systems (e.g., landlocked
basins).
The specific rainfall depths and durations recommended for the Floodplain and
Infrastructure Assessment will be made upon completion of Task 1 GIS-Based Flood Hazard
Assessment.
Starting Water Levels
Given that 2019 was the wettest year ever recorded for central Washington County, 2015 -
2019 was the wettest 5 years on record, and 2010-2019 was the wettest 10 years on record,
using current (2021) water levels when modeling lakes, wetlands, ponds, and depressions
would be a reasonable approach to simulating a conservative worst-case scenario.
Since the watershed districts are monitoring a portion of the waterbodies, it will be important
to conduct a synoptic water level monitoring event to get a one-time snapshot of existing
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water levels in the three watersheds. Alternatively, the watershed districts could fly the area
to get a more comprehensive picture of starting water levels/conditions in all waterbodies
and depressions in the watersheds. This scope of services includes a cost estimate to monitor
water levels manually.
Completion of this task will involve:
• Utilize existing staff gauges installed throughout all districts and rely on lake volunteers
(CAMP)/ others to read the staff gauges the day of the effort. This will save time and serve
as an outreach opportunity.
• Reach out to landowners around waterbodies of interest, particularly those on private
property to see if the landowner would be willing to mark the water level the day of the
planned effort. They could use something as simple as wood lath, a stick, or anything to
poke in near the edge of the wat erbody to mark the water level. EOR would visit the site
with the survey-grade GPS and survey the marked water levels at a later date. We would
recommend the landowners mark the location with a ribbon or other material so the
location can be easily located by our survey crew.
• For other waterbodies not captured by the above effort, we would utilize other EOR/
district staff to go out on the same day and simply mark water levels to be surveyed at a
later date.
With this approach, it is assumed we could obtain over 100 waterbody elevations assuming
a decent number of volunteers are willing to mark water levels.
Collecting information about many water levels across the study areas at once will provide a
“snapshot in time” that will not only allow for a higher level of confidence in the results of the
Flood Vulnerability Assessment but can also be used for subsequent H/H model calibration
and validation efforts, as well as future water quality and groundwater studies. Since the
2011 statewide lidar data collection event constitutes another snapshot in time almost
exactly 10 years prior to this effort, such a data collection effort could also be used to help
analyze long-term trends in surface water and shallow groundwater levels.
Role of Groundwater and Infiltration
In 2020, the BCWD recognized the need to better understand the role that abnormally high
groundwater levels play in flooding. Recent high-water levels on Kimbro Basin caused
flooding of County Road 12 as well as flooding of a neighboring residence. Historically, the
BCWD has experienced localized flooding as a result of elevated groundwater levels (e.g.,
flooding of the Goggin’s/School Section Lakes system). The CLFLWD also has received several
landowner complaints of minor flooding on their property due to high-water levels in nearby
wetlands. Historic aerial photographs also indicate that several large wetlands in the
CLFLWD have become wetter over time with a corresponding loss of woody vegetation. While
it is recognized that the watershed districts would like to gain a better understanding of the
role groundwater plays in local flooding issues, this component of the water balance would
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not be addressed explicitly by this assessment. Rather, we will likely make assumptions based
on existing studies and local groundwater monitoring data.
In December of 2020, the modeling work completed by Barr to support the Metropolitan
Council’s evaluation of potential hydrogeologic conditions changes under a range of
projected future climate scenarios was finalized and summarized in a Technical
Memorandum. This work was completed to provide information about possible infiltration
and groundwater conditions for a range of potential future climate changes projected to the
mid and end of century. The evaluation found that modeled long-term infiltration (recharge)
was lower under future climate scenarios (2020-2039, 2040-2069, 2070-2099) than under
historic conditions 2010-2019 using the baseline Metro Model 3 results.4
While long term trends indicate Minnesota is generally getting wetter, we don’t necessarily
know how these wetter conditions will look. The precipitation data from the climate models
are skewed more towards frequent and small precipitation events (e.g., less than 0.5 inches)
which likely results in more precipitation being captured via interception and/or shallow
storage and evapotranspiration, versus the infiltration and groundwater recharge associated
with less frequent but bigger precipitation events. Additionally, infiltration trends over time
are lacking as any differences expressed by individual climate models are canceled out when
the entire climate model ensemble is evaluated. The technical memorandum notes that
additional work is needed to furthe r evaluate the drivers behind the simulated decreases in
infiltration and groundwater levels.
In discussions with Lanya Ross, Environmental Analysis - Water Supply Planning,
Metropolitan Council, it was made apparent that while long-term trends may indicate a
decreasing pattern in infiltration and groundwater recharge, we should be prepared for
shorter-duration wet cycles such as the 2010-2019 period of record. As a result, the
watershed districts may want to consider the following recommendations to bette r
understand the risk from flooding due to abnormally high groundwater levels:
• Expand existing groundwater monitoring programs to include additional residential
wells in areas identified as a higher priority for flood risk (outcome of Task 1). This
will enable the District’s to track groundwater levels more comprehensively to
anticipate their impact on flooding.
• Conduct a preliminary assessment of groundwater flood risk by approximating the
capacity of the surficial aquifer to accept recharge as an equivalent rainfall depth.
• Develop maps of areas vulnerable to groundwater emergence from consolidated
aquifers to reflect the groundwater conditions experienced in 2019-2020. These
maps, covering all consolidated aquifers in the east metro, could be presented as a
provisional set of risk maps that, with further refinement, could be utilized in
regional planning decisions and for flood risk management .5
Task 2.2. Technical Workshop
This second workshop with District staff, member communities, Washington County and state
agency representatives will aim to solidify a vision and plan for what the next steps could look like:
Emmons & Olivier Resources, Inc. - page 12 of 14
− What are the opportunities that could increase resilience?
− What are the challenges or barriers to pursuing these opportunities?
− What specific steps can address those barriers and allow each watershed district to take
priority actions?
Scope & Timeline
These scopes of services include the steps for Task 1 of the Floodplain Mapping / Flood Risk
Assessment as well as the synoptic water level monitoring which would be used in Task 2 , H/H
Floodplain and Infrastructure Assessment. The difference in cost for Task 1 between the three
watershed districts is due to differences in the size of the watersheds, the completeness of existing
data and models, and the number of waterbodies and depressions (landlocked basins) that we
anticipate including in this assessment.
Table 3. Cost Estimate for Task 1. GIS-Based Flood Hazard Assessment - BCWD
Task Estimated
Hours
Estimated
Cost
Schedule
Task 1.1. Flood Hazard Layer
• Hydro correcting LiDAR
• Generating Preliminary Flood Hazard Layer
• Meetings (2) to review FHL internally and with
District Staff and member communities
• Validate FHL and Generate Classified FHL
• Water Balance for LLBs
112 $14,924 2-3 months from
start date
Task 1.2. Flood Risk Assessment
• Select spatial datasets
• Review datasets with District Administrators
• Evaluate intersections of flood hazard with stacked
datasets
24 $3,612 2-3 months from
start date
Task 1.3. Review Findings with Member
Communities and Workshop Participants
• Planning and Workshop Coordination
• Meetings (2) to prioritize areas for further
assessment internally and with District Staff and
member communities
• Facilitate workshop with larger stakeholder group
106 $15,414 4 months from the
start date
Task 1.4 Pivot to Task 2
• Technical Memorandum
• Meetings (2) with District Staff to define needs of
Task 2
34 $4,606 4-5 months from
start date
Sub-Total 276 $38,556
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Task 2.1 – Starting Water Levels: Synoptic water
level measurements
24 $2,856
As soon as possible
– contingent on
timing of rainfall,
staffing, and
volunteer
coordination
Sub-Total 24 $2,856
TOTALS 300 $41,412
Table 4. Cost Estimate for Task 1. GIS-Based Flood Hazard Assessment – CMSCWD*
Task Estimated
Hours
Estimated
Cost
Schedule
Task 1.1. Flood Hazard Layer 170 $20,314 2-3 months from start date
Task 1.2. Flood Risk Assessment 30 $4,296 2-3 months from start date
Task 1.3. Review Findings with
Workshop Participants 74 $10,346 4 months from the start date
Task 1.4 Pivot to Task 2 52 $6,610 4-5 months from start date
Sub-Total 326 $41,566
Task 2.1 – Starting Water Levels:
Synoptic water level
measurements
54 $5,946
As soon as possible – contingent
on timing of rainfall, staffing,
and volunteer coordination
Sub-Total 54 $5,946
TOTALS 380 $47,512
* See Table 3 for more detailed description of the individual tasks.
Table 5. Cost Estimate for Task 1. GIS-Based Flood Hazard Assessment – CLFLWD*
Task Estimated
Hours
Estimated
Cost
Schedule
Task 1.1. Flood Hazard Layer 139 $16,899 2-3 months from start date
Task 1.2. Flood Risk Assessment 26 $3,654 2-3 months from start date
Task 1.3. Review Findings with
Workshop Participants 78 $10,930 4 months from the start date
Task 1.4 Pivot to Task 2 38 $5,002 4-5 months from start date
Emmons & Olivier Resources, Inc. - page 14 of 14
Sub-Total 281 $36,485
Task 2.1 – Starting Water Levels:
Synoptic water level
measurements
30 $3,390
As soon as possible – contingent
on timing of rainfall, staffing,
and volunteer coordination
Sub-Total 30 $3,390
TOTALS 311 $39,875
* See Table 3 for more detailed description of the individual tasks.
Board Action
1. Approve the scope of work for Task 1 of the Floodplain Mapping / Floodplain Risk
Assessment as presented in this memorandum for your watershed district.
2. Approve the scope of work for Task 2.1 – Starting Water Levels: Synoptic water level
measurements to conduct this work in all three watershed districts.
References
1. Field CB, Barros V, Stocker TF, Dahe Q, editors. Managing the Risks of Extreme Events and
Disasters to Advance Climate Change Adaptation: Special Report of the Intergovernmental Panel on
Climate Change. Cambridge: Cambridge University Press; 2012.
http://ebooks.cambridge.org/ref/id/CBO9781139177245. doi:10.1017/CBO9781139177245
2. Global Covenant of Mayors for Climate and Energy. Global Covenant of Mayors. 2021 [accessed
2021 Mar 21]. https://www.globalcovenantofmayors.org/
3. Rincón D, Khan U, Armenakis C. Flood Risk Mapping Using GIS and Multi -Criteria Analysis: A
Greater Toronto Area Case Study. Geosciences. 2018;8(8):275. doi:10.3390/geosciences8080275
4. Barr Engineering. Regional Modeling - Climate Scenarios with SWB and Metro Model 3. Barr
Engineering; 2020.
5. Morris SE, Cobby D, Parkes A. Towards groundwater flood risk mapping. Quarterly Journal of
Engineering Geology and Hydrogeology. 2007;40(3):203–211. doi:10.1144/1470-9236/05-035