Item 3.5 Otsego Creek Hydrologic and Hydraulic Analysis
Request for
City Council Action
DEPARTMENT INFORMATION
ORIGINATING DEPARTMENT REQUESTOR: MEETING DATE:
Public Works City Engineer Wagner June 24, 2024
PRESENTER(s) REVIEWED BY: ITEM #:
Consent City Administrator/Finance Director Flaherty 3.5 – Otsego Creek Study
STRATEGIC VISION
MEETS: THE CITY OF OTSEGO:
Is a strong organization that is committed to leading the community through innovative communication.
X Has proactively expanded infrastructure to responsibly provide core services.
Is committed to delivery of quality emergency service responsive to community needs and expectations in a
cost-effective manner.
Is a social community with diverse housing, service options, and employment opportunities.
Is a distinctive, connected community known for its beauty and natural surroundings.
AGENDA ITEM DETAILS
RECOMMENDATION:
City staff recommends the City Council accept and adopt the Otsego Creek Hydrologic and Hydraulic Analysis.
ARE YOU SEEKING APPROVAL OF A CONTRACT? IS A PUBLIC HEARING REQUIRED?
No No
BACKGROUND/JUSTIFICATION:
The City Council authorized updating the Otsego Creek Storm Water Study. An update of the study was necessary due to:
• Existing report being over 20 years old
• Better modeling programs available
• 100-year storm events have changed significantly
• Existing infrastructure land use and proposed land use can be input
• Expansion of W-WWTF is complete allowing continued development
• Update storm water fee with required improvements
The study reviewed several iterations:
• Pre-development conditions. Pre-development conditions show the extent of flooding or a base line prior to any
development and establish flood plain areas.
• Current conditions. Current conditions locate areas of concern.
• Full build no improvements. Full build no improvements adds future areas of concern.
• Full build with improvements. Full build with improvements determines methods of improvement to alleviate
issues and allows flooding at limits of preset parameters.
Knowing the needed improvements, cost estimates were derived and divided amongst developable land to determine an
updated Otsego Creek Storm Water Impact Fee. The change to this fee was included in an omnibus package of amendments
to the City Code at the June 24, 2024, City Council meeting.
The Public Works Subcommittee was presented with the findings of this study at their May 15, 2024, meeting and provided
a recommendation for consideration of adoption by the City Council.
The attached report includes only the narrative summary of the study, but due to the size of the document, the exhibits
were not included within the packet. A full copy of the report will be available upon request to City staff.
SUPPORTING DOCUMENTS ATTACHED:
• Otsego Creek Hydrologic and Hydraulic Analysis (Summary Version)
POSSIBLE MOTION
PLEASE WORD MOTION AS YOU WOULD LIKE IT TO APPEAR IN THE MINUTES:
Motion to accept and adopt the Otsego Creek Hydrologic and Hydraulic Analysis dated April 19, 2024.
BUDGET INFORMATION
FUNDING: BUDGETED:
Fund 408 – Otsego Creek Watershed
Yes
OTSEGO CREEK
HYDROLOGIC AND
HYDRAULIC ANALYSIS
Otsego, Minnesota
April 19, 2024
Prepared for:
City of Otsego
AE2S Project #: P05409-2022-003
P05409-2022-003 Page i
OTSEGO CREEK HYDROLOGIC AND HYDRAULIC
ANALYSIS
April 2024
Professional Certification
I hereby certify that this report was prepared by me or under my direct supervision and that I am
a duly Registered Professional Engineer under the laws of the State of Minnesota
Name:
Justin Klabo, P.E.
Company: Advanced Engineering and Environmental Services, Inc. (AE2S)
Date: April 19, 2024 Registration Number: 48701
Prepared By:
Advanced Engineering and Environmental Services, Inc. (AE2S)
Water Tower Place Business Center
6901 E Fish Lake Road, Suite 184
Maple Grove, MN 55369
Otsego Creek Hydrologic and Hydraulic Analysis
Table of Contents
April 2024
P05409-2022-003 Page ii
Table of Contents
Chapter 1 Introduction and Summary of Recommendations ......................................... 1-4
1.1 Project Background ........................................................................................................................................ 1-4
1.2 Summary of Findings .................................................................................................................................... 1-5
Chapter 2 Methodology ...................................................................................................... 2-6
2.1 Modeling Software ........................................................................................................................................ 2-6
2.2 Rainfall ............................................................................................................................................................... 2-6
2.3 Data Used ......................................................................................................................................................... 2-6
2.4 Hydrology ........................................................................................................................................................ 2-7
2.5 Hydraulics ......................................................................................................................................................... 2-7
Chapter 3 Existing Conditions Analysis ............................................................................. 3-8
3.2 Hydrology ........................................................................................................................................................ 3-8
3.2.2 Sub-watershed Areas .................................................................................................................... 3-8
3.2.3 Curve Numbers ............................................................................................................................... 3-8
3.2.4 Time of Concentration.................................................................................................................. 3-8
3.3 Hydraulics ......................................................................................................................................................... 3-9
Chapter 4 Existing Conditions Results ............................................................................. 4-10
Chapter 5 Pre-Development Analysis .............................................................................. 5-11
5.1 Hydrology ........................................................................................................................................................ 5-11
5.1.1 Time of Concentration................................................................................................................. 5-11
5.2 Hydraulics ........................................................................................................................................................ 5-11
Chapter 6 Pre-Development Results ................................................................................ 6-12
Chapter 7 Full Build Analysis ............................................................................................ 7-13
7.1 Hydrology ....................................................................................................................................................... 7-13
7.1.1 Sub-watershed Areas .................................................................................................................. 7-13
7.1.2 Curve Numbers ............................................................................................................................. 7-13
7.1.3 Time of Concentration................................................................................................................ 7-13
7.2 Hydraulics ....................................................................................................................................................... 7-14
Chapter 8 Full Build Results and Regional Improvements ............................................ 8-15
8.1 Full Build – No Regional Improvements ............................................................................................... 8-15
8.2 Performance Standards ............................................................................................................................. 8-15
8.3 Full Build – Regional Improvements ...................................................................................................... 8-16
8.4 Road Crossing Categorization ................................................................................................................ 8-17
8.5 Improvements Discussion and Closing Summary ............................................................................ 8-19
Otsego Creek Hydrologic and Hydraulic Analysis
Table of Contents
April 2024
P05409-2022-003 Page iii
List of Tables
Table 2.1 Summary of NOAA Atlas 14 Rainfall Depths (inches) ..................................................... 2-6
Table 2.2 Summary of Rainfall Depths Used in HydroCAD Study (inches) ................................. 2-6
Appendices
Appendix A: Hydrology: Tabular Summaries
Appendix B: Natural Resources Conservation Service (NRCS) Soils Data
Appendix C: Hydrology Figures
Appendix D: Existing Conditions Results Figures
Appendix E: Pre-Development Results Figures
Appendix F: Full Build With No Improvements Results Figures
Appendix G: Full Build With Improvements Results Figures
Appendix H: Improvement Cutsheets
Appendix I: Future Storm Sewer Trunkline Concept Plan
Appendix J: Electronic Modeling Files
Otsego Creek Hydrologic and Hydraulic Analysis
Chapter 1 – Introduction and Summary of Recommendations
April 2024
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Chapter 1 Introduction and Summary of
Recommendations
1.1 Project Background
The watershed for Otsego Creek has been developing over the past two decades and growth
continues throughout the City of Otsego. With this growth, the City raised concerns over this
large scale land use change and its effects on stormwater management. They quantify
stormwater infrastructure needs through an infrastructure impact fee based on the amount of
improvements needed. The City wanted to achieve a better understanding of the improvements
needed and thereby help envision the proactive plan of the City for long term outlook. To do
this, they needed to reinvestigate their city stormwater model.
The City of Otsego has a hydrologic and hydraulic (H&H) model for Otsego Creek that was
developed in 2003 using the HydroCAD modeling software. HydroCAD is a runoff
(subwatershed) to pond routing software that has limitations when analyzing larger regional
areas. Also, the 2003 HydroCAD model has not been updated since its creation and doesn’t
include landuse changes as a result of development and improvements completed by the City
throughout the watershed. Therefore, the City wanted to take this opportunity to develop an
updated model to:
· Accurately reflect drainage conditions. Many properties have been developed within
the watershed over the past couple decades that are not included in the City’s 2003
HydroCAD model.
· Account for larger flood flows. The hydraulic storage volume and overland flow
conduits are able to be included that enable current rainfall events (100-year, Atlas-14)
as well as great flows to mimic potential future changes in precipitation amounts to
understand the resiliency of the system.
· Streamline floodplain review. The Federal Emergency Management Agency (FEMA) has
released preliminary floodplain mapping for Otsego Creek (Zone A floodplain) which will
become regulatory in June 2024. The Zone A floodplain designation will require the City
to ensure future development within the floodplain does not increase flood impacts. The
updated model will use FEMA approved modeling software and standards and allow the
City to easily administer floodplain regulations.
The City hired AE2S in 2022 to evaluate historic, existing, and future Otsego Creek drainage
conditions using XPSWMM. XPSWMM is a commonly used modeling software for analyzing
both hydrology and hydraulics within a watershed and is approved by FEMA to evaluate
floodplains and develop defined flood elevations. This report summarizes the methods and
assumptions used to develop the model, key model results, and recommended improvements.
Otsego Creek Hydrologic and Hydraulic Analysis
Chapter 1 – Introduction and Summary of Recommendations
April 2024
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The overall goals for this H&H analysis include:
· Provide the City an understanding of the peak flows and peak flood elevations
throughout the Otsego Creek drainage basin.
· Identify existing and future areas impacted by flooding.
· Provide the City a tool to evaluate alternative improvements and support future planning
and design of drainage infrastructure.
· Develop regional improvement recommendations to be taken into account for
stormwater utility fees as well as improvement projects needed for any given
development.
1.2 Summary of Findings
The following list highlights the primary findings of this analysis:
· The development that has occurred within the Otsego Creek watershed over the past
20+ years has not led to significant changes in flood risk.
· Under current conditions, 15 roads overtop and approximately 48 structures are at risk of
flooding in the 100-year event.
· The flood risk to property adjacent to Otsego Creek may increase with additional
development in the watershed.
· Additional stormwater standards have been determined to measure the effectiveness of
regional improvements.
· Regional improvements may be implemented to mitigate or minimize potential future
impacts or flood risk.
· Concept level trunkline storm improvements (pipes and channels) have been developed
to provide a long-term plan for full build out conditions within the watershed.
Otsego Creek Hydrologic and Hydraulic Analysis
Chapter 2 – Methodology
April 2024
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Chapter 2 Methodology
The following section describes the methodology used to complete the H&H modeling.
2.1 Modeling Software
XPSWMM software (version 2024.1) is the modeling program used for both hydrology and
hydraulics in the historic (i.e., pre-development), existing, and future (i.e., full build) model
scenarios. Results were generated by solving the Runoff and Hydraulics modes simultaneously.
2.2 Rainfall
NOAA Atlas-14 rainfall depths and the MSE 3 rainfall distribution were used in the models to
simulate rainfall. Table 2.1 summarizes the rainfall depths for the 2-, 10-, and 100-year design
storms used in this analysis. Table 2.2 summarizes the rainfall depths for the same storms used
in the 2003 HydroCAD study for reference.
Table 2.1 Summary of NOAA Atlas 14 Rainfall Depths (inches)
Rainfall Duration 2-Year 10-Year 100-Year
24-Hours 2.85 4.24 7.00
Table 2.2 Summary of Rainfall Depths Used in HydroCAD Study (inches)
Rainfall Duration 2-Year 10-Year 100-Year
24-Hours 2.70 4.10 5.85
2.3 Data Used
The following data were used in this analysis:
· NRCS Web Soil Survey
· Aerial Imagery (MnGeo WMS Service, 2021 Metro Region)
· Existing Topography LiDAR (MnDNR, MnTOPO, 2011)
· City Development Plans and Record Drawings
· Survey Data (provided by City February 2023)
Otsego Creek Hydrologic and Hydraulic Analysis
Chapter 2 – Methodology
April 2024
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2.4 Hydrology
Runoff computations follow TR-20 methodology. Time of concentration (Tc) for existing
conditions was calculated using the TR-55 methodology. All inputs were based on the best
available data at the time of model creation.
2.5 Hydraulics
All elevations within the models are based on the North American Vertical Datum of 1988
(NAVD 88). Most of the storm sewer nodes and links within the models are georeferenced based
on GIS record data. However, some nodes and links (notably flared end sections) have been
shifted for proper 1D connectivity and should not be used to approximate structure locations.
Nodes and links representing the creek and rural areas are approximately located. Natural
channel cross section geometry use MnTOPO LiDAR. Culverts along Otsego Creek were
surveyed by the City in February 2023.
Otsego Creek Hydrologic and Hydraulic Analysis
Chapter 3 – Existing Conditions Analysis
April 2024
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Chapter 3 Existing Conditions Analysis
The base scenario evaluated is referred to as Existing Conditions. This scenario represents the
current (as of 2023) land use and drainage infrastructure. This section describes the Existing
Conditions model development.
3.2 Hydrology
The SCS curve number method was used to develop runoff hydrographs for each sub-
watershed. The following is a summary of the information/process used for the existing
conditions hydrology.
3.2.2 Sub-watershed Areas
The sub-watersheds were delineated based on MnTOPO LiDAR (2011) and GIS storm sewer data.
Where MnTOPO topography did not appear to include newly constructed developments, the
development’s record drawings were used to delineate drainage areas.
3.2.3 Curve Numbers
A land use shapefile was created to generate curve numbers for each sub-watershed. This land
use shapefile included information for:
· Hydrologic soil group (HSG) (see Appendix B);
· Water surface and wetland areas (see Appendix C Figure 2); and
· Land use areas (see Appendix C Figure 2).
The pervious and impervious breakdown for each land use was estimated by either manually
delineating coverage or by measuring coverage for representative areas (i.e. where available
aerial did not yet match the known development). Appendix C Figure 2 includes the
imperviousness for each land use. Sub-watershed runoff was calculated separately for pervious
and impervious areas. Weighted pervious curve numbers were generated following HSG and all
impervious area (including water and wetland) was assigned a curve number of 98.
3.2.4 Time of Concentration
The time of concentration was calculated using two different methods.
For the urban areas, the time of concentration was estimated as follows: 8.7 minutes plus 0.2
minutes per acre of sub-watershed area. The 8.7 minutes was derived using TR-55 sheet flow,
assuming 80 feet of lawn at 2% slope. This was estimated to represent the flow from homes to
either the closest street or storm sewer. The remaining 0.2 minutes per acre within the sub-
watershed represents the timing of water conveyed by the streets or the storm sewer pipes. The
Otsego Creek Hydrologic and Hydraulic Analysis
Chapter 3 – Existing Conditions Analysis
April 2024
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value of 0.2 was derived by calculating five developed sub-watershed Tcs using TR-55
methodology and determining the correlation between shallow concentrated flow time,
channel/pipe flow time, and the sub-watershed area. This flow time was consistently 0.2 minutes
per acre for the tested sub-watersheds. Therefore, 0.2 minutes was used for all developed sub-
watersheds. See Appendix A for the Hydrology Summary.
For the rural areas, the time of concentrations were calculated for each sub-watershed using TR-
55 methodology. Sheet flow was capped at 100 feet of flow length, and shallow concentrated
flow was capped at 500 feet.
3.3 Hydraulics
Hydraulic element information (e.g., invert, material, size) was gathered from multiple sources.
The majority of storm sewer information came from record drawings provided by the City. Major
culvert crossings and critical elevations were surveyed by the City. Overland conveyance features
(e.g. roadway overtopping, channels/ditches) were estimated using LiDAR. Where no survey or
record drawing information existed, assumptions were made to complete model element input.
The data source is recorded in the model for each link within the “notes” field.
Cross section geometry for Otsego Creek and its major tributaries was generated using LiDAR.
The cross section channel bottom was modified to account for the area below water surface or
where LiDAR was not fine enough to capture accurate topography. An additional 1-2 feet deep
by 3-4 wide rectangular section was added to cross sections.
Detention basins and other large storage areas were modeled using depth-area curves which
were developed using record drawings or MnTOPO LiDAR.
Otsego Creek Hydrologic and Hydraulic Analysis
Chapter 4 – Existing Conditions Results
April 2024
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Chapter 4 Existing Conditions Results
The 24-hour 100-year flood event inundation was developed from model node high-water-
levels (HWL). This raster was manually digitized based on model HWLs and the corresponding
surrounding LiDAR data. Appendix D includes figures showing the 100-year inundation and
labels HWLs at detention ponds and depressions. The digital model can be used to find results
at specific locations within the system.
There are multiple locations where current conditions results in flood risk. During the 100-Year
event, approximately 48 structures are at risk of flooding with a freeboard of 1.5 feet or less. The
majority of these structures are: Arbor Creek Development Phases 1 and 2 flooded by Otsego
Creek between Labeaux Ave and MacIver Ave, and Magnolia Landing Development and Otsego
Preserve Development both adjacent and flooded by the undeveloped farmland west of
McAllister Ave and north of 60th St.
Alongside the structures at risk, multiple road crossings overtop during the 100-Year event. 15
crossings overtop during the 100-Year event, with some allowing over 400 cubic feet per second
(cfs) of flow to go over the road.
It is important to note the lake approximately in the middle of the study area. The lake, just
downstream of the majority of current development in the study area, has a HWL over 3 feet
below the lowest nearby structure elevation. This information is relevant to the regional
improvement discussion and stormwater standards seen in Chapter 8.
Otsego Creek Hydrologic and Hydraulic Analysis
Chapter 5 – Pre-Development Analysis
April 2024
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Chapter 5 Pre-Development Analysis
In addition to the Existing Conditions Analysis, a Pre-Development scenario was modeled to
evaluate flooding extents prior to when the majority of the residential subdivisions were built.
This scenario represents generally the watershed and infrastructure conditions in the early
1990s.
The primary question this analysis attempts to answer:
How much does the past 20+ years of residential development contribute to the flood
extents?
5.1 Hydrology
The method for hydrology computations is generally the same as the existing conditions
analysis. Developed land use was replaced with farmland. Sub-watersheds were aggregated
based on assumed routing for 1991 condition – for example, at that time Lannon Avenue did
not exist so sub-watersheds in that area were grouped under Pre-Development Conditions.
Appendix E includes maps displaying which roads did not exist in Pre-Development Conditions.
Curve numbers remained the same as in existing conditions with the exception of the reverted
areas. Areas that are developed in Existing Conditions were reassigned to farmland and the
composite CN of each affected drainage area was recalculated accordingly.
5.1.1 Time of Concentration
Time of concentration was estimated in a similar manner as the urban sub-watersheds in
Existing Conditions. The Pre-Development sub-watersheds whose boundaries were unchanged
kept the same Tc. Tc for the aggregated area had to be estimated as those areas do not have
elevation data available for that time period, therefore the slope of the flow path could not be
calculated nor could the flow path itself be determined with any certainty. Sub-watersheds that
were aggregated were assigned a new Tc using the formula: 9.8 minutes plus 0.5 multiplied by
the total area. The 9.8 minutes was derived from 100 feet of farmland (with crop residue) at 3%
slope. The 0.5 minutes per acre of area was estimated from the correlation of shallow
concentrated flow/channel flow Tc time to watershed area.
5.2 Hydraulics
The only change to hydraulics from the existing condition was the exclusion of road crossings
and detention basins that did not exist in the early 1990s. At locations where a crossing was
removed, the channel link was simply continued with the upstream channel segment routed
directly to the downstream channel segment.
Otsego Creek Hydrologic and Hydraulic Analysis
Chapter 6 – Pre-Development Results
April 2024
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Chapter 6 Pre-Development Results
The 100-year flood inundation was developed for Pre-Development Conditions. Appendix E
includes maps comparing the Pre-Development and Existing Conditions inundation and HWLs.
The Pre-Development flooding is higher and lower than Existing Conditions in some locations
but is generally consistent with Existing Conditions. The most noticeable changes are near
roadways that are excluded in the Pre-Development analysis. In these cases, the Pre-
Development HWL is lower upstream and higher downstream. The main conclusion drawn from
this analysis is that changes in flooding between Pre-Development and Existing Conditions are
driven by the development of road crossings rather than changes to land use.
Otsego Creek Hydrologic and Hydraulic Analysis
Chapter 7 – Full Build Analysis
April 2024
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Chapter 7 Full Build Analysis
Full Build Conditions were modeled to understand the potential impact of additional
development within the watershed and help plan and size regional drainage infrastructure
improvements. Two Full Build scenarios were modeled: with Regional Improvements and
without Regional Improvements.
Regional Improvements were designed to meet basin-specific criteria established by City staff
and AE2S. These criteria and the concept improvements are discussed in more detail in Chapter
8.
7.1 Hydrology
The hydrologic method used in the Full Build Conditions is the same as the Existing Conditions
with a few minor adjustments discussed in the following sections.
7.1.1 Sub-watershed Areas
Sub-watershed drainage boundaries were maintained from Existing Conditions. The drainage
boundaries were unchanged because no proposed plans are available to delineate future
developments. Additionally, while developments will slightly move the drainage divides, overall,
the total drainage areas to each wetland, channel, pond, etc. will likely be roughly the same as
existing.
7.1.2 Curve Numbers
Land use was updated using the future land use map and shapefile provided by the City. Each
land use was assigned an impervious percentage. The remaining pervious area was assigned as
grass. See Appendix A and C for land use and assigned impervious percentages. Wetland areas
were maintained in Full Build Condition, as those areas are likely to remain undeveloped.
7.1.3 Time of Concentration
The time of concentration of each “to-be developed” sub-watershed was manually increased
such that the proposed peak runoff rate was between 95%-100% of the existing peak runoff rate
for the 100-year event. This adjustment of the time of concentration simulates the future
development onsite detention basins. Future water quality abstraction was not included in the
analysis. This method for estimating future runoff maximizes the potential impact of the fully
developed watershed from a runoff volume standpoint and gives an accurate assessment of
what would happen if current minimum stormwater management standards were met.
Otsego Creek Hydrologic and Hydraulic Analysis
Chapter 7 – Full Build Analysis
April 2024
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7.2 Hydraulics
Hydraulics were unchanged between Full Build and Existing Conditions with the exception of the
proposed regional improvements which are further discussed in Chapter 8.
Otsego Creek Hydrologic and Hydraulic Analysis
Chapter 8 – Full Build Results and Regional Improvements
April 2024
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Chapter 8 Full Build Results and Regional
Improvements
8.1 Full Build – No Regional Improvements
The Full Build Condition was first analyzed without any regional improvements to determine the
performance of the system if current rate control regulations were followed.
Full Build figures include the modeled high water levels of each pond/depression as well as the
peak flow rates through and over critical open channel crossings. These figures can be found in
Appendix F. Flooding increases across the majority of the study area ranging from 0.1-0.5 feet
with the highest increases (~0.9 feet) occurring between Kalland and Labeaux Avenue and
between Lannon and Maciver Avenue.
If current stormwater regulations (i.e., conventional onsite pre-post discharge rate control) were
followed, increased flooding would impact several areas within the Study Area and increase flow
rates downstream of the lake. To mitigate these potential increased flood impacts, alternative
solutions were developed and evaluated.
8.2 Performance Standards
In discussions with the City staff, standards were developed to provide target performance levels
to measure the effectiveness of alternative improvements. These performance standards are
specific for this analysis and are not currently intended to be used for development criteria, but
they may be used in the creation of future policies. These standards include:
· The 100-Year HWL shall be at least 1.5 feet below the nearest adjacent low opening
· The EOF of all storage areas or road crossings shall be 1.5 feet below the nearest
adjacent low opening
· A maximum bounce of 1.0 feet shall be enforced between existing and full build
conditions throughout the model area, except at the lake
· All CSAH and major road crossings shall not overtop during the 50-Year storm event
o Streets covered under this standard include: 85th St NE, MacIver Ave NE, and
CSAH 38 (70th St NE), and CSAH 19 (Labeaux Ave NE)
· Non-CSAH and minor road crossings shall not overtop by more than 0.5 feet for a
duration of 12 hours or more during the 100-Year storm event
Otsego Creek Hydrologic and Hydraulic Analysis
Chapter 8 – Full Build Results and Regional Improvements
April 2024
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8.3 Full Build – Regional Improvements
In coordination with the City, key areas for regional improvement were determined based on the
Full Build model results. These improvements were designed to reduce flooding in areas
identified as at risk and to meet the performance standards outlined in the previous section. A
list of regional improvements is detailed below. Full Build results with regional improvements
can be found in Appendix G. Summaries of each improvement, with descriptions and model
results, can be found in Appendix H.
List of Regional Improvements
· 1: Lake Outlet
o This improvement creates additional flood storage at the Lake with an outlet
designed to attenuate the increase in runoff from Full Build development
upstream and to meter outflow which dampens the flow rates in the Main Branch.
· 2: Southeast Regional Pond
o This improvement creates a regional pond just west of McAllister Avenue to help
reduce HWLs adjacent to the Magnolia Landing subdivision. The regional pond
was sized such that the 100-year HWL has a freeboard of 3.0 feet since there is
no overland emergency overflow route. The outlet is a 60-inch RCP pipe crossing
McAllister Ave discharging to the East Branch of Otsego Creek. This improvement
lowers the Existing Conditions HWL by 2.6 feet.
· 3: Berm Near Labeaux Avenue and 80th Street
o Breakout flow from a wetland complex west of Labeaux Ave routes east where it
contributes to a culvert that crosses Labeaux Ave eastward to a channel system,
which passes through development before reaching Otsego Creek’s West Branch.
The channel system is already at capacity in the Full Build Condition, therefore
reducing incoming flow where feasible is favorable. This improvement eliminates
the breakout flow and instead forces all outflow from the wetland complex south
in its primary drainage pattern and into Otsego Creek’s West Branch. With a berm
at the breakout point and a new dedicated southerly drainage system (either pipe
or open channel), this improvement alleviates the flow in the already stressed
channel system.
· 4: Regional Storage at the West/Main Branch Junction
o In the Full Build Condition the developments in the center of the study area
would see increased HWLs potentially inundating structures. A regional storage
area located at the confluence of the West and Main Branch, coupled with
additional improvements, provides enough detention volume to lower the HWLs
to be within target standards.
· 5: Metering Outflow from Mud Lake into the Main Branch
o The Otsego Creek Authority Agreement between the Cities of Otsego and
Albertville states that the outflow from Mud Lake shall be limited to a maximum
flow of 17.4 cfs. Under existing conditions, this criterion is not met because there
Otsego Creek Hydrologic and Hydraulic Analysis
Chapter 8 – Full Build Results and Regional Improvements
April 2024
www.ae2s.com Page 8-17
is no flow control structure. The existing culvert underneath 70th Street
discharging into City limits is simply a 36-inch RCP. The culvert was modeled as
flowing full in existing conditions for the 100-year event – 69 cfs. This regional
improvement proposes to develop an outlet structure at the head of the 70th
Street culvert to meet the flow limit established in the Agreement.
8.4 Road Crossing Categorization
With the Full Build Model and the standards outlined in the above sections, it was important to
gage the Full Build Model with Improvements and the performance in relation to those
standards. This section outlines areas of note that are split into levels. These levels are:
· Category 1 – Crossings along Otsego Creek and its branches that do not overtop during
the 100-Year event. These crossings are of minimal to no concern and no further action
is needed.
· Category 2 – Crossings that meet the design standards but overtop during the 100-Year
event, in which case the extent of overtopping is reported.
· Category 3 – Crossings or Areas that do not meet part of the design standards but were
not addressed for various reasons explained below.
The list below details each area in the Full Build Model with Improvements, with figures detailing
the location of each area seen in Appendix G.
· Level 1 – Crossings that do not overtop during the 100-Year event:
1. Labeaux Ave NE
2. 73rd St NE
3. Lancaster Ave NE
4. 77th St NE/Lannon Ave NE
5. MacIver Ave NE (at Otsego Creek crossing)
6. 67th St NE
· Level 2 - Areas that meet the standard but overtop roadway during 100-Year Event:
7. Kadler Ave: This crossing overflows with a flow depth of 0.1 ft for 50 hours during
a 100-Year Event, with a max depth of 0.2 ft.
8. 80th Street NE: This crossing overflows with a flow depth of 0.5 ft for 1.5 hours
during a 100-Year Event, with a max depth of 0.7 ft.
9. McAllister Ave and 67th St Junction: This crossing overflows with a flow depth of
0.5 ft for 1.5 hours during a 100-Year Event, with a max depth of 1.2 ft.
10. 70th Street NE: This major roadway crossing does not overtop during the 50-Year
Event (meeting the standard) but does overtop during the 100-Year Event with a
flow depth of 0.1 ft for 3 hours.
11. 77th St NE [West]: This crossing overflows with a flow depth of 0.5 ft for 1.5 hours
during a 100-Year Event, with a max depth of 0.6 ft. However the peak flow is
over 200 cfs across the road, increasing the risk during vehicular passage.
Otsego Creek Hydrologic and Hydraulic Analysis
Chapter 8 – Full Build Results and Regional Improvements
April 2024
www.ae2s.com Page 8-18
12. 77th St NE [East]: This crossing overflows with a flow depth of 0.5 ft for 2.5 hours
during a 100-Year Event, with a max depth of 1.1 ft. However the peak flow is
over 400 cfs across the road, increasing the risk during vehicular passage.
· Level 3 - Areas that do not meet the standard:
13. Prairie View Schools: the farmland just west of MacIver and the school has a
depression that fills with runoff roughly near the school driveway entrance. This
depression drains through storm sewer into the trunkline under MacIver flowing
north. However, both this depression and the rate control basin adjacent to the
baseball field overflow MacIver during the 50-year event and greater. While the
rate control basin is on school property and cannot be readily altered, the farm
depression can be converted into a storage pond to lower the water level around
the school entrance. Approximately 30 ac-ft of cut, or a pond live storage volume
of 24.6 ac-ft below 940.0 will reduce the depression’s HWL to below MacIver,
leaving only the overflow from the rate control basin impacting road passage,
which will be about 0.3 ft for less than 1 hour during the 50-Year event.
14. Kittredge Crossing Development: 2 twinhome structures are located just
southwest of the Otsego Creek West Branch’s junction with Kalland Ave. The HWL
upstream of this crossing is 946.8 while the low opening of these 2 structures is
947.0. Preliminary feasibility for a regional improvement was explored at this
location but it was determined extensive changes had to be implemented to
address the flood risk for 2 structures. These changes include upsizing both the
Kalland Ave and Labeaux Ave creek crossings along with storage expansion in
between these two crossings. With this in mind, the chosen action item for this
area is to receive priority for temporary flood protection such as sand-bagging
when forecasts predict large storm events.
15. 83rd Street NE: the first crossing downstream of the lake overtops during the 100-
Year Event for 0.5 ft for 36 hours, with a max depth of 0.6 ft. It is recommended
to add an additional culvert crossing east of the intersection with Mason Ave to
reduce flow over the roadway.
16. 87th Street NE: This crossing overtops during the 100-Year Event for 0.5 ft for 42
hours, with a max depth of 1.4 ft. With the improvement of 85th Street to the
south (which does not overtop during the 100-Year Event) and the likely
development of this area, a new road servicing this area connecting to 85th
without crossing the floodplain will be proposed. With a new road, the current
87th Street can be abandoned and removed which will improve floodplain
conveyance in this area.
Otsego Creek Hydrologic and Hydraulic Analysis
Chapter 8 – Full Build Results and Regional Improvements
April 2024
www.ae2s.com Page 8-19
8.5 Improvements Discussion and Closing Summary
With these improvements, Otsego can greatly decrease the flood risk for the study area and also
downstream given the reduction in peak flow for Otsego Creek. However, it is important to note
that these improvements are concepts only. Proper design should be conducted before any
plans are ready for construction. With that said, these improvements are flexible to change. The
improvements have the ability to be altered if the City deems it necessary, whether its due to
land acquisition complications, coordination with the City of Albertville in the case of
Improvement 5, a change in target standards, etc.
AE2S recommends to further investigate each of these improvements from both a design and
construction feasibility perspective, with the ultimate constructed solution meeting the
standards the City accepts at the time(s) of construction.