5.1 Otsego Creek Hydraulic Study
Request for
City Council Action
DEPARTMENT INFORMATION
ORIGINATING DEPARTMENT REQUESTOR: MEETING DATE:
Public Works City Engineer Wagner December 12, 2022
PRESENTER(s) REVIEWED BY: ITEM #:
City Engineer Wagner City Administrator/Finance Director Flaherty 5.1 – Otsego Creek
Hydraulic 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 approve an amendment to a professional services agreement.
ARE YOU SEEKING APPROVAL OF A CONTRACT? IS A PUBLIC HEARING REQUIRED?
Yes No
BACKGROUND/JUSTIFICATION:
The City of Otsego needs to complete an Otsego Creek Rehabilitation Project in conjunction with the
Prairie Pointe Development. The DNR had some additional modeling requirements to obtain permits for
the rehab project.
Otsego Creek has a preliminary FEMA flood plain modeled over it. Previous FEMA flood maps did not
include Otsego Creek. Although the preliminary FEMA flood plain has yet to be officially adopted
(anticipated 2024), the DNR insisted the City of Otsego must prove zero stage increases on the 100-year
flood using HEC-RAS.
The City of Otsego’s previous contract with AE2S was for modeling using XPSWM which is very good at
modeling storm flows when passing through various storm pipe networks and storm ponds. HEC-RAS
specializes in creek and river flows. Preparing a second model over the same area essentially doubles the
scope of work necessary, thus the amendment for an additional $18,500.
This item was previously discussed at the November 28, 2022 City Council meeting, with the topic being
tabled by the City Council for additional discussion at a future meeting. Engineering staff have provided
additional information included in the packet materials and will be present at the meeting for discussion.
SUPPORTING DOCUMENTS ATTACHED:
• AE2S Technical Memorandum
• Background and Justification Memorandum
• Amendment to Professional Services Agreement
POSSIBLE MOTION
PLEASE WORD MOTION AS YOU WOULD LIKE IT TO APPEAR IN THE MINUTES:
Motion to approve Amendment No. 1 to the Professional Services Agreement for storm water hydraulic
modeling of Otsego Creek with Advanced Engineering and Environmental Studies, LLC (AE2S) in the amount
of $18,500.
BUDGET INFORMATION
FUNDING: BUDGETED:
Fund 408 – Otsego Creek Watershed
No; however, there is sufficient funding
accumulated from Stormwater Impact Charges
collected through developments to pay for the
study.
AE2S Project Number: P05409-2022-002 Page 1 of 7
Think Big. Go Beyond . www.ae2s.com
https://ae2s.sharepoint.com/sites/Pioneer/Shared Documents/Otsego/Report/2022.06.17/PrairiePointe_Memo_2022.06.21.docx
TECHNICAL MEMORANDUM
To: Ron Wagner, PE - Otsego City Engineer
From: Justin Klabo, PE – Senior Water Resources Engineer
Tony Miller, PE, – Water Resources Engineer
Re: Prairie Pointe Hydraulic Study – 83rd St. NE Culvert Analysis
Date: June 21, 2022
A 75-acre parcel is being proposed for development within the City of Otsego. The land will be
converted from agricultural to single-family residential. The development, Prairie Pointe, is
located northeast of the intersection of county roads 38 and 19. Refer to Figure 1 for exact
project location and extents.
Prairie Pointe Development Site Location
Otsego Creek runs through the south side of the parcel as well as along the east side. In
addition, there is a lake/wetland (DNR waterbody 86035100) located in the southeast corner of
the parcel, with the creek serving as the outlet of the lake. As the creek flows north, it crosses
83rd St. NE in two culverts, ultimately discharging to the Mississippi River. This creek does not
currently have a FEMA Flood Insurance Rate Map, but preliminary mapping has been conducted
and is available through the FEMA Map Service Center. Refer to Figure 2 for an aerial view of
the reach. County Road 19 MacIver Ave NE County Rode 38
80th St NE
83rd St NE
Technical Memorandum
Re: Prairie Pointe Hydraulic Study – 83rd St. NE Culvert Analysis
June 21, 2022
AE2S Project Number: P05409-2022-002 Page 2 of 7
Think Big. Go Beyond . www.ae2s.com
83rd St. Reach Aerial Image
The City has retained AE2S to conduct a hydraulic analysis of the creek and the impact of both
the Prairie Pointe development and the two culvert extensions at the 83rd St. NE Road Crossing.
The creek section will be labeled as “83rd St. Reach” for the remainder of this memo.
SUMMARY
The impact of the development adjacent to 83rd St. Reach and the two culvert extensions under
83rd St. NE is less than the allowable 0.5-feet of rise (as discussed with the DNR). Therefore, no
mitigation is necessary. Further information regarding the data, modeling, assumptions, and
results can be found within the body of this memorandum.
DATA OBTAINED
The following data was used for this analysis:
DNR waterbody 86035100
83rd St. NE Mason Ave NE 83rd St. NE
83rd St. Reach
Technical Memorandum
Re: Prairie Pointe Hydraulic Study – 83rd St. NE Culvert Analysis
June 21, 2022
AE2S Project Number: P05409-2022-002 Page 3 of 7
Think Big. Go Beyond . www.ae2s.com
• Aerial Imagery (Google, 2022)
• Existing Topography (DNR, MNTOPO, 2020)
• Prairie Pointe CAD files (Pioneer, May 2022)
• Field Survey (AE2S, April 2022)
• Field Survey (Pioneer, November 2021)
• Preliminary HEC-RAS model (DNR, January 2011)
• 83rd St. NE Construction Plans (Pioneer, March 2022)
AE2S conducted a site visit to verify the existing conditions and to collect survey information on
key stormwater features that would affect the modeling.
The developer provided AE2S with their draft plans, digital files of the existing survey, and
surface files for both the existing and proposed conditions.
The DNR supplied AE2S with the preliminary HEC-RAS model for this area.
CREEK MODELING
AE2S created our own HEC-RAS model for the 83rd St. Reach using the survey data discussed in
the Data Obtained section of this report and the 100-year flow (500-cfs) from the preliminary
HEC-RAS model provided by the DNR. Cross sections were generated from the existing LiDAR
and the development’s CAD surface. Culvert crossings were modeled using survey information
gathered by AE2S and Pioneer. Figures 3 and Figure 4 (below) display the model cross sections,
surface contours, and inundation mapping for both the existing and proposed conditions.
The existing 83rd St. NE crossing is two RCP pipes: one 36-inch diameter and one 54-inch
diameter. Both pipes will be extended about 35-ft in proposed conditions. See Figures 3 and
Figure 4 for the 83rd St. NE Crossing details in and existing and proposed conditions,
respectively.
Technical Memorandum
Re: Prairie Pointe Hydraulic Study – 83rd St. NE Culvert Analysis
June 21, 2022
AE2S Project Number: P05409-2022-002 Page 4 of 7
Think Big. Go Beyond . www.ae2s.com
83rd St. Reach Existing Conditions
DNR Waterbody 86035100
Technical Memorandum
Re: Prairie Pointe Hydraulic Study – 83rd St. NE Culvert Analysis
June 21, 2022
AE2S Project Number: P05409-2022-002 Page 5 of 7
Think Big. Go Beyond . www.ae2s.com
83rd St. Reach Proposed Conditions
DNR Waterbody 86035100
Technical Memorandum
Re: Prairie Pointe Hydraulic Study – 83rd St. NE Culvert Analysis
June 21, 2022
AE2S Project Number: P05409-2022-002 Page 6 of 7
Think Big. Go Beyond . www.ae2s.com
MODELING RESULTS – HYDRAULIC ANALYSIS
The model shows a water surface elevation (WSE) increase of less than the allowable 0.5-feet (as
discussed with the DNR) at every crossing in 83rd St. Reach. A summary of the model results is
provided below. See Appendix A for a complete table of the modeling results.
Table 1.1 shows the WSE changes at the upstream and downstream extents of 83rd St. Reach
along with the maximum WSE increase seen within the reach.
Table 1.1 Summary of 83rd St. Reach Hydraulic Results
Location Cross
Section
Existing
Conditions
100-Year WSE
(ft)
Proposed
Conditions
100-Year WSE
(ft)
Difference
(ft)
Downstream Extents
(400-ft north of 83rd St.
NE)
9988 921.06 921.06 0.00
Approximately 700-ft
south of 83rd St. NE 9995 924.76 924.77 0.01*
Upstream Extents
(Lake Outlet) 10000 925.39 925.39 0.00
* Maximum WSE Increase
The 0.01-foot increase in WSE is a result of fill from the development (loss of conveyance
capacity) and extending the culvert on 83rd St. NE (reduced capacity based on lengthening the
culvert). The WSE increase is only 0.01-feet, despite these two impacts. This is because the
existing 100-year event overtops 83rd St. NE. The creek’s conveyance capacity is governed by
both the culverts and the road (which acts as a large weir). Any increased WSE due to fill or
culvert extension is minimal due to the large amount of conveyance capacity provided by the
overtopping road.
Table 1.2 shows the flow summary at the 83rd St. NE crossing, detailing the flow contribution of
each culvert and the roadway separately. As seen in the table, over half of the flow from the
100-year event is overtopping 83rd St. NE in both the existing and proposed conditions. A
summary of the road inundation during the 100-year event is shown in Table 1.3.
Technical Memorandum
Re: Prairie Pointe Hydraulic Study – 83rd St. NE Culvert Analysis
June 21, 2022
AE2S Project Number: P05409-2022-002 Page 7 of 7
Think Big. Go Beyond . www.ae2s.com
Table 1.2 83rd St. Crossing Flow Summary
Condition 36” RCP Flow
(cfs)
54” RCP Flow
(cfs)
Road
Overtopping
Flow (cfs)
Total Flow (cfs)
Existing 65.1 173.9 261.0 500.0
Proposed 65.3 166.1 268.6 500.0
Difference +0.2 -7.8 +7.6 0.0
Table 1.3 83rd St. Crossing Road Overtopping Summary
Condition 100-Year WSE
(ft)
Road
Overtopping
Elevation (ft)
Inundation
Depth (ft)
Inundated
Road Length
(ft)
Existing 924.7 924.0 0.7 320
Proposed 924.7 924.0 0.7 320
Difference 0.0 0.0 0.0 0.0
HEC-RAS River: PrairiePointe_DN Reach: 83rd St Reach Profile: PF# 1
Reach River Sta Profile Plan Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width Froude # Chl
(cfs) (ft) (ft) (ft) (ft) (ft/ft) (ft/s) (sq ft) (ft)
83rd St Reach 10000 PF# 1 Existing 500.00 919.50 925.393 921.42 925.42 0.000129 1.42 522.01 172.63 0.11
83rd St Reach 10000 PF# 1 Proposed 500.00 919.50 925.395 921.42 925.42 0.000129 1.42 449.46 130.52 0.11
83rd St Reach 9999 PF# 1 Existing 500.00 919.54 925.346 922.32 925.40 0.000320 2.10 424.04 160.31 0.17
83rd St Reach 9999 PF# 1 Proposed 500.00 919.54 925.347 922.32 925.40 0.000324 2.12 365.16 124.07 0.17
83rd St Reach 9998 PF# 1 Existing 500.00 919.23 925.196 922.50 925.33 0.001072 2.96 169.13 95.86 0.29
83rd St Reach 9998 PF# 1 Proposed 500.00 919.23 925.198 922.50 925.33 0.001070 2.95 169.26 95.93 0.29
83rd St Reach 9997 PF# 1 Existing 500.00 919.08 924.993 922.05 925.14 0.001106 3.13 187.97 121.25 0.29
83rd St Reach 9997 PF# 1 Proposed 500.00 919.08 925.001 922.05 925.15 0.001033 3.15 188.88 121.61 0.29
83rd St Reach 9996 PF# 1 Existing 500.00 919.93 924.768 924.93 0.001027 3.50 180.54 81.44 0.30
83rd St Reach 9996 PF# 1 Proposed 500.00 919.93 924.781 924.95 0.001028 3.52 181.63 82.00 0.30
83rd St Reach 9995 PF# 1 Existing 500.00 919.92 924.761 924.83 0.000467 2.37 306.66 181.05 0.20
83rd St Reach 9995 PF# 1 Proposed 500.00 919.92 924.774 924.84 0.000480 2.39 291.17 144.11 0.20
83rd St Reach 9994 PF# 1 Existing 500.00 919.77 924.759 924.78 0.000178 1.44 594.94 372.18 0.12
83rd St Reach 9994 PF# 1 Proposed 500.00 919.77 924.742 922.03 924.78 0.000319 1.93 377.69 198.25 0.17
83rd St Reach 9993 PF# 1 Existing 500.00 919.31 924.731 924.75 0.000154 1.41 654.94 376.22 0.12
83rd St Reach 9993 PF# 1 Proposed 500.00 919.31 924.699 922.86 924.73 0.000255 1.80 450.02 205.78 0.15
83rd St Reach 9992 PF# 1 Existing 500.00 918.98 924.717 924.72 0.000058 0.93 941.50 410.68 0.07
83rd St Reach 9992 PF# 1 Proposed 500.00 918.98 924.659 922.00 924.68 0.000151 1.46 519.77 193.35 0.11
83rd St Reach 9991 PF# 1 Existing 500.00 918.45 924.709 921.58 924.71 0.000047 0.88 1010.33 421.28 0.07
83rd St Reach 9991 PF# 1 Proposed 500.00 918.45 924.647 921.58 924.66 0.000084 1.17 729.00 293.25 0.09
83rd St Reach 9990 PF# 1 Existing 500.00 918.43 924.706 921.22 924.71 0.000020 0.54 1796.48 956.59 0.04
83rd St Reach 9990 PF# 1 Proposed 500.00 918.43 924.645 921.19 924.65 0.000025 0.59 1622.17 874.79 0.05
83rd St Reach 9989.43 Culvert
83rd St Reach 9989 PF# 1 Existing 500.00 916.69 921.321 921.46 0.001424 3.10 193.28 173.09 0.33
83rd St Reach 9989 PF# 1 Proposed 500.00 916.69 921.319 921.46 0.001469 3.14 191.43 185.42 0.34
83rd St Reach 9988 PF# 1 Existing 500.00 916.29 921.060 920.51 921.24 0.002607 3.93 195.29 231.95 0.43
83rd St Reach 9988 PF# 1 Proposed 500.00 916.29 921.060 920.51 921.24 0.002607 3.93 195.29 231.95 0.43
83rd St Reach 9987 PF# 1 Existing 500.00 915.93 920.189 919.28 920.50 0.003851 4.71 139.68 154.10 0.53
83rd St Reach 9987 PF# 1 Proposed 500.00 915.93 920.189 919.28 920.50 0.003851 4.71 139.68 154.10 0.53
Appendix A: HEC-RAS Model Results Summary
Background/Justification
Otsego Creek Storm Water Fund
The City of Otsego began requiring developers pay a Storm Water Fee for any work completed within
the Creek in 2004. Since Otsego Creek is a DNR waterway, any work or crossings requires a permit from
the DNR. Due to that reason, and the fact that it is the largest trunk water system in Otsego, all work
within the Creek was deemed to be completed by the City not the developer, hence the original report
in 2003 and the subsequent fee. The City has been collecting these fees for the purpose of paying for
items needed for overall development of the watershed (ie storm water modeling, engineering design,
culvert crossing, obtaining DNR permits, actual creek rehab, etc). At this point the Otsego Storm water
fee is $3,985 per net acre. The City is obligated, by collecting these fees, to complete the storm water
projects in their entirety for the necessity of the developments it has approved. To this end, the City has
completed several projects and obtain various permits for developers to obtain crossings and for the
City to rehab the creek. The Otsego Creek Fund has a cash balance of $2,075,504 (as of December 8,
2022). Below and attached is AE2S and Hakanson Anderson more complete explanation of the
addendum to AE2S’s modeling contract.
Original Contract ($18,500)
AE2S was hired to assist the City with a Hydraulic Study for the Prairie Pointe development in the winter
of 2022 (agreement dated January 14, 2022). The purpose of the study was to evaluate the proposed
culvert crossing at 77th Street NE and the existing crossing at 83rd Street NE to confirm culvert sizes. The
following are the major tasks of the agreement:
1. Develop a Prairie Pointe HEC-RAS model
a. One (1) single model with both crossings
2. Run alternatives
a. One (1) alternative for each location
3. Develop report with design recommendation
a. One (1) report with all of the information
Summary of Extra Services ($18,500)
The following summary outlines the additional work for the study as required to obtain the MnDNR
Public Waters permit.
1. Project Management
a. Additional three (3) coordination meetings with the City and DNR staff.
2. Hydraulic Study
a. Review the federal preliminary floodplain maps and models.
i. Compare data used in the FEMA model as compared to the City model.
ii. Identify any discrepancies between the two models and provide a summary for
the MnDNR.
b. Model Accuracy Review
i. Compare the FEMA model compared to the City model to determine which
model is more accurate.
ii. Take the alternatives developed and analyzed in the City model and integrate
them into the FEMA model. Update designs for each culvert crossing based on
the FEMA model to provide a comparison between the City & FEMA models.
iii. Update recommended preliminary alternative for both Reach 1 and Reach 2.
c. Encroachment Analysis
i. Evaluate Reach 2 to confirm the development fill along the channel is not
impacting the high water levels of the floodplain.
3. Report
a. Prepare a standalone report for the 83rd Street NE crossing detailing the methodology,
findings, and recommendation for the analysis.
Background
Otsego Creek is designated a Public Water by the MnDNR. As a result, any culvert or improvement
within the channel needs to be permitted through the MnDNR with a Public Waters Work permit.
During the development of the HEC-RAS model (City model) in the spring of 2022 for the two crossing
locations, a coordination meeting between the City, MnDNR and AE2S was conducted to collaborate on
the analysis based on the Public Waters Work permit. During the discussion with the MnDNR they
provided new modeling information on the floodplain, that wasn’t available previously, developed as
part of the preliminary FEMA mapping effort to update the floodplain. The FEMA preliminary model was
reviewed and compared against the City model. It was found the hydrologic and hydraulic data of the
FEMA model was not consistent with the most accurate data available. As a result, the MnDNR required
the City to complete a review to confirm which model more accurately details the high water levels of
the floodplain. This included evaluating the existing conditions and the culvert alternatives for the
crossing locations. As part of this analysis, the alternatives needed to be re-designed to conform with
State and Federal floodplain standards based on the differences in the findings. Based on the findings of
the model review, MnDNR required our team to move forward with the crossing design based on the
City model. Overall, the modeling collaboration between the City, MnDNR and AE2S started in April of
2022 with final direction provided in July 2022.
During the permitting process with the MnDNR, the developer requested the culvert design for the 83rd
Street NE crossing to be completed independently from the 77th Street NE culvert so they could move
forward with grading a portion of the development. AE2S collaborated with MnDNR to confirm they
agreed to splitting the analysis into two separate designs. To facilitate this work, a separate Report
(dated June 21, 2022) was developed detailing the analysis, findings and recommendation for the
culvert crossing. In addition, the MnDNR required an Encroachment Analysis to be completed for Reach
2 in conjunction with the preliminary mapping to demonstrate the development wouldn’t be causing a
rise in the base flood elevation for the area adjacent to the development.
In addition to the culvert crossings proposed concurrently with the Prairie Pointe development, a
section of the channel is proposed to be restored along the south end of the development. Under the
original scope of work, the culvert design (for both crossings) was proposed to be developed at the same
time as the channel restoration. However, the channel restoration work is proposed for 2023 and the
final crossing design delayed until final plans are developed in conjunction with the channel restoration.
Timeline of Work Completed to Date
1. March
a. Developed existing conditions Prairie Point HEC-RAS model.
2. April
a. Evaluated preliminary alternatives for each crossing.
b. Met with MnDNR
i. Obtained FEMA model for preliminary floodplain.
c. Reviewed preliminary FEMA model.
i. Discovered the FEMA model had inaccurate hydrology and hydraulic data. Based
on this discovery, MnDNR required an accuracy review of the FEMA versus City
model for the floodplain including alternatives for each culvert crossing.
3. May
a. Meeting with the MnDNR to discuss findings of the model accuracy review.
b. Supplemental modeling was completed based on MnDNR comments.
i. Review un-permitted farm access road and its impacts to the floodplain
modeling (City model vs. FEMA model).
ii. MnDNR required an encroachment analysis for Reach 2.
4. June
a. Follow-up meeting with the MnDNR to discuss findings.
b. The modeling was split into two projects so the culvert adjacent to the development
grading can be done at the same time and not delay the grading.
i. Evaluate Reach 2 culvert crossing final design alternatives.
ii. Developed a report with design recommendations for Reach 2, 83rd Street NE
culvert crossing.
5. July
a. Direction from the MnDNR on the model to use (City model).
b. Project on hold until Final Design started for channel restoration work.
Remaining Work to be completed
1. Final Design of Reach 1, 77th Street NE Culvert Crossing
a. Evaluate culvert crossing in conjunction with the final plan development of the channel
restoration final design. Run alternatives for Reach 1.
2. Final Report
a. Develop report with design recommendation for Reach 1