2026-02-27_240743_Stormwater Report MMW
STORMWATER
MANAGEMENT
NARRATIVE
PREPARED FOR:
Gym, Stage, and Science Addition and Alterations
Minnetonka Middle School
Minnetonka Public Schools District 276
6421 Hazeltine Blvd
Excelsior, MN 55331
February 13, 2026
Revised February 27, 2026
Prepared by:
2300 Berkshire Lane North, Suite 200
Plymouth, MN 55441
Table of Contents
Summary Stormwater Summary
Appendix A Rainfall Analysis Data
Appendix B Water Quality Volume Calculations
Appendix C Drainage Maps
- Existing Drainage Map
- Proposed Drainage Map
Appendix D Existing vs Proposed Site Areas
Appendix E Existing vs Proposed Rate and Volume Summaries
Appendix F MIDS Diagram and Calculation Summary
Stormwater Summary
The Minnetonka Middle School West Gym, Stage, and Science Addition Project in Excelsior, MN is located at 6421
Hazeltine Blvd. This project consists of replacing an existing grass sports field with a synthetic turf field, relocating the
existing running track, and constructing a new gym and science addition.
A HydroCAD stormwater model is being developed to estimate peak runoff rates and volumes for the existing and proposed
drainage conditions. The SCS Curve Number Method was used with rainfall distribution of a Type II 24-hour storm. The
hydrographs were based on the updated NOAA Atlas 14 data for the area. This rainfall data is shown in Appendix A. Curve
Numbers were determined from the National Resource Conservation Service (NRCS) Web Soil Survey, a topographic survey
from Civil Site Group Project Number 23189.02, dated October 11, 2025, aerial images, and the Geotechnical Evaluation
Report from Braun Intertec Project Number B2508902, dated January 27, 2026.
Existing Conditions
The existing project site consists of a ~130,000 SF school and supporting parking facilities along with recreational facilities.
It also includes a grass sports field surrounded by a running track and other track and field components. The draft
Geotechnical Evaluation Report by Braun Intertec indicated that the native soils at this site consist of alluvially deposited
clay loam and organic soils.
According to the survey from Civil Site Group Surveying, the site drains to 6 separate subcatchments. Four of the
subcatchments drain to onsite BMP’s (Subcatchments 1-4), and 2 drain offsite untreated (5&6). A 7th Subcatchment in the
NW portion of the site drains overland and is conveyed by City storm drain at Hazeltine Blvd. See the Existing Drainage
Map in Appendix C.
The site is located in Minnehaha Creek Watershed District (MCWD) and therefore follows the MCWD permitting
regulations. The majority of the site drains to ditch on Hazeltine Blvd which discharges to a pond south of Lake Luc y Road.
This pond is within the hydrological boundary in MCWD but is within the jurisdictional boundary of Riley Purgatory Creek.
LIDAR TOPOGRAPHY (from MNTOPO) HYDROLOGICAL AND DISTRICT BOUNDARIES
(from MNTOPO)
Proposed Conditions
The proposed site modifications consist of replacing the grass sports field and the running track with a new synthetic turf
field and the running track. This is designated on both the Existing and Proposed Drainage Maps as Subcatchment 6S. This
will change the hydrologic boundary in the SE corner of the site closer to the jurisdictional boundary line due to the new
sports field drainage now being directed west. Also apart of the improvements are 2 new building additions and associated
parking.
Three new stormwater management facilities will also be constructed. The first is an underground filtration system (Pond
3A-P) under the sports field, the second/third (Pond 3B-P and Pond 7) are above ground filtration systems that will treat
roof and parking runoff. See the Proposed Drainage Map in Appendix C.
Stormwater Objectives
Based on the site size, the disturbance and the amount of increased impervious surface, the Minnehaha Creek Watershed
District requires phosphorus control, rate control, and volume control for the additional impervious surface. With the site
being predominantly D soil, filtration will be considered versus infiltration. For filtration, the required volume is for 2” of
runoff (calculation for 2” treatment in Appendix B). Total Phosphorus (at least 60%), Total Suspended Solids (at least 90%) ,
and rate control will be provided using filtration.
The size of the site is 30.7 acres, with 7.7 acres of existing impervious surfaces (7.4 acres of buildings and pavements, 0.3
acres of water). Approximately 8.2 acres will be disturbed on the site, which is less than 40% of the site (8.2/30.7=27%).
There will be an increase of approximately 2.9 acres of impervious surface with the synthetic turf, new addition and parking
improvements, the total of which is less than a 50% increase in impervious surface.
For the treatment under the field, the aggregates and sand are modeled as having 30% voids. Providing a minimum of 3ft of
sand for filtration beneath the synthetic turf, there will be 80,640 cf of filtration provided. The volume requirement is
obtained in the first foot on the sand.
The volume control requirement in the synthetic turf area will require 8,000 cf of abstraction. Fifty percent of the filtration
volume equates to an abstraction volume credit of 40,320cf, which is higher than the requirement.
Filtration Basin #3 provides the required storage for the ~70,500 SF of impervious that is routed to it, plus additional storage
that will allow ~18,400 SF of impervious in the future.
Filtration calculations are shown in Appendix B. HydroCAD calculations summaries are provided Appendix E and
demonstrate that flow rate is reduced in the 2,10, and 100 yr storm events to all 3 points of discharge. The volume in 6S is
reduced due to the reduction in area free drains offsite, but the total volume does increase due to the added impervious. The
full HydroCad calculations are provided in a separate document.
A MIDS model was completed to determine TP and TSS removal for the 3 new BMP’s. Assuming that all of the new and
disturbed ground is routed to the new BMP’s, the TSS removal is 9 5% and TP is 86%. The undisturbed ground that is
currently routed to existing BMP’s was ignored for this calculation. The MIDS summary can be found in Appendix F.
Erosion and Sediment Control
Temporary Control -A construction exit will be installed. Seeding will be used on disturbed areas to prevent erosion,
and to allow permanent stabilization of the grassy areas in the future. Inlet protection will be used at all inlets to prevent
sediment from entering into the storm sewers. Temporary soil stockpiles will have silt fence or other effective sediment
controls, and will not be placed in surface waters, including storm water conveyances such as conduits and ditches.
Stockpiles that remain on the site for more than 7 days shall be seeded, mulched, and surrounded by silt fence.
Permanent Control – Natural turf will be established for complete stabilization on all disturbed pervious areas
outside of the track and field. Synthetic turf will be placed over the aggregates within the track.
Appendix A: Rainfall Analysis Data
Rainfall Data
Storm Event Rainfall
(in)
1-Year 2.47
2-Year 2.84
10-Year 4.20
100-Year 7.18
Appendix B: Water Quality Volume Calculations
NOTE: Additional volume in 3B-P allows for 18,413 SF of future impervious
NOTE: Additional volume in 7-P allows for 236 SF of future impervious
Appendix C: Drainage Maps
EXISTING DRAINAGE MAP
PROPOSED DRAINAGE MAP
Appendix D:
EXISTING VS PROPOSED SITE AREAS
Appendix E:
EXISTING VS PROPOSED RATE AND VOLUME SUMMARIES
Appendix F:
MIDS DIAGRAM AND CALCULATION SUMMARY