26-10 Car833409A (Carroll) RFB

February 26, 2026
Thrills Hills Wetland Design Report
Site No. CAR833409A
Carroll County, Iowa
Project Overview
This wetland improvement is planned to provide nutrient reduction benefits to a part of the watershed
of an unnamed tributary of Middle Raccoon River in Carroll County, Iowa. Nitrates will be converted to
nitrogen gas through natural processes as the water passes through the wetland.
The project location is on the property of Thrills Hills LLC, roughly four miles southeast of Carroll, Iowa. A
wetland pool will be created by a combination of excavation and the construction of an embankment.
Water from the stream will be pumped into the wetland to fill the pool. Diversion berms are proposed
within the permanent pool to encourage mixing and reduce short-cutting of water out of the wetland.
Water will be released from the pool through the rectangular wetland outlet structure and be carried
through a 36-inch reinforced concrete pipe, to be discharged into the stream. During large storm events,
the pool will rise, and water will flow over the earthen auxiliary spillway, directing the outflow to the
stream. Constructing the wetland will not have adverse effects on the hydrology of the neighboring
properties.
Site Selection and Analysis
This site was deemed viable because of the size of the stream watershed (2,106 acres) and its
topography. Initial payment information was presented to the landowner, who wished to proceed with
the project.
Bolton & Menk reviewed the soil survey data for the location and found the soils to be primarily loam.
The web soil survey data was confirmed with soil samples from three locations on the site. The wetland
watershed was analyzed using LiDAR data. A map of the contributing watersheds is included with this
report. Curve number calculations are provided in the attached hydrology report, and runoff analysis is
provided in the attached HydroCAD output.
The site was surveyed with survey-grade GPS equipment, and additional topographic information was
provided from a publicly available LiDAR database. LiDAR was verified with survey data. Elevations are
referenced to NAVD88, and the Iowa State Plane North (Survey Foot) coordinate system was used for
horizontal coordinates. Survey data was collected electronically and is available in .csv files.
Design Standards
Iowa NRCS Conservation Practice Standard (CPS) 656, dated December 2023, was used to develop this
project. CPS 378 (Pond) was used to design the primary outlet, auxiliary spillway, and the embankment.
The project is NRCS Job Class III under CPS 656 because the direct watershed is less than 300 acres.
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Name: Thrills Hills Wetland Design Report
Date: February 26, 2026
Page: 2
Project Layout
A permanent pool elevation and embankment location were selected based on the existing topography
and the ability to maximize pool area while minimizing excavation and the height of the embankment.
The pool was also sited to minimize impacts to the stream it is adjacent to. There is not an established
regulatory floodway for the stream, but the embankment is out of the channelized area and its only
encroachment into the regulatory floodplain is in a backwater area.
Watersheds were developed using contour and DEM data from a publicly available database. The
wetland watershed is that from which surface water directly enters the wetland pool. The gravel pit
watershed is that from which surface water enters the adjacent existing gravel pit. The gravel pit was
included in the hydraulic analysis but it was shown that it will not overflow into the wetland pool. The
stream watershed, which is used for the wetland treated area analysis, was determined from
StreamStats. The watershed information and rainfall data were used to develop hydrographs in
HydroCAD. The storm distribution used was MSE 3, with rainfall depths from the National Engineering
Handbook. These hydrographs and the proposed stage-storage information were used to size the
primary outlet and auxiliary spillway for the wetland. Exceeding the minimum design standards of NRCS
CPS 378, the auxiliary spillway is placed above the 25-year storm, and the embankment is designed to
provide at least one foot of freeboard during the 100-year storm. HydroCAD outputs are provided in the
appendix.
Dam Regulation
A dam construction permit is required by the Iowa DNR if it (1) has a height of at least 25 feet and a
storage of 15-acre feet at the top of the dam elevation; or (2) has a storage of at least 50 acre-feet at the
top of dam and a height of at least 6 feet; or (3) is assigned a hazard potential of high hazard. This dam
does not require a dam construction permit because the height is 6.5 feet, the storage at the top of the
dam is 15.4 acre-feet, and the hazard potential is low hazard.
The dam is classified as "low hazard" if failure of the dam would result in no probable loss of human life,
low economic losses, and low public damages. This dam meets the requirements of a low hazard dam.
Land Rights and Permits
The permanent easement for this project will be on the property of Thrills Hills LLC. IDALS-DSWQ is
currently in the process of updating the abstracts and will have a signed easement recorded for the area
as shown in the plans. No additional land rights are needed.
The project will be permitted under Army Regional Permit 39. The permit will be obtained by the Iowa
Department of Agriculture and Land Stewardship (IDALS). A floodplain permit application will also be
obtained by IDALS if necessary. Portions of the project are within the regulatory floodplain; there is no
established floodway at this location.
Sincerely,
Bolton & Menk, Inc.
Brandon C. Short, PE
Water Resources Project Engineer
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Name: Thrills Hills Wetland Design Report
Date: January 8, 2026
Page: 3
Appendices:
A: Soil Survey Map
B: Watershed Map
C: Hydrology Report
D: HydroCAD Output
E: Pump Curve
F: Pipe Capacity Calculations
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Name: Thrills Hills Wetland Design Report
Date: January 8, 2026
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Appendix A: Soil Survey Map
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Name: Thrills Hills Wetland Design Report
Date: January 8, 2026
Page: 5
Appendix B: Watershed Map
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Name: Thrills Hills Wetland Design Report
Date: January 8, 2026
Page: 6
Appendix C: Hydrology Report
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Report Summary
Design Parameters and Procedures:
Design hydrology and hydraulic study of this site was completed. The drainage area was determined and a stage-area-storage
curve was developed based on the topographic survey, Lidar data available from the Iowa Department of Natural Resources, and
grading design for the wetland. Land use and treatment were determined through a site visit and aerial photo review. Hydrologic
data was compiled, the structure was flood routed using the SCS TR-55 method in HydroCAD, and all the output was reviewed.
Rainfall events used for this site were MSE 3 distribution, 24-hour, 5-, 10-, 25-, and 100-year events. The primary spillway is a
rectangular outlet structure with six-foot-wide openings on all sides that discharges to the existing stream on site. The auxiliary
spillway is a 30-foot earthen spillway placed above the 10-year rainfall event high water level, that also discharges to the stream.
Hydrology Analysis Summary
25-year Event: 5.70'' 100-year Event: 7.90''
* Hydrologic Soil Groups: B, C, C/D
* Runoff Curve Number: 83.0 (based on Land use shown in this report)
* Primary Spillway Weir Coefficent 3.0
* Tc = 1.0 hours determined by the sum of sheet, shallow concentrated and channel flow times.
* Normal pool area is 3.49 acres.
* Surface watershed area is approx. 50.4 acres or 0.08 square miles.
Summary Table:
Overflow Elevation: 1235.75 (Data for Surface Flow)
Q In Q Out Max Elev Duration - Hours at elevation
max max
(cfs) (cfs) (ft) 1217.50 1218.00 1218.25
5-year 61.99 43.80 1217.69 1.3 0.0 0.0
10-year 80.07 59.84 1217.83 1.7 0.0 0.0
25-year 111.78 87.06 1218.06 2.2 0.5 0.0
100-year 170.73 108.89 1218.72 2.7 1.3 0.8
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IDALS ID Number: CAR833409A
Job: Thrills Hills
BMI Project Number: 25X.137945
Designed By: BCS Date: 1/16/2026
Rainfall Data
Surface-Drained DA = 50 ac
= .08 sq mi
The following rainfall data was used for the hydrology analysis of the site. Rainfall depths from the NEH Title 210
Figure IA2-25 were used, and two additional sources are shown for reference only.
County: Carroll
Storm Distribution: MSE 3
Iowa Climactic Section (SUDAS): West Central
24-hr Rainfall Depths (in)
Return Data Source
Period NEH Title 210 SUDAS Design Manual
(Years) (Figure IA2-25) 2025 Edition NOAA Atlas 14
2 3.00 3.01 3.06
5 3.80 3.74 3.79
10 4.50 4.45 4.52
25 5.70 5.59 5.69
50 6.70 6.58 6.73
100 7.90 7.67 7.88
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IDALS ID Number: CAR833409A
Job: Thrills Hills
BMI Project Number: 25X.137945
Designed By: BCS Date: 1/16/2026
Tributary Watershed Data
Area Area Time of Concentration
Watershed (ac) (sq. mi.) Tile/Surface Drained (min)
Direct 50 0.08 Surface 59.6
Stream (Pumped) 2106 3.29 Tile & Surface
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IDALS ID Number: CAR833409A
Job: Thrills Hills
BMI Project Number: 25X.137945
Designed By: BCS Date: 1/16/2026
Runoff Curve Number
Surface-Drained DA = 50 ac
= .08 sq mi
Soil Portion
Name1 HSG1
Cover Type and Hyd. Condition of W/S
CN2
Product
percent
Ida silt loam; Terril loam B Straight Row Crops, Good 28.7% 78 22.386
Colo silt loam; et al. C Stright Row Crops, Good 37.7% 85 32.045
Burchard-Adair clay loams; et al. C/D Straight Row Crops, Good 33.6% 85 28.560
Totals: 100.0% 83.0
References
1 USDA - NRCS Soil Survey of Carroll Co., Iowa.
2 USDA - NRCS Technical Release 55, Chapter 2, June 1986.
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