20272001_A1_Attachment C_Cuthbert Stormwater Report.pdf

Cuthbert Amphitheater Site Improvements – Eugene, Oregon Stormwater Report

June 22, 2026

Contents

1.0 Project Overview and Description

1.1. Executive Summary

1.2. Size and Location of Project Site

1.3. Existing Site Conditions

1.4. Watershed Description

1.5. Permits Required

2.0 Methodology

2.1. Drainage at Existing Site

2.2. Potential Impacts

2.3. Mitigation Techniques

2.4. Depth to Groundwater

2.5. Infiltration Testing

2.6. Proposed Stormwater Management

3.0 Analysis

3.1. Design Assumptions

3.2. Computation Methods and Software Used

3.3. Basin Analysis

3.4. Hydrology Calculations

3.5. Conveyance Requirements and Design

3.6. Flood Control Point of Discharge and Receiving System Capacity

4.0 Conclusions

4.1. Compliance with Stormwater Management Manual

4.2. Requirements Satisfied

Tables Table 3-1: Hydrologic Soil Group Ratings Table 3-2: Rainfall Intensities Table 3-3: Post-Developed Flow Rate Summary Table 3-4: Pre- and Post-Developed Flow Rates

Figures

Figure 1: Pre-Developed Basin Map Figure 2: Post-Developed Basin Map

Appendices Appendix A: HydroCAD Summary Appendix B: Web Soil Survey Appendix C: Geotechnical Report Appendix D: Facility Sizing – Presumptive Approach

STORMWATER REPORT

Cuthbert Amphitheater Site Improvements

Eugene, Oregon

1.0 Project Overview and Description

1.1. Executive Summary

This project consists of improvements to the Cuthbert Amphitheater (Cuthbert) within Alton Baker Park.

Improvements include construction of approximately 69 new paved parking spaces, new sidewalk connections to the pedestrian entry, resurfacing of the gravel backstage area, and utility relocation.

Stormwater from new impervious surfaces will be treated and managed using an Dry Detention Pond (Extended Filtration Facility) designed in accordance with the 2025 City of Eugene Stormwater Management Manual.

The facility provides water quality treatment, supplemental infiltration, and controlled discharge to the existing channel north of the site. The design meets all applicable requirements for water quality, flow control (where required), and flood control.

1.2. Size and Location of Project Site

The project is located in Alton Baker Park along Day Island Road in Eugene, Oregon, Tax Lot 300, Lane County Assessor’s Map 17 03 29 34. The total site area is approximately 6 acres.

1.3. Existing Site Conditions

Currently, the site contains an amphitheater, a gravel area for travel busses, and a grassy area behind the seating bowl of the amphitheater. A gravel path connects Day Island Road to the ticket booth and runs along the chain link fence. Pre’s Trail, a wood chip jogging trail, crosses the site.

1.4. Watershed Description

The site lies within Basin WKQF-020 per the City of Eugene Storm Drainage Basins – HUB. The amphitheater is located within FEMA Flood Zone AE.

1.5. Permits Required

The project requires:

City of Eugene Commercial Erosion Prevention Permit

DEQ 1200-C(A) Erosion and Sediment Control Plan (ESCP)

2.0 Methodology

2.1. Drainage at Existing Site

The southern portion of the site drains toward an existing wetland west of the amphitheater, sloping from approximately elevation 423.5 to 420.0. The northern portion drains toward a channel behind the amphitheater at elevation ~417.0. The site ultimately drains to the Willamette River.

Soils, per the Natural Resources Conservation Service (NRCS) Soil Resource Web Survey (Appendix B), include Camas gravelly sandy loam and Newberg fine sandy loam (HSG A) and Cloquato silt loam (HSG B).

2.2. Potential Impacts

Project improvements increase impervious area by approximately 50,000 square feet, resulting in:

Increased runoff volume and peak flow rates

Potential for increased pollutant loading (sediment, hydrocarbons, nutrients, metals)

Reduced time of concentration

Potential impacts to the downstream channel if unmanaged

These impacts are mitigated through the proposed Dry Detention Pond, vegetated filtration strips, and controlled overflow routing.

2.3. Mitigation Techniques

Mitigation measures include:

Dry Detention Pond providing water quality treatment and extended detention

Filtration strips for areas not tributary to the pond

Emergency overflow routed to the existing channel north of the site

Erosion control BMPs during construction

2.4. Depth to Groundwater

A geotechnical investigation was done by Foundation Engineering, Inc. on June 17, 2025. At the time of drilling, groundwater was not encountered within any of the test hole explorations. Refer to page 3 of the Geotechnical Report (Appendix A) for additional depth to groundwater discussion.

2.5. Infiltration Testing

Infiltration testing was performed by Foundation Engineering, Inc. on June 18, 2025, at three locations.

Testing was performed at depths from ±1 to ±3 feet below existing site grades. The subgrade soil encountered within the test holes consisted of sandy silt and silty sand. The infiltration rate used for this analysis is 0.5 in/hr.

While the infiltration rate used for this analysis is at the minimum allowed rate for a dedicated infiltration facility, a Dry Detention Pond was selected and sized with the understanding that infiltration will continue to occur.

2.6. Proposed Stormwater Management

Stormwater will be managed using:

Dry Detention Pond sized to treat runoff from ~38,000 sq ft of impervious area

Filtration strips for remaining impervious areas

Controlled overflow to the existing channel

The pond is located to allow future expansion if the parking lot is enlarged.

3.0 Analysis

3.1. Design Assumptions

The design of the stormwater system was analyzed for runoff generated by the 10-year storm event per 2025 City of Eugene Stormwater Management Manual. The 10-year storm is 4.46 inches in a 24-hour period for flood control.

The following table outlines the Hydrologic Soil Group rating for the soil types:

Table 3-1: Hydrologic Soil Group Ratings NRCS Map Unit Identification

NRCS Soil Classification (Percentage of Site)

Hydrologic Soil Group Rating

22 Camas Gravelly Sandy Loam (3.7%) A 29 Cloquato Silt Loam (2.3%) B 95 Newberg Fine Sandy Loam (94.1%) A

Runoff curve number (CN) for the developed conditions were obtained from Table C-2 of the City of Eugene Stormwater Management Manual. The following curve number (CN) was used for this analysis:

98 for Paved Parking Lots, roofs, driveways, etc. for Hydrologic Group A/B 76 for Gravel for Hydrologic Group A 72 for Dirt (Pre-Trail) for Hydrologic Group A

A time of concentration of 6 minutes was used for the post-developed runoff rate.

Table 3-2 summarizes the 24-hour rainfall intensities that were used for each design storm.

Table 3-2: Rainfall Intensities Recurrence Interval (Years) Total Precipitation Depth (Inches)

Water Quality 1.40 2-year 3.12 5-year 3.60

10-year 4.46 25-year 5.18

100-year 6.48

3.2. Computation Methods and Software Used

The design storm events were analyzed with HydroCAD (v10.0.22) using the Santa Barbara Urban Hydrograph (SBUH) method to generate the runoff generated from the events. The stormwater pond has been provided to fully infiltrate the 10-year storm event.

3.3. Basin Analysis

The project site was divided into four drainage basins — Basin 2S, Basin 3S, Basin 4S, and Basin 5S see figure 1 — for the purposes of stormwater flow control and water quality treatment analysis. Basin 2S encompasses the proposed sidewalk and parking lot areas, where runoff is collected via catch basins and conveyed to a dry detention pond. The pond is designed to provide both flow control and stormwater treatment. Runoff from concrete paths in Basins 3S and 4S is directed over filtration strips, which provide stormwater treatment through surface filtration. Basin 5S consists primarily of gravel surface areas, where runoff is managed through sheet flow dispersion over adjacent landscaped areas, allowing for natural filtration and passive infiltration into the underlying soils.

3.4. Hydrology Calculations

Table 3-3 below summarizes the post-developed flow rate and basin areas for the proposed site.

Table 3-3: Post-Developed Flow Rate Summary

Basin Area (Acres)

Curve Number

(CN)

Design Storm Event

Time of Concentration

(minutes)

Rainfall Intensity (in/hr)

Runoff Rate (cubic feet per second)

1.0 98 Water Quality 6.0 1.40 0.02 2-year 6.0 3.12 0.04 5-year 6.0 3.60 0.05 10-year 6.0 4.46 0.07

The Dry Detention Pond was sized using:

Impervious area: ~38,000 sq ft

Infiltration rate: 0.5 in/hr with factor of safety = 2

Water quality storm: 1.4 inches

Flood control storm: 10-year, 4.46 inches

3.5. Conveyance Requirements and Design

See Table 3-4 for the comparison between pre- and post-developed flow rates.

Table 3-4: Pre- and Post-Developed Flow Rates Design Storm Pre-Developed Flow Rate (cfs) Post-Developed Flow Rate (cfs)

2-year 0.04 0.00 5-year 0.07 0.00

10-year 0.15 0.00

3.6. Flood Control Point of Discharge and Receiving System Capacity Emergency overflow is directed north of the stormwater pond to the channel and away from the future parking lot. The channel has adequate capacity based on observed conditions and City drainage mapping.

4.0 Conclusions

4.1. Compliance with Stormwater Management Manual

The project complies with the 2025 City of Eugene Stormwater Management Manual through:

Use of an approved Extended Filtration Facility

Proper sizing for water quality treatment

Adequate conveyance and overflow routing

Avoidance of increased peak flows

4.2. Requirements Satisfied

Water Quality: Fully satisfied via Dry Detention Pond

Flow Control: No increase in peak flows

Flood Control: 10-year storm fully infiltrated with overflow routed to existing channel

CHKD:

DRWN:

DATE:

AKS JOB:

DW

G:

BA

SI

N

M

AP

S

FI

G

06/19/2026

12082

ADM

RCW

PRE-DEVELOPED BASIN MAP

FIGURE

1CUTHBERT AMPHITHEATER SITE IMPROVEMENTS

AKS ENGINEERING AND FORESTRY, LLC

3700 RIVER RD N, STE 1

KEIZER, OR 97303

503-400-6028 WWW.AKS-ENG.COM

LEGEND

CHKD:

DRWN:

DATE:

AKS JOB:

DW

G:

BA

SI

N

M

AP

S

FI

G

06/19/2026

12082

ADM

RCW

POST-DEVELOPED BASIN MAP

FIGURE

2CUTHBERT AMPHITHEATER SITE IMPROVEMENTS

AKS ENGINEERING AND FORESTRY, LLC

3700 RIVER RD N, STE 1

KEIZER, OR 97303

503-400-6028 WWW.AKS-ENG.COM

LEGEND

Appendix A: HydroCAD Summary

1S

Pre-Developed Basin

2S

PARKING LOT

1P

DRY DETENTION

POND

Routing Diagram for 12082 SW Calcs Prepared by AKS Engineering & Forestry, Printed 6/19/2026

HydroCAD® 10.00-22 s/n 01338 © 2018 HydroCAD Software Solutions LLC

Subcat Reach Pond Link

Type IA 24-hr 2-YR Rainfall=3.12"12082 SW Calcs Printed 6/19/2026Prepared by AKS Engineering & Forestry

Page 3HydroCAD® 10.00-22 s/n 01338 © 2018 HydroCAD Software Solutions LLC

Summary for Subcatchment 2S: PARKING LOT

Runoff = 0.50 cfs @ 7.94 hrs, Volume= 0.165 af, Depth= 2.01"

Runoff by SCS TR-20 method, UH=SCS, Weighted-CN, Time Span= 0.00-60.00 hrs, dt= 0.05 hrs Type IA 24-hr 2-YR Rainfall=3.12"

Area (sf) CN Description

* 36,155 98

* 6,700 39

42,855 89 Weighted Average 6,700 15.63% Pervious Area

36,155 84.37% Impervious Area

Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs)

6.0 Direct Entry, Subcatchment 2S: PARKING LOT

Runoff

Hydrograph

Time (hours) 605856545250484644424038363432302826242220181614121086420

F lo w

(c fs

0.55

0.5

0.45

0.4

0.35

0.3

0.25

0.2

0.15

0.1

0.05

Type IA 24-hr

2-YR Rainfall=3.12"

Runoff Area=42,855 sf

Runoff Volume=0.165 af

Runoff Depth=2.01"

Tc=6.0 min

CN=89

0.50 cfs

Page 4HydroCAD® 10.00-22 s/n 01338 © 2018 HydroCAD Software Solutions LLC

Summary for Pond 1P: DRY DETENTION POND

Inflow Area = 0.984 ac, 84.37% Impervious, Inflow Depth = 2.01" for 2-YR event Inflow = 0.50 cfs @ 7.94 hrs, Volume= 0.165 af Outflow = 0.04 cfs @ 24.06 hrs, Volume= 0.143 af, Atten= 93%, Lag= 967.4 min Discarded = 0.04 cfs @ 24.06 hrs, Volume= 0.143 af Primary = 0.00 cfs @ 0.00 hrs, Volume= 0.000 af

Routing by Dyn-Stor-Ind method, Time Span= 0.00-60.00 hrs, dt= 0.05 hrs Peak Elev= 419.52' @ 24.06 hrs Surf.Area= 3,235 sf Storage= 4,891 cf

Plug-Flow detention time= 1,203.2 min calculated for 0.143 af (87% of inflow) Center-of-Mass det. time= 1,118.4 min ( 1,875.8 - 757.4 )

Volume Invert Avail.Storage Storage Description

#1 417.50' 15,390 cf Custom Stage Data (Prismatic) Listed below (Recalc)

Elevation Surf.Area Inc.Store Cum.Store (feet) (sq-ft) (cubic-feet) (cubic-feet)

417.50 1,605 0 0

422.00 5,235 15,390 15,390

Device Routing Invert Outlet Devices

#1 Discarded 417.50' 0.500 in/hr Exfiltration over Horizontal area #2 Primary 420.75' 10.0' long Sharp-Crested Rectangular Weir 2 End Contraction(s)

Discarded OutFlow Max=0.04 cfs @ 24.06 hrs HW=419.52' (Free Discharge) 1=Exfiltration (Exfiltration Controls 0.04 cfs)

Primary OutFlow Max=0.00 cfs @ 0.00 hrs HW=417.50' (Free Discharge) 2=Sharp-Crested Rectangular Weir ( Controls 0.00 cfs)

Page 5HydroCAD® 10.00-22 s/n 01338 © 2018 HydroCAD Software Solutions LLC

Pond 1P: DRY DETENTION POND

Inflow Outflow Discarded Primary

Hydrograph

Time (hours) 605856545250484644424038363432302826242220181614121086420

F

0.55

0.5

0.45

0.4

0.35

0.3

0.25

0.2

0.15

0.1

0.05

Inflow Area=0.984 ac

Peak Elev=419.52'

Storage=4,891 cf

0.50 cfs

0.04 cfs

0.04 cfs

0.00 cfs

Type IA 24-hr 5-YR Rainfall=3.60"12082 SW Calcs

Page 6HydroCAD® 10.00-22 s/n 01338 © 2018 HydroCAD Software Solutions LLC

Summary for Subcatchment 1S: Pre-Developed Basin

Runoff = 0.07 cfs @ 22.10 hrs, Volume= 0.056 af, Depth= 0.12"

Runoff by SCS TR-20 method, UH=SCS, Weighted-CN, Time Span= 0.00-60.00 hrs, dt= 0.05 hrs Type IA 24-hr 5-YR Rainfall=3.60"

Area (ac) CN Description

* 0.100 39

* 0.100 61

* 4.400 39

* 0.900 76

* 0.100 72

5.600 46 Weighted Average

5.600 100.00% Pervious Area

Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs)

41.5 Direct Entry, Subcatchment 1S: Pre-Developed Basin

Runoff

Hydrograph

Time (hours) 605856545250484644424038363432302826242220181614121086420

F

0.08

0.075

0.07

0.065

0.06

0.055

0.05

0.045

0.04

0.035

0.03

0.025

0.02

0.015

0.01

0.005

Type IA 24-hr

5-YR Rainfall=3.60"

Runoff Area=5.600 ac

Runoff Volume=0.056 af

Runoff Depth=0.12"

Tc=41.5 min

CN=46

0.07 cfs

Page 7HydroCAD® 10.00-22 s/n 01338 © 2018 HydroCAD Software Solutions LLC

Summary for Subcatchment 2S: PARKING LOT

Runoff = 0.62 cfs @ 7.93 hrs, Volume= 0.201 af, Depth= 2.45"

Runoff by SCS TR-20 method, UH=SCS, Weighted-CN, Time Span= 0.00-60.00 hrs, dt= 0.05 hrs Type IA 24-hr 5-YR Rainfall=3.60"

Area (sf) CN Description

* 36,155 98

* 6,700 39

42,855 89 Weighted Average 6,700 15.63% Pervious Area

36,155 84.37% Impervious Area

Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs)

6.0 Direct Entry, Subcatchment 2S: PARKING LOT

Runoff

Hydrograph

Time (hours) 605856545250484644424038363432302826242220181614121086420

F

0.65

0.6

0.55

0.5

0.45

0.4

0.35

0.3

0.25

0.2

0.15

0.1

0.05

Type IA 24-hr

5-YR Rainfall=3.60"

Runoff Area=42,855 sf

Runoff Volume=0.201 af

Runoff Depth=2.45"

Tc=6.0 min

0.62 cfs

Page 8HydroCAD® 10.00-22 s/n 01338 © 2018 HydroCAD Software Solutions LLC

Summary for Pond 1P: DRY DETENTION POND

Inflow Area = 0.984 ac, 84.37% Impervious, Inflow Depth = 2.45" for 5-YR event Inflow = 0.62 cfs @ 7.93 hrs, Volume= 0.201 af Outflow = 0.04 cfs @ 24.07 hrs, Volume= 0.159 af, Atten= 93%, Lag= 968.4 min Discarded = 0.04 cfs @ 24.07 hrs, Volume= 0.159 af Primary = 0.00 cfs @ 0.00 hrs, Volume= 0.000 af

Routing by Dyn-Stor-Ind method, Time Span= 0.00-60.00 hrs, dt= 0.05 hrs Peak Elev= 419.92' @ 24.07 hrs Surf.Area= 3,556 sf Storage= 6,243 cf

Plug-Flow detention time= 1,271.3 min calculated for 0.159 af (79% of inflow) Center-of-Mass det. time= 1,141.9 min ( 1,888.4 - 746.5 )

Volume Invert Avail.Storage Storage Description

#1 417.50' 15,390 cf Custom Stage Data (Prismatic) Listed below (Recalc)

Elevation Surf.Area Inc.Store Cum.Store (feet) (sq-ft) (cubic-feet) (cubic-feet)

417.50 1,605 0 0

422.00 5,235 15,390 15,390

Device Routing Invert Outlet Devices

#1 Discarded 417.50' 0.500 in/hr Exfiltration over Horizontal area #2 Primary 420.75' 10.0' long Sharp-Crested Rectangular Weir 2 End Contraction(s)

Discarded OutFlow Max=0.04 cfs @ 24.07 hrs HW=419.92' (Free Discharge) 1=Exfiltration (Exfiltration Controls 0.04 cfs)

Page 9HydroCAD® 10.00-22 s/n 01338 © 2018 HydroCAD Software Solutions LLC

Pond 1P: DRY DETENTION POND

Inflow Outflow Discarded Primary

Hydrograph

Time (hours) 605856545250484644424038363432302826242220181614121086420

F

0.65

0.6

0.55

0.5

0.45

0.4

0.35

0.3

0.25

0.2

0.15

0.1

0.05

Inflow Area=0.984 ac

Peak Elev=419.92'

Storage=6,243 cf

0.62 cfs

0.04 cfs

0.04 cfs

Type IA 24-hr 10-YR Rainfall=4.46"12082 SW Calcs

Page 10HydroCAD® 10.00-22 s/n 01338 © 2018 HydroCAD Software Solutions LLC

Summary for Subcatchment 1S: Pre-Developed Basin

Runoff = 0.15 cfs @ 19.94 hrs, Volume= 0.150 af, Depth= 0.32"

Runoff by SCS TR-20 method, UH=SCS, Weighted-CN, Time Span= 0.00-60.00 hrs, dt= 0.05 hrs Type IA 24-hr 10-YR Rainfall=4.46"

Area (ac) CN Description

* 0.100 39

* 0.100 61

* 4.400 39

* 0.900 76

* 0.100 72

5.600 46 Weighted Average

5.600 100.00% Pervious Area

Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs)

41.5 Direct Entry, Subcatchment 1S: Pre-Developed Basin

Runoff

Hydrograph

Time (hours) 605856545250484644424038363432302826242220181614121086420

F

0.16

0.15

0.14

0.13

0.12

0.11

0.1

0.09

0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.01

Type IA 24-hr

10-YR Rainfall=4.46"

Runoff Area=5.600 ac

Runoff Volume=0.150 af

Runoff Depth=0.32"

Tc=41.5 min

0.15 cfs

Page 11HydroCAD® 10.00-22 s/n 01338 © 2018 HydroCAD Software Solutions LLC

Summary for Subcatchment 2S: PARKING LOT

Runoff = 0.84 cfs @ 7.91 hrs, Volume= 0.267 af, Depth= 3.26"

Runoff by SCS TR-20 method, UH=SCS, Weighted-CN, Time Span= 0.00-60.00 hrs, dt= 0.05 hrs Type IA 24-hr 10-YR Rainfall=4.46"

Area (sf) CN Description

* 36,155 98

* 6,700 39

42,855 89 Weighted Average 6,700 15.63% Pervious Area

36,155 84.37% Impervious Area

Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs)

6.0 Direct Entry, Subcatchment 2S: PARKING LOT

Runoff

Hydrograph

Time (hours) 605856545250484644424038363432302826242220181614121086420

F

0.9

0.85

0.8

0.75

0.7

0.65

0.6

0.55

0.5

0.45

0.4

0.35

0.3

0.25

0.2

0.15

0.1

0.05

Type IA 24-hr

10-YR Rainfall=4.46"

Runoff Area=42,855 sf

Runoff Volume=0.267 af

Runoff Depth=3.26"

Tc=6.0 min

0.84 cfs

Page 12HydroCAD® 10.00-22 s/n 01338 © 2018 HydroCAD Software Solutions LLC

Summary for Pond 1P: DRY DETENTION POND

Inflow Area = 0.984 ac, 84.37% Impervious, Inflow Depth = 3.26" for 10-YR event Inflow = 0.84 cfs @ 7.91 hrs, Volume= 0.267 af Outflow = 0.05 cfs @ 24.08 hrs, Volume= 0.186 af, Atten= 94%, Lag= 969.7 min Discarded = 0.05 cfs @ 24.08 hrs, Volume= 0.186 af Primary = 0.00 cfs @ 0.00 hrs, Volume= 0.000 af

Routing by Dyn-Stor-Ind method, Time Span= 0.00-60.00 hrs, dt= 0.05 hrs Peak Elev= 420.58' @ 24.08 hrs Surf.Area= 4,087 sf Storage= 8,757 cf

Plug-Flow detention time= 1,353.3 min calculated for 0.185 af (69% of inflow) Center-of-Mass det. time= 1,173.0 min ( 1,904.6 - 731.6 )

Volume Invert Avail.Storage Storage Description

#1 417.50' 15,390 cf Custom Stage Data (Prismatic) Listed below (Recalc)

Elevation Surf.Area Inc.Store Cum.Store (feet) (sq-ft) (cubic-feet) (cubic-feet)

417.50 1,605 0 0

422.00 5,235 15,390 15,390

Device Routing Invert Outlet Devices

#1 Discarded 417.50' 0.500 in/hr Exfiltration over Horizontal area #2 Primary 420.75' 10.0' long Sharp-Crested Rectangular Weir 2 End Contraction(s)

Discarded OutFlow Max=0.05 cfs @ 24.08 hrs HW=420.58' (Free Discharge) 1=Exfiltration (Exfiltration Controls 0.05 cfs)

Page 13HydroCAD® 10.00-22 s/n 01338 © 2018 HydroCAD Software Solutions LLC

Pond 1P: DRY DETENTION POND

Inflow Outflow Discarded Primary

Hydrograph

Time (hours) 605856545250484644424038363432302826242220181614121086420

F

0.9

0.85

0.8

0.75

0.7

0.65

0.6

0.55

0.5

0.45

0.4

0.35

0.3

0.25

0.2

0.15

0.1

0.05

Inflow Area=0.984 ac

Peak Elev=420.58'

Storage=8,757 cf

0.84 cfs

0.05 cfs

0.05 cfs

Type IA 24-hr WQ Rainfall=1.40"12082 SW Calcs

Page 14HydroCAD® 10.00-22 s/n 01338 © 2018 HydroCAD Software Solutions LLC

Summary for Subcatchment 1S: Pre-Developed Basin

[45] Hint: Runoff=Zero

Runoff = 0.00 cfs @ 0.00 hrs, Volume= 0.000 af, Depth= 0.00"

Runoff by SCS TR-20 method, UH=SCS, Weighted-CN, Time Span= 0.00-60.00 hrs, dt= 0.05 hrs Type IA 24-hr WQ Rainfall=1.40"

Area (ac) CN Description

* 0.100 39

* 0.100 61

* 4.400 39

* 0.900 76

* 0.100 72

5.600 46 Weighted Average

5.600 100.00% Pervious Area

Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs)

41.5 Direct Entry, Subcatchment 1S: Pre-Developed Basin

Runoff

Hydrograph

Time (hours) 605856545250484644424038363432302826242220181614121086420

F

Type IA 24-hr

WQ Rainfall=1.40"

Runoff Area=5.600 ac

Runoff Volume=0.000 af

Runoff Depth=0.00"

Tc=41.5 min

Page 15HydroCAD® 10.00-22 s/n 01338 © 2018 HydroCAD Software Solutions LLC

Summary for Subcatchment 2S: PARKING LOT

Runoff = 0.12 cfs @ 7.99 hrs, Volume= 0.046 af, Depth= 0.56"

Runoff by SCS TR-20 method, UH=SCS, Weighted-CN, Time Span= 0.00-60.00 hrs, dt= 0.05 hrs Type IA 24-hr WQ Rainfall=1.40"

Area (sf) CN Description

* 36,155 98

* 6,700 39

42,855 89 Weighted Average 6,700 15.63% Pervious Area

36,155 84.37% Impervious Area

Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs)

6.0 Direct Entry, Subcatchment 2S: PARKING LOT

Runoff

Hydrograph

Time (hours) 605856545250484644424038363432302826242220181614121086420

F

0.13

0.125 0.12

0.115

0.11

0.105

0.1

0.095

0.09

0.085

0.08

0.075 0.07

0.065

0.06

0.055

0.05

0.045

0.04

0.035

0.03

0.025 0.02

0.015

0.01

0.005

Type IA 24-hr

WQ Rainfall=1.40"

Runoff Area=42,855 sf

Runoff Volume=0.046 af

Runoff Depth=0.56"

Tc=6.0 min

0.12 cfs

Page 16HydroCAD® 10.00-22 s/n 01338 © 2018 HydroCAD Software Solutions LLC

Summary for Pond 1P: DRY DETENTION POND

Inflow Area = 0.984 ac, 84.37% Impervious, Inflow Depth = 0.56" for WQ event Inflow = 0.12 cfs @ 7.99 hrs, Volume= 0.046 af Outflow = 0.02 cfs @ 21.91 hrs, Volume= 0.046 af, Atten= 81%, Lag= 835.6 min Discarded = 0.02 cfs @ 21.91 hrs, Volume= 0.046 af Primary = 0.00 cfs @ 0.00 hrs, Volume= 0.000 af

Routing by Dyn-Stor-Ind method, Time Span= 0.00-60.00 hrs, dt= 0.05 hrs Peak Elev= 417.87' @ 21.91 hrs Surf.Area= 1,905 sf Storage= 652 cf

Plug-Flow detention time= (not calculated: outflow precedes inflow) Center-of-Mass det. time= 342.8 min ( 1,178.0 - 835.2 )

Volume Invert Avail.Storage Storage Description

#1 417.50' 15,390 cf Custom Stage Data (Prismatic) Listed below (Recalc)

Elevation Surf.Area Inc.Store Cum.Store (feet) (sq-ft) (cubic-feet) (cubic-feet)

417.50 1,605 0 0

422.00 5,235 15,390 15,390

Device Routing Invert Outlet Devices

#1 Discarded 417.50' 0.500 in/hr Exfiltration over Horizontal area #2 Primary 420.75' 10.0' long Sharp-Crested Rectangular Weir 2 End Contraction(s)

Discarded OutFlow Max=0.02 cfs @ 21.91 hrs HW=417.87' (Free Discharge) 1=Exfiltration (Exfiltration Controls 0.02 cfs)

Page 17HydroCAD® 10.00-22 s/n 01338 © 2018 HydroCAD Software Solutions LLC

Pond 1P: DRY DETENTION POND

Inflow Outflow Discarded Primary

Hydrograph

Time (hours) 605856545250484644424038363432302826242220181614121086420

F

0.13

0.12

0.11

0.1

0.09

0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.01

Inflow Area=0.984 ac

Peak Elev=417.87'

Storage=652 cf

0.12 cfs

0.02 cfs

0.02 cfs

Appendix B: Web Soil Survey

Soil Map—Lane County Area, Oregon

Natural Resources Conservation Service

Web Soil Survey National Cooperative Soil Survey

4/22/2026

493770 493810 493850 493890 493930 493970 494010

493770 493810 493850 493890 493930 493970 494010

44° 3' 26'' N

3°

' 4 0'

' W 44° 3' 26'' N

3°

' 2

8' ' W

44° 3' 14'' N

3°

' 4

0' ' W

44° 3' 14'' N

3°

' 2

8' ' W

N

Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 10N WGS84 0 50 100 200 300

Feet 0 25 50 100 150

Meters Map Scale: 1:1,730 if printed on A portrait (8.5" x 11") sheet.

Soil Map may not be valid at this scale.

MAP LEGEND MAP INFORMATION

Area of Interest (AOI) Area of Interest (AOI)

Soils Soil Map Unit Polygons

Soil Map Unit Lines

Soil Map Unit Points

Special Point Features Blowout

Borrow Pit

Clay Spot

Closed Depression

Gravel Pit

Gravelly Spot

Landfill

Lava Flow

Marsh or swamp

Mine or Quarry

Miscellaneous Water

Perennial Water

Rock Outcrop

Saline Spot

Sandy Spot

Severely Eroded Spot

Sinkhole

Slide or Slip

Sodic Spot

Spoil Area

Stony Spot

Very Stony Spot

Wet Spot

Other

Special Line Features

Water Features Streams and Canals

Transportation Rails

Interstate Highways

US Routes

Major Roads

Local Roads

Background Aerial Photography

The soil surveys that comprise your AOI were mapped at 1:20,000.

Warning: Soil Map may not be valid at this scale.

Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and accuracy of soil line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale.

Please rely on the bar scale on each map sheet for map measurements.

Source of Map: Natural Resources Conservation Service Web Soil Survey URL:

Coordinate System: Web Mercator (EPSG:3857)

Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area, such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required.

This product is generated from the USDA-NRCS certified data as of the version date(s) listed below.

Soil Survey Area: Lane County Area, Oregon Survey Area Data: Version 24, Sep 10, 2025

Soil map units are labeled (as space allows) for map scales 1:50,000 or larger.

Date(s) aerial images were photographed: May 17, 2023—Jun 3, 2023

The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident.

Soil Map—Lane County Area, Oregon

Natural Resources Conservation Service

Web Soil Survey National Cooperative Soil Survey

4/22/2026

Map Unit Legend

Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI

22 Camas gravelly sandy loam, occasionally flooded

0.5 6.2%

29 Cloquato silt loam 0.4 4.4%

95 Newberg fine sandy loam 7.8 89.5%

Totals for Area of Interest 8.7 100.0%

Soil Map—Lane County Area, Oregon

Natural Resources Conservation Service

Web Soil Survey National Cooperative Soil Survey

4/22/2026

Appendix C: Geotechnical Report

Appendix D: Facility Sizing – Presumptive Approach

Stormwater Surface Filtration/Infiltration Facility Sizing Spreadsheet 24 Hour Storm, NRCS Type 1A Rainfall Distribution City of Eugene

Version 2.1 Project Information

Instructions:

Design Requirements:

Choose "Yes" from the dropdown boxes below next to the design standards requirements for this facility.

Pollution Reduction (PR) Yes Flow Control (FC) Yes

Destination (DT) Yes *An infiltration facility must be chosen as the facility type to meet destination requirements

Site Data-Post Development

Total Square Footage Impervious Area= 5144 sqft Total Square Footage Pervious Area= 9153 sqft

Impervious Area CN= 98 Pervious Area CN= 66

Total Square Footage of Drainage Area= 14297 sft Time of Concentration Post Development= 6 min

Weighted Average CN= 78

Site Data-Pre Development (Data in this section is only used if Flow Control is required)

Pre-Development CN= 46 Time of Concentration Pre-Development= 41.5 min

Soil Data

Tested Soil Infiltration Rate= 0.5 in/hr (See Note 4) 0.25 in/hr Design Soil Infiltration Rate= 0.5 in/hr

Design Storms Used For Calculations

Design Storm

1.4 inches Water Quality

3.6 inches Flood Control

3.6 inches Flood Control

Facility Data

Facility Type= Facility Surface Area= 3860 sqft

Surface Width= 20 ft Facility Surface Perimeter= 426 ft

Surface Length= 193 ft Facility Bottom Area= 3860 sqft

Facility Side Slopes= 0 to 1 Facility Bottom Perimeter= 426 ft

2 in Basin Volume= 611.2 cf

18 in Ratio of Facility Area to Impervious Area= 0.270

Permit Number:

Catchment ID:

Destination Design= Soil Infiltration Rate

24-hour Rainfall DepthRequirement

Pollution Reduction

Flow Control

4/27/2026

Depth of Growing Medium (Soil)=

3S & 2P

Date:

1. Complete this form for each drainage catchment in the project site that is to be sized per the Presumptive Approach.

Richard Walker, PE, Principal

AKS Engineering and Forestry

Eugene, OR 97401

Project Address:

Designer:

Company:

Cuthbert Amphitheater Site Improvements

Infiltration Stormwater Planter

Project Name:

200 Day Island Rad

Destination

2. Provide a distinctive Catchment ID for each facility coordinated with the site basin map to correlate the appropriate calculations with the facility.

3. The maximum drainage catchment to be modeled per the Presumptive Approach is 1 acre (43,560 SF)

4.For infiltration facilities in Class A or B soils where no infiltration testing has been perfromed use an infiltration rate of 0.5 in/hr.

For all facilities use a maximum soil infiltration rate of 2.5 in/hr for topsoil/growing medium.

Max. Ponding Depth in Stormwater Facility=

4/24/2026-11:54 AM 1

Pollution Reduction-Calculation Results

Peak Flow Rate to Stormwater Facility = 0.004 cfs Peak Facility Overflow Rate= 0.000 cfs

214 cf Total Overflow Volume= 0 cf

Max. Depth of Stormwater in Facility= 0.0 in

Drawdown Time= 0.2 hours

Yes Facility Sizing Meets Pollution Reduction Standards?

YES Meets Requirement of No Facility Flooding?

YES Meets Requirement for Maximum of 18 Hour Drawdown Time?

Flow Control-Calculation Results

Peak Flow Rate to Stormwater Facility = 0.118 cfs Peak Facility Overflow Rate= 0.000 cfs

1827 cf Total Overflow Volume= 0 cf

Max. Depth of Stormwater in Facility= 0.3 in N\A cfs

Drawdown Time= 0.2 hours

Pre-Development Runoff Data

Peak Flow Rate = 0.005 cfs

Total Runoff Volume = 144 cf

Yes Facility Sizing Meets Flow Control Standards?

YES Meets Requirement for Post Development offsite flow less or equal to Pre-Development Flow?

YES Meets Requirement for Maximum of 18 Hour Drawdown Time?

Destination-Calculation Results

Peak Flow Rate to Stormwater Facility = 0.118 cfs Peak Facility Overflow Rate= 0.000 cfs

1827 cf Total Overflow Volume= 0 cf

Max. Depth of Stormwater in Facility= 1.3 in

Drawdown Time= 4.7 hours

Yes Facility Sizing Meets Destination Standards?

YES Meets Requirement of No Facility Flooding?

YES Meets Requirement for Maximum of 30 hour Drawdown Time?

Total Runoff Volume to Stormwater Facility =

Total Runoff Volume to Stormwater Facility =

Total Runoff Volume to Stormwater Facility =

Peak Off-Site Flow Rate Filtration Facility Underdrain=

4/24/2026-11:54 AM 2

Stormwater Surface Filtration/Infiltration Facility Sizing Spreadsheet 24 Hour Storm, NRCS Type 1A Rainfall Distribution City of Eugene

Version 2.1 Project Information

Instructions:

Design Requirements:

Choose "Yes" from the dropdown boxes below next to the design standards requirements for this facility.

Pollution Reduction (PR) Yes Flow Control (FC) Yes

Destination (DT) Yes *An infiltration facility must be chosen as the facility type to meet destination requirements

Site Data-Post Development

Total Square Footage Impervious Area= 3688 sqft Total Square Footage Pervious Area= 27841 sqft

Impervious Area CN= 98 Pervious Area CN= 62

Total Square Footage of Drainage Area= 31529 sft Time of Concentration Post Development= 6 min

Weighted Average CN= 66

Site Data-Pre Development (Data in this section is only used if Flow Control is required)

Pre-Development CN= 46 Time of Concentration Pre-Development= 41.5 min

Soil Data

Tested Soil Infiltration Rate= 0.5 in/hr (See Note 4) 0.25 in/hr Design Soil Infiltration Rate= 0.5 in/hr

Design Storms Used For Calculations

Design Storm

1.4 inches Water Quality

3.6 inches Flood Control

3.6 inches Flood Control

Facility Data

Facility Type= Facility Surface Area= 4800 sqft

Surface Width= 30 ft Facility Surface Perimeter= 380 ft

Surface Length= 160 ft Facility Bottom Area= 4800 sqft

Facility Side Slopes= 0 to 1 Facility Bottom Perimeter= 380 ft

2 in Basin Volume= 760.0 cf

18 in Ratio of Facility Area to Impervious Area= 0.152

2. Provide a distinctive Catchment ID for each facility coordinated with the site basin map to correlate the appropriate calculations with the facility.

3. The maximum drainage catchment to be modeled per the Presumptive Approach is 1 acre (43,560 SF)

4.For infiltration facilities in Class A or B soils where no infiltration testing has been perfromed use an infiltration rate of 0.5 in/hr.

For all facilities use a maximum soil infiltration rate of 2.5 in/hr for topsoil/growing medium.

Max. Ponding Depth in Stormwater Facility=

Project Address:

Designer:

Company:

Cuthbert Amphitheater Site Improvements

Infiltration Stormwater Planter

Project Name:

200 Day Island Rad

Destination

4/27/2026

Depth of Growing Medium (Soil)=

4S & 3P

Date:

1. Complete this form for each drainage catchment in the project site that is to be sized per the Presumptive Approach.

Richard Walker, PE, Principal

AKS Engineering and Forestry

Eugene, OR 97401

Permit Number:

Catchment ID:

Destination Design= Soil Infiltration Rate

24-hour Rainfall DepthRequirement

Pollution Reduction

Flow Control

5/6/2026-8:26 AM 1

Pollution Reduction-Calculation Results

Peak Flow Rate to Stormwater Facility = 0.003 cfs Peak Facility Overflow Rate= 0.000 cfs

68 cf Total Overflow Volume= 0 cf

Max. Depth of Stormwater in Facility= 0.0 in

Drawdown Time= 0.2 hours

Yes Facility Sizing Meets Pollution Reduction Standards?

YES Meets Requirement of No Facility Flooding?

YES Meets Requirement for Maximum of 18 Hour Drawdown Time?

Flow Control-Calculation Results

Peak Flow Rate to Stormwater Facility = 0.094 cfs Peak Facility Overflow Rate= 0.000 cfs

2263 cf Total Overflow Volume= 0 cf

Max. Depth of Stormwater in Facility= 0.1 in N\A cfs

Drawdown Time= 0.2 hours

Pre-Development Runoff Data

Peak Flow Rate = 0.011 cfs

Total Runoff Volume = 317 cf

Yes Facility Sizing Meets Flow Control Standards?

YES Meets Requirement for Post Development offsite flow less or equal to Pre-Development Flow?

YES Meets Requirement for Maximum of 18 Hour Drawdown Time?

Destination-Calculation Results

Peak Flow Rate to Stormwater Facility = 0.094 cfs Peak Facility Overflow Rate= 0.000 cfs

2263 cf Total Overflow Volume= 0 cf

Max. Depth of Stormwater in Facility= 1.5 in

Drawdown Time= 5.7 hours

Yes Facility Sizing Meets Destination Standards?

YES Meets Requirement of No Facility Flooding?

YES Meets Requirement for Maximum of 30 hour Drawdown Time?

Total Runoff Volume to Stormwater Facility =

Peak Off-Site Flow Rate Filtration Facility Underdrain=

Total Runoff Volume to Stormwater Facility =

Total Runoff Volume to Stormwater Facility =

5/6/2026-8:26 AM 2