Blue_Pool_Prelim_Stormwater_Management_01-12-23.pdf

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661 Phone 360-619-7700

Fax 360-619-7846

PRELIMINARY STORMWATER MANAGEMENT PLAN

To: Mathew Miller, WFLHD Project Manager

From: Vernon Bevan, WFLHD Hydraulics Engineer

Date: January 12, 2023

Project: Blue Pool Safety and Access Enhancement Project, Willamette National

Forest – OR DOT 126(1)

Introduction

The Federal Highway Administration (FHWA), United States Forest Service (USFS), and Oregon

Department of Transportation (ODOT) are partnering to address critical safety and access concerns at the McKenzie River National Recreation Trailhead (McKenzie River Trailhead). The proposed project is located on OR-126 between milepost (MP) 10.5 and 10.8. The highway is a two-lane paved road which provides access to the McKenzie River Trail which leads to the popular destination of the Blue Pool (Tamolitch Falls). The road also provides access to the Eugene Water and Electric Board (EWEB) facilities.

Over the past 10 years there has been a significant increase of visitors and the current trailhead parking is unable to accommodate demand. Overflow parking has resulted in safety concerts along

OR-126 as visitors frequently park on the shoulder and cross the highway at a blind curve in the road. Cars parked along access road block emergency ingress and egress to the trailhead which is need multiple times per year. Parked cars also inhibite access to EWEB facilities located at the southern end of the project area on the west side of the highway.

This report discusses stormwater treatment best management practices (BMPs) relevant for this highway improvement project.

Project Description

This project proposes three measures to mitigate traffic, parking and access issues associated with the McKenzie River Trailhead. The first is construction of a new, paved parking area which would have capacity for approximately 100 vehicles including RVs and trailers. The parking lot complex includes a short connector to the to the trailhead. The proposed area is generally flat and would require standard leveling and drainage. No large cut and fill areas are required. The project site is located west of OR-126 and situated approximately 500 feet east of the McKenzie River.

The second measure is construction of a paved access road (NF-735) between OR-126 and the new parking lot. As peak use of the trailhead often exceeds 100 vehicles, additional space will created along this road for overflow parking.

The project includes improvements to approximately 1,900 feet of OR-126. The main improvement is the creation of a left hand turn land for northeast traffic at the intersection of OR-126 and NF-735.

Memo: Stormwater Management Plan 2

October 14, 2022

Two significant waterbodies are situated in the project area. At the southern end, the McKenzie

River runs along the west side of OR-126 for approximately 600 feet (Station 108+00 to Station

102+00). The channel of the river is located approximately 70 feet from the west shoulder of the highway. On the east side, Payne Creek flows along the edge of the road between Station 108+40 and Station 102+40 before passing under the highway through a 72-inch culvert and discharging to the McKenzie River.

A drainage ditch runs along the eastern edge of the highway from the upper project limit to approximately 108+40 where it enters a 24-inch culvert. Flow from the ditch is carried under OR-

126 where it is dispersed into the forest. This structure will be replaced with a new culvert of the same size with an improved alignment.

Under present conditions, direct discharge from OR-126 does not enter surface waters. The OR-126 roadway is sloped and directs the majority of runoff as sheet flow into adjacent vegetated areas where it infiltrates into undisturbed soils. Some of the runoff may be directed to the drainage ditch where it is carried under the highway and dispersed into undisturbed soil.

Potential Pollutants

During construction, the primary potential pollutant is sediment/total suspended solids originating from soils exposed onsite. Sediment may be entrained in stormwater runoff from areas that are excavated and graded for roadway widening as well as areas that are cleared, excavated and graded for the proposed access road (NF-735) and parking area.

Low concentrations of petroleum products, suspended and dissolved metals, and other pollutants normally found in runoff from roadways should be expected both during and after construction.

Following construction, these pollutants are not expected to exceed those already generated by existing usage.

Receiving Waters

The project area has little to no direct connection with receiving waters as both the McKenzie River and Payne Creek are located sufficiently distant from OR-126 and the proposed parking area. The majority of stormwater runoff from OR-126 and FR-735 will be directed to undisturbed soils as sheet flow where infiltration will occur. Water which has infiltrated on the west side of OR-126 may travel subsurface and eventually reach the McKenzie River.

On the east side of OR-126, runoff may be directed toward the drainage ditch in the northern half of the project area or to undisturbed soils in the southern half. Sheet runoff directed to the ditch will be carried under the highway and dispersed to undisturbed soil where it will infiltrate. From there, it has the potential to travel subsurface and may eventually reach the McKenzie River. Sheet flow in the southern half of the project will be directed to undisturbed soil on the side of the highway.

Stormwater which has infiltrated into the soil in this area may travel as subsurface flow and reach

Payne Creek

The vast majority of stormwater runoff in the parking lot will be directed inwards towards bioswales.

Stormwater which leaves the parking lot will be directed as sheet flow towards undisturbed or re-vegetated soils. Once infiltration has occurred, the water may travel subsurface and discharge to the

McKenzie River.

Memo: Stormwater Management Plan 3

Soil Evaluation

A subsurface investigation was developed to characterize soil conditions and to inform design.

Testing was initially proposed at two locations. However, a field observation following heavy rainfall indicated that soils in the project area highly permeable.

On June 14, 2022, Matt Helstab of the USFS witnessed a high-intensity rainfall event at the project location. According to NOAA estimates, 3.15 inches of rain fell within the project area. This was preceded by 2.30 inches of rain which fell in a 48-hour period between June 11 and June 12 while heavy equipment was on site in support of tree clearing operations. No pooling of water or rutting of soil was observed on June 14, despite the intense rainfall. Based on this observation, it was determined that the soils have a high infiltration capacity and further testing was not required.

Stormwater Treatment Best Management Practices (BMPs)

This project will employ natural dispersion to address the concern that untreated stormwater could enter surface waters such as Payne Creek and the McKenzie River. The natural undisturbed forest soil/duff layer and vegetation within the project corridor provide an important stormwater function by providing for infiltration; nutrient, sediment, and pollutant adsorption; sediment and pollutant biofiltration; water interflow storage and transmission; and pollutant decomposition. Natural sheet flow dispersion onto undisturbed vegetated slopes provides the simplest and most effective method for attenuation and treatment of runoff. Based on field observations, the soils have a high infiltration capacity required for natural dispersion. Constructing engineered stormwater facilities would require clearing additional forest and excavation of previously undisturbed soils. This would produce additional sediment during construction while not providing improved stormwater treatment after construction.

Natural dispersion is appropriate for both OR-126 and NF-735 where roadway runoff flows onto adjacent forested areas. Within the proposed parking area, the majority of stormwater runoff will be directed inwards towards bioswales. Some runoff will be directed towards the parking lot entrance and onto NF-735. Stormwater that reaches the road will be directed as sheet flow to undisturbed soils in the surrounding forest.

Maintenance

Maintenance should be conducted on all BMPs to insure their effective operation and will generally follow the guidelines found in the ODOT’s Road Maintenance Guide (ODOT 2004). Maintenance of vegetation in ODOT’s Road Maintenance Guide is generally described as:

“Activities designed to restore sight distance, minimize or remove shading that may cause icy road conditions, and control or prevent slope failure. These activities are also designed to control unwanted vegetation, control noxious weeds, comply with city, county or local ordinances, reduce fire danger, reduce snowdrift accumulation near roadways, and to maintain a clear zone along the roadway. These actions involve mechanical mowing, trimming, removal of brush and cleanup.”

ODOT will be responsible for the ongoing BMP maintenance on OR-126 while the USFS will be responsible for ongoing BMP maintenance on NF-735 and the parking area. Both agencies will

Memo: Stormwater Management Plan 4 coordinate with WFLHD project managers and to ensure all objectives are being met.

Conclusion

Stormwater treatment proposed for the roadways and parking area provides pollutant removal to the maximum extent practicable while minimizing disturbance of forested areas. Promoting the use of the natural forested areas for dispersion helps to reduce the removal of trees and native soils which would be required to construct additional stormwater treatment facilities.

The proposed biofiltration swales placed within the parking area further minimize cut and fill construction as well as eliminate the need to disturb additional forested areas. These facilities have been designed to provide treatment needed to reduce the amount of pollutants leaving the parking area and from ultimately reaching entering the McKenzie River.

Attachment: Appendix A – Project Area Map cc: Sven Leon, WFLHD Senior Hydraulics Engineer

Appendix A – Project Area Map

BLUE POOL SAFETY AND ACCESS ENHANCEMENT PROJECT AREA

Approximate

Extent of Project

Payne Creek

McKenzie River

Project Location

NF-735

(Access Road)

OR-126

EWEB Facilities