Flood Control Zone District

History of Slide Ridge

Located on the east facing slope on the Chelan Mountain terrain, Slide Ridge, also known as Granite Ridge, produces frequent, violent discharges of soil and rocky material. Hundreds of years of these violent discharges created an alluvial fan, or a fan-shaped deposit of soil and rocks, at Lake Chelan.

The elevation change in the Slide Ridge area is drastic, from 1,100 feet at the edge of Lake Chelan to 1,800 feet at the apex of the alluvial fans and over 5,000 feet at the uppermost relief that contributes direct water runoff to the alluvial fan area at Lake Chelan. 

The slide area is frequently hit by intense rain and thunderstorms that deliver more than an inch of rainfall in an hour’s time. Rainfall on Slide Ridge results in water flowing down its steep slopes, where it dislodges rock down the slope to Lake Chelan. The storms have left a mix of woody, rocky and muddy debris as high as 12 feet on South Lakeshore Road, the only north-south corridor for residents living along the west side of Lake Chelan, one of the largest tourist destinations in the state.

In 1995, in attempts to better protect public and private property, an earthen-engineered drainage channel was constructed in the slide area of Slide Ridge. The drainage channel conveys material to a catch basin on the west side of South Lakeshore Road, where a 72-inch corrugate metal culvert is intended to convey material under the roadway to the east, along its historical path into Lake Chelan. During smaller storm events, the culvert operates as planned; however, it requires cleaning. During larger storm events, which occur about every two years, the volume of material frequently overwhelms the culvert and causes it to fail. This causes material to backup, fill the stilling basin and flow on top of South Lakeshore Road, stranding people north of the culvert and prohibiting emergency access into the area. The design and subsequent construction of the engineered channel was an attempt to solve the problem; however, ultimately, it did not provide a sustainable solution.

Most recently, storms have occurred at Slide Ridge in 2003, 2005, 2006, 2010, 2011, 2014, 2015, 2017 and 2019. The destructive events close South Lakeshore Road, typically for two days until a one-lane, unpaved bypass road can be cleared and re-opened. Cleanup costs of removing the material, hauling it offsite and re-establishing the existing engineered channel have cost Chelan County taxpayers nearly $975,000 from 2003 to 2019.

The Chelan County Flood Control Zone District (FCZD) proposes removing the existing roadway and 72-inch culvert and constructing a bridge over the earthen channel to allow mud and debris flows to freely pass under – not over – the roadway.

Project Description

In 2017, the FCZD hired an engineering team consisting of KPFF Inc., RH2 Engineering, PanGeo, and Indicator Engineering to evaluate the site and design a bridge structure for this location.  A bridge will reduce the risk of damage to life and property through the conveyance and control of mud and debris flows originating from Slide Ridge.  A bridge also will ensure a year-round, continuous corridor for the community and visitors, and it will be a proactive solution to a problem that Chelan County has dealt with for decades.

The single-span bridge (Bridge Alternative 1) will be about 108 feet long and 38 feet wide to accommodate the debris flow and to meet required roadway width (including shoulders). Utilities poles also will be removed and placed in conduits along the eastern side of the bridge, eliminating the potential of damage and failure from slide events. It should be noted that because Slide Ridge is an alluvial fan, material will continue to be conveyed through the project area into Lake Chelan as it currently does.

The project also consists of filling in a portion of the existing debris basin, removing the existing culverts, raising the roadway, improving residential access points, regrading and making slight alignment changes to the existing channel, and extending berms and side slopes to the low-water mark of Lake Chelan.

The consultants estimate that within 200 feet of the lake’s ordinary high-water mark, there will be 1,000 cubic yards of channel excavation and 4,100 yards of fill (extension of the berms and side slopes.)

In addition, extensive debris modeling determined that extending the conveyance berms and side slope will need to occur below the ordinary high-water line down to the low-water line to better protect adjacent private property from impacts during higher debris flow events. This will result in an estimated 80 cubic yards of channel excavation and 5,500 cubic yards of rock fill. This work will occur between November and May, when Lake Chelan is lowered, so no in-water work will occur.

The following resources provide more information:

• Project Vicinity Map
• Proposed Channel Design
• Photo Page of Previous Events
• Slide Ridge Bridge Feasibility Report
• Slide Ridge Bridge Feasibility Report: Appendices
• SEPA Checklist October 2020
• Determination of Non-Significance: Slide Ridge

Project Benefits

South Lakeshore Road serves about 400 residential lots, U.S. Forest Service land and the Lake Chelan National Recreation Area. People become stranded when debris covers the road, and emergency services do not have access into the area via South Lakeshore Road.  Existing utilities also are within the debris zone and are typically struck when a slide occurs.

With the construction of this project, the FCZD is attempting to be proactive by solving a reoccurring flooding and debris flow problem that cannot be eliminated.  At this time, Chelan County is acting in a reactionary way – cleaning the site up after an event occurs – then waiting for the next event to occur. 

When the project is completed, the closure of South Lakeshore Road from mud and debris flows will be eliminated, which is invaluable to Chelan County and the residents who use South Lakeshore Road.  A bridge will allow this community uninterrupted emergency response, utilities that will not fail and access to state and federal lands. It also eases concerns that one day someone may become trapped in the mud and debris flows. 

Construction Schedule

The project has been awarded to KRCI of East Wenatchee for more than $2.2 million. Construction starts June 21, 2021. During construction, a detour route will be established around the construction site on the unpaved, one-lane bypass lane that currently runs adjacent to Slide Ridge. An unmanned traffic light may be used to direct traffic. Minor delays may occur.

Because the roadway will not be closed, there should be minimal impact on services, such as garbage and delivery services. We will contact emergency services as well as Zippy Disposal and the Chelan branch of the U.S. Post Office to alert them to the construction project.

Funding

The construction portion of the project will cost about $3.2 million. It will be paid for with a nearly $1.3 million grant from FEMA and with money from the FCZD.

Receive Project Updates

As we have updates on the project, incuding the construction start date, we will send them out on the project's listserv. Sign up for the listserv online or email us at FCZD@co.chelan.wa.us.

Frequently Asked Questions

Posted: 10/27/2020 03:49 PM
Last Updated: 04/19/2023 11:12 AM