A portion of the South Prairie Creek, which recently received updates to its flood thresholds. Photo courtesy the Puget Sound Estuary Program

A portion of the South Prairie Creek, which recently received updates to its flood thresholds. Photo courtesy the Puget Sound Estuary Program

Flood information improved for pair of local waterways

Both the White River and South Prairie Creek received flooding threshold updates.

To help residents in this area prepare for flood events on the White River and South Prairie Creek, differing flood stages have been updated on the former and added for the first time to the latter.

The changes were added to the National Weather Service Advanced Hydraulic Prediction Services Map in January at the request of Pierce County.

The identification of various flood stages (including minor, moderate, and major, for example) warn residents about possible flooding and help officials determine when to implement evacuations and anticipate road closures.

SOUTH PRAIRIE CREEK ADDITIONS

New thresholds indicating minor, moderate, and major flooding on South Prairie Creek have been added. This was based on impacts from a moderate flooding event in Orting and the surrounding area in February 2020. The presence of sandbags around homes and property within the town of South Prairie indicates how frequently the rising creek is an issue.

The thresholds are 4,000 cubic feet of water per second (CFS) for minor flooding, 5,500 CFS for moderate flooding, and 8,000 CFS for major flooding.

WHITE RIVER CHANGES

Pierce County and local cities also agreed to increase the threshold for minor flooding on the White River by 1,000 CFS to 7,500 CFS, while the threshold for moderate flooding was raised by 2,000 to 10,000 CFS and the threshold for major flooding was raised by 2,000 to 12,000 CFS. These changes were requested after monitoring water releases from Mud Mountain Dam. The changes more accurately reflect when flooding may occur.

National Weather Service maps show information from different points on local waterways. Viewed last week were the Carbon River near Fairfax, the White River near Mud Mountain Dam and South Prairie Creek near the town of South Prairie – all showing no current signs of flooding.

FLOOD MANAGEMENT IN PIERCE COUNTY

Pierce County rivers looked very different more than a century ago when small farms dotted the valleys. The Puyallup, White, Carbon and Nisqually rivers were important resources to the communities that grew along them. But the rivers were also hazards due to frequently shifting channels covering fields and communities with water.

1906 – On Nov. 14, a large flood caused a debris jam to form forcing the White River to alter its course into the Stuck River.

1913 – A legal settlement between King County and Pierce counties was the start of several projects dedicated to the straightening of the lower Puyallup and White rivers over several decades. Levees and revetments were built during this time, and many still exist today.

1914-1940 – Construction of levees and revetments along the lower Puyallup and White rivers are built to contain flood flows.

1933 – Floodwaters overtopped portions of the new levees and revetments during the largest flood on record in Pierce County.

1948 – The U.S. Army Corps of Engineers finished Mud Mountain Dam to regulate flow of the White River into the Puyallup River.

1960s – More levees and revetments were constructed and portions of the Carbon and mid–upper Puyallup rivers were straightened in an attempt to flush sediment and gravels quickly through the river system.

1962-present – Major flooding has led to 15 presidentially-declared disasters in Pierce County.

1991 – First Puyallup River Basin Comprehensive Flood Control Plan adopted.

1990s – Major floods during the decade resulted in five presidential declarations and confirmed the need to change the philosophy and practices for managing Pierce County’s rivers.

2000s – Two presidential declarations and two major flood events. Pierce County moved away from limiting the rivers’ movements with levees and revetments and began allowing the rivers, where possible, to return to historical channels.

2012 – The Pierce County Flood Control Zone District is created by the County Council to focus on flood management needs and projects.

2013 – The Pierce County Rivers Flood Hazard Management Plan is adopted.

Learn more about flooding in Pierce County, including ways to prepare, at www.piercecountywa.gov/flooding.


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