Flood Modeling, Water Retention Studies Underway


Over the last century, major floods have occurred about twice per decade.

The Chehalis River Basin Flood Authority and Chehalis Work Group do not — and cannot — stop flooding.

But they can, they say, lessen the danger and destruction of catastrophic flooding.

At the Great Wolf Lodge on Wednesday, the flood mitigation partners held a workshop, allowing public access to updates on the variety of projects occurring throughout the basin.

The gathering, the first of three workshops, included members of the Chehalis River Basin Flood Authority, the Chehalis Work Group and representatives from the Department of Ecology and Fish and Wildlife.

In one year, the Chehalis Work Group, a flood-policy group appointed by the governor, must announce whether or not it will recommend moving forward with a water retention structure on the Upper Chehalis River Basin.

To prepare for that decision, the flood leaders are examining water retention from all sides.

They are looking at the technical feasibility of a dam, including its size and functions, and how it would deal with fish and debris; they want to better understand the geology of the area and determine whether a dam would be safe and could withstand natural hazards like earthquakes or landslides.

They also are looking to the future. Should the dam be constructed, the group wants to know, how would it affect aquatic species, the environment and man-made infrastructure.

Though the proposed dam would be built on the Upper Chehalis River near Pe Ell, the scope of the group’s work encompasses the entire basin, an ecosystem with diverse, complex needs; a system where a positive measure in one area can have a negative effect in another.

In one month, the Chehalis Work Group must produce a technical memo and approve the proposed methodology.

Its draft report must be completed by June, finalized by August and presented in November 2014.


Gov. Jay Inslee earlier this year approved the Work Group’s funding request: $9.2 million for the study and design of a dam and other long-term projects to improve Interstate 5; $10.7 million for local flood protection projects; $4.4 million for projects that reduce flooding while benefitting fish; $1.75 million for reducing damage to residences and other structures in the floodplain; $1.2 million for operation of the basin program and for project management; and $950,000 for state agency technical assistance and project permitting.

Members of the Work Group include: Vickie Raines, the Flood Authority chairwoman; Karen Valenzuela, the vice chair of the Flood Authority; J. Vander Stoep, an alternate to the Flood Authority; David Burnett, the chairman of the Chehalis Tribe; Jay Gordon, the head of the Washington Dairy Farmers Association; and Keith Phillips, an adviser to the governor.


To model the future, you must be discerning in choosing data from the past, Jeff Johnson, a partner at Watershed Science and Engineering, said on Wednesday.

Johnson over the last several months has worked on creating a hypothetical flood that will be used to determine the costs and benefits of proposed projects throughout the basin.

“Our job is to recommend the type of flood that should be looked at for economic analysis,” Johnson said. “This is key information for economists. They need to know what type of flood they are looking at, how deep it will be, so they can calculate what would be damaged.”

In developing a flood model, Johnson took a basinwide approach.

Analysis of historic high water events indicated that for the Upper Chehalis to flood, there must, be flooding on all its major tributaries, with one exception — the Skookumchuck. Past high water events, including the 2007 flood, have occurred without its help, and Johnson’s model will take into account that “you can’t count on the Skookumchuck,” he said,

In the simulated flood model, all measurements will be taken at the Grand Mount Gage.

Johnson also will model three actual floods: the 2007, 1996 and 2009 floods.

“We’re going to model three events that you’ve observed for two reasons: to make sure we calibrate our hydraulic model well, and, these are the events that your constituents and people in the Basin saw and know. They’re going to want to ask you, ‘in 2007, if that alternative was chosen, what would have been the benefit to me?’”

“So you can go and say, with this alternative, this is how the 2007 flood would have looked to you,” Johnson said.

Over the last several months, the hydraulic engineers at Watershed Science and Engineering have reviewed and fine-tuned hydrology data from the Chehalis River gage in Doty.

The engineers’ analysis of the water flow measurement gage in Doty repeats work done already by USGS — it’s a sort of second opinion to ensure total accuracy before moving forward.

USGS’ conclusions, both on peak flow and volume, were slightly too high, Johnson said.

“We have to know how large these flows are, not just from the peak of the event, the maximum discharge, but the volume of the event, how much water actually comes down the river, over a period of time, during one of these events? It’s really important to understand,” he said, “because if one of the options is to put a dam in the upper watershed, we need to know how much it needs to store.”

Johnson presented the information on behalf of his colleague Larry Karpack, who is the lead on the project.


Keith Ferguson, an engineer and fishery scientist with HDR Engineering on Wednesday provided detailed, up-to-date information on building a water retention structure.

Over the last 20 years, those seeking flood solutions have returned time after time to building a dam — a large one or a small one, one that only holds water or one that serves several purposes.

Based on a series of investigations, one configuration has emerged as the frontrunner: a large upstream dam on the main stem of the Chehalis River above Pe Ell.

Such a structure, the Chehalis Work Group says, could hold back storm flows when the mainstem of the Chehalis is the principal source of major flooding, and could hold back mainstem flows when the tributaries like the Skookumchuck and Newaukum are flooding.

Should a dam be built on the Upper Chehalis it likely will loom 230 to 290 feet high, Ferguson told those at the workshop Wednesday.

The Pe Ell site appears to be best suited for one of three dam types, roller compacted concrete, central clay core rockfill or composite, Ferguson said.

Roller compacted, or RCC, is a type of concrete gravity dam, with a sloped, staircase-like face, that is relatively quick and cheap to build.

Additional benefits, according to Ferguson, include integrated structural elements, effective seepage barriers and crack control strategies. Concrete dams, in general, provide the most flexible range of flood operations and fish passage options, he said.

A central clay core rockfill dam, a type of embankment day, could effectively utilize existing natural resources, according to Ferguson. The triangular dam, with an imperious clay center, would be particularly good at handling seismic pressure, he said.

An RCC/Embankment Composite dam — one that combines the RCC design with a simple embankment slope — would be another convenient and cost effective option, Ferguson said Wednesday.

“Now we have to match the dam with a hydraulic structure to make it safe and make it operable,” he said.

Several options for hydraulic structure, in terms of fish passage, are on the table, including a passive system, like a tunnel through a concrete section or a tunnel through an abutment.

“There may be ways topographically that we might be able to bring some fish passage up to a certain level, then through a tunnel. We’re going to be looking at those things,” Ferguson said.

More active types of fish passage management systems, trap and haul or four-way collection, for example, also are possibilities

“Whatever we do with the dams, debris and sediment management is going to be a big consideration, making sure our hydraulic structures can handle that,” Ferguson said.

Attendee Vince Panesko earlier that day also called attention to the issue of sediment and debris.

Panesko, the owner of a tree farm in Pe Ell which currently is used for fish studies and could become the site of the dam, is intimately familiar with the Chehalis River, he said.

“I’ve made this comment before, but I wanted to emphasize: that river is changing every year. It’s already changed this year. Nobody in this study has any clue, all the sand and gravel that moves,” he said,

Already, he’s lost a football-field worth of sediment, and over the years, he’s lost tons and tons of timber.

“The river takes bites out every year,” he said.

By early December, HDR Engineering will finalize the criteria it will use to configure the dam project, and then will use that information to put together a portfolio of options.