Optimizing Generation: Removing Sediment at Monroe Street Dam

To deal with heavy debris build-up at Monroe Street Dam in 2012, Avista Utilities worked to remove and relocate large rocks and sediment from the dam’s intake screens, ensuring optimal flow and steady generating capacity.

By Karen Terpak

Avista Utilities’ first hydroelectric development, the Monroe Street Dam on the Spokane River in downtown Spokane, Wash., has been producing power since 1890 – longer than any other hydroelectric development currently in operation in the state. Avista, then known as Washington Water Power, constructed the original run-of-river 800-kW facility at a natural waterfall at Spokane’s Lower Falls. The original timber dam was severely damaged during a high water event (34,000 cubic feet per second) and was replaced in 1974 by a new gravity concrete dam that exists today.

Over the decades, the powerhouse was upgraded in many stages; however, as the facility approached its 100th birthday, it was due for an overhaul. In 1992, the old powerhouse was decommissioned and replaced with a new 15-MW powerhouse with a single 14-foot-diameter penstock and a single vertical Kaplan turbine.

The debris pulled from the intake trashrack at Monroe Street Dam included non-natural materials - shopping carts, concrete pilings, wood embedded with railroad spikes, and bicycles - that had to be disposed of.
The debris pulled from the intake trashrack at Monroe Street Dam included non-natural materials – shopping carts, concrete pilings, wood embedded with railroad spikes, and bicycles – that had to be disposed of.

Monroe Street Dam is a 21- foot by 217-foot low-head concrete gravity structure with hydraulic capacity of nearly 3,000 cfs, while the mean annual flow of the Spokane River at that location is nearly 7,000 cfs. The watershed upstream of the dam is subject to peak run-off conditions from the west side of the Rocky Mountain crest in Idaho. As a result, flows have reached as high as 49,000 cfs at the dam, and significant spill is a yearly event.

While finer sediments from upstream pass through the turbines or over the spill crest, heavier cobbles, rocks and boulders travel down the river as bedload and build up behind the dam and against the penstock screens. These materials originate primarily from the Spokane Valley floodplain, which filled with mixed cobbles, boulders and coarse sediments during periods of glaciation and flooding thousands of years ago.

If left in place, these materials can reduce inflow into the penstock, reducing generation capacity, as well as cause damage to the intake screens and potentially to the turbine. To keep the facility generating optimum power, Avista dredges these natural materials on an as-needed basis and relocates them downstream. This approach, done in close coordination with environmental and fisheries agencies, provides a source of spawning gravel to move naturally downstream for trout habitat. Avista has typically conducted dredging every few years, although back-to-back years of dredging have been needed during years of significantly high flows.

During the spring of 2012, high and extended river flows caused large amounts of rocks, gravel, and other materials to accumulate at the dam. Avista hired Associated Underwater Services (AUS) divers to perform an underwater video inspection of the dam’s trashrack and six intake gates in August 2012. Divers measured the buildup upstream of the intake trashrack to create a profile to help determine if dredging would be required. The profile AUS developed showed the build-up varied from 3 feet to 10 feet in thickness – conditions that warranted dredging to be performed.

The divers also found the bottom 6 feet of the intake trashrack had rocks wedged in between the 2.5-inch spacing between the flat bars. They were able to remove most of the wedged rocks during the dive inspection.

The old Monroe Street powerhouse and dam in 1925.
The old Monroe Street powerhouse and dam in 1925.

Preparing for the project

To reduce project and contracting costs and interruption to the plant’s operations, Avista decided to take a more aggressive approach with the year’s rock and sediment dredging activities, compared to previous efforts. The primary changes, as reflected in the request for proposals issued, were as follows:

– A site visit was mandatory for all bidders prior to submitting a proposal. The project engineer, plant manager, chief operator, mechanical supervisor, environmental lead, contract manager and construction inspector attended the meeting to address bidders’ questions and ensure key issues were identified. Such issues included avoiding interference with operation of the city of Spokane’s “Skyride.” The Skyride is a gondola ride available to the public year round to view the dam and river and it travels adjacent to the powerhouse and drops downstream of the dam so the public can have an up-close view of the Spokane Falls. A spotter was required to assist the crane operator throughout the project because the crane boom was maneuvering only 15 feet from the gondola.

– The crane operator had to submit a resume to prove critical skills and experience in working near dams. This reduced risk of damage to the dam and associated structures, such as the festooning on the trashrake. A spotter for the crane operator was required during the dredging operation to ensure safety for the operator and equipment.

– A minimum boom length of 210 feet was specified in order for the dredged material to be properly relocated downstream near the center of the river, to avoid rock and sediment build-up along the sides of the riverbank. The bidders were required to provide the crane and bucket information to Avista as part of the proposal. The intent was to be able to move more materials effectively and efficiently using a larger crane and bucket. This consisted of a 250-ton crane with a 290-foot boom and a 1.5 cubic yard bucket. In 2011, a smaller 125-ton crane and 0.75 cubic yard bucket was utilized for the work.

Avista worked under five environmental reviews for the project:

– Hydraulic Project Approval from the Washington Department of Fish and Wildlife (WDFW);
– Shoreline Conditional Use Permit from the city of Spokane;
– State Environmental Policy Act, Determination of Non-Significance from WDFW;
– 404 Permit from the U.S. Army Corps of Engineers; and
– 401 Water Quality Certification from the Washington Department of Ecology.

Avista also coordinates such project work closely with the Federal Energy Regulatory Commission, through FERC’s Portland Regional Office. Monroe Street Dam is located in the heart of Spokane and the work site is adjacent to Spokane City Hall. In preparation for the work, Avista’s corporate communications team informed customers and the community of the upcoming activity through various channels, including news releases and social media tools.

The actual dredging process took 15 days. Notice the dredged material is being relocated near the center of the river so that it can move naturally downstream for trout habitat.
The actual dredging process took 15 days. Notice the dredged material is being relocated near the center of the river so that it can move naturally downstream for trout habitat.

Rock removal and relocation process

Avista contracted with a local Spokane based company, Rhodes Crane, for the crane operations. Rhodes Crane began mobilizing a 250-ton crane on Sept. 7, 2013, and completed mobilization and setting up the staging area on Sept. 10. To ensure adequate room to maneuver this large equipment, Rhodes Crane coordinated with the City of Spokane and acquired a street closure permit. Staging the area included fencing it off to protect pedestrians and designating a temporary disposal area for non-natural debris.

The Hydraulic Project Approval permit specified no water disturbance after Sept. 30 to protect the mountain whitefish, which are winter spawners. Starting and keeping the project on time was critical. Work began Sept. 10 and continued until Sept. 28 (15 days of dredging activity). During this time, the crane was located at the south abutment of the dam.

Natural materials, including rocks, cobbles and gravels were collected from the forebay using the crane and then placed back into the river below the dam, per an edict from WDFW that requires Avista to replace natural materials back into the river. Redistributing the rock and gravel materials downstream of the dam provides a potential gravel source for spawning habitat in the lower Spokane River.

Typically, non-natural debris that has accumulated at the dam’s intake trashrack structure is also removed from the river and disposed of during the operation. Approximately 18 cubic yards of debris was collected, including shopping carts, concrete pilings, wood embedded with railroad spikes, and bicycles. The debris was disposed at the Graham Road Recycling & Disposal which is a Construction, Demolition & Land Clearing Debris (CDL) site in Medical Lake, Washington.

As a permit condition, Avista was required to follow best management practices throughout the dredging activities. Avista conducted water quality sampling at an upstream and a downstream location to ensure the dredging activities did not exceed turbidity standards or affect dissolved oxygen levels in the river. In addition, due to concerns about upstream sources of contaminants, Avista must periodically complete full physical and chemical analyses of the dredged material. The analyte list was based on guidelines included in the U.S. Army Corps of Engineers’ Dredging Manual.

In the unlikely event the analysis indicated the concentrations were above the screening values, Avista would have been required to transfer the dredged material off-site. To date, test results have indicated the material meets sediment standards for being returned to the river.

The contract allowed Rhodes Crane three days for demobilization due to the size of the equipment and the removal of the debris. The work was complete and the contractor was completely demobilized by Oct. 1.


The use of a larger crane and bucket in 2012 produced measurable results in terms of materials moved. In past years, a 125-ton crane and 3/4 cubic yard bucket were used, limiting the amount of material that coule be removed. Nearly 1,800 cubic yards of materials were removed in 2012, compared with 810 cubic yards in 2011, 890 cubic yards in 2010 and 1,450 cubic yards in 2008. The volume of dredged material was calculated from the daily dredging sheets.

Due to the successful outcome of this project, Avista plans to incorporate the modified requirements in future dredging projects at the Monroe Street Dam. The successful operation is attributed to the contractors’ equipment and experienced crane operators, along with Avista’s planning and coordination with environmental agencies and the public.

As a follow up, Avista anticipates dredging to be required in 2014 because of two reasons – dredging was not required in 2013 (as anticipated with the aggressive efforts in 2012 and a peak day average flow of 19,120 cfs in 2013) and it’s forecasted the Spokane River will have a high spring runoff. Avista estimates that 6 to 8 feet of rocks will build up at the intake trashrack during runoff this spring.

Karen Terpak, M.S., P.E., is a generation maintenance engineer at Avista Utilities.


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