Hydro power plants and fish rarely enjoy a harmonious relationship but construction of the Dworshork small hydro power plant in the US is proving otherwise. Suzanne Moxon reports

The great old days of constructing large scale hydro power projects in the US are fast becoming a distant memory. In the twenty-first century economies of scale are changing — US hydro developers are reported to be downsizing. Faced with an increasing regulatory burden to address environmental issues, modern-day hydro developers are realising that big is not always beautiful when counting megawatts. ‘The larger sites have been developed in the US,’ says Jim Francfort from Idaho National Engineering and Environmental Laboratory, which has recently conducted a statewide survey of undeveloped US hydro potential (see article on pp28-9 of this issue). ‘Smaller sites of 25MW or less are really the future,’ he adds, ‘and hydro developers are mainly looking to utilise these now.’

One such scheme currently under construction is located on the Clearwater river in Northern Idaho. The Idaho Department of Water Resources (IDWR) is developing a 2.9MW plant downstream from the existing Dworshork dam and hydroelectric facility. After ten years of concerted effort a licence to construct the facility was finally issued by the Federal Energy Regulatory Commission (FERC) on 4 August 1998.

The ten-year wait has been attributed to the land on which the plant is being built. Owned by various agencies of the federal government, as well as being within a Native American reservation, approval had to be sought from several agencies. The permit is valid for four years but IDWR is confident the plant will be completed by August 2000.

The Dworshork small hydro power project is a unique design as it will utilise the flow in pipelines to fish hatcheries located one mile downstream of Dwor-shork reservoir. Originally built for ana-dromous fish mitigation at Dworshork, the hatcheries will now allow for the construction of a small hydro plant.

‘We decided to develop the plant as we wanted to utilise the wasted energy at this site,’ Ralph Mellin from IDWR says. ‘The opportunity was there and there was a good positive cost-benefit ratio.’ A 30-year PPA has been secured with Bonneville Power Administration and IDWR is expected to contract out the operation of the facility when completed.

Two pipelines (36in and 18in) flow down from Dworshork reservoir. The 18in pipe comes out of the middle reservoir level at a fixed point and the 36in pipe takes water from the top of the reservoir. The two waters can be blended to achieve a suitable temperature for the fish hatcheries. The separate pipelines lead down to a distributor box and from here they divide into four — two go to Clearwater and two to Dworshork National Fish Hatcheries. The actual small hydro plant will be built on top of the distributor box.

Larry Bennett from Montgomery Watson, the project designer, explains that the distributor box was designed about twelve years ago with the intention of building a hydro power facility on the top. ‘But the design had to be changed,’ he says. ‘The original power facility was only to have one 2MW turbine generator but now we have one of 2.5MW and one of 0.4MW.’

Montgomery Watson was concerned that the 50% increase in equipment capacity would exceed the structural strength of the existing structure. The original distributor box had a thin cover that was designed for a 3ft thick concrete slab and the 2MW of equipment. To hold the heavier equipment, the design of the existing structure was re-analysed. The concrete slab thickness was increased to 4ft 3in with additional steel reinforcing. A mezzanine was added to provide space for the additional control equipment.

‘Aesthetics were also an important design consideration,’ Bennett says. ‘The plant will be visible across the river to a nearby town. So we had to use colours and textures that would blend into the existing natural rock in the area.’

As the facility was to be built on an existing structure there would be minimal environmental impact. ‘Obviously FERC was very interested in ensuring that the operation of the fish hatcheries was not affected in any way,’ Mellin said. As a result, various conditions have been written into the small hydro plant’s licence to safeguard the hatcheries.

The most important consideration is that the pipelines have to be operational all year round. During construction a continuous flow of water had to be maintained to feed the active hatcheries and so sequential procedures were devised to ensure that the water flowed without interruption. Once the facility is on line if generation problems occur the water has to be diverted around the turbines. Bennett does not see this as being a problem in the future and explains that sophisticated instrumentation is being used to monitor the facility. In addition an operator will be located near the facility at all times to respond quickly in the event of an emergency.

Other FERC licence provisions cater for design considerations, construction methods and entrained gas levels (which could impact the hatcheries). Biodegradable oil is also being used in the lubrication system and oil overflow sumps were added to ensure that oil does not cause downstream contamination.

Obviously a lot of consideration has been given to the impact of the small hydro facility on the hatcheries, but will the Dworshork hydro power dam impact the new smaller plant? ‘The two will co-habitate nicely,’ Bennett says. ‘The water for the fish hatcheries, and so used for the small hydro facility, is separate to that used for power at Dworshork. The two plants will not have any impact on one another.’

Dworshork small hydro power plant

2.9MW – 1×2.5MW and 1×0.4MW.
Generators: Alconza (Spain).
Turbines: Gilkes (UK).
Mechanical and electrical equipment: Yankee Energy (US).
Construction and installation: contractors Northwest (US).
Dworshork reservoir: elevation of 410-570ft. Active storage of 2,016,000acre-feet.
Flow through the 36in pipeline is 90cfs and through the 18in pipeline 25cfs.
The US Army Corps of Engineers owns the fish hatcheries and Dworshork dam.