Hydroelectric facilities are prime examples of infrastructures that are dependent on raw water. A monitoring program in such facilities needs to target both the external source water and the internal raw water conduits. It will be cost-effective over the long run if monitoring and control strategy designs are integrated. Integration of the two will allow immediate implementation of appropriate control measures if the monitoring program detects zebra mussels in the system.
Sensitive Components within Hydroelectric Facilities:
Hydroelectric facilities generally have the following structures, which need special attention when implementing a monitoring program:
1. Water Intake System: This may be either a submerged or a surface intake. Submerged intakes often involve a cage structure to protect the intake and the piping which carry the water into the facility; surface water intakes usually involve a forebay or channel and a pumphouse. Submerged intake cribs are structures particularly vulnerable to both physical clogging by mussels and secondary clogging by frazil ice, the formation of which is thought to be accelerated by the presence of zebra mussels. Other sensitive components within the water intake system that mussels frequently attach to are the traveling screens and trash bars. Mussel druses can also be carried by water currents and become entrained in the screens. Pipes and valves, due to their relatively small diameter, can become easily clogged or have their flow impeded.
2. Penstock: This specialized intake structure (usually located at the receiving end of the water intake system) channels incoming water through a vertical drop into turbines. They are at most risk if water is allowed to sit in them during dewatering periods. Their normal operating condition with high flow rates is generally adequate to keep mussels from attaching to their walls and interfering with flow.
3. Service Water System: These plumbing systems serve many functions within the facility and are particularly vulnerable to infestation because of their relatively small diameters. They are most vulnerable to infestation when water is allowed to sit in them. If service water systems are fed from city water sources, there is little danger of zebra mussel infestation. However, if service water is drawn directly from infested source water, problems may arise if the system is accessed periodically and then refreshed. Under conditions of intermittent water velocity, mussels could be introduced into pipes by periodic water refreshment, allowed to settle in low flow conditions, and nourished by oxygen and food provided during refreshment.
Recommended Monitoring Strategies for Hydroelectric Facilities:
1. Proactive or Reactive: The type of monitoring required should keep control in mind. Reactive control does not need monitoring for veligers. Proactive control will emphasize monitoring for veligers.
2. Mainstream Source Water Monitoring: Determine the amount of infestation by zebra mussels that can be tolerated. The more sensitive the facility, the more frequent the sampling for zebra mussels will need to be and the wider the variety of sampling gear that will have to be used. For veliger monitoring, sampling should be done when ambient water temperatures are above 10°C (8°C in areas where quaggas are present). If no zebra mussel colonization can be tolerated, continue sampling for attached mussels year-round. Be consistent in sampling methods to ensure comparability of samples between years. Depending on the life stage being monitored, sample, either with plankton nets or artificial and natural substrates, upstream, downstream, and within fore- and discharge bays. Sampling programs focusing on veligers employ plankton nets and/or filamentous substrates. The type of filamentous substrate used will depend on how frequently the samplers are exchanged, and how much suspended organic matter is present. Very eutrophic sites may require substrates of a large mesh size and replacement of samplers on a weekly or biweekly basis. Once the filamentous substrate is completely clogged with organic debris, its effectiveness at collecting zebra mussels of any life stage rapidly declines. Filamentous samplers do not have to be conditioned (i.e., allowed to develop biofilm) before use. Sampling programs focusing on settlement of larvae (i.e., the initial biofouling stage) should use filamentous artificial substrates, which should be checked frequently. Again, the size of mesh used and the replacement frequency will be determined by the amount of suspended matter present. Settlement can occur as early as 8 days after veligers are found in the water column, although 21 days are more commonly required for completion of pelagic development.
3. System Monitoring: Sidestream samplers should be installed and maintained at the following areas: upstream from where control measures (e.g., chemicals, fine mesh filters, etc.) are being applied, at the furthest point(s) that control effectiveness is desired, at any sensitive or critical areas (such as tube sheets), and near water quality equipment. Sidestream samplers are discussed extensively by Mackie (1991) and by Claudi and Mackie (1994). A variety of substrates, both solid and filamentous, should be considered in any sidestream sampler. See the section on Bioboxes for further details.