Mitigating rapid, short-term variations in flow (hydro-peaking operations)

Introduction

Figure 1: The figure illustrates the hydropeaking parameters flow ratio (amplitude of change), speed of change and frequency of change in water flow/level.

Figure 2: Fish using the areas identified by red colour will have a risk of stranding when the water level drops rapidly.

Hydropeaking refers to hydropower operations that are characterized by more rapid and frequent changes in power production than typical base-load hydropower production. If the water is released into a river, rapid fluctuations in discharge and water level may cause negative impacts to the riverine ecosystem. The severity of the hydropeaking can be categories according to the magnitude of changes in flow, the frequency of the changes, and the timing of the changes, i.e. what time of the day and year the hydropeaking happens.

Bakken et al. (2016) defines measures to reduce the impacts from hydropeaking operations into three types; i) operational measures that involves adjusting the magnitude, speed of change, frequency and timing of change to the better for the ecosystem exposed to hydropeaking, ii) physical changes in the river system downstream the outlet of the hydropower plant. Such measure will to a some extent be similar to those habitat measures presented in Section YYY, and iii) technical measures directly on the power plant/infrastructure, i.e. a technical setup that allows a wider range of turbine discharges, slower stop and start-up, etc.

The measures presented in the following are all related to changes in flow.

Methods, tools, and devices

During planning

A group of experienced Norwegian scientists summed of a large research project on environmental impacts from hydropeaking operations (CEDREN EnviPEAK - www.cedren.no) by defining a categorisation system for environmentally-adapted hydropeaking operations. The system was developed based on research from EnviPEAK, other similar studies in Norway and across the world, and the expert judgment of the involved scientists. The system was developed primarily based on knowledge about salmonids’ response to hydropeaking. This system is a set of recommendations, seem to have been adopted as standard requirements for hydropeaking operations in Norway, when the environmental terms of operations are revised, or new licences granted. COSH-tool can be used to find the relevant hydropeaking parameters.

The full categorisation system published in Bakken et al. (2016) and Harby et al. (2016) also includes the dimension of assessing the vulnerability of the ecosystem exposed to hydropeaking. The vulnerability is given by assessing factors such as effective population size, degree of limitations in recruitment, habitat degradation, reduced water temperatures and percentage of impacted river length compared to total length. The combined assessment of the effect factors and the vulnerability will give the total and overall impact assessment.

During implementation

If the technical system is properly designed with respect to providing environmentally adapted hydropeaking operations, the introduction of a new flow regime would simply be to release the defined water flows at the right speed and time of the day and year. If the existing infrastructure cannot release support slower ramping rates, adjusted water flow and the flexibility of the frequency and timing, a re-building of the infrastructure might be needed.

During operation

A new flow regime better adapted to the ecosystem tolerance of hydropeaking operations would normally not introduce any extra maintenance. Hydropower operators must normally document that the environmental restrictions are followed, and would have standard monitoring systems (gauging stations) in place. It should be ensured that the monitoring is made with sufficiently high resolution to capture the rapid changes in water flow due to hydropeaking.

The positive environmental effect of the released environmental flow must be done by assessing the development of the fish population, e.g. by monitoring juvenile fish densities, number of smolts, etc.

Classification table

Classification
Fish species for the measure	All
Does the measure require loss of power production	Operational (requires flow release outside turbine)
	Operational (requires flow release through turbine)
	Recurrence of maintenance	Never
Which life-stage of fish is measure aimed at	Spawning / Recruitment
	Juvenile habitat (0+)
	Juvenile habitat (1+)
	Juvenile habitat (older than 1+)
	Adult fish
	Movements of migration of fish
Which physical parameter is mitigated	-
	-
	-
	-
	-
	Water velocity
	Water depth
Hydropower type the measure is suitable for	Plant in dam
	Plant with bypass section
Dam height (m) the measure is suitable for	All
Section in the regulated system measure is designed for	-
	Bypass section
	Downstream outlet
River type implemented	Steep gradient (up to 0.4 %)
	Fairly steep with rocks, boulders (from 0.4 to 0.05 %)
	Slow flowing, lowland, sandy (less than 0.05 %)
Level of certainty in effect	Very certain
Technology readiness level	TRL 9: actual system proven in operational environment