Abstract
The role of hydropower as a renewable and balancing power source is
expected to significantly increase in a Net Zero Emissions by 2050
scenario. As a common phenomenon in hydropower plants, hydropeaking will
become more prominent, resulting in additional stresses on the
ecological status of rivers. Here we propose a novel approach to design
and operate auxiliary reservoirs called re-regulation reservoirs that
aims to mitigate the adverse impacts of hydropeaking on rivers. A
re-regulation reservoir aims at smoothing flow fluctuations caused by
hydropeaking by diverting and retaining parts of high flows and
returning them back to river corridors during low flows. The regulatory
performance of re-regulation reservoirs is a function of its geometry
and volume availability. It is defined (and optimized) by restricting
various flow components thresholds. Using actual data from a
hydropeaking-influenced river system, the operation and efficiency of
potential re-regulation reservoir have been investigated by employing a
range of thresholds for hydropeaking mitigation. A methodology and an
open-access algorithm to operate re-regulation reservoirs, by
establishing a hierarchy of conditions to restrict peak flow, minimum
flow, up-ramping rates, and down-ramping rates was developed. Our
calculations show clear theoretical possibilities for regulating
hydropeaking with re-regulation reservoirs, while offering several
advantages, including greater flexibility and adaptability to changing
environmental conditions, power, and water demand without increasing the
operational cost of power systems.
Introduction
Hydropower is one of the largest renewable electricity sources in the
European Union (EU), accounting for 36% of renewable electricity and
10% of gross electricity production as of 2018 (Alsaleh et al., 2023).
These figures are expected to grow as the European Commission has
proposed a European Green Deal to make Europe climate-neutral by 2050.
As a result, with hydropower being a flexible power source, its role in
balancing and stabilizing the power market will grow (Ashraf et al.,
2018). However, hydropower’s flexibility causes sudden variations in
sub-daily flows in rivers, i.e., hydropeaking, defined as an artificial
river flow regime caused by the cyclical release of water. This is due
to the rapid switching between increasing and decreasing power
generation in hydropower plants (HPP), in response to the power market
fluctuations (Bieri et al., 2014). Manipulating the power demand may
indirectly alter flow conditions and intensify hydropeaking regimes in
rivers (Ashraf et al., 2022). In turn, artificial flow fluctuations
induced by human activities are considered a primary threat to aquatic
ecology (Bunn et al., 2002). Altering natural flow conditions can result
in ecological stresses as they are widely recognized as significant
drivers of ecological sustainability of rivers along with their
associated floodplains (Poff et al., 1997).
Hydropeaking leads to a high flow pattern variability significantly
impacting river ecosystems (Meile et al., 2011). On a temporal scale,
variations in flow patterns driven by hydropeaking are significantly
more prominent than other forms of flow variations, such as seasonal
flow changes or daily flow fluctuations (Bejarano et al., 2018).
Furthermore, the magnitude of hydropeaking releases can be much larger
than those of natural flows, leading to significant changes in water
depth and velocity (Shen et al., 2010). Aiming at mitigating the adverse
impacts of hydropeaking, governing authorities usually impose
environmental constraints on HPPs operations, such as setting minimum
environmental flows and limits on flow change rates. It has been
demonstrated that implementing such environmental constraints can
effectively mitigate sub-daily flow fluctuations (Olivares et al.,
2021). However, these constraints will result in economic losses for
HPPs (Pérez-Díaz et al., 2010; Guisández et al., 2016). Alternatively,
the introduction of a re-regulation reservoir (RRR) can mitigate the
loss of operational flexibility caused by environmental constraints.
This strategic measure enhances the plant’s operational flexibility
while simultaneously mitigating economic losses arising from operational
constraints (Pérez-Díaz et al., 2010).
Bieri et al. (2014) conducted a study on RRRs in the upper Aare River
basin, Switzerland, focusing on mitigating rapid flow changes,
specifically ramping rates, rather than altering peak discharge or
off-peak discharge. The study examined four RRR volumes (50,000; 60,000;
80,000; 100,000 m3) and demonstrated significant
reductions in flow ramping rates compared to existing values or future
projected rates. Similarly, Tonnolla et al. (2017) utilized ecological
indicators to evaluate the same retention volumes at the Innertkirchen
HPP. The results revealed that volumes of 80,000 m3and 100,000 m3 led to the most significant ecological
improvement. On the other hand, Anindito et al. (2019) studied the
cost-effectiveness of re-regulation reservoirs (RRRs) in addressing
ecological impacts from sub-daily hydropeaking. They evaluated the
techno-economic performance of a 360,000 m3 RRR to
mitigate hydrological alterations caused by HPPs. The study explored
different investment costs for RRR and provided comprehensive
recommendations for profitability. Popa et al. (2019) conducted a
qualitative analysis on the feasibility of constructing a RRR downstream
of the Golesti HPP. The study aimed to release water, with or without
smoothed fluctuations, through a small HPP to generate green
electricity, while minimizing adverse effects on the riverbed and
downstream ecosystem. However, the study did not address ecological and
hydrological concerns associated with this approach. Olivares et al.
(2021) assessed the impact of small RRRs situated downstream of HPPs,
with a focus on the tradeoffs between flow flashiness (ramp rate) and
power system costs. Using a system-wide cost-minimization model, the
study revealed that small RRRs successfully mitigate ramping rates while
minimizing the cost increase caused by operational constraints. In a
recent study, Reindl et al. (2023) examined on a hydropeaking diversion
HPP at the Swiss/Austrian border. The analysis supported the use of a
reservoir with a volume of 300,000 m3, which was
deemed optimal for the site. Larger retention basins
(>1,000,000 m3) theoretically could
provide superior effects, but land availability constraints rendered
their construction infeasible (Reindl et al., 2023). Most of the prior
research has predominantly examined RRRs from ecological or economic
perspectives for specific localities. Studies incorporating hydrological
aspects often centered on ramping rates (flow flashiness, rate of flow
change, etc.) neglecting the consideration of flow magnitude and were
limited by land availability (RRR volume) or economic constraints. In
contrast, our study exclusively concentrates on the theoretical design
of RRRs from a hydrological standpoint, encompassing both flow magnitude
and ramping rates as essential factors for analysis. The objectives of
this study are to; 1) Design a model to determine the required volume of
the RRR to shave the peak flow, increase minimum flow, and limit ramping
rates. 2) Examine the potential of deploying RRR downstream of HPPs by
utilizing the model developed in this study; 3) Validate the model and
optimize its applicability towards a re-regulation strategy for RRR
operation. To the best of our knowledge, this is the first study that
attempts to develop a model exclusively focused on the design of RRRs
from a hydrological standpoint.
Materials and Methods
To address the key research objectives of this study, a mixed type of
methodological approach was employed. The first and second parts consist
of a desk review of published literature in peer reviewed journals,
periodicals, proceedings, and book chapters on the concept of RRRs with
a snapshot of main mechanisms involved in their operation. The desk
review also covered policies, regulations, and standards of ecological
criteria of RRR operation. The third part provides a comprehensive
description of the development of a model-based design of RRRs.
Re-regulation Reservoir (RRR) Conceptual Approach
A RRR stores a portion or the entirety of excess flow of a river regime
and releases it back at an adequate rate to smooth out any sub-daily
alterations in the regime (Figure 1). Figure 1.a. is a schematic that
visually illustrates the operation of the RRR. The main idea of the RRR
is to restore the regulated river regime back as much as possible to its
natural regime. This could be achieved by storing the excess flow (red
phase in Fig. 1) from the HPP during up-ramping events (i.e., starting
or increasing the turbine power production), to be released later during
down-ramping events, i.e., stopping or decreasing the turbine power
production (blue phase, Fig. 1), through two automated gates that
regulate the flow into and out of the reservoir. These gates are
controlled by a re-regulation algorithm developed in this study, which
determines the appropriate timing to open the inlet and outlet gates.
Under ideal circumstances, a RRR can potentially fully restore a river
regime to its natural state and ensure a continuous minimum flow to
accommodate ecological requirements of watercourses. However, this might
require a large RRR volume which might not always be feasible due to
economic or land availability constraints. As such, when the RRR volume
is limited, it should be operated to achieve the priority objectives and
then fulfill the secondary objectives whenever possible. A simplified
hydrograph of a natural river regime versus a regulated river regime at
a sub-daily scale is displayed in Figure 1.b. The hydrograph
demonstrates excess water occurrence that could be stored in RRRs (i.e.,
red), and potential flow release back into the river (i.e., blue).