Flow Regime Changes: From Impounding a Temperate Lowland River to Small Hydropower Operations
|The Mode of Operation or Development Type||Water Retention Time in Reservoir (D)||Comments|
|Run-of-river (RoR)||D ≤ 2 h (~0.1 day)||No possibilities to significantly regulate flow|
|Pondage||2 h < D < 400 h (~17 days)||Daily or weekly river flow regulation. Stores water at off-peak times and releases water through turbines at peak times|
|Storage||D ≥ 400 h||Long-term impounding of water to meet seasonal and annual fluctuations in water availability. Not typical for an SHP|
|Water Retention Indicator K||1||10||100||200||500||1000||2000|
|Reservoir Useful Capacity, Vu (Percentage of Volume of Annual Inflow)||100||10||1||0.5||0.2||0.1||0.05|
|Water Retention Time (Days)||365||36.5||3.65||1.82||0.73||0.36||0.18|
|Water Body State||“Stagnant” (lake)||“Running” (stream or river)|
- Turbines use the natural flow of the river with very little alteration to the terrain stream channel at the site and little impoundment of the water.
- A type of hydro project that releases water at the same rate as the natural flow of the river (outflow equals inflow).
- Past research focusing on SHPs operating in run-of-river mode on temperate lowland rivers was reviewed;
- Historical flow and stage data to quantify the changes in flow regime pre- and post-river impoundment and after SHP construction was collected;
- Hydrograph ramping key characteristics using the hourly data of flow/stage downstream power plants to determine the causes was assessed;
- Measures to reduce the effects of SHPs by adapting turbines to the river natural flow were proposed.
2. Materials and Methods
|SHP Name||Distance of the Mouth, km||Catchment Area A km2||Mean annual Flow Q0 m3/s||Year of Construction Reservoir/SHP||Reservoir||SHP||Reservoir Filling Period “D”|
|Surface Area km2||Total Volume Mm3||Installed Power MW||Turbine Type and Number||SHP Discharge QT m3/s||Head, m||QT/Qe|
|Angiriai||25||1050||6.0||1980/1998||2.48||15.6||1.3||Propeller 2||10.2 (5.1 + 5.1)||14.5||15||23|
|Vaitiekunai||60||799||5.1||1979/2001||1.42||5.0||0.37||Cross-flow 2||5.5 (5.2 + 0.3)||9.7||1.4||15|
|Gauging Station||Distance from the Mouth, km||A, km2||Data Series Length||Q0||Q0 V-XI||Qmax||Qmax/Q0|
3. Results and Discussion
3.1. Pre- and Post-Impoundment Hydrologic Changes of the Susve River
- Free flowing river (1956–1978);
- Regulated river (2 impoundments in place) (1981–1997);
- 2 SHPs deployed (2003–2014).
|Period||Gauging Station||Mean||Min||Max||STD||Coefficient of Variation||Skewness||Kurtosis|
|Period||FDC Parameters||90 Day Minima||180 Day Minima|
|Mean Q0 m3/s||Q95 m3/s||RMSE||Bias m3/s||RMSE||Bias m3/s|
3.2. River Flow Alterations by Upstream Storage
3.3. Flow (Stage) Ramping
3.3.1. Mechanics of Ramping
- Operators, believing that they are producing more power are deliberately starting/shutting down turbines frequently during day/night periods for a certain number of hours. During the remaining time, turbines are operating at minimum flow or they can be completely stopped to comply with the prescriptions of instream flow. This mode of operation is inappropriate because energy output will be the same if turbines are operated at a reduced capacity but with stable patterns during 24 hours.
- Turbines are not well adapted to the natural streamflow regime. This means that the design discharge of the turbines is too high, and control of the discharge flowing through the turbines is not flexible. In particular, this is evident for Angiriai SHP, which has a very high design discharge of propeller-type turbines (QT ≈ 2 Q0).
3.3.2. Downstream River Stage Fluctuations
3.4. Assessment of Turbine Types with Regard to Possible Alterations of River Flow
|Turbine Type||Description of Most Suitable River Flow Regime|
|Kaplan and propeller||Propeller (unregulated)||Only for low variation in flow regime, not suitable for flashy rivers. This means that the low flow has a considerable proportion of the mean flow. The FDC must have a very flat slope. *|
|Kaplan (single regulated)||Moderate variation in flow regime|
|Kaplan (double regulated)||Any variation in flow regime (any shape of FDC)|
|Francis||Only for low variation in flow regime. Not suitable for streams with initially steeply sloped FDCs|
|Cross-flow (Ossberger, Banki-Michell, Cink)||Any variation in flow regime|
- The basic statistical test allows us to conclude that no significant trends were detected in the long-term hydrological data series and that the employed data represents necessary hydrological cycles.
- The FDC derived from long-term records consisting of pre-post impoundment and SHP development does not markedly differ in the first and second period, but differs in the third period, especially the lower part of the curve, this points to possible impact of the Angiriai SHP.
- Water retention time (D and K) is a simple and good indicator to evaluate the significance of probable impacts of an impoundment on river flow. Despite this fact, the proposed threshold values need to be based more scientifically.
- Downstream river flow (stage) ramping is an environmental issue for large hydro, and small hydropower alike. The operation of small run-of-river power plants that do not necessarily follow energy peak demand could possibly result in this negative phenomenon.
- More intensive fluctuations in the downstream river flow and stage can be observed in SHPs with high turbine design flows, unregulated types of turbines and a low number of turbines.
- When an SHP is not intended for operations covering peak energy demand, its design turbine flow should not exceed the mean annual river discharge. There are currently a large number of advanced turbines available. The turbines have a wide range of capacities, there are also advances in turbine design (e.g., double regulated or cross-flow). These features will allow for adapting higher values of design flows with a minimum risk of flow ramping.
- By applying simple turbine operational measures—step-wise turbine start up and shut-down together with varying their number and capacities during 24 h—river flow ramping rates can be substantially alleviated.
- Recommended turbine types are most suitable for a particular natural flow regime. However, total avoidance of downstream hydrograph ramping is not possible without applying structural measures (involving physical constructions) for run-of-river projects with impoundments.
Conflicts of Interest
small hydropower plant
design flow of turbine
environmental (instream) flow
water retention time in a reservoir (reservoir filling period)
water retention indicator related to water retention time in a reservoir
mean daily flow duration curve
run-of-river (HP operation mode)
river flow and stage gauging station
drawdown depth of a reservoir needed for power generation
water level, reservoir normal water level
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Punys, P.; Dumbrauskas, A.; Kasiulis, E.; Vyčienė, G.; Šilinis, L. Flow Regime Changes: From Impounding a Temperate Lowland River to Small Hydropower Operations. Energies 2015, 8, 7478-7501. https://doi.org/10.3390/en8077478
Punys P, Dumbrauskas A, Kasiulis E, Vyčienė G, Šilinis L. Flow Regime Changes: From Impounding a Temperate Lowland River to Small Hydropower Operations. Energies. 2015; 8(7):7478-7501. https://doi.org/10.3390/en8077478Chicago/Turabian Style
Punys, Petras, Antanas Dumbrauskas, Egidijus Kasiulis, Gitana Vyčienė, and Linas Šilinis. 2015. "Flow Regime Changes: From Impounding a Temperate Lowland River to Small Hydropower Operations" Energies 8, no. 7: 7478-7501. https://doi.org/10.3390/en8077478