Low-level Outlets Research Articles

Intrusive effects of dual-tip conductivity probes on bubble measurements in a wide velocity range

High-velocity air-water flows in hydraulic structures, such as spillways and low-level outlets, are characterised by high turbulence levels and strong self-aeration. In air-water flows, the void fraction varies from values close to zero at the invert up to near unity in the upper spray region. Because of the strong aeration, non-intrusive optical measurement techniques can only be applied to a limited extent, and intrusive phase-detection probes are widely used to estimate air-water flow properties, including void fraction, interfacial velocity, and chord sizes. However, these probes are often used without independent validation. Herein, this study systematically investigated the uncertainties of phase-detection probe measurements of individual bubbles traveling at a wide range of velocities (up to 7.5 m/s) in a vertically upward bubbly pipe flow. Velocities and chord times of air bubbles were simultaneously measured with an intrusive dual-tip conductivity probe and stereo-view high-speed videos, and a comparative analysis identified various effects of bubble-probe interactions on the measurement accuracy of the bubble velocities. The bubble deceleration due to these bubble-probe interactions were quantified. For standard interactions (i.e., bubbles undergoing no significant deformation and slowing down), the velocity estimates compared well with a theoretical model (errors less than 2 %), while larger errors were found for non-standard interactions such as those characterised by crawling, drifting, and waking mechanisms. In addition, a novel correction scheme for chord times was proposed, improving the accuracy of chord length, bubble size and void fraction estimations in air-water flows.

Air–Water Flow Patterns and Shockwave Formation in Low-Level Outlets

Air–Water Flow Patterns and Shockwave Formation in Low-Level Outlets

Generation of Supersaturated Total Dissolved Gases from Low-Level Outlets at Hydropower Facilities

Generation of Supersaturated Total Dissolved Gases from Low-Level Outlets at Hydropower Facilities

Managing reservoir sedimentation through coordinated operation of a transboundary system of reservoirs in the Mekong

Sedimentation is one of the major challenges for the long-term sustainable operation of a hydropower reservoir. Trapping of sediment reduces its storage capacity and consequently diminishes hydropower production. Here first the development of a REServoir Sediment MANagement routine (ResSMan) and its integration into a hydrological model, the Soil and Water Assessment Tool (SWAT) is presented. ResSMan has functions to determine sediment accumulation in multiple reservoirs, and its impacts on the storage capacity and hydropower production under user-specified operation policies. It allows to compute the restoration of storage capacity due to the removal of sediment by flushing, i.e. the removal of sediment from a reservoir by passing water and sediment through the low-level outlets, and sluicing, i.e. passing sediment before suspended sediment solids have settled down in the reservoir. The capabilities (flushing and sluicing) of ResSMan were evaluated through a comparison with the Sediment Simulation Screening (SedSim) model, a well-tested sediment management simulation model and the resulting R2 values of 0.99 validated its capabilities. Subsequently, ResSMan was applied to assess and manage reservoir sedimentation using different management strategies in a complex system of 19 reservoirs (12 currently existing) in the Sesan and Srepok (2S) basin of the Mekong River for 2021–2120. The unregulated mean annual sediment yield at the outlet of the 2S basin was estimated as 7.24 million tonnes/year (Mt/y) and it will be reduced to 0.11 Mt/y due to the operation of the 19 reservoirs. In total 924 Mt of sediment will accumulate in these 19 reservoirs over 100 years, resulting in an average trapping efficiency of 74% (ranging from 11% to 97%). System-wide sediment management coordination simulations demonstrated that bi-annually and 5-yearly flushing of alternate reservoirs are effective options for efficiently releasing sediment in the 2S basin. However, the use of frequent flushing (annual or bi-annual) may be more favourable to minimize adverse impacts due to release of higher sediment loads on downstream ecosystems. The analysis is an initial step towards the coordination of sediment management plans and policies for the multi-reservoirs system in the Mekong basin.

Study on the dynamic interaction between an arch dam and its appurtenant structures

This paper aims to analyze the effect of appurtenant structures on the seismic response of arch dams. For this purpose, a fine-grade finite element model is developed for the Sanhekou dam in China as a case study, which includes the discharge structure, the intake tower, the elevator shaft, and the viewing platform. The results indicate that different appurtenant structures have different influences on the seismic response of the dam. The crest spillways and the low-level outlets perforated in the dam body, significantly affect the seismic response of the local region of the arch dam body, while the appurtenant structures extending from the dam body, such as the intake tower, the elevator shaft, and the sightseeing buildings, have little influence on the seismic response of the dam body. In addition, the local vibration amplitudes of the high cantilever water inlet and the high-rise elevator shaft are significant, and thus they may be damaged under seismic loads.

Influences of upstream reservoir stratification and downstream tidal fluctuations on the summer thermal regime of a regulated coastal river

AbstractThis study focused on the effects of upstream reservoir thermal dynamics and downstream tidal influences on temperatures in a 25‐km reach of Alouette River (coastal British Columbia, Canada) below a control dam and upstream of its confluence with Pitt River. Temperature was monitored during summer 2013 using 25 sensors. Water was released from the reservoir through a low level outlet at approximately 2.7 m3 s−1, except during late spring when a higher flow was released over the dam spillway. Temperature variations in the lowest section of Alouette River, and in the lower portion of a tributary, were distinct from those further upstream due to backwatering effects associated with a semi‐diurnal tide, which can cause flow reversals in Pitt River. An internal seiche was identified in the reservoir during mid‐summer that resulted in oscillating releases of warmer and cooler water with an amplitude of up to 6°C and a period of approximately 12 hr. Wavelet analysis and band‐pass filtering indicated that the 12‐hr signal declined in strength with downstream distance, but remained detectable about 15 km below the dam. In contrast, the 24‐hr diel signal increased in strength with distance below the dam. Travel times computed via cross‐correlation of the 12‐hr signals with that at the low level outlet were within ±10% of those estimated from measured mean velocities. Lagrangian tracking of water parcels using the derived travel times indicated that the cooling effect of periodic releases of cold water during the seiching period persisted to the lower extent of the non‐tidal reach. The tidally influenced locations experienced higher temperatures than those recorded in the non‐tidal portion of Alouette River, although the relative roles of heating in the upstream tidal reach versus upstream advection of water associated with tide‐driven flow reversals in Pitt River require further study.

Air Demand of Low-Level Outlets for Large Dams

AbstractLow-level outlets are key safety elements of reservoir dams, especially for structures with high heads. Their main purpose is the regulation and—if required—rapid drawdown of the reservoir ...

Study on Sediment Management Strategy of Sandy River Reservoir

The establishment and operation of one hydropower complex will change the movement law of water flow and sediment of the river way, and sediment deposition is difficult to avoid. Due to the characteristics of “high utilized water head, small reservoir storage, high sediment content and particle hardness” of proposed Upper Arun Hydroelectric Project, this paper applies one-dimensional hydrodynamic & sediment model to studying the movement law of flow and sediment of 3 different sediment management strategy, then finding the rational one which can not only maintain a sustainable live storage of the reservoir by sediment flushing, but also reduce sediment abrasion to turbines. The results show that, when the proposed fully open of all low-level outlets for sediment flushing being implemented during high flow periods, the difference of sedimentation is relatively minor, which all 3 options can effectively maintain the live storage of the reservoir thereby minimizing deposition of sediment at the places that will affect project operation. Therefore, the selection of sediment management strategy should depends on the comprehensive technical and economic comparison. Due to the advantages of relatively short outage periods, conventional technology of construction and the largest Net Present Value, the appropriate sediment desilting operation mode is applying a sediment bypass tunnel with its intake located in the middle of reservoir, and meanwhile utilizing the reservoir’s sedimentation effect, to reduce sediment content transported through the turbines.

High-velocity air-water flows downstream of sluice gates including selection of optimum phase-detection probe

High-velocity air-water flows occur commonly in hydraulic structures including low-level outlets. Novel experiments of air-water flow patterns and properties were conducted along a tunnel chute downstream of a high-head sluice gate for a wide range of flow conditions including Froude numbers between 10 and 45, Reynolds numbers up to 2.4 × 106 and flow velocities up to 22 m/s, limiting potential scale effects at close to prototype scale. The study documents the evolution of distribution shapes and characteristic values for a range of air-water flow properties along the tunnel chute. The results show an increase in flow aeration and bubble break-up processes with increasing upstream head and flow velocity. The measurement of air-water flow properties was conducted with a side-by-side double-tip conductivity probe, amongst others, which in a comprehensive comparative analysis of four types of phase-detection intrusive probe designs was identified as the most suitable one in high-velocity air-water flows as studied herein. The performance of the conductivity probe for the present flow conditions suggests that the probe may be a promising instrumentation for future prototype measurements of high-velocity air-water flows.

One-Dimensional Modeling of Suspended Sediment Dynamics in Dam Reservoirs

AbstractContinuous sedimentation of fine sediments in dam reservoirs triggers considerable economic and ecological impacts worldwide. The definition of appropriate reservoir management procedures requires accurate tools for predicting the sediment dynamics both in the reservoir and downstream reaches. In this article, a one-dimensional numerical model aiming to reproduce suspended sediment dynamics is presented. To take into account water intake locations and vertical distribution of suspended sediments in areas of larger water depths close to dams, an additional module is developed. The model is applied to a reach of the French Upper Rhone River, including two reservoirs. The calibrated model provides accurate flux estimates and reproduces erosion and deposition patterns observed in the reservoir for several scenarios. The additional module allows to improve the estimation of the sand load released by low-level outlets. Eventually, the model could be used either to provide insights on the outcome of past...

댐 비상방류 설계기준 선정을 위한 수리수문학적 검토(I)

It is well known that emergency outlet works have to be provided for the safety of dams. However, concept of emergency outlet works did not applied for the design of the most dams in Korea. Korean design standard for low-level outlet works does not provide enough design criteria which could be used in design of emergency outlet works. In this research, as-built status and hydraulic design criteria of outlet works, such as drawdown rate or hydraulic pressure due to the impounded water depth, were examined. Another relationship between drawdown rate and the dam slope stability was also examined with SEEP model. It was found that 25% reduction of impounded water depth decreases the pressure forces about 50%. Therefore, outlet works should be designed to drawdown properly at the beginning of the emergency. Seepage analysis of dam bodies showed that most of Korean dams could safely stand for 1m/day drawdown rate. Higher drawdown rate could result high discharge so the drawdown rate must be related with the flood risk of downstream. Finally, multi-stage design was recommended that faster discharge for the initial 25% of water depth in 7-10 days than the rest of it in 1-2 months.

댐 비상방류 설계기준 선정을 위한 수리수문학적 검토(II)

It is well known that emergency outlet works have to be provided for the safety of dams. However, concept of emergency outlet works did not applied for the design of the most dams in Korea. Korean design standard for low-level outlet works does not provide enough design criteria which could be used in design of emergency outlet works. In this research, as-built status and hydraulic design criteria of outlet works, such as drawdown rate or hydraulic pressure due to the impounded water depth, were examined. Another relationship between drawdown rate and the dam slope stability was also examined with SEEP model. It was found that 25% reduction of impounded water depth decreases the pressure forces about 50%. Therefore, outlet works should be designed to drawdown properly at the beginning of the emergency. Seepage analysis of dam bodies showed that most of Korean dams could safely stand for 1m/day drawdown rate. Higher drawdown rate could result high discharge so the drawdown rate must be related with the flood risk of downstream. Finally, multi-stage design was recommended that faster discharge for the initial 25% of water depth in 7-10 days than the rest of it in 1-2 months.

Optimizing Selective Withdrawal from Reservoirs to Manage Downstream Temperatures with Climate Warming

AbstractSelective withdrawal systems can take advantage of thermal stratification in reservoirs to manage downstream temperatures. Selective withdrawal might also help adapt operations to environmental changes, such as increased stream temperatures expected with climate change. This exploratory study develops a linear programming model to release water from different thermal pools in reservoirs to minimize deviations from target downstream temperatures. The model is applied with representative thermal dynamics to Lake Spaulding, a multipurpose reservoir on the South Fork Yuba River in California with climate warming represented by uniform increases in air temperature. Optimization results for thermal pool management with selective withdrawal are compared to a single, low-level outlet release model. Optimal selective withdrawal hedges the winter release of cold water to decrease summer stream temperatures. With climate warming, selective withdrawal can help lessen stream warming in the summer but at a cost...

Preliminary Investigation and Design Considerations for the Rehabilitation of Trash Screen at Jor Reservoir Low Level Outlet

Jor reservoir was constructed to assure flow regulation for the Woh hydroelectric power station, as well as to provide storage for flood control. The reservoir outlet is frequently operated once every three months to flush the deposited sediments accumulated within the reservoir. Preliminary testing and inspection on the reservoir outlet revealed that the trash screen located at upstream of reservoir outlet had been damaged and dislodged creating a large opening at the trash screen thus permitting entry of larger logs which blocks the passage of water through the reservoir outlet. Bathymetric survey showed the total height of sediment deposition above the reservoir outlet covering the trash screen was estimated at approximately 13 m. The sediment volume estimated to be cleared from above the reservoir outlet area is 15,000 m3. Soil investigation conducted shows that the sediments deposited within the reservoir exhibits various different profile and characteristics. The average sediment distribution at deeper depths closer to the trash screen was found to be of 27% clay, 26% silt, 43% sand and 4% gravel. The rehabilitation works on structure as the low level outlet serves as an important feature for reservoir sedimentation management as well as for safe flood discharge release as designed for the dam. It is important that the function of the reservoir outlet structure is maintained in accordance to the International Dam Safety Standards.

Computing Air Demand Using the Takagi–Sugeno Model for Dam Outlets

An adaptive neuro-fuzzy inference system (ANFIS) was developed using the subtractive clustering technique to study the air demand in low-level outlet works. The ANFIS model was employed to calculate vent air discharge in different gate openings for an embankment dam. A hybrid learning algorithm obtained from combining back-propagation and least square estimate was adopted to identify linear and non-linear parameters in the ANFIS model. Empirical relationships based on the experimental information obtained from physical models were applied to 108 experimental data points to obtain more reliable evaluations. The feed-forward Levenberg-Marquardt neural network (LMNN) and multiple linear regression (MLR) models were also built using the same data to compare model performances with each other. The results indicated that the fuzzy rule-based model performed better than the LMNN and MLR models, in terms of the simulation performance criteria established, as the root mean square error, the Nash–Sutcliffe efficiency, the correlation coefficient and the Bias.

Determining Air Demand for Small- to Medium-Sized Embankment Dam Low-Level Outlet Works

Air vents in low-level outlet works are installed downstream of control gates or valves to mitigate subatmospheric pressures locally and reduce the potential for damaging cavitation. Published air-vent-sizing methods, which have been limited to large-dam outlet geometries (in-line vertical slide gate), are not applicable for small- to medium-sized embankment dam low-level outlet geometries in which the slide gate is installed on the upstream sloping face of the dam. The water-discharge and air-demand characteristics of a lab-scale low-level outlet works, with the inlet installed on a 3H:1V sloping floor (representing the upstream face of an embankment dam), were evaluated experimentally with respect to gate design (round or rectangular), gate opening, driving head, and outlet submergence; the results are reported herein. Discharge coefficients for predicting low-level outlet works discharge rates (vented or nonvented), and a methodology for air-vent sizing for small- to medium-sized embankment dams are al...

Improving Concrete Containment Structures Associated with Fixed-Cone Valves

Fixed-Cone valves are often used to dissipate energy and regulate flow at the low level outlet works of dams. Fixed-Cone valves, also known as Howell-Bunger valves, create an expanding conical jet allowing the energy of the water to dissipate over a large area. However, in many applications constructing the large stilling basin necessary for these valves is either not possible or not feasible. In order to reduce the relative size of the stilling basin, hoods or concrete containment structures have been used in conjunction with Fixed-Cone valves. This paper discusses the use of baffles in concrete containment structures in order to dissipate energy in a considerably confined space. It was determined that using baffles, in place of a deflector ring and end sill (Used in traditional containment structures.), significantly improves the function of containment structures by reducing downstream flow velocities and improving flow patterns and stability. This information will be useful to engineers allowing them to minimize scour and erosion associated with concrete containment structures.

Low level outlets 1: formula for target capacity

It has for some time been apparent that there is little consistency of approach in the United Kingdom to the question of low level outlet capacity or valve arrangements. The author suggests a new formula for low level outlet capacity for higher category earthfill dams in the United Kingdom which may also be of use for small reservoirs with earthfill dams overseas. The question of fitting upstream valves and/or lining of pipes is also discussed.

Low level outlets 2: risk assessment

It has been apparent for some time that there is little consistency of approach in the United Kingdom to the question of low level outlet capacity or valve arrangements. This paper indicates a risk based approach that may be adopted for detailed assessment.

Low level outlets 3: British Waterways' approach

British Waterways' portfolio of old dams is described and contemporary valve layouts are discussed. The risks associated with such layouts are reviewed as are recent recommendations by inspecting engineers for mitigating these risks. British Waterways' emergency drawdown policy is stated and three recent examples of pipelining are described. Finally, lessons learnt by British Waterways from the Boltby and Ulley incidents are summarised.