Hydraulic Model Investigation Research Articles

Extreme Pressures and Risk of Cavitation in Steeply Sloping Stepped Spillways of Large Dams

Stepped spillways have been increasingly used to handle flood releases from large dams associated with hydropower plants, and it is important to evaluate the fluctuating pressure field on the steps. Hydraulic model investigations were conducted on three 53° (1V:0.75H) sloping and relatively large-stepped chutes to characterize the mean, fluctuating, and extreme pressures acting on the most critical regions of the step faces, near their outer edges. The pressure development along the chutes is presented, generally indicating an increase of the modulus of pressure coefficients up to the vicinity of the point of inception of air entrainment, and a decrease further downstream. The extreme pressure coefficients along the spillway are fitted by an empirical formula, and the critical conditions potentially leading to cavitation on prototypes are calculated. The correlation between the cavitation index and the friction factor is also applied for predicting the onset of cavitation on prototypes, and the results are compared with the pressure data-based method. Generally, the results obtained from those methods yield typical values for the cavitation index in the vicinity of the point of inception, varying approximately from 0.8 to 0.6, respectively. In light of these results, maximum unit discharges of about 15–20 m2/s are considered advisable on 53° sloping large-stepped spillways without artificial aeration, for step heights ranging from 0.6 to 1.2 m. For much higher unit discharges, a considerable reach of the spillway may potentially be prone to the risk of cavitation damage.

Hydraulic Model Investigation on Stepped Spillway's Plain and Slotted Roller Bucket

Hydraulic Model Investigation on Stepped Spillway's Plain and Slotted Roller Bucket

Hydraulic Model Investigation on Stepped Spillway's Plain and Slotted Roller Bucket

In ogee spillway, the released flood water from crest to toe possesses a high amount of kinetic energy causing scour and erosion on the spillway structure. The dam projects normally have a stilling basin as an energy dissipater which has specific energy dissipation limitations. The stepped spillway is a better option to minimize kinetic energy along the chute and safely discharge water in the river domain. The Khadakwasla dam is situated in Pune, Maharashtra (India), and has scouring and erosion issues on the chute of ogee spillway and on the stilling basin. The present study develops a physical hydraulic model for the dam spillway with steps, plain and slotted roller bucket as per IS Code 6934 (1998) and IS Code 7365 (2010). Experiments were performed at heads of 4m (low head) and 6m (high head) on the developed physical models, namely on the plain and slotted roller bucket model for the ogee spillway and the plain and slotted roller bucket model for the stepped spillway. It was found that the plain roller bucket of ogee spillway dissipates 81.26% of energy at the low head, whereas the stepped spillway with slotted roller bucket dissipates the 83.86% of the energy at the high head.

Shaft spillway with deflector downstream of vertical bend

This study considers the hydraulic design of a shaft spillway with a constriction at the upstream end of the vertical bend. The effect of key factors relating to the capacity of the crest, shaft and tunnel is assessed using hydraulic model investigations of several spillways and available experience from other relevant research. As a result, new expressions for the discharge and velocity coefficients have been established. An algorithm for the design procedure, based on previous experience, is also presented.

Developing Flow Region and Pressure Fluctuations on Steeply Sloping Stepped Spillways

The hydrodynamic pressure field is important for the design and safety of steeply sloping stepped spillways, which are typically designed for considerably lower maximum specific discharges than smooth spillways. The hydraulic performance of stepped spillways at high velocities may compromise its use due to major concern with safety against cavitation damage. Hydraulic model investigations were conducted in different large-size stepped chutes to characterize the nonaerated flow region which is potentially prone to cavitation damage and the pressure field acting on the step faces. The clear water depths and energy dissipation in the developing flow region are described in terms of integral measures of the turbulent boundary layer. Expressions for the location of and the flow depth at the inception point of air entrainment are derived. Pressure distributions on the horizontal and vertical faces of the step along the spillway are presented. Measurements indicated a different behavior of the pressure field in the aerated and nonaerated flow region. The mean and fluctuating pressure coefficients along the spillway are approximated by a regression function. The vertical face near the outer step edge close to the inception point of air entrainment is identified as a critical region for predicting cavitation inception in flow over stepped spillways. From the analysis of the pressure fluctuations in that region a maximum velocity of 15 m/s is proposed as a criterion to avoid extreme negative pressures in typical prototype steeply sloping stepped spillways, eventually leading to the occurrence of cavitation in the nonaerated flow.

Australian and German experiences on the use of geotextile containers

Geotextile containers and tubes are used for flood emergency protection in dams and dikes, and also as construction elements for erosion control, bottom scour protection and scour fill artificial reefs, groynes, seawalls, breakwaters and dune reinforcement. New shore protection structures, especially at sandy coasts, are increasingly needed. However, due to the increasing storminess associated with climate changes some of the existing dunes must be protected/reinforced. Hydraulic model investigations were carried out at Leichtweiss-Institute for Hydromechanics and Coastal Engineering (LWI) of the Technical University of Braunschweig, to establish reliable stability formulas for sand containers applied as dune protection/reinforcement subject to storm waves.

Stepped Spillway Hydraulic Model Investigation

An experimental hydraulic model investigation was conducted to evaluate the performance of a stepped overflow spillway . The spillway tested has a standard ogee profile with continuous steps cut into a straight portion of downstream face from tangential point (P.T) to the toe . Five constant dimensions (h/l=1.4) steps were used . The objectives of the study are to evaluate the effectiveness of steps on the amount of energy loss under nappe , transision and skimming flow regime . A preliminary comparison with a standard spillway (WES) , that the average increase in energy dissipation at toe is approximately 37% , that would imply a significant reduction of the size of downstream stilling basins . The experimental results indicate that dissipation is highest for the small values of yc/Nh tested (i.e at nappe to transition flow) , also the dissipation ratio increase with increasing N . However , the results are compared to the earlier investigation , for commonly used ranges of the steps and flow condition a general relation is established between the energy loss and the parameter yc/Nh , which may be useful as a predictive formula in a practical applications .

Hydraulic model investigation of downstream erosion of large regulators: a case study

Flow conditions, which could not be explained, occurred in the stilling basin and outfall channel of the Feni Regulator sited at the western end of the Feni River closure dam. This regulator controls outflows from the upstream reservoir which supplies irrigation water to Muhuri Project in Bangladesh. Analysis of flood discharge data revealed that the design discharge for the structure was not exceeded; yet abnormal scour occurred in the outfall channel and the brick block rip-rap placed thereon was damaged. A model study was conducted to understand the causes of such unusual local erosion downstream of the stilling basin and to provide answers to two main questions: Using a 1:30 scale model, the probable maximum scour was simulated, and the performance of alternative rip-rap designs including that of the existing one were examined. The results of this study supplemented by field scour data collected during subsequent flood seasons indicated that even if the flow rate through the regulator approaches the design flood discharge, the downstream scour is not likely to extend up to an elevation of − 10.7 m, a scour level observed in the previous year at a lower discharge. It also showed that the existing rip-rap blocks were marginally undersized and consequently the rip-rap was prone to failure if flow conditions departed from uniform. A suitable method of scour protection downstream of the stilling basin at the regulator exit also evolved from the study.

Discussion of “ Stepped Spillway Hydraulic Model Investigation ” by Robert M. Sorensen (December, 1985, Vol. 111, No. 12)

Discussion of “ <i>Stepped Spillway Hydraulic Model Investigation</i> ” by Robert M. Sorensen (December, 1985, Vol. 111, No. 12)

Discussion of “ Stepped Spillway Hydraulic Model Investigation ” by Robert M. Sorensen (December, 1985, Vol. 111, No. 12)

Discussion of “ <i>Stepped Spillway Hydraulic Model Investigation</i> ” by Robert M. Sorensen (December, 1985, Vol. 111, No. 12)

Stepped Spillway Hydraulic Model Investigation

A physical hydraulic model investigation was conducted to evaluate the performance of a stepped overflow spillway. The spillway has a standard ogee profile with continuous steps cut into the spillway face from just below the crest, to the toe. The steps significantly increase the rate of energy dissipation on the spillway face, thus eliminating or greatly reducing the need for a large energy dissipation basin at the spillway toe. Primary objectives of the investigation were to evaluate the effectiveness of the flow transition from the smooth crest profile to the steps, to quantify the energy dissipation on the spillway face, and to define the flow characteristics on the steps. The investigation demonstrated that this stepped spillway is quite effective at dissipating energy and that smooth flow transition from the spillway crest to the stepped face is easily achieved.

INVESTIGATIONS ON ORBITAL VELOCITIES AND PRESSURES IN IRREGULAR WAVES

This paper deals with the results of hydraulic model investigations of orbital velocities and pressures in irregular waves. Different simulation methods in the time and frequency domain were checked or developed, and the theoretical results compared with measurements. Using simulation methods based on linear wave theory, results with good correlation are obtained, at locations near the water surface, however, a tendency towards over- or underestimation exists.

Umfolozi Road Bridge Hydraulic Model Investigation

A movable-bed hydraulic model study was carried out of a new road bridge across the Umfolozi River in Zululand, Natal. The model was designed in accordance with the shear/settling velocity criterion, V\d〈\dr=W\dr, and crushed anthracite was used as bed material (170υ\N). The horizontal and vertical scales were 1 in 250 and 1 in 100 respectively. The velocity scale was based on Froude’s law and, with the large tractive force in this steep river, it was found necessary to deviate from the criterion Re\d〈\dr=1. Tests were run to determine the best river crossing, the required bridge length and the optimum orientation of the bridge piers, particularly with regard to minimum river bed scour. When a major flood occurred during the early stages of bridge constuction, an opportunity arose to check on the predicted scour depths. Scour depths recorded in nature were reproduced to an accuracy of about 10 percent in the model and, because of the greater confidence which could be placed on the model results, considerable savings were effected in the bridge foundations.

Hydraulic model investigations of spillway structures for the Chirkey hydroelectric station

1. For hydrodevelopments located on mountain rivers with a narrow, tortuous channel deeply incised into the rock, the problem of providing a maximum range of ejection of the flow and simultaneously eliminating its considerable effect on the canyon slopes in the case of a dam spillway with a frontal ski-jump is quite complex. Variants with a spillway tunnel ending with a lateral overflow ski-jump are considerably more flexible. 2. The use of a lateral overflow ski-jump and uniformly distributes the discharges over a sufficiently long stretch of the channel and eliminates the fall of large quantities of water onto the canyon slopes through a wide range of discharges and heads. In this case the intensity of the effect of the discharge on the channel bed can be reduced substantially in comparison with the frontal ski-jump.

Water Waves Generated by Landslides in Reservoirs

A hydraulic model investigation has been conducted to ascertain the magnitude of wave height and runup to be expected from potential landslides in Lake Koocanusa at Libby Dam, Montana. Although this investigation is of a particular site, it furnishes valuable information and insight into general problems of waves resulting from landslides that might be pertinent to other areas. The model study results show the wave heights and runup to be expected at various pool elevations as well as the effect and corrective measures proposed to reduce wave heights to an acceptable level. This paper presents the Libby prototype problem, model concepts, some experimental data, and conclusions pertinent to the prediction of landslide generated water waves.

INTERNAL VELOCITIES IB THE UPRUSH AND BACKWASH ZONE

In an earlier paper the authors reported the results of both field and hydraulic model investigations into the reaction of beaches to the action of waves of low phase difference. These waves are those whose time of uprush on the beach is equal to or less than 0.7 T, where T is the wave period. The present paper summarises the study of the internal velocity field under similar conditions. By making certain simplifying assumptions based on observations of the profile of the wave during uprush and backwash, the surging wave is open to quantitative description. For very low phase differences the uprush is approximately the reverse of the backwash, but to a different time scale. However, as the phase difference approaches the limit for the surging condition, the uprush starts and may remain in the form of a bore.

Design of Condenser Cooling Water System for a Nuclear Power Plant Located on a Large Estuary

A coordinated program is described for developing information needed for designing a condenser cooling water system for a nuclear power plant located on a large estuary to meet State water quality standards and minimize any adverse effects on aquatic life. The paper discusses estuarine conditions pertinent to the design of the intake and discharge structures, the heat assimilative capacity of the estuary, application of the momentum jet theory to the condenser cooling water discharges, and hydraulic model investigations to determine mixing and dispersion patterns.

DEVELOPMENT OF HYDRAULIC AND SHOALING CHARACTERISTICS OF SAVANNAH HARBOR, GEORGIA, BY PROTOTYPE STUDIES AND HYDRAULIC MODEL

Maintenance of Savannah Harbor became progressively more critical as the deepening of the channels caused heavy shoaling concentrations in a highly industrialized section of the harbor where spoil disposal areas are limited. Studies were performed to evaluate the shoaling and develop means of reducing the high cost of maintaining 31 miles of deep-water navigation channels. A hydraulic model investigation substantiated the conclusions reached through analysis of prototype data, greatly expanded the knowledge of hydraulic conditions and shoaling processes, and developed an improvement plan which involved a tide gate to induce the deposit of sediment in an off-channel sediment basin.

Hydraulic Model Investigation of Miter Gate Operation

Hydraulic Model Investigation of Miter Gate Operation