Showing papers on "Spillway published in 1989"

Study of air entrainment and aeration devices

Abstract: Cavitation erosion damage to spillway surfaces may be prevented with the use of aeration devices (aerators) introducing air in the layers close to the channel bottom in order to reduce cavitation damage. The performance of a spillway aerator are studied on steep spillway model with high velocities (from 4 m/s up to 15 m/s) and new air concentration and velocity measurements were performed. The data are presented and analysed developping new equations. The behaviour of the air demand relationship between the air discharge and the subpressure in the cavity beneath the nappe is analyzed. All these results provide a better understanding of the air entrainment processes above a spillway aerator and the quantity of air entrained can be obtained. Experimental data at the impact point of the jet are shown. The results indicate a strong deaeration process occuring in the impact region and reducing the effect of the aeration occuring above the aerator.

Flow downstream of an aerator - aerator spacing

Abstract: Cavitation erosion damage to spillway surfaces may be prevented with the use of aeration devices (aerators), introducing air in the layers close to the channel bottom. A study of the flow downstream of a spillway aerator was performed on the Clyde dam spillway model. The results are presented and a comparison between the free-surface aeration and the flow downstream of an aerator is developed. Such a method enables to compute the flow parameters at any point downstream of an aerator, in particular the air concentration near the spillway bottom. The computed results are compared with experimental data. This analysis contributes to a method for the determination of aerator spacing and aerator location.

Extreme Historical UK Floods and Maximum Flood Estimation

Abstract: Historical flood events recorded as flood marks on bridges and houses or reported in newspapers and journals provide valuable information on the maximum size of floods which are likely to occur in the UK. For hydrological analysis peak water level must be converted to peak discharge, which is problematic even when the flood has been recorded at a flow gauging station. This paper describes some of the methods used to calculate the magnitude of extreme floods. Sixty-nine historical UK floods are plotted on a graph of discharge versus drainage area and compared with curves published in 1933 and 1960 which were used to estimate the Normal Maximum Flood required for dam spillway design before 1975. From that date the design standard became the Probable Maximum Flood. PMF is calculated for 17 sites where extreme floods have occurred. For six events the observed peak discharge exceeded PMF. Possible reasons for these findings are discussed.

Stepped protection blocks for dam spillways

Abstract: Stepped block protection has considerable potential as a low-cost method for the construction of chute spillways and the protection of embankments from erosion by overtopping flow. Design practice developed in the USSR as a result of extensive model studies and prototype trials is being evaluated by CIRIA with a view to producing a design guide. Dam engineers in the UK and USA are particularly interested in using stepped blocks for auxiliary spillways to increase inadequate spillway capacity at existing dams. The results of research and experience gained are discussed here

Hood Characteristics for Siphon‐Shaft Spillways

Abstract: A shaft spillway is equipped with a hood in order to operate as a siphon. Appropriate hood dimensions and shape are searched by using cylindrical hoods of different dimensions for a standard bellmouthed shaft. The hood, sized and streamlined according to experimental results, showed a good performance concerning the discharge and subatmospheric pressure conditions. Design charts for discharge coefficient as well as critical hood and shaft‐crest pressures are presented for such a siphon‐shaft spillway, and a design example is given.

Proposed Swedish Spillway Design Guidelines Compared with Historical Flood Marks at Lake Siljan

Abstract: A comparison between the proposed Swedish spillway design floods and historic flood marks made at lake Siljan in central Sweden, is shown. Frequency analysis is performed incorporating pregauge information on water levels together with a sensitivity analysis of modelling assumptions. A water level of 0.42 to 0.75 metres above the highest historic flood mark (166.10 m.a.sl., 1659) was obtained when routing the design spring flood through lake Siljan. The design autumn flood lifted the lake to 1.56 to 1.52 metres below the highest flood mark. Return period for the design spring and autumn flood was estimated to about 1,000 years. The uncertainty in frequency analysis proved to have larger impact than modelling assumptions on estimating the risk of the design flood.

Proposed Swedish spillway design floods in relation to observations and frequency analysis

Abstract: Proposed guidelines for the computation of spillway design floods in Sweden are described. The guidelines are based on the transformation of extreme hydrological conditions to floods by use of conceptual hydrological models. Floods computed according to the guidelines are compared to observations for both autumn conditions when rainfall dominates and for snowmelt conditions in spring. The highest floods are in the order of 60% of the proposed guidelines. Comparisons with frequency analysis show that the computed floods are well beyond the 10,000 year flood according to the Gumbel and Lognormal 2 distribution functions, while the results according to Lognormal 3 are more uncertain.

Mechanics of an Earth Spillway Failure

Abstract: Severe storms in Mississippi during May of 1983 produced flooding that resulted in one of the few flood control reservoir auxiliary spillway failures on record. Subsequent analysis of the breach using state of the art procedures reproduced the downstream observations of high water, but failed to reproduce the observed timing of the failure or conditions observed at the site during spillway operation and failure. The present effort uses the observed failure conditions, site geology, and a simplified model of failure mechanics to develop an independent estimate of the breach outflow hydrograph. Agreement with the previously estimated peak discharge is surprisingly good. The analysis, although site specific, clearly demonstrates the important role of surface and soil discontinuities in the failure process.

Designing aerators for high velocity flow

Abstract: Airflow through aerators has been measured in several recently built structures: the Foz do Areia and Emborcacao spillways in Brazil; the Amaluza spillway in Ecuador; the Colbun spillway in Chile; and, the Tarbela Tunnel No. 3 outlet in Pakistan. The corresponding basic data have been listed and their water flow and aerator conditions are compared and analysed. Analysis of the prototype experience confirms the role of some basic parameters in the air dragging mechanism, and the results may be used as a practical tool at the design stage of aeration devices

Low-Head Hydropower Impacts on Stream Dissolved Oxygen

Abstract: A method to evaluate the effect of hydropower development on downstream dissolved oxygen (DO) is presented for a low head dam. Water, previously aerated during release over spillways and under gates, is diverted through the hydropower facility without further aeration. The oxygen transfer that occurs as a result of air entrainment at the various release points of a dam is measured. Oxygen transfer efficiencies are calculated and incorporated into an oxygen transfer model to predict average release DO concentrations. This model is used to systematically determine the effect of hydropower operation on downstream DO. Operational alternatives are investigated and a simple operational guide is developed to mitigate the effects of hydropower operation. Combinations of reduced generation and optimal releases from the dam allow the hydropower facility to operate within DO standards.

Design of Efficient Side‐Channel Spillway

Abstract: The paper describes a simplified method of designing an efficient side‐channel spillway, with particular reference to an L‐shaped weir. The method can be used with minor modifications for a straight weir or one U‐shaped in plan. The invert of the collecting trough slopes down from the crest of the end weir to the chute. The invert is designed to be as high as practicable so as to reduce construction costs. An additional benefit from this design method is that the spillway operates with little wave action. The usual momentum equation has been extended to include the downstream component of velocity of approach of lateral inflow. The spillway trough profile is designed directly by an iterative process which has been computerized. The design shape should be checked by hydraulic modeling, but, in the example given, was found to require very little alteration.

Movable pitchfork gate dam

Abstract: The utility model discloses a movable pitchfork gate dam which is fit for empoldering low-head water power resource The utility model is characterized in that the movable pitchfork gate dam has functions of water fending, drainage controlling, sand discharging, energy eliminating and stability increasing The movable pitchfork gate dam can be installed using prefabricated parts, and the opening and closing of a gate can be controlled by filling and draining water Facilities of foundational construction and energy eliminating can be saved; constructions can be carried out in water, avoiding cofferdams and diversion constructions, and constructions can be carried out both in high water and low water periods The utility model has the advantages of low cost, long service life and surpassing 10 meters applied dam height The movable pitchfork gate dam is suitable for any foundations and seismic regions, can successfully eliminate the deposited silts and drifts behind the gate, and is easy to be examined, repaired and dismantled; the components of the movable pitchfork gate dam can be repeatably used in other places In spite of being suitable to be the gate dam, the cofferdam and the gate of a spillway, the movable pitchfork gate dam can be developed into a jury hydroelectric power station

Simplified earthquake analysis of gated spillway monoliths of concrete gravity dams

Abstract: : The simplified procedure for earthquake analysis of concrete gravity dams developed earlier for nonoverflow monoliths is extended in this report to gated spillway monoliths. Standard data are presented for the vibration properties of such monoliths and for all parameters that are required in the analysis procedure. The use of the simplified analysis procedure and of a computer program that facilitates implementation of the procedure is illustrated by examples. Keywords: Concrete, Dams (gravity), Design, Dynamics, Earthquakes, Earthquake resistant structures, Foundation interaction, Hydrodynamic pressure, Spillway.

A checklist of the fishes of the Kwando River, Selinda (Magwegkwana) Spillway, Lake Liambezi and Chobe River systems with notes of their biology and distribution

Abstract: Van der Waal & Skelton (1984) reviewed the early history of fish collections, mainly based on collections from Caprivi. Important collections from the Chobe River near Kasane were made by the Vernay-Lang Kalahari Expedition in 1930 (Fowler 1935). The second Barnard Carp Expedition in 1952 made collections on the Kwando River (Jubb 1958). Collections of fish from Lake Liambezi and the Chobe River near Kazangula were made for the Queen Victoria Museum, Zimbabwe, in November and December 1961 (Guy 1962). Fox & Watt (1976) made a brief visit to the Linyanti River at Shaile in order to observe the effects of a dense growth of the exotic aquatic plant Salvinia molesta on the composition of the fish community in infested and non-infested lagoons. Van der Waal & Skelton (1984) provide the only comprehensive checklist of fishes of the Caprivii.

A study of air entrainment. Application to spillway design

Abstract: Cavitation erosion damage to spillway surfaces may be prevented with the use of aeration devices, called aerators. These serve to introduce air in the layers close to the channel bottom in order to reduce the cavitation damage. The performances of spillway aerator are studied with data obtained on a steep spillway model and high velocities (from 4 m/s up to 15 m/s). Air concentration and velocity profiles at various cross sections above and downstream of the aerator are presented and analyzed. They provide a better understanding of the air entrainment processes occurring above a bottom aerator. A new analysis the flow downstream of an aerator is presented and this contributes to a new method for the determination of aerator spacing.

Etude des phénomènes d'entraînement d'air. Application aux évacuateurs de crue

Abstract: Cavitation erosion damage to spillway surfaces may be prevented with the use of aeration devices, called aerators. These serve to introduce air in the layers close to the channel bottom in order to reduce the cavitation damage.The performances of spillway aerator are studied with data obtained on a steep spillway model and high velocities (from 4 m/s up to 15 m/s). Air concentration and velocity profiles at various cross-sections above and downstream of the aerator are presented and analyzed. They provide a better understanding of the air entrainment processes occurring above a bottom aerator.A new analysis of the flow downstream of an aerator is presented and this contributes to a new method for the determination of aerator spacing.

Storing Sediment and Freeing Fish

Abstract: Ten years after the volcanic eruption in the State of Washington, 3.8 billion cu yd of debris deposited in Fork Toutle River Valley has been eroding and choking a system that includes the Cowlitz and Columbia rivers. An earth and rockfill dam is currently being erected on the North Fork Toutle River to trap and store any sediment that threatens the river system. Known as the sediment retention structure (SRS), the project must not only collect the sediment, but also allow for the migration of salmon and steelhead to those rivers. The main features of the SRS, the embankment dam, the spillway, the outlet works, and a facility to aid fish migration are described. Details of the construction are described including the fish collection facility (FCF), which consists of a fish barrier across the river, a fish ladder, a collection pool and a lock. The details of the structural design are described. The survival rate of fish migrating downstream will determine any changes to the design or construction of the spillway and outlet channels.

Improvement of Dissolved Oxygen Levels at Shepaug Hydro Station

Abstract: Discharge from Shepaug Dam in Connecticut fall below the state standard for dissolved oxygen in summer and early fall, because of thermal stratification of the upstream eutrophic reservoir. A study of methods of increasing dissolved oxygen in the Shepaug discharges was performed. Alternatives considered included reservoir destratification, spillway releases, intake modifications, turbine air injection, oxygen injection, and adding a new generating unit with a high level intake. The various options were assessed from an economic and environmental point of view. The two most attractive schemes were assessed using a mathematical water quality model, and the intake modification option was selected for more detailed evaluation.

Scour at the Straight Drop Spillway with High Tailwater

Abstract: Tests made to determine the effect of high tailwater and long duration flows on scour downstream of the straight drop spillway are described in this paper. Results are presented on performance with the standard design basin, the basin with a nappe-shaped weir crest, and basins with rounded abutments at the entrance to the stilling basin. The basins with the rounded abutments performed best and the standard design basin performed poorest. Based on these tests, from a hydraulic performance standpoint, the straight drop spillway should be designed using rounded abutments with a 2H or larger radius of rounding (H is the specific head in the approach channel).

Spillway Discharge Calculations in NWS DAMBRK

Abstract: The hydraulic computations for spillway discharges employed by the National Weather Service computer program NWS DAMBRK are presented. Important assumptions and limitations of the equations are discussed. The techniques have been applied in several dam-breach studies in the United States.

Service spillway of the Kambarata No. 1 hydroelectric station

Abstract: 1. The use of the scheme of a tunnel spillway with dissipation of energy inside the conduit in a shaft stilling basin under conditions of the Kambarata No. 1 hydrostation makes it possible to reduce the volume of earthworks and concrete in comparison with alternative variants and to protect the downstream stretch of the river valley from collapse of the slopes and substantial erosion of the channel. 2. Investigations confirm the efficiency, reliability, and safety of the spillway and all its components.

Navigation Conditions at Oliver Lock and Dam, Black Warrior River Project; Hydraulic Model Investigation

Abstract: : William Bacon Oliver Lock and Dam is located on the left descending bank of the Black Warrior River about 3463 river miles above Mobile, AL, in the corporate limits of Tuscaloosa, AL The principal existing structures are a 700- ft-long fixed-crest spillway and a 95- by 460-ft lock The dam forms a run of the river pool that extends 88 miles upstream to Holt Lock and Dam During high pool elevation and river discharges, tows bypass the lock and navigate over the fixed-crest weir Oliver Lock has the smallest chamber (95 by 460 ft) on the Black Warrior-Tombigbee Waterway, and the present plan of development is to replace the existing lock and dam with a new structure located about 2,300 ft downstream The replacement structure will provide a 110- by 600-ft lock chamber and an 815-ft-long fixed-crest spillway A fixed-bed model reproduced about 28 miles of the Black Warrior River channel and adjacent overbank area to an undistorted scale of 1:100 (rrh)

Warragamba Dam Flood Protection Program

Abstract: Warragamba Dam supplies most of Sydney's water. Recent estimates suggest that the dam is in jeopardy at approximately 60% of the probable maximum flood. Damage or failure of the dam would pose disastrous consequences for the Hawkesbury/Nepean Valley and the general Sydney community. The Water Board has embarked on a priority, two-stage program to upgrade the safety of the dam. Stage one is expected to be completed in late 1989. Stage two involves further protection of the dam and a possible improvement to the dam's limited ability to mitigate floods. Options under consideration include an auxiliary spillway, and various flood mitigation alternatives, such as further raising of the dam in concrete or construction of a new rock fill dam. Significant progress has been achieved since the commissioning of an EIS for Stage 2 in November 1987. At this stage, there appears to be wide spread community acceptance of the need for full PW protection of the dam. The Hawkesbury/Nepean Valley is particularly vulnerable to large floods, such as the flood of 1867. Large floods would cause severe damage and would pose difficult flood warning/evacuations problems. Attention has been focused on the adequacy of current one in hundred chance per year flood policy for development purposes. Consequently, there is increasing interest in schemes providing substantial flood mitigation. The Board has budgeted an additional $80m to construct a spillway to protect the dam. Should the Government and community require a more expensive flood mitigation option to be constructed funds in excess of the $80 million will need to be generated by other sources.

A Study of Air Entrainment: Application to Spillway Design = Etude des Phenomenes d'Entrainement d'Air: Application aux Evacuateurs de Crues

Abstract: Cavitation erosion damage to spillway surfaces may be prevented with the use of aeration devices, called aerators. These serve to introduce air in the layers close to the channel bottom in order to redue the cavitation damage. The performances of spillway aerator are studied with data on a steep spillway model and high velocities from 4 m/s to 15 m/s. Air concentration and velocity profiles at various cross-sections above and downstream of the aerator are presented and analysed. They provide a better understanding of the air entrainment processes above a bottom aerator. A new analysis of the flow downstream of an aerator is presented and this contributes to a new method for the determination of aerator spacing.