The latest issue of Dam Engineering (Volume XXXII, Issue 1) has been published:
Assessment of Material Properties of Concrete Gravity Dams – An Indian Case Study
P N Ojha, Brijesh Singh, V V Arora, Pramod Narayan, Amit Trivedi & Mantu Gupta
Abstract: In India more than 5000 large dams have an average age of 40 years wherein assessment of dam health is an emerging requirement. Due to ageing, the materials are subjected to severe weather impacts that affect the strength and, ultimately, the operational performance of the dam. This paper presents the case study of a concrete gravity dam currently being investigated under the ongoing Dam Rehabilitation & Improvement Project (DRIP) of the Central Water Commission in India. The dam chosen is more than 50 years old. To study the unusual deflection of this dam detailed assessment of concrete and rock properties for conducting advanced Finite Element Analysis (FEA) of a gravity dam was needed. This study was undertaken through a multi-disciplinary approach which involves testing of the dam material, studying available instrument readings and developing a 3D numerical model of the dam, its foundation and reservoir to perform sequential thermal, static and linear dynamic analysis. It covers a complete material property investigation in order to carry out an accurate and reliable analysis. The complete investigations of mechanical properties included a field assessment, thermal properties, and expansion issues. Concrete core samples from the dam were used to determine mechanical properties, chemical properties, petrographic studies, surface morphology study of concrete samples using the Scanning Electron Microscopic (SEM) method, including detection of ettringite presence and fracture pattern, X-Ray diffraction analysis and Alkali Aggregate Reactivity. Testing on rock samples from the left and right bank on the downstream side of the dam for determining volumetric weight, Modulus of Elasticity, Poisson’s Ratio, Uni-axial Compressive Strength, Ultrasonic Pulse Velocity (UPV) (P-wave only) and permeability co-efficient of the rock mass (laboratory test only) are also presented in this paper. Based on the study, it is seen that the overall quality of concrete is sound and the experimental test results of compressive strength, modulus of elasticity, Poisson’s Ratio and split tensile strength are of a similar range to the designed values for these parameters. The test results of the resistivity test and humidity meter indicate that moisture content in the upstream side is higher than the downstream side. The impact of aggregate and other concrete constituents on the deterioration of concrete, which can have a significant impact on the durability and safety of the dam, are discussed. The chloride, sulphate, pH and other chemical parameters are within the permissible limits given in IS: 456-2000[7] for both concrete and water.
Flood Frequency Estimation of Toddbrook Following the Whaley Bridge Incident
Dr Colin Clark, CHRS, Bruton, UK
Abstract: Following intense rainfall caused by a deep area of low pressure centred over southern England, nearly 100mm of rainfall was recorded over the catchment of Toddbrook on 30-31 July 2019. The catchment is located in the Derbyshire Peak District just above the town of Whaley Bridge. A 24m dam impounds the run-off from an area of 17.04km2. The auxiliary spillway discharged about 18-20m3/sec which under good maintenance would not have been damaged. However, much needed maintenance had not been carried out with the result that the left flank of the spillway partly collapsed. A state of emergency was declared and many townsfolk were evacuated for fear of a major dam-break incident. This paper assesses the hydrological evidence gathered after the flood and gives an estimate of the magnitude and frequency of the event. Three methods of analysis have been used. The first is a field survey and 2D flow modelling; the second is a joint probability method for estimating flood frequency; and the third is the application of a non-linear flow model which uses soil hydraulic conductivity data gathered in the field. The results showed that the July 2019 flood had a rarity of about 1 in 30 years, and that if the 1 in 100-year event had taken place the dam would probably have been completely breached. Additional results gave an estimate of 290m3/sec for the probable maximum flood (PMF) at the dam site. The author believes this result alone calls for a major redesign of the spillway – now underway – but says the dam undertakers and Dams & Reservoirs Ltd have not been willing to discuss the present estimate of the PMF, even though it is more than 100m3/sec higher than the figure being used in the construction of the new spillway. Newly revised estimates of probable maximum precipitation (PMP) for the British Isles[23] show that the standard values given in the Flood Estimation Handbook are far too low and cannot reproduce the discharges of many historic floods. Since the original PMP estimates[10] for Britain are now over 20 years old, the need for national dam safety appraisal is now urgent.
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