Water Gaging for Low-Head Units of High Capacity

Abstract

Abstract In the field of hydroelectric construction there has been in recent years considerable advance in the development of low-head units of high specific speed. This tendency in design has been conducive to the development of low-head sites which were at one time considered uneconomical. It became, therefore, increasingly important to find methods of measuring the discharge of low-head plants where the water quantities are large and the water passages particularly short and unsuited to other methods of testing. American engineers had developed methods of measurement of flow in long penstocks and then attempted to extend these methods to plants with shorter water passages. European engineers have shown a preference for current meters, at one time commonly used on both sides of the Atlantic for measurements in large intakes. The paper discusses the status of the current-meter method in Europe at the time this method was selected for the tests at Safe Harbor and the application of the two-type meter method. It shows how this method was applied at the outset and how certain modifications were made for increasing the reliability and decreasing the time and effort required for making tests. These made possible new methods of compilation which have been developed to facilitate greatly this phase of the testing program. The successful application of these methods should make current-meter measurements more feasible than ever before. In preparation for these tests, investigations were conducted at the Bureau of Standards and elsewhere to determine the characteristics and limitations of the meters under various conditions of uniform, oblique, pulsating, and turbulent flow. It has been shown that water passages with parallel sides for metering and approach sections do not insure parallel flow, and that even under as nearly ideal conditions as it is practical to secure, the use of two types of meters will indicate an effective angularity. However, it is believed that accurate measurements of oblique flow can be made by the use of two types of meters, each affected to a different extent by angularity. The use of this method will greatly enlarge the field of current-meter testing. There are pointed out the advantages of making index tests in conjunction with current-meter measurements; not only to extend them to a wide range of operating conditions, but also as a measure of the consistency of the discharge measurements.