ASFM Measurement In An Open Spillway

Inadequate spillway capacity is one of the major reasons for dam failure in embankment dams. New Swedish guidelines for the design flood, adopted in 1990, have led to an extensive work with upgrading of the discharge capacity in a number of dams. A large number of dams have such a spillway design that they do not easily allow for accurate theoretical calculations of the discharge capacity due to e.g. oblique inflow conditions, head losses in the adjacent spillway area and uneven water levels in front of the spillway openings. Hydraulic models have been the traditional way to determine the discharge capacity in those cases where a theoretical approach is judged to give an uncertain result. In some cases even a hydraulic model is suspected to give an uncertain result, e.g. when friction losses play an important role in the discharge capacity. For these reasons, a requirement exists to develop a field measurement method to determine spillway capacity. A test with an Acoustic Scintillation Flow Meter to directly measure discharge was made at the Mellanfallet Dam on the river Dalalven in Sweden in August 1999. New Swedish Guidelines In 1985, the same year the work started with the new Swedish guidelines, heavy rain and overtopping of the dam crest caused dambreak in Noppikoski dam. The major cause of the dambreak was a jammed spillway gate that couldn’t be opened. Hydrological simulations showed that the dam was in danger of overtopping even if the gate had been opened. The Noppikoski case has been reported by Enfors & Eurenius, 1988. The new Swedish guidelines were adopted in 1990 and preliminary calculations have, in some cases, substantially increased the design flood. Only a few power plants have so far been subject to more detailed investigations. The new guidelines are described by Bergström Examples are given by Yang, 2000 in which the increase in design flood for Bergeforsen dam in river Indalsälven is almost 50 % from 2300 to 3400 m/s. Determination Of Spillway Capacity Equal important to determine the design flood is the determination of the real discharge capacity in a dam’s spillway. So far hydraulic models have been the predominant tools to determine spillway capacity especially when the probability for a successful determination with a theoretical approach is low. In the last few years, new techniques with field measurements and mathematical modelling have increased the possibilities to determine discharge capacities with high accuracy. When it comes to field measurement, it is of course only possible to measure on existing structures but it is an important tool to verify old discharge capacity curves with unknown origin and to compare, and in some cases to verify, the results from hydraulic models. Mathematical modelling has in recent years become a serious alternative to hydraulic models. In a comprehensive Swedish study, mathematical modelling of spillway discharge capacity for six different spillways was compared in a blind test to corresponding hydraulic modelling in a flume. The result showed good agreement, except for some cases, with those exceptions being due to the mathematical model describing the flow situation and not weaknesses in the program code. In the same study a theoretical approach was made to determine the discharge capacity. The results from that test showed, in many cases, substantial deviations from the correct value. This study is described by Cederström. Hydraulic model tests are judged to have a maximum error in discharge capacity of about 3 to 4% in a well-designed model. The accuracy is suspected to perhaps be lower if inertia and gravitational forces do not solely govern the discharge capacity, as it should in a true Froude model. This could be the case if friction losses and the development of the boundary layer in the approaching flow to the spillway affect the discharge capacity. Discharge Measurement With An Acoustic Scintillation Flow Meter (ASFM). Acoustic scintillation drift is a technique for measuring flows in a turbulent medium, such as water or air, by analyzing the variations (with position and time) of sound which has passed through it. Scintillation in this context refers to random variations in the intensity of the sound caused by the variations in the refractive index of the water produced by the turbulence, which is always present in any natural flow. The ASFM measures the speed of the current from the transverse drift of the acoustic scintillation’s observed across two relatively closely spaced propagation paths. The method has been used for many years to measure winds in the atmosphere and ionosphere (Ishimaru, 1978; Lawrence, Ochs & Clifford, 1972; Wang, Ochs & Lawrence, 1981), more recently for measuring currents and turbulence in ocean channels (Clifford & Farmer, 1983; Farmer & Clifford, 1986; Farmer, Clifford & Verrall, 1987; Lemon & Farmer, 1990) and in hydroelectric plants (Birch & Lemon, 1993; Lemon, 1995; Lemon & Bell, 1996); its derivation is well-established. The ASFM measures the lateral (i.e. along-path) average of the component of the flow perpendicular to the acoustic path. It is therefore well suited for collecting data for discharge measurements, since the product of the path length with the lateral average of the normal component of flow gives the element of discharge at the depth of the path. Sampling at several levels in the vertical and integrating then gives the discharge.