ISO 9123:2017 pdf free

ISO 9123:2017 pdf free.Hydrometry一Stage-fall-discharge relationships
Most programmes for collecting records of discharge of streams are based on the fact that a relatively simple relationship exists between gauge height and discharge so that, by simply recording gauge height and developing the stage-discharge relationship, a continuous record of discharge can be computed.Several factors, however, can cause scatter of discharge measurements about the stage-discharge relationship at some stations. Backwater is one of these factors and is defined as a condition whereby the flow is retarded so that a higher gauge height is necessary to maintain a given discharge than would be necessary if the backwater were not present. Backwater is caused by constriction such as narrow reaches of a stream channel or downstream structures such as dams/ bridges, downstream tributaries or tidal reach of a stream. All these factors can increase or decrease the energy gradient for a given discharge and cause variable backwater conditions. For example, in tidal streams the energy gradient during flood tides is less than the energy gradient during ebb tides.
Constant backwater, as caused by section controls for instance, will not adversely affect the stage-discharge relationship. The presence of variable backwater, on the other hand, does not allow the use of simple stage-discharge relationships for accurate determination of discharge. Regulated streams may have variable backwater virtually all of the time, while other streams will have only occasional backwater from downstream tributaries, vegetal growth, from the return of overbank flow or backwater from the sea.
Actually, the method is valid for steady, gradually varied flows. Large errors occur when the flow is unsteady, and/or it is rapidly varied. Then, the computed fall between the two gauges can be hydraulically meaningless. In such situations, other techniques including water volume balances might be used, not the stage-fall-discharge method. Further, this methodology appears to assume a constant linear relationship in terms of hydraulic gradient between the auxiliary and reference sites. It is possible for this relationship to be compromised by, for example, variable weed growth between the sites. Hence,the flow gaugings at these sites should be taken simultaneously or at least during conditions when the flow is the same at both sites.ISO 9123 pdf free download.