Absolute flow measurements to obtain depth against discharge for sewers using bromine-82 and lithium as tracers

The methods used by the Water Research Centre Laboratory to obtain absolute discharge data in respect of partially full sewers are presented. Discharge recorders and depth gauges were calibrated by dilution methods using lithium and bromine-82 as water tracers. The Laboratory's 'extended gulp' technique is described and the relative merits of the dilution techniques and tracers used are discussed. Bromine-82 is considered to be the most suitable tracer for measurements at very high flowrates and where high accuracy is required. Lithium can be used at short notice and is simpler to apply in field situations. A description is given of several sets of flow measurements carried out in the Stevenage outfall to obtain estimates of the ultimate carrying capacity of the sewer - a concrete pipe of 1 m diameter. For part of the study the natural flow was augmented by releasing water from a water meadow to obtain a range of depths of liquid above the normal maximum and thus determine the complete depth/discharge characteristic. Data relating to sewers 0.3 to 1.5 m diameter are also given; these illustrate that absolute measurement data, ever over very restricted ranges of depth, can be most useful in estimating carrying capacities as a function of depth when utilized with theoretical formulae. Composite pipe roughness values have been estimated and the experimental data compared with predictions based on Manning's formula and the Colebrook-White equation. (author)