Irregularities and corrections

In the middle and lower reaches of rivers where the beds consist of sands and gravels and there is no stable channel control, the stage-discharge relationship may be unreliable owing to the alternate scouring and depositing of the loose bed material. If the general long-term rating curve remains reasonably constant, Stout's technique allows individual shift corrections to be made to evaluations of Q in accordance with the most recent gaugings by adjusting the stage readings (Fig. 7.24).

Discharges depend both on the stage and on the slope of the water surface; the latter is not the same for rising and falling stages as a non-steady flow passes a gauging station. From gaugings at a particular stage value there are thus two different values of discharge. From gaugings made on a rising stage, the corresponding discharges will be greater than those measured at the same stage levels on the falling stage. Thus, there can be produced a looped rating curve. When the river sustains a steady flow at a particular stage, an average of the two discharges may be taken, but otherwise the values of the relevant rising or falling side of the looped rating curve should be used.

Other irregularities in the stage-discharge relationship may be caused by nonuniform flow generated by interference in the channel downstream of the gauging section, thus overriding the flow control. Such interference can result from the backing up of flow in a main channel owing to flow coming in from a downstream tributary, or from the operation of sluice gates on the main channel. Vegetation growth in the gauging reach (particularly in lowland rivers in southern England) will also interfere with the Q versus H relation. In certain rivers it may be advisable to have rating curves for different seasons of the year. More permanent interference may be provided by changes in the cross-section owing to the scouring or the deposition of an exceptional flood and, if discharges have been obtained for stages which include inundation of the flood plain, any further developments on the flood plain would affect these values.

0 0

Post a comment