Effect of Velocity on Seepage and its Measurement

The direct effect of velocity in a canal on seepage rates and the effect of the velocity pattern distortion around a seepage-meter bell on the local head environment of the bell were studied in a laboratory flume. Velocity did not directly affect seepage rates, but the velocity distortion around the bell caused the average head around the seepage meter to be less than that caused by the free water surface in the canal. Correction factors to convert the free water surface to the proper reference level for seepage measurement with bell-type devices were evaluated. It is shown that correcting the free water surface for velocity-induced pressure differences is only necessary in case of low seepage gradients. The absence of a direct effect of velocity on seepage indicates that measuring seepage from flowing canals by means of still water bodies (in ponded sections or inside seepage bells) is correct in principle.