1. Investigations showed that the existing standards for selecting graded filters for draining hydraulic structures do not reflect the physical aspect of phenomena occurring in filters during fluctuating percolation. 2. Intense intralayer re-formation of the filter occurs during fluctuating percolation. Under the effect of pressure pulses particles of the filter which are even larger than the average diameter of the voids in the filter layer are brought into motion. However, this motion does not harm the work or stability of the filter. Under the effect of alternately directed pulses there occurs a periodic (as jerky) motion of small particles, and with each fluctuation the particles descend ever lower into the near-contact zone (with both a vertical and horizontal direction of the fluctuating pulses). Here they fill ever larger voids and become clogged, preventing the removal of sand particles from the foundation into the filter. As a result of such re-formation the interlayer coefficients decrease considerably, and the contact stability of the “filter-foundation” layers improves markedly. 3. On the basis of the experiments conducted it can be stated with certainty that in the first layer of a graded filter laid on sand and consisting usually of rubble of size 0.1–10 mm, the presence of small sand fractions measuring 0.1–0.5 mm in an amount of 10–15% and more is not only allowable but also desirable. Therefore, it is necessary to wash out only the silty and clay particles, if their content exceeds 5%. 4. The selection of the composition of the first layer of a graded filter under a horizontal revetment operating under fluctuating-flow conditions can be made by means of Fig. 4, being guided by the considerations expressed above. This graph is suitable, probably, also for selecting the next layers of the filter, but this requires checking.