The Sakura River flows in a narrow alluvial channel with gentle slope, draining the southeast area of Ibaraki Prefecture (Figs. 1 and 2). The bed consists of arcosic sand and gravel which reflect the geological condition of the upper drainage basin of the river.
The investigation of flood deposits in the upper reaches of the river shows that the flood deposits have reverse grading structure (becoming coarser grain from bottom to top of a bed) and that a sedimentary unit, the lower mud layer and the upper sand layer, is formed during a single flood. It is the purpose of this paper to explain the depositional process of the reverse graded bed from the characteristics of sediment transport in the Sakura. Main results are as follows.
The flood deposits are thick only on the cut-sides of the river channel, which are attacked by the main flow of the flood (Fig. 6). Levees are better developed in these areas. Medium and fine sand make up the thick flood deposits on the natural levee (Fig. 7). It begins with the deposition of a thin silt layer and is followed by a thick sand layer which displays no internal structures. Sand sizes, φ₁ as well as φ₅₀, gradually increase toward upward. Fig. 8 shows the repeated cycles of coarsening upward sequence of flood deposits on the natural levee. The maximum diameter of each flood deposit has the upper limit of about 10(0. 5mm) . The floods which left the thick flood deposits are shown in the time-stage curves of Fig. 9. Reverse grading is also developed among the thin flood deposits on the lower part of the flood plain (Fig. 7). The lowest clayey silt layer with a thickness of a few milimeters suddenly changes the upper sand layer which usually has fine lamination.
Fig. 9 shows the observational results on the hydraulic regimes and the suspended sediment transports in some floods in the Sakura River. It is the common fact to all observations that the maximum concentration of suspended load precedes the peak discharge. Furthermore, it is clear from Fig. 10 that the amount of the finer sediment load of silt-clay and very fine sand is extremely high at the leginning of each flood and rapidly decreases with the rising of waterlevel.
The depositional process of reverse graded bedding in flood deposits can be explained as follows and Fig. 12 schematized them.
1. Deposition of the mud layer (Fig. 12-I)
It is very likely that the lower muddy part of reverse graded bed is deposited during the beginning of the flood when the concentration of finer grained suspended sediments, the socalled “wash load”, is extremely high. Flood water with large quantities of wash load is diverted into the flood plain and the stagnation of flows on the flood plain causes the deposition of this finer sediments. It can be considered that the reason why mud layers on the natural levee are very thin and rather indistinct (Figs. 7 and 8) is that the natural levee is not covered by the inundated water at this stage. Boils along the bank begin to occur in the mean while.
2. Deposition of the sand layer (Fig, 12-II)
The upper sandy part is presumed to be produced by the deposition of suspended sand after the rapid decrease of wash load concentration. The upward coarsening structure in this sand layer may be produced under the condition of gradual increase of flow intensity. The sand make up the natural levee is coarser than that of suspended up to the bankfull level at the center of the channel (Figs. 7, 8 and 11). This fact is explained as follows. Boils over the bank become more and more violent. This suggests that the upward flows (kolks) near the bank become violent with increases of discharge. It is supposed that these kolks cause the coarser sediment grains to suspend near the bank.
3. Preservation of the reverse graded bed (Fig. 12-III)
With the beginning of water level falling, the inundated flow turns back from the flood plain to the channel.