Effect of Rock Properties on the Longitudinal Profiles of River Beds

Comparison of the Mountain Rivers in Granite and Paleozoic Sedimentary Rock Basins

Abstract

Longitudinal profiles of mountain rivers have been thought to be controlled by the type of bedrock. Thus we verified the relation between the rock properties and the longitudinal profiles of river beds by field measurements and laboratory tests. We chose two rivers which have basins of almost equivalent sizes (2 km²) and relative heights (700 m), located near each other in Niigata Prefecture, central Japan (Fig. 1). The studied reaches in both basins are 900 m in horizontal length and 130‰ in gradient. They differ only in the bedrock type, and are approximately the same in other characteristics, including climatological and geomorphological ones.
A field survey of longitudinal profiles showed that a stepwise longitudinal profile, with a number of falls, formed of bedrock not of gravel, was found in succession in the granite basin, while a smooth and straight profile without a fall was found in the Paleozoic sedimentary rock basin (Fig. 2, Photo 1-a, b).
Compressive strengths (S_c) and tensile strengths (S_t) of Paleozoic sedimentary rocks were 3 or 4 times stronger, respectively, than granite (Table 1). These values were obtained for cylindrical specimens by measurement in a laboratory. Schmidt rebound numbers also are large for Paleozoic sedimentary rocks, in which no fall was found (Fig. 3). In the reach of the granite, the histogram of longitudinal wave velocity measured in the field shows that there are at least two parts with high frequency (Fig. 4): the part with lower velocities (0.9-1.5km/s) to fracture zones or open jointed rocks, and that with higher velocities (1.8-2.1km/s) to intact or jointed rocks. On the other hand, the histogram for Paleozoic sedimentary rocks shows that the frequency tends to increase as the velocity decreases (Fig. 4). Weathering properties obtained by wetting-drying tests (Fig. 5) and dissolution tests (Fig. 6) indicated that both rock types were almost equally resistant to weathering. Therefore it is presumed that the formation of falls is not related to the strength of fresh rock specimens or susceptibility to weathering.
Fracture zones or open joints were found at intervals of 10_??_20 meters in the reach of granite. On the contrary, no significant fracture zones could be found in the reach of Paleozoic sedimentary rocks, which existed as bedding planes densely at intervals of several centimeters. Hence, it can be said that weak zones such as joint planes, bedding planes and fracture zones in the Granite reach are distributed clearly in a different mode from that in the Paleozoic sedimentary rock reach.
It is an empirical fact that discontinuities such as joint planes, bedding planes and fracture zones slow the longitudinal wave velocity in the rock mass. Index K, which expresses the degree of fissuring, is given by V_pf/V_pl, in which V_pf is the longitudinal wave velocity of bedrock measured in the field and V_pl is that of cubical specimens without visible discontinuities. We can express the rock mass strength (S_tm) affected by discontinuities as follows
S_tm =K²×S_tl,
where S_tl is the mechanical strength measured for cylindrical specimens without visible discontinuities (Ikeda, 1979).
Applying index K, we can calculate values of two other quantities K²×S_c and K²×S_t, which express the rock mass strength for each rock facies (Table 2). In the reach of granite the rock mass strength of intact rocks or jointed rocks was 10_??_25 times as large as that of fracture zones or open jointed rocks.