Hydrogeomorphological characteristics of a zero-order basin

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1 Erosion and Sedimentation m the Pacific Rim (Proceedings of the Corvallis Symposium, August, 1987). IAHS Pubi. no Hydrogeomorphological characteristics of a zero-order basin INTRODUCTION YOSHINORI TSUKAMOTO & HIROHIKO MINEMATSU Department of Forestry, Tokyo University of Agriculture and Technology, Fuchu, Tokyo 183, Japan ABSTRACT The topographical development of a zero-order basin is clarified through discussions of the relationship between slope types, soil properties, mass slides, storm runoff processes and the occurrence of saturated overland flow. The depth of the subsurface flow in a slope is a determining factor in the development of topography of a zero-order basin. An important result is that variable source area is predominant in low relief mountains and liquefaction of colluvial deposits is predominant in high relief mountains. A zero-order basin is a slope unit that conjoins a slope and a stream. Geomorphologically, a zero-order basin is a conjunctive area of hillslope and fluvial processes. In the former, gravitational processes are predominant. In the latter, hydraulic processes are predominant. Hydrologically, a zero-order basin is an area where subsurface storm runoff appears on the ground as saturated overland flow. This means that this area becomes part of a stream during a storm. The conjunctive site of slope and stream shifts in a season as well as in a storm. A seasonal shift occurs in areas where dry season alternates with wet season. A shift during storms occurs in areas of heavy storms. In the latter case, the shifting length is small, but important and drastic phenomena such as dynamic source area and liquefaction of slope deposits are brought about. The objective of this paper is to give a consistent explanation to hydrological and geomorphological phenomena occurring in zero-order basins. The discussions in this paper are based on the writers' field observations and field measurements on slopes of three rock types. SURFACE TOPOGRAPHY OF HILLSLOPES AND SURFACE SOIL CHARACTERISTICS Soils on a slope Weathered materials on hillslopes are classified into three categories: residual soil, creeping soil, and colluvial soil. A typical pattern of soil mantles on a slope is indicated in Figure 1. A middle slope is mostly composed of residual soil and creeping 61

2 62 Yoshinori Tsukamoto & Hirohiko Minematsu Lower slope Concave upwards Colluvial soil «z*$y L i No cree p yjgrî'js X*r\ Creep or water transport FIG.l A typical slope profile and soils. soil. The former overlies the latter. On an upper slope, residual soil predominates. On a lower slope, creeping soil predominates and is composed of colluvial deposits. Relative depths of residual and creeping soil on a middle slope vary on a slope and differ greatly from slope to slope. This variation and the difference of the depths of two soil layers brings about hydrologlcal characteristics of slopes and differences in slope processes. Slope types Microtopography of slope surfaces is classified into nine slope types with curvatures of contour lines and flow lines (Suzuki, 1977) (Figure 2). Convergence and divergence of storm flow and weathered materials on these slope types greatly affect surface soil depths, soil properties, soil moisture, tree growth, slide types, and other slope characteristics. Countour line Divergent Straight Convergent FIG.2 Basic slope types (after Suzuki, 1977).

3 Slope units and zero-order basins Hydrogeomorphological characteristics 63 The whole slope in a watershed is divided into three slope units, that is, convergent, plane, and divergent. The importance of convergent slope units was pointed out from the viewpoint of hillslope hydrology and hillslope processes (Tsukamoto et al., 1982). This convergent slope unit is called a O(zero)-order basin (Tsukamoto, 1973). Zero-order basins are classified into two categories. One is a zero-order basin at the head of the first-order channel. The other is a zero-order basin on a side slope. Both zero-order basins possess the characteristics summarized in Table 1. TABLE 1 A zero-order basin and slope type At the head of the lst-order channel On side slopes Standard shape of zero-order basin Combination of VII+VIII+IX VII+VIII+IX slope type or VII+VIII or VII+VIII Length/width Small Large Gradient Gentle Steep Length of VIII slope type Long Short Special shape of zero-order basin. Case I VII * Rare Case II IX * Rare * Observed at large landslide scars or at hard rock cliffs. Case I may not be called a zero-order basin. * Covered with deep colluvial deposits. Periglacial effects are frequently observed. TOPOGRAPHICAL DEVELOPMENT OF A ZERO-ORDER BASIN Formation of potential sliding mass Weathering and soil creep from the upper slope increase soil depth which finally reaches a critical depth in balance with slope gradient. Slide scars recover their soil depths in a comparatively short period on granite slopes (Shimokawa, 1984). Figure 3A is a result of measurements of critical soil depths with a simple cone penetrometer (Tsukamoto & Minematsu, 1987). Figure 3B is a schematic illustration of soil recovery in slide scars. There are two types of recovery of soil depths in slide scars. One is by weathering of bed materials and the other is by soil creep from the upper slope. Actually, both proceed simultaneously. The former

4 64 Yoshinori Tsukamoto & Hirohiko Minematsu type seems to belong to the saturated interflow runoff process type discussed later. Another effect of weathering on slope stability is the internal change of soil structure within a slope. Evidence of eluviation and illuviation are widely recognized in soil horizons. Nishida (1985) observed them in the lateral direction in a granite slope. The upper slope eluviated clayish materials and the lower slope illuviated them. These processes greatly affect hydraulic conductivities of surface soils. The former promotes infiltration of the upper slope surface and the latter functions as an impeding zone of saturated interflow in the lower slope, which results in pore pressure increase and promotion of piping on the slope Slope gradient Year FIG.3 A schematic illustration of formation of potential sliding mass. (A): An example of the relationship between slope gradients and critical soil depths measured on granite, Tertiary and Paleozoic slopes. (B): Developing process of surface soil depths on creeping slopes. Shaded areas are the widths of variation estimated based on the writers' observation. Runoff process Two agents are observed in the development of topography of a zeroorder basin. One is saturated interflow and the other is ground water outflow (saturated lateral flow) through underlying rocks. Which of the two agents is predominant on hillslopes is determined by the permeability of weathered layers below the surface soil (A horizon + B horizon). An illustration is given in Figure 4. Saturated interflow type: Pore pressures develop as the result of convergence of saturated interflow. The characteristics of this type are the rapid weathering of bed materials and the short recurrence interval of surface soil slides. In addition, typical and distinct topography of zero-order basins (such as numerous small spoon-like basins) is observed on slopes of this type. Zero-order basins in weathered granite areas and young Tertiary areas are of this type. Ground water type: Ground water outflow through deeply weathered bed rock, ruptured zones, and/or large fissures seem to be the more common mode of development of zero-order basin topography than

5 Hydrogeomorphological characteristics 65 saturated interflow in the surface soil. This type of zero-order basin is observed on slopes in Paleozoic terrain. Storm rainfall No Vegetation cover On bare surface Extremely large drainage density Saturated interflow J: Ground water outflow through base rocks 4- i 1 l Slide on Slide on Scars of Slide of creeping hollow old deep deeply weathered soil deposit slide rocks T ' I i Shallow slide Shallow slide Deep slide short hillslopes i Large drainage density on hollow 4- deposit T New ' zero order I On long hillslopes basin I Small drainage density I: storm intensity, i: infiltration, i u : pearmeability of the underlying layer below the surface soil and i r : pearmeability of base rocks. FIG.4 Relationship between runoff process and erosion types on hillslopes. Two types of slides A zero-order basin is a slope unit characterized by frequent discharge of weathered materials. The topographical development of a zero-order basin occurs with the following two types of slides: (a) A slide on the creeping and/or residual slope: This type of slide occurs on the lower end of VII slope type which is near the boundary of a residual slope and a creeping slope. This steepest slope segment is a conjunctive point of two slope curvatures and is called an erosional front. The tip of the residual slope and the upper creeping slope recede by this slide. This slide is caused by the convergence of saturated interflow on the residual slope of the VII slope type, ground water outflow through bedrock, or combined actions of both flow types. The size of the slide is typically small. (b) A slide on a colluvial or alluvial slope: This type of slide occurs mostly on IX slope type which is composed of colluvium or alluvium. The slide is comparatively large and frequently develops into a debris flow that devastates downstream channels. The convergence of weathered materials by creeping is the primal cause of this slide. The convergence of saturated interflow on the upper

6 66 Yoshinori Tsukamoto & Hirohiko Minematsu slope of VII, VIII, and IX slope types and the convergence of ground water outflow on the creeping slope are the second cause of this slide. This slide is referred to as liquefaction of colluvial deposits. According to the analysis by Sassa (1986), liquefaction of colluvium and alluvium is greatly accelerated by loading caused by a sliding mass from the upper creeping and/or residual slope. On an actual slope, typical slides of the above types can be discriminated. However, many slides on hillslopes are intermediate between these two types or occur in a united sliding mass. According to the analysis by Takeshita (1971), 95% of slides occur on slopes that are at least in part concave upward. Occurrence sites are shown in Figure 5. Approximately 60% of slides occur on colluvial slopes. Nearly half of them occur in soils on creeping slopes. Most slides on convex-upward slopes are associated with concave-upward slope segments. For example, the slide of B-l type on a concave-upward slope is connected with slides of A-1, A-2, or A-3 type on a convex-upward slope. Slide areas in zero-order basins are generally large because most slides of the above two types occur simultaneously. An interesting result of our analysis is that the average sizes of slide areas are closely related to the average areas of zero-order basins (Tsukamoto et al., 1982). Occurrence site f A-1 f A-2 f A-3 / A-4 f A-5 Other Percentage(%) Occurrence site Percentage(%) J B J B y B Other 4.9 FIG.5 Relationship between longitudinal slope profiles and occurrence sites of slides (after Takeshita, 1971). Tree root effect on slope stability The effect of tree roots on slope stability differs greatly with slope type. The greatest effect is expected on a convex-upward or a straight slope because these slopes are covered with shallow and comparatively dry surface soil that promotes the development of lateral root networks and the penetration of vertical roots into the underlying residual soil. The tree root density is greatly reduced on a concave slope that is covered with deep, moist colluvial soils. The deep surface soil reduces the effectiveness of vertical roots in stabilizing soil and the moist soil also reduces the density of lateral roots Therefore, deforestation of creeping slopes exerts a

7 Hydrogeomorphological characteristics 67 greater influence on slope stability than deforestation of a colluvial soil (Tsukamoto & Minematsu, 1987). A ZERO-ORDER BASIN AS A CONJUNCTION OF A SLOPE AND STREAM The upward shift of the initiating point of streamflow in a hollow brings about the expansion of saturated overland flow area in a storm. This shift is important from- two viewpoints, that is, the expansion of source area contributing to stormflow generation and the sliding or liquefaction of hollow deposits. In this section, the conditions of source area formation and liquefaction of hollow deposits are discussed and compared. The results of discussions are summarized in Figure 6. Source area Variable source area is the expansion and shrinkage of saturated overland flow area in a hollow (Hewlett, 1967). The conditions favoring frequent occurrence of widespread expansion of saturated overland flow area in a hollow without sliding or liquefaction are as follows: (a) Bed materials under the surface soil in a hollow are composed of impermeable, clay-rich materials or impermeable rocks. (b) Ground water levels in a hollow are high enough to generate saturated overland flow in small storms. For this, the hollow must be poorly drained, having gentle slopes and fine-textured soils. (c) The area of saturated overland flow greatly expands in hollows with gentle gradients and wide, flat bottoms. These conditions are satisfied in mountains with low relief where hollows are gentle and contain fine-textured soil. Low relief results in high drainage density which provides a large number of stream heads. From this viewpoint, lower relief mountains are more favorable for source area contributions in storm runoff. Liquefaction of hollow deposits The liquefaction of colluvial deposits in a hollow without loading of other slides requires a great amount of water supplied to the deposits and occurrence of saturated overland flow (Takahashi, 1977). The conditions of liquefaction of hollow deposits only occur in extraordinary heavy storms and are as follows: (a) No saturated overland flow occurs in ordinary storms. Hollow deposits must be well-drained, which requires steep gradients and coarse materials. (b) Hollow deposits must be on a steep, impermeable layer for the initiation of motion. (c) Liquefaction of hollow deposits in areas with lower relief would necessitate external impacts such as loading by slide masses from upslope. Conditions (a) and (b) occur in mountain areas with high relief.

8 68 Yoshinori Tsukamoto & Hirohiko Minematsu Low relief mountain (short hillslope) Relative importance or possibility of occurrence Contribution to storm runoff from hollow areas Saturated overland flow (source area) Contribution to storm hydrograph Discharge from source areas High relief mountain (long hillslope) Liquefaction of deposits Infiltration 'or pipeflow through colluvial deposits Discharge from hillslopes FIG.6 Effect of relief on relative magnitude of hydrologie phenomena occurring on hollows and slopes. UNCHANNELED EPHEMERAL STREAM LENGTH DURING A STORM It is highly probable that most of a hollow area is covered with saturated overland flow when the hollow deposit is liquefied and slides. In this circumstance, the hollow can be regarded as an ephemeral stream. The theoretical relationship between exterior links (EX and EX ) and interior links (IN) in Figure 7 satisfies the following equation under the assumption of EX/IN = 1.0: EX Q /IN l+lx(%) 1 +lx(%) 2 + = 2 EX, EX, and IN were measured on maps with the scale of 1:25000 in granite mountain are as with differing relief. The heads of first-order channels we re determined by the intersection angle of the side slopes of firs t-order basins. Figure 7 shows that EX /IN is approximately equal to the theoretical value, but EX/IN is far larger than the theoret ical value. Three areas in the s tudy areas have maps of the scale of 1:2500. Zero-order basins and s lides were delineated in the maps. Assuming the heads of slides wer e the heads of saturated overland flow areas, the measurement of EX, EX, and IN in zero-order basins was carried out. Even though it is taken into account that only half the number of slides in zero-order basins occur on hollow deposits and the heads of the slides are regarded as the heads of exterior links, Figure 7 seems to indie ate that the hollow area satisfying EX/IN < 1 is covered with saturât ed overland flow in heavy storm periods. This means that slope 1 engths in zero-order basins during heavy storms decrease to appr oximately one-half of those of dry periods.

9 Hydrogeomorphological characteristics 69 EXO IN 2.0 EX IN A Cy, ~~~ A o""~" 9 «*P A 0 o A o 0 o o o s _L X* (B) fis; S ^ Liquefaction or sliding of colluvial hollow deposits ; *) Sliding of creeping '" or residual soils A A Exterior links ore 1st order channols on the maps of 1:$5000 r Exterior links ass zero order channels on the maps of 1ÏF2500. Ao.-^O A EX IN IN. : Mean / - ---: Theoretical FIG.7 Relationship between interior links and exterior links in first- and zero-order basins. CONCLUSION Characteristics of a zero-order basin are clarified through the discussion on topography of slope surfaces, slides, saturated overland flow, and liquefaction of hollow deposits. An important remaining problem is the process of formation of the present surface soil mantles and hollow deposits that were greatly affected by the past climatological conditions and volcanic activities. REFERENCES Hewlett, J.D. & Hibbert, A.R. (1967) Factors affecting the response of small watersheds to precipitation in humid areas. In: Intern. Symp. on Forest Hydrology, Sopper and Lull (Eds), Pergamon Press, Oxford, Nishida, K. (1985) Change of soil properties and surface slides. In: Evaluation of slope instability taking into account weathering of bed rocks and their local variability (Natural disaster research project No. A-60-2, ) (in Japanese). Sassa, K. (1986) The mechanism of debris flows and the forest effect on their prevention. Proc. of 18th IUFRO World Congress, Div 1, vol. 1, Shimokawa, E. (1984) Natural recovery process of vegetation on landslide scars and landslide periodicity in forested drainage basins. Proc. Symp. on effect on forest land use on erosion and slope stability, East-West Center, Honolulu, Hawaii,

10 70 Yoshinori Tsukamoto & Hirohiko Minematsu Suzuki, R. (1977) Slope profiles and classification of slope types. Surveying No. 7, (in Japanese). Takahashi, T. (1977) A mechanism of occurrence of mud-debris flows and their characteristics in motion. Annuals, Disaster Prevention Research Institute, Kyoto Univ., No. 20, B-2, , (in Japanese). Takeshita, K. (1971) Estimations of mountain disasters occurrence and their location analysis on the Kyushu. Bull, of Mountain Conservation I, Fukuoka Forest Expt. Sta (in Japanese). Tsukamoto, Y. (1973) Study on the growth of stream channel (I). Relationship between stream channel growth and landslides occurring during heavy storm. J. Jpn. Erosion Control Soc. 25(4) (in Japanese). Tsukamoto, Y., Ohta, T. & Noguchi, H. (1982) Hydrological and geomorphological studies of debris slides on forested hillslopes in Japan, IAHS Pub. No. 137, Tsukamoto, Y. & Minematsu, H. (1987) Evaluation of the effect of deforestation on slope stability and its application to watershed management. Proc. Symp. on forest hydrology and watershed management, IAHS, Vancouver (in press).

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