Elsevier

Geomorphology

Volume 70, Issues 1–2, August 2005, Pages 147-162
Geomorphology

Slope of alluvial fans in humid regions of Japan, Taiwan and the Philippines

https://doi.org/10.1016/j.geomorph.2005.04.006Get rights and content

Abstract

This study constructed a database for 690 alluvial fans in humid regions of Japan, Taiwan and the Philippines, and analyzed their slopes and other morphometric parameters such as the area and relief ratio of the source area. This type of comprehensive geomorphological research on alluvial fans in humid regions has been limited, although numerous studies have dealt with fans in arid and semi-arid regions. Semi-conical depositional landforms larger than 2 km2 and steeper than 0.11° (0.002 m/m) were selected as alluvial fans. About 60% of the fans formed during the Holocene and the rest (40%) formed during the Pleistocene. Mean fan slopes for both all the Quaternary fans and Holocene fans follow a lognormal frequency distribution. The distribution also fits slopes of fans belonging to a certain areal range. The area and relief ratio of the source area, which correlate well with the fan slope, also follow a lognormal distribution, indicating the dimensions of fan/basin systems vary gradually rather than abruptly. These findings contrast with a previous notion that depositional landforms rarely have slopes of 0.5° (0.009) to 1.5° (0.026) and hence only semi-conical landforms steeper than 1.5° should be called alluvial fans.

Introduction

Semi-conical depositional landforms along mountain piedmonts are generally referred to as alluvial fans. The strict definition of alluvial fans, however, is still an open question. A possible factor affecting the definition is surface inclination, because very flat or very steep depositional landforms are usually called floodplains or talus rather than alluvial fans. Therefore, the slope or gradient of semi-conical depositional landforms needs to be analyzed to establish a common definition of alluvial fans.

Studies in arid regions especially in the American Southwest during the 1950s and 1960s suggested that alluvial fans are relatively steep, since fans formed by debris flows were the main research targets. Blissenbach (1954) classified alluvial fans in semiarid regions into steep (>  = 0.087 m/m), gentle (2° to 5° = 0.035 to 0.087) and flat (<  = 0.035) types, and implied that landforms gentler than 1°(0.017) are not alluvial fans but floodplains. Anstey (1965) examined data for more than 4,000 alluvial fans in the arid American Southwest and Pakistan, and pointed out that most fans have slopes between 1°(0.017) and 5°(0.087). Hooke (1968a) suggested that the slope of alluvial fans ranges from 2°(0.035) to 12°(0.206).

In humid regions, however, gentler depositional landforms have been identified as alluvial fans. Tomita (1951) compiled morphometric data for 29 alluvial fans in eastern Taiwan whose lowest slope is 0.5°(0.009). Toya et al. (1971) noted that 153 alluvial fans in Japan have slopes lower than 0.57°(0.01). Such gentle fans have been formed by fluvial processes rather than debris flows (Kadomura, 1971). In Alaska and Iceland, semi-conical depositional landforms gentler than 1°(0.017) have also been called fans (Boothroyd, 1972, Boothroyd and Nummedal, 1978). Evans (1991) suggested that slopes of alluvial fans in humid tropics range from 0.057°(0.001) to 0.57°(0.010).

Much gentler depositional landforms in humid regions have also been regarded as alluvial fans. According to Davis (1898), a very gentle (0.014° = 0.0002) depositional landform formed by the Yellow River in China is one of the largest alluvial fans in the world. Another large and gentle landform formed by the Kosi River at the foot of Himalayas (0.019° = 0.0003; Stanistreet and McCarthy, 1993) has been called a wet fan (Schumm, 1977), mega-cone (Parkash et al., 1980), large flat fan (Wells and Dorr, 1987), and megafan (Gohain and Parkash, 1990). Such gentle fans with many distributary channels, which have been sedimentologically called fluvial distributary systems (Nicols and Hirst, 1998), also occur in the other Himalayan piedmonts and along the Andes (Singh et al., 1993, Iriondo, 1993, Sinha and Friend, 1994, DeCelles and Cavazza, 1999). One of the flattest alluvial fans so far reported is the Okavango fan in northern Botswana, with a mean slope of 0.013°(0.0002; McCarthy et al., 1991). Stanistreet and McCarthy (1993) named it “losimean fan” (low sinuosity/meandering fan) because the river on it is freely winding.

The observations of such gentle alluvial fans were challenged by Blair and McPherson (1994) based on data for the 237 piedmont depositional landforms previously classified as alluvial fans. They indicated that alluvial fans have average slopes between 1.5°(0.026) and 25°(0.411), whereas rivers in sedimentary basins are significantly gentler, rarely exceeding 0.4°(0.007). They noted that this classification is validated from morphology, sedimentary facies, hydraulics and sedimentary processes, and assumed alluvial fans are constructed mainly by catastrophic fluid gravity flows or sediment gravity flows. Thus, 18 piedmont depositional landforms gentler than 0.4°(0.007), formerly classified as alluvial fans or fan-deltas, were renamed rivers or river deltas. For example, gentle fans in Alaska and Iceland are gravel-bedded rivers; the Kosi River fan is a sand-bedded river; and the Okavango fan is a mud-dominated river.

Blair and McPherson (1994) also concluded that depositional slopes of 0.5°–1.5° (0.009–0.026) are uncommon in aggrading alluvial basins, and called this interruption “the natural depositional slope gap”. McCarthy and Cadle (1995) as well as Kim (1995) cast doubt on the occurrence of the gap, and ascribed it to the incomplete choice of data by Blair and McPherson (1994). In reply to such criticisms, Blair and McPherson, 1995a, Blair and McPherson, 1995b further insisted that their inferences are valid, because they thought that the additional data presented by Kim (1995) and McCarthy and Cadle (1995) are inappropriate.

Saito et al. (2003) surveyed 123 alluvial fans in Death Valley, California, based on 1 : 24,000 topographic maps and satellite images and revealed that almost all the fans there are steeper than 1.5°, while their surrounding floodplains and playas are generally gentler than 0.5°. This finding suggests that, if data are taken only from Death Valley, Blair and McPherson's (1994) depositional slope gap can be detected. However, Blair and McPherson (1994) suggest that their slope gap is universal and applicable to any climatic regions.

The presence or absence of a universal depositional slope gap is crucial for not only understanding the nature of fluvial sedimentation, but also the proper definition of alluvial fans. If the gap can be recognized in many places, it seems natural to allocate different terms to steeper and gentler piedmont depositional landforms. In contrast, if the gradient of semi-conical depositional landforms tends to vary gradually, all of them can be called alluvial fans. To discuss this issue on a global scale, geomorphological information from humid regions is indispensable, because previous studies rarely dealt with a large dataset of alluvial fans in humid regions. Although Blair and McPherson (1994) analyzed data for both arid and humid regions, the number of the fans from humid regions examined seems to be small.

This paper constructs a database for 690 alluvial fans in Japan (annual precipitation is ca. 1,000–3,000 mm), Taiwan (ca. 2,000–3,000 mm) and the Philippines (ca. 1,000–4,000 mm), to analyze fan slopes and other morphometric properties in humid regions based on a large data set. Attention is directed toward whether the depositional slope gap exists, to test the validity of Blair and McPherson (1994) from a geomorphological viewpoint. We also collected morphometric data for the source areas of alluvial fans and analyzed them in relation to the fan slope, because many previous studies noted the effects of source-area properties on the form of alluvial fans.

Section snippets

Data collection

Alluvial fans in the three countries were identified using the complete sets of topographic maps: 1 : 25,000 maps for Japan were published by the Geographical Survey Institute with a contour interval of 5 or 10 m; the 1 : 50,000 maps for Taiwan were complied by Gakuseisha (1982) with a contour interval of 10 or 20 m; and the 1 : 50,000 maps for the Philippines were published by the Board of Technical Surveys and Maps with a contour interval of 10 or 20 m.

We identified semi-conical depositional

Examples of alluvial fans

Unlike alluvial fans in some arid regions such as the American Southwest, fans in Japan, Taiwan and the Philippines have not been well known to international researchers. Therefore, this section introduces several typical examples of alluvial fans in the three countries with contour maps. As this paper focuses on fan slopes, the examples are divided into three categories based on their slopes.

Frequency distributions of fan slope and source area properties

The fan slope data for Japan, Taiwan, and the Philippines were arranged to examine their frequency distribution, one of the most basic statistical properties of a numerical dataset. Fig. 5A shows the histogram of the slopes of all the 690 alluvial fans with a 0.1° (partly 1°) slope bin. The negatively skewed histogram shows that slopes between 0.2°(0.003) and 0.8°(0.014) are the most frequent. For larger slopes, the frequency of fans gradually decreases with an increasing slope. The histogram

Discussion

The 690 alluvial fans in Japan, Taiwan, and the Philippines cannot be divided logically based on their slopes, because the slopes follow the lognormal frequency distribution without distinct gaps. The lognormal distribution also applies to the area and relief ratio of the source basin, indicating that the shape of the fan/basin systems varies gradually rather than abruptly. Such geomorphological continuity casts doubt on Blair and McPherson's (1994) concept of the natural depositional slope gap

Acknowledgements

We thank Thad Wasklewicz for reviewing an early draft of this paper and correcting English, and Adrian Harvey and Martin Stokes for their useful review comments. We also thank Gary Weissmann and Justin Wilkinson for encouraging us to write this paper. This study was supported by the Grants-in-Aid for Scientific Research, from the Ministry of Education, Culture, Sports, Science and Technology (Exploratory Research # 15650188).

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