Alluvial fan

Japanese: 扇状地 - せんじょうち（英語表記）alluvial fan

A semi-cone-shaped gravel deposit formed by a river that starts at the mouth of a valley in a mountain or hill and opens out like a fan toward the lowlands. In the foothills of arid regions, the bedrock can be covered with a thin layer of gravel due to lateral erosion by rivers that flow only during rainfall, resulting in the development of an eroded landform that is morphologically similar to a fan. This is called a rock fan, and the former is sometimes called an alluvial fan. Small fans are also called alluvial fans, and steeply sloping fans are also called alluvial cones.

[Shingo Juen]

Origin and distribution of alluvial fans

When a river suddenly emerges from a narrow valley into a wide, gently sloping lowland, it floods the surrounding area and becomes shallower, so it encounters a lot of frictional resistance from the riverbed, its flow rate slows, its carrying capacity weakens, and the coarse sand and gravel it carried during floods is deposited along the riverbed. As the river changes course in a steeper direction, it moves radially from the mouth of the valley with each flood, and the deposited gravel overlaps and forms a semi-conical depositional landform with the mouth of the valley as its apex, i.e. a fan. The apex of the fan, which is the key point of the fan, is called the fan apex, the central part is called the fan center, and the end is called the fan tip.

Globally, alluvial fans are well developed in orogenic belts such as the Alps and Himalayas, as well as in arid regions, around the Pacific Rim (except in equatorial regions). Many develop on the fringes of mountainous areas that have been subject to crustal movement such as faults and uplift, but they are also common around fault basins in mountainous areas, and can also be seen at the mouths of tributaries that flow into main valleys. On the other hand, alluvial fans do not develop well in areas of land-building movement or in rivers in tropical regions. This is related to the fact that in the tropics, chemical weathering of rocks is severe, making it difficult for gravel to form.

[Shingo Juen]

Relationship between alluvial fans and rivers

The size of an alluvial fan is positively related to the width of the upstream basin of a river, but the upstream basin of a large alluvial fan often has fault zones and collapsed areas with exposed weathered rocks. The development of an alluvial fan is related to the following factors: (1) the presence of rocks and strata that are easily denuded or eroded in the upstream basin, which supply a large amount of debris, (2) the presence of many steep slopes that are convenient for the movement and transportation of debris, (3) the lack of vegetation cover and the high level of surface runoff, (4) the high frequency of heavy rainfall, and (5) the presence of a wide lowland at the mouth of the valley where the river can flow freely, and where the sand and gravel that accumulates there is not removed or submerged. Generally, alluvial fans created by large rivers are larger than those created by small rivers.

The slope of an alluvial fan is related to the flow velocity and volume of the river and the particle size and amount of the material being transported, and is steep at the top of the fan and gentler towards the end. Its cross section is a concave curve, with the river's transport capacity being appropriate for transporting the load. The slope (average slope) of an alluvial fan is steeper the greater the relative elevation of the upstream basin when the area of the upstream basin is the same, and conversely, when the relative elevation is the same, the larger the drainage basin area is, the gentler the slope tends to be. Also, when a mountain valley is still in its infancy and has not yet reached equilibrium, the slope of the alluvial fan is gentler than the cross section of the valley, but when a mountain valley forms a valley floor plain, it is often steeper than the cross section. However, the larger the alluvial fan, the gentler the slope of the alluvial fan is in general.

Where the alluvial fan transitions to the downstream natural levees, the surface gradient decreases abruptly and the fan tip is often relatively clearly visible. This is because the river sediments change from gravel to sand discontinuously, and the alluvial fan is made up of gravel.

[Shingo Juen]

Alluvial fan deposits

The deposits of an alluvial fan consist mainly of poorly sorted large and medium sized gravel, with sand, silt, and clay being secondary. The gravel is coarse at the apex of the fan and becomes finer towards the end. The thickness of the deposits is often greater in the center of the fan than at the apex.

Because alluvial fan rivers have thick sand and gravel, the flowing water seeps through and flows underground, generally appearing as rivers without water or rivers without end. Alluvial fan waterways are usually called washes. When a waterway near the apex of the alluvial fan is dug relatively deeply, it is called an alluvial fan ditch. Because alluvial fan rivers transport and deposit large amounts of coarse sand and gravel during floods, the riverbed is shallow and almost flush with the alluvial fan surface. As a result, flowing water overflows during floods and often changes course. If levees are built to keep the course constant, deposition only occurs inside the levees, and the riverbed gradually rises, creating an overhead river that often leads to more serious flood damage.

During the formation of a fan, when the indentation of the river flow exceeds the uplift of the foothills, the flowing water channel excavates the fan, and the area of most active deposition continues to move toward the end of the fan, forming a long, narrow fan that extends downstream.

[Shingo Juen]

Types of alluvial fans

After a fan develops in front of the foot of a fault mountain, if the mountain side rises or the fan side sinks, the new fan will overlap the previous one in a kagamimochi style, forming a double fan, as typified by the Musashino Plateau in Tokyo. If the flow rate of the river that formed the fan increases, the amount of material it carries decreases, or the alluvial surface becomes steeper or uplifted due to tilting, the alluvial fan will be dissected by the river and become a dissected fan. The alluvial fans in the Isawa River basin in southwestern Iwate Prefecture are an example of this. Rivers that dissect alluvial fans often form new alluvial fans downstream. Such old and new alluvial fans connected one behind the other are called parent-child alluvial fans, and can be seen in Omura City, Nagasaki Prefecture. Double and parent-child alluvial fans are called composite fans. In contrast, when two or more alluvial fans are adjacent to each other and connected to the foot of a mountain, this is called a confluent fan. Large-scale confluent alluvial fans form foothill alluvial plains, and are also called foothill alluvial surfaces. When heavy rain falls irregularly at the foot of fault mountains in semi-arid to arid regions with little vegetation cover, large-scale confluent alluvial fans called bajadas (bajada in Spanish) often develop.

The underground water from alluvial fans gushes out as springs at the tips of the fans, so settlements form here and rice paddies often develop nearby from an early stage. Water is relatively easy to obtain at the tops of the fans, so settlements often developed there from an early stage. However, the center of the fan has deep underground water and development was delayed, leaving it as wilderness for a long time. In Japan, development finally began from the end of the Edo period through the Meiji period, and the area has been used for cotton, mulberry, tea, wheat and orchards.

[Shingo Juen]

Formation of alluvial fans
(1) When the uplift of the mountain front exceeds the indentation of the river, the top of the fan is not deeply indented by the river, and deposition continues close to the mountain front, forming a fan. (2) When the indentation of the river exceeds the uplift of the mountain front, the waterway is indented into the fan, deposition occurs on the lower side of the fan, and a new fan is formed .

Formation of alluvial fans

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

山地や丘陵の谷口を頂点とし、低地に向かって扇状に開く河川によってつくられた、半円錐(えんすい)形の砂礫(されき)の堆積(たいせき)地形。乾燥地域の山麓(さんろく)には、降雨のときだけ流出する河流の側方侵食によって、岩床が薄く砂礫で覆われ、形態的に扇状地に似た侵食地形が発達することがある。これを岩石扇状地とよぶので、前者を沖積扇状地とよぶこともある。小さな扇状地は沖積扇(せん)、傾斜の急な扇状地は沖積錐(すい)ともよばれる。

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扇状地の成因と分布

狭い谷間から急に勾配(こうばい)の緩い広い低地に出ると、河流は周囲に氾濫(はんらん)して浅くなるので、河底の摩擦抵抗を多く受けて、流速が遅くなり運搬力も衰えて、出水時に運搬してきた粗粒の砂礫を流路沿いに堆積する。河流は勾配の急な方向へ流路を転ずるので、出水ごとに谷口を中心として放射状に移動し、堆積する砂礫は重なり合って、谷口を頂点とする半円錐形の堆積地形、すなわち扇状地を形成する。扇状地の扇の要(かなめ)にあたる頂点を扇頂、中央部を扇央、末端を扇端という。

　世界的にみると、扇状地は赤道地方を除く環太平洋や、アルプス、ヒマラヤなどの各造山帯と乾燥地域によく発達している。多くは断層や曲隆などの地殻運動を受けた山地の周縁に発達しているが、山地中にも断層盆地の周囲に多く、また、主谷に注ぐ支谷の谷口に発達をみることもある。一方、造陸運動地域や熱帯地域の河川には、扇状地の発達が悪い。熱帯では岩石の化学的風化が著しく、礫ができにくいことと関係がある。

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扇状地と河流の関係

扇状地の大きさは河川の上流域の広さと正の関係を示すが、大きな扇状地の上流域には、断層破砕帯や風化岩石の露出する崩壊地などが発達することが多い。扇状地の発達には、(1)上流域に削剥(さくはく)や侵食されやすい岩石や地層があって、多量の岩屑(がんせつ)を供給する、(2)岩屑の移動、運搬に都合のよい急斜面が多い、(3)植被が乏しく表面流出が多い、(4)豪雨の頻度が高い、(5)谷口に河川が自由に移動できる広い低地が開け、そこに堆積する砂礫が排除されたり水没したりしない、ことなどが関係する。一般に大きな河川のつくる扇状地は、小さな河川のつくる扇状地より大きい。

　扇状地面の傾斜は、河流の流速や流量と、運搬物質の粒径や多少とに関係し、扇頂部に急で扇端部に向かって緩くなる。その縦断面形は、河流の運搬能力が荷重を運搬するに適当なうえに凹の曲線を示す。扇状地面の勾配（平均傾斜）は、上流域の面積が同じ場合は上流域の比高が大きいほど急で、逆に比高が同じ場合には、流域面積の大きいほど緩やかな傾向を示す。また、山中の谷がまだ幼年期の状態にあって、平衡状態に達していない場合には、扇状地面の勾配は谷の縦断面形のそれよりも緩やかで、山中の谷が谷底平野をつくっているような場合には、その縦断面形よりも急であることが多い。ただし、扇状地面の勾配は、一般に大きな扇状地ほど緩やかであることが多い。

　扇状地から下流側の自然堤防帯に移行する所では、表面勾配が急に減少し、扇端部が比較的明瞭(めいりょう)に認められる場合が多い。これは、河川堆積物の礫から砂への粒径変化が不連続的におこり、扇状地が礫からなるためである。

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扇状地の堆積物

扇状地の堆積物は、淘汰(とうた)の悪い大礫や中礫が主体で、砂、シルト、粘土などは従属的である。礫は扇頂部に粗粒で、扇端部にいくにつれて細粒となる。堆積物の厚さは扇頂部よりむしろ扇央部に厚いことが多い。

　扇状地の河川は砂礫が厚いため、流水は浸透して伏流し、一般に水無川や末無川の状態を呈する。扇状地の水路は通常ウォッシュwashとよばれている。扇頂近くの水路が比較的深く掘り込まれているときは、扇頂溝とよばれる。扇状地の河川は洪水時に多量の粗大な砂礫を運搬堆積するから、河床は浅く扇状地面とほとんど等高である。したがって、洪水時には流水が溢出(いっしゅつ)し、しばしば流路を変えることが多い。堤防を設けて流路を一定に保つと、堆積は堤防内部のみで行われ河床はしだいに高くなり、天井川を生じてかえって大きな水害を招くことが多い。

　扇状地形成中、河流の下刻(かこく)が山麓(さんろく)の隆起に勝るときは、流水路は扇状地を掘り進み、堆積のもっとも盛んな所は扇端部へ移動し続け、下流に延長する細長い扇状地が形成される。

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扇状地の種類

扇状地が断層山麓前面に発達後、山地側が隆起するか、扇状地側が沈降すると、以前の扇状地の上に新しい扇状地が鏡餅(かがみもち)式に重なり、東京の武蔵野(むさしの)台地に代表されるような二重式扇状地が形成される。また、扇状地を形成した河川の流量が増加したり、運搬物質が減少したり、さらに傾動で扇状地面の傾斜が急になったり、隆起したりすると、扇状地は河川によって開析されて開析扇状地となる。岩手県南西部の胆沢(いさわ)川流域の扇状地がこれにあたる。扇状地を開析した河川は、その下流側に新しい扇状地を形成することが多い。このような前後に連なる新旧扇状地を親子扇状地といい、長崎県大村(おおむら)市にみられる。二重式扇状地や親子扇状地は合成扇状地composite fanとよばれている。これに対し、二つ以上の扇状地が横に相接して山麓に連なるときは、これを合流扇状地confluent fanという。大規模な合流扇状地は山麓沖積平野を形成し、山麓沖積面ともよばれている。植被も乏しい半乾燥～乾燥地域の断層山麓に、まれに不定期的に集中豪雨が降ると、バハダbahada（スペイン語ではbajada）とよばれる大規模な合流扇状地の発達をみることが多い。

　扇状地の伏流水は扇端に泉となって湧出(ゆうしゅつ)するので、ここに集落ができ、付近には水田が早くから発達していることが多い。扇頂部も比較的水が得やすく、集落が早くから発達していることが少なくない。しかし、扇央部は地下水が深く開発が遅れて長く原野のまま放置されてきた。日本では、江戸時代末期から明治にかけてようやく開発が進められ、ワタ畑、クワ畑、チャ畑、ムギ畑、果樹園などに利用されてきた。

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沖積扇状地の形成
(1)山地前面の隆起が河流の下刻より勝るときは、扇頂部は河流で深く掘り込まれることはなく、堆積は山地前面に接近して行われ続けて扇状地が形成される(2)河流の下刻が山地前面の隆起に勝るときは、流水路が扇状地の中に掘り込まれて、堆積が扇状地の低所側に行われて新しい扇状地が形成される©藤田正純">

沖積扇状地の形成

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