河型成因研究

Formation of river patterns

不能用某些河性作为河型成因。决定冲积河流河性河型的是来水来沙条件和外加的边界条件;后者主要指变动的侵蚀基面。不同的来水来沙关系对滩、槽的冲、淤部位不同,导致河槽向单股窄深或向多汊宽浅发展,从而形成不同的河型。用Q=k(QJ)~m 表示来水来沙关系,m>2.5的形成单股窄深蠕动河道,m<2.5的形成多汊宽浅游荡河道。但若来沙偏小,单股窄深蠕动河型因裁弯后新、故河长期并存而派生成多汊窄深蠕动河型,多汊宽浅游荡河型也因支汊不易冲淤而派生成多汊宽浅蠕动河型。故河型可分为四类。冰冻使小水期输沙能力增大,河床易冲而河岸较稳定,相当于使m值加大。因此长期冰冻易于发展成单股窄深蠕动河道。

Characteristics and patterns of rivers are governed by incoming water/ silt discharges and given boundaries, i.e. solid bank and datum. For alluvial rivers, less solid banks would exist. Scour/siltation on different parts of channel cross sections depends on water-silt discharge relation which can be expressed by Q5=kQm. If the flow hydrograph and silt amount remain unchanged, large value of m will result in more silt discharge in flood period and less in low-water period. This would lead to more siltation on flood plain and less siltation or more scour in channel. Consequently, a deep-narrow channel would be developed. On the contrary, smaller value of m will result in shallow-broad channel. Field data coincide well with the above mentioned analysis. The demarcation value of m may be 2.5. If the erosion datum changes continuously, slope J must be taken into account by using following expression, Qs = k(QJ)m, Severely affected by back water, lower Jingjiang and Lower Hanjiang of Yangtse River have much smaller slopes in flooa season than those in low-water season, their values of m increase to 3.1 and 4.1 respectively. Eventually, they become famous meandering rivers. Less incoming silt discharge causes more stable channel branches, which in turn can develope into two derivative river forms. As a result, four patterns of river would exist, namely, single-deep narrow-meandering, multiple-deep narrow-meandering,multiple-shallow broad-meandering, multiple-shallow broad-wandering.Freezing of water tends to increase the capacity of silt transportation and strengthen the channel banks during low-water season. Thus, river with long ice period tends to form a single-deep narrow channel.