A Field Guide to British Rivers. George Heritage

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СКАЧАТЬ continuum from bedrock to alluvial forms, each influenced by a spatially and temporally variable set of controlling parameters. The result is that each published classification throws up issues when used as a general tool for determining a river type. The work of Leopold and Wolman (1957), for instance, concentrated only on a relatively narrow range of low to moderate energy alluvial systems, so failing to cover steep upland channel types, whereas Montgomery and Buffington (1997) and SEPA (2012) do extend their typology to steep channels and to channels where bedrock dominates over alluvium.

Table 2.3 Functional channel types defined by SEPA (2012).

      Source: Based on SEPA (2012). © John Wiley & Sons.

Geology	Slope	Sinuosity	River type
Bedrock	Any	Any	Bedrock, Cascade
Not bedrock	>0.1	Any	Bedrock, Cascade
	0.03> <0.1	Any	Step‐pool, Plane bed
	0.005> <0.03	<1.1 >1.1	Step‐pool, Plane bed Plane riffle, Braided, Wandering
	0.001> <0.005	Any	Plane riffle, Braided, Wandering
	0.0005> <0.001	<1.4 >1.4	Plane riffle, Braided, Wandering Active meandering
	0.001> <0.005	Any	Active meandering
	<0.0001	Any	Passive meandering

Table 2.4 General character of functional channel types as defined by SEPA (2012).

      Source: Based on SEPA (2012). © John Wiley & Sons.

Channel type	Description
Bedrock channels	Most found in upland areas, though bedrock lined reaches can occur in certain lowland environments. They generally have little if any bed sediment and have limited hydraulic connection with the riparian zone. Channel gradients tend to be high, resulting in a high transport capacity but limited sediment supply. These factors, together with the high degree of bank strength, result in quite stable channels.
Cascades	Restricted to upland areas with steep slopes and are characterised by disorganised bed material typically consisting of cobbles and boulders constrained by confining valley walls. The riparian zone is usually extremely small in extent and interactions with the channel are limited. The large size of bed and bank material, together with high levels of energy dissipation due to the bed roughness, dictates that the largest bed load only becomes mobile in extreme floods (circa > 25‐year return interval). Bedrock outcrops are common and small pools may be present among the boulders.
Step‐pool channels	Have a steep gradient and consists of large boulder splays which form discrete sediment accumulations across the channel, forming a series of “steps” which are separated by intervening pools containing finer sediment (typical spacing 1–4 channel widths). The stepped channel morphology results in zones of turbulence interspersed by more tranquil flows. As with cascade reaches, the high degree of channel roughness and large sediment on the channel bed and banks results in stable channels that respond only in very large flood events. The stream is generally confined by the valley sides, and there is limited development of terraces or floodplains.
Plane bed channels	Generally moderate gradient streams with relatively featureless gravel/cobble beds, but which include units ranging from glides, riffles, and rapids. Sediment size and channel gradients are smaller than step‐pool channels and deeper pool sections tend to be lacking. The riverbed is generally armoured and, thus mobilized only in larger floods. Although channels are typically stable, they are more prone to channel change than any of the preceding channel types. With relatively more frequent bedload movement, they represent transitional channels between the more stable types listed above and the following more dynamic types of channel. Channels are generally straight and may be confined or unconfined by the valley sides. However, the banks – which generally comprise material resistant to lateral migration – constrain the channel from migrating laterally and developing alternate bars or riffles.
Pool‐riffle and Plane‐riffle channels	Meandering and unconfined channel that during low flows are characterised by lateral oscillating sequences of bars, pools, and rifles, resulting from oscillations in hydraulic conditions from convergent (erosive) to divergent (depositional) flow environments (typical spacing 5–15 channel widths). The gradient of such channels is low to moderate, and the width to depth ratio high. The bed is predominantly gravel with occasional patches of cobbles and sand. Accumulation of sediments in gravel bars indicates increasingly transport‐limited conditions, though most large floods will produce some bedload movement on an annual basis, thus reducing the stability of the channel. In such channels, interactions between the stream and the riparian zone become more obvious with extensive over‐bank flood flows and wetland areas often characterising the riparian zone. The banks are typically resistant to erosion, and lateral migration of the channel is limited, resulting in relatively narrow and intermittently deep channels. Plane‐riffle channels form an intermediate channel type between those of plane‐bed and pool‐riffle channels. They retain many of the attributes of pool‐riffle channels; however, they generally have less defined pools, coarser (armoured) substrate, and less‐extensive bar features. They are a common channel form in the United Kingdom, although it is unclear whether their presence is natural or whether they represent a degraded form of the pool‐riffle channel. For management purposes, it is suggested that they are treated as a pool‐riffle channel type.
Braided channels	Braided reaches can occur in a variety of settings. They are characterised by relatively high gradients (but ones that are less than upstream reaches) and/or abundant bedload. Sediment transport is usually limited under most conditions and the channel splits into several threads around instream bars. Nevertheless, poor bank strength renders them highly dynamic and channels will generally change even in relatively small flood events.
Wandering channel	These reaches exhibit characteristics of braided and meandering channels and typically switch between divided and undivided channel types. Wandering channels may also be susceptible to channel avulsions during high‐flow events, where the channel switches to a historical planform. Wandering channels typically occur where a reduction of bed material size and channel slope is combined with a widening of the valley floor. In sediment transport terms such reaches are bedload channels, but the number of competent transport events in any year will vary greatly according to bed material size and the associated entrainment function. Generally, СКАЧАТЬ В начало < 7 8 9 10 11 12 13 14 15 16 > В конец