A Field Guide to British Rivers. George Heritage

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, they can be viewed as a transition channel type between braided and lowland meandering channels. Low‐gradient actively meandering Unconfined low‐gradient meandering channels with a bedload dominated by sand and fine gravel; hence, the channel bed has marked fine sediment accumulations that are mobile in most flood events. These occur in higher‐order (i.e. typically lowland) channels exhibiting more laminar flow hydraulics, with turbulent flows being uncommon. The fine bed sediment, erodible banks, and unconfined settings means that such channels are dynamic and prone to change. They also often have extensive riparian zones and floodplains which are linked to the channel. Bars and pools may be present and are associated with bends and crossing of the meander pattern. Groundwater‐dominated channels Groundwater‐dominated rivers have low‐gradient channels and are characterised by a stable flow regime, although limestone rivers with cave systems may display hydrological characteristics like freshet rivers. This stable regime is a product of the permeable catchment geology and consequent reduction in overland flow that characterises groundwater‐dominated streams. Bed movement is infrequent, and sediments are predominantly transported in suspension. Typically, sediments are derived from catchment sources, although large macrophyte beds provide a source of in‐stream organic detritus. As bed disturbance is infrequent, deposited sediments may remain in the gravel for extended periods, promoting the accumulation of large quantities of fine sediment. Substrate generally comprises gravels, pebbles and sands, and glides and runs are the dominant flow types. Localised areas of riffle habitat may be present particularly where woody debris is available. Low‐gradient passively meandering These channels are typically found at lower extremities of the channel system. Generally, they flow through high‐resistant materials, for instance clays and coarse deposits. They are generally sinuous; however, as the banks comprise materials that are resistant to erosion, they typically display “fixed” planform geometry. Thus, these channels are often incised and display low width to depth ratios. The beds typically comprise fine sedimentary materials (sands and silts), although pockets of gravel can be present, particularly in poorly formed bar deposits. These channels are typically deep, and flows are dominated by glides, although runs may be associated with meander bends. Riparian vegetation is influenced by the clay soils and is often sparser than in other channel types, comprising grasses shrubbery and smaller pockets of woody growth. Primary production is strong in these channels and coupled with stable beds and extensive growth of macrophyte vegetation.

      A key issue arises when discussing the scale over which any classification system is applied. In Scotland, pool‐riffle units appear to have been subsumed into an “active meandering channel type” by SEPA (2012), whilst remaining explicit in many other classifications. Similarly, rapids and associated morphologic units become difficult to classify. Anabranching or anastomosed channels, which are often rare across temperate regions where the majority of the classifications have been developed, are also under‐represented in typologies with only the River Styles framework (Brierley and Fryirs 2005) explicitly considering this channel type, illustrating how geography has influenced the typologies.

Recognising the above and the absence of a “one size fits all” methodology, this book follows Montgomery and Buffington’s well‐established (1997) fluvial framework, expanding it where necessary to include rarer, but nonetheless important, river types encountered across the United Kingdom. We use this based on a collective 90 years’ plus of river science and river management experience in the authorship team, but particularly as this classification is fundamentally based around characteristic process controls (Table 2.5) and morphological assemblages which are summarised in Figure 2.5 and Tables 2.6 and 2.7 all of which are described in greater detail in Chapter 3.

Table 2.5 Basic character and processes operating in the channel types used in this book.

Channel type	Processes
Diffuse Upland	Channels are characteristically small with steep organic or fine sediment bank profiles. The channel bed may be organic or fine sediment with occasional bedrock or boulder steps separating long pools upstream. Seasonal channels may be vegetated and generally are less well defined promoting diffuse surface flow.
Bedrock	Bedrock channels are most associated with mountain areas but can occur almost anywhere along a watercourse. They are morphologically robust and have a high flood competence. Valley bottom area is often narrow. Bedrock and/or boulder step pool sequences dominate with the steps acting to moderate gradient providing stable channel conditions. The pools often contain limited finer sediments due to the low‐energy conditions created by the backwater effects of the steps.
Wandering	Wandering channel types are associated with upland zones. They are often found flowing across an extended valley floor in moderate‐to‐high gradient systems where sediment loads are high. Often, they are associated with relict fluvio‐glacial valley bottom material with the river forming several channels through a mix of sediment dominated by large material rarely mobilised by the Скачать книгу В начало < 7 8 9 10 11 12 13 14 15 > В конец