River channel adjustments downstream from channelization works in England and Wales

Abstract

Relatively little attention has been given to river channel adjustments that occur downstream from channelization works. This study is concerned with the nature of channel adjustments downstream from a total of 46 channelization works located in low and high energy environments in England and Wales. Channel changes are identified principally by the method of field survey and by reconstructing the original positions of eroded beds and banks. Use is also made of maps, aerial photographs, and engineering drawings of different dates and the technique of space-for-time substitution is applied.Enlargement of channel cross-sections through erosion had occurred downstream from a variety of types, sizes, and dates of channelization works. The maximum increase of channel size was 153 per cent. Out of a total of 14 sites with enlarged channel cross-sections, seven had undergone a change of width only, at a further three width increased rather than depth, and at the remaining four sites depth increases were dominant. These sites all have relatively high stream powers.Factors causing spatial variation of erosion included tree roots locally binding bank sediments and the Occurrence of bends. Planform change had taken place at only one site. A further three high stream power sites had downstream reaches incised into bedrock and therefore did not exhibit adjustment.Channel enlargement is explained in terms of increased flood flows downstream from channelization works causing higher stream velocities, which in turn cause erosion, thereby increasing channel width and/or depth. Examination of flow records for 35 stations revealed flood events which would formerly have spread overbank but are now confined by the Channelization works and are therefore likely to alter downstream flows. At sites with downstream change it is proposed that the energy of increased flows was sufficient to exceed a threshold required for erosion of perimeter sediments. By contrast the absence of change at a majority of sites in low energy lowland areas could be a reflection of both the incompetence of increased flows to erode and resistance provided by perimeter sediments. Sites with erosion features appear not to have yet attained new equilibrium conditions.

Effects of sediment dredging on water quality and zooplankton community structure in a shallow of eutrophic lake

“…But most of the previous studies related to effects of dredging on water quality, release of nutrients and heavy metals, benthos, algal-periphyton, bacteria, larval zooplankton, birds and aquatic plants (Brookes, 1987;DeCoursey and Vernberg, 1975;Howarth et al, 1982;Lewis et al, 2001;Lohrer and Wetz, 2003;Nayar et al, 2004;Spencer et al, 2006). Information on the effects of dredging on zooplankton community structure is rare but also inconsistent (Wang et al, 2005;Li et al, 2007;Wu et al, 2008).…”

Reduced fine sediment flux and channel change in response to the managed diversion of an upland river channel

“…Although this declined rapidly, concentrations were still 510 % greater 2 hours after the breakthrough, with downstream concentrations of 359 mg L −1 . Such disturbances have been observed elsewhere; with sediment loads immediately downstream of in-stream works measuring between 40 and 150 % more than those immediately upstream (Brookes, 1987;Sear and Archer, 1998). The observed duration and concentrations may have short-term impacts on the primary productivity and the free-living ecology of the river such as reduced natural penetration of light; increases in the rate of drift and reduced abundance of invertebrates (Rosenberg and Wiens, 1978;Doeg and Milledge, 1991); and modified salmonid feeding and foraging behaviour (Robertson et al, 2007).…”

“…Although previous studies have documented the immediate and instantaneous impact of such disturbances (e.g. Brookes, 1987;Sear and Archer, 1998); few provide a sustained assessment of the impacts of modification (e.g. Gilvear and Bradley, 1997).…”

Quantifying River Channel Stability at the Basin Scale

“…Based on streams in England, Wales and Denmark, Brookes [114,115] revealed that post-project adjustment of channelised rivers can be related to stream power, whereby values less than approximately 15-25 W·m −2 correspond to projects that experienced deposition and values greater than approximately 25-35 W·m −2 correspond to projects that responded predominantly through erosion. This approach was developed into the 'stream power screening tool' [4,35], a bankfull-level assessment with the purpose of rapidly identifying potential sediment issues along a river network and prioritising reaches for further geomorphological investigation.…”