Channel straightening is a technique whereby channels are reformed creating a straighter channel – usually distinctive in sinuous channels where meanders are cut-off (foreshortening) by building artificial straight channels. The aims of straightened channels are to create a more navigable channel to improve flood control; increasing flow velocity within the straightened section which was shown in the Skunk River and Squaw Creek in Iowa (Noble & Palmquist, 1968). Channel straightening also leads to the widening and deepening of the channel’s cross-section (also known as channel re-sectioning) to facilitate greater volumes of water and allow greater hydraulic efficiency. As with all engineering and modification techniques, channel straightening has significant impacts on stream power, conveyance, sediment transport and channel adjustment and throughout this essay, these four components will be looked at. Stream power is a fundamental concept pioneered by Bull (1979) and is the product of the density of the water (y), stream discharge (Q) and slope (S) expressed as yQS (Bull, 1979). The threshold of critical power is where critical power and stream power equal 1.0, however, where stream power exceeds critical power over a given period of time (>1.0), additional sediment obtained through processes of vertical downcutting into bedrock increasing sediment load and transport results in the degradation of the river. Net lateral erosion increases in streams with high discharges. Where stream power is less than critical power (<1.0), sediment is deposited causing the river to aggrade (fig.1). It can be argued that when a channel becomes straightened, stream power increases compared to a sinuous channel. The narrowing of the channel constricts flow in one direction, causing flow to increase. Artifical cut-offs of meanders results in the reduction of friction, increasing flow velocity. However, it can be inferred that increases in stream power in straightened channels result in increased incision, causing the deepening of the channel. Wyżga (1993) examined the river responses to the Raba River, Poland which was subject to straightening in the 19th century following severe flooding. As an attempt to minimise flood hazard and stabilise the river, straightening occurred at meander cut-offs in the middle-to-lower courses alongside other engineering methods. The study found that up to 3m of incision occurred on a gravel-bed stream leading to increased degradation both in upstream and downstream reaches resultant from varying rates of headcut retreat and the affects mid-channel bars had on river-energy dissipation. Continuing outwashing of fine grain materials from the bed led to reduction in sediment yield of the basin which saw increased in stream power (pg.541).
Conveyance is the process following the river’s capability to pass water through its cross-section and interaction with friction directly impacting the velocity of flow. Conveyance is calculated using manning’s equation; k2=A2R4/3/n2 where n=manning’s roughness coefficient and R=hydraulic radius. In channelized rivers, conveyance is increased where flow velocity is also increased. Factors likely to increase conveyance include gradient of the channel and bed roughness. Knapp et al. (1989) found that concrete along the perimeter of the channel linings improved the efficiency of flow. It can be argued that where there are increases in conveyance, there is a reduction in erosion processes of the bed and banks which result in a reduction in sediment supply.
Downs and Gregory (2004) look at the six-stage model (fig.2) of channel adjustment following channel straightening adapted from Hupp and Simon (1991). The model shows how overtime, a straightened channel modifies itself in order to reach quasi equilibrium. High sheer stresses on straightened channels causes the knickpoint to migrate upstream, eroding the channel bed to the point where the banks are unable to withstand their own weight causing large scale mass failures following saturation during flood events causing the widening of the channel (Downs and Gregory, 2004, pg. 142). Large masses of accumulated sediment are transported back to the formerly incised channel through upstream erosion causing the channel to reform itself until it reaches a relaxation period.
Bank stabilisation
Bank stabilisation is a method used to control erosion of the river banks and slopes as well as controlling flow velocity and sediment transport. Common engineering methods include stabilising banks with groynes/wing dykes, stone gabions, rip-rap and vegetation.
Groynes and Wing-Dykes
Prasad et al., (2016) described groynes in rivers (wing-dykes) as structures which are constructed perpendicular to river flow to protect the banks by deflecting flow. Groynes are either permeable or impermeable and typically made out of wood, concrete, stone or rock piles. The construction of groynes in rivers results in variations in hydrodynamic and morphodynamic responses which was explored by Pinter & Heine (2005) on the Lower Missouri River in the US. Using gaging measurements, data was collected to construct the changing channel geometry overtime. Analysis of stage and discharge data of the Lower Missouri river showed stages at low flows trended downward systematically while larger discharges presented an opposite trend (pg. 89). Overtime, all gages document systematic changes in flow velocity, depth and elevation, correlating with wing-dam construction, implying the channel’s gradual response to modification to reach equilibrium. Significant losses in channel conveyance was observed during flood flows, indicating the channel’s inability to pass water and sediment through its cross-section likely due the groynes acting as a barrier, trapping sediment which caused localised erosion and scouring. Przedwojski (2010) found that bed topography and local scour holes in concave bends at groyne heads orientated upstream with equal spacing and geometric shape were formed where flow velocity was equal or less than mean annual discharge.
Huthoff et al., (2013) analysed the impacts of flood stages caused by wing-dykes (fig. 3). T0 shows the channel with no wing-dykes while T1 shows the channel immediately after wing-dyke construction while T2 shows the response of the channel to wing-dykes after several years, where a new equilibrium is recognised (pg.552). It was argued that response times typically lasted between 1-5 years with a range of discharges following the construction. Despite higher rates of incision post wing-dykes, the argument was that the net effect of wing-dykes resulted in higher flood levels.
Prasad et al., (2016) found that groynes tend to move the erosion zone to a different location in the river, predominantly downstream, and interferences in flow led to an increase in water depth, velocity and turbulence which led to scouring in the nose of the groyne (pg.48). Deposition was dominant upstream as a result of reductions in flow velocity and substantial erosion occurred on the opposite bank to the groyne. Their observations concluded that for maximum bank protection, an angle of 135 is more reliable compared to a groyne with an angle of 45. These results will aid engineers in designing and constructing of groynes in the future.
Rip-rap and Gabions
Rip-rap is a form of revetment used as another form of channel stabilisation; where large boulders or concrete blocks are placed on edges of banks preventing lateral erosion of rivers (Reid & Church, 2015). Gabions are stone-filled wire-mesh walls placed on the river banks and used to construct revetment walls to groin deflectors and grade-control sills (Thompson et al., 2016, pg.,779). In comparison with rip-rap, “gabions utilize local materials, are strong, flexible, and can be used on steeper slopes with less thickness for bank protection” (Thompson et al., 2016, pg.,780). Gabions typically last for 25 years but there have been instances in projects where they have lasted between 75 to 100 years. Both rip-rap and gabions prevent channel migration and inhibit local sediment supply to the channel resulting in scour and substrate coarsening, leading to downstream erosion. Placement of rip-rap has also resulted in the removal of organic material from the riparian zone affecting input into the channel. Flow in these reaches are usually decreased also impacting channel conveyance.
Vegetation
Riparian vegetation is common in channels due to their impacts on hydrological, fluvial and biological processes. Dense vegetation obstructs flow, reducing friction thus reducing conveyance capacity (Uotani et al., 2014). Vegetation has the ability to sustain biological services; providing habitat for species and cycles of organic and inorganic substances. The balance of these is important in understanding future channel dynamics as it has been shown that distributed riparian vegetation can adjust singular channels from braided where vegetation stabilises banks and diverts discharge from floodplains to channels (Van Dijk et al., 2013). In regards with soil mechanics, vegetation results in greater infiltration rates and reduces hortonian overland flow into catchments which is important during flood events where runoff into catchments are a secondary source of water to catchments. Rowiński et al., (2018) emphasises the importance of vegetation being incorporated into channel designs and flood management plans to improve resilience to flooding and to also develop better ecological status (pg. 1). Vegetation is a natural and cheaper alternative compared to other bank stabilisation methods mentioned.
Levées are a common mechanism acting as embankments to raise the channel’s height and increasing bankfull discharge. Levées are natural or artificially constructed though the majority are artificial created from concrete and similar materials resistant to erosion. Remo et al., (2018) argued that levées “increase flood discharges by reducing static and transient water storage through the confinement of water flows to a levée-defined floodway that speeds up downstream propagation of flood wave” (pg.90). The likelihood of flooding is reduced in areas where levées are present and it can be argued that the material the levée is made out of i.e. concrete exacerbates flood discharges by reducing friction. This implies that stream power in catchments with levées are likely to increase due confinement which force the water to flow through a narrower cross-section thus increasing flow velocity; also increasing conveyance. When levées breach, it can be argued that the velocity at first will be high but once the floodwaters spill out onto the floodplain, velocities then begin to decrease. Heine and Pinter (2012) discussed the alteration in river stages by levées in rivers in Iowa and Illinois. The model (fig.4) shows how upstream (B-B) before the levée, there is a slight increase with levées and how the increase in stage at the levée site (A-A) is significantly greater to B-B. Further downstream at C-C, the stage decreases with levée. The reason for the slight increase in stage at B-B upstream of the levée indicates backwater effects; whereby velocity is slowed resulting in the ponding of water. Once a levée is breached, rivers may adjust immediately, resulting in flows to navigate elsewhere from the main channel onto floodplains which was the case in Italy where the Po Plain, the largest alluvial plain became inundated by several levée breaches (Govi & Maraga, 2005).
Rahimi et al., (2018) examined 30,000 miles of levées in the US, constructed for the purpose of severe flood control and wave action during storm events. 70% of these levées failed during the Midwest floods of 1993. The USACE found that the majority of levées in the US are susceptible to very high risk of failure and an estimated $80 billion is required for repairs and maintenance of levées in the next 10 years. Fell & Kousky (2015) looked at the relationship between the value of levée protection and commercial properties in areas with and without levée structures to see whether commercial property sales were affected with levées nearby. The paper found there was no difference with properties not at risk of flooding. They found that firms with ‘less to lose’ would move to areas of levée protection which impacted in boosting property sales.
Pinter et al., (2008) focused on flood trends in the Mississippi river and documented contributors relating to increased flooding was resultant from levée construction combined with wing-dykes affecting navigation. Long-term impacts of engineering on flood control have been correlated with decreased conveyance of flood flows as shown in fig.5 below.
From fig.5, instream changes affect flood magnitudes over long periods and stage
trends of the middle, upper and lower Missouri river conclude increases in flood levels resultant from engineering (pg.3).
Dredging
Dredging involves removing large masses of sediment (commonly silts and sands) from the river bed to remove accumulated sediment, increasing channel capacity and velocity at bankfull. Dredging however has significant impacts on river courses including changes in channel slope both upstream and downstream, reduced sediment availability leading to vertical erosion and river bank instability causing over-deepening and widening as well as increases in stream power causing coarse sediment to be transported downstream resulting in bed armouring. Pinter et al., (2004) discusses dredging in the middle and upper Mississippi river documenting patterns of bed aggradation on a spatial and temporal scale. The study found that dredging occurred in five places; channel bifurcations, mouths of tributaries, thalweg crossing, meander bends and straight reaches in rivers (pg.293). The aggradation of unregulated sediment in all these locations caused by persistent shoaling results in dredging.
Dredging has led to a variety of ecological concerns illustrated by Meng et al., (2018) where their study revealed that responses of macroinvertebrates and local environment to dredging caused a 50% reduction in taxa richness and increased water depth, turbidity and changes to sediment composition (pg.1350). Decreases in water nutrient levels were observed and a general decline in biodiversity and composition of benthic fauna; while enrichment of fauna was observed 500m from the dredged site.
Van Vuren et al., (2015) examine the aftermath river restoration works for the Rhine in the Netherlands involving plans known as Room for River (RfR) program and the European Water Framework Directive (EUWFD) designed to increase flood conveyance capacity and improve ecology and biodiversity (pg.172). The paper finds that intervention works designed to control flood levels during storm events led to an increase in dredging amounts by 10%. Current works in the upstream reach of the Waal resulted reduced sediment transport rates due to lowered flow in the main channel causing the river to aggrade and dredging quantities to increase. The importance in reducing dredging and allowing the river system to become navigable by itself is expressed.
Weirs
Weirs are seen as underwater dams aiding in preventing flooding and altering flow of the river. They are placeed in certain parts of the river to manage water accumulation. Weirs can create backwater effects causing deposition reducing flow velocity and conveyance. Bednaríck et al., (2017) discusses how weirs affect methane dynamics (CH4) and found an overall increase in CH4 resultant in increased microbial activities in accumulating weir sediments increasing emissions (CO2), affecting aquatic organisms intolerant to increased gas emissions – exacerbated in winter conditions by longer surface water residence time causing CH4 oxidation (Atkins et al., 2017). There is an over-emphasis on flood control with the use of weirs, however, they are used to maintain flow during low-flow or drought periods. Ahn et al., (2018) evaluated how weirs resulted in highly efficient water resource management and water use during drought conditions contributing to positive environment.
Culverting
Culverting involves movement of water under artificial structures i.e. bridges, roads, railroads or trails using pipes, reinforced concrete or other materials embedded in soil for stabilisation (Taylor, 2010). Significant risks associated to culverting include the possibility of the culvert becoming blocked with sediment; increased during a flood event complicating maintenance as access to the culvert itself is difficult, meaning management is impractical. However, it can be argued that culverts are more effective in dryland channels where morphological change is less, suggesting efficient throughput of water and sediment (Chin et al., 2017). Chin looks at how urbanization in Fountain Hills; characteristic of a desert, dryland environment, has fragmented the wash of sediments into channels during heavy rains and morphological responses has led to scouring downstream and accretion further downstream. Urbanisation has affected sediment conveyance and thus the installation of culverts under has been favoured.
Conclusion
This essay has demonstrated how different channel engineering and modification strategies impact channel processes over a range of timescales. The interaction between transporting water and sediment determines the behaviour of rivers; where low flows causes sediment to be accreted while high flows result in erosion (Seed, 1997). Accretion and erosion alter channel conveyance which increases risks of flooding and these risks may be reduced or indirectly exemplified. Studies have shown that post construction of levées, wing-dykes and channelization, rivers naturally adjust overtime to re-achieve a state of equilibrium. Spatially, where engineering has occurred in an area of the river, other engineering intervention may be required either upstream or downstream. This has been recognised in the construction of levées where backwater affects led to accretion of sediment causing dredging to occur to remove the build-up of sediment.
Climate change is having significant impacts on the intensity and frequency of flood events and it is down to engineers to construct effective river management plans to minimise the risk of large-scale floods. Kesel & Yodis (1992) argued how humans impacted fluvial systems. The Buffalo and Homochitto rivers in SW Mississippi received increased sediment yields from forest clearance and poor land-use practices which amplified soil erosion. The impacts land-use and urbanisation near catchments which increase runoff, promoting erosion and instability must be recognised within individuals and planners. While we continue to implement methods with the intention of minimising flood risk and improving discharge, improvements are not always beneficial and not immediately effective.