Research Paper on River Geomorphology and Civil Engineering

Introduction

Fluvial geomorphology is the study of river processes and forms. It investigates how the shape of the earth is modified and shaped by flowing water through the process of erosion and deposition. For centuries, rivers have been transformed to satisfy the social demands of human beings. The active drainage network of rivers controls the recession flow in a basin. Recession flow is characterized by a decrease in streamflow over time occurring during the no rain season or dry season. The geomorphology of large rivers in America is a reflection of the extensive control of water infrastructure with more than 75000 dams constructed (Alcayaga, Mao & Belleudy, 2018). The large dams with a capacity of up to 1.2km3 can affect the flow of all large rivers in the country (Alcayaga, Mao & Belleudy, 2018). Also, the process of flood management affects the fluvial flow process of rivers and has geomorphic consequences for floodplains and rivers as well. The effect of traditional river engineering practices is evident in most developed countries.

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There has been a progressive change of rivers and streams from headwaters to mouth, i.e., from steep, narrow, and turbulent flow to deep, wide, and meandering slow-flowing channels (Burk, 2018). Watershed comprises an open system with sediment zones and water production, storage and transfer. Changes in river geomorphology have led to adverse impacts on resources, human interest and the environment. It is, therefore, essential to develop proper management practices to solve the problems created by river transformation and to give way to new conceptions and goals in the civil engineering field. This paper will hence investigate how sustainable long-term management strategies can be used to solve impacts caused by the transformation of rivers to resources, environment and human interest.

Literature Review

Traditional river engineering practices have led to significant impacts on the environment. It is essential to understand the river and stream geomorphic to know how to respond to various natural and human-caused disturbances to enable effective management, rehabilitation and conservation of waterways to allow for accommodation of multiple and often conflicting needs. Channel changes due to engineering activities may have implicated protection of water supply, structures and property, habitat and navigation. Erosion of channel banks that happen during channel migration on a flood plain can be damaging to structures and property located near the river (Newson & Newson, 2000). Natural and human-caused disturbances cause instability to which the streams and rivers have to adjust. Some of the civil engineering activities causing disorders include channel extraction of gravels and sand, channelization, and urbanization. Geomorphological responses to such disturbance include degradation of the channel bed, deposition of material, also referred to as channel bed aggradation, widening of the river channel, and channel straightening (Gregory, 2006). The adjustments are a result of the river trying to develop an equilibrium position.

River development has been practiced for centuries and has been on the increase in the past decades as a result of rapid economic growth. Soft river maintenance practices have been established to help mitigate the adverse effects caused by traditional river engineering practices. Channel adjustments are a concern to river geomorphology. First, extensive lowering of the channel bed is threatening to foundations of bridge pier as well as buried cables and pipelines (Newson & Newson, 2000). It also increases bank height leading to bank instability, which may lead to the widening of the channel. Aggradation of the river channel reduces the capacity of the river, raises the bed elevation, which increases the likelihood of flooding. Channel bed changes due to engineering practices on rivers may also move upstream to the river tributaries threatening structures, habitat and structures. Long term adjustments of the river channel may initiate or worsen local scouring problems.

In the 19th century, rivers were mainly considered as vital in the evacuation of wastewater and navigation (Gholami et al., 2019). They were also considered to be risky due to flooding, sediment transport and frequent erosion. For this reason, civil engineers found it necessary to establish bank protection and realignment of the channel and found it socially and economically beneficial. For this reason, the building of strong embankments commenced, and in some place's diversion works were done, leading to siltation in floodplains. Joint efforts by the public and engineers to improve the control of rivers were made to reduce the vulnerability of river margins. During this period, floodplains were densely populated and heavily cultivated by poor communities in rural areas. Engineering was a succession of piecemeal work constructed without any basin-wide perspective whereby locally induced bedload transit led to uncontrolled aggradation and widening of channel downstream (DiazRedondo et al., 2017).

In the past few decades, the policy has been abandoned on large rivers, which are now almost wholly modified. The most common engineering interventions in rivers include the extraction of gravels or bank protection. Gravel extractions are now used as raw materials for the construction industry and also reduces flooding. Most of the rivers have undergone channel metamorphosis with straight patterns and freely meandering flow replacing the previous braiding. High energy rivers respond rapidly to peak flow changes and supply of bed loads, entrenchment, aggradation and channel narrowing or widening, which are characteristics of fluvial systems in recent history. From the 1980s, it was necessary to compensate for river bed disequilibrium and excessive mining through expensive protection of old dams and bridges by controlling grade using bottom weir construction. Reduction in water table levels affects irrigation, and it has to be compensated for by establishing new water resources (Thorne, Hey & Newson, 2005).

Method

Various cases will be used to determine how rivers are affected by a lack of coarse sediments or excessive extraction of gravel due to engineering practices. Most of the investigations focus on how rivers and streams respond to civil engineering activities such as construction and maintenance of reservoirs, and channelization. The research conducted by the US Geological Survey from 1995 documented channel changes, determine geomorphic process rates (Stone, Byrne & Morrison, 2017). This investigation can assist in the prediction of future changes in the river channel. Various methods have been used and include multidate aerial photography, stream gauge data, onsite data collection to establish timing, location, direction, magnitude, rate of channel change and duration. The drome river was identified for this study as it has an unstable flow and has high energy streams. The river is also affected by snowmelts effect and heavy rainfall, which affects the river hydrology. The river has undergone channel metamorphosis leading to prior braiding of the river getting replaced by free meanders and straight patterns (Burk, 2018).

Findings

The drome river, which drains at diois massif, is connected to watershed sediment sources (Charlton, 2007). The realignment of the river channelled to land availability in valley bottoms for farming, to develop roads and railways and to assist in flood control. 37% of drome river banks have been embanked to protect agricultural lands, villages and floodplains (Charlton, 2007). Excessive extraction of gravels which peaked to about 250000 m3 per year (Charlton, 2007). The annual bedload transportation was about 35000 m3 per year (Charlton, 2007). The sediment balance deficit results in bed incision along the river and its tributaries. Analysis done on the river over a period indicate that there have been 75% degradation of the channel (Charlton, 2007). Over time, the resulting loss in sediments was due to extraction and was evaluated to be at 75% (Charlton, 2007). The river is also affected by the shortage of sediment supply from its tributaries. Air photos indicate that the number of branches contributing to the drome river declined from 24 to 11 over 43 years, from the year 1948 to 1991 (Charlton, 2007). It resulted in a reduction in flood peaks, which in turn decreases bedload transport potential. These changes are attributed to the effects of public reforesting and the reclamation of land at the watershed scale.

Discussion

The adverse effects brought about by the reclamation of land are manifold for both human and natural environments. Outcrops of limestone exhumed from 6% of the river led to a decline in the diversity of the habitat and aquatic fauna (Carranza, 2017). The incision has also led to the dropping of water table levels between one and five meters. Engineering activities also resulted in damage of 7.5km of the river embankment, and 17 transverse weirs bridges build to control grade, and flow diversion is threatened (Carranza, 2017). The cost of rehabilitation is estimated to be between $0.8 million for 1km of the dike (Carranza, 2017). To restore the aging Pues weirs will cost around $1.9 million (Carranza, 2017). To evaluate the perspective of engineering geomorphology, a geomorphic survey is necessary. The concept behind this perspective is that it is possible to reverse bed degradation through sediment supply. It can be achieved by establishing alternative methods of managing watersheds. The bed refill potential of the rushing river was measured at the watershed scale. An essential thing to be considered by civil engineers is the continuity and necessity of bedload transport throughout the river.

Maintenance of bedload transportation in the tributary torrents is essential and also where elastic supply is possible, connections between active landslides and eroding slopes and talwegs should be done at the watershed scale (Ali et al., 2017). Corrective works in the areas where the supply of bedload is possible should be stopped in the long term. The objective is to obtain bedload transport on torrents flowing to the most affected or degraded reaches of the river. Slope protection should focus only on the marly slopes that produce suspended sediments that are harmful to aquatic fauna (DiazRedondo et al., 2017). It is also essential to preserve the stable and slightly aggrading reaches to ensure compatibility with flood protection requirements. Refill of the degraded reaches can be achieved through the mobilization of sediments during floods. Eroding banks should be tolerated where possible, and riverine management purchases considered where erodible and eroding banks would benefit fluvial dynamics (Brierley & Fryirs, 2013). Enhancement of bedload transit must be enforced in aggrading reaches. To reduce pressure on the banks, sinuous channels should be constructed through a vegetated gravel bar. Removal of gravel from active channels should be avoided except in areas where the pejoration of flooding risks have emerged (Brierley & Fryirs, 2013).

Conclusion

River geomorphology has facilitated the understanding of the interaction between sediment and water transport process and physical shapes of rivers and the landform they develop. It is essential to understand the flow of rivers and their interactions with the landscape. Streamflow acts as a reflection of the volume and size of bed material and the transport capacity of the river. The alteration of sediment budgets and river conditions affect braided rivers, which become relict landscapes.