Impacts of river engineering on river channel behaviour : implications for managing downstream flood risk

Although knowledge of sediment transport has improved over the last 25 years, our understanding of bedload transfer and sediment delivery is still based on a limited set of observations or on models that make assumptions on hydraulic and sediment transport processes. This study utilises repeat lidar survey data of the River Caldew above the City of Carlisle in the UK to investigate the balance of erosion and deposition associated with channel switching from an engineered and managed single thread channel to a naturalising incipient wandering system. Over the 11-year survey period (four bankfull flood events) around 271,000 m3 of sediment were delivered to the river and floodplain and 197,000 m3 eroded suggesting that storage rates of around 7000 m3/annum occurred. The balance of erosion and deposition is influenced by channelisation with very restricted overbank sedimentation and only limited local and transient in-channel bar deposition along the engineered reach (8000 m3 eroded). This contrasts with the activity of the naturalising reach downstream where a developing wandering channel system is acting to store coarse sediment in-stream as large bar complexes and the associated upstream aggrading plane bed reaches and overbank as splay deposits (87,000 m3 stored). Such behavior suggests that naturalisation of channelised systems upstream of flood vulnerable urban areas can have a significant impact on sediment induced flooding downstream. This conclusion must, however, be moderated in the light of the relatively small volumes of material needed to instigate local aggradation in over-capacity urban channels

Recent changes to floodplain character and functionality in England

Regime analysis suggests that temperate alluvial watercourses overtop their banks on average once every 1.5 years transferring water and sediment across the valley floor to form floodplains helping maintain a strong hydrological connection between in-channel and overbank form and process. Flooding also causes erosion, sediment transfer and deposition creating a variety of floodplain morphologic units and functional connectivity with the main river. The result is a morphologically and ecologically varied wetland dominated ecotone where diversity is sustained by the action and flooding and shallow groundwater processes. Floodplains are, however, sensitive to disruption and many have been significantly degraded since the Bronze Age as a result of activities that alter flooding and groundwater processes and manage vegetation communities. The current (2015) floodplain condition and trends of change since 1990, for England are presented here using land use data for 1990, 2000, 2007 and 2015. Floodplain system degradation has been found to be both widespread and severe across the whole of the country. The 1990 data set showed that intensive agriculture occupied around 38% of floodplain zones expanding to 53% by 2000 before slowing slightly to covering 62% in 2007. Between 2007 and 2015 the coverage remained relatively static (64%) with some suggestion that arable areas were being transformed to pasture. Wetland areas in the form of fen, marsh, swamp and bog have been devastated with the data sets indicating that these fundamental floodplain units have been all but lost. Upland and lowland areas are both severely impacted with a near ubiquitous loss of natural floodplain functioning. Despite this some 31% of rivers in England are classified as good or better under the European Water Framework Directive classification system calling into question the UK WFD status classification process

A new methodological contribution for the geodiversity assessment: applicability to Ceará State (Brazil)

The concept of geodiversity aggregates the abiotic elements of nature and promotes the geoconservation. The main objective of this work is to contribute to the upgrade of the method for the assessment and quantification of geodiversity proposed by Pereira et al. (2013). The method is based on the superposition of a regular grid of 12 × 12 km on different maps (lithology, geomorphology, soil, paleonthology, mineral and geological energy resources) at scales of 1:250,000 to 1:600,000. In addition to other up- grades, the water resources are regarded here as a new com- ponent to consider when quantifying geodiversity. The sum of these maps generated the quantitative Map of Geodiversity Indices and the Map of Geodiversity Assessment, ranging from very low to very high geodiversity. The analysis of the geodiversity map of the State of Ceará (Brazil) shows the applicability and advantage of this method, highlighting two regions with higher levels of geodiversity (Northwest and South) and another region with the lowest levels (Sertões Cearenses). The results also allowed the characterization of the State of Ceará concerning the individual components of the geodiversity, especially the water resources. Geodiversity indices and maps are comprehensive and user-friendly data in the territorial planning, considering the geodiversity either as a whole, or each of its components, especially the more sensi- tive such as fossil conservation, and water, mineral, and non- renewable energy resources management.The authors express their gratitude to the Brazilian research fostering institution "Coordenação de Aperfeiçoamento de Pessoal de Nível Superior" (CAPES) for awarding the Ciência Sem Fronteiras (CsF) PhD scholarship that enabled this work. This work was partially co-funded by the European Union through the European Regional Development Fund, based on COMPETE 2020 (Programa Operacional da Competitividade e Internacionalização), project ICT (UID/GEO/04683/ 2013) with reference POCI-01-0145-FEDER-007690 and national funds provided by Fundação para a Ciência e Tecnologia

The influence of contemporary flow regime on the geomorphology of the Sabie River, South Africa.

The existence of a channel-forming, 'dominant' or 'bankfull' discharge has been applied, with some success, to a variety of alluvial river systems in temperate areas. This paper presents the results of an investigation into the relationship between flow magnitude and frequency, and the geomorphological units of the Sabie River in the Mpumalanga Province, South Africa. The river is perennial, however, it exhibits an extreme seasonal flow regime. The river has been subject to incision (100,000-10,000 years ago), which exposed bedrock areas as topographic highs within a wide macro-channel. Zones of deposition have created alluvial sections resulting in a morphologically diverse river system. Stage-discharge relationships have been observed and constructed for 23 monitoring sites on the Sabie River. These have been used in conjunction with a 62-year calibrated simulated daily discharge record, to generate inundation frequencies for the morphological units present at each site. This includes results using annual maximum series data and time spans for activation, using the continuous daily flow data. The results reveal that the overall form of the cross-section is not related to a single channel-forming discharge, instead, a complex relationship exists whereby the section is influenced by the entire flow regime. The results do however suggest a poorly defined division of morphological units within the incised macro-channel, namely, those influenced by perennial, seasonal and infrequent flows. This relationship is best demonstrated by the morphological units associated with the perennial, active channels in alluvial sections, which correlate with the low flow regime (1-2-year return period on the annual maximum series), and larger macro-channel deposits which are related to rarer higher flows. Climatic wet and dry cycles, human influences and the physical effect of bedrock in the macro-channel further complicate the inundation pattern, resulting in disequilibrium conditions between channel form and contemporary flow regime on the Sabie River. Macro-channel sedimentary deposits, in particular, show no consistent relationship with a particular segment of the flow regime, being inundated by flows of between a 10- and 50-year return period

Morphological classification of fluvial environments: an investigation of the continuum of channel types

Bedrock-controlled channel systems exhibit considerable morphological variation. Both bedrock-influenced and alluvial morphological units coexist to form a system of changing channel patterns in response to changes in the relative influence of the controlling process variables. This article investigates the morphological composition of the bedrock-influenced Sabie River, Mpumalanga Province, South Africa, mapping 25 km of river channel at the scale of individual morphological units. Cluster and discriminant analyses define objective reach–scale "channel type" assemblages based on morphological unit composition. A number of robust clusters emerged that could be broadly classified into five channel types, namely, bedrock anastomosed, mixed anastomosed, pool-rapid, braided, and alluvial single-thread. The cluster analysis revealed that these channel types fit on a continuum from bedrock-dominated channels to fully alluvial systems. Each channel type could also be characterized by a certain set of "dominant" morphological units, which changed from bedrock-influenced at one end of the continuum (bedrock anastomosed, pool-rapid) to alluvial deposits at the other (braided, alluvial single-thread). An investigation of the role of controlling process variables in defining these channel types revealed a broad link between the degree of bedrock influence and the amount of available energy within the system as defined by indices such as the flow regime and water-surface slope variation. It is clear that the bedrock-dominated channel types are characterized by energy levels in excess of those accepted for alluvial systems, and an extended river classification is presented for the Sabie River that includes these bedrock channels. The mixed anastomosing channel type on the Sabie River is characterized by higher available energy levels than braided or alluvial single-thread reaches. As such, it appears to be a higher-energy example of an anabranched system, probably formed as a result of sediment accumulation on top of a high-energy bedrock anastomosed channel template