Measuring sediment exchange rates on an intertidal bank at Blacktoft, Humber Estuary, UK

Fine-scale quantification of stream bank geomorphic volume loss caused by cattle access

Unrestricted cattle access to streams and rivers can be a significant source of pollution in fluvial systems, contributing to bank erosion and fine sediment inputs. Despite this pressure, observational data are scarce. This study quantified stream bank geomorphic modifications caused by cattle access at fine scale using motion-capture cameras and Terrestrial Laser Scanning (TLS) campaigns. Continuous monitoring of rainfall, discharge, conductivity and turbidity further augmented this dataset. The application of these techniques extended over a five-month grazing period in agricultural sub-catchments with intensive cattle production. At low flow, high-resolution water quality data showed that the frequency of cattle activity in and around stream margins was associated with elevated turbidity signals downstream. However, when elevated turbidity coincided with high flow events, it was not possible to distinguish between local erosion and upstream sediment transfers. TLS results indicated a loss of 0.141 m³ to 1.035 m³ stream bank material, which equates to 0.067 m³ m^-2 - 0.092 m³ m^-2 of stream bank area (between 27% and 41% in the <2 mm fraction) over the study period from sites with 130 to 1154 discrete cattle access hits. Multiple linear regression showed that the observed geomorphic volume loss could not be explained by natural processes alone (hydrometeorology), but was more significantly related to cattle-access frequency as the principal driver. The geomorphic volume loss had the potential to impact 29 m² to 197 m² of stream bed with fine sediment (<2 mm) from the three study sites. Grazing parcels adjacent to streams in the study sub-catchments were enumerated at 18.4 parcels km^-2 and so the results of this investigation potentially scale to a considerable fine sediment risk. Regulations and time-limited incentives to exclude cattle access to stream channels should therefore expect to reduce sediment pressures where these measures are targeted at access points.

Erosion and sedimentation pattern of fine sediments and its physical characteristics in a macrotidal estuary

The seasonal and fortnightly erosion and sedimentation pattern as well as the bonding characteristics of the mud deposit at the estuarine turbidity maximum (ETM) zone of the macrotidal Chikugo River estuarine channel were studied during 2005-2008 using the periodical topographic surveys and mud sampling along with continuous monitoring of water level, turbidity and flow velocity. The results revealed that the estuary was influenced by the tidal forcing during the dry season. This tidal forcing accelerated the net landward sediment transport and sedimentation takes place in the estuarine channel. On the contrary, the river discharge dominated the estuary during the rainy season which eroded those mud deposit in the estuarine channel and exported to the downstream and the sandy base layer appeared at the bottom. During the dry season, a mud deposit of 1.5-1.8 m thick was formed which was characterized by 90% of mud (fine silt and clay) and 10% of fine sand. The high viscosity ratio and loss on ignition of this mud deposit revealed that its consolidation begins at the early stages of deposition due to thixotropy and the presence of organic matter. This consolidation process in the natural environment will be several times higher than that of the disturbed mud, which will help the sediment surface to maintain stability against erosion even at high velocities. Moreover, the consolidation of the mud deposit has a strong impact on the seasonal changes in estuarine morphology other than external forcing such as river discharge, tides, wind and availability of sediments.

Seasonal morphodynamic evolution in a meandering channel of a macrotidal estuary

The spatio-temporal morphodynamics in relation to the fortnightly tidal variations and seasonal river discharge variations in a meandering estuarine channel were investigated. Intensive topographic surveys (longitudinal and transverse) every 3-4 months using a digital sonar system equipped with a differential global positioning system and continuous monitoring of salinity and turbidity with moored instruments were carried out during 2009-2012 in the upstream section of the highly turbid macrotidal Chikugo River estuary, Japan. The results revealed that the tidal flow dominated in the estuary for most of the year except for the rainy season in which river flow dominates. During the low flow season, tidally induced net upward sediment transport dominates and deposition takes place in the upstream section especially in the inner part of the meander. It contributes towards the strengthening of the existing estuarine turbidity maximum (ETM) in the upstream section of the estuary, and the channel capacity reduced gradually during this period. However, large flood events led to the breakdown of this ETM zone in the upstream during the rainy season and exported the sediments to the downstream tidal flat which resulted in the rapid increase of channel capacity. This further exhibited that the ETM zone was migrated to the downstream part of the estuary during the rainy season. There were significant differences in the seasonal trends of morphological evolution in the estuary which further greatly influenced the channel capacity. The drastic changes in channel morphology due to the sediment export by the high river flow during the flood season was approximately recovered through the sediment import by the fortnightly tidal cycle during the dry season over a seasonal basis. The study concludes that the morphological changes in the tide dominated estuaries are strongly influenced by the fortnightly tidal variations as well as the seasonal river discharge variations.