Future impacts of climate change on sediment influx rate in hydropower reservoir using SWAT

Climate change causes more frequent and intense rainfall events, leading to severe erosion in the catchment and sediment transferred into rivers and reservoirs. This study focus on long term sediment load in major rivers in Cameron Highlands and prediction of annual sediment inflow into Ringlet Reservoir from 2000 to 2030. Soil Water Assessment Tool (SWAT) is used as the simulation tool, utilising future gridded rainfall 2017 to 2030 under CCSM and future land use 2030. Future annual rainfall is minimum at 1551 mm (in 2030) and maximum at 3150 mm (in 2029). The future projected annual sediment load into Ringlet Reservoir from 2017 to 2030 is averaged at 354,013 m3/year, ranging from 216,981 to 461,886 m3/year. Comparing between the historical period of 2000 to 2016 and future projection (2017–2030), annual sediment load shows an increase of 12 %. To combat the increase sediment yield, catchment management such as erosion control plan, drainage and runoff control must be developed to minimise sediment yield and subsequent effect of high sediment load transport via rivers and drainage network.

Modelling and forecasting reservoir sedimentation of irrigation dams in the Guinea Savannah Ecological Zone of Ghana

Effective management of reservoir sedimentation requires models which can predict sedimentation of the reservoirs. In this study, linear regression, non-linear exponential regression and artificial neural network models have been developed for the forecasting of annual storage capacity loss of reservoirs in the Guinea Savannah Ecological Zone (GSEZ) of Ghana. Annual rainfall, inflows, trap efficiency and reservoir age were input parameters for the models whilst the output parameter was the annual sediment volume in the reservoirs. Twenty (20) years of reservoirs data with 70% data used for model training and 30% used for validation. The ANN model, the feed-forward, back-propagation algorithm Multi-Layer Perceptron model structure which best captured the pattern in the annual sediment volumes retained in the reservoirs ranged from 4-6-1 at Karni to 4-12-1 at Tono. The linear and nonlinear exponential regression models revealed that annual sediment volume retention increased with all four (4) input parameters whilst the rate of sedimentation in the reservoirs is a decreasing function of time. All the three (3) models developed were noted to be efficient and suitable for forecasting annual sedimentation of the studied reservoirs with accuracies above 76%. Forecasted sedimentation up to year 2038 (2019–2038) using the developed models revealed the total storage capacities of the reservoirs to be lost ranged from 13.83 to 50.07%, with 50% of the small and medium reservoirs filled with sediment deposits if no sedimentation control measures are taken to curb the phenomenon.

Assessing hillslope sediment generation potential by tree throw: a preliminary field study along a small river valley in the Jura Mountains, northwest Switzerland

A preliminary field-based investigation was undertaken in a small (< 10 km2) river valley located in the mountainous Jura region of northwest Switzerland. The aims of the work were to assess sediment generation and annual sediment transport rates by tree throw on forested hillslopes and to document surface hydrology characteristics on four fresh tree throw mounds associated with recent tree throws over a 24 d monitoring period. For the tree throw mounds, average sediment recovery ranged from 7.7–28.2 g (dry weight), equivalent to a suspended sediment concentration of 145.2–327.8 g L−1, and runoff coefficients ranged from 1.0 %–4.2 %. Based on a soil bulk density value of 1044 kg m−3, upslope runoff generation areas were denuded by an average of 0.14 mm within the 24 d monitoring period, representing an erosion rate equivalent to 2.1 mm a−1. This means that a ca. 50 cm high tree throw mound could theoretically persist for around 200–250 years. For tree throw work, the dimensions of 215 fallen trees were measured and their locations mapped in 12 separate locations where tree throw was prominent along the river valley, representing a cumulative area equivalent to 5.3 ha (average density equivalent to 43 trees ha−1). The 215 tree throws generated a total of 20.1 m3 of fine sediment (< 2 mm dia.), or the equivalent of 3.8×10-4 m3 m−2. The process of tree throw was originally attributed to two extreme weather events that occurred across west and central Europe in late December 1999. Taking the 18-year period since both storms, this represents an annual sediment transport rate of 2.7×10-5 m3 m−1 a−1. Exploring the relationship with wind on fall direction, however, 65.5 % of mapped tree throws (n= 143) generally fell in a downslope direction irrespective of hillslope aspect on which they were located. Given the similar fall orientation for most trees, this infers that severe storms may not have been responsible for the majority of tree throws, but instead, their upheave might be related to root failure. Given the relative maturity (average age 41 years) of fallen trees in this river valley, our data suggest that once trees attain a certain age, their physiognomy (i.e. height, mass, and centre of gravity) compromises their ability to remain securely anchored. We tentatively attribute this possibility to the presence of bedrock close to the surface, and to the shallow soil profile overlaying the steep rocky slopes. More in-depth studies are required to firstly confirm our findings, and secondly, tree throw studies should be undertaken in other Jura mountain river valleys to assess whether these results are representative.

The Variation on Sediment Reworking with Influencing Factors by a Sand Bubbler Crab, Scopimera globosa, in Intertidal Sediments of the Anmyeon Island, Korea

Though biological and ecological characteristics of Scopimera globosa have been intensively investigated, little has been understood on bioturbation, especially sediment reworking. This study was designed to evaluate variation on sediment reworking of S. globosa based on feeding pellet production (FP) and burrowing pellet production (BP) with influencing factors and estimating the chlorophyll content reduction within the surface sediment by its feeding. The FP and BP largely fluctuated according to chlorophyll a concentration and crab density, but both were not influenced by temperature. The FP was enhanced by chlorophyll a concentration, whereas both FP and BP were restricted by crab density. The daily individual production was highest in spring, followed by fall and summer, with values of 25.61, 20.70 and 3.90 g ind.−1 d−1, respectively, while the total daily production was highest in fall, followed by summer and spring 2150, 1660 and 660 g m−2 d−1, respectively. The daily sediment reworking based on the FP and BP of Scopimera was highest in fall, followed by summer and spring, with values of 1.91, 1.70 and 0.77 mm d-1 and the annual sediment reworking rate of this species was calculated 40 cm year−1 based on its density in this study area. The chlorophyll a reduction ratio was estimated from 11 to 24% in one day by its feeding. These results imply that the sediment reworking of S. globosa is regulated by food abundance and its density, and Scopimera is an important bioturbator, greatly influencing biogeochemical changes in the intertidal sediments.

Temporal variability of annual suspended sediment yield estimates and their uncertainties

&lt;p&gt;Suspended sediment contributes to the vast majority of the annual sediment load transported by rivers to the global oceans. At the same time, this large fraction is transported just in a fraction of time. Towards achieving sustainable sediment management and healthy fluvial systems, identifying the impact of the temporal variability on annual load estimates becomes indispensable in order to reduce uncertainties.&lt;/p&gt;&lt;p&gt;We aim to estimate the temporal variability of suspended sediment transport and the uncertainty of annual suspended sediment loads. Our approach is based on high-resolution time series (15 min sampling interval) of discharge and suspended sediment concentration (SSC) at four monitoring stations with different degrees of discharge variability. The quantification of the variability of discharge and sediment yield is achieved through the exceedance time. The uncertainty of the annual sediment load is estimated using a bootstrap approach. We assess the impact of the sampling interval and link the optimal sampling interval to different SSC-variability. Further, the impact of rating parameters on the uncertainty of annual loads is investigated.&lt;/p&gt;&lt;p&gt;Our results indicate an increase in SSC-variability with decreasing discharge, leading to a negative relationship with the contributing catchment area. The 80 % exceedance times for the annual sediment load range from less than 10 % for the river Ammer (catchment area 608 km&amp;#178;) between 10 &amp;#8211; 20 % for the rivers Ilz (765 km&amp;#178;) and Moselle (27 088 km&amp;#178;) to more than 40 % for the river Rhine (109 806 km&amp;#178;). Simultaneously, the variability increases with a decrease in sampling frequency. Our preliminary results indicate a negative exponential relationship between exceedance time and uncertainties in annual load estimates. This relationship can be used to estimate the uncertainty of annual loads estimated based on low frequency sediment sampling at the continental to global scale.&lt;/p&gt;

Spatio-temporal Analysis of Sediment Yield Estimation Using Integrated RUSLE and GIS Technique: Case of Nashe Dam Reservoir, Abay Basin, Ethiopia

Sediment accumulation in a dam reservoir is a common happening environmental problem throughout the world. Topographic conditions, land use land cover change, the intensity of rainfall, and the soil characteristics are the major driving factors for sedimentation to occur. The effect of sedimentation in a dam reservoir is very visible in the watershed as a result of hilly topographic conditions, high rainfall intensity, thin land cover, and less soil infiltration capacity. In this paper, an integrated RUSLE and GIS technique was implemented to estimate a mean annual sediment yield based on spatial and temporal variations in Nashe dam reservoir situated in Fincha catchment, Abaya River basin, Ethiopia. Spatial and temporal estimation of mean annual sediment yield was estimated using the Revised Universal Soil Loss Equation (RUSLE) model and GIS. Historical 6-year (2014-2019) rainfall for the temporal variations and other physical factors such as soil erodibility, slope and length steepness, management and land used land cover, and support practice for spatial variations were used as sediment driving factors. The mean annual sediment yield ranges from 0 to 2712.65 t ha-1 year-1 was seen. Spatially, Very high, high, moderate, low, and very low sediment yield severity with total area coverage with 25%, 10%, 30%, 15%, and 20% in 2017, 2015, 2019, 2014, and 2018 respectively. The information about the spatial and temporal variations of the severity of sediment yield in RUSLE model has a paramount role to control the entry of sediment into the dam reservoir in this watershed. The results of the RUSLE model can also be further considered along with the watershed for planning strategies for dam reservoirs in the catchment.