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Downstream trends of alluvial sediment composition and channel adjustment in the Llano River watershed, Central Texas, USA : the roles of a highly variable flow regime and a complex lithologyHeitmuller, Franklin Thomas 05 February 2010 (has links)
This study investigates the downstream controls of alluvial sediment
composition and river channel adjustment in the Llano River watershed, Central Texas,
USA. The Llano River watershed is characterized by a highly variable, flood-prone
flow regime and a complex lithology of Cretaceous carbonate rock, Paleozoic
sedimentary rock, and Precambrian igneous and metamorphic rock. Sedimentary
variables for this study include particle size, sorting, carbonate content, and magnetic
susceptibility. Channel adjustment includes the planform dimension and cross-sectional
dimensions of bankfull- and macro-channels. Nineteen sites along the Llano River and
selected tributaries were visited to measure cross-sectional channel geometry and
sample bed, bank, and overbank sediment. Laboratory analyses of sediment and
hydraulic analyses of cross sections were accompanied by analyses of partial-duration
flood frequency, flow resistance, hydrography, digital elevation models, and statistical correlation. Findings include: (1) channel-bed material reduces in size with downstream
distance, despite increasing valley confinement and bedrock exposure; (2) the
downstream decrease in particle size is more evident for channel-bar deposits than for
low-flow-channel (thalweg) deposits; (3) an abrupt gravel-to-sand transition occurs
about 20 kilometers downstream of the Paleozoic-Precambrian contact; (4) an abrupt
coarse- to fine-gravel transition occurs between 75 and 90 kilometers downstream the
North Llano and South Llano Rivers; (5) channel-bank material increases downstream,
contrasting with decreases in bed material; (6) carbonate content and magnetic
susceptibility of alluvial sediment are inversely related, with carbonate content peaking
near Junction; (7) four general categories to classify reaches of the North Llano, South
Llano, and Llano Rivers are based on hydrology, planform morphology, lithology, and
valley confinement; (8) mean depth increasingly compensates for bankfull discharge in
a downstream direction; (9) mean depth compensates more than width for macrochannels;
and (10) the return periods for bankfull and macro-channels are about 1 to 2
years and greater than 10 years, respectively. The results of this study will contribute to
fluvial geomorphic theory of downstream trends in sediment composition and channel
adjustment; as well as inform applied efforts related to aquatic biology, flood hazards,
infrastructure design, and riparian and water-resource management in the region. / text
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Sustainability of water abstraction by hand drilling in the floodplain of River Benue of Yola, NE NigeriaApagu, Buba Ankidawa January 2014 (has links)
The aim of the research is to assess the sustainability of groundwater supply and the suitability of hand-drilling techniques for accessing groundwater for irrigation practices along the shallow alluvial floodplains of River Benue, NE Nigeria. Hand-drilling techniques are affordable means for the farmers to abstract water from these shallow aquifers. Determining the most sustainable hand-drilling techniques (taking into account the hydrology and sedimentology of the floodplain) will improve farming activities and food security in this region and the country at large. Hydrological data (obtained from fieldwork and modelling) demonstrate that the River Benue is the main source for recharge of the shallow alluvial aquifers of the floodplain during the dry season period. Water table heights were estimated by resistivity survey using ABEM Terrameter equipment and measured by automatic piezometer instruments. Floodplain sedimentology and hydrogeology were assessed at seventeen natural riverbank outcrops and twelve hand-drilled boreholes. At each location, sediment samples were collected from every exposed sedimentological unit. Locations and elevations were measured using a ProMark3 dual frequency GPS instrument, to create a detailed topographic map with updated contours. Twenty-four electrical resistivity sounding profiles and twelve-groundwater measurement were also obtained to explore the groundwater level of the floodplain. The resistivity results confirm the availability of water in the alluvial aquifers of the floodplain. In order to determine the most appropriate hand drilling techniques, a Field Shear Vane Tester was used to measure sediment shear strength at twelve different borehole locations. Shear strength forces were higher on clayey silt and sandy silt, and lower on sand formations. It appeared that in some areas of the floodplain, the farmers are already above the shear strengths that can be provided by human power. Hence, any increase of the hardness of the surface of the sediment would make low-cost hand drilling impractical. Particle size analysis for the sediment samples showed that the samples were largely sandy in nature, which enables easy movement of water through the layers for aquifer recharge. Magnetic susceptibility (used to classify the source of sediment and the process of their formation) revealed that the main source of the sedimentary materials was upstream of the study site and varies little over time. The groundwater level of the study area decreased away from River Benue valley during the dry season period. One perched aquifer formations and possibly two others were observed in three different locations, which reflects a low-permeability stratigraphic unit (such as lens of clayey silt) within alluvial sands. These should be avoided by farmers, as they are likely to provide water only in the short-term. Finally, groundwater modelling was undertaken (with various scenarios) for the River Benue floodplain using acquired sedimentology and hydrology data integrated into MODFLOW software. The results revealed that low-cost hand-drilling techniques such as augering and jetting remain possible for abstracting the shallow alluvial aquifers on the floodplain for irrigation farming in the study area, unless the most likely low river water stages in River Benue, over-exploitation of the shallow alluvial floodplain groundwater and drought scenarios occur.
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