The Amazon River is the largest by discharge and basin area globally, annually it delivers large volumes of water to the oceans through a mono modal flood cycle. During this cycle an unknown quantity of water is exchanged with the adjacent floodplain. The volume and duration of this exchange plays an important natural function for sedimentological, biogeochemical and ecological processes. It is known that hydraulic processes within the floodplain, including water depths, flow discharges and inundation timings, influence this exchange of water between the Amazon River and its floodplain. Hence the overall aim of this thesis was to investigate how water was routed through the Amazon floodplain and the hydraulic processes behind this routing. Floodplain water routing and hydraulics are influenced by three key aspects in the Amazon: 1) the volume of water, 2) the pathways along which this volume is routed, and 3) the influence oflocal water sources in the floodplain upon water derived from the Amazon River. Addressing these aspects led to three key scientific questions around which this thesis was structured: 1) What volume of water was exchanged between the Amazon River and its floodplain? 2) Along what flow pathways does this exchange occur? and 3) How does water from local hydrological sources influence the floodplain hydraulic processes? Investigations in this thesis were focussed upon a 30,000 km2 study site situated in the middle reach of the floodplain, ~250 km upstream of Manaus, Brazil. Within this area a two-dimensional hydraulic model was constructed from and tested against data from insitu gauges, satellite data and field work observations. The model was then used to address each of the above scientific questions. Before investigating the first question, it was necessary to develop a method to minimise the vegetation canopy distortion errors which had degraded the accuracy of the Digital Elevation Model (OEM), used by the model to represent floodplain topography. After this method, volumetric exchanges between the river and the floodplain were quantified by assessing the volume of water which filled and drained annually from the floodplain. Then the volume of flow routed through the floodplain as a proportion of the total flow was assessed by identifying cross sectional flow differences from simulations with and without floodplain storage components. Between 1995 - 1997 the floodplain filled during rising water by 93 - 113 km3 , whilst during falling water the floodplain drained 75 - 113 km3. Annual flows through the floodplain represented 6.4% - 10.7% of the total flow via the Amazon River, this increased to 9.0% - 18.8% at high water and decreased to 1.3%- 2.3% at low water. The role of floodplain channels as preferential flow pathways for water entering and exiting the floodplain was investigated by incorporating their geometry into the hydraulic model using a subgrid methodology. The overall effect of all these channels was to increase the duration of connection between the Amazon River and the floodplain. Consequently floodplain inundation volume increased by 7.3% - 11.3% at high water, and decreased at low water by 4.0% - 16.6%. These floodplain channels tended to function in networks, whereby smaller channels delivered flow into the floodplain, these collected into larger channels which then drained back into the main river. In total, these networks served as a net drainage mechanism taking flow from the floodplain back to the main river. The influence of local hydrological water inputs from terrafirme catchments which bounded the floodplain, were represented from simulated outflows from a hydrological rainfall-runoff model. Over the entire domain floodplain hydraulics were insensitive to these additional inputs, however a more significant influence could be observed int the areas surrounding the input locations. Temporally, the most significant influence was during falling and low water, when some areas witnessed flow increases of 15% - 400%. During this time, water in the floodplain could be composed of up to 10% - 15% from these local water sources. In answering the thesis aim, the work that will be presented shows that the Amazon floodplain routes a large proportion of the total flow within the study domain. This large exchange of water is aided by the existence of flow pathways, afforded from floodplain channel features, which play an important role in connecting the river and floodplain during early rising water and late falling water. Floodplain hydraulic processes are subtly influenced by local water inputs, which can be important in sustaining flows during falling and low water.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:683694 |
| Date | January 2014 |
| Creators | Baugh, Calum Alexander |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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