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3D Numerical Modelling of Sediment Fluxes at Paso Ancho Hydropower Project

Bijukchhe, Pratik
Master thesis
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no.ntnu:inspera:43469946:25261035.pdf (11.24Mb)
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http://hdl.handle.net/11250/2624547
Utgivelsesdato
2019
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  • Institutt for bygg- og miljøteknikk [2850]
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The purpose of this thesis was to setup a three-dimensional numerical model on SSIIM 2

for the peaking pond of the Paso Ancho Hydropower Project. Not limited to numerical

model, failure reasons for the desander is also studied. The main aim of the numerical

model is to obtain the distribution of bed sediment in the pond and compare with the

data from the field.

The peaking pond has severe sediment problem due to the poor efficiency of the

desander located at the headworks. All the sediments entering through the intake passes

through the desander and enter in to the pond. This has led to loss of capacity of the

pond and extra financial cost for implementation of different sediment removal methods.

At present, 200mm dredge installed by SediCon is removing the sediments from the

pond. It was installed in August 2018 and by January 2018 had removed around

14458m3 volume of sediments. The total capacity of pond is 70000m3.

A survey was done by Acoustic Doppler Current Profiler (ADCP) to obtain the bathymetry

and velocity profile of the peaking pond. Sediment samples from the bed of the pond was

analyzed in the lab to obtain the grain size distribution of the samples. The discharge in

river was measured to obtain the inflow in the pond. These field data are compared with

the results from the SSIIM model.

A three-dimensional numerical model has been setup to perform hydraulic and sediment

transport simulation. A first model is setup with the measured discharge and surveyed

geodata to perform a water flow simulation. The obtained velocity magnitude is

compared with the surveyed velocity profile data. The hindrance most encountered was

in shaping the inflow structure for the peaking pond to imitate the same flow pattern as

actual in the site. The model showing similar pattern of flow is chosen and two additional

cases of water flow simulation are performed which later is used for sediment transport

simulation. Second model is run with the higher discharge corresponding to the discharge

of wet season at which the pond receives high sediment inflow. And third model is run

again on the higher discharge but the geodata of original geometry of the pond is used,

which has no sediment deposition. The second and third cases of water flow simulation

are used to the run the sediment transport simulation. From the sediment simulation, the

bed grain size distribution is obtained and compared with the sediment samples taken

from the pond.

Number of changes to the geometry and location of the inflow were done and different

algorithms and parameters were used simultaneously to produce a likely result. The

result showed similar flow patterns but different bed sediment distribution when

comparing with the field data. There are still uncertainties concerning the input data,

geometry of inflow and outflow. The sediment deposition and distribution pattern in

pond is affected by the daily fluctuation of water level in the pond. There is a daily

lowering down of water level to the lowest possible operation level since this is a daily

peaking type project and has different inflow and outflow discharge. The numerical model

is run in the scenario assumed that in the wet season, when discharge is high in the

river, the level of water at the pond is maintained at constant normal water level

throughout the day and has an equal inflow and outflow discharge.

To conclude, the model requires more calibration and validation from the field data and

further work on the geometry of model is also recommended to use the model for the

prediction of sediment distribution.
 
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