الفهرس 
THEORETICAL APPROACHOnly 14 pages are availabe for public view
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Abstract
The formation of local scour holes around bridge piers is almost an unavoidable problem in alluvial channel beds subjected to the erosive action of oncoming river flow. The design and construction of bridges spanning across alluvial channels requires the knowledge, or at least an accurate estimate of maximum scour depth which might occur during the anticipated life of the bridge near the piers. A unifying theory for estimating scour depth at piers is still in an embryonic stage, mainly due to the complex nature of the scour problems. The local scour at bridge pier continues for a sufficiently long time until the hydrodynamic forces in the scour hole are no longer able to remove particles from hole. At this condition, the scour hole reaches an equilibrium condition, and the scour depth does not change appreciably unless the flow conditions change.
Debris, consisting of floating aquatic weeds and other resources, can have a substantial impact on bridge scour in several ways. A build-up of materials can reduce the size of the waterway under a bridge causing contractions scour in the channel. A build-up of debris on the abutment can increase the obstruction area and increase local scour. Debris can deflect the water flow, changing the angle of attack, increasing local scour. Debris might also shift the entire channel around the bridge causing increased water flow and scour in another location. Information regarding methods for computing forces imposed on bridge superstructures by floating debris is lacking despite the fact that debris causes or contributes to many failures.
Numerous experimental and analytical investigation of local pier scour were conducted in alluvial channels and series of prediction equations were developed by researchers to estimate the maximum scour depth at bridge piers under different approach flow, sediment size and gradation, pier type and size conditions.
The main interest of this research is to experimentally study the magnitude of the equilibrium local scour at bridge pier placed in sandy soil due to existence of weed racks and weeds accumulation. The research presented in this dissertation investigates the scour depth using literature review, theory, and through laboratory experiments.
Also in this research, Artificial Neural Network (ANN.) modeling using back-propagation learning technique was formulated to predict the maximum scour depth around bridge piers due to installation of aquatic weeds racks. The data used to train the ANN was obtained from a test series of physical model. The discharge, velocity, Froude number and opening area percentages; unoccupied area of the rack, were used as input parameters to ANN while scour hole depth as the output parameter. Results of ANN show good estimation of maximum scour depth compared to the measured data from physical model. An advantage of the use of ANN in the prediction of maximum scour hole depth and length that it will certainly decrease the cost and time for physical modeling and help in simulating different hydraulic conditions.
To accomplish the goals of this investigation, three physical models were conducted and 145 scour test runs were carried out in the Hydraulics Laboratory of Channel Maintenance Research Institute, National Water Research Center. The scope of work entailed three different sets of experiments for three barriers models with three variable depths; for each barrier depth, five different running discharges were used.