1. UKM Learning and Research Repository
2. PHD Thesis / Tesis PHD
3. Faculty of Engineering and Built Environment / Fakulti Kejuruteraan dan Alam Bina

Title:	Temporal and spatial local scour profile of cylindrical pier in cohesive sediment mixture
Authors:	Mojtaba Porhemmat (P69798)
Supervisor:	Wan Hanna Melini Wan Mohtar, Dr.
Keywords:	Sediment transport
Issue Date:	28-May-2018
Description:	In the past decades, several researches have investigated the local scour phenomenon around hydraulic structures such as bridge piers. To date, most studies have focused on the effects of non-cohesive sediment on the local scour, while very limited number of research have tried to investigate the effect of cohesion. Furthermore, most of the natural sediments in the rivers contain some percentage of cohesive sediments such as silt and clay, which change the characteristics of the incipient motion of sediments. Most studies on a laboratory scale faced difficulties in obtaining a holistic local spatial variability around bridge pier, especially for continuous small-time interval, and only few studies have measured the spatial and temporal local scour development. Therefore, the main objective of this research is to further investigate the effects of cohesion on the spatial and temporal local scour characteristics around the cylindrical piers. In addition, a low cost automated data recording system (ADRS) controlled by Arduino microcontroller was designed and developed to obtain spatial and temporal data in different time intervals. The ADRS was programmed to monitor the development of the local scour by capturing photos in adjustable distances and angles from the original positions in different time intervals. The comparison between manual measurements and the ADRS confirmed that it is capable of recording data with high accuracy and the ADRS is able to provide a holistic view of both spatial and temporal variability in the development of local scour on a laboratory scale. Two sizes of sands (with median grain size 𝐷50 = 0.59 mm and 0.99 mm) with different percentages of cohesive materials (0-30%) were used. The cohesive material contains 90% kaolinite and 10% bentonite. The median grain size of the cohesive sediments is 0.0038mm and 𝐷90 = 0.012mm, which indicate that the particles are within the cohesive range. Varying flow velocity to critical velocity ratio (𝑉/𝑉𝑐) was applied to evaluate the spatial and temporal local scour development in the cohesive-sand mixture. The results show that the location of maximum scour depth varied for different time intervals. Although a very high rate of scour was observed at initial stage of experiments, the rate decreased with the development of local scour hole reaching to nearly 0.0 mm/hour at the end of each experiment. The percentage of clay in the sediment mixture does not necessarily influence the maximum ultimate value of scour depth. The maximum ultimate scour depth was occurred at the upstream, usually at the pier nose, while the minimum ultimate scour depth was observed to occur at the downstream for the sand-clay sediment mixtures with up to 20% of cohesive materials. However, an adverse profile was observed for the sediment mixture with 30% of cohesive material where the maximum scour depth occurred at the downstream. Increasing 𝑉/𝑉𝑐 caused an increment in the ratio of minimum to maximum scour depth (𝑦𝑠𝑢𝑚𝑖𝑛/𝑦𝑠𝑢𝑚𝑎𝑥) for cohesionless sediment mixtures, which resulted in a more uniform peripheral scour. For sand-clay sediment mixtures with up to 20% cohesive materials, 𝑦𝑠𝑢𝑚𝑖𝑛/𝑦𝑠𝑢𝑚𝑎𝑥 ≈ 0.7. However, by adding 30% of cohesive materials, the ratio of 𝑦𝑠𝑢𝑚𝑖𝑛/𝑦𝑠𝑢𝑚𝑎𝑥 became significantly lower to 0.2. The side length of scour hole was observed to have higher value than the upstream length for all experiments. This study showed that the influence of cohesive material becomes significant for 30% clay fractions, where less cohesive fractions exhibit similar scour profile to the non-cohesive sediment. Even so, the effect of cohesion becomes critical for temporal scour characterization and the determination of critical velocity.,Ph.D.
Pages:	160
Call Number:	TC175.2P647 2018 3 tesis
Publisher:	UKM, Bangi
Appears in Collections:	Faculty of Engineering and Built Environment / Fakulti Kejuruteraan dan Alam Bina

Items in DSpace are protected by copyright, with all rights reserved, unless otherwise indicated.