Experimental Study on Settling Behavior of Toe Protection Elements of River Bank Protection Works

Abstract

River bank erosion has always been a challenging problem in Bangladesh. Conventional method of designing erosion protection structures are governed by the hydraulic loads resulting from currents and waves. The effectiveness of design of protection works mainly depends on its constructional aspects. The appropriate method of construction depends on mechanism of settling behavior of protection elements. In practice, toe protection elements are dumped into flowing water and settle somewhere on the river bed to form an apron. But the placement of elements at designated positions is difficult to ensure. The present study has been undertaken to investigate experimentally two important aspect of underwater construction such as the settling behavior and threshold condition of toe protection elements. The experiments are conducted to determine the fall velocity in a square shaped settling column of 30 cm X 30 cm X 130 cm and in the large tilting flume of the Hydraulics and River Engineering Laboratory of Water Resources Engineering Department, BUET. Sixteen different sizes of elements ranging from 1.6 cm X 1.6 cm X 1.3 cm to 4.1 cm X 4.1 cm X 2.7 cm have been used to conduct 64 experimental runs with the discharges range from 0.033 m3/s to 0.203 m3/s. During experimentation various observations are made and the measured data are used to obtain various relationships for the settling behavior of the toe protection elements. Experimental results are analyzed to develop relationships between the relative size and flow parameters. Developed empirical relationships can be used to predict the settling velocity, horizontal settling distance and incipient condition for selected types and sizes of toe protection elements. The proposed relationships are also compared with the equations available in previous studies. Comparisons show that the predictive capacity of the proposed relationships is found to be satisfactory i.e. for fall velocity prediction equation, the error is 3.91% for CC block and 2.38% for geobags. To estimate settling distance results show that the developed equation also performed better compared to Zhu et al. (2004). Also verification of the proposed equation has been done using the independent set of laboratory data and result shows satisfactory agreement with an error of 3.80%. It is hoped that the outcome of the present study can be used as a tentative guideline for under water construction of toe protection elements in river bank protection works.