Experimental study on incipient condition of toe Protection elements of river bank protection works

Abstract

The present study has been undertaken to investigate experimentally the incipient condition of to e protection elements. Relationships for governing parameters of the incipient velocity of toe protection elements have been theoretically analyzed. These relationships are then verified using laboratory data. The experiments are conducted in the large tilting flume of the Hydraulics and River Engineering Laboratory of Water Resources Engineering Department, BUET.A total of eighteen experimental run consisting of five different sizes (ranging from 20mm-24mm) of geobags and three different sizes (ranging from 16mm-23 mm) of CC blocks have been conducted in laboratory flume for discharges varying from 0.066 m3/s to 0.187 m3/s.During experimentation various observations are made and measured data are used to obtain relationships for the incipient condition of the toe protection elements.

An empirical relationship (equation 4.1) to determine the size of CC block as toe protection element based on incipient condition is developed with a coefficient of correlation (R2) is 0.951. Also, an empirical relationship (equation 4.3) to determine the size of geobag as toe protection element based on incipient condition is developed with a coefficient of correlation (R2) is 0.53.

Improved empirical relationship is found when both current and previous data are taken into account. For CC block, equation 4.5 (R2 = 0.854), and for geobag, equation 4.6 (R2 = 0.321), are proposed. For incipient condition, CC blocks can withstand at higher velocity at higher depths (Figure 4.8). Proposed relationships show indication to selection of larger protection unit than the conventional formula (Figure 4.9).

It is found from the study that for a given velocity the required thickness of protection decreases with the increase of depth of flow for CC block. However, reverse condition is observed for geobag (Figure 4.10). This discrepancy may be due to the fact that the geobags are relatively flat and less dense, and thus their underwater functional behavior becomes more composite as a group. Also, the specific gravity of geobag is almost half of the CC block.

Experimental results are analyzed to develop relationships between the relative size and flow parameters. Improved empirical relationship is found when both current and previous data are taken into account. Developed empirical relationships can be used to predict incipient condition for selected type and size of toe protection elements. The proposed relationships are also compared with the equations available in previous studies. Proposed relationships show indication to selection of larger protection unit than the conventional formula. It is hoped that the outcome of the present study can be used as a tentative guideline for design of toe protection elements in river bank protection works.