Hydraulic Resistance Induced by Deposition of Sediment Inporous Medium

The deposition of the instantaneously released sediment into a gravel matrix and the hydraulic resistance induced by the rapid siltation are investigated herein. The experimental results reveal that the depositional patterns of the sediment are governed by the gravel-sediment size ratio, the amount of sediment released, and the seepage flowrate. The observations also indicate that the stable stage of sediment deposition is reached shortly after the occurrence of sudden slump. A regression relation is developed to quantify the stable-stage hydraulic resistance with the major governing factors. Given the hydraulic resistance, one can use the siltation equation to evaluate the hydraulic conductivity of the silted porous medium. With such information, planning of the engineering alternatives to meet the seepage flow requirements is made possible. FIG. 1. Sketch of Experimental Setup Showing Deposition of Instantaneously Released Sediment into Gravel Filter INTRODUCTION The deposition and accumulation of fine sediments in gravel-bed streams may affect the ground-water recharge and the benthic ecosystem (ASCE 1992; Schälchli 1995). This can occur as a slow, insidious process with the continuing deposition of small quantities of sediments or be triggered as a rapid, almost catastrophic, event, exemplified by a sudden slump or landslide (Reiser et al. 1989). The gradual siltation process of riverbeds has been intensively investigated by Schälchli (1995), and a quantitative relationship describing the reduction of hydraulic conductivity with time has been proposed as follows: gL K(t) = (1) 2g h 2 rCt where K(t) = hydraulic conductivity of bed material after time t; L = seepage length; = hydraulic resistance of unsilted riverbed = gL/K0 ; K0 = hydraulic conductivity of unsilted bed material; g = gravitational acceleration; = kinematic viscosity of water; h = pressure head; r = specific hydraulic resistance; and C = concentration of suspended load (by weight). The specific hydraulic resistance r varies as a function of the uniformity of bed material, Reynolds number, hydraulic gradient of infiltrating water, and bed shear stress (Schälchli 1995). It is pointed out that the r values for the siltation process in rivers lie in the range between 2 10 and 2 10 m/kg. Eq. (1) has been developed to evaluate the variation of the hydraulic conductivity of a riverbed for the slow siltation process; however, the deposition of a sudden slump of sediments and the consequential hydraulic resistance induced by the rapid siltation has rarely been addressed previously (Schälchli 1995). Since the mechanisms of these two siltation processes are fairly different and the physical parameters used to define the systems are not identical, there is a need for a quantitative relationship that models the rapid siltation process. The objective of this experimental study is to investigate the deposition of instantaneously released sediments in a porous medium and the hydraulic resistance induced by such a rapid siltation process. Asst. Prof., Dept. of Agric. Engrg. and Hydrotech Res. Inst., National Taiwan Univ., Taipei, Taiwan, R.O.C. Grad. Res. Asst., Inst. of Envir. Engrg., Nat. Taiwan Univ., Taipei, Taiwan, R.O.C. Note. Discussion open until December 1, 2000. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this technical note was submitted for review and possible publication on November 8, 1994. This technical note is part of the Journal of Hydraulic Engineering, Vol. 126, No. 7, July, 2000. ASCE, ISSN 0733-9429/00/0007-0547–0551/$8.00 $.50 per page. Technical Note No. 9564. EXPERIMENTS The laboratory experiments were conducted with a 25 25 cm column filter (shown in Fig. 1). The gravel filter is 30 cm in length (i.e., the seepage length L = 30 cm). The experimental setup consists of a column filter, a sand supplier, and a recirculating water supply system. The valves and flowmeter in the system allow us to maintain a constant seepage flow. For each trial, a predetermined quantity of sand was instantaneously released to the system and the mixture infiltrated into the gravels with an instantaneous concentration Ci. After a sufficient period of time (usually within a few minutes), sediment deposition reached a stable stage due to the clogging effect. Flow was then terminated, and the water in the system was drained. The sand remaining atop the filter surface and the sand accumulating within the gravel filter were sampled and their quantities (designated as m0 and m in Fig. 1, respectively) were measured, where m is the sum of the sediment deposits taken from the six 5-cm-thick layers of the gravel filter. A review of previous studies (e.g., Behnke 1969; Sakthivadivel and Einstein 1970; Cunningham et al. 1987) suggests that three major factors govern the siltation of porous media; they are the grain sizes of sediment and gravel, sediment concentration, and seepage flow velocity. The gravel/