Jihn-Sung Lai
Assistant Research Fellow, Hydrotech Research Institute,
National Taiwan University, Taipei, Taiwan，China
Address: 158 Chow Shan Road, Taipei 106, China
Telephone: (886-2) 23644512
Fax: (886-2) 23644512
E-mail:
jslai@hy.ntu.edu.tw
Abstract:
Many researches have been focused on the transport
and fate of sediments during flushing operations in reservoirs. A successful
operation of flushing sediment through a reservoir depends on the control of
water-sediment interaction. One of the important processes during flushing is
the critical condition for erosion of previous deposits. To understand the
critical conditions of the reservoir deposits in erosion processes, experiments
were conducted. It is found that each fitting curve of measured velocity or
shear stress against erosion time corresponding to a given erosion depth will
approach a constant value which may be defined as the critical condition to
initiate the movement of cohesive deposits.
Keywords: cohesive deposits, critical shear stress, flushing operations
Due to steep slope, high intensity of rainfall, etc, in the watersheds of reservoirs in Taiwan, serious reservoir sedimentation problems had reduced the storage capacity about 1.46 x 107 m3 per year, which was estimated officially in 1995[2]. Except making efforts on soil conservation and removing sediment by dredging, reservoir desiltation by flushing sediment deposits has been practiced in the Tapu reservoir and Jen-sang-pei reservoir successfully. The A-kung-tien reservoir located in southern Taiwan is planned to restore its capacity by dredging and then flushing sediment by emptying the reservoir during flood season.
In Taiwan, sediment deposits in many reservoirs
contain a great amount of fine-grained materials. Generally speaking, about 10%
of clay in the soil mixture will suffice to assume control soil properties with
cohesiveness [2]. Figure 1 shows three size distributions of sediment deposits
sampled in different reservoirs. As shown in Figure 1, more than 70%
of the sediment deposits pass the No. 200 sieve. Obviously,
the reservoir deposits contain large portion of the fine-grained sediments.
The flushing process takes place when water and sediment are released from the reservoir to pass sediment entering from upstream or erode previous reservoir deposits. During flushing, the water-sediment interactions are complicated. One of the important processes during flushing is the critical condition for resuspension of previous deposits. Under critical shear stresses acting on bed to initiate the movement of deposits, sediments can be eroded/resuspended into the flow. For understanding the critical conditions of the reservoir deposits from the Tapu reservoir in erosion/resuspension processes, experiments were conducted in a tilting flume. The initiation criteria of cohesive deposits was defined by McNeil et al. (1996), which is examined by the experimental data measured in this study and discussed herein.
Most of the reservoirs in Taiwan have no sediment release outlets for desiltation by means of hydraulic methods. However, only few reservoirs have conducted desilting operations by flushing. Due to water drawdown, flushing sediment by flood or by emptying the reservoir is still not well understood, especially flushing on cohesive sediment deposits. The Tapu reservoir located in northern Taiwan was completed in 1960. It is a 20.9m high and 98.9m long concrete gravity dam with four spillways (8.3m high, 8m long each) and one sluiceway (3.8m high, 8m long) controlled by tainter gates. The design capacity of the four spillways is 1,937 m3 /s, and of sluice gate is 440 m3 /s. With the maximum level of 69.6 m, the reservoir pool has about 8 km in length and water surface area of 135 ha. The initial total reservoir storage was 9,258,000 m3 and its active storage was 7,960,000 m3. Water supply and flood control are the main functions of the Tapu reservoir. Due to intensive mining activities in its watershed, cumulative deposits filled up 51.0% of the initial capacity according to reservoir bed elevation survey in 1987. Serious sedimentation has raised the flood stage along the tail reach of the reservoir and forced local people to build levees for protection. With a sediment sluice gate, it practiced several flushing operations by emptying the reservoir to remove a great amount of deposits.
According to the density of the deposit materials, the water-sediment mixture of the bed may be roughly classified into to four states: mobile suspension, stationary high- density suspension, consolidating (soft) deposit and consolidated (firm) bed [4]. With no mechanical strength, the mobile suspension and stationary suspension barely have resistance to the bed shear resulting from the flowing water. Consolidating processes result in a settled bed with a lower water content, higher shear strength, and more stable water-sediment mixture of the bed. The critical shear stress, tc, is the bed shear strength to initiate the movement of the cohesive deposits, and usually is utilized as an indicating parameter referring to erosion mode and erosion rate.
For a site-specific reservoir, the inflowing sediment with cohesiveness will settle and consolidate to form the bed deposit. A consolidating bed causes the time-varying density of the bed deposits. The accompanying density increase with time as well as in depth changes the erodibility of the consolidating bed in bed shear strength. Many researchers have found that the erosion potential with respect to bed shear strength of bed can be related to the dry density of deposits [4]. The relationship with an exponential form is not unique for every type of deposits. Although this relationship is very approximate and site-specific, it is useful for estimating the critical shear stress by eroding water (or bed shear strength with respect to erosion) in the absence of a better correlating to properties characterizing bed structure.
Comparing with the non-cohesive deposits, the initiation of cohesive sediment movement is not easily determined due to the difficulty on the observation of incipient motion on the bed surface. According to the mode of erosion, surface erosion is initiated as a particle by particle or small aggregate by aggregate separation from the soil surface [1]. McNeil et al.(1996) described that for very low shear stresses sediment may not move. As the shear stress slowly increased, a few particles can be seen to roll along the surface. If the shear stress increases further, a small amount of erosion occurs as bursts of sediment at many small areas over the entire surface. The above descriptions of cohesive sediment erosion are qualitative, however, it is still not clear to define the critical condition to initiate the deposit. Based on the experimental measurements, a procedure was proposed by McNeil et al.(1996) to define the critical condition quantitatively. If more than 1 mm depth of sediment deposits is eroded at a certain shear stress in 2 minutes, the flow rate is decreased to give a new value of shear stress when erosion occurs, and at this moment the shear stress is defined as the critical shear stress. On the other hand, if less than 1 mm sediment deposits is eroded in 15 minutes, the flow rate is slightly increased to erode the deposits, and the shear stress under such flow condition is also defined to critical shear stress. Under this quantitative procedure, the critical shear stress for erosion as that shear stress causes an erosion rate between 10–3 and 10-4 cm/s [3].
For further understanding of the critical shear stress acting on the bed to initiate the movement of the sediment deposits, experiments were conducted in the 12m long, 0.3m wide and 0.6m high tilting flume to examine the above definition of the critical condition of sediment erosion. With observation windows of the flume, the sediment erosion can be observed. The soil sample with given dry density was paved in a box (15cm long, 6cm wide and 1 cm deep). The surface of the box with soil sample was carefully placed and had same elevation with the bed. Various discharges were applied from the upstream to generate different shear stresses exerting the each soil sample with desired dry density.
For the deposits sampled near Shiwu-lau in the Tapu reservoir, the experimental results of critical shear stress and critical velocity to initiate the bed deposits are plotted in Figure 2 and 3, in which the criteria of 1mm deep erosion on the bed surface was examined. As shown in Figure 2, the shear stress is plotted against time duration of generating 1mm erosion depth from the bed surface. It is found that each fitting curve will approach a constant value that may be defined as the critical velocity to initiate the resuspension of cohesive deposits. In Figure 3, the similar pattern of the curve approaching a constant value is obvious to define the critical shear stress. As mentioned previously, 1 mm depth of sediment deposits is eroded at a certain shear stress (or velocity) between 2 minutes and 15 minutes to obtain a quantitative critical conditions. At present, 2mm erosion depth is also measured for the examination of McNeil ‘s criteria, and the shear stress is plotted against time duration of generating 2mm deep surface erosion.
As described previously, the critical shear stress (tc, shear strength of the deposit) can be an exponential function of dry density (rd ). According to the experimental results shown in Figure 6, the relationship of dry density and shear stress as well as velocity can be obtained by regression.
The interactions between flow shear stress and bed deposits are complicated during flushing in a reservoir. One of the important processes during flushing is the critical condition for erosion of previous deposits. Experiments were conducted to understand the critical conditions of the cohesive reservoir deposits in erosion processes. Based on the experimental data, the initiation criteria defined by McNeil et al (1996) was examined and discussed in this study. It is found that the quantitative criteria proposed by them can be applied reasonably.
Acknowledgments
Financial support under the grant NSC 89-2211-E-002-050 by the National Science Council, Taiwan, is highly appreciated.
References
[1] Arulanandan K., Loganathan P. and Krone R. B., (1975), Pore and eroding fluid influences on surface erosion of soil, Journal of Geotechnical Engineering Division, ASCE, pp. 51- 66.
[2] Lai, J. S., (1998), A study on the critical erosion conditions for the A-kung-tien reservoir deposits, Journal of Taiwan Water Conservancy, Vol.46, N0.3, Sept. pp.76-83. (in Chinese)
[3] McNeil, J., Taylor, C. and Lick, W., (1996), Measurements of erosion of undisturbed bottom sediment with depth, Journal of Hydraulics Division, ASCE, Vol. 122, No. 6, pp.316-324.
[4] Mehta A. J., et al., (1989), Cohesive sediment transport. I：process description, Journal of Hydraulics Division, ASCE, Vol. 115, No. HY8, pp.1076-1093.
Fig. 1 Grain size distributions of reservoir deposits
Fig. 2 Shear stress against erosion duration at various dry density
Fig. 3 Velocity against erosion duration at various dry density
Fig. 4 Shear stress against erosion duration at various dry density for 2mm erosion depth
Fig. 5 Velocity against erosion duration at various dry density for 2mm erosion depth
Fig. 6 The critical shear stress plotted against the dry density for reservoirs