Analysis of Suspended Sediments and Flow Under Sub-Tropical Environment of Taiwan
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Recent Advances in Energy and Environment Integrated Systems Analysis of suspended sediments and flow under sub-tropical environment of Taiwan SAMKELE S. TFWALA1, YU-MIN WANG2*, AND YU-CHIEH LIN2 1Department of Agriculture and International Cooperation 2Department of Civil Engineering National Pingtung University of Science and Technology 1 Shuefu Road, Neipu, Pingtung, 91201 TAIWAN * Email: [email protected] Abstract: - The total amount of suspended load carried by a stream during a year is usually transported during one or more extreme events related to high river flows and intense rainfall, leading tohigh-suspended sediment concentrations (SSC). Therefore, the purpose of the study is to build a model to estimate sediment discharge based on observed flow and SSCfrom torrential rainfallevents.Price meter and depth-integrating suspended- sediment sampler, model DH-59, are used to measure flow velocity and SSC, respectively during sixevents in Shi-wen River, southern Taiwan. These data were collected every two hours in the period, July 2011 to August2012.The results evidenced that sampling high flows is sufficient to generate SRC’s with higher confidence (R2=0.87) in estimating sediment loadas it underestimated sediment load from Typhoon Nanmadol by 18% compared to 73% from the 10-day interval SRC (R2=0.26) developed using data collected January, 2011 to August, 2012. Key-Words: - suspended sediment, sediment discharge, flow, torrential rain, Shi-wen River, sub-tropical 1 Introduction essential. However, measuring and estimating Being located in a sub-tropic area, Taiwan faces suspended sediments concentrations (SSC) in rivers serious attacks from torrential rains accompanied have long been subject to confusion and uncertainty. with typhoons during a summer-autumn season Many methods have been developed for collecting (June to August), and has a mean precipitation of data and estimating sediment discharge in rivers, 2500 mm/year and reaches 3000-5000 mm/year in which suggests complexity in river studies. the mountainous regions. The frequency of typhoon According to [7], river study is necessary for with torrential rainfall happened one time for every reliable forecasting, but it is a difficult task due to two years before year of 2000, once a year after the complexity and inherent non-linearity of its 2000 and three to four times since 2008 [1]. These hydrological system. Regardless of the complexity, typhoons induce severe hazards in the form of correct estimation of SSC is becoming more and flooding [2,3]. By comparing the rivers around the more essential in engineering design, planning, world, the ones in Taiwan have the steepest slopes, maintenance, and operation of water resources [8]. largest discharge per unit drainage area and the In addition, suspended sediment has been identified shortest time of concentration [4]. The watersheds by [9] as a leading direct cause of river impairments. are formed by fragile sandstone, and downstream For example, high concentration of suspended reaches of rivers have heavy deposits from the poor sediment can increase cost of water treatment; geologic upstream catchments by these extreme damage pumps and turbines; fill reservoirs; impede events. Consequently, sediment concentrations are navigation and increase frequency and severity of large, as evidenced by the fact that 7 of the 10 floods by reducing channel capacities. The global rivers with the highest sediment yields are sediments in a river are usually caused by erosion Taiwanese [5]. These high sediment concentrations on upstream section, washing of soil within worsen the severity of these disasters as illustrated catchment area or because of man interfering with by [6] and [2]. In addition, damage caused by riverbed. typhoon Morakot in 2009 was mostly due to There are two common approaches for sediment related disasters [1]. estimating suspended load; first one being the In the wake of such events, the ability to interpolation procedure [10]. This requires regular accurately quantify sediment loads in rivers is sediment sampling over a relatively long period. ISBN: 978-1-61804-181-4 86 Recent Advances in Energy and Environment Integrated Systems The load is estimated as a product of mean sediment 2 MATERIALS AND METHODS concentration and water discharge over some time 2.1 Study Area such as 1 hr, 1 day, or 1 month. The second Our study area was in Shiwen River, located (21°34 approach is the regression analysis (Walling and 48 North latitude and 120°47 56 East longitude) Webb 1981), the most popular of which is the in the southern part of Taiwan (Fig.1). The length of sediment-rating curve (SRC) [11, 12]. A sediment- the main stream is about 22.3 km and the basin rating curve is an empirical relationship between covers 89.61 km2. The average slope is about 0.03 suspended sediment concentration, C (mg/L) and with a design flood at 1300 m3/s. the associated discharge Q (m3/s), and since SSC and sediment discharge often vary over several orders of magnitude the rating curve is expressed in a form of a power function: C=aQb (1) Where C is the suspended sediment concentration; Q is water discharge; and a, b are coefficients. The coefficients a and b are dependent on each river and are determined by mechanism of transport. The rating curve is assumed to represent a continuous relationship over the entire range of water discharge. Thus one can apply a full range of Q records to interpolate (sometimes extrapolate) suspended loads over a given time period. Using the established SRC and the available continuous Fig.1 Shi-wen river basin discharge data, sediment load for a given period maybe calculated by summing daily loads or by the magnitude frequency method [13]. The main 2.2 Monitoring suspended sediment advantage of the rating curve method is that long- concentration term historical data can be stratified and more than The Taiwan Water Resources Agency (WRA) one rating curve for specific time intervals can be manually measures discharge and corresponding constructed [14]. concentrations of suspended sediment three times a Most suspended sediments move during month (10 days interval) at Shiwen River as part of infrequent high flows that collectively account for river management. Based on these measurements, only a small portion of the measurement period [15]. rating curves of flow discharge and sediment According to [16], one variability in the sediment discharge were obtained. For the same station, we transport that should be considered is the season of collected the same data during six torrential rainfall the year and the relative magnitude of the events for estimating sediment discharge through precipitation. [11] earlier report this when they establishment of a sediment-rating curve. These demonstrated a decrease of the coefficient of events included four typhoons, namely, Nanmadol determination (R2) from 0.85 in summer to 0.39 in (28 August to 31 August 2011), Talim (20 June to 21 winter. The associated high transport rates and June 2011), Saola (2 August to 3 August 2012) and variances dictates that most data be collected during Tembin (24-25 August and 28 August 2012). During high flows as suggested by [17] in his study on these events, samples were collected every two concentration and transport in Slovene rivers. hours using DH-59 and price meter for SSC and However, the infrequency and brevity of the high flow velocity respectively. [18] give the operation flow periods combined with measurement and and descriptions of this equipment. The mid-section access cause acute problems in collecting data. method of measurement as shown by Fig.2 was Nonetheless, data collected during high flows are employed. This method of measurement assumes still essential to the development of good rating that thevelocity samplesat each depth samplingpoint curves. Hence, the purpose of the paper is to (Di), represents the mean velocity in aparticular estimate sediment discharge through rating curves rectangular cross sectional area.The total depth of developed from data collected during torrential that particular vertical determined the measurement rainfall events. depths of velocity. If depth was less than 75 cm, measurements were carried at 0.6 depth of vertical. ISBN: 978-1-61804-181-4 87 Recent Advances in Energy and Environment Integrated Systems If depths were more than 75 cm, measurements 3 Results and discussion were done at 0.2 and 0.8 depths of verticals.The 3.1 Behaviour of suspended sediment during partial area extends laterally from half the distance the torrential events from the preceding meter location to half the In this study, suspended sediment samples were distance to the next, and vertically from the water collected every two hours during torrential events. surface to the measured The sediment discharge was estimated accordingly. The important hydraulic characteristics of the six torrential events are given in Table 1. Among the six events, event 3 (Typhoon Nanmadol) had the highest duration (64 hours). Thus, observed concentration of suspended sediments were extremely high, with maximum observed value at 50014 ppm. The least SSC was observed in event 5 (Typhoon Saola), having 6920 ppm in a duration of Fig.2 Sketch of mid-section method for computing 24 hours. Event 1, which was not a typhoon, cross-sectional area for discharge resulted to a peak discharge (Q) of 72 cubic metre per second (cms) and maximum SSC of 16547 ppm. depth and the computations are done using equation Table 1 shows that maximum Q does not always 2 to equation 4 based on Fig.2. coincide with maximum Qs. For example, maximum Q for event 1 was 72 cms, but maximum Q ̻ Ɣ ʚ͑ dz ̾ ʛ … . ƍʚ͑ dz ̾ ʛ (2) s $ $ ) ) occurred when Q was 70 cms. The same happened ͐ Ɣ 0.68 Ɛ ͈ ƍ 0.005 (3) for event 2.