MARCH 2018 L I E T A L . 625 The Time Delay of Flow and Sediment in the Middle and Lower Yangtze River and Its Response to the Three Gorges Dam YANGYANG LI,YINGXIN ZHU,LEI CHEN, AND ZHENYAO SHEN State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing, China (Manuscript received 5 August 2017, in final form 3 January 2018) ABSTRACT As the largest hydropower project in the world, the Three Gorges Dam (TGD) has drawn extensive concern in terms of its impact on downstream areas. In this study, an improved time delay estimation and wavelet analysis were used to investigate the influence of the TGD on the streamflow and sediment in the middle and lower Yangtze River, using time series of the daily discharge and sediment concentration data from three hydrological stations downstream of the dam. The results indicated that all of the time series at the three stations have prominent annual cycles, but the cycle of daily mean sediment concentration was nearly non- existent after the impoundment of the TGD. Changes in discharge and sediment between the Yichang and the Hankou stations are larger than those between the Hankou and the Datong stations, which is mainly at- tributed to the streamflows of tributaries and Dongting Lake and the flood diversion area of Jingjiang. The transmission time of discharge for the whole Yichang–Datong river section is approximately 6 days. In ad- dition, the attenuation of discharge from the Yichang station to the Datong station is 20%–30%. In contrast, the transmission of suspended sediment is slower than that of discharge, which takes 7–7.5 days to move from the Yichang station to the Datong station. The attenuation of sediment is approximately 30% in the Yichang– Datong river section and shows a clear increasing trend after 2006, mainly because a large amount of sediment was trapped by the TGD, and the dynamic balance of sediment was disturbed. 1. Introduction Variations in discharge and sediment at stations in the main stream or tributaries of the Yangtze River during The Three Gorges Dam (TGD), the world’s largest recent decades have been documented in previous dam, was completed and began storing water in 2003. studies. Dai et al. (2008) examined the impacts of the Large dams disrupt river continuity and unavoidably in- TGD impoundment and serious droughts on river dis- duce alterations in flow, sediment, and water temperature charge reduction in 2006. A sharp decrease of sediment regimes (Chen et al. 2016; Li et al. 2011; Syvitski et al. 2005; load after the impoundment of the TGD was reported Wang et al. 2016). With the construction of the TGD, by many studies (Li et al. 2011; Q. Zhang et al. 2012, considerable attention has been focused on how the dam 2013), and the significant downward trends in sediment impacts the river regime, especially the environment flux were likely to be dominated by human activities, downstream in the middle and lower Yangtze River (Chen especially dam construction (Zhao et al. 2012). Gao et al. 2016; Stone 2008).ThemiddleandlowerYangtze et al. (2013) investigated flow regime changes in the River basin is one of the most developed and densely middle and lower Yangtze River between the pre- and populated areas in China. The flow regime, changed by the postimpoundment periods and found that the TGD the TGD, may have a great influence on water supply significantly reduced the mean flow in October, while and economic development. Furthermore, sediment de- river discharge increased in February. Compared to position in the reservoir has reduced the amount of sedi- climate variability impacts on the catchment, the ment in the middle and lower reaches and has even TGD has a much greater influence on the seasonal resulted in significant topographic changes. Therefore, it is (September–October) dryness of Poyang Lake and has necessary to understand the impact of the TGD on the flow further altered the relationship between the river and and sediment regime downstream. the lake (Guo et al. 2012; Zhang et al. 2014). Mei et al. (2015) analyzed hydrological data from the Yichang, Corresponding author: Lei Chen, [email protected] Hankou, and Datong stations and noted that the DOI: 10.1175/JHM-D-17-0119.1 Ó 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 10/01/21 07:51 PM UTC 626 JOURNAL OF HYDROMETEOROLOGY VOLUME 19 hydrology of the Yangtze River is mainly controlled by the Using the time series of daily mean discharge and sedi- TGD. Luan and Jin (2016) calculated and analyzed the ment concentration data from the Yichang, Hankou, and impacts of the TGD impoundment on sediment based on a Datong hydrological stations, the present study was con- water-sediment transport numerical algorithm. These ducted with the following objectives: 1) to assess the pe- studies imply that the TGD may have a significant influ- riodic variation in discharge and sediment concentration ence on river discharge and sediment transport in the caused by the TGD and investigate the dependencies middle and lower Yangtze River. Although the seasonality among stations and 2) to quantify the lag time and atten- of the altered levels prevailed throughout the downstream uation of discharge and sediment in the main stream. reaches of the Yangtze River, the time of occurrence and magnitude of level changes varied among the stations (Wang et al. 2013). Understanding the lag time and 2. Materials and methods transmission factors of river discharge and sediment be- a. Description of the study area tween upstream and downstream is critical for building precise models and evaluating and improving control The Yangtze River, the longest river in China, plays a strategies for pollutants transported by the river. Never- vital role in Chinese economic development and environ- theless, there are insufficient studies on the lag time, river mental conservation. The expansive Yangtze River basin is discharge transmission factor estimation (TFE), and sus- divided into the upper, middle, and lower Yangtze reaches, pended sediment in the Yangtze River. though we concentrate on the middle and lower Yangtze Several methods have been used to investigate the im- River in this paper. The upper reaches extend from the river pacts of dam construction, such as the Mann–Kendall test source to the Yichang station, which is located just down- (Assani 2016; Chen et al. 2016), the continuous wavelet stream of the TGD. The middle reaches flow from the transform (CWT) technique (White et al. 2005; Zhang Yichang to Hankou stations with a length of 950 km. The et al. 2012), time series analysis (Li et al. 2011), scanning lower reaches are 600 km long, flowing from Hankou to t test (Q. Zhang et al. 2013), indicators of hydrologic alter- the river mouth, though the river section between the ation (IHA), and the range of variability approach (RVA; Hankou and Datong stations is defined as being within the Alrajoula et al. 2016; Wang et al. 2016). However, previous lower reach in this study to avoid the influence of tides studies on the Yangtze River focused more on analyzing (Fig. 1). There are two large lakes in the middle and lower individual stations, and few studies have evaluated the Yangtze River: Dongting Lake and Poyang Lake. Ex- quantitative relationships between upstream and down- changes of water and material between the two lakes and stream regions. A more detailed investigation is required the main channel support flood control, the ecosystem, and into the alterations of discharge and sediment regimes re- water supply in the middle and lower basin (Gao et al. 2013). sulting from the TGD with more accurate methods. Time The Yangtze River basin is characterized by a subtropical delay estimation (TDE) between the transmissions of an monsoon climate, and the monsoon-driven precipitation array of sensors has been utilized in different areas. For causes seasonal variability in the river flow, with high water example, Hocking and Kelly (2016) quantified the time lag and sediment discharge in the wet season from May to between rainfall and recharge in groundwater, and the re- October (Li et al. 2011). The construction of the TGD has sults showed that the significant delay should be in- resulted in a series of changes in flow and sediment regimes. corporated into numerical groundwater models or that it For example, after the impoundment of the TGD, a sharp was a source of a calibration error. DeWalle et al. (2016) decrease in the sediment load was widely reported. There- estimated the lag times between atmospheric deposition fore, understanding the impact of the TGD on downstream and stream chemistry by cross-correlating monthly data areas is necessary. In this paper, we use wavelet analysis to from four pairs of stream and deposition monitoring sites, assess the periodic variation of hydrological features and the noting that understanding lag times between changes in dependencies of discharge and sediment regimes among chemical inputs and watershed responses is critical to eval- stations. We improve the time delay estimation algorithm to uating and refining pollutant control strategies. Xia et al. quantify the lag time and attenuation of discharge and (2016) proposed a high-resolution time delay estimation sediment between upstream and downstream sites. scheme to obtain the complete time sequence structure of b. Data source the geometric scattering of an underwater target. TDE is also used in optics, seismology, fault location, and bio- Data were sourced from the hydrological yearbook of medical engineering to evaluate lag time and attenuation China, consisting of discharges and sediments at the (Liao et al. 2013; Ling et al.