The Application of a Multi-Beam Echo-Sounder in the Analysis of the Sedimentation Situation of a Large Reservoir After an Earthquake

water

Article The Application of a Multi-Beam Echo-Sounder in the Analysis of the Sedimentation Situation of a Large Reservoir after an Earthquake

Zhong-Luan Yan 1,2, Lei-Lei Qin 1,2, Rui Wang 2,3, Jia Li 2, Xiao-Ming Wang 4, Xi-Liang Tang 4 and Rui-Dong An 2,* 1 Postdoctoral Research Station, China Three Gorges Corporation, Beijing 100038, China; [email protected] (Z.-L.Y.); [email protected] (L.-L.Q.) 2 State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University, Chengdu 610065, China; [email protected] (R.W.); [email protected] (J.L.) 3 Power China Chengdu Engineering Corporation Limited, Chengdu 610072, China 4 China Three Gorges Projects Development Co., Ltd., Chengdu 610094, China; [email protected] (X.-M.W.); [email protected] (X.-L.T.) * Correspondence: [email protected]; Tel.: +86-136-6623-2326

Received: 22 March 2018; Accepted: 23 April 2018; Published: 26 April 2018

Abstract: The Wenchuan Earthquake took place in the upper reach catchment of the Min River. It resulted in large amounts of loose materials gathering in the river channel, leading to changes in the sediment transport system in this area. The Zipingpu Reservoir is the last and the largest reservoir located in the upper reach of the Min River. It is near the epicenter and receives sediment from upstream. This paper puts forward a study on the reservoir sedimentation and storage capacity of the Zipingpu Reservoir, employing a multi-beam echo-sounder system in December 2012. Then, the data were merged with digital line graphics and shuttle radar topography mission data in ArcGIS to build a digital elevation model and triangulate the irregular network of Zipingpu Reservoir. Via the analysis of the bathymetric data, the results show the following: (1) The main channels of the reservoir gradually aggrade to a flat bottom from the deep-cutting valley. Sedimentation forms a reach with a W-shaped longitudinal thalweg profile and an almost zero slope reach in the upstream section of the reservoir due to the natural barrier induced by a landslide; (2) The loss ratios of the wetted cross-section surface are higher than 10% in the upstream section of the reservoir and higher than 40% in the natural barrier area; (3) Comparing the surveyed area storage capacity of December 2012 with March 2008, the Zipingpu Reservoir has lost 15.28% of its capacity at the dead storage water level and 10.49% of its capacity at the flood limit water level.

Keywords: multi-beam echo-sounder; Zipingpu Reservoir; sediment deposition; topographical change; capacity loss

1. Introduction Dam construction breaks the sediment balance in a natural river, creating an impounded river reach [1]. As the water level rises, the flow speed decreases, and so does the sediment transport capacity [2–5]. Therefore, the reservoir will gather sediment and lose capacity until a balance is once again achieved, normally after the sediment fills up the impoundment [6]. An estimated value of 0.5–1% of the global reservoir storage is lost each year, while the sedimentation rate varies between 0.1% and 2.3% [7]. The worldwide loss of storage caused by sedimentation each year is greater than the increased capacity from newly built reservoirs [8]. Continual sedimentation can no longer assure reservoir capacity, flood control, power generation, irrigation, and other benefits related to

Water 2018, 10, 557; doi:10.3390/w10050557 www.mdpi.com/journal/water Water 2018, 10, 557 2 of 16 the storage capacity [9–11]. Regarding power generation, when the reservoir sedimentation reaches a high level, the energy production will decrease. Regarding irrigation, reservoirs hold and store the sediment and sediment-associated nutrients, resulting in a decline of nutrients downstream [12]. Schmitter et al. [13] found a decreased field nutrient status in the uplands of northwestern Vietnam because local people established a reservoir as a sink for sediment, leading to a nutrient-rich sediment in the reservoir, causing a low level of nutrients downstream and endangering the balance of the ecosystem. Furthermore, sedimentation in a reservoir may also influence the security of the waterway systems and hydraulic structures. For instance, sediment transported by the turbidity current may block or damage the intakes and outlets. Then, it will decrease the efficiency and increase the maintenance costs [14,15]. Hydrographic surveys are the most accurate method to measure the distribution patterns of sediment and the volume occupied. The depth with horizontal coordinates (X, Y) must be obtained in hydrographic surveys. In recent years, there have been many ways to measure the underwater topographic data, like the single-beam echo-sounder (SBES), the multi-beam echo-sounder (MBES), and remote sensing methods like air-borne light detection and ranging (LiDAR), and so forth. The deposition pattern of Tahtali Reservoir in Izmir, Turkey, was surveyed via the dual frequency (28/200 kHz) SBES. The bathymetry and sediment thickness were revealed through the estimation from the difference of depths measured by transducer [16]. Haregeweyn et al. [17] surveyed the Angereb Reservoir in northwestern Ethiopia by employing the SBES and estimated the total annual capacity loss during different periods. Kubinský et al. [18] employed a double-beam sonar and external GPS with a measured step of 5 m to study and analyze the changes of the storage capacity of the vel'ký kolpašký water reservoir from the year 1889–2012. Wang [19] calculated the storage capacity curve of White Oak Bayou watershed utilizing the high-resolution LiARD method. Lima et al. [20] adjusted regression models between field bathymetry values and Landsat 5 TM satellite data, providing satisfactory accuracy for estimating the bathymetry values of the Saco reservoir in Serra Talhada-PE, Brazil. Although the MBES method is commonly used in submarine topography surveys [21,22] and marine scientific investigations [23], a few reports about its application in reservoir surveys can be found. The economic and societal importance of water storage makes sedimentation in reservoirs an active and expanding field of research. It is necessary to study the rate, pattern, and volume of sediment deposition in order to find out the accurate storage capacity of a reservoir. However, in China, no research has focused specifically on the detailed topographic changes, the precise sedimentation situation, and the accurate capacity loss of the Zipingpu Reservoir after the Wenchuan Earthquake. This paper aims to apply the MBES in the reservoir topographic survey and conduct research within the Zipingpu Reservoir using the MBES bathymetric data and other supplemented data to derive reliable terrain and accurate reservoir capacity. The historical bathymetric data and the MBES data were compared to study topographic changes, sedimentation situation, and reservoir capacity loss that were directly or indirectly induced by the Wenchuan earthquake for the reservoir management.

2. Material and Methods

2.1. Study Area Zipingpu Reservoir is located in Dujiangyan, Sichuan, Southwestern China, as shown in Figure1. The reservoir was constructed in 2001 and began to impound water in September 2005. It was built on a mountainous river reach of the Min River, where the rock-cut channel with narrow and V-shaped cross-sections was set in the deep valley. The Min River is the largest tributary of the Yangtze River. It originates from the Min Mountain, with a mean annual discharge of 469 m3/s and total annual runoff of 1.48 × 1011 m3/year. The Zipingpu Reservoir has a catchment area of 22,700 km2 above the dam site, accounting for the drainage area of 98% of the Min River upstream. It effectively controls 98% of the sediment inflow and 90% of the flood inflow of upstream reaches. Downstream of the dam, Water 2018, 10, x FOR PEER REVIEW 4 of 17 Water 2018, 10, 557 3 of 16 of fan-shaped pulses directly towards the bed and achieves a depth data profile. The depth is computed while accounting for the beam angle. Along the survey line, the MBES has a better the reservoir links up with the traditional Dujiangyan Irrigation System. The Zipingpu Reservoir meets coverage of the bed and can obtain topographic data of a certain area, while the SBES can only themeasure irrigation a line and of municipaldata underneath water the supply transducer of the, Dujiangyan which cannot irrigation represent area the andterrain. Chengdu The MBES district has as itsmany main purposes,advantages and such its otheras saving purposes field are observation balancing floodtime. control, However, power the generation, SBES is still environmental used in protection,hydrographic and surveys tourist industry,worldwide and due so to forth. the relatively Therefore, low the cost safety in terms of reservoir of money directly and manpower relates to in the safetyChina. and property of people downstream. Other basic engineering characteristics of the Zipingpu Reservoir are shown in Table1.

D B

M5 M1 D M6 M3

M2 M7 M12 M11 M4 M30 M14 M13 M29 M8 M17 M10 M18 M15 M9 M28 M16 M27

M26 M19 M20 M25 M21

M24 M22 M23 S3 S1 S2 S4

Figure 1. The location, survey area, and transects. (A) A map of China; (B) Sichuan province; (C) a map Figure 1. The location, survey area, and transects. (A) A map of China; (B) Sichuan province; (C) a showingmap showing the location the location of the Zipingpuof the Zipingpu reservoir reservoir and the and upper the Minuppe Riverr Min basin;River (basin;D) the (D survey) the survey area and cross-sections (M) denotes the main stream, S denotes the tributary Shouxi River). (A–C) are from the National Administration of Surveying, Mapping and Geoinformation; (D) is from Google Earth. Water 2018, 10, 557 4 of 16

Table 1. The basic engineering characteristics of Zipingpu Reservoir.

Coordinate Type of Dam Height (m) Length (m) Installed Capacity (MW) Embankment 31◦02007” N Concrete-face 156 663 760 103◦34026” E Rock-ﬁll Designated Dead Designated Flood Designated Utilizable Designated Total Basin Area (km2) Storage (m3) Regulation Storage (m3) Storage (m3) Storage (m3) 2.27 × 104 2.24 × 108 5.39 × 108 7.74 × 108 1.11 × 109

At 14:28 (UTC + 8) on 12 May 2008, the catastrophic Wenchuan Earthquake (Magnitude = 8.0) took place on the Longmenshan fault zone. The Longmenshan fault consists of the Wenchuan-Maowen fault, the Yingxiu-Beichuan fault, and the Pengzhou-Guanxian (Dujiangyan) fault [24]. Those three faults are located in the catchment of the Zipingpu Reservoir. The Yingxiu-Beichuan fault mainly induced the earthquake. Yingxiu, the earthquake epicenter, is about 17 km from the dam site. The Ministry of Land and Resources of China carried out a systematic investigation and observed landslides, rock falls, and debris flows induced by the earthquake in 15,000 sites [25]. Some occurred in the catchment of Zipingpu Reservoir. For example, the Niujuan landslide was the largest landslide near the epicenter and was located at the end of the reservoir backwater. It destroyed roads and highways, delivered sediment to the Min River, blocked the Niujuan Valley, and formed a barrier lake [26]. Since this was the largest and most destructive earthquake that occurred in this mountainous area in the last 100 years in China, secondary disasters such as landslides, rock falls, debris flows, and so forth, altered the river transport system and increased the sediment discharge in the upper reaches of the Min River. As the last and largest reservoir on the upper reaches of the Min River, the Zipingpu Reservoir received the sediment from upstream and lost its storage capacity. Once the sedimentation reaches a certain degree, it will influence the reservoir operation or cause safety problems, and it may lead to disasters, affecting people downstream in the coming decades. Therefore, the change of the storage capacity of the reservoir caused by sedimentation must be analyzed, and hydrographic data of the reservoir must be obtained.

2.2. Bathymetric Theory We employed the SBES in the hydrographic survey in the reservoir in 2008 and 2011, and the MBES in the 2012. Both the SBES and the MBES are acoustic depth sounding equipment, measuring the pulse travel time from the transducer to the bottom. The surveys also need to measure the speed of sound in the water column, the horizontal position (X, Y), and the reference water elevation. In this way, the relative depth can be calculated by multiplying the travel time and the speed of the pulse. Then, the space coordinate can be obtained by means of integrating the horizontal position and relative depth data. Although the two pieces of equipment have the same fundamental bathymetric algorithm theory, they still have obvious differences as shown in Figure2. The SBES may have transducers either with a single transducer or an array, while the number of depth data obtained in one ping equals the number of sensors. With an array of sensors built-in, the MBES transmits a series of fan-shaped pulses directly towards the bed and achieves a depth data proﬁle. The depth is computed while accounting for the beam angle. Along the survey line, the MBES has a better coverage of the bed and can obtain topographic data of a certain area, while the SBES can only measure a line of data underneath the transducer, which cannot represent the terrain. The MBES has many advantages such as saving ﬁeld observation time. However, the SBES is still used in hydrographic surveys worldwide due to the relatively low cost in terms of money and manpower in China. Water 2018, 10, x FOR PEER REVIEW 5 of 17

area and cross-sections (M) denotes the main stream, S denotes the tributary Shouxi River). (A–C) are from the National Administration of Surveying, Mapping and Geoinformation; (D) is from Google

Water 2018Earth., 10 , 557 5 of 16

Water Surface SBES MBES

River Bed

Figure 2. A sketch of the differences between the SBES and the MBES. Figure 2. A sketch of the differences between the SBES and the MBES. 2.3. Equipment 2.3. Equipment The MBES system is an integrated system. It consists of four parts: The acoustic system, auxiliary system,The data MBES collection system andis an visualization integrated system. system, It and consists post-processing of four part system.s: The acoustic In this paper,system, the auxiliary survey employedsystem, data R2sonic collection 2022 MBESand visua as anlization acoustic system, system and in December post-processing 2012. The system. R2sonic In 2022this paper, MBES canthe transfersurvey employed pulses and R2sonic receive backscatter2022 MBES data. as an Then, acou itstic calculates system the in hydrographicDecember 2012. data The profile R2sonic normal 2022 to theMBES trajectory can transfer of the shippulses via and built-in receive microchips. backscatter Parameters data. Then, and configurations it calculates the of acoustic hydrographic transducers data canprofile be adjustednormal to while the trajectory operating of the the Sonic ship Control via built-in software microchips. (Sonic Control Parameters 2000, R2Sonic,and configurations LLC, Austin, of TX,acoustic USA) transducers during the survey.can be adjusted The auxiliary while system operatin consistsg the ofSonic devices Control that software can measure (Sonic the Control sound speed 2000, inR2Sonic, water, theLLC, position, Austin, the TX, heading, USA) during and the the attitude survey. of The the auxiliary vessel. Those system data consists are of crucialof devices importance that can inmeasure post-processing. the sound Becausespeed in the water, speed the of position, sound changes the heading, with temperature, and the attitude pressure, of the and vessel. salinity, Those it willdata affectare of the crucial survey importance results if it isin not post-process correct. Minos-X,ing. Because produced the speed by the AMLof sound Company changes (Sidney, with Britishtemperature, Columbia, pressure, Canada), and providessalinity, it a measurewill affect of the the survey vertical results distribution if it is of not sound correct. speed Minos-X, for the soundproduced propagation by the AML track Company calibration. (Sidney, The VS British 111 GPS Columbia, compass toolkit,Canada), produced provides by a Hemisphere measure of GPS the Inc.vertical (Scottsdale, distribution AZ, USA),of sound is utilized speed tofor provide the so theund horizontal propagation position, track heading,calibration. and The GPS VS time 111 for GPS the spacecompass coordinate toolkit, calculation.produced by The Hemisphere Trimble R8 GPS GPS Inc. (Trimble, (Scottsdale, Sunnyvale, AZ, USA), CA, USA) is utilized is utilized to provide to provide the ahorizontal RTK-GPS position, solution heading, to position and the GPS horizontal time for and the verticalspace coordinate position ofcalculation. the transducers. The Trimble The IMU-108 R8 GPS attitude(Trimble, sensor, Sunnyvale, produced CA, USA) by the is SMC utilized Company to prov (Gzira,ide a RTK-GPS Malta), issolution also used to position to collect the the horizontal attitude dataand ofvertical the vessel. position All theof datathe transducers. collected by theThe transducers IMU-108 attitude are integrated, sensor, transported, produced andby the saved SMC in aCompany computer (Gzira, using theMalta), software is also Navipac used to andcollect Naviscan the attitude (developed data of by the EIVA, vessel. Skanderborg, All the data Denmark, collected www.eiva.comby the transducers) as aare data integrated, collection transported, and visualisation and saved system. in a Navipac computer is mainlyusing the used software to monitor Navipac and recordand Naviscan the trajectory (developed of the by ship, EIVA, and Skanderborg, the Naviscan Denmark, is used to www.eiva.com) integrate and save as a thedata data collection sets while and visualizingvisualisation the system. data as Navipac 3D point is clouds mainly in used the topography to monitor and underwater. record the Table trajectory2 shows of the the specifications ship, and the ofNaviscan the devices is used in theto integrate MBES system. and save The the Hydrographic data sets while Information visualizing Processingthe data as System3D point and clouds Sonar in Informationthe topography Processing underwater. System Table (HIPS&SIPS) 2 shows the software specifications (Version of 7.2, the CARIS, devices Fredericton, in the MBES NB, system. Canada) The wasHydrographic used in the Information post-processing Processing to derive System the topographic and Sonar Information data. Processing System (HIPS&SIPS) software (Version 7.2, CARIS, Fredericton, NB, Canada) was used in the post-processing to derive the topographic data. Table 2. The specifications of devices in the MBES system.

No. Device Range Accuracy Description 1 Sonic 2022 0.5 m–500 m (depth) 1.25 cm Echosounder ±30◦ (angle) 0.03◦ RMS (angle) 2 IMU-108 Attitude sensor ±10 m (heave) 5 cm or 5% (heave) <0.02 m (position) 3 VS111 toolkit - GPS compass <0.10◦ RMS (heading) 0.5 m–6000 m (depth) ±0.05% (depth) 4 Minos-X Sound velocity Proﬁler 1375 m/s–1625 m/s (sound velocity) ±0.025 m/s (sound velocity) <0.01 m (horizontal position) 5 Trimble R8 - RTK-GPS <0.02 m (vertical position)

Water 2018, 10, x FOR PEER REVIEW 6 of 17

Table 2. The specifications of devices in the MBES system.

No. Device Range Accuracy Description 1 Sonic 2022 0.5 m–500 m (depth) 1.25 cm Echosounder ±30° (angle) 0.03° RMS (angle) 2 IMU-108 Attitude sensor ±10 m (heave) 5 cm or 5% (heave) <0.02 m (position) 3 VS111 toolkit - GPS compass <0.10° RMS (heading) 0.5 m–6000 m (depth) ±0.05% (depth) Sound velocity 4 Minos-X 1375 m/s–1625 m/s ±0.025 m/s (sound velocity) Profiler (sound velocity) <0.01 m (horizontal position) Water5 2018,Trimble10, 557 R8 - RTK-GPS6 of 16 <0.02 m (vertical position)

2.4. Field Survey In March March 2008 2008 (before (before the the Wenchuan Wenchuan Earthquake Earthquake)) and and March March 2011, 2011, hydrographic hydrographic data, data,only onlyincluding including the cross-section the cross-section depth depthprofiles, proﬁles, were obtained were obtained utilizing utilizing the real-time the real-time kinematic kinematic global globalpositioning positioning satellite satellite (RTK-GPS) (RTK-GPS) and the and SBES. the SBES.The traditional The traditional volume volume calculation calculation method method was wasemployed employed to calculate to calculate the storage-capacity the storage-capacity curve. curve. In December 2012, the MBES system was was employed employed to to survey survey the the reservoir. reservoir. Before Before the the survey, survey, the MBES system was mounted on a vessel as shown in Figure 3 3.. TheThe relativerelative positionspositions ofof eacheach sensorsensor to the transducer and the mounted angle offset werewere measured. As the sound speed in water has a fundamental effecteffect on on the the accuracy accuracy of depth,of depth, the distribution the distribution of the soundof the speedsound in speed water wasin water measured was usingmeasured the Minos-Xusing the during Minos-X the during surveys. the Aftersurveys. measuring After measuring the sound the speed, sound the speed, ﬁeld the survey field started.survey Duringstarted. theDuring survey, the the survey, water the elevation water waselevation measured was hourlymeasured at each hourly survey at regioneach survey by recording region theby waterrecording gauge the reading water gauge near thereading shore. near Those the shore. data were Those used data to were correct used the to hydrographic correct the hydrographic raw data in theraw post-processingdata in the post-processing to derive the to terrainderive the of the terrain reservoir. of the Thisreservoir. survey This was survey conducted was conducted over six days, over andsix days, the survey and the area survey covered area 15 covered km2 as 15 shown km2 as in Figureshown1 in. However, Figure 1. theHowever, other areas the other in the areas backwater in the areabackwater of the reservoirarea of the were reservoir not surveyed, were not considering surveyed, considering the safety. the safety.

R2 Sonic2022 GPS Antenna

VS111 Receiver IMU-108

Figure 3. The MBES system on the vessel during the survey.

2.5. Data Processing The datadata post-processing post-processing was was ﬁrstly firstly done done in HIPS&SIPS. in HIPS&SIPS. The hydrographic The hydrographic data with data a resolution with a ofresolution 2 m × 2 of m 2 were m × derived2 m were from derived HIPS&SIPS. from HIPS&SIPS. Then, we imported Then, we the imported data into the ArcGIS data into (V10.2, ArcGIS Esri, Redlands,(V10.2, Esri, CA, Redlands, USA) for CA further, USA) processing. for further The processing. ArcGISsoftware The ArcGIS has thesoftware capability has the for capability 3D modeling for and3D modeling visualizing and the visualizing data with the a variety data with of analyst a variety tools. of analyst Triangulated tools. Triangulated Irregular Network Irregular (TIN), Network one of the(TIN), most one widely of the usedmost 3Dwidely terrain used models 3D terrain for creatingmodels for and creating representing and representing surfaces with surfaces mass with data worldwide,mass data worldwide, was used to was create used the topographicto create the surface topographic of the Zipingpusurface of Reservoir the Zipingpu in ArcGIS Reservoir [27,28 ].in ArcGISDue [27,28]. to the following reasons, the MBES has a blind area the swath cannot cover in the survey: (1) ConsideringDue to the following the safety, reasons, the MBES the shouldMBES has not a be blind applied area inthe shallow swath cannot water zonescover within the complex survey: terrain;(1) Considering (2) Because the thesafety, mounting the MBES of theshould echo-sounder not be applied requires in shallow a draft ofwater water, zones the with MBES complex cannot obtain data above the transducer; (3) Owing to the swath angle and the distance between the survey vessel and shore, the MBES system will result in a blind area on the shore terrain outside the swath coverage. Considering the safety of the equipment, we generally avoid the bank survey and use digital line graphics (DLG) with a resolution of 1:50,000 and the Shuttle Radar Topography Mission (STRM) 90 m DEM data from the Computer Network Information Center in the Chinese Academy of Science (http://srtm.datamirror.csdb.cn/)[29,30]. In this paper, we created the TIN and DEM (refer to Figure S1 and Figure S2 in Supplemental Material) of the reservoir and its catchment using the data mentioned above.

2.6. Storage Capacity Calculation Methods Nowadays, in China, the topographic data of the river channel reservoir are generally surveyed using the SBES and RTK (GPS). Considering the small amount of data, the storage capacity of the river channel reservoir is calculated by a cross-section method (Figure4A). The cross-section method is Water 2018, 10, x FOR PEER REVIEW 7 of 17 terrain; (2) Because the mounting of the echo-sounder requires a draft of water, the MBES cannot obtain data above the transducer; (3) Owing to the swath angle and the distance between the survey vessel and shore, the MBES system will result in a blind area on the shore terrain outside the swath coverage. Considering the safety of the equipment, we generally avoid the bank survey and use digital line graphics (DLG) with a resolution of 1:50,000 and the Shuttle Radar Topography Mission (STRM) 90 m DEM data from the Computer Network Information Center in the Chinese Academy of Science (http://srtm.datamirror.csdb.cn/) [29,30]. In this paper, we created the TIN and DEM (refer to Figure S1 and Figure S2 in Supplemental Material) of the reservoir and its catchment using the data mentioned above.

2.6. Storage Capacity Calculation Methods Nowadays, in China, the topographic data of the river channel reservoir are generally surveyed Water 2018, 10, 557 7 of 16 using the SBES and RTK (GPS). Considering the small amount of data, the storage capacity of the river channel reservoir is calculated by a cross-section method (Figure 4A). The cross-section method isrecommended recommended in in the the code code for for the the reservoir reservoir hydrologic hydrologic and and sediment sediment survey survey (issued (issued by by the the Ministry Ministry of ofWater Water Resources Resources of of the the People’s People’s Republic Republic of of China). China). It It ﬁrstly firstly dividesdivides thethe riverriver channel into several prismsprisms according according to to the cross-sections. Each Each prism prism is is divided divided into into several several trapezoids trapezoids with with the the bases bases divideddivided on on the the cross-sections. The The volume volume of of the the tr trapezoidsapezoids can can be be calculated calculated with with the the height height and and basebase areas. areas. The The volume volume of of the the prism prism between between the the two two cross-sections can can be be calculated by by summing thethe volume volume of of the the trapezoids. trapezoids. Hence, Hence, we we can can also also calculate calculate the the storage storage capacity capacity of of the the river river channel channel reservoirreservoir by by summing summing the the volume volume of of those trapez trapezoidsoids (refer (refer to Formula 1-3 1–3 and and Figure Figure S3A S3A in in SupplementalSupplemental Material). Material).

(A) (B)

(C)

Figure 4. The storage capacity calculation method. (A) Cross-section Method; (B) Triangle Prism Method; and (C) Triangulated Terrain between M13 and M14.

With the high-density survey data of the river channel reservoir and the rigorous terrain modeling method, TIN, the whole survey area is constructed into a triangular network. To calculate the capacity of the river channel reservoir, we can divide the river channel reservoir into several tri-prisms with TIN terrain and the speciﬁed water surface as their bases. The volume of the tri-prism can be calculated by multiplying the base area (on the speciﬁed water surface) with the average height (Figure4B). To calculate the capacity of the river channel reservoir, we only need to sum all the volumes of the tri-prisms (refer to Formula 4–6 and Figure S3B in Supplemental Material). This method can be implemented in ArcGIS. Water 2018, 10, 557 8 of 16

3. Results

3.1. Comparison of the Calculation Method A straightforward case was carried out to determine the method to calculate the storage capacity of the Zipingpu Reservoir. The reach between the cross-section of M13 and M14 was chosen. A TIN of the reach was built and can be viewed in Figure4C. The storage of the river reach under different water levels ranging from 820 m to 850 m was calculated by increments of 5 m and is shown in Table3. Here, we defined that the storage calculated by the cross-section method is V1 and the storage calculated by the triangular prism method is V2. The results indicated that the relative error (the formula is shown in Table3) of the calculated storage ranges from 2.37% to 8.04% and increases with the water level. The triangular prism method can provide more detailed information of the terrain with the sufficient MBES data, while the cross-section method can only generalize the volume into several prismoids with trapezoid bases, straighten the centerline between the cross-sections along the reach, and smooth the rough into a slope with insufficient data. Therefore, the cross-section method is less accurate, and the result is less reliable than that of the triangular prism method.

Table 3. The computational results comparison.

Storage Calculated by Storage Calculated by Triangular Relative Error(%) No. Elevation (m) Cross-Section Method V (m3) Prism Method V (m3) V1−V2 × 100% 1 2 V2 1 820 2,015,360 1,968,752 2.37 2 825 2,905,420 2,774,756 4.71 3 830 3,840,031 3,633,206 5.69 4 835 4,816,596 4,535,003 6.21 5 840 5,846,240 5,4746,08 6.79 6 845 6,932,660 6,449,968 7.48 7 850 8,061,938 7,461,832 8.04

3.2. Accuracy and Reliability of Terrain Data Obtained by the MBES A spatio-temporal analysis of the consecutive hydrographic survey will contribute to understanding the physical dynamics of the Zipingpu Reservoir. Important indicators for reservoir management monitoring, such as the coastline changes, the reservoir sedimentation situation, and the terrain changes, are also required for the analysis of the lifetimes of the reservoir. For an accurate analysis, the accuracy and reliability of the hydrographic data must be ensured. Compared with the 785 SBES survey points of the river bank, 87.8% of the MBES points were with 0.2 m, 9% were within 0.5 m, and the rest were within 1 m. The achieved accuracy is sufﬁcient for this study. It meets the International Hydrographic Organization (IHO) S-44 Standards for Hydrographic Surveys [31], where the horizontal accuracy is 2 m. In this study, we established the underwater terrain of the Zipingpu Reservoir using the MBES data and HIPS&SIPS software with a 2 m spatial resolution. For the ﬁrst time, the underwater terrain of the Zipingpu Reservoir has been revealed. We found that the terrain beneath the water surface was imaged well with detailed topographic features obtained by the MBES as shown in Figure5, which can show the reliability of the terrain. It is obvious that the MBES survey is more reliable, while the SBES survey only obtains the relatively discrete depth data along the track sketched as the black line in Figure5. Compared with the images of the same reaches taken in April 2011 from Google Earth, the 2D terrain images are consistent in the morphology of the shallow water zones (shown in the red circle in Figure6), such as terraces, gullies, overbanks, and so forth. Those consistencies suggest the high resolution, accuracy, and superior performance of the MBES in hydrographic surveys. WaterWater 2018 2018, ,10 10, ,x x FOR FOR PEER PEER REVIEW REVIEW 99 of of 17 17 ofof the the Zipingpu Zipingpu Reservoir Reservoir has has been been revealed. revealed. We We found found that that the the terrain terrain beneath beneath the the water water surface surface waswas imaged imaged well well with with detailed detailed topographic topographic features features obtained obtained by by the the MBES MBES as as shown shown in in Figure Figure 5, 5, whichwhich can can show show the the reliability reliability of of the the terrain. terrain. It It is is obvious obvious that that the the MBES MBES survey survey is is more more reliable, reliable, while while thethe SBES SBES survey survey only only obtains obtains the the relatively relatively discre discretete depth depth data data along along the the track track sketched sketched as as the the black black lineline in in Figure Figure 5. 5. Compared Compared with with the the images images of of the the same same reaches reaches taken taken in in April April 2011 2011 from from Google Google Earth,Earth, the the 2D 2D terrain terrain images images are are consistent consistent in in the the morphology morphology of of the the shallow shallow water water zones zones (shown (shown in in thethe redred circlecircle inin FigureFigure 6),6), suchsuch asas terraces,terraces, gulgullies,lies, overbanks,overbanks, andand soso forth.forth. ThoseThose consistenciesconsistencies suggestsuggest thethe highhigh resolution,resolution, accuracy,accuracy, andand superisuperioror performanceperformance ofof thethe MBESMBES inin hydrographichydrographic Water 2018, 10, 557 9 of 16 surveys.surveys.

FigureFigure 5.5. A AA map map showing showing the the underwater underwater terrain terrain of of th thethee ZipingpuZipingpu ReservoirReservoir andand thethe topographictopographic proﬁlesprofilesprofiles at at transects transects M3, M3, M13, M13, M19, M19, M28, M28, and and S1 S1 (M (M denotes denotes the the main main stream, stream, S S denotes denotes the the tributary tributary ShouxiShouxi River). River).

(A)(A)

(B)(B)

Figure 6. The comparison between the Google image (A) and the MBES terrain (B).

3.3. Characteristic Deposited Feature on the River Bed

3.3.1. Longitudinal Bed Profiles along the Thalweg The underwater channel thalweg is a preferential zone of the erosion and deposition. Axial thalweg erosion and deposition occur in the channel during the cycles of the water level lowstand and highstand. The longitudinal bed profile along the thalweg was quantified using sequential topographic data of the reservoir in March 2008, March 2011, and December 2012. The thalweg profiles Water 2018, 10, 557 10 of 16 and the slopes were analyzed. The configuration of the cross-sections measured in the reservoir is shown in Figure1 and Table4. The direct sediment supply to the Zipingpu Reservoir increased the axial thalweg deposition within the channel, resulting in the characteristic deposited features on the river bed.

Table 4. The distance from the dam to each cross-section on the main stream and the Shouxi River (M denotes the main stream, S denotes the tributary Shouxi River).

NO. Distance from the Dam (km) No. Distance from the Dam (km) No. Distance from the Dam (km) M1 0.23 M12 6.62 M23 12.73 M2 0.63 M13 6.95 M24 13.03 M3 1.05 M14 7.66 M25 13.37 M4 1.64 M15 8.21 M26 14.13 M5 2.29 M16 8.75 M27 14.61 M6 2.82 M17 9.38 M28 15.25 M7 3.57 M18 9.97 M29 15.81 M8 4.27 M19 10.41 M30 16.46 M9 4.97 M20 11.16 S1 13.15 M10 5.65 M21 11.92 S2 13.65 M11 6.21 M22 12.39 S3 14.15 S4 14.64

The longitudinal thalweg proﬁles evolve with time, as shown in Figure7, in which the thalweg elevation at each cross-section is plotted against the corresponding distance from the dam. The comparison of the two previous thalweg lines (March 2008 and March 2011) indicates that the thalweg proﬁle of March 2011 has been raised by an averaged value of 9.25 m, with a range of −6.70 m to 33.84 m along the main channel. At the reach between M3 (1.05 km) and M7 (3.57 km) close to the dam site, the elevation of the thalweg along its length increases from 4.45 m to 7.61 m, with an averaged value of 6.68 m. Between M8 (4.27 km) and M11 (6.21 km), the range of the thalweg increment is from 4.34 m to 5.98 m, with an average of 5.09 m. At the reach between M12 (6.62 km) and M14 (7.66 km), the thalweg increases by a maximum value of 33.84 m and the average of 27.32 m in the 1 km reach, where there are counter-slopes and steep slopes because the landslide took place on the left bank of the steep hill. This landslide volume is estimated to be 5.3 × 106 m3. As such large quantities of materials slid into the river channel, a stable natural natural barrier formed. From M15 (8.21 km) to M26 (14.13 km), the thalweg increases from 7.00 m to 13.40 m and it shows a stream-wise growth along its length. The thalweg of the reach between M26 (14.14 km) and M30 (16.46 km) has elevation increments ranging from 3.00 m to 3.95 m, with an average of 3.41 m. According to the information above,Water 2018 it, can 10, x be FOR concluded PEER REVIEW that in the upper reaches of M11 (6.21 km), the thalweg has greater elevation11 of 17 increments than those of the lower reach.

Figure 7. The evolution of the longitudinal bed proﬁlesprofiles along the thalweg.

Comparing the thalweg profile of 2012 to 2011, the averaged increment along the main channel is 3.78 m. The averaged increment of thalweg of the reach between M3 (1.05 km) to M7 (3.57 km) is 3.39 m. The increment ranges from −1.27 m to 1.64 m with average of −0.03 m between M8 (4.27 km) and M13 (6.95 km). In the reach between 7.65 km (M14) and 11.16 km (M20), the thalweg elevation increases from 3.02 m to 14.38 m, with an average of 7.89 m. The increment of thalweg between M21 (11.92 km) and M26 (14.13 km) ranges from 1.42 m to 8.35 m, with an average of 4.89 m. The thalweg of the reach at the reservoir tail has an increment varying from 0.88 m to 3.56 m. Via analyzing the increment of thalweg profiles, one may draw a conclusion that the variation mainly occurs at the upper reach of M11 (6.21 km). The average annual rate of increase in the thalweg elevation falls from 2 m/a (March 2008 to March 2011) to 1.17 m/a (March 2011 to December 2012) in the reach between M3 (1.05 km) and M10 (5.65 km). A similar trend could be noted in the upper reach. However, the thalweg along the reach between M14 (6.95 km) and M20 (11.16 km) can be characterized to be W-shaped. The upper reach between M20 (11.16 km) and M26 (14.13 km) has an almost zero slope.

3.3.2. Sectional Shape and Wetted Cross-Section Surface Loss Since there are remarkable changes in the thalweg profile, the river transects vary along the main stream. Figures 5 and 8 show the typical transects on the main stream and tributary (the profiles of all transects are shown in Figure S4), indicating that before the earthquake, the original morphology shape of the Zipingpu Reservoir channel presents as deep cutting V-shaped cross-sections. After the earthquake, the channel shows a greater variation in the bed morphology with U-shaped cross- sections. Figures 5 and 8 show that the bank slopes vary very little, while the deposition of the river bed increases. The bottom of each cross-section on the main stream rises and the main channel has been filled up to be level with sediment. However, compared to other reaches, the topographic changes of the cross-sections in the reach between M9 (4.97 km) and M13 (6.95 km) are different. There are few variations in the topography in this reach during March 2011–December 2012. To analyze the area changes of the cross-sections, here we define the area loss of the wetted cross-section surface as the flow area reduction due to the river bed lifting caused by sedimentation under a certain water level. Via analyzing all the topography of transects, it can be found that the main topographic changes took place under the water level of 850 m. The area loss of wetted cross- section surface under the water level of 850 m is calculated and shown in Figure 9. Compared with the area loss of March 2008–March 2011 and March 2011–December 2012, the highest area loss is 7270 m2 at M13 (6.95 km) during March 2008–March 2011, and 5491 m2 at M6 (2.85 km) during March 2011–December 2012. The average area loss during March 2011–December 2012 in the upstream cross section at M15 (8.21 km) is greater than that of March 2008–March 2011. Therefore, the upstream of

Water 2018, 10, 557 11 of 16

Comparing the thalweg proﬁle of 2012 to 2011, the averaged increment along the main channel is 3.78 m. The averaged increment of thalweg of the reach between M3 (1.05 km) to M7 (3.57 km) is 3.39 m. The increment ranges from −1.27 m to 1.64 m with average of −0.03 m between M8 (4.27 km) and M13 (6.95 km). In the reach between 7.65 km (M14) and 11.16 km (M20), the thalweg elevation increases from 3.02 m to 14.38 m, with an average of 7.89 m. The increment of thalweg between M21 (11.92 km) and M26 (14.13 km) ranges from 1.42 m to 8.35 m, with an average of 4.89 m. The thalweg of the reach at the reservoir tail has an increment varying from 0.88 m to 3.56 m. Via analyzing the increment of thalweg proﬁles, one may draw a conclusion that the variation mainly occurs at the upper reach of M11 (6.21 km). The average annual rate of increase in the thalweg elevation falls from 2 m/a (March 2008 to March 2011) to 1.17 m/a (March 2011 to December 2012) in the reach between M3 (1.05 km) and M10 (5.65 km). A similar trend could be noted in the upper reach. However, the thalweg along the reach between M14 (6.95 km) and M20 (11.16 km) can be characterized to be W-shaped. The upper reach between M20 (11.16 km) and M26 (14.13 km) has an almost zero slope.

3.3.2. Sectional Shape and Wetted Cross-Section Surface Loss Since there are remarkable changes in the thalweg profile, the river transects vary along the main stream. Figures5 and8 show the typical transects on the main stream and tributary (the profiles of all transects are shown in Figure S4), indicating that before the earthquake, the original morphology shape of the Zipingpu Reservoir channel presents as deep cutting V-shaped cross-sections. After the earthquake, the channel shows a greater variation in the bed morphology with U-shaped cross-sections. Figures5 and8 show that the bank slopes vary very little, while the deposition of the river bed increases. The bottom of each cross-section on the main stream rises and the main channel has been filled up to be level with sediment. However, compared to other reaches, the topographic changes of the cross-sections in the reach between M9 (4.97 km) and M13 (6.95 km) are different. There are few variations in the topography in this reach during March 2011–December 2012. To analyze the area changes of the cross-sections, here we define the area loss of the wetted cross-section surface as the flow area reduction due to the river bed lifting caused by sedimentation under a certain water level. Via analyzing all the topography of transects, it can be found that the main topographic changes took place under the water level of 850 m. The area loss of wetted cross-section surface under the water level of 850 m is calculated and shown in Figure9. Compared with the area loss of March 2008–March 2011 and March 2011–December 2012, the highest area loss is 7270 m2 at M13 (6.95 km) during March 2008–March 2011, and 5491 m2 at M6 (2.85 km) during March 2011–December 2012. The average area loss during March 2011–December 2012 in the upstream cross section at M15 (8.21 km) is greater than that of March 2008–March 2011. Therefore, the upstream of the main stream has been more seriously deposited during March 2011–December 2012. At the downstream of the main stream near the dam site, although the area loss mainly concentrates in the reach between M3 (1.05 km) and M8 (4.27 km) in both periods, the elevation of the bed along the river had a small increment due to the enlargement of the river and increment of the water depth (shown in Figures5 and7). It can be concluded that the general area loss trends for the two periods are similar along the reservoir, except for the reach between M11 (4.95 km) and M13 (6.95 km). Water 2018, 10, x FOR PEER REVIEW 12 of 17

the main stream has been more seriously deposited during March 2011–December 2012. At the downstream of the main stream near the dam site, although the area loss mainly concentrates in the reach between M3 (1.05 km) and M8 (4.27 km) in both periods, the elevation of the bed along the river had a small increment due to the enlargement of the river and increment of the water depth (shown Water 2018in Figures, 10, 557 5 and 7). It can be concluded that the general area loss trends for the two periods are similar12 of 16 along the reservoir, except for the reach between M11 (4.95 km) and M13 (6.95 km).

900 900 900 MAR 2008 MAR 2008 880 880 880 MAR 2011 MAR 2011 860 DEC 2012 860 DEC 2012 860 ）））））） m m m m m 840 m 840 M8 840 （（（ M8

（ M4 （ M4 （ 820 820 820 M14 800 800 800 Elevation

Elevation MAR 2008 Elevation Elevation

Elevation MAR 2008 780 Elevation 780 780 780 780 780 MAR 2011 760 760 760 DEC 2012 740 740 740 0 500 1000 1500 0 500 1000 1500 0 500 1000 1500 （）（）（） Distance （m） Distance （m） Distance （m） (A) (B) (C) 900 900 900 880 880 880 860 860 860 ）））））） m m m m m m 840 840 840 （（（（（（ 820 820 820 800 M15 800 M18 800 M24 Elevation Elevation Elevation MAR 2008 MAR 2008 Elevation Elevation Elevation MAR 2008 MAR 2008 MAR 2008 780 780 MAR 2011 780 MAR 2011 MAR 2011 MAR 2011 MAR 2011 DEC 2012 DEC 2012 760 DEC 2012 760 DEC 2012 760 DEC 2012 740 740 740 0 500 1000 1500 0 500 1000 1500 0 500 1000 1500 （）（）（） Distance （m） Distance （m） Distance （m） (D) (E) (F)

FigureFigure 8. The 8. The topographic topographic proﬁles profiles of of some some typicaltypical cross-sections. (A ()A M4;) M4; (B) (M8;B) M8; (C) (M14;C) M14; (D) M15; (D) M15; (E) M18; (F) M24. (E) M18;(E) M18; (F) M24. (F) M24. )

) 10000 2 10000 2 MAR 2011 - DEC 2012 MAR 2008 - MAR 2011 7500

5000

2500

0 Sectional area loss (m loss area Sectional Sectional area loss (m loss area Sectional 0 5 10 15 20 25 -2500 0 5 10 15 20 25 Transect number

FigureFigure 9. The 9. The wetted wetted cross-section cross-section surfacesurface loss loss under under the the water water level level of 850 of 850m. m.

3.3.3.3.3.3. Sedimentation Sedimentation Situation Situation ConsideringConsidering the the prevention prevention of of sediment, sediment, the oper operation,ation, and and the the management management of the of reservoir, the reservoir, the area with severe sedimentation must be located to ensure the security and reasonable operation the areathe witharea with severe severe sedimentation sedimentation must must be be located located to to ensure ensure thethe securitysecurity and and reasonable reasonable operation operation of of the reservoir. In this paper, we define the loss ratio of the wetted cross-section surface as the wetted the reservoir.of the reservoir. In this In paper, this paper, we deﬁnewe define the the loss loss ratio ratio of of the the wettedwetted cross-se cross-sectionction surface surface as the as wetted the wetted cross-section surface loss during 2008–2012 over the flow area of the cross-section under the water cross-section surface loss during 2008–2012 over the ﬂow area of the cross-section under the water level level of 850 m in 2008. According to the aforementioned results, the loss ratio of the wetted cross- of 850 m in 2008. According to the aforementioned results, the loss ratio of the wetted cross-section surface of each cross-section under the water level of 850 m was calculated and interpolated to the whole survey area as shown in Figure 10. Obviously, the most severe reach is between M12 (6.62 km) and M14 (7.66 km), where the natural barrier is located, and the loss ratio of the wetted cross-section surface exceeds 40%. From the dam site to M11 (6.21 km), the downstream section of the reservoir, the loss ratio of the wetted cross-section surface is lower than 10%. The reach, with a loss ratio of wetted cross-section surface between 10% and 20%, is located between M14 (7.66 km) and M18 (9.97 km). All the remaining reaches have severe depositions with a loss ratio of wetted cross-section surface higher than 20%. Therefore, along with the main stream, the natural barrier divided the reservoir into Water 2018, 10, x FOR PEER REVIEW 13 of 17

section surface of each cross-section under the water level of 850 m was calculated and interpolated to the whole survey area as shown in Figure 10. Obviously, the most severe reach is between M12 (6.62 km) and M14 (7.66 km), where the natural barrier is located, and the loss ratio of the wetted cross-section surface exceeds 40%. From the dam site to M11 (6.21 km), the downstream section of the reservoir, the loss ratio of the wetted cross-section surface is lower than 10%. The reach, with a loss ratio of wetted cross-section surface between 10% and 20%, is located between M14 (7.66 km) Water 2018and, 10M18, 557 (9.97 km). All the remaining reaches have severe depositions with a loss ratio of wetted13 of 16 cross-section surface higher than 20%. Therefore, along with the main stream, the natural barrier divided the reservoir into two sections, the upstream section with a loss ratio of wetted cross-section two sections,surface higher the upstream than 10%, sectionand the downstream with a loss sectio ration, of with wetted a loss cross-section ratio of wetted surface cross-section higher surface than 10%, and theless downstream than 10%. section, with a loss ratio of wetted cross-section surface less than 10%.

FigureFigure 10. The 10. distributionThe distribution of the of loss the ratio loss ofratio the of wetted the wetted cross-section cross-section surface surface in the in Zipingpu the Zipingpu Reservoir. Reservoir. 3.4. Storage Capacity 3.4. Storage Capacity Figure 11 shows the storage-capacity curve of the survey area of design, March 2008 (before the Figure 11 shows the storage-capacity curve of the survey area of design, March 2008 (before the WenchuanWenchuan Earthquake) Earthquake) and Decemberand December 2012. 2012. It is It obvious is obvious that that the storage-capacitythe storage-capacity curves curves of of the the design and Marchdesign 2008and almostMarch 2008 coincide almost and coincide seriously and deviate seriousl fromy deviate that offrom December that of 2012.December Via comparison2012. Via to the storage–capacitycomparison to the curve storage–capacity of March 2008 curve before of March the Wenchuan 2008 before Earthquake, the Wenchuan the storageEarthquake, capacity the loss ratiosstorage are found capacity to be loss less ratios than are 20% found at each to be depth. less th Thean 20% dead at storageeach depth. is 1.858 The ×dead10 8storagem3 with is 1.858 a loss × ratio of 15.28%108 m compared3 with a loss toratio the of designate 15.28% compared curve at to the the waterdesignate level curve of 817 at the m. water At the level ﬂood of 817 restricted m. At the water level offlood 850 restricted m, the storage water level capacity of 850 is m, 5.032 the ×storage108 m capacity3 and it is had 5.032 aloss × 10 ratio8 m3 and of 10.49%.it had a However,loss ratio of those are approximate10.49%. However, results those owing are to approximate the incomplete results survey owing and to imprecise the incomplete storage survey capacity and ofimprecise March 2008, storage capacity of March 2008, calculated by the cross-section method. However, it is still obvious calculated by the cross-section method. However, it is still obvious that most of the loss of the storage that most of the loss of the storage capacity occurred after the 2008 Wenchuan Earthquake, and mostly capacity occurred after the 2008 Wenchuan Earthquake, and mostly under the level of dead water. Waterunder 2018 the, 10 level, x FOR of PEER dead REVIEW water. 14 of 17

Capacity loss MAR 2008 - DEC 2012 (108 m3)

9000.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

880

860

840

820

800 Design Capacity Water level (m) 780 Capacity - MAR 2008 Capacity - DEC 2012 760 Capacity loss MAR 2008 - DEC 2012

740

0246810 8 3 Capacity (10 m )

FigureFigure 11. The11. The storage–capacity storage–capacity curve curve ofof thethe survey area area of of March March 2008 2008 (before (before the theWenchuan Wenchuan Earthquake)Earthquake) and and December December 2012. 2012.

4. Discussion In this paper, the results clearly demonstrate that the MBES method is suitable to provide a detailed map and visualization of a large reservoir. The accuracies and reliabilities of this method were checked by comparison with Google images and MBES terrain, and MBES data and SBES data. The MBES method also allowed for the survey in the shallow-water zone, which can hardly be mapped in detail (Figures 5 and 6), and it can accurately detect the features of the topography by covering a large area. This is important since the SEBS method only obtains the discrete depth data along the cross-sections. The higher the water level, the larger the reservoir area the MBES method can cover. This means that the MBES method should be employed at a high water level for the purpose of “full coverage” survey. Generally, the full coverage survey cannot be achieved because of the complex river terrain that the survey cannot reach. The MBES method cannot detect the terrain above the water level. However, this problem could be solved if we consider using LiDAR to detect both the shallow water zone and the terrain above water. In this case, we compared the two storage-capacity calculation methods and the computed results show a relative error ranging from 2.37% to 8.04%. The main source of this relative error was due to the cross-section method. This is because the cross section method straightens the centerline between the cross-sections along the reach and smooths the rough into a flat slope by generalizing the natural river into prismatoids with trapezoid bases. Another source of error is the quantity of the topographic data. The more data the survey obtains, the more detailed terrain we can establish and the more precise storage capacity result we can get. Given that the storage capacity curves are calculated by different calculation methods and bathymetric data, the design capacity and capacity of March 2008 should be less than the real reservoir capacity. The influences of the sedimentation on the reservoir storage can be considered negligible before March 2008. The capacity loss under the water level of 870 m should be larger than the calculated value due to the low capacity of March 2008 calculated by the cross-section method. Unfortunately, we cannot determine those capacities and the capacity loss values limited by the SBES data. Changes of longitudinal bed profiles along the thalweg are indicative of a temporal sequence of sedimentation in the Zipingpu Reservoir (Figure 7). In the reach between M11 (6.62 km) and M14 (7.66 km), the thalweg elevations dramatically rise during March 2008–March 2011 and remain constant during March 2011–December 2012. However, changes of the rest of the cross-sections differ. Their thalweg elevations keep rising during March 2008–December 2012. Similar results can be

Water 2018, 10, 557 14 of 16

4. Discussion In this paper, the results clearly demonstrate that the MBES method is suitable to provide a detailed map and visualization of a large reservoir. The accuracies and reliabilities of this method were checked by comparison with Google images and MBES terrain, and MBES data and SBES data. The MBES method also allowed for the survey in the shallow-water zone, which can hardly be mapped in detail (Figures5 and6), and it can accurately detect the features of the topography by covering a large area. This is important since the SEBS method only obtains the discrete depth data along the cross-sections. The higher the water level, the larger the reservoir area the MBES method can cover. This means that the MBES method should be employed at a high water level for the purpose of “full coverage” survey. Generally, the full coverage survey cannot be achieved because of the complex river terrain that the survey cannot reach. The MBES method cannot detect the terrain above the water level. However, this problem could be solved if we consider using LiDAR to detect both the shallow water zone and the terrain above water. In this case, we compared the two storage-capacity calculation methods and the computed results show a relative error ranging from 2.37% to 8.04%. The main source of this relative error was due to the cross-section method. This is because the cross section method straightens the centerline between the cross-sections along the reach and smooths the rough into a flat slope by generalizing the natural river into prismatoids with trapezoid bases. Another source of error is the quantity of the topographic data. The more data the survey obtains, the more detailed terrain we can establish and the more precise storage capacity result we can get. Given that the storage capacity curves are calculated by different calculation methods and bathymetric data, the design capacity and capacity of March 2008 should be less than the real reservoir capacity. The influences of the sedimentation on the reservoir storage can be considered negligible before March 2008. The capacity loss under the water level of 870 m should be larger than the calculated value due to the low capacity of March 2008 calculated by the cross-section method. Unfortunately, we cannot determine those capacities and the capacity loss values limited by the SBES data. Changes of longitudinal bed profiles along the thalweg are indicative of a temporal sequence of sedimentation in the Zipingpu Reservoir (Figure7). In the reach between M11 (6.62 km) and M14 (7.66 km), the thalweg elevations dramatically rise during March 2008–March 2011 and remain constant during March 2011–December 2012. However, changes of the rest of the cross-sections differ. Their thalweg elevations keep rising during March 2008–December 2012. Similar results can be concluded from Figures5,8 and9. Hence, the topographic changes of reach between M11 (6.62 km) and M14 (7.66 km) can mainly be generated by the landslide triggered by the earthquake, while the topographic changes of the remaining reaches are formed mainly due to the sediment inflow after the earthquake. It should be noted that this result lacks effective support of timely topographic survey after the earthquake, and the contribution of landslide and sediment inflow to sedimentation cannot be quantitatively distinguished.

5. Conclusions The underwater topographic terrain of the Zipingpu Reservoir is derived and revealed for the ﬁrst time. The inﬂuences of the earthquake on the Zipingpu reservoir are investigated and presented as follows: (1) A natural barrier is generated by a landslide, with a bed lifting of 33.84 m at 7.21 km upstream of the dam, and division of the reservoir channel into the upstream and downstream section. In the upstream section, the main channel is substantially silted by the sediment, and gradually changes from a V-shaped channel into a U-shaped channel. Besides, there are zero slopes and W-shaped slopes forms in the longitudinal direction along the thalweg in the upstream reservoir section; (2) The most important deposition area is the reach where the natural barrier located, with a loss ratio of wetted cross-section surface over 40%. The loss ratios of the wetted cross-section surface are lower than 10% in the downstream reservoir section, ranging from 10% to 20% in the reach between 7.66 km and 9.97 km, respectively, and higher than 20% in the remaining reaches in the upstream reservoir section; Water 2018, 10, 557 15 of 16

(3) The calculated results of the storage capacity curve of March 2008 and December 2012 show that the storage capacity loss ratios are 15.28% at the dead storage water level and 10.49% at the ﬂood limit water level owing to the sedimentation after the Wenchuan Earthquake.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4441/10/5/557/s1. Figure S1. The storage capacity calculation method. (A) Cross-section Method; (B) Triangle Prism Method; and (C) Triangulated Terrain between M13 and M14; Figure S2. The TIN terrain of Zipingpu Reservoir; Figure S3. The DEM and 3-dimensional visualization of the Zipingpu Reservoir. (A) The DEM of the reservoir; (B) The 3-dimensional visualization of the reservoir; Figure S4. The topographic profiles of all cross sections. Author Contributions: Zhong-luan Yan and Rui-dong An performed the experiments, analyzed the data and wrote the manuscript. Jia Li designed the experiments and revised the manuscript. Lei-lei Qin, Rui Wang, Xiao-ming Wang and Xi-liang Tang processed the data and revised the manuscript. The authors also thank the anonymous reviewers for their help in improving the scientific content of the manuscript. Acknowledgments: This research was supported by the National Natural Science Foundation of China (Grant No. 51579164 and 91547211), China Three Gorges Projects Development Co., Ltd. (Contract No. JG/18019B and JG/18020B) and Sichuan Province Zipingpu Development Co., Ltd. Conflicts of Interest: The authors declare no conflict of interest.

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