12th International Conference on Hydroscience & Engineering Hydro-Science & Engineering for Environmental Resilience November 6-10, 2016, Tainan, .

Assessment of Dam Removal in Dajia River Basin with Numerical Modeling

Keh-Chia Yeh1, Yen-Yu Chiu1, Chung-Ta Liao1, Wei-Cheng Lo2, Jiing-Yun You3, Dong-Sin Shih4 1Department of Civil 2Department of Hydraulic and 3Department of Civil 4Department of Civil Engineering, National Chiao Ocean Engineering, National Engineering, National Chung Engineering, National Taiwan Tung University Cheng Kung University Hsing University University Hsinchu, Taiwan Tainan, Taiwan , Taiwan Taipei, Taiwan

ABSTRACT numerical simulation. Numerical simulation has the advantages of time saving and high efficiency, which facilitate application to river During flood periods in Taiwan, a substantial amount of sediment is engineering projects. Strictly speaking, Flows necessitate 3-D created in river basins, often causing deposition on riverbeds, especially simulation. However, 3-D simulations must represent the in the impounding regions of dams and weirs. Dams, weirs, and river- diversification of each factor in the 3-D space, which not only causes crossing structures cause substantial local scouring in mid-downstream the analysis to be considerably complex but also necessitates more time areas. Some of these structures progressively lose function and and techniques for calculation. Most vast territorial waters such as lakes, approach their service life limitation. Therefore, for the purpose of river mouths, and river segments typically have larger horizontal environmental restoration, structure removal is an option for dimensions than vertical dimensions; hence, the flow field can be management and administration. represented by the depth-averaged value. Therefore, 2-D hydraulic models are sufficient to simulate them. Compared with 1-D models, This research used Dajia River as a study site and analyzed the effect which regard flow as one dimension and simply use the cross-section- on morphology of removing Shihgang Dam, which is located in the averaged velocity to simulate cases quickly and roughly, 2-D models middle of Dajia River. First, the study area and dam are introduced. are relatively detailed and require longer calculation time and higher- Second, the practical application of this study is described. resolution terrain data. According to the demands of different cases, 1D, Subsequently, 1-D and 2-D numerical sediment models used to 2D, or 3D hydraulic numerical models should be chosen. simulate the adjacent reach of Shihgang Dam and the effect of dam removal are presented. Finally, the local scouring near the piers of This study applied 1D, 2D, and 3D models to analyze an engineering Dongshih Bridge observed through 3-D simulation is described. project involving removal of a dam from a river. The models address sediment supplies associated with bed erosion to estimate sediment KEY WORDS: Dam removal, Numerical model, Dajia River, yield during storm events. The difference between retaining and Shihgang Dam removing a dam (Shihgang Dam) was determined. Bed migration was analyzed at large and small scales through models with different INTRODUCTION dimensions.

Dam removal is a contemporary concern that has been discussed since CASE STUDY–DAJIA RIVER the 1970s. Dams negatively affect the aquatic environments of rivers that they are located on; however, these effects are often balanced by Dajia River, located in north-central Taiwan, originates in Hsuehshan the purpose of the dam, such as hydroelectric power production or and Nanhu Mountain in the and flows for 124 flood control. In the United States, roughly 900 dams have been km, traveling through Taichung City before emptying into the Taiwan removed since the 1990s, with 50 to 60 more being removed each year. Strait. Techi Reservoir and a sequence of five dams on Dajia River In Taiwan, Chijiawan Dam was the first to be removed. produce up to 1,100 mW of hydroelectric power and generate more than 2.4 billion kWh per year. They provide municipal water, generates Because of Taiwan’s topography, rivers are steeply sloped and surface hydroelectric power, prevent flooding, and are used for recreation. In runoff is carried to the ocean at high speed with large discharges. sequence, beginning with the highest, the five dams are Qingshan Dam, Consequently, the effects of dam removal must be analyzed carefully. Kukuan Dam, Tienlun Dam, Ma'an Dam, and Shihgang Dam. The locations of these structures are shown in Fig. 1. The methods for studying water flow and sediment in rivers can be classified into three groups: theoretical analysis, physical modeling, and Water resource utilization is the main function of Shihgang Dam. After 12th International Conference on Hydroscience & Engineering Hydro-Science & Engineering for Environmental Resilience November 6-10, 2016, Tainan, Taiwan. the 921 earthquake in 1999, the structure of the dam was partly with sediment transport. The finite element transformation enables the damaged. Although repaired over the years, the dam is located on a model to be applied to cases with complex natural geometric and fault zone and hence safety is still a concern. Shihgang Dam serves as a topographic domains. The advantages of this model are the small-scale dividing point between scouring and siltation. The riverbed in the lower near field, detailed flows, sediment transport analyses, and engineering reaches of Dajia River, especially downstream of Shihgang Dam, applications. exhibits a constant scouring trend, whereas the upstream reach displays a depositing trend. These phenomena not only are caused by floods but PRACTICAL APPLICATION also are results of the 921 earthquake. The dam may negatively affect the river environment because it blocks the transfer of sediment from The three models were calibrated and verified to ensure accuracy by the upstream to the downstream reaches; consequently, the bridges and using historical floods in 2009–2013. Of these floods, that during the dikes located downstream are endangered by the aggravated Typhoon Soulik (2013) brought the largest discharge to Dajia River. scouring nearby piers and banks. For environmental restoration, The flood peak at Shihgang Dam was approximately 7,000 cm, which structure removal is an option for management and administration that was close to the 20-year return period (Q20). Fig. 2 depicts the requires numerous tools for thoroughly analyzing flood control and discharge and water level hydrograph during Typhoon Soulik. safety. Structures (bridge and dam), the bed material distribution, and the sediment flux from the upstream reach of the Dajia River were NUMERICAL MODELS FOR SIMULATING FLOW AND considered in the models. The computed results with two scenarios SEDIMENT (dam retained and dam removed) for Typhoon Soulik (Q20) are discussed in the following sections. Three models were applied in this study. First, for the 1-D model, the 1- D module of WASH123D (Watershed Systems of 1-D Stream-River The 1-D Module of WASH123D model Network, 2-D Overland Regime, and 3-D Subsurface Media) was adopted because of its ability to simulate flow and sediment transport. The 1-D WASH123D module was applied to estimate the sediment This model has been applied to several projects including the transformation of a 22 km long area from Tienlun Dam to the estuary. Comprehensive Everglades Restoration Project and Biscayne Bay Fig. 3 illustrates the difference between removing and retaining Coastal Wetland. Shihgang Dam for the event (Q20). The longitudinal difference of the bed level is depicted. After dam removal, sediment upstream from As the 2-D model, ARAM-2D (Alluvial River-Movable Bed-2-D Shihgang Dam has an increased tendency to be transported downstream. Model) was employed. Governing equations are depth-averaged According to the simulated scenarios, the elevation between Houfeng continuity and momentum equations for computational fluid dynamics. Bridge and Shihgang Dam is 0.34 m higher than it is when the dam is The MacCormack explicit finite-difference method was adopted as its retained. In addition, the bed level between Shihgang Dam and numerical scheme. Dongshih Bridge is 0.24 m lower in the removal scenario. The result indicates that the problems of scouring in the lower reaches and Finally, the CCHE3D model was chosen as the 3-D model. It is a deposits upstream of Shihgang Dam might be moderated as expected. software package designed for simulating free surface turbulent flows

Fig. 1 . Locations of structures at Dajia River

12th International Conference on Hydroscience & Engineering Hydro-Science & Engineering for Environmental Resilience November 6-10, 2016, Tainan, Taiwan.

Fig. 2 Discharge and water level hydrograph of Typhoon Soulik

Fig. 4 Result of riverbed change after Typhoon Soulik

Fig. 3. Longitudinal difference of bed level between dam retention and dam removal

ARAM-2D

The ARAM-2D model was applied to analyze the reaches within 2 km Fig.5 Local scouring of a pier of Dingshih Bridge of Shihgang Dam. In the Shihgang Dam removal scenario, the bed morphology after Typhoon Soulik was changed dramatically in a short period of time. Silt was stored in the impounding region of the dam before transport and then deposited downstream where bedrock was exposed. This result implies that the removal of Shihgang Dam would ensure the continuity of transport and protect the exposed bedrock area. Moreover, the silt in the dam area could be delivered naturally, without necessitating dredging (Fig. 4).

CCHE 3D

The 3-D flow field and local scouring of bridge piers are standard 3-D topics. Because of the limitations of field data recording, Dingshih Bridge, located 6.5 km upstream from Shihgang Dam, is the structure that was studied. Each pier of this bridge is consist of 4 pile shown in Fig. 6 Local scouring distribution of Dingshih Bridge Fig. 5. The effects of the bridge piers, were accounted for in the simulation. Fig. 5 illustrates how local scouring appeared on the streambed nearby the piers after the flood event. Sediment accumulated CONCLUSIONS in front of the first pile of each pier because of stagnation. This phenomenon was evident on the middle piers in the main channel and Flow and sediment numerical models are convenient and cost-effective appeared on all piers (Fig. 6). tools for analyzing flood processes and designing engineering projects. Simulation of the effect of Typhoon Soulik on Dajia River by using WASH123D, ARMB2D, and CCHE3D provided abundant information regarding phenomena such as the flow depth, velocity, and sediment transport processes. With these results, the morphology after dam

12th International Conference on Hydroscience & Engineering Hydro-Science & Engineering for Environmental Resilience November 6-10, 2016, Tainan, Taiwan. removal could be assessed. The simulations revealed that concerns Chen S.C and Chao Y.C. (2010). Locations and orientations of large regarding the exposed rock bed in the downstream channel would be woody debris in Chichiawan Creek. Interprevent. International addressed and the silt in the dam region would be transported to the Symposium in Pacific Rim, Taipei, Taiwan, April 26–30. lower reaches and deposited. In addition, local scouring on the piers of Ding, Y. and Wang, S.S.Y. (2012). Optimal control of flood diversion Dongshih Bridge was simulated using CCHE3D and the changes in watershed using nonlinear optimization. Advances in Water around the piles of the piers were illustrated. By combining the results Resources, 44, 30-48. of each model, the changes in the morphology of Dajia River after Jia, Y. and Wang, S.S.Y. (1997), CCHE2D model verification tests storms can be predicted in large and small scales. Thus, the applied documentation. Technical Report CCHE TR-4, Center for models are useful for estimating the change of riverbeds and managing Computational Hydroscience and Engineering, The University of rivers. Mississippi. Jia, Y., and Wang, S. S. (1999). Numerical model for channel flow and ACKNOWLEDGMENTS morphological change studies. Journal of Hydraulic Engineering, 125(9), 924-933. This study was supported by the Ministry of Science and Technology Jia, Y., Wang, S. and Xu, Yichen (2002). Validation And Application (103-2625-M-009-001). The authors gratefully acknowledge the Of A 2D Model To Channels With Complex Geometry. International support. Journal Of Computational Engineering Science, Vol. 3, No.1 (March 2002), Pages 57-71. REFERENCES Jia, Y., Chao, X.B., and Zhu, T.T. (2013), CCHE3D Technical Manual. NCCHE TR-01-2013, National Center for Computational Hydroscince

and Engineering, University of Mississippi, MS 38677. Chang, H.H., (2008). Case study of fluvial modeling of river responses Tsai, C. H. and Tsai, C. T. (2000). Velocity and concentration to dam removal. Journal of Hydraulic Engineering, ASCE, 134(3), distributions of sediment-laden open channel flow. Journal of the 295–302. American Water Resources Association, Vol. 36, No. 5, pp. 1075-1086. Chen, C. N., Yeh, C. H., Tasi, C. H. and Tasi, C. T. (2002). The Valiani, A., Caleffi, V., and Zanni, A. (2002). Case Study: Malpasset simulation of disharge and suspended sediment concentration Dam-Break Simulation using a Two-Dimensional Finite Volume hydrographs in river basin. Proceeding of the 5th Taiwan-Japan Joint Method. Journal of Hydraulic Engineering, ASCE, 28:5(460), 460-472 Seminar on Natural Hazards Mitigation, Tainan, Taiwan, pp. 99-108.