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 1. Department of Civil Engineering, National Chiao Tung University Hsinchu, Taiwan 2. Department of Hydraulic and Ocean Engineering, National Cheng Kung University Tainan, Taiwan 3. Department of Civil Engineering, National Chung Hsing University , Taiwan 4. Department of Civil Engineering, National Taiwan University Taipei, Taiwan

ABSTRACT saving and high efficiency, which facilitate application to river engineering projects. Strictly speaking, Flows necessitate 3-D During flood periods in Taiwan, a substantial amount of sediment is simulation. However, 3-D simulations must represent the created in river basins, often causing deposition on riverbeds, especially diversification of each factor in the 3-D space, which not only causes in the impounding regions of dams and weirs. Dams, weirs, and river- the analysis to be considerably complex but also necessitates more time crossing structures cause substantial local scouring in mid-downstream and techniques for calculation. Most vast territorial waters such as lakes, areas. Some of these structures progressively lose function and river mouths, and river segments typically have larger horizontal approach their service life limitation. Therefore, for the purpose of dimensions than vertical dimensions; hence, the flow field can be environmental restoration, structure removal is an option for represented by the depth-averaged value. Therefore, 2-D hydraulic management and administration. models are sufficient to simulate them. Compared with 1-D models, which regard flow as one dimension and simply use the cross-section- This research used Dajia River as a study site and analyzed the effect averaged velocity to simulate cases quickly and roughly, 2-D models on morphology of removing Shihgang Dam, which is located in the are relatively detailed and require longer calculation time and higher- middle of Dajia River. First, the study area and dam are introduced. resolution terrain data. According to the demands of different cases, 1D, Second, the practical application of this study is described. 2D, or 3D hydraulic numerical models should be chosen. Subsequently, 1-D and 2-D numerical sediment models used to simulate the adjacent reach of Shihgang Dam and the effect of dam This study applied 1D, 2D, and 3D models to analyze an engineering removal are presented. Finally, the local scouring near the piers of project involving removal of a dam from a river. The models address Dongshih Bridge observed through 3-D simulation is described. sediment supplies associated with bed erosion to estimate sediment yield during storm events. The difference between retaining and KEY WORDS: Dam removal, Numerical model, Dajia River, removing a dam (Shihgang Dam) was determined. Bed migration was Shihgang Dam analyzed at large and small scales through models with different dimensions. INTRODUCTION CASE STUDY–DAJIA RIVER Dam removal is a contemporary concern that has been discussed since the 1970s. Dams negatively affect the aquatic environments of rivers Dajia River, located in north-central Taiwan, originates in Hsuehshan that they are located on; however, these effects are often balanced by and Nanhu Mountain in the and flows for 124 the purpose of the dam, such as hydroelectric power production or km, traveling through Taichung City before emptying into the Taiwan flood control. In the United States, roughly 900 dams have been Strait. Techi Reservoir and a sequence of five dams on Dajia River removed since the 1990s, with 50 to 60 more being removed each year. produce up to 1,100 mW of hydroelectric power and generate more In Taiwan, Chijiawan Dam was the first to be removed. than 2.4 billion kWh per year. They provide municipal water, generate hydroelectric power, prevent flooding, and are used for recreation. In Because of Taiwan’s topography, rivers are steeply sloped and surface sequence, beginning with the highest, the five dams are Qingshan Dam, runoff is carried to the ocean at high speed with large discharges. Kukuan Dam, Tienlun Dam, Ma'an Dam, and Shihgang Dam. The Consequently, the effects of dam removal must be analyzed carefully. 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 numerical simulation. Numerical simulation has the advantages of time the 921 earthquake in 1999, the structure of the dam was partly damaged. 12th International Conference on Hydroscience & Engineering Hydro-Science & Engineering for Environmental Resilience November 6-10, 2016, Tainan, Taiwan.

Although repaired over the years, the dam is located on a fault zone and model to be applied to cases with complex natural geometric and hence safety is still a concern. Shihgang Dam serves as a dividing point topographic domains. The advantages of this model are the small-scale between scouring and siltation. The riverbed in the lower reaches of Dajia near field, detailed flows, sediment transport analyses, and engineering River, especially downstream of Shihgang Dam, exhibits a constant applications. scouring trend, whereas the upstream reach displays a depositing trend. These phenomena not only are caused by floods but also are results of the PRACTICAL APPLICATION 921 earthquake. The dam may negatively affect the river environment because it blocks the transfer of sediment from the upstream to the The three models were calibrated and verified to ensure accuracy by downstream reaches; consequently, the bridges and the dikes located using historical floods in 2009–2013. Of these floods, that during downstream are endangered by the aggravated scouring nearby piers and Typhoon Soulik (2013) brought the largest discharge to Dajia River. banks. For environmental restoration, structure removal is an option for The flood peak at Shihgang Dam was approximately 7,000 cm, which management and administration that requires numerous tools for was close to the 20-year return period (Q20). Fig. 2 depicts the thoroughly analyzing flood control and safety. discharge and water level hydrograph during Typhoon Soulik. Structures (bridge and dam), the bed material distribution, and the NUMERICAL MODELS FOR SIMULATING FLOW AND sediment flux from the upstream reach of the Dajia River were SEDIMENT considered in the models. The computed results with two scenarios (dam retained and dam removed) for Typhoon Soulik (Q20) are Three models were applied in this study. First, for the 1-D model, the 1- discussed in the following sections. D module of WASH123D (Watershed Systems of 1-D Stream-River Network, 2-D Overland Regime, and 3-D Subsurface Media) was The 1-D Module of WASH123D model adopted because of its ability to simulate flow and sediment transport. This model has been applied to several projects including the The 1-D WASH123D module was applied to estimate the sediment Comprehensive Everglades Restoration Project and Biscayne Bay transformation of a 22 km long area from Tienlun Dam to the estuary. Coastal Wetland. Fig. 3 illustrates the difference between removing and retaining Shihgang Dam for the event (Q20). The longitudinal difference of the As the 2-D model, ARAM-2D (Alluvial River-Movable Bed-2-D bed level is depicted. After dam removal, sediment upstream from Model) was employed. Governing equations are depth-averaged Shihgang Dam has an increased tendency to be transported downstream. continuity and momentum equations for computational fluid dynamics. According to the simulated scenarios, the elevation between Houfeng The MacCormack explicit finite-difference method was adopted as its Bridge and Shihgang Dam is 0.34 m higher than it is when the dam is numerical scheme. retained. In addition, the bed level between Shihgang Dam and Dongshih Bridge is 0.24 m lower in the removal scenario. The result Finally, the CCHE3D model was chosen as the 3-D model. It is a indicates that the problems of scouring in the lower reaches and software package designed for simulating free surface turbulent flows deposits upstream of Shihgang Dam might be moderated as expected. with sediment transport. The finite element transformation enables the

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 Fig.5 Local scouring of a pier of Dingshih Bridge

The ARAM-2D model was applied to analyze the reaches within 2 km 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 Fig. 6 Local scouring distribution of Dingshih Bridge 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 CONCLUSIONS Fig. 5. The effects of the bridge piers, were accounted for in the simulation. Fig. 5 illustrates how local scouring appeared on the Flow and sediment numerical models are convenient and cost-effective streambed nearby the piers after the flood event. Sediment accumulated tools for analyzing flood processes and designing engineering projects. in front of the first pile of each pier because of stagnation. This Simulation of the effect of Typhoon Soulik on Dajia River by using phenomenon was evident on the middle piers in the main channel and WASH123D, ARMB2D, and CCHE3D provided abundant information appeared on all piers (Fig. 6). regarding phenomena such as the flow depth, velocity, and sediment transport processes. With these results, the morphology after dam removal could be assessed. The simulations revealed that concerns

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

regarding the exposed rock bed in the downstream channel would be Ding, Y. and Wang, S.S.Y. (2012). Optimal control of flood diversion addressed and the silt in the dam region would be transported to the in watershed using nonlinear optimization. Advances in Water lower reaches and deposited. In addition, local scouring on the piers of Resources, 44, 30-48. Dongshih Bridge was simulated using CCHE3D and the changes Jia, Y. and Wang, S.S.Y. (1997), CCHE2D model verification tests around the piles of the piers were illustrated. By combining the results documentation. Technical Report CCHE TR-4, Center for of each model, the changes in the morphology of Dajia River after Computational Hydroscience and Engineering, The University of storms can be predicted in large and small scales. Thus, the applied Mississippi. models are useful for estimating the change of riverbeds and managing Jia, Y., and Wang, S. S. (1999). Numerical model for channel flow and rivers. morphological change studies. Journal of Hydraulic Engineering, 125(9), 924-933. ACKNOWLEDGMENTS Jia, Y., Wang, S. and Xu, Yichen (2002). Validation And Application Of A 2D Model To Channels With Complex Geometry. International This study was supported by the Ministry of Science and Technology Journal Of Computational Engineering Science, Vol. 3, No.1 (March (103-2625-M-009-001). The authors gratefully acknowledge the 2002), Pages 57-71. support. Jia, Y., Chao, X.B., and Zhu, T.T. (2013), CCHE3D Technical Manual. NCCHE TR-01-2013, National Center for Computational REFERENCES Hydroscince and Engineering, University of Mississippi, MS 38677. Tsai, C. H. and Tsai, C. T. (2000). Velocity and concentration distributions of sediment-laden open channel flow. Journal of the Chang, H.H., (2008). Case study of fluvial modeling of river responses American Water Resources Association, Vol. 36, No. 5, pp. 1075- to dam removal. Journal of Hydraulic Engineering, ASCE, 134(3), 1086. 295-302. Valiani, A., Caleffi, V., and Zanni, A. (2002). Case Study: Malpasset Chen, C. N., Yeh, C. H., Tasi, C. H. and Tasi, C. T. (2002). The Dam-Break Simulation using a Two-Dimensional Finite Volume simulation of disharge and suspended sediment concentration Method. Journal of Hydraulic Engineering, ASCE, 28:5(460), 460- hydrographs in river basin. Proceeding of the 5th Taiwan-Japan Joint 472. Seminar on Natural Hazards Mitigation, Tainan, Taiwan, pp. 99-108.

Chen S.C and Chao Y.C. (2010). Locations and orientations of large

woody debris in Chichiawan Creek. Interprevent. International Symposium in Pacific Rim, Taipei, Taiwan, April 26-30.