Numerical Simulation of the Salty Water Intrusion After the Construction of Houshan Reservoir, China

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1 Advanced Materials Research Submitted: ISSN: , Vols , pp Accepted: doi: / Online: Trans Tech Publications, Switzerland Numerical Simulation of the Salty Water Intrusion After the Construction of Houshan Reservoir, China XIE Mingxiao 1, a, XU Ting 1,b and CAI Yin 1,c 1 Key Laboratory of Engineering Sediment of Ministry of Transportation, Tianjin Research Institute for Water Transport Engineering, China a crabsaver@163.com, b shanshan.hhu@163.com, c caiyin@whu.edu.cn Keywords: Numerical simulation; Salty water intrusion; Reservoir;. Abstract. The seawater intrusion of the Gou River, China and the impact of Houshan Reservoir are studied using numerical simulations. The results show that the natural seawater intrusion length is 1.66km for Gou River estuary, and once the Houshan reservoir is constructed the intrusion length increases to 1.98km, which is 0.32km longer than the natural condition. At present, the highest salinity at the estuary is around 18, and recovers to fresh water environment in 3.0km. After the construction of the reservoir, the salinity value increase to 20 at the estuary. To around 2.0km from the estuary, the salinity recovers to the natural condition. Introduction The Gou River locates in the Suizhong County, Liaoning Province, China, and it directly flows to the western coast of Liaodong Bay without convergence of other branches. The length of the river is 86.7km, and the drainage area is 539km 2. At present, the utility percentage of the water resource for Gou River is only 17.2%, which can not meet the urgent requirement of the agricultural irrigation in the downstream especially in the dry seasons. Considering the above reason, the local government plans to construct a reservoir called the Houshan Reservoir with a storage capacity m 3 to better regulate the water resource, see Fig.1. The river length in the upstream of the reservoir is 47.9km, and the controlling area is 377km 2. The designed normal storage elevation of the reservoir is 134.0m, and the dead storage elevation is 107.0m. However, the construction of the reservoir significantly changes the hydrology condition in the downstream, especially reduces the flow discharge during wet seasons, and consequently the salty water from the coastal area could intrude from the estuary to the upstream bringing in the salinity environment variation in the estuarine area. Therefore, a comprehensive evaluation should be pre-made before the construction of Houshan reservoir. For the historical researches, the numerical simulation has been used as an effective tool to predict the salty water intrusion [1-4]. In this paper, a two-dimensional hydrodynamics and salinity diffusion model is applied, and the salinity distribution and intrusion length before and after the construction of Houshan Reservoir is simulated, respectively, and some discussions are made. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (# , Pennsylvania State University, University Park, USA-15/09/16,23:32:40)

2 Advanced Materials Research Vols SUIZHONG COUNTY, LIAONING Gou River Houshan Reservoir T3 7# LIAODONG T1 T2 3# 1# 4# 2# 6# Model domain 8# BAY Tidal level station Tidal current station Fig.1 Locations of the reservoir, model domain and in-situ measurement stations Numerical Model Establishment Model theory. The governing equations of the hydrodynamics follow the depth-averaged shallow water equations derived from the original three-dimensional Reynolds form, see Eq. 1 to Eq. 3. h hu hv + + = 0 t x y. (1) 2 2 hu hu huv η gh ρ sx bx + + = fv gh + + ht + t x y x 2ρ0 x ρ0 ρ0 x y ht 2 2 hv hvu hv η gh ρ sy by + + = fu gh + + htxy + t x y y 2ρ y ρ ρ x y ht ( xx ) ( xy ) ( ) ( yy ). (2). (3) where h is the total water depth; x and y are horizontal coordinates, respectively; η is the surface elevation; t is time; g is gravity acceleration; U and V are velocity components, respectively; f is the Coriolis force; ρ is the seawater density; E x and E y are horizontal eddy viscosity components, respectively; bx and by are bed shear stress components, respectively; T is the diffusion term. The governing equation of the salinity follows the form of advection-diffusion equation, see Eq. 4. S us vs S S t x x x x y y + + = Dh + Dh (4) where S is salinity, D h is the horizontal diffusion coefficient, Model validation. Fig. 2 presents the established model domain, and the upstream boundary of the model is set at, and the ocean open boundaries reach the -30m depth contour to the offshore (see Fig.1). The domain discretization applies the unstructured triangular mesh system. The upstream boundary condition is set to flow discharge, and the ocean boundaries are set to tidal levels. The hydrodynamics validation is based on the in-situ measurement datasets from representative spring, medium, and neap tidal cycles carried out during to (see Fig.1 for locations of the observation stations). Fig.3 gives the comparisons of measured and simulated tidal level, current speed, and current direction under the spring tide condition. For a shorter paragraph, readers

3 1072 Environmental Protection and Resources Exploitation II of interest could refer to Xie et al. [5] for more details. Through comparisons, the simulated hydrodynamic results agree well with that of the observed data, which prove the validity of the model. h(m) T2 芷锚湾 计算计 sim. 实测 obs v(m/s) 1.5 angle :00 14:00 20:00 2:00 8:00 t(h) 8:00 14:00 20:00 2:00 8:00 t(h) v(m/s) 7# angle 7# :00 14:00 20:00 2:00 8:00 t(h) 8:00 14:00 20:00 2:00 8:00 t(h) Fig.2 Comparisons of the measured and simulated tidal level, current speed, and current direction t(h) Numerical Modelling of the Salty Water Intrusion Variation of the intrusion length. Fig.3 compares the discharge values at at present and after the construction of the reservoir, and it indicate that the under the regulation of the reservoir, the downstream discharge decreases significantly in the wet season from July to September. Because the most sensitive month of the estuarine aquatic environment at the Suizhong coast is September, therefore in this paper the salinity distribution is simulated by the season of September. Discharge (m 3 /s) Before After Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Fig. 3 Annual discharge variation before and after the reservoir construction at Theoretically, the intrusion length of the salty water depends on the mutual interaction between the upstream discharge and the tidal level in the coastal area. In order to investigate the least favorable condition, the intrusion length is defined as the distance from the location where the velocity is close to zero to the estuary under the highest tidal level of the spring tide. Table 1 lists the variation of the discharge values and the intrusion lengths before and after the construction of the reservoir, and Fig.4 illustrates the current field structures under both conditions. After analysis, some conclusions are made as follows. (1) Because the river bed slope is steep for the Gou River (3.41 from to the estuary), generally the natural intrusion length is not large. Under the spring tide condition, the maximum intrusion length is only 1.66km in September. (2) After the construction of Houshan Reservoir, the discharge from the upstream decreases under the regulation effect. As a result, the sea water gets into the estuarine section of the river more easily, and then makes the intrusion length larger than the natural condition. From the simulation results, the intrusion length increases to 1.98km. Overall, after the construction of the Houshan Reservoir, because the upstream discharge gets smaller, the intrusion length of the salty water becomes 0.32km longer than the natural condition.

4 Advanced Materials Research Vols 大官帽闸 P=50% 建库前 BEFORE 建库前 海水上溯位置 顶托点 Intrusion point 狗河河口 (a) Before the construction of reservoir 大官帽闸 P=50% 建库后 AFTER 建库后 海水上溯位置 Intrusion 顶托点 point 狗河河口 (b) After the construction of reservoir Fig.4 Simulated current fields before and after the construction of reservoir (September) Table 1 Comparison of the estuarine discharges and intrusion lengths (September) Discharge at estuary (m 3 /s) Intrusion length (km) Before After Difference Before After Difference Variation of the estuarine salinity distribution. Once the coastal currents move into the river estuary, the salty water mixes with the discharged fresh water in the upstream, and then changes the salinity value in the estuarine area. According to the simulation results (September for instance also), the average salinity value distribution in the estuarine area is illustrated in Fig.5. Through discussions, some conclusions are made as follows. (1) For the present condition, the salinity has highest value at the estuary, which is around 18, and gradually decreases to the upstream until recovers to fresh water environment. The length of the brackish zone is approximately 3.0 km.

5 1074 Environmental Protection and Resources Exploitation II (2) After the construction of the reservoir, the salinity distribution feature remains the same as that of the natural condition only with the sanity values increase, and at the estuary the sanity value is 20. To around 2.0km from the estuary, the salinity recovers to the natural condition. Salinity (PSU) Before After Distance from estuary (km) Fig.5 Salinity distributions before and after the construction of reservoir (September) Summary By applying the numerical simulations, the salty water intrusion length of the Gou River, China is studied, and the impact of Houshan Reservoir is also evaluated. Through discussions, some conclusions are made as follows. (1) The natural seawater intrusion length is not large at the river estuary with only 1.66km, but after the construction of Houshan Reservoir, the discharge from the upstream decreases and makes the intrusion length increases to 1.98km, which is 0.32km longer than the natural condition. (2) For the present condition, the highest salinity at the estuary is around 18, and gradually recovers to fresh water environment in 3.0km. After the construction of the reservoir, the salinity distribution feature remains the same as that of the natural condition only with the sanity values increase, and at the estuary the sanity value is 20. To around 2.0km from the estuary, the salinity recovers to the natural condition. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Grant No ). References [1] N. Rahmawati, J. Vuillaume and I.L.S. Purnama. Journal of Hydrology, Vol. 494, No. 28 (2013), p [2] M. Cobaner, R. Yurtal. A. Dogan. and L.H. Motz. Journal of Hydrology, Vol. 464, No. 25 (2012), p [3] Z. Ren, Q. Chen. and X. Zhao. China Environmental Science, Vol. 33, No. S1 (2013), p [4] M. Xie, T. Xu. and Y. Cai. Report of Tianjin Research Institute for Water Transport Engineering, China (2014). [5] C. Lu, L. Yuan, S. Gao, R.Chen and B. Su. Advances in Water Science, Vol. 24, No. 2 (2013), p