A quasi-2D and quasi-steady hydraulic model for physical habitat simulations Sung-Uk Choi,* Sanghwa Jung and Seung Ki Kim Yonsei University, Department of Civil and Environmental Engineering, Seoul, Korea ABSTRACT This study presents a new hydraulic model for simulating the physical habitat of a river. The model is based on the quasi-steady assumption that the flow is steady, but the channel morphology changes during the computational time step. The model is also capable of simulating flow distribution in the lateral direction, making the model quasi-two-dimensional. The physical habitat simulations are carried out for a reach including a bend in the Dal River in Korea. Zacco platypus is used as the target fish. The habitat suitability index model for Zacco platypus, based on field monitoring, is used for the habitat simulation. The hydraulic model simulates morphological changes caused by a flood and the result is compared with the two-dimensional model. Simulation results obtained using the developed model compares favorably to calculations obtained using the two-dimensional model. Distributions of the habitat suitability index are provided for various flows by the present model. It is found that the morphological change by the flood increases the habitat suitability significantly by decreasing the flow depth and increasing the velocity in the study reach. In addition, the time change of the weighted usable area based on the mobile-bed computation is given, indicating that the flood decreases the habitat suitability seriously. Copyright © 2014 John Wiley & Sons, Ltd. KEY WORDS physical habitat simulation; quasi-steady model; quasi-two-dimensional model; morphological change; habitat suitability index; weighted usable area Received 3 February 2014; Revised 23 April 2014; Accepted 24 April 2014 INTRODUCTION A physical habitat simulation is a numerical procedure that relates changes in flow to changes in physical habitat availability in a river. Aquatic habitat simulation models have been utilized since the 1970s. In 1981, US Fish and Wildlife Service introduced Physical Habitat Simulation System (PHABSIM), and PHABSIM and similar models have since been used widely. A physical habitat simulation is based on the idea that the abundance of a certain species in a river can be given by the value of the habitat (Maddock, 1999; Milhous, 1999). The value of the habitat is now the product of habitat quantity and quality. Normally, the surface area of a river reach and the suitability of the target species serve as habitat quantity and quality, respectively. The procedure used to simulate physical habitat is comprised of two parts, namely, hydraulic simulation and habitat simulation. Hydraulic simulation provides information regarding water surface elevation and velocity for a certain discharge. With prior information on suitability or preference of the target species, a habitat simulation provides habitat values such as the Weighted Usable Area (WUA). A physical habitat simulation can be useful in water management to assist in the establishment of instream flow requirements to support water control and water allocation activities (Williams, 1996; Maddock, 1999; Parasiewicz, 2003). The hydraulic model for simulating physical habitat can be divided into three groups depending on the number of spatial dimensions the model covers, namely, 1D, 2D, and 3D models. The 1D model is a cross section-averaged model that considers the velocity only in the streamwise direction. This model is useful in river modeling because of the geometric feature of the river, that is, dimensional disparity in three directions. Physical habitat modelings in which 1D hydraulic computations have been used include Moir et al. (2005), Tomsic et al. (2007), and Almeida and Rodriguez (2009). The 2D model is a depth-averaged model that deals with two-directional flow in a horizontal plane. Such a model is useful if the flow can be assumed to be uniform in the depth direction. Examples of the use of 2D hydraulic computations in physical habitat modeling include studies reported by Ghanem et al. (1996), He et al. (2006), Yi et al. (2010), and Im et al. (2011). The 3D model considers all three velocity components and does not involve governing equations, neither averaged over the *Correspondence to: Sung-Uk Choi, Yonsei University, Department of Civil and Environmental Engineering, Seoul, Korea. E-mail: schoi@yonsei.ac.kr ECOHYDROLOGY Ecohydrol. 8, 263–272 (2015) Published online 30 May 2014 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/eco.1504 Copyright © 2014 John Wiley & Sons, Ltd.