SRH stands for Sedimentation and River Hydraulics. It refers to a family of numerical models developed mainly by the U.S. Bureau of Reclamation, with long-term technical collaboration and application development involving Taiwan’s Water Resources Agency. The SRH modeling system is designed to address hydraulic, sediment transport, reservoir, coastal, and watershed-related problems.
Development Background
The SRH model family has been led by Dr. Yong G. Lai of the U.S. Bureau of Reclamation. To address Taiwan’s water resources challenges, including limited water availability, steep river systems, high sediment loads, and riverbed stability issues, the Water Resources Agency began cooperation with the Bureau of Reclamation in 2006 under the Taiwan–U.S. water resources technical support framework.
Through this cooperation, SRH models have been tested and expanded using Taiwan’s river conditions as important case studies. The collaboration has supported technology transfer, model improvement, field application, and the development of modules that are more suitable for Taiwan’s hydrological and geomorphic environment. Related research and applications have also been presented in international journals and conferences.
Applications and Integrated Development
After many years of development, SRH-2D has been widely used by government agencies, universities, and engineering consulting firms in Taiwan and abroad. In the United States, SRH-2D has also been connected with the Surface Water Modeling System (SMS) platform and used in hydraulic analysis related to river crossings, bridge scour, and floodplain studies.
To broaden the application of the SRH modeling system, the Water Resources Agency and the Bureau of Reclamation have continued to develop related tools for watershed rainfall-runoff and soil erosion analysis, coastal and estuarine hydraulics, and sediment transport. These developments have contributed to a more integrated modeling framework, including SRH-Watershed, SRH-Coast, and SRH-ONE, which aim to support whole-basin analysis from watershed runoff to river hydraulics, sediment movement, and coastal processes.
Major SRH Model Types
SRH-1D
SRH-1D is a one-dimensional mobile-boundary hydraulic and sediment transport model. It can simulate steady and unsteady flow conditions, internal boundary conditions, river networks, cohesive and non-cohesive sediment transport, bank erosion, and lateral inflow.
The model uses river cross-section data to estimate changes in river channels caused by sediment movement. With given sediment inflow, bed material, hydrological conditions, and hydraulic boundary conditions, it can estimate sediment concentration and channel response along a river reach. SRH-1D uses a finite-difference numerical method.
SRH-2D
SRH-2D is the most widely applied model in the SRH family. It is a two-dimensional river hydraulics and sediment transport model developed to meet practical needs in river engineering and mobile-bed simulation.
SRH-2D solves the two-dimensional dynamic wave equations, also known as the depth-averaged Saint-Venant equations. It can simulate subcritical flow, supercritical flow, mixed-flow transitions, steady flow, and unsteady flow. The model uses a finite-volume numerical method and can work with structured, unstructured, or hybrid computational meshes.
After being introduced and applied in Taiwan, SRH-2D was further expanded to better represent local river conditions. Important modules include soft-rock erosion, bank retreat, density currents in reservoirs, additional sediment transport formulas, hydraulic structures, bridge hydraulics, pressure flow, and bridge scour analysis. These functions make SRH-2D useful for complex river environments, especially in Taiwan’s steep, sediment-rich, and highly variable river systems.
The major SRH-2D modules include hydraulic simulation, sediment transport, vertical erosion of soft rock, bank retreat, reservoir density currents, in-stream hydraulic structures, pressure flow, bridge scour, watershed modeling, and coastal or estuarine modeling.
SRH-3D
SRH-3D is a three-dimensional hydrostatic model developed to simulate river and reservoir hydraulics. It solves three-dimensional mass and momentum conservation equations and is designed to address situations where vertical flow structure and density effects are important.
The model assumes hydrostatic pressure distribution, applies the Boussinesq approximation, and treats the water-sediment mixture as incompressible. Its vertical grid system is designed to respond to the predicted bed elevation, free water surface, and density interface. This helps reduce limitations found in traditional sigma-coordinate and z-coordinate grid systems, especially when modeling density currents, near-bed sediment transport, and bed shear stress.
U2RANS
U2RANS is a high-resolution three-dimensional model that solves unsteady Reynolds-averaged Navier-Stokes equations using structured, unstructured, or hybrid three-dimensional meshes. It is suitable for detailed hydraulic problems involving complex geometry, such as hydraulic structures, fishways, bends, and local flow patterns around engineering facilities.
Because U2RANS is mainly used for fixed-bed simulations, it does not directly simulate bed changes or sediment transport. Its strength lies in detailed flow field analysis where accurate representation of three-dimensional geometry and turbulence-related flow behavior is required.
SRH-Coast
SRH-Coast is designed for coastal, estuarine, and nearshore environments. It includes hydrodynamic, wave, and sediment transport modules. The hydrodynamic module solves shallow-water equations to obtain current and water level information. The wave module estimates wave conditions using a spectral wave approach similar in concept to SWAN. The sediment transport module can simulate non-uniform and non-equilibrium sediment movement.
SRH-Coast uses unstructured grids and can simulate the interaction among waves, currents, and sediment transport. It is suitable for applications in tidal flats, beaches, lagoons, river mouths, and other coastal environments where wave-driven processes influence sediment movement and bed changes.
SRH-Watershed
SRH-Watershed is designed for rainfall-runoff, soil erosion, and sediment transport analysis at the watershed scale. It includes modules for meteorology, evapotranspiration, subsurface flow, overland runoff, channel networks, and sediment transport.
The model solves key variables and parameters on two-dimensional computational grids. It can be applied to both small watersheds of less than 10 square kilometers and large watersheds of more than 1,000 square kilometers. For small watersheds, finer grids can be used to capture local topographic features. For large watersheds, coarser grids and parameterized processes can be used to represent broader hydrological and sediment-related behavior.