The two-dimensional (2D) hydrodynamic modeling approach consists of:
- generating detailed digital terrain models (DTM) of the study sites (Figure 6),
- collecting substrate and cover data to create polygons to overlay onto the DTMs for modeling hydraulic roughness and for modeling aquatic habitat,
- collecting water surface calibration data, and
- 2D modeling of flow fields over a range of flows.
The 2D modeling in turn consists of:
- selecting a 2D hydrodynamics model,
- generating computational meshes,
- water surface modeling, and
- velocity and depth modeling.
A number of two-dimensional flow models are used for analysis (FESWMS, River2D, Mike21, etc.), and new models are continually being created and adapted to address the concerns listed below. In general, the models are incorporated into a multi-dimensional surface modeling system, based on three-dimensional riverbed topography, flow rate, and downstream stage (i.e., water surface elevations) boundary conditions to calculate flow, velocities, water surface elevations, and boundary shear stresses in the channel. These have been used in channels with or without islands in both high and low Froude number flows (i.e., sub-critical and super-critical flow conditions).
The 2D modeling approach uses habitat criteria values (typically depth, velocity, and cover) from the literature in the same manner as IFIM and PHABSIM to determine the habitat-flow relationship for analysis species and life stages for the study reach. See Section 6.4.3 for a discussion of the use of 2D modeling to assess instream flows for amphibians.
4.1.3.1 Advantages and disadvantages of approach
Use of 2D modeling to assess instream flows has advantages and disadvantages relative to the other approaches evaluated, as discussed below:
Advantages
- Approach requires intensive collection of topographical data, and thus no extrapolation between transects is required.
- Can account simultaneously for multiple habitat values (depth, velocity, substrate, and cover).
- Allows modeling of any simulated flows (within range of measured flows) and as many species and life stages as needed.
- Species habitat criteria can be changed and the sites can be remodeled if habitat criteria information is updated.
- Standardized application, which is also repeatable.
- Results in a highly visual output that is easy to interpret.
Disadvantages
- All water velocity measurements are based on vertically-averaged water velocities, which often do not represent locations where aquatic species hold or feed.
- Although it can account simultaneously for multiple habitat variables, it is difficult to account for additional metrics, such as the focal velocity of an organsims that resides on the substrate.
- Hydraulic calibration of the model emphasizes mass balance, and there is little confirmation of the accuracy of velocity or depth simulation.
- This approach uses habitat suitability criteria which are often biased because they are based on site-specific habitat conditions and use of vertically-averaged velocities, and the criteria fail to account for habitat preferences that vary based on the scale that is considered. For example, the focal point (or "nose") water velocities used by a fish are often much lower than the average water velocity at the same location.
- There is a weak tie to population response, and interpretation of results almost always assumes that minimum flows limit the abundance and/or distribution of populations. There is a limited ability to integrate results with a limiting factors analysis, so discerning which species or life-stage should drive flow selection is unreliable. Does not provide total amount of preferred or usable habitat available at varying flows, so the data can not be used in population modeling (see Section 4.6).
- Does not allow clarification of statistical error bounds on the predicted habitat-flow relationships; therefore the reliability of the results can not be estimated and considered when making management decisions.
4.1.3.2 Site-specific considerations and applicability
Many of the advantages and disadvantages of this approach will depend on site-specific considerations, as discussed below:
- 2D models are less reliable in hydraulically complex areas due to poor ability to address spatial shifts in water velocity. 2D approaches are best applied in systems without substantial bed roughness, or without hydraulic complexity. Increasing the precision of the topography can compensate for errors when applied in systems with substantial bed roughness.
- The density of the mesh screen used to generate topography affects accuracy of results. With adequate mesh densities, has the ability to model hydraulics accurately.
- Cannot be applied to a larger area than is modeled. Study reaches tend to be short. A total habitat area for the reach is not estimated.
- Intensive data collection can be very expensive.
- Has been applied on the Oak Grove Fork of the Clackamas River at the Clackamas River Hydroelectric Project (FERC Project No. 2195), Oregon; at the Klamath Reclamation Project, on the Klamath River (FERC Project No. 2082), California; on LaVerkin Creek, Utah; on the Pit River Hydroelectric Project (FERC Project No. 2687) on the Pit River, California; and on the Spring Lake and San Marcos River system, Texas.
4.1.3.3 Selected references
Addley, R.C. and J.A. Ludlow 2001. Kerr Dam Ramping Rate Study Lower Flathead River. Final Report for the Confederated Salish and Kootenai Tribes. Utah Water Research Lab, Utah State University, Logan, Utah. 86pp.
Hardy, T.B. and Addley, R.C. 2001. Evaluation of interim instream flow needs in the Klamath River, Phase II Final Report (draft). Institute for Natural Systems Engineering, Utah State University.
Hardy, T. B., and R. C. Addley. 2003. Instream flow assessment modelling: combining physical and behavioural-based approaches. Canadian Water Resources Journal 28: 273-282.
Nelson, J.M. 1996. Predictive Techniques for River Channel Evolution and Maintenance. Water, Air and Soil Pollution 90:321-333.
Nelson, J.M. and J.D. Smith. 1989. Flow in meandering channels with natural topography, in River Meandering, Water Resour. Monog., Vol. 12, edit by S. Ikeda and G. Parker, pp. 69-102, AGU, Washington, D.C.
Steffler, P., and J. Blackburn. 2002. River 2D: two-dimensional depth averaged model of river hydrodynamics and fish habitat. Introduction to depth averaged modeling and user's manual. Prepared by University of Alberta.
Thompson, D.M., Nelson, J.M., and Wohl, E.E., 1998, Interactions between pool geometry and hydraulics: Water Resources Research, v. 34, no. 12, p. 3673-3681.
