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6 models where only peak discharges are evaluated and accuracy is less critical. 1D vs 2D 1D and 2D model geometries are developed based on topographic data like DEMs and TINs (sourced from various federal, state and local agencies) as well as site-specific surveys. 1D models feature a select number of cross-sections at specific locations within a study reach (refer to Figure 3), whereas, 2D models feature a mesh that covers the entire study reach (refer to Figure 4). Figure 3. Example of 1D HEC-RAS model geometry [4] A 1D model may be best suited when the following are true: • Flow is generally one-directional (i.e., downstream) and does not spread dramatically into the floodplain; • The reach is relatively uniform with limited expansion or contraction; • The floodplain is narrow relative to the main channel. The width of the floodplain should be less than three times the width of the main channel; • The channel is well defined and bounded by steep slopes and channel flow is well connected to overbank flow. Channels that are raised above the floodplain may not be well suited for a 1D model; and • The desired outputs are simple profile characteristics along the main channel such as energy grade line, average channel flow depth, and velocity, channel shear stress, etc. • Other benefits of a 1D analysis include: • Flow characteristics at individual cross-sections are more readily available than they are with a 2D analysis; • Run times are typically much shorter with a 1D analysis, especially for very large models; • Pressure flow at bridges can be modeled in a 1D analysis, which is a feature that is not yet available in 2D; • Industry familiarity with 1D, which has been used for decades as opposed to 2D, which is a relatively new feature; and • Model result verification and detailed checking can be completed much more easily for 1D scenarios as compared to 2D scenarios. Figure 4. Schematic of typical 2D HEC-RAS computational mesh. [4] Conversely, a 2D model may be best suited when the following are true: • Flow is multi-directional and is expected to spread dramatically. This includes reaches with abrupt expansions/contractions, urban areas with buildings and other flow obstructions, and narrow bridge crossings; • The floodplain is wide compared to the main channel. A 2D model may be most appropriate if the width of the main channel is greater than three times the width of the main channel; • The terrain is very flat such as wetlands, estuaries, deltas, etc.; and • The study is focused on a stream network with multiple watercourses and junctions or lateral structures.