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