[18pt] ras2fim v2.0 wiki page

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Updating ras2fim wiki page for version 2.

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ras2fim flowchart:

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5 - Create Depth Grids and Synthetic Rating Curves

Script names: create_fim_rasters and worker_fim_rasters

This is where the bulk of the RAS2FIM workflow is performed. The provided HEC-RAS models are 'cut' to a spatial limits between cross sections that approximate the conflated limits of the National Water Model reach. From these cut models, a synthetic rating curve is computed by running multiple flow rates down the river and computing a reach averaged depth for each discharge. The provided terrain is used in conjunction with RAS Mapper to simulate the model and generate flood depth grids at the prescribed even reach averaged depth increments.

In the conflation output XXXXXXXX_interim_list_of_streams.csv in step #2, each feature-id from the National Water Model that conflates to a HEC-RAS model will note the (1) path to the corresponding HEC-RAS model [ras_path], (2) the cross section limits upstream [us_xs] and downstream [ds_xs] and (3) the peak flow rate for all cross sections and the profiles within these cross sections [peak_flow].

For example, feature-id #6974668 conflates to the Hunter_Cr model between cross sections 22805 and 20261 with a peak discharge of 135.29 cms.

A new HEC-RAS model is created for the feature-id that is 'clipped' to the limits determined in the conflation process. If available, two (2) additional cross sections area added upstream and downstream of the limits. This is to minimize flood inundation mapping gaps when compositing adjacent reaches' depth grids.

A first pass steady flow file is created for the new HEC-RAS geometry model. The create_fim_rasters script is hard coded to multiply the provided peak discharge by 1.2 and split that resulting value into value into 75 flow values. For example, a flow of 135.29 cms is broken into flow values at an interval of 2.16 cms. A reach averaged depth is simulated for 75 flow values [2.16 cms, 4.32 cms, 6.48 cms .... 157.68 cms, 159.84 cms, 162.0 cms]

For each of these flow values, a reach average depth is calculated and a 'first-pass synthetic rating curve' is determined. From the first-pass rating curve, flows are interpolated at points where it is assumed that the HEC-RAS model will compute flood depths at the desired depth intervals. For RAS2FIM flood depth grids are evaluated at every 0.2 meters (metric) or 0.5 feet (SI).

From these interpolated values, a revised flow file is created and HEC-RAS is recomputed. For this second pass, a RAS Mapper files is written that requests that a flood inundation grid be generated for each depth interval. Note that for all HEC-RAS simulations, flow rates are constant down the reach. All flow is assumed to be sub-critical and downstream energy is set at 'normal depth' with a slope of 0.5%.

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