Dispersion Model) Clause Samples

Dispersion Model). The inputs for the dispersion modeling followed EPA’s 1992 Guidance for CO determinations using CAL3QHC (version 04244) and were consistent with the approach used by the Federal Highway Administration (FHWA) Carbon Monoxide (CO) Categorical Hot-Spot Finding (FHWA, February 2014) for intersections. As for emissions modeling, dispersion modeling was performed using conservative underlying assumptions and, in many cases, worst-case inputs and assumptions. These inputs and assumptions are outlined below:  As a conservative assumption, the wind speed was set to 1.0 m/s (the lower limit of CAL3QHC meaningful input).  Wind direction was modeled every ten degrees from 0 to 350 degrees.  A mixing height of 1000 m was used, consistent with standard modeling procedures. Sensitivity testing has shown that due to the close proximity of the receptors, mixing height has negligible influence on dispersion analysis.  For urban modeling, a surface roughness (z0) of 108 cm was used, corresponding to a single family residential setting. The single family residential setting is the least rough setting for an urban environment and is conservative. The recommended surface roughness in urban areas can vary from 108 to 370 cm. For rural areas, a surface roughness of 1.0 cm was used, which corresponds to a moderately short grass height (6-8 cm) as identified in prairie grass.16 Shorter grass heights are unlikely to be found in most rural locations.  The 1992 EPA CO Guidelines specifies a stability class of D (neutral) for urban areas and E (stable) for rural areas. These guidelines were applied in the model.  Receptor Placement o Freeways and Arterials:  Receptors were modeled per the CAL3QHC and 1992 EPA Guidance. For both freeways and arterials, receptors were placed on both sides of the roadway extending out to 295 feet from the roadway and were modeled to establish decreasing CO concentrations with distance.  For freeways, receptors were located beginning at 30 feet from the outside lane to account for off-road safety clearance.  For arterials, receptors were located beginning at 10 feet from roadway edge (where the general public has access and is within the limitations of the model to predict valid concentrations). o Intersections and Interchanges:  Receptors were modeled per the CAL3QHC and 1992 EPA Guidance and began at 10 feet from roadway edge.  A grid of receptors was used in each quadrant to ensure the worst case concentrations were identified. The grid spaci...