5.5. SR 08-06-2007 - Laserfiche WebLink

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evaluation came primarily from peer-reviewed manuals, with additional information and discussion based on Barr's own experience and professional judgment. Table 1 shows that the only limitation with the P8 model, as it relates to the modeling requirements for the loading assessment, is that it is not intended to be used to determine pollutant loadings from non- urban land uses. However, the Simple Method, PONDNET and GWLF can be used to determine pollut- ant loadings from both urban and non-urban land uses. Table 1 also shows that the Simple Method, PONDNET and GWLF lack the ability to model the BMPs that would typically be considered for imple- mentation by the City (such as vegetated drainage ways, extended detention, rain gardens and street sweeping). SLAMM is more detailed than P8 with respect to porous pavements and distinguishing source loading areas (such as driveways, parking lots, lawns, etc.). XP-SWMM is more complex, significantly more expensive, and BMP modeling is more cumbersome, less accurate and less intuitive than the P8 model. For the purposes of developing our proposed scope of work and cost estimate, we have assumed that we will use the Simple Method to determine the pollutant loadings and runoff volumes from each of the land uses within each watershed and then use the P8 model to account for the effects of BMP implementation for the time periods of interest to the MPCA. The following statements provide justification for choosing the Simple MethodIP8 model combination for the loading assessment modeling, in comparison to SLAMM, POND NET and XP-SWMM: · Based on a meeting, on behalf of several Barr clients, MPCA staff were briefed on this modeling approach for the Loading Assessment and indicated that it would be accepted · Modeling approach will support potential development of a Nondegradation P~an in the future to address any new or expanded discharges · All of the Simple Method inputs can be directly derived within GIS · PONDNET does not model TSS loadings and is only intended for wet detention pond modeling · P8 exceeds the capabilities of SLAMM when it comes to networking of watershedslBMPs and many of the graphics and advanced features · P8 provides routines for perforrniD.g sensitivity analyses and can also be run in design mode to de- termine required sizes of BMP(s) to meet treatment criteria · P8 has highest peer acceptance in Minnesota for urban runoff and BMP water quality modeling · P8 is free, user-friendly and easy to learn with its menu driven system · P8 allows for hydrologic calibration within the program · P8 allows for some GIS compatibility via ASCII text fIle import of watershed data and export of results · P8 reads in hourly precipitation data, while SLAMM only reads in the total precipitation and du- ration of each rainfall event Barr Engineering Company Page 7