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lemo To: File From: Laura Chap Emall: Phone: Date: Feb 20, 2014 Ref: cc: Subject: Comments regarding potential appeal of flood hazard boundaries in the City of Centerville and Lino Lakes Background New Zone A modeling and mapping was provided for the Clearwater Creek watershed in Anoka County. it was partially incorporated into the preliminary DFIRMs, however the effective LOMR 95 -05 -339P for the City of Centerville was not superseded with this new data as it includes detailed study and a floodway, and detailed study may not be replaced with approximate analyses. The effective LOMR is based on TR -20 and HEC -2 analyses, and was delineated on USGS topographic data. The purpose of this review is to provide the City of Centerville with guidance on what information would need to be submitted to upgrade the existing Zone A modeling within Anoka County to a detailed study. For purposes of this review, Atkins used the exported XP -SWMM synthetic event models. Atkins ran these in version 2011. Additionally, the time interval in the global rainfall data was corrected (it appears the 24 hour data is given in 0.1 hr increments, not 1 hour increments). While an overall review was performed for the model, the focus of the review was on the areas within Anoka County_ Comments 1. The effective data is Zone AE and includes a floodway. The current Zone A model includes the 10-, 50-, and 100-yr storms. The revised modeling would need to include a 500 -year profile as well as a floodway run_ Please see the following document regarding floodway modeling in SWMM: httol/www.fema. media library-d ata/20130726- 1553- 204904M80/frm swmm pdf 2. Only the 100 -yr floodpiain delineation was provided. Please provide the 500 -yr delineation as well as a mapped floodway. 3. The model was developed in lnfOSWMM, and the synthetic storms were exported to XPSWMM. However, notes included with the model indicate that the XP -SWMM model results do not match the InfoSWMM results. InfoSWMM is not on the FEMA approved models list In order for this model to be the effective detailed model for the area, the model will need lo be exported to either EPASWMM or XPSWMM and run in one of those models. 4. The report states that the 100 -yr maximum profile was created by choosing the maximum elevations from the 24 -hour rainfall and the 10-day runoff event. Please provide the 10-day runoff model. Also, please note that this method may cause difficulties with the floodway analysis, and it may be better to break the model into segments so that only one event per segment is being used for the 100 -year profile. 5. Please provide a map of the sub -basin delineation which includes the topographic data_ This can be digital (provide the digital contours) or a pdf /paper map. 6. For flood insurance studies, FEMA requires that XP -SWMM models are calibrated to actual events. The review memo included in the supplemental data indicates that the model cannot be considered calibrated for flood events due to the lack of data, and that Clearwater Creek cannot be compared to regression values as the drainage areas are too urbanized. Please provide comparisons of modeled flows to either regression equations with urban aclustments, or rational method calculations, as appropriate. oMM Documentl