5.0 Flood Hazard Area Mapping

5.1 Approach

Two dimensional floodplain analysis.

5.2 Assumptions

The flood hazard area maps are non-regulatory, but identify structures at risk of flood damage based on surface flow depths. The flooding depicted on this map is a uniform storm event across all watersheds at the same period of time and same duration.

Free flow. These depths do not account for debris or hail or other impediments in storm water conveyance systems. Debris flow modeling is possible, but is beyond the scope of this master plan study.

Storm sewer systems. Small diameter storm sewer systems were not accounted for in the hydraulic modeling and resulting floodplain mapping for this master plan study. The impact of a 4 or 8-inch or even 18-inch culvert on flood depths is limited when considering storm events beyond the 2 year recurrence interval. As a sensitivity analysis, the 18-inch storm sewer pipes and inlets were accounted for in the Lyons Village area. The resulting flood depths were unchanged for all but the 2 year recurrence interval. In the 2 year recurrence interval storm, the flood depths were reduced by approximately 3 inches. The floodplain mapping is drawn as a quilt of interlocking 10 foot squares with depth averaged across that area. There were less than 40 squares (4,000 square feet) removed from flooding when accounting for the 18-inch storm sewer.

It is important to note, this is not a referendum on the existing 18-inch storm sewer system. Clearly, an 18-inch storm sewer conveys water and a has a beneficial impact on the flood risk in the neighborhood. This is a clear indication of the limitations of master plan level, town wide, flood risk mapping. The uncertainty variables at the master plan level results in floodplain maps depicting general, conservative areas of risk. When areas such as developed neighborhoods with existing storm sewer systems show up on the master plan flood maps attention should be given to determine if a higher level of study is necessary. In the case of Lyons Valley, the storm is functional and clearly benefits the properties more than the master plan flood hazard maps indicate. However, when combined with anecdotal stories about spring and summer street flooding from overwhelmed inlets, this may be an area to consider for additional detailed field survey and hydraulic analysis.

5.3 Regulatory Floodplains

Conversion of these flood hazard areas to regulatory floodplains is a complicated, long, but well documented process. At this time, the master plan makes no specific recommendation for submitting these floodplain delineations to FEMA for formal adoption. As flood recovery projects are completed and budgets are set for priority capital improvements, a more formal Flood Insurance Study can be considered to accurately map the flood hazards to FEMA specifications.