04-18-2022 Agenda - Laserfiche WebLink

CASS COUNTY ROADWAY 18 EXTENSION STUDY – CASS COUNTY, ND 45 7.4.2 DRAINAGE/FLOODING IMPACTS 7.4.2.1 EXISTING CONDITIONS Local drainage and flooding have always been concerns in the Red River Valley, especially when roadways are intended to be altered, such as in the alternatives presented in this report. Upstream of this study area, throughout Richland and Cass Counties, the flooding generally originates from either the Sheyenne River or the Wild Rice River. As the capacity of these rivers is exceeded, flood waters tend to break out of the channel banks and flow overland, backing up behind roadways prior to overtopping and continuing to flow from section to section in a northeasterly direction. Using the existing hydraulic models created for the Fargo-Moorhead Area Diversion Project, existing conditions flooding in the study area was reviewed. The model used for this study simulates 100-year riverine flooding from the Sheyenne River and the Wild Rice River. Near the study site, water breaks out of the west banks of the Sheyenne River and it flows overland to 52nd Street SE, west of the Norman Lutheran Church. Water also breaks out of the Sheyenne River to the east as it cascades north and east along 52nd Street SE. Further to the east, along the existing County Road 18, water breaks out of the Wild Rice River and Drain 37 prior to overtopping County Road 18. Water also overtops County Road 18 at the Wild Rice River structure. The existing conditions flooding was presented at the public meeting and several residents concurred with the overall drainage patterns and overtopping representations. 7.4.2.2 PROPOSED CONDITIONS WITH ALTERNATIVES From a flooding perspective, future design of County Road 18 should include a detailed hydraulic analysis to minimize impacts from the project. For this study, no detailed design or analysis was conducted, however, the previously created FM Diversion model (existing conditions) was used to simulate the effects of the potential alternatives (A, B, and C) in the vicinity of the Norman Lutheran Church. Figure 22, Figure 23, andFigure 24 present the flooding extents and impacts for the various alternatives. 7.4.2.3 IMPACTS OF THE FARGO-MOORHEAD DIVERSION PROJECT The FM Diversion Project will not affect the proposed extension of County Road 18 or the Sheyenne River crossing near the Norman Lutheran Church. However, additional flooding depths are expected during diversion operations for the current stretch of County Road 18 near the Wild Rice River. The depth of this flooding is dependent on the frequency of event (50-year, 100-year, etc.), and based on historic records the project will not have ever operated during the growing season. Figure 25 presents expected existing conditions flooding for the 10-, 20-, and 100-year flood events, without the diversion project, and Figure 26 presents flooding with the diversion in place and operating.