20230830 Environmental Board Packet Combined

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Amelia Ditch Study August 10, 2023 Page 3 Culvert C is likely submerged year-round except during very dry conditions, as such, the condition of this pipe could not be verified during the survey. Modeling does not indicate that HWLs reach a point that the road overtops, even in the 100-year event. West of 20th Ave, the ditch turns to the southwest and follows a straight alignment through agricultural land. There is a fair degree of accumulated sediment downstream of the pool off Culvert C that settles from larger washout events. The ditch section is well define d here, with a width of 25 feet and depths of 3-5 feet. The channel appears to be inconsistently dry, likely depending more on downstream water levels. Outside of the channel is largely flat, HWLs are likely to spill out from the channel and into the surrounding fields. This ditch section drains through Culvert D on the 6483 20th Ave property. This culvert is 24” CPP with positive drainage, flowing beneath an access path on the property between the eastern and western sides of the ditch. Downstream of Culvert D, the ditch flows through a grassed and lightly forested area in a well- defined channel with spoils piled on either side. The channel section is 25-35 feet wide and 4-6 feet deep. There is a small tributary ditch north of the property line between 6443 and 6483 20th Ave that runs west to east into the channel that has formed a significant sediment delta, likely impeding flows upstream. The ditch likely has water in this section year-round due to poor downstream drainage. Modeled high water levels are c ontained in the channel with the exception of a small inundation area in the 6483 20th Ave property. This section of ditch drains through Culvert E on the 6433 20th Ave property. This culvert is 36” CPP that is back sloped and installed above an old culvert that was left in place. As such, Culvert E is higher than the others in the ditch and likely impounds water upstream. Through the 6483 20th Ave property, the ditch flows in grassed and lightly forested areas in a well-defined channel that has spoils piled on its north side. The channel section is 30-35 feet wide and 5-6 feet deep. Through the 1826 Birch St property, the ditch flows through more agricultural land, although there are some trees lining the channel. The channel section there is 30-40 feet wide and 4-5 feet deep, with flatter topography to the south that floods based on modeled HWLs. Both of these ditch sections likely have water year-round due to the poor condition of the Culvert F and tailwater effects from Lake Amelia. This culvert is a 30” CMP that is in poor condition. The middle of the pipe is heaved upwards, impounding water upstream. Additionally, water was observed flowing beneath the pipe, indicating the potential for Culvert F to wash out in the event of major flows through the ditch. Downstream of Culvert F, there is a section of ditch that eventually blends into Lake Amelia. The channel is roughly 40 feet wide and 3-5 feet deep where it is still defined as a ditch between agricultural fields. For the sake of this study, the end of the ditch was defined at the boundary between the fields and wetlands surrounding the lake. The water level in this section is functionally hydrostatic with the lake based on its profile and proximity. In existing conditions, there are known drainage concerns with the low-lying fields around ditch near its crossing of 20th Ave. While a fair amount of these HWLs can be attributed to the very flat nature of the ditch, the problems are exacerbated by sediment loading in the channel and culverts that are too high or in poor condition. The entirety of the ditch is shown as FEMA Zone A floodplain with no flood elevation; however, Lake Amelia has a mapped flood elevation of 910.2. In addition, there are currently no easements over the ditch which has led to confusion around which parties are responsible for its maintenance or impacts to upstream properties when changes are made.