20230830 Environmental Board Packet Combined

Home Search

Amelia Ditch Study August 10, 2023 Page 4 PROPOSED CONDITIONS Improvements to the ditch are recommended to alleviate drainage problems and more efficiently convey flows. These recommended improvements generally fall into two categories, short-term and long-term. Short-term work is recommended within the next year or two and would consist of less significant work that would provide benefits to ditch conveyance with the goal of reducing flooding and ponded water. Long-term work is recommended with the eventual development of the area. This work would consist more of floodplain storage, BMP construction, and ditch realignment/meandering with the goal of floodplain and water quality improvements. This long- term work would likely come from the City as a recommended concept plan that would then be brought into final design and construction by a developer of the area. Short-Term Work The majority of proposed short-term improvements would consist of earthwork within and adjacent to the ditch channel, as shown in the Proposed Conditions Map of Appendix A. This work will be limited to the ditch alignment between 20th Ave and Culvert F. The City would obtain easements along the ditch to conduct the work and allow for future maintenance. Excavation is recommended to remove accumulated sediment from upstream loading, channel bed and bank scour, and settling around culverts. Much of this excavation would be within the top foot of the ditch bottom, up to 2 feet where more sediment has accumulated. The main intent of earthwork on the bottom of the channel is to provide a more consistent slope through the ditch. This would increase conveyance and reduce future sediment settling. Through much of the residential property (6433, 6443, and 6484 20th Ave), the channel slopes are sparsely vegetated and often steeper than 3:1. The proposed earthwork would bring these slopes to 3:1 at most in order to improve stability and reduce sediment loading from bank erosion. Decreasing these slopes would also slightly widen the channel section, further increas ing conveyance capacity of the ditch and reducing flood levels. This earthwork on the banks would only expand the channel footprint into the existing spoil piles and not impact the usable lawn areas observed during survey. This earthwork would also involve vegetation management. In areas not farmed, most of the vegetation observed during survey were lower quality trees and scrub along the banks of the ditch. During earthwork, many of these trees will need to be removed. This will provide an opportunity to replant with native grasses and other vegetation that is more resistant to erosion. Removal of trees will also bring more sunlight to the channel, further improving the uptake of new seeding. This proposed new vegetation would provide more consistent soil cover, stabilizing the banks and reducing sediment loading to the ditch. Another major component of the short-term work would be replacement of Culvert F. The pipe is recommended to be replaced with a 36” RCP with flared end sections instead of the existing 30” CMP. This replaced culvert would be placed lower to ensure it is not impounding water upstream like it currently does. Additionally, a properly bedded RCP will be more resistant to washing out unlike the risk posed to the current pipe. There is also potential to reseat the back sloped Culvert E as part of this project, but that is not a major priority since it was installed recently and seems to be functioning well. There is also an opportunity to excavate a small settling basin upstream of Culvert F. This would serve to provide additional settling and pollutant removals before the discharge to Lake Amelia. Expansion to the west would impact upland agricultural area and not require wetland impacts. As this basin would be relatively small with respect to the remaining floodplain and proposed channel