20230830 Environmental Board Packet Combined

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Amelia Ditch Study August 10, 2023 Page 5 excavation, it is not anticipated to impact flood levels in any appreciable manner. Approximately 700 cubic yards of excavation would be needed to create this basin, with an estimated cost of $21,000. The modeling results presented in Appendix B show the drainage improvements expected as a result of this work. Since the short-term work takes place downstream of the 20th Ave road crossing, that’s where the majority of the modeled drainage improvements are shown. Within DA 11, the modeled 100-year HWL drops approximately 0.5’ from existing conditions due to increased downstream conveyance. This HWL decrease goes up to 3.4’ upstream of Culvert F. This pattern is reflected by the increased max flow rates in these same drainage areas. Modest increases from existing flows are shown for areas upstream of Culvert E, while major flow increases are seen leading up to and including the proposed replaced culvert. This will all serve to improve conveyance in the ditch and reduce water levels in major rain events . While the proposed modeling shows increased flow rates to Lake Amelia, adverse impacts are not anticipated to the lake’s hydrology or ecosystem. The ditch is proposed to be restored to as- built conditions, not a major rework of the channel. Much of the increase in flow capacity comes from improved ditch conveyance and replacing a failing culvert. These increased flows will not appreciably increase velocities into the lake due to the hundreds of feet of ditch downstream of the improvements attenuating flows. Additionally, the improved grading and stabilization will better dissipate energetic flow than in current conditions, reducing pollutant loading from erosive forces. This phase of the project is solely focused on conveyance of water to alleviate flooding concerns and does not propose to increase the volume of water reaching Lake Amelia in flood conditions. The increase in flow rates may slightly alter the lake’s flood hydrograph, but relative to the rest of the lake’s tributary area it is unlikely to increase high water levels. The majority of work in the short-term would consist of grading the ditch to improve conveyance, clearing nuisance vegetative canopy to encourage understory growth, and the replacement of Culvert F. In all, this work is estimated to cost $252,500 to complete. This cost includes a 30% contingency factor to account for potential changes to ditch conditions prior to work beginning. The estimate does not account for any potential costs for easement acquisition, if they are needed. A detailed breakdown of this estimate can be found in Appendix C. Long-Term Work As opposed to the short-term improvements, the long-term work is proposed for the current agricultural areas along the ditch. These properties, namely the 6518 20th Ave and 1926 Birch St parcels, are anticipated to develop into single family residential housing in the next 10-20 years. Development of these properties is currently limited by the lack of sanitary sewers. It is expected that the extension of the sewer from the west will drive development of these parcels. For development, the City could require out lots and easements along the existing ditch alignment to construct these more significant improvements. Alternatively, the City would still request the out lots, but instead recommend these improvements and the developments would take them into consideration for their requisite stormwater management facilities. With this larger project, the ditch alignment would be proposed to meander similar to a natural stream, as opposed to the current sharp turns and straight channel sections. This meandering portion would have a wider and more gently sloped channel section. The ditch would have improved ecological value as the meander provides a variety of flow conditions and water depths for plant and animal life. In addition, a meandering alignment improves the aesthetics and recreational value of the area. With the potential trail extending through the area from the west, this region could serve as a greenway space for the City.