19 - Drainage Presentation - Sandy Creek Upper Tributaries 4 and 6

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"While this solution mitigates a major cause offlooding in this area and is significantly less than the $10.2M improvement plan proposed by CF, the cost (monetarily and politically) would still be high. Based on our experience, culvert improvement projects can be expected to range from $175k to $225kper crossing. Add to that the home/property buyouts at $80k to $115kper (six total) and the cost to demolish and increase (excavation) the floodplain capacity along the west side of the channel and the City would be looking at a project in the $1.75M to $2.OMprice range. " HEI pointed out that their options did not address the flooding of Tributary 6 (or 4) due to the undersized channels there. "The Tributary 6 channel, immediately downstream of Culbertson, decreases in size. Upstream of Culbertson, the concrete lined trapezoidal channel has a 5 -foot bottom width and 15 -foot top width; however downstream of the Culbertson Street crossing the bottom width decreases to 2.5 - feet and the top width decreases to 9 -feet. While the proposed solution indirectly addresses this issue by reducing the backwater effect at Simpson Street, the flooding in this area could be further mitigated with channel modifications downstream of Culbertson. " This is the section shown in Images 10 and 11 earlier in this report. While widening this smaller channel is necessary, the flooding downstream of it will likely worsen, and should be further studied prior to excavation. HEI also proposed adding storm drain pipes and inlets along 26th Street between Simpson and Collegiate Drive. These pipes would probably need to be 4 feet in diameter and require half of the street to be excavated to be installed. I estimate construction costs to install 4 foot diameter storm systems and inlets along 2601 at approximately $300,000. More studies and precise drainage calculations would need to be performed before implementing any of these strategies. A stormwater system in this location would likely only make sense if the channel was widened from the Confluence of Tributaries 4 and 6 (shown in Image 13) up to Culbertson Street (Image 10). City Action following the Cobb Fendley Report and Hayter Memorandums: Following the studies and analysis, City Leadership asked the Public Works Department to reinforce their efforts to clean sections of drainage ditches throughout the city. Some of the areas where they cleaned and improved the side channels of the ditches and tributaries are as follows: 15