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Engineers tell council Sunnyside drainage system undersized; recommend detention pond and phased rebuild
Summary
Maclaurie Engineering presented a Sunnyside drainage study finding undersized inlets and trunk lines across the 212-acre basin and proposed three scenarios — including a detention pond that could reduce overall costs — with phased implementation recommended to mitigate flooding at Williams and Anderson.
Maclaurie Engineering presented the Sunnyside Drainage Basin Study at the Aug. 6 city council meeting, concluding the basin’s existing storm infrastructure is undersized for the city’s 5-year design event and recommending phased improvements.
Ryan Murtha, senior project manager, summarized the study area as 212 acres with 103 inlets, 57 storm sewer pipes and multiple undersized trunk lines. He said modeling showed many pipes operating under pressure during the five-year storm event and singled out the Williams and Anderson area as a persistent problem area. “We end up with a significant number of pipes throughout the system that are surcharged,” Murtha said.
The consultants presented three scenarios: (1) a detention pond near the county jail plus targeted piping to provide immediate relief in the Williams–Anderson area; (2) the detention pond plus replacement/upsizing of pipes across the basin and two trunk lines (54-inch east trunk, 48-inch west trunk); and (3) a stand-alone option that upsizes pipes without a detention basin. Murtha said including a detention pond “will likely save a couple million dollars” by allowing smaller pipe sizes downstream.
Councilors raised follow-up questions about detention-pond footprint and depth, safety and jurisdictional constraints (the proposed pond footprint is on county-owned land). The presenter noted some design decisions are constrained by limited land availability in a fully developed basin and that downstream capacity on Dry Run Creek must be examined if trunk lines are upsized.
Cost estimates provided were preliminary and based on generalized assumptions (including water-main and sanitary replacements and some road reconstruction); Murtha emphasized phasing to manage budget and construction timing. The study will be folded into a broader master drainage plan and returned to council with additional reports.
Next steps: staff will combine this study with other infrastructure reports for council review and master planning; phasing and land-acquisition questions remain open.
