Story, City, IA

South Story City Drainage Improvements

  • Client City of Story City
  • Type Stormwater
  • Services Evaluation, Design, Construction Administration
  • Construction Cost $720,000 (50% CDBG)
  • Project Manager Scott Williams, P.E.

Challenge

Story City initiated a study of its stormwater facilities after major flooding, most recently in 2008 and 2010. On the southern side of the city, flooding caused significant damage to residential properties and jeopardized operations at a Bethany Life assisted living and nursing care facility, home to 200 elderly residents. A study determined that runoff from agricultural areas south of the city, along with normal flows within the city’s storm sewer system were combining to overload the city’s storm sewer network during major storm events. This resulted in storm runoff overtopping streets, flowing overland through yards, and basement flooding in residential and commercial buildings.

The two solutions typically considered to alleviate flooding involve conveyance or detention improvements. Conveyance improvements “make the pipes larger”.  Detention improvements “hold back water”.  Installing larger storm sewer pipes to create a new outfall to the Skunk River would be very costly.

Creating stormwater detention to hold back runoff from the agricultural areas was another possibility. However, without an adequate outlet, the size and depth of the planned detention basin needed to be very large to capture runoff from over 600 acres during a 100-year storm event; and it would require convincing an adjacent property owner to remove a large amount of land from active row-crop farming to sell it to the city. Based on construction costs and impacts to adjacent property owners, neither of the solutions scored highly.

Solution 

Using technology common to wastewater treatment facilities, FOX designed a hybrid solution that provided a smaller detention area footprint, along with electronic controls to throttle outlet flows to the existing storm sewer network. A berm was constructed to raise the temporary ponding depth in a natural drainage swale in the adjacent farm field. This did not require massive earthwork to build a detention basin and allowed the area to remain in crop production. The city purchased 4 acres of land to construct the berm and control structure; they also acquired a stormwater detention easement for 32 acres with the condition that stormwater would pond for no more than 72 hours following a major storm event.

Electronic sensors were placed in downstream pipes to monitor flows in the storm sewer network and a motor-operated sluice gate was installed to control flows from the detention area. The gate is normally open to allow tile flows from the farm field to pass through an outlet to the city’s storm sewer. During large storm events the gate closes, allowing water to temporarily back up into the field. The city’s normal storm flows can pass before the detained flows are then released to the storm sewer network.

The sluice gate opens and closes to regulate the flows, taking advantage of all available downstream pipe capacity to empty ponded water from the detention area. Operation of the gate is automated and can be monitored by city staff via the internet.

Outcome

Construction was completed in 2015, and the system received its first test in December 2015. Rainfall from a major storm event combined with frozen ground to generate massive runoff, unusual for this time of year. The system functioned as designed, allowing water to pond temporarily in the farm field (see below). Ponded water was released within 48 hours as capacity in the downstream storm sewer became available. City staff indicated that this type of storm event would have resulted in street overtopping and flooding prior to construction of the improvements.

The city was awarded a Community Development Block Grant under the Community Facilities & Services category, for 50% of the project costs.