In 2012, the City of Great Falls and TD&H Engineering began a study on the main storm drain interceptor for a 1,156-acre drainage basin south of Central Avenue. A stormwater computer model of the area was created in order to analyze the relationship between the subbasins, stormwater inlets, surface conveyance, storm drain pipes, and the Sand Hills detention pond.
The 18th Street South storm drain interceptor had been in use for several decades, but the aging infrastructure was proving to be inadequate for the increased runoff caused by urban development and several severe rain events which caused flooding in areas along the interceptor.
The storm drainage computer model was used to analyze the existing infrastructure and identify locations prone to flooding. In some areas, designing additional surface inlets and increasing the interceptor pipe capacity were enough to reduce the chance of flooding; however, this did not solve all of the problems predicted by the model. Several intersections in the study area are located in a physical “sag”, or bowl, where the water cannot naturally drain away in the gutters or streets and have experienced flooding the past during severe rain events. In sag intersections, stormwater is removed through surface inlets or by providing a surface overflow to direct the water towards a detention pond. During severe storms, the inlets may have inadequate capacity to transmit the large flow rates into the underground storm drain pipes; the pipes may also become surcharged, resulting in a reduced capacity.
To relieve the flooding in the sag intersections, two new detention ponds will be designed to receive excess surface runoff from the gutters and streets and to accommodate storm drain pipe surcharging. The Sand Hills detention pond will be expanded to provide greater storage capacity during large storm events.
The improvements to the 18th Street South Storm Drain have been split into several phases. Phase 1A – Sand Hills Detention Pond Expansion was completed in 2013. Phase 1B – UGF, 11th Ave. South & 18th St. South consists of storm drain pipe and inlets; it has been bid and will begin construction soon. Additional phases involving new storm drain pipe, inlets and two detention ponds are in the design stage.
For more information on how TD&H Engineering can help you with your storm drain or run off issues please feel free to call and talk to one of our many qualified engineers at any of our seven locations.
Contribution by Camille Johnson, EIT
Posted on May 21, 2014