Managing Stormwater for the Next Generation

Stormwater Treatment

Managing stormwater runoff is important to urban planning and environmental conservation. The site of Beacon Bluff drains an area of approximately 163 acres of stormwater that flows into the Phalen Creek tunnel and ultimately the Mississippi River. An existing 48 inch storm pipe, located in Duchess Street, connects to the 14 foot diameter Phalen Creek Storm Tunnel that discharged untreated stormwater directly into the Mississippi River.

The Port Authority and the City of Saint Paul were challenged to design a way to divert the first few inches of runoff from heavy rainfall, typically containing high levels of sediment and contaminants, into a stormwater treatment system.

The stormwater treatment system uses new materials in its construction. It implements an integrated monitoring system for long-term collection of data to measure how clean the water is before it enters the groundwater system and how much stormwater is infiltrating the ground and recharging groundwater aquifers. This interpretative panel is intended to promote understanding of stormwater runoff and the benefits of proper treatment to our lakes, rivers, and groundwater sources.

The Next Generation Treatment System 

The Next Generation stormwater system is one of the largest underground infiltration systems in the state of Minnesota with a footprint about the size of two basketball courts end-to-end and a storage capacity of over 150,000 cubic feet (slightly less than two Olympic-sized swimming pools or over one million gallons). This system consists of three rows of 10-foot diameter perforated corrugated metal pipes totaling over 650 feet in length.

Pipes, wrapped in a filter fabric, are laid in sand below a bed of shredded tire chips (substituting for stone aggregate), sand, engineered topsoil, and plantings. These pipes are connected by a sump manhole structure fitted with a baffle system developed by the Saint Anthony Falls Laboratory (SAFL) at the University of Minnesota to keep sediment out of downstream water bodies. This is the first time a SAFL baffle was incorporated into a Next Generation stormwater drainage system, a first in the nation!

The SAFL baffle design uses holes in a grate to slow down the velocity of water and allow sediment in the water to settle into the sump manhole. The settling process removes up to 90 percent of the sediment and contaminants attached to the sediment. The captured sediment and pollutants are removed periodically by vacuum truck.

Vertical pipes contain monitoring devices to measure infiltration rates and to sample water quality.

Treatment Monitoring and Data

Water quality and water volume are monitored at 21 separate locations. The monitoring devices include storm sewer manholes, lysimeters installed in underground barrels, pressure gauges to measure water height in the 10 foot diameter pipes, and an onsite rain gauge.

Monitoring data will provide documentation to determine if the system is performing as anticipated. The following data are being tracked: the volume of water diverted from the 48 inch pipe and captured in the Next Generation system; the reduction in pollution; the infiltration rates into the soil; and the amount of water recharged into the groundwater aquifer.

The first large rain storm recorded in 2012 recharged the groundwater aquifer with over one million gallons. Data collected from the monitoring devices since 2012 show that the system is meeting or exceeding expectations.