Case Studies > Haddam Demonstration Site > Rain Garden > Results > Flow

Flow

Most influent left the rain gardens as subsurface flow (95.4%). Only 0.8% of the inflow water overflowed during the entire study period (Table 2). The residual volume (3.7 %) was assumed to be evapotranspiration (ET) from the gardens. The soil matrix used in this rain garden was the native loamy sand soil. It is expected that if no liner were present, the volume of percolate that left through the underdrains would exfiltrate into the surrounding soils. This means that if a rain garden did not have an underdrain in soils such as these, over 99% of flow and associated pollutants would be retained by this garden, which was sized to contain 1 inch of runoff. This retention may be lower if the surorunding soils have a lower permeability.


Table 2 below , precipitation and flow data for one event in October 2003 demonstrate the ability of the rain garden to reduce peak flow rates and increase the time to peak. The timing and shape of the inlet (roof runoff) hydrograph follows the precipitation hyetograph closely. However, the underdrain outflow shows a lower peak flow rate and a delayed response to the precipitation event.

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