Rain Garden Prototype at Boys Choir Lot

WHAT IS BEING STUDIED AT THE BOYS CHOIR LOT?

The goal of this research project is to understand how soil composition affects rain garden performance. By using a variety of sensors, we can better understand key relationships between precipitation, soil moisture, infiltration and runoff.

This experiment is testing two types of soil: Standard Stormwater Soil (SSS), and High Sand Content Soil (HSS).

WHAT IS A RAIN GARDEN?

A rain garden receives stormwater generated on surrounding surfaces,  helping to infiltrate it into the soil. Rain gardens can take on water from a roof, patio or yard, and are also being installed in sidewalks to capture runoff water generated from the street. Rain gardens are a type of Green Stormwater Infrastructure (GSI) being installed throughout Philadelphia to improve water quality in the Schuylkill and Delaware Rivers and their tributaries.

Rain gardens are one of the simplest and most effective ways to reduce stormwater pollution, while providing extra benefits such as beautifying the urban landscape and creating habitat for pollinators!

HOW DOES THIS EXPERIMENT WORK?

The bottom 18 inches of the barrel is filled with gravel, on top of which is  a geotextile mesh and 12 inches of either SSS or HSS soil. HSS has more porous space, so water can move through it more quickly.

Some barrels have a tarp to simulate a rain garden built in the sidewalk that also receives runoff water from street. This increases its hydraulic loading ratio (or HLR).

As rain flows into the barrel and tarp, it travels through the sand and gravel, and some flows out the discharge pipe. The scale measures how much of the water is retained in the soil.

Site Layout

The original site layout (below) had 12 barrels to simulate 4 soil and HLR combinations. The layout changes occasionally according to available and working sensors.

HOW DO WE COLLECT DATA?

Monitoring systems at this site include soil Decagon soil moisture probes inside the barrel soil, scales under the barrels, and outflow measurement box in from of the barrel. These inputs are wired for each scale into a microcontroller (called an Arduino Uno) with a data logging shield, which stores all the data locally on an SD card.  A set of shallow and deep soil moisture probes are located in the soil at 6cm and 20 cm depths to monitor the infiltration time. Data from the scale helps for mass balance analysis before and after rain events to approximate soil water retention. Finally the outflow measurement system is designed to measure the outflow rates from the system which indicates the field capacity of the rain garden and helps closing the mass balance. 

A solar-powered weather station collects atmospheric data such as temperature and precipitation, and stores that information in a data logger. This allows the research team to use local precipitation data to correlate rainfall patterns with soil weight and moisture.

WHAT DOES THE DATA LOOK LIKE?

Below is a timeline of precipitation and soil weight (measured from the scale). The top chart shows the cumulative rate of rain application to the barrels (white region). The light gray shading is the period immediately after rain when the barrels are still draining. Most drainage has ended once the dark region is reached. The bottom chart shows the soil moisture and scale response to the precipitation in one of the barrels over the same time period.

Below are other monitoring results from the experiment:

WHAT HAVE WE LEARNED SO FAR?

HSS Soil:

  • Retains less precipitation than SSS.
  • Conveys rainwater downward through the soil more rapidly than SSS, and thus will be able to manage more stormwater.
  • May dry out faster in between storms.

SSS Soil:

  • Conveys water less rapidly.
  • Temporarily stores rainwater in upper soil region.
  • Makes more water available to plants for transpiration.
  • Manages less stormwater than SSS and dries out more slowly in between storms.

This experiment is ongoing! Future research will statistically compare rates of evapotranspiration and incorporate findings for different HLRs.

Sweet Coneflowers in bloom in the rain barrels