Rain-gardens – the secret tool to reduce stormwater runoff
Posted on: March 30, 2020

Temperatures are slowly starting to rise and the grass is starting to make an appearance, some may suggest that spring is on the way. With the onset of spring comes getting outside, digging in the dirt, biking to work, and busting out a pair of shorts. But the onset of spring also brings spring floods, melting snow, and precipitation in the form of rain. This results in the amount of runoff going into waterways to increase. Runoff can potentially be harmful because it increases flows that can increase rates of erosion. Runoff can also carry possible pollutants such as sediments, nutrients, hydrocarbons, bacteria and other pollutants from the land to our aquatic ecosystems.

Runoff is accelerated when it comes into contact with impervious surfaces such as sidewalks, driveways, roads, or any other surface that black the percolation of water. One way that is relatively inexpensive that reduces runoff into waterways is through rain gardens.

Rain gardens are a relatively inexpensive way to prevent excess runoff from entering the waterways. Rain gardens reduce and retain stormwater runoff as well as increase percolation and evapotranspiration which can reduce storm flows and pollutants from entering the water.

Raingardens typically replace areas that once had a non-permeable surface. They are usually in a depression which contains a soil mixture of sandy loam, mulch, and plants that can withstand wet and dry water conditions. The mulch layer can sometimes be omitted, but it has been found that the mulch is a good mechanism for increasing pollutant absorbency. Raingardens can connect to a natural groundwater recharge system, or they can have an impervious boundary at the bottom to collect water.

Most studies on rain gardens take place in a temperate region, but Thunder Bay typically has below zero temperatures from October to April and depending on the season freezing conditions can be even longer. A study from Norway looked at the role of rain gardens in a cold-environment and found that things behave a little differently when temperatures drop below freezing. In cold-climates such as Thunder Bay, the soil moisture prior to freezing plays a critical role in the effectiveness of reducing runoff which depends on infiltration rates.

Raingardens work because water can percolate through them rather than running off them like a sheet. In order for water to infiltrate effectively through frozen or partially frozen soil, the soil must be frozen with certain moisture content. Frozen soil takes three general forms, concrete, granular, and porous. Concrete soil occurs when saturated soil is frozen and creates an impermeable layer. Granular soil occurs when unsaturated soil freezes and creates a permeable layer that can sometimes outperform unfrozen soil. Porous soil is in between concrete and granular. Rain gardens are effective in cold climates when concrete soil conditions are avoided. One way to increase winter infiltration is to increase the sand content of your soil.

We are currently in a unique time where gardening can serve as a tool to get outside for some sunshine while socially distancing yourself from others. Building a rain garden is a great activity that will benefit you and the environment. Just remember to practice advice from healthcare providers if you need to venture out to get supplies.

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