Storm water and water run off serve as a major source of pollution as well as minor flooding. Water
Resource Engineers deal with the issues created by storm water every day. Engineers in this field work to mitigate damage created by storm water and control flow volumes, one of these tools is known as a
Bioretention Garden. A bioretention garden is a best management practice (BMP) that uses the combination of a standard water collection basin and plant life to better manage storm water. Since the gardens employ natural processes they are becoming an increasingly considered BMP alternative
Basic schematic of a bioretention garden. Note the use of soil layers.
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The most important part of any bioretention basin is the soil placed underneath it. These soils and aggregates serve as a foundation for the plant life as well as simplistic water filters to eliminate pollution that might reach ground water sources. There are three main parts to the soil basin are as follows:
Gravel Layers, Sand Layers, Planting Soil.
Gravel Layers:
After much of the existing soil bed is removed the first layer of soil placed is a layer of gravel. This gravel removes larger contaminants by allowing the bacteria and other organisms to colonize on the individual stones. Typically this layer is fairly thick and placed closest to the existing soil bed, serving as the last filter to protect any sources of ground water underneath. It also creates a solid foundation to build the rest of the garden on and evenly distributes water infiltrating the existing soil.
1 Image from: https://www.pinterest.com/pin/125889752053600951/

Sand Layers:
The sand layer is where the majority of filtration occurs. Just like a Slow Sand Filtration system, the sand has a high content of silica or anthracite to react with contaminants and actively remove them from the water. The sand is tightly packed slowing how quickly water seeps through to ensure that all contaminants are eliminated.
Planting Soil:
Planting soil is the uppermost layer that serves as the foundation for plant roots and the pond bed. The soil is tightly packed and absorbs large amounts of water to allow the plants to soak up and transpire the deposited water. This layer also limits the infiltration rate of the water to help the sand layer better manage the pollutants it needs to remove.
The number of layers in the soil basin can change from garden to garden. Different combinations of the layers will work better for different area depending on a number of factors like the number of storm events and how large the events are among other things.
Water will always move in the path of least resistance seeking the lowest elevation possible. To guide water to and from the retention garden inlet and outlet spillways are constructed.
Inlets:
Spillways made of concrete or large riprap stone lead water from roadways, parking lots, or even buildings. Typically these spillways will have some sort of baffle devices to dissipate energy created by the water and protect the soil layers of the garden. From these spillways water can begin to pool up in the basin and feed the plants.
Outlets:
During heavy storm events it is not uncommon for gardens to become too full and become completely submerged. In such cases emergency spillway weirs and overflow pipes are used to remove some of the water. The overflow weir’s usually lead to a smaller secondary basin constructed in a similar manner to manage the left over water. Meanwhile the overflow pipe, labeled in the diagram above as an open throat catch basin, usually spills water into storm drains to be treated elsewhere.
Plant life is an essential part to the bio retention garden, the plants absorb the water deposited in the basin and remove it via transpiration. This not only benefits the foundation of the garden and removes pollutants but keeps the plant life alive. Naturally, some of the water is also evaporated by heat, however the combined removal of water, known as evapotranspiration, in most cases removes the deposited water with ease. The plant life from garden to garden may also vary, in places like Central
Pennsylvania and Maryland pond lilies and cat tails are usually employed, however in western states like
Arizona or New Mexico plants like aloe and cacti are more likely to be used. The kind of plants used is dependent on how much water needs to be absorbed and how fast. In areas with flakey earth like the mid-west it’s best to absorb the water rather quickly to protect from mudslides, on the eastern sea board where much of the ground is already moist and supported by a layer of limestone bed rock this is not as important.

As with any BMP, bioretention gardens have their pros and cons. The limitations and scope of each water management project can vary widely, so the following should be taken into consideration when choosing the most efficient BMP.
Advantages:
Bioretention gardens are extremely flexible and can fit almost any scenario. Everything from the soil basin, to the size, and the plants employed can change depending on your needs. To the right are two examples of completed gardens, one in a parking lot and another in an urban setting.
(2,3) Note how they have been adjusted to better suit the environment they are placed in. These gardens require very limited maintenance as well. With the exception of extreme circumstance such as draught or frozen earth, the plants should survive with ease and return each season. And since the basin is made entirely of natural materials they do not need to be deconstructed when they’ve surpassed their usable life.
Disadvantages:
Gardens typically are a slightly more expensive than a standard drainage ditch since a high volume of planting soil and plant life needs to be purchased. This also means they might require a little more planning as well, as some plants need space to grow and might need fertilizers. However, in most cases the price difference is not enough to eliminate it as a valid alternative to a standard drainage ditch.
Using natural processes bioretention gardens are becoming the best option when choosing a BMP. With the need for environmentally friendly engineering continuing the rise these gardens will show up more and more in rural and urban areas to better handle storm water and water runoff.
2 Image from: http://bluegreenbldg.org/wp-content/uploads/2011/01/LowesIndWay20110110_03.jpg
3 Image from: http://greenroofconsultancy.com/can-swamp-forests-come-to-town/rain/