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Author: Darren Hamrick, PE, MS, LEED AP, Leader/Civil Engineering Division
A relatively new topic, green infrastructure for drainage has become increasingly popular in recent years, as the desire to take care of our watersheds becomes even more important. In addition, new stormwater ordinances and more stringent criteria for state permit renewals are contributing to this shift in site planning and design. A green infrastructure design uses site planning techniques to manage stormwater closer to its natural pattern as well as reduce runoff.
Rain in natural environments soaks into the ground and is filtered by soil and surrounding plants. In developed areas, rain can’t be absorbed (roads, buildings, etc.) and more water, called runoff, is formed. The problem with runoff is that it can cause issues such as flooding, stream bank erosion, and increased water pollution.
In a green infrastructure site plan, stormwater is directed into natural areas, which allows infiltration and natural detention. The timing and overall rate of water running off the site would more closely match what would have happened before the development occurred, which is one of the main benefits to green infrastructure: post-development hydrology meeting pre-development hydrology.
There are many things to consider in site planning with a green infrastructure design; volume in a post development and sediment transport in the system. For example, a piece of property that is developed as a shopping center or an office complex, may have a retention pond for stormwater management, but it may release water and only address the peak-flow rates discharged from the pond for a specific designed storm. In actuality, it may not mimic the pre-development hydrology and how water flowed off the site. A green infrastructure design is intended to more closely mimic runoff characteristics to the pre-development conditions.
Sain is currently in the midst of a new project involving a green infrastructure design in the Birmingham area. The project involves an area where a railroad was built decades ago that was not sufficiently designed for full development of the watershed. Over the years, the developing watershed was not required to have stormwater management, which resulted in peak runoff rates far exceeding the capacity of the railroad culvert and caused flooding. As we analyzed the overall areas being flooded, we determined that increasing the culvert size under the railroad would not solve the larger problem, and it would simply push the problem downstream.
Based on this analysis, we felt like we needed to provide storage in the watershed, to provide a place for the excess runoff to be detained without flooding streets or buildings. We were able to find a large vacant area adjacent to the railroad which was unused due to overhead transmission lines, and could be graded to provide the additional storage for this excessive amount of runoff. In the past, this area had not been maintained and was unsightly due to abandoned cars being visible from the road.
Sain’s recommended approach and a creative solution to this problem is to convert this area into a greenspace area, which will allow for stormwater storage and filtration. This way, the water can flood naturally, not in the streets and buildings. This design approach provides the opportunity for the runoff to infiltrate into the ground and filter through the grass, while adding more natural, green space to this area.
This green infrastructure improves the flooding problem and costs less, which is a common misconception about green infrastructure. Many people believe this technique to be costly, a premium, but the truth is quite the opposite. Studies have shown that green infrastructure costs less because it creates more attractive, revenue-generating spaces.
As new laws and standards are implemented regarding water quality, runoff rates and stormwater volumes, green infrastructure for drainage will likely continue to grow in popularity. It is a great tool to reduce flooding in developed and developing areas, while respecting the natural environment.