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February 10, 2012

Soil: A Foundation for Achieving Net Zero Runoff

You might recall our Net Zero discussion began with a short article entitled "Taking Irrigation Off the Grid".  Part I of this 3 part series centered around capture and reuse irrigation technique. Picking up our discussion regarding essentials to achieving Net Zero, this article focuses on the need for proper soil management for proper plant health and water management function. You will recognize some necessary criteria required by the Contractor during construction that may be new to you.  Soil testing is essential. If we don't know the materials or conditions we are starting with, we cannot achieve expected results and we cannot anticipate a predictable outcome!

Soil: A Foundation for Achieving Net Zero Runoff by Eric Davis RLA, ASIC, LEEP AP

Net Zero projects require Landscape Architects and our allied professionals to carefully consider the water cycle of the project site.  Net Zero projects need to evaluate the water demands of the new construction, the waste water that will be generated, and the runoff produced by normal rains.  Techniques for reducing water demand and waste generation in the building are best tackled by Architects and MEP Engineers, but Landscape Architects must have a well-stocked 'toolbox' at their disposal for addressing the water cycle between the building walls (and roof) and the boundaries of the site.  As discussed in a previous article, one method (or tool) for reducing runoff is to capture it as a supply source for irrigation.  Methods for large-scale reduction in runoff revolve around keeping the water on the site and allowing it to percolate through the soil, with an eye toward water quality improvement and groundwater recharge.

Absorption Zones

Landscape Architects have done a good job in promoting these techniques and the most common are becoming a staple of sustainable design.  Not that long ago raingardens and bioswales were 'fringe' ideas struggling to gain popularity; today they find themselves indoctrinated into community development ordinances.  As methods for collecting and absorbing excess rainwater, these 'absorption zones' are perfectly suited for incorporation into a net zero project.  The drawback to absorption zones is that they function better at maturity than at installation.

As the plants in absorption areas/BMP's grow, they extend an impressive root mass into the soil, opening it structurally and providing new pathways for water infiltration and lateral movement.  The plants, if selected appropriately, will 'ramp up' the performance of the raingarden as the roots mature, reaching peak performance within two or three growing seasons.  Problems arise when raingardens are installed in clay soils with limited pore space (porosity) and a corresponding slow infiltration rate.  In these instances growth of the plants can be slowed, retarding the maturation of the raingarden, and there is also a chance that the new plants will drown due to the slowed infiltration rate. Landworks Studio, in an effort to minimize the risk to the plants and maximize absorption, has incorporated several new soil-related techniques into our recent projects..

Importing Soil 

Importing suitable topsoil for every project is simply not a responsible or sustainable practice.  Imported topsoil must be stripped from an existing location which, in addition to destabilizing the borrow site, adds additional environmental impacts through motorized harvesting and transportation.  Landworks Studio prefers to work with the soils available on the site to reduce these impacts and keep costs manageable.  

Our Approach

Landworks Studio's approach on these types of projects is to: analyze the existing soil's properties, develop an amendment program to achieve a baseline performance, and test and verify the amended soil.  In addition to amendments such as sand and organic matter, Landworks will specify an amendment recipe that includes organic fertilizers and soil inoculations of mycorrhizae to further encourage vigorous root growth. The QA/QC procedures in the specifications will verify consistent proportions for all the materials and Landworks Studio will work closely with the testing lab and contractor to make any adjustments, as they may be needed.  These processes must be clearly defined in the technical specifications and the Contractor must be on board with all of them from the moment they are awarded the project.

In conclusion, while selecting good plants for a bioswale or raingarden is necessary, it may only be of secondary importance to the overall goal of maximizing water absorption.  Analyzing the existing soil and developing a specification to loosen and open the soil structure is, at least to Landworks Studio, of primary importance.  Raingardens with amended soils will absorb more water, have healthier plants, and reduce more runoff than those constructed from in-situ topsoil or even those with a haphazard approach to amendments.  Testing, specifying and verifying throughout the design and construction phases helps maximize the potential for success.