UPDATE: Potential Drainage Issues with Podium Landscaping (on Slab) Including Green Roofs

The District of Squamish has found that podium style landscaping, landscaping placed on top of a slab or other hard surface, can result in stormwater nutrient levels that exceed allowable limits.

Plants need nutrients in order to grow and thrive, and landscaping soils are specifically designed with this in mind. When water passes through the soil, it absorbs some of these nutrients and carries them away from the landscaping. When the water goes directly into the ground, there are natural processes that absorb or treat the nutrients. But when the stormwater discharges directly into watercourses such as streams, sloughs, rivers, or the ocean, or when it leads to the District storm system which consists of hard pipes and ditches that lead to watercourses, there is the potential for these nutrients to harm the environment and negatively affect fish health.

These high nutrient levels are not unique to specific soils. We have found that any soil with enough nutrients for plants to grow, including lower nutrient “riparian soils,” will result in high levels of nutrients in the runoff.

Drainage runoff from podium landscaping can be treated for high nutrient levels prior to discharge to the municipal system through natural filtration via infiltration into the ground, mechanical filtration, or bio filtration using a special media. Through research on filtration options, comparison with policies of other municipalities, feedback from the development community, and our own observations, we recognize that mechanical and bio filtration using a special media is not always a practical approach.

The Solution:
The District requires, where possible, that drainage from podium landscaping be directed to ground where it can be filtered through natural biological processes before it reaches watercourses and sensitive habitat. Where this is clearly demonstrated to not be possible, we may accept storm runoff without additional filtering. This will require prior review and approval from the District Engineer.

Questions?
Please contact the Engineering Department for more information.

March 10, 2021

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