Urban Rainwater: from Problem to Resource

Over the past two plus years I have felt privileged to work closely with the Minneapolis Convention Center (MCC) – leading their facilities and management team through the LEED v4 Existing Buildings: Operations & Maintenance (EBOM) certification process and helping them pursue and track some aggressive energy and water conservation goals. The MCC is a physically massive and complex building, consisting of over 1.6 million square feet of convention, meeting, and support spaces, that sits on approximately 10 blocks at the southern edge of Minneapolis’ central business district.

Because of the sustainable operations already in practice at the MCC, the EBOM certification process was primarily about documenting existing practices. However, more than one opportunity for further conservation was discovered through the LEED process.

One very impactful discovery was made in pursuit of the “Site Improvement Plan” LEED credit. This credit prompts a master planning exercise, with the focus on assessing the site’s conditions and finding unique opportunities to improve the sustainability of the site as a whole. During this process, we identified a perfect synergy for an urban rainwater harvesting system.

Over 90% of the MCC’s total site – or nearly 1.2 million square feet – is impervious. Impervious surfaces are materials that rainwater cannot infiltrate through, meaning that rain falling on these surfaces immediately becomes stormwater runoff. Impervious surfaces – buildings, sidewalks, roads, even highly compacted grassy areas – are unfortunately common in urban settings and have significant negative environmental impacts that include water pollution and the urban heat island effect.

In addition to the environmental impacts from urban runoff, the excessive amount of storm water on these impervious surfaces results in significant operational costs. We knew from the start that the environmentally responsible and cost effective solution to the runoff from the site’s impervious surfaces would be to capture the rainwater and divert it from the city’s storm sewer system. As is often the case on urban sites with over a century of historic development and complex site constraints (buried utilities, underground structures, and past issues with soil contamination) it was clear that infiltrating rainwater onsite could have more negative impacts than positive ones.

The MCC’s site also includes some landscaped areas with high irrigation requirements. The outdoor summer water demand for these areas was posing a significant hurdle to the MCC’s potable water reduction goals. Instead of infiltrating the water onsite in underground systems, the team recognized the opportunity to harvest some of it, and use it to offset the drinking water used in irrigation applications.

The result was a multidisciplinary project led by LHB’s Civil Engineering group, with input from our Landscape Architecture and Performance Services teams. The project outcome included a system that will collect rainwater from 400,000 square feet, or nearly half, of the MCC’s rooftop, and directing it to a 250,000-gallon underground storage system. The water will be collected and stored year-round, and then used to irrigate the site’s landscaped areas during the summer. It is estimated that this system will prevent 4-5 million gallons of stormwater runoff from entering the city’s storm sewer system and draining into the Mississippi River each year, while saving another 4-5 million gallons of drinking water in the process.

Photos courtesy of Minneapolis Convention Center

Construction is completed and came fully online in the spring of 2017. I look forward to seeing the impact of this system on future potable water consumption at the MCC. The latest version,  LEED v4 Existing Buildings: Operations & Maintenance (EBOM) is significantly harder than the versions prior. LHB congratulates the MCC for stepping up to the challenge of becoming the first convention center in the country to pursue and achieve this aggressive LEED v4 EBOM certification, but even more so, for their unwavering commitment to being sustainability leaders – and responsibly operating and managing this remarkable facility.

Authored by Thea Rozenbergs, September 18, 2017

 

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