New commercial development in Moncton meets strict runoff regulations
The new McLaughlin Place retail centre found the solution to meeting the local mandate of eliminating any increase in storm water runoff while maximizing the number of parking spaces. Instead of using a detention pond or sump, the designers decided to use a system of chambers under the parking lot that would collect and hold storm water runoff from the lot and rooftops. The one hectare commercial development with five buildings will open this year and is located near the Université de Moncton. Plans call for apartment buildings to be added in the future.
"Based on the city’s design criteria, there could not be any increase in storm water runoff into Moncton's storm sewer system from the McLaughlin Place," said Denis LeBlanc of WSP Canada Inc., the project engineer. "This is in an older, fully developed part of the city with the infrastructure -- roads, storm sewers -- already there around it. The city has a zero net increase storm water policy, which means that when you develop a site your post-development flows have to equal the pre-development flows. In our case there had been some old houses and an old skating rink on the property that had been demolished a few years before but the downstream storm sewer was still limited by capacity so we had to go above and beyond the zero increase. In our design we actually had to reduce pre-development flow conditions due to the undersized storm sewer that was downstream of our site."
Moncton is the geographic centre of the Maritime provinces and has a population of nearly 70,000. Home to the Université de Moncton with some 5,000 students making it Canada's largest French-language university outside of Quebec, the area is also known as the 'Hub City'.
"Here in Moncton we do a lot of open, dry detention ponds," he explained. "The reason for that is because land value isn't that elevated here meaning that developers could usually afford to lose a bit of land to put a pond in. In this case we didn't have any available land and our site was fully covered by buildings or parking lots and underground storage was our choice for detention."
To satisfy the city's Zero Net Increase for Storm Water Runoff law P#215 and meet the site's storage capacity requirement of 485 cubic metres, the underground system used 87 StormTech MC-3500 chambers in a 26.8 x 17.9-metre area.
According to Advanced Drainage Systems, Inc. (ADS), the MC-3500 chamber is designed to meet the full scope of design requirements of CSA B184 standards. For this project, the structural design of the chambers, backfills and installation methods met CL-625 (H-20, HS-20) loads. The chamber also meets Section 12.12 of the AASHTO LRFD Bridge Design Specifications and is produced to the requirements of ASTM F 2418 ”Standard Specification for Polypropylene (PP) Corrugated Stormwater Collection Chambers”. It also provides the full AASHTO safety factors for live loads and permanent earth loads.
Each StormTech MC-3500 chamber is 2286 long x 1956 wide x 1143 mm high with minimum installed storage capacity of 5.06 cubic metres. The open graded stone around and under the chambers provides a significant conveyance capacity ranging from approximately 23 l/s to 368 l/s. Actual conveyance capacity is dependent upon stone size, depth of foundation stone and head of water.
The excavation was 3.3 metres deep that allowed for 2.13 metres of cover above the chambers. Gravel bed was ¾ to two-inch washed rock. A rock slinger was used in order to place the stone faster. A non-woven geotextile separates the native soil from the washed rock. To convey the water from the catch basins, ADS N-12 corrugated high-density polyethylene (HDPE) pipe was used to create a 600 x 450-mm manifold connected to the first four chamber rows. Products came from the local ADS distribution centre in Moncton and supplied by Wolseley Canada - Waterworks Division. The system was installed by Yves Excavating.
"We're not calling it retention but detention," LeBlanc continued. "The water is not infiltrating into the ground. Our soils are all clay here so there's little to no infiltration. This means you need to still outlet it to a pipe or a sewer. Our chambers are on the bed of gravel with the geotextile under it to prevent the clay from interacting with the gravel. From there the water goes into a control structure at the downstream of the chambers and that is basically just a manhole with an orifice in it, and then from there it goes into the municipal storm sewer."
To convey water from the underground detention system to the municipal storm sewer , a 250-mm diameter solid wall DR17 high-density polyethylene (HDPE) pipe was horizontally directionally drilled (HDD) nearly 16 metres under Morton Avenue, a major road. "This was needed because the city couldn't shut down any lanes of the road," he explained.