The Alton Mill is located in Caledon, Ontario, situated alongside a cold water fishery which discharges into the Credit River and ultimately into Lake Ontario. The Alton Mill Initiative is a multi-faceted, public-private partnership aimed at converting a century-old plus heritage stone mill and pond complex into an artisan-oriented tourism destination.
The stormwater management servicing plan for this initiative was designed to reduce development impacts on adjacent properties and the receiving watercourse. The design featured a low-impact infiltration gallery, along with 120 m (390 ft) of storm sewer and swale collection system network throughout the 1.1 ha (2.7 ac) site. A recent version of the U.S. EPA StormWater Management Model (SWMM5) was used to represent the hydrology and hydraulics of the site and stormwater facilities. The design objective was to preserve the natural infiltration features of the underlying soils on site and discharge runoff from frequent rainfall events into a central infiltration gallery. The gallery was designed to temporarily store surface runoff for subsequent percolation and groundwater recharge. Runoff is discharged evenly through a manifold perforated pipe system into the gallery, which is filled with continuous 50 mm (2 in) clear stone and covered with native soil material. Runoff exceeding the capacity of the collection system or gallery is designed to discharge via overland sheet flow into the receiving watercourse.
Modeling results indicated that stormwater runoff for all storms up to the 10-year return period event can be contained on site and dispersed through the infiltration gallery. It was also confirmed that the peak hydraulic gradeline remains below basement floor slab elevations in all adjacent buildings for all design storms up to the 5-year return period event. A scenario was simulated that represented a failure of the infiltration gallery under frozen/clogged media conditions, in which there was no infiltration and the storage within the gallery (void space in the porous media) was greatly reduced. Model results confirmed the resulting overflow depths and velocities were within acceptable limits for the 100-year return period events. While design storm events were used to establish the basis for design, continuous hydrologic simulation was also undertaken to evaluate the long-term water balance and to verify the system’s hydraulic performance during average annual conditions.
The servicing plan was completed as part of the submittal for site plan approval and permitting in October 2006. Construction of the stormwater facilities was completed in September 2008. Continuous simulation to confirm the long-term hydraulic performance of the infiltration gallery was completed in January 2010. This paper highlights the key project features and critical lessons learned from all phases of the project, including hydrologic/hydraulic modeling, design, permitting, construction, and inspection.