Hybrid stormwater management facility

By Stephen Mitchell, C.E.T. Corm Construction Sewer & Watermain Ltd.

Accurate grading during construction should never be taken for granted, even when constructing a stormwater quality, (end-of-pipe) facility where water flow and drainage remain critical design elements.

Because of the unique design and challenging construction activities associated with the Cane Parkway Stormwater Management Facility in Newmarket, Ontario, Corm Construction decided to use its own resources for construction surveying, since grade setting and checking would be an ongoing service requiring immediate use of equipment at any given time. The window for construction was limited, and the contractor had little opportunity in the construction schedule to depend upon outside geomatics services. Since the project was located adjacent to an existing wetland and within the regulated floodplain of the Holland River, there was no room for grading errors that may cause delays.

Hydraulic grade-line was of greatest concern to the consulting engineer, Cumming Cockburn Limited, due to the closeness to outlet to river. Cumming Cockburn was hired by the Town of Newmarket Public Works and Environmental Services Department, in conjunction with the Lake Simcoe Region Conservation Authority, to prepare the detailed engineering design and Holland River walkway extension and improvement. The walkway improvement included installation of two new 30,000 lb footbridges that can accommodate maintenance vehicle access to the site.

The challenge to the consulting engineer was to achieve maximum storage in the wet pond within design parameters governed by the existing hydraulic grade line.

The Cane Parkway Stormwater management facility is a hybrid facility consisting of a flow splitter, a sediment forebay, a wet pond, and a constructed wetland with micro-pools and submerged outlet culvert. Located immediately west of the Municipal Office on Mulock Drive, the facility and walkway system provides a focal point in the community. The hybrid facility is also adjacent to the 900 mm diameter York-Durham sanitary forcemain that influenced the facility design in the vicinity of the constructed wetland and pools. Not only does the facility improve water quality treatment for the 102 hectares of developed urban land immediately upstream, it also improves the quality of the Holland River ecosystem. All interested parties supported the project and had opportunity to contribute to the design of the facility through a “Schedule B” Class Environmental Assessment.

Constructed at a cost of $430,000, the hybrid facility is functioning as designed, within a critical hydraulic grade line of approximately one metre.

A 1650 mm diameter storm sewer discharges water into the site from the upstream development. In the vicinity where stormwater enters, Corm constructed a weir with armour stone revetment to direct water over a shallow nine-metre channel into the forebay of the wet pond. The elevation of the inlet weir was 23 cm below the top of a flow splitter, and 23 cm below a cofferdam that stored water behind its armour stone structure at an elevation that allowed the backed up flow to enter a concrete pipe into the existing wetland, as well as the forebay. As water enters the wetland it passes through the dense, indigenous vegetation where sediment and nutrients are removed naturally before discharge into the Holland River. The clarity of the discharge from the wetland contrasts significantly with the cloudy appearance of the river.

In a major rainfall event, water flows over the weir into the sediment forebay, and may spill over the cofferdam into an existing drainage channel that discharges directly to the river over a gradient of approximately one metre. The hybrid facility is not designed for flood control of the upstream development.

The elevations of the top of the weir, revetment, flow splitter and cofferdam were critical to the ability of the facility to cleanse stormwater. As water flows over the shallow weir into the forebay, sediment settles before the flow continues over a second, wider weir at a lower elevation. The forebay is designed for cleanout every 7.5 years.

Once over the second weir, water is now in the 2.5 metre pond. The 7,532 m³ permanent wet pond was designed with a 7:1 slope on its eastern shore, adjacent to the public pathway. This gentle gradient is a safety feature that is designed to enable anyone who slips into the pond to get out without great difficulty. The gentle shoreline eliminates the need for a chain link fence to keep people and animals out of the wetland. The pond has an extended detention capacity of 5,683 m³.

Water leaves the wet pond through a controlled 375 mm diameter pipe and concrete headwall at an invert elevation that is about 60 mm below the top elevation of the shallow weir at the entrance of the forebay. Devices can be installed on the headwall to sample water leaving the wet pond for water quality.

The drainpipe is directed to a manhole that redirects flow to the first micro-pool at the head of the constructed wetland adjacent to the Holland River. The in vert of the outlet structure is 400 cm below the invert of the outlet from the wet pond.

The first micro-pool (plunge pool) is approximately 1.3 metres deep and is connected to a second micro-pool with a weir that is about 0.3 metres below the water level of the river. A 1.0 metre high berm separates the micro-pools from the river, but water is channeled to the river through a submerged 375 mm (level-grade) pipe from the second pool. The pipe passes under a gabion matt spill way allowing fish to pass between the micro-pool and the river. Any remain ing sediment entering the micro-pools is settled out before entering the river.

In the event of a major storm, where the hydraulics of the submerged pipe are inadequate to carry the flow, water enters the river directly over the 1.7 metre-wide spillway. This design feature prevents flooding of the hybrid facility and potential erosion damage to the constructed wetland facility, sanitary forcemain and walkway system.

Since construction was in the floodplain, the Department of Fisheries and Oceans requested that no in-water work occur between March 15 and July 1,2001, to avoid any adverse impact on fish habitat, and to protect local fish populations during spawning and nursery periods. All excavation and in-water work were completed in the winter months and before the end of January 2001.

During construction, necessary erosion and sediment control measures, as well as vehicle control measures were taken to prevent substances such as soil particles and fuel from entering the river and affecting fish habitat. Stockpiled materials and disturbed areas were immediately stabilized, vegetated or revegetated during construction to keep sediment and any other substances away from the river.

The Town of Newmarket awarded the contract to construct the facility in October. 2000. Completed by June 2001, the new hybrid facility already contributes significantly to the water quality of the Holland River. Terrestrial and aquatic vegetation has regenerated more quickly than anticipated, attracting a wide variety of wildlife. Constructed at a cost of $430,000, the hybrid facility is functioning as designed, within a critical hydraulic grade line of approximately one metre.