The Day the Rains Came

Sunday, July 26, 2009 started out like any other southern Ontario summer day — hot and humid. At first it seemed as though the Canadian Open golf tournament in Oakville would finally get a dry day after each of the preceding days had been played in very wet and stormy weather. Ground conditions across the Golden Horseshoe were saturated, area creeks were flowing steadily, and most stormwater management facilities across the city of Hamilton were full and still draining.

At approximately 11 a.m. the skies in Hamilton darkened as heavy clouds approached from the southwest. In the next three hours, an extremely intense storm moved across the Red Hill Creek watershed from its headwater to its outlet, releasing well over 100 mm (4″) of rainfall. The only rain gauge which remained operational after the storm, recorded an astonishing 91 millimetres in one hour alone.

The extent and magnitude of the flooding, particularly in the Red Hill Creek watershed, was unprecedented. Portions of the new Red Hill Valley Parkway became a torrent as stormwater came cascading down the Niagara Escarpment, overwhelming ditches, sewers and roads. At its lower end towards the Canadian National Railway, water overtopped the parkway, trapping motorists and closing the lecting data immediately following the storm. Gary Moore, P. Eng., who previously led Hamilton’s special projects office and design for the parkway and now leads the Public Works Division, together with AMEC staff, conducted an inspection less than 24 hours following the storm.

Evidence of damage was widespread — downed fences, debris lines, eroded creek sections. However, the road was open and traffic was moving at normal speeds. The roadway appeared safe and fully functional.

In the next few days, the AMEC engineers surveyed the high water marks and tied these into a geodetic datum. They gathered rainfall data for the storm, both from ground stations (rain gauges) and radar images. Finally, they used a numerical model to recreate the runoff response to the July 26 storm event in order to determine its magnitude compared to the design conditions.

The data collected from rain gauges on the ground was used as input for the initial assessment. As is evident in the radar image, however, the storm’s intensity and spatial distribution were far from uniform. Consequently, this assessment approach was quickly abandoned in favour of a more resolute method, which used the radar imaging to produce data for numerous locations in the watershed. This approach simulated the actual spatial and temporal variability in rainfall (in road for up to eight hours. Farther downstream, the Red Hill Creek breached its banks and spilled across the Queen Elizabeth Way, closing this 400-series highway that carries 125,000 vehicles per day for approximately three hours. Elsewhere in the community over 6,000 homes suffered various forms of flood damage.

So, what had happened?

The new expressway in Hamilton’s east end had opened in 2007, over 50 years after it was first planned. The seven-kilo-metre, four-lane, highway runs through the Red Hill Valley Creek. AMEC (then Philips Engineering) had led a team of specialists to realign and rehabilitate the creek. They also designed the roadway’s overall drainage and flood protection system. ¹

The integrated network of flood control systems was designed to protect the roadway from the risk of floods. The engineering team based the design on protecting the parkway so that it could safely withstand a 100-year storm. Did this drainage system fail?

The first steps in answering this question involved col-effect a total of 11 pseudo-gauges were created from the radar image). It ultimately led to the results in Table 1.

Shocking results

The results were truly shocking. The analysis suggested that flow rates at Albion Falls (the point at which the headwater drains over the Niagara Escarpment) were at, or slightly above, the predicted 100 year flow rate. Farther downstream towards the Red Hill Creek outlet, flow rates increased in magnitude to the point where by the time the flood wave reached the QEW, flow rates exceeded 150% of the 100 year peak flow! This was a storm event of unparalleled magnitude in the city of Hamilton.

Hydraulic models of the creek system, including its bridges and culverts, corroborated the simulated flow rates with the field-measured high water levels. This correlation suggested that the design tools were not at fault, and provided an even greater level of confidence in the numerical modelling results. Rather than being a design failing, the design condition had actually been exceeded by an alarming 50%.

What did we learn?

So, how did the system fare and what did we learn?

• Storm direction can play a key role in the magnitude of flooding. By tracking from the headwater to the outlet, the flood intensity was compounded as the downstream flood peak coincided with the most intense period of the storm.

• Design standards can be exceeded. Despite the best of intentions, all engineers must realize that design standards can be exceeded, especially where nature is involved.

• Identifying the weakest points in the system and reinforcing these is important. The Red Hill Valley Parkway overtopped in predictable locations; reinforcing these against washout and structural damage would add a level of protection to the roadway.

• Emergency response is key — vehicular access gates were not used in time. Despite the presence of gates at ramp entrances, cars were not prevented from accessing the parkway; a better flood warning system is required.

• Major flood control systems in natural areas through sensitive design can be highly effective in providing flood protection. The Davis Creek flood control facility was not yet built (it is currently under construction); by controlling the flows from the Red Hill Creek’s largest tributary at King St., the facility would have reduced the flooding risk by 15% or more.

• Radar, rather than point rainfall data, is very important to assess spatially varied rainfall. The variability in rainfall intensity reduces the utility of ground based rainfall stations particularly when the gauge density is low; designers need to recognize this when assessing watershed response.

• Can these storms be an indication of Climate Change? Climate change has been suggested as a cause of more frequent and more intense extreme weather. These changing patterns will need to be monitored carefully in the future to ensure that rainfall design parameters are kept current and relevant.

In summary, the drainage system for the parkway performed exactly as designed; it flooded exactly where it was predicted to, in the amount that it was predicted to. So the question remains — Should expressways be designed to higher or different standards?

Only time will tell. With the spectre of climate change on the horizon, there will need to be close attention to standards, and analytical tools will need to keep pace.

Ron Scheckenberger, P. Eng. is the unit manager of the Burlington Infrastructure office of AMEC Earth & Environmental. He was project manager with AMEC for the design of the Red Hill Valley Parkway drainage system as well as the Red Hill Creek realignment and restoration project.

¹McCormick Rankin, Stantec Consulting and Philips (now AMEC) together led the engineering design team for the Red Hill Valley Parkway design and construction.