A Municipal Hydraulics course available at McMaster University strongly reinforces the importance of the “Restoring Resilient Spaces” workshops promoting citizen activism and vigilance with stormwater management solutions within the City of Hamilton.
As mentioned in an earlier blog, a major portion of Hamilton’s sewer infrastructure is very old, particularly in the downtown regions of the city, where over 600 kilometres of combined sewer systems (sanitary sewage and stormwater collected together in a single pipe) wind underneath our feet. During periods of heavy rainfall, inflow from the combined sewer system can overwhelm the wastewater treatment facilities, leading to sewer overflow into Hamilton Harbour or the surrouding watersheds. These overflows often contain the really nasty stuff as well. A overflow of just 3% of the combined sewage for a given time period can contain 35% of the suspended solid material within the volume. It’s important to remember that on top of the environmental concerns for local wildlife, these wastes are getting dumped into Lake Ontario, where we source our drinking water from!
To Hamilton’s credit, work has been done to minimize the number of combined sewer overflows. Storage tanks have been set up to contain the overflow for later treatment when the system is back underneath capacity. The locations and capacities of these tanks are below:
This has reduced the quantity and frequency of untreated outflow by roughly 90%. Outflows that would be used more than two dozen times a year are now only used 2-3 times a year – far from a perfect solution, but its a step in the right direction.
It is still a temporary solution. Hamilton’s downtown population will continue to grow, especially much of the core now zoned for high-rise development. With more residents, there will be more sanitary sewer flow. Additionally, as climate change produces more frequent and violent storms, more stormwater performance will be demanded of our aging water systems.
During LRT construction, some of the pipes along the downtown corridor will be replaced by like-for-like pipes, or in some cases, upgraded to larger capacity pipes. These replacements will still be a combined sewer network – it would likely be far too costly and disruptive to switch from a combined sewer network to separated parallel networks for sanitary sewage and stormwater. Much of the 600 km network would have to be updated simultaneously for this monumental task.
Over time, the additional combined sewage will impact the wastewater treatment plants, forcing more plant and storage upgrades as the quantity of combined sewer overflow would rise to pre-intervention levels.
In Hamilton’s newer areas, there is a separated sewer system, with two parallel pipes, one for sanitary sewage collection, and one for stormwater uses. The sewage is sent to the water treatment plants, while the stormwater bypasses the treatment plant and is sent directly back into the watershed. In theory, this is much better than a combined sewer system because it reduces the energy and infrastructure costs. Two narrow pipes running side by side may require less pipe material than a single pipe that must be wide enough to carry both, and the wastewater treatment plant doesn’t need to have as large of a capacity to handle the inflow since its coming only from sanitary sewers.
However, these seperated sewer systems come with their own sets of problems. For one, the stormwater is directed back into our waterways completely untreated, but it can be far from clean. As it runs along the path of least resistance to the storm sewers, it collects dirt, asphalt, litter, oil, and other debris. Drains have covers to ensure larger debris doesn’t go through, but the materials that do pass through the grates can negatively affect our aquatic environments.
A partial solution to this are stormwater ponds (as shown above), where small debris can settle at the bottom of a man-made pond before the water is sent into the waterways. These ponds do require maintenance, such as clearing away the sediment on a timely basis, to sustain their mitigating effects on water pollution.
Another problem comes with intentional and unintentional cross-connections. Either deliberately or from a DIY project gone wrong, untreated sewage can get sent into our waterways. Garbage, fecal contamination and odour issues coming from the Red Hill Creek stormwater outflows spured Environment Hamilton’s “PipeWatch” community activism in 2002, with an unexpected revival one decade later in 2012.
As well, creating independent stormwater infrastructure is typically not done as conservatively or with the accuracy of sanitary sewage systems or combined sewer systems. Underestimating the capacity of these systems would mean that raw sewage could get pushed up onto the streets. This is a problem that most modern cities haven’t faced very frequently since the early days of municipal wastewater treatment in the 1800’s.
In comparison, stormwater sewers are considered “lower-risk” since in the event of a heavy storm exceeding stormwater capacity or causing a backup in the system, the water flooding your street (and your basement!) wouldn’t pose a significant health risk. The flow rate used to estimate s pipe sizes is calculated from a simple equation:
C – the runoff coefficient, is based on the land use and the associated soil infiltration and ground permeability, all of which depend on complex factors like the environment and interactions with the built infrastructure. For example, a more built-up area with the ground covered by pavement would have a higher C value. This is because less of the rainwater would infiltrate through the road compared to a natural environment, and as a result, more reliance would be put on the sanitary sewer system for drainage. Ranges for the runoff coefficients based on land use are given below:
I – the rainfall intensity, is determined from past rain events, and selected based on the “duration” and “design frequency”. “Duration” refers to the length of the rainfall event, and this value is usually matched to the length of time it takes water to clear out of the stormwater sewer system once it hits the ground. “Design frequency” is how often, on average, a storm of greater intensity is expected to happen. The standard in Ontario is 5 years, which is to say that stormwater pipes are under-developed to handle exceptional storms that happen, on average, once every 5 years. This is scary, because when the pipes cannot handle any more water, you get flooding! In addition, with climate change, these rare “5-year storms” are getting increasingly common. Additional urban development means that permeable soils that allow for natural protection against flooding will be replaced by asphalt and concrete. This will effectively increase the value of “C” – the runoff coefficient, and put more demand on the under-built stormwater system that was designed before development took place.
A – the drainage area, is also variable. Infrastructure and disruption of the natural flow patterns can change the effective drainage area. For example, if an area is leveled out for condo development, this can completely change the direction and speed the water flows. Some areas will inevitably find themselves with a larger drainage area than previously predicted.
Hopefully this has given you a bit more of an appreciation for the complexity and importance of our sewer networks, along with an indication of the fragility of these systems and the problems Hamilton has faced and will face in the coming years. With our growing city and aging infrastructure, it is more important than ever to citizen scientists to revitalize rain gardens and make the most of our green space.
Course Notes from Civil Engineering 3M03 – Municipal Hydraulics at McMaster University – Prof. Yiping Guo