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Quantity Control:
The increase of impervious surfaces (surfaces that water can't flow through), such as asphalt, directs stormwater into the City's stormwater systems (ditches, pipes, gutters, and catch basins). These conveyance systems all have a design capacity, which allows or restricts the amount of water that can be transmitted through the system. City staff engineer the system based on a rate storm event such as a 1 in 10 year storm, and the ability of the existing landscape to move water (slope). There are several areas within the City of Orillia that are considered flood prone areas, which is due to the elevation of the lands in comparison to lake levels (see map for these areas).
Elevation, maintenance, increase intensified precipitation events such as those predicted from climate change all impact the ability of the designed system to convey proper quantities of stormwater. As a result, events can occur outside the design of the conveyance system which results in flooding.
Development also impacts the quantity of water that supplies the groundwater aquifers. Much like the engineered pipes, soils also have a specific flow rate based on the voids, or spaces between the grains. Gravel and sandy soils typically allow for good water movement through the ground, whereas silt and clay have restrictive flows. A typical practice in development is to compact soils, which produces a smooth surface or lawn. By compacting or paving over these soils the void spaces in the soils is reduced which restricts the ability of the soil to flow water downwards. Two impacts occur with the reduction of water being returned to the ground:
- A reduction in groundwater sources which provides both drinking water and a consistent source of water for most streams, rivers, ponds, wetlands and lakes.
- An increase in flow over a short duration to rivers, streams, ponds and wetlands due to the stormwater systems discharging most water to these natural drainage courses.
Storm sewer design incorporates storm events at the time of the design. Climate change has increased the frequency and intensity of larger storm events in which the storm sewers are not designed for. This in turn produces increased flooding events and increased lake levels producing further restrictions on outfall flows. You may wonder why we might be receiving higher intensity rainfall events. Well if we look back to the water cycle the first stage is evaporation. The rate of evaporation is highly dependent on temperature, wind and surface area. Climate change has reportedly increased Ontario's temperature 1.4°C (Ontario Ministry of the Environment, 2011) which accounts for higher temperatures. In lake areas you may notice that it is very rare when the wind is not blowing, and with Orillia being surrounded by two large lakes this increases the evaporation within the area. The next point is the key to tie in the water cycle. With urban development runoff is increased causing a significant increase in water entering the lakes and rivers. This increased water creates a rise in water levels which means that shorelines are reduced. If we think of a slanted bowl (illustrated below) and we start to increase the water level within the bowl we are creating an increase in surface area.
All three factors increase the evaporation rate and in turn create larger amounts of water being condensed into clouds at a shortened rate, which then produces an intensified rainfall event. By reducing runoff and promoting groundwater recharge in the City a reduction in evaporation can occur reducing the intensified storm events.
Click here to learn about the new stormwater rates that took effect January 1, 2017.
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