Urban Surfaces Reduce Groundwater Recharge and Increase Runoff
In typical urban areas, one third to one-half of the surfaces are impermeable. When precipitation falls on road, roofs and paved surfaces, it picks-up pollutants and quickly runs off. If urban runoff (stormwater) is simply piped underground and dumped into the nearest stream, it triggers:
Man dwarfed by huge
stormwater outfall into Rouge
After Rainfall, Water Flows Dangerously
High and Fast at Stormwater Outfall
- flash floods and increased drowning risks for children;
- loss of groundwater recharge leading to lower aquifer and water table levels;
- loss of stream base flow and wetlands;
- erosion and loss of adjacent public parkland and private property;
- rapid flushes of pollution and bacteria into our Lake, particularly if stormwater and sanitary sewers are combined in one pipe;
- pollution of our beaches and drinking water source;
- siltation and damage to fish habitat through rapid changes in water flow, temperature and chemistry;
- additional costs to tax payers for erosion control and pollution abatement.
In Toronto, the cost to clean-up past stormwater mistakes will probably exceed a billion dollars. However, the status quo is even more costly in terms of risks to public health and impairment of beneficial water uses such swimming, recreation, tourism, fishing and safe drinking water.
Polluted (black) Sediment from Road
Runoff at Malvern Stormwater Outfall
Polluted and Dangerous Flash Flood
from Malvern Stormwater Outfall
After rainfall, Malvern stormwater
outfalls increase stream depth from
less than 30 cm (1 foot) to more
than 150 cm, tripling stream velocity
and creating serious drowning hazards
Malvern Outfalls Dump Stormwater
into streams leading to costly erosion.
Scarborough Golf Course sued the
City for millions of dollars
to armour Highland Creek stream banks in
order to protect their property.
Polluted water flows into Rouge River
from stormwater outlet contaminating
the fish being caught by this fisherman.
Fish covered in fungus due to stress and
pollution from storm water and pollution.
Case Study – Morningside Tributary – Mistakes
and Remediation Opportunities
In the 1960s, the Morningside Creek was small enough to jump across and it supported large spawning runs of trout and other migratory fish. It flowed through a beautifully wooded valley, with two to three foot diameter maple, cherry and pine trees, before joining the Rouge River Valley beside the Toronto Zoo's current location.
Malvern Stormwater Harms Rouge Stream
In the early 1970, the federal government created an affordable housing project in Malvern between Markham Road and Morningside Avenue and Sheppard and Finch Avenues. Runoff from this large area was simply funneled into two huge 2.5 to 3.0 metre diameter underground pipes and then dumped into the Morningside tributary.
Serious Drowning, Pollution and Erosion Hazards
After a heavy thunderstorm, the two big Malvern stormwater outfalls change the gently flowing stream into a raging torrent of polluted water that could sweep a child or adult to their death. At the mouth of the Malvern outfalls, the Morningside stream is stained with pollutants such as oil, rubber, plastics and heavy metals from driveways, roads and parking lots. Immediately downstream from the lower Malvern outfall, a large concrete channel and stone filled gabion baskets have been undermined by the force of the frequent flash floods. A little farther downstream, the banks of the stream are failing and hundreds of trees are being uprooted and swept away. Another kilometre downstream, a high quality spawning area for trout and salmon is suffering significant harm from the flash floods and pollution.
Tragedy or Lawsuit
The Malvern stormwater outfalls create dangerous flash floods in Morningside Creek near two primary schools (Fleming and St. Bede). With a bit of luck, the City will remediate these dangerous outfalls before a drowning tragedy occurs or a lawsuit.
On another Scarborough stream, Highland Creek, a lawsuit occurred. The Scarborough Golf Course successfully sued the City for millions of dollars to repair the erosion and loss of property enjoyment caused by flash flooding and erosion in Highland Creek. Since that time, many more millions of dollars have gone to damage control. These costs to taxpayers could have been avoided through good stormwater management.
Reducing Flash Flooding
Steps can and should be taken to reduce flash flooding. Vacant lands near the Malvern outfalls could be used to create a storm water storage and treatment area. The creation of a wetland area, perhaps in the shape of coiled snake, would reduce the flash flood flows and remove pollutants. A large stormwater storage pipe could also be buried under an abandoned railway spur line near the outfalls. This pipe could store the peak flows and allow the water to slowly leak into the soils of the adjacent Morningside stream valley. The surface of the rail line could be converted into a bicycle and hiking trail for the community and the adjacent Rouge Park.
Reducing Runoff at its Source
The best measures for storm water management involve good watershed planning and reducing runoff near its source. Maintaining and restoring forest cover is one of the best ways to reduce runoff and protect water quality. Keeping paved or hardened surfaces to less than 10% of the watershed area is another important strategic step.
At the local level, many small steps can be taken to reduce stormwater. Small diagonal ridges can be installed on driveways to deflect runoff into boulevard areas to water the grass and trees. Roof down spouts can be directed to rain barrels or cisterns that then discharge to lawn and garden areas. Owners of large parking areas can be encouraged to create perimeter cattail ditches to store and purify runoff and provide water for trees and grass. These are just a few of the innovations necessary to reduce the flash flooding and its costly damages.
Improving Stormwater Ponds and Creating Wetlands
Many new urban developments use large ponds to reduce flash flooding. However, if these ponds are built in a stream valley or flood plain, they destroy sensitive soils and habitat. Furthermore, if ponds are created with berms within a flood plain, they can aggravate flood damage during a major storm by clogging the flood plain with silt and debris.
Stormwater ponds should not be located within stream flood plains, valleys and environmentally sensitive areas.
Meeting Surface Water Quality Standards
Stormwater ponds may be a step forward, but they are failing to remove enough pollutants to meet provincial water quality standards. Water quality should be improved further by having stormwater ponds drain into large constructed wetlands full of cattails. Cattails are natural water purifiers.
Avoiding Nuisance Bird Problems
Another problem is the attraction of nuisance numbers of Canada geese and mallard ducks to stormwater ponds. In large numbers, the excrement from these waterfowl leads to bacterial and nutrient pollution. Ponds surrounded by tall trees and dense shrubs are less attractive to ducks and geese because they are more difficult for landing and take-off and they provide hiding locations for predators. Unfortunately, few ponds are currently designed to have fast growing trees around their perimeter.
Forests Reduce Runoff and Recharge Aquifers, Streams and Wetlands
There is little surface runoff (storm water) within a healthy forest. In forests, precipitation is temporarily stored in the sponge-like leaf litter layer and depressions in the forest floor. This stored water percolates underground, slowly flowing towards aquifers, wetlands, streams and lakes. This groundwater flow supports stable and clean stream flows in contrast to the flash-flooding and low flow cycles of deforested or urbanized watersheds.
Modern scientific studies reveal that water quality, stream flow, and fish and wildlife habitat, undergo a serious decline when a watershed contains less than 30% forest cover, or more than 10% impermeable surfaces.
Large trees are also important to provide shade and reduce solar heating of the water. Nuisance blooms of algae grow in warm, sun-exposed, nutrient rich water. When these nuisance algal blooms decay, they release a foul odour and they remove oxygen from the water, leading to a less attractive environment for people and aquatic life. Open ponds also reduce natural stream flow because wind and sunlight increase evaporative water loss.
Good watershed planning can help us to avoid costly stormwater management mistakes. We can do a much better job of treating stormwater and returning it to the groundwater system to recharge our aquifers, streams, wetlands and lakes. We can take significant steps to remediate past stormwater mistakes so that we can enjoy the benefits of clean and healthy water. However, we need to understand and respect the carrying capacities of our watersheds or we will degrade the fresh water cycle that is essential to our health and well-being. We cannot pave-over ever-increasing areas of our watersheds and expect to maintain healthy water and healthy communities.
With current technologies, it appears that we should be striving for less than 10% hard surfaces within our watersheds and more than 30% forest cover, in order to protect our precious fresh water resources from pollution and depletion.