Everything You Need to Know About SWM

Legislative Gaps, South March Highlands

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Storm Water

Storm water is created when land development alters the natural water balance:

  1. A large portion of the rainfall is naturally absorbed by the trees, vegetation and ground surface and this significantly reduces the amount of water which will flow offsite.
  2. Forests perform a vital function to the city as “green infrastructure” that soaks up storm water, as well as combating air pollution and carbon emissions.
  3. Wetlands perform a vital function to the city as “green infrastructure” that helps store storm water to prevent flooding elsewhere.

When natural vegetation/trees and soil is replaced with roads, buildings, parking lots, less rainfall infiltrates into the ground, less gets taken up by vegetation/trees, and more becomes surface runoff:

  1. Infiltration is 14 – 35% higher for forests as compared to urban lawns for the same soil type. Looking at it the other way, runoff is up to 35% higher when forests are replaced by subdivisions [MOE SWM Design Manual 2003 ch 3 Table 3.1].
  2. Runoff is 37 -88% greater when forest vegetation is replaced by urban lawns for the same soil type.   [MOE SWM Design Manual 2003 ch 3 Table 3.1]
  3. Water holding capacity is 4 – 5x greater for forests than for urban lawns for the same underlying soil type  [MOE SWM Design Manual 2003 ch 3 Table 3.1]

Surface water flow is affected by land slope – the higher the land, the faster it flows:

  1. It is difficult, if not impossible, to control storm water flowing down hilly lands or an escarpment.
  2. The City of Hamilton knows this and was involved in a court case when the homes at the bottom of the escarpment flooded.

Surface water is affected by imperviousness of the soil and infiltration – bedrock is impervious and most of the South March Highlands have bedrock less than 1 m below the surface.

Storm water is dangerous – a well-recognized risk to human life and to property. 

SWM Basics

Storm water management (SWM) is supposed to manage:

  1. Volume of water to prevent flooding.
  2. Timing of water flow to prevent flooding and erosion.
  3. Pollution since runoff is often contaminated by oil, grit, etc. from city streets.
  4. Temperature since runoff is typically warmer than the streams that the water flows into and this can damage fish habitat.

Storm water management also handles heavy spring water flow caused by snow melt – in addition to water flow caused by storms.

Cutting trees without storm water management controls in place will cause storm water to:

  1. Flow uncontrolled offsite.
  2. Flow in much greater quantity.
  3. Flow at a faster rate than previously.
  4. Erode land, creating risk of mudslides on steep slopes.
  5. Erode stream and river banks, damaging property and changing the path of stream/river meander patterns.

While surface water runoff may not cause problems when a few trees are cut on flat lands, this is not true for highly forested and rocky areas, escarpments, or lands near floodplains and Urban Natural Features or wetlands – especially in Provincially Significant Wetlands.

In the absence of adequate SWM control:

  1. Urbanization usually dramatically increases surface runoff volumes and rates.
  2. Urbanization would convert a 100-year discharge to a 2-year discharge [Walesh 1989 pg 65].
  3. Peak flows after development are seen to be over 5 times peak flows before development [Walesh 1989 pg 58].

Servicing studies identify two systems to handle storm water runoff:

  1. The Minor System (the storm sewers) handle the runoff up to a 5 year storm.
  2. The Major System (roads, land, ditches) handle the overland flow as understood by a 100 year storm.
  3. Both of these systems are designed to flow to a stream or river, often via a SWM pond so the that flow is moderated over time.
  4. The regulatory flood level is the expected crest of the water during a 100 year storm.

Storm water management ponds are built to store storm water temporarily so it doesn’t flow downstream too fast and cause flooding:

  1. SWM without quantity control only handle the minor system flow and are prone to flooding (e.g. 351 homes in Stittsville in 2008).
  2. SWM without quality control do not deal with pollution or temperature impacts.

Watershed and subwatershed plans are required by the province to set the storm water management criteria and establish catchment areas:

  1. Water flows downhill.
  2. Loss of water retention resulting in increased runoff in highlands due to new development can flood lowlands with pre-existing developments.
  3. All development in a watershed or subwatershed is supposed to account for the cumulative effect of storm water as it flows downhill.
  4. Flow is not supposed to be diverted between watersheds and subwatersheds – a Schedule C Environmental Assessment (EA)is required if this is to be done as a last resort.

To learn more about SWM, read the Ontario Understanding SWM booklet produced by the MoE.

Ottawa SWM

The City’s storm water management program does not adequately take cumulative effect into account:

  1. An environmental management plan is required for managing cumulative effects yet none exist.
  2. A storm water policy is required for the escarpment and hilly lands, yet no policy exists.
  3. Ottawa is the only major city in Ontario without a site-alteration by-law that prevents tree removal prior to adequate SWM facilities or plans in place.

The City should require developers to use computer models to determine the impact of tree-cutting for and demonstrate in SWM plans that controls do not need to be in place prior to any removal of trees.  Otherwise, developers should be required to implement SMW controls prior to tree removal to protect public safety.

The City routinely appears to make decisions to develop lands and build roads based on incorrect models which will increase flooding in the West Urban Community:

  1. The current Carp River flood model is so incorrect that it shows water flowing upstream during a storm!
  2. No increase – not even 1 cm – should ever be permitted in a regulatory flood level because no one can predict all the engineering decisions that were made based on that flood level when prior developments were planned. 
  3. Yet the Carp River regulatory flood level has been allowed to increase by nearly 1 meter.  If it increases much more, or if the models are wrong, the Carp could actually overflow into Kizell and down Watt’s Creek.
  4. An increase in a regulator flood level increases flood risk both upstream as well as downstream.
  5. Further development along the Carp River without adequate flood plain compensation and improved SWM controls is a risk to residents in Carp, Kanata North, Kanata South and in Stittsville.

Flood Risk

The South March Highlands are the aquifer for Kanata North and for parts of West Carleton: 

  1. That means that development in this area has pervasive effects on both land and water downhill from it.
  2. For example, construction in Morgan’s Grant caused wells to go dry in March Rural.
  3. Loss of water retention in the highlands will substantially increase flood retention in the low lands were existing subdivisions are.

Flooding has been a chronic problem in North Kanata since the 1970s.  Beaver Pond was built as a dam to control flooding in the 1980s – especially to protect the MDS Nordion nuclear facility and NCC lands.

The Shirley’s Brook /Watt’s Creek Subwatershed study shows the drainage in this area is split 3 ways into SB3, KD1 and KD2 catchment areas.  Tree cutting will increase flow to all 3 of these drainage areas if no storm water management controls are in place.


The Beaver Pond and lands immediately north are in the KD1 catchment area:

  1. Pathways around Beaver Pond flooded on 24 July 2009 and flood routinely every spring due to snow melt.
  2. Increasing flow into Beaver Pond without mitigation will cause increased flooding and backup into the Kizell Wetlands.

Part of KNL Phase 9 lands are also in the KD2 catchment area: 

  1. Flooding occurred downstream in this catchment area on 24 July 2009 at several of the 9 culverts which cross the Kizell Drain. 
  2. Adding additional flow may increase risk of flooding downstream and diversion to Shirley’s Brook may be the only alternative – a violation of the subwatershed plan.

Part of KNL Phase 9 lands are in the SB3 catchment area:

  1. These lands need to be coordinated in the Shirley’s Brook Diversion EA but are not in scope.
  2. City staff says only impacts KNL’s Phase 7 and 8 but are unable to produce documentation to back this up after 6 months of community questioning.

KNL’s 2006 Servicing Study ignores the Shirley’s Brook Watershed/Subwatershed Study recommendations:

  1. KNL’s Servicing Study requires a diversion – a violation of the subwatershed study.
  2. The subwatershed study does not allow for the removal of any water from Shirley’s Brook or the addition of any water to Kizell/Watt’s Creek.
  3. Condition 59 of KNL subdivision approval requires compliance with the subwatershed study.
  4. City staff say that KNL has somehow met all conditions of subdivision approval but are unable to explain how the servicing is in compliance with the subwatershed study.
  5. KNL’s Phase 5 appears to have diverted storm water from the Carp River Watershed to the KD1 catchment area without an EA being done.
  6. KNL  plans to apparently divert storm water from KD2 and SB3 to the Beaver Pond in KD1 without an EA being done.

Open Issues

KNL apparently plans to cut trees with no storm water controls in place. It is difficult to understand how this does not increase flood risk.

The Richardson Ridge tree-cutting is also proceeding on the Hazeldean Escarpment adjacent to the Compensation Lands and First Line Road which drain to the Kizell Wetlands.  This tree-cutting will result in increased runoff to the Kizell Wetlands and thus to the Beaver Pond, as well as to the Carp River.

The entire area needs to be studied at the same time – which is why the provincial policy requires an Environmental Management Plan – which has never been done despite the fact that this policy has been in place for almost 10 years.

The city’s West End Flood Investigation report identified a lot of culvert crossing problems further south in the KD3 portion – if these are fixed, then more flow would go downstream to the Watts Creek junction, and could impact the KD1/Beaver Pond drainage as well as flooding the NCC’s land.

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