Storm Water Management

Why Stormwater Ponds?

The greatest challenge to maintaining the natural features of the upper Carp River wetland is managing the stormwater flowing into it from the rapidly expanding less water permeable urbanizing neighbourhood. This requires employing a complex set of measures related to::

i) minimizing the nuisance effects urban runoff can have on neighbourhoods;

ii) minimizing the risk of flooding of private property;

iii) avoiding aggravating existing flood levels in receiving watercourses;

iv) avoiding creating erosion and sedimentation problems in receiving watercourses;

v) avoiding reducing the quality of runoff in receiving watercourse;

vi) minimizing changes to the water cycle (infiltration, in particular) etc.

This set of measures requires management of runoff from its source (i.e. from each developed lot), downstream to its outlet in one of the stream flowing into the Carp River.

Hydrologic and hydraulic computer models are developed to simulate runoff conditions and conveyance of runoff to the receiving watercourse. These models are prepared to represent existing conditions and post development conditions, and assist in the identification and evaluation of mitigation measures required to reduce the potential impact of urbanization on engineered and natural drainage systems. The post-development models need to account for all of the effects of urbanization and the many stormwater management measures included in the design to control runoff from its source to the outlet.

Because of the large cost involved in the design, construction and maintenance of stormwater ponds, centralized “SWM ponds” that control runoff from several plans of subdivision are commonly recommended in Master Servicing Studies – rather than constructing a SWM pond in each subdivision. While centralized SWM ponds provide an opportunity for potential cost savings, particular care is required in the coordination of all development and servicing approvals within the catchment areas of the SWM Ponds.

As noted above, there are two types of models: hydrologic models and hydraulic models. Hydrologic models simulate the amount and rate of runoff from an area generated after a rain storm (usually referred to as the runoff hydrograph). The runoff hydrographs are then used as inputs to the hydraulic models, which simulate the rate at which the hydrographs are collected and conveyed downstream to the stormwater ponds and outlets to the receiving watercourse. Hydraulic models are also used to simulate flood levels in the receiving watercourses.

The percent imperviousness of land cover conditions on a land parcel is usually the parameter which creates the greatest potential runoff, and hence impact of urbanization on drainage systems. The greater the imperviousness, the greater the runoff potential. Hydrologic models account for the effect of imperviousness on runoff rates in a number of ways, for example, by considering how much of the impervious areas (such as roof drainage from eaves trough downspouts) are drained across pervious areas (which increases infiltration, reduces runoff volumes, and slows the rate at which runoff from roofs drains onto the streets). The models also account for the average distance drainage from the impervious and pervious areas must travel before they drain to the streets.

Once the drainage reaches the streets, the modelling transitions from a hydrologic model to a hydraulic model. Considerable design considerations are made in the grading and servicing of subdivisions to avoid nuisance flooding conditions on streets and to minimize the risk of runoff surcharging storm sewers that could result in basement flooding. These design considerations include installing “inlet control devices” at catchbasins that limits the drainage at catchbasins to a rate that does not exceed the capacity of storm sewers – that could otherwise result in basement flooding. When the runoff rate exceeds the design capacity of ICDs, runoff starts ponding on streets. The maximum depth of ponding is normally restricted to 0.3m. This limits the risk of flood levels rising on streets to a point where cars could become flooded and where flooding within the road rights-of-way could spill onto private property. Managing the depth of street flooding is also important to provide an area along the crown / centre of roads for vehicles to travel safely (in particular emergency vehicles) during extreme weather conditions.

To limit ponding depths to 0.3m during very large rainfall events, the roads themselves are graded such that excess runoff is conveyed in a systematic manner, eventually outletting to either a stormwater pond or to a receiving watercourse.

Master planning of storm drainage in new communities often starts many years in advance of development of the individual subdivisions. As noted above, careful coordination is required in the planning, design, and construction of centralized storm drainage systems that consist of large trunk storm sewers (the “minor” system) that collect drainage from local storm sewers from the individual subdivisions; conveyance of overland flow (the “major” system) along roads and ditches; and the stormwater management pond and outlet.

The Master Planning process through which the preferred stormwater management approach is developed and individual projects are planned - such as trunk sewers and SWM ponds - usually requires approval under the Environmental Assessment Act. To maintain the integrity of these projects - projects that may be constructed several years in advance of the planning of individual subdivisions - it is imperative that what is ultimately approved and constructed in the subdivisions under the Planning Act is consistent with the conditions on which the previous community-level projects had been designed, approved, and constructed.

A current example of potential problems that could result due to inconsistencies in stormwater projects planned, designed and constructed at the community planning level and what is being approved at an individual subdivision level - and in some cases after the subdivisions and their storm drainage infrastructure have been built - is the plan to allow homeowners in suburban areas to widen their driveways to 50% of their lots to accommodate additional off-street parking.

The effect of the additional / unplanned hardening of land cover associated with wider driveways is an increase in runoff volumes and flow rates. In many of the subdivisions where wider driveways are to be allowed, other planning approvals have been issued in recent years that has seen a reduction in minimum lot area and lot width to permit development at higher housing density. This increase in density in new communities in Barrhaven, Stittsville and Kanata has occurred years after stormwater infrastructure had already been planned, approved and constructed based on lower densities in earlier Master Servicing Studies.

The impact of unplanned higher density development in combination with the reduction in pervious areas and greater runoff resulting from wider driveways could effectively undermine the entire basis of stormwater planning at the lot level, street level, subdivision level, trunk sewer level, SWM Pond level, and watershed level. These impacts could increase the frequency and level of nuisance flooding, increase the risk of basement flooding, increase runoff volumes and rates to SWM ponds, reduce the efficiency of Water Quality and Erosion controls of the SWM Ponds (unless costly changes are made), increase peak discharges to the receiving watercourse which could aggravate flooding and erosion and sedimentation, and the increased imperviousness will reduce infiltration, which will have a negative effect on the water cycle.

The lesson to be learned from this experience is that when planning of community level stormwater infrastructure is undertaken, it is important that conservative design parameters are employed in the planning of stormwater infrastructure. In so doing, a cushion can be provided to accommodate unanticipated increases in runoff from more intense development, changes to land cover, and other potential causes like climate change.

Features

In the upper Carp River catchment area there are currently 26 existing stormwater management ponds. Nine additional ponds will be built in the Restoration area in addition to closing the 3 acting interim ponds.

Monitoring

What impact does the existing and future stormwater ponds have on the riverine wetland from Stittsville to the Robertson Sideroad? Several aspects of the pond design control how well it improves water quality and reduces peak flows, these include permanent pool storage volume, active storage volume, length-to-width ratio, presence of a forebay or cells, and the location of the inlet and outlet.