by Jan Hardstaff, PCL Housing & Development

Note: This article was originally published in the December 2016 edition of the Parkallen News.


In November, the City of Edmonton released their City-wide Flood Mitigation Study, and Flood MapsThe Parkallen map appears on page 8.

What does the City-Wide Flood Mitigation Study map show? (Parkallen is on page 12)

This study looked at neighbour­hoods built before 1980; many, including Parkallen, experienced flooding, surface ponding and sewer backups following severe storms in 2004 and 2012. These “once-in-a-hundred-years” storms occurred eight years apart and overwhelmed the drainage system. The study was conducted by creating a computer simulated four-hour long rainstorm modeled over four large mature neighbourhood areas of the city to study the results of worst-case scenario”.

The dashed line shows the area boundary. Looking at Parkallen, you will see a cluster of dots indicating houses that reported basement flooding in previous rain events colour coded by year. For example, flooding reported in 2012 is shown by a red dot and in 2004 by a blue dot.

The map also shows blotches of colour that show how deep water can get during a catastrophic rain storm. Green areas show ponding less than 0.35 m (about mid calf), orange 0.35 – 0.50 m (up to your knees), red areas 0.50 – 0.75 m (mid thigh) and maroon depths greater than 0.75 (waist high or more, deep enough to submerge a vehicle).

Finally, there are solid-coloured lines that indicate how far below the surface drainage pipes in the area are, and if they would reach capacity during a heavy rain storm. Most of Parkallen shows red lines. This means our drainage pipes are less than 1.5 m below the surface. The shallower pipe shown in red would mean storm water drainage exceeds capacity and the result would be more surface ponding in the street and sewer back up in the basement. Orange lines show depth of pipe is 1.5 – 2.5 m deep and green lines show pipe is more than 2.5 m below ground.

City Macro Scale Flood Mitigation Solutions
– Dry Ponds

The Flood Map Release Package, p. 8, shows the Hydraulic Results for the existing drainage system in Parkallen during a 1:100 year 4-hour rainfall. It clearly shows that the centre of Parkallen was once a pond 0.50 – 1.25 m deep with the deep end in Ellingson Park near the playground.

Dry ponds are macro flood mitigation solutions that collect storm water drainage during catastrophic rain events. In newer neighbour­hoods both dry ponds and wet ponds or man-made lakes are used to capture storm water overflow. The map on page 26 shows the Proposed Storm Water Management Facility (SWMF) for Parkallen. This includes the construction of a dry pond and proposed upgrades to the drainage utility to provide macro scale flood mitigation solutions. The City Drainage Department indicates there may be funding for the design phase as soon as 2017. Following the design phase, the Parkallen Community would be consulted. The PCL Executive will work to protect all of our current recreational amenities for use in the future.

The dry pond is represented by a dashed red line in the NE corner of our central park, east of the school and southwest of the intersection of 68 Avenue and 111 Street. (currently, baseball diamonds and soccer fields) This will be similar to the dry pond constructed in Lendrum and will include overland regrading to provide an inlet/outlet to the dry-pond which will also be con­nected to the existing and future improvements to the storm-water sewer system. This may not address all of the flooding issues in Parkallen, but will help to resolve flooding in areas con­nected to the dry pond. It will take 2-3 years to construct and landscape the dry pond. It is estimated that this pond would have a storage potential of 24,400 cubic metres of water up to 2 metres deep. The cost will be just over $6 million. It will be designed as a dual use facility to allow the area to continue to be used for recreation.

Why is this information important to mature neighbourhoods like Parkallen? 

Parkallen was built where a large wetland pond once existed, the borders of which are clear if you look at the flood mitigation maps. To the north, in McKernan, there was an even deeper lake.

Neighbourhoods built in the 1950’s or earlier have drainage called combined sewer systems. This system was designed to
collect roof runoff, then dis­charged into roof leaders (downspouts) directed into the side of the house to the main drain in the basement, where it then combined below the floor with wastewater and sewage into the home’s sewer drain. This combined mixture of runoff and wastewater then connected to the main sewer pipe under the street or lane. Next to the main sewer pipe is a separate stormwater pipe which collects runoff from the street and other ground surfaces in catch basins located below curb grates in the street.

The ‘weak link’ of the combined sewer system is the combined sewer overflow (CSO). This is discharged directly into the storm water drainage pipe when the capacity of the sewer is exceeded during a catastrophic rain event. What does this mean? The overflow causes sewer backup into basements; the combined runoff and wastewater from homes may be discharged directly into the storm water drain and ultimately to the watershed. Additionally, when the storm-water main cannot keep up, water backs up into basements and street catch basins don’t drain, resulting in surface ponding and overland flooding.

City Initiatives to Reduce Impact on the Drainage System 

In the late 1970’s and early 1980’s the City required the roof leaders from homes in mature neighbourhoods to be redirected to surface drainage and concrete splash pads were provided. You will see many of these in Parkallen today. The benefit of this initiative was that rainwater was no longer combined with the sewer drain and the chance of sewage overflowing into the storm water drainage system was reduced. Instead, it was redirected to overland surface drainage where it can be absorbed by the landscaping and ultimately excess storm water drained to the City Right of Way, to a catch basin below the street and eventually into the storm water drain.

The problem with this initiative is that the grading of many of these older homes was not designed for surface drainage. When the properties were graded, there was no need. Consequently, many lots in mature neighbourhoods have poor grading and a low-grade differential (the difference in height from the high to low point of the property) resulting in a lowgrade percentage calculated by dividing the grade differential by the distance to the City right of way. A 2% grade ensures water drains from the property, but grades of 1% or lower will not allow water to drain. To make this worse, during the years that water drained into the basement drain, the clay subsoil around many homes dried out and shrunk, resulting in settling which may have caused cracks in the foundation cracks and often created negative grade around the home, which means water drains toward the foundation rather than away from it… -
(this is what happened with the grade actually drains water towards to foundation rather than away from it) This made homes vulnerable to surface flooding when the roof leaders (downspouts) were redirected to surface drainage to the City right of way (the front yard).

The City’s Flood Prevention Program for Mature Neighbourhoods

Did you know you can have a drainage expert come to your home to do a free Flood Prevention Check-Up? Call 780.944.7777. They will identify drainage and grading problems and make recommendations on how to address these. You can also learn how to grade around your home by checking online for the Homeowners Guide to Flood Prevention. The rule of thumb: grade should be 10% for 2 m around the foundation to a side yard swale that should have a 2% grade to the City right of way. The City also provides a publication called Residential Lot Grading Guidelines. (http://bit.ly/Grading_YEG) These guidelines are especially important to understand if you plan to re-landscape your yard or plan a significant renovation or infill development on your property or have one planned next door. The City also has a Backwater Valve Subsidy Program so homeowners can prevent sewer backup in their basements.  

Landscaping & Low Impact Development Micro Scale Solutions on Residential Lots

Landscaping is more than the pretty face of your property. There is an important functional role for trees, garden beds, and lawns. Landscaping increases site absorption, keeping water out of your basement, reduces erosion, shades, and shelters the property and cools the air temperature. The establishment of mature trees, shrubs, and lawns to increase site absorption, the creation of bio-retention swales and rain gardens, the site capture and storage of rainwater, all contribute to reducing stormwater run-off. The City has many other low impact development recommendations that provide every homeowner with the information needed to implement micro flood mitigation solutions at the residential lot level and reduce storm water run off. You can learn more about many of these LID ideas here. The PCL Housing and Development Committee plans to hold a workshop to provide more information on LID solutions for flood mitigation on every lot. If you are interested in attending, or helping with planning, please let us know

Landscaping of residential infill developments

This is very important! In June of 2016, the City established new Landscaping Requirements for infill. The percentage of impervious site coverage by a home, garage and other hard surfaces (once about one-third of the original lot) is effectively doubled when an infill home is built to the maximum 40% site coverage allowed. The mature landscape that once covered two-thirds of the lot is removed during development. Afterward, with only a third of the site is left for landscaping. This has a huge impact on site drainage. When you double the covered area and reduce, by half, the site absorption, storm water runoff can increase fourfold. This also increases the impact on the storm water drainage infrastructure. As infill increases on a street, this can create a cumulative effect that pushes the infrastructure beyond its capacity. Is this sustainable in the long term? The City drainage department should consider establishing an “infill capacity” for a street or area, where catastrophic rain events already pose a considerable risk of surface ponding and flooding during catastrophic rain events. Now is the time for Parkallen to make sure the City of Edmonton considers ALL the downstream (pardon the pun) impacts of infill development. Our community was designed based on assum­ptions that are no longer relevant. We want and need new families and new housing in Parkallen. The City of Edmonton needs to ensure that we get this right to avoid problems in the future.

Be sure to join us for the Drainage Discussion & Flood Mitigation Meeting on March 7, 2017, from 6:30 pm - 8:30 pm at Parkallen Community Hall, 6510 - 111 Street.