Conventional stormwater management relies on large-scale facilities such as dry ponds, wet ponds, constructed and natural wetlands to detain flows from large storm events.
These facilities mitigate the impact of downstream flooding. However, these types of facilities are only part of the picture in protecting our watersheds and managing stormwater. Creating an environmentally and hydrologically functional landscape requires consideration of volume, frequency, evapotranspiration, recharge and discharge. Detention facilities only address discharge. What about these other characteristics of a hydrologically functional landscape and how do they relate to flood mitigation?
Low Impact Development (LID) refers to a desired outcome: land development that causes so little harm that it can be sustained. An LID approach integrates microscale nature-mimicking features and processes across the landscape. Rather than a prescription, LID offers a toolbox of practices to supplement conventional stormwater management in order to achieve more goals than merely discharge-rate control.
|The Sunnyside community in Calgary during the 2013 floods. Photo: The City of Calgary|
One of the simplest things that can be done to attenuate flooding is to give urban soils more attention. In urban development, soil has tended to be treated - at best - as a veneer rather than a vital, living component of a hydrologically functioning landscape. Combined with deeprooted vegetation (read: NOT Kentucky Bluegrass), our yards and open spaces have immense potential to work for us to deliver water quantity and quality benefits. The City of Calgary has recently implemented a 300- mm topsoil depth guideline for new development and redevelopment throughout the city, in accordance with the recommendations of the Bow Basin Watershed Management Plan. Has your jurisdiction?
Deeper topsoil is seen as one of the most cost-effective ways to achieve multiple objectives, including flood attenuation. Topsoil holds water in its void spaces, thereby delaying the onset of runoff and the severity of flooding. More topsoil has more void space - a simple case of “more is more.” While organic amendments enhance the absorptive capacity of soil, organics shrink as they break down, and are better used in planting beds where settling will not be noticeable (as opposed to lawns). Ultimately, the magnitude of infiltration is dominated by soil structure and soil thickness, not by the presence of organics. However, organics provide the glue to help with the establishment of soil structure, so a truly organic-depleted soil may benefit from amendment.
Along with the characteristics of the soil itself, vegetation, especially woody vegetation, assists with the creation of macropores, which enhance infiltration and infiltration depth (as do underground creature passages - but these infiltration ‘enhancements’ are usually not counted). How significant is void space in soil for flood mitigation? With 300-mm-thick topsoil, rough estimates are that it can handle all precipitation from a 1-in-25-year storm with a 24-hour duration, or all precipitation from a 1-in-100-year storm with a 12-hour duration.
Green roofs, rain barrels and cisterns, downspout dispersion, reduced lot grading, reduced hard surfaces, microdepressions (rain gardens), underground storage, bioretention, bioswales, increased amounts of soil and vegetation – the storage adds up in proportion to the extent of implementation.
Infiltration trench and swale, Canada Lands Company’s Currie Barracks redevelopment in Calgary. A gravel chamber overlain by a thin layer of absorptive soil and conventional sod replaces the storm sewer. Photo: Alberta Low Impact Development Partnership
What about the flooding in 2013? How would the implementation of these practices have made a difference? The presence of these practices would, at first glance, have had little impact during the 2013 flood, since rainfall was occurring in the foothills and mountains, well outside of our urban areas. Even if the rainfall had happened within urban areas, the amount of precipitation would have overwhelmed these features since, once saturated, absorptive surfaces behave as if they are impervious surfaces. However, nuisance flooding from overland areas could have been reduced. In fact, this is where LID for flood attenuation shines. More frequent storm events, the kind that can deposit 20 to 50 mm of rain in a couple of hours - which are usually associated with summer thunder storms - result in most of the nuisance flooding in our urban areas. At least some of these impacts would have been reduced in the 2013 flood, if LID features had been implemented.
Even though the presence of LID features in our urban areas would not have reduced the magnitude of the flow coming into our cities and towns in 2013, they could have reduced the second instance of flooding that occurred in the Sunnyside community in Calgary in early July 2013. This event occurred when about 20 to 25 mm of rain hit the Sunnyside catchment and the gates at the outfalls of the storm sewer system into the Bow River were still partially closed following the June flood. The absence of LID measures in the catchment above Sunnyside has contributed to the need to make a multi-million dollar investment in automated gates and pump stations to protect this community from future flooding events.
LID measures would have helped in two other ways during the June 2013 flood. First, in urban catchments, the repeated erosive forces exerted on the bed and banks of receiving streams during frequent, small storms is such that vegetation along the stream does not have adequate time to repair itself and restore the capability of its roots to hold the bank together when the next erosive event hits. The banks lose their capacity to resist the much larger erosive forces of the magnitude that occurred in June 2013. The resulting erosion subsequently threatened adjacent public infrastructure including pathways, roadways, water mains, sanitary trunk sewers, etc, as well as private property. Some erosion would have occurred naturally, and we do not have supporting studies to indicate to what extent this small-storm erosive phenomenon occurs in larger streams. However, this phenomenon is well-known for our smaller creeks and streams like Nose Creek and Pine Creek that are, to a large extent, impacted by the increased runoff from urban areas draining into them. The adoption of runoff volume targets in, for example, the Nose Creek Watershed Water Management Plan was done to minimize the erosive effects of the increased runoff from urban areas. These runoff volume targets require the wholesale implementation of LID in the headwaters.
|Memorial Drive in Calgary, July 2013. Photo: The City of Calgary|
Second, one of the original messages of the Alberta Low Impact Development Partnership (ALIDP) is that LID is much more than the implementation of an array of features commonly referred to as “source control.” The first principle of LID is to conserve natural features. Development should stay outside of any environmentally sensitive or hazard areas, or in the case of flood issues, avoid riparian areas that would be inundated during flood events. Staying outside of the meander belt of a stream equates to giving it the necessary room to naturally move, that is, truly “Room for the River.” This then minimizes the risk to adjacent infrastructure and property, and removes the need for extensive river training works. The ALIDP is glad to see that the “Room for the River” concept, which the ALIDP has been advocating since its inception, is now getting due attention as one avenue to minimize the impacts of future floods. Even with Room for the River, runoff volume control is still needed if land development continues, since increased flows will increase stream power and increase the width of the meander belt.
A frequent question is why attention should be given to these sorts of measures for urban areas when they account for only a small fraction of the watershed. The insurance industry tells us that the greatest flood losses are incurred in urban areas because that is where the most people and property are. To illustrate the insurance industry’s acknowledgement of this, you only have to look as far as the ALIDP’s March 2015 training course in Calgary. The Insurance Bureau of Canada has sponsored us to heavily subsidize AUMA-member municipalities to attend a full-day session on financing municipal stormwater, including a review of LID incentive programs.
In summary, the implementation of LID features would not have prevented the riverine flooding experienced in June 2013, since it originated from rainfall in the foothills and mountains. However, the implementation of LID will mitigate the impacts of severe storm events - especially summer thunderstorms - when they hit our urban centres, by attenuating peak flows, reducing runoff volumes and maintaining the resiliency of receiving streams. Developing outside of environmentally sensitive and hazard areas such as riparian areas would have greatly reduced the damage incurred in 2013.
|Rain gardens installed as part of the Village of Alix’s main street revitalization project. Photo: Alberta Low Impact Development Partnership|
Besides flood resilience, LID can play a key role in objectives such as biodiversity, carbon sequestration, water quality, water security and water conservation. Taking a holistic approach to choosing cost-effective solutions through a lens of multiple considerations is something we expect to see more of. But it doesn’t happen on the back of a napkin or in a few stakeholder engagement sessions. Work such as the Cooperative Stormwater Management Initiative demonstrates the complexity of grappling with even a subset of these objectives. We need to better understand the performance of LID features in our climate in order to validate assumptions and make design decisions that will succeed. Along with education and training, the ALIDP is working through its partners to establish a research program to investigate practices that will assist with achieving water quality objectives. But that’s another story...
Visit the ALIDP website at: https://alidp.org/
Leta van Duin is Executive Director at Alberta Low Impact Development Partnership. For more information contact Leta at leta[at]alidp.org or visit https://alidp.org