Exploring Stormwater Wetland Design Objectives and
Wetland Design Elements
In areas of Western Canada, where wetland losses have been high and the effects of outrageous weather events are commonly experienced, the demand for creating or restoring wetlands has become louder and louder. Just after the turn of the century, there was a shift in environmental management in Western Canada, likely in response to the economic boom that occurred in the early-2000s, coupled with the tangible effects of climate change, where magnitude and occurrence of major weather events has amplified.
Outcries for more consistent environmental management from landowners, land users and land managers were heard, resulting in updates to many provincial/territorial water-related regulations or policies to better protect wetlands. Wetlands are keystone ecosystems[1] that provide landscape services to areas within a wetland’s catchment (upland areas that are larger than the extent of the wetland[2]), as well as downstream or downslope lands. So, whenever the land around or within a wetland is modified through a land use activity, there may be a direct, permanent loss of wetland area, as well as an indirect impact to surrounding lands due to changes to water behaviour within that wetland’s catchment (e.g., flooding, drying).
Generally, environmental managers aim to ensure ‘no net loss’ of wetlands to reduce the liability/human safety risk associated with loss of wetland services. Many jurisdictions in Western Canada (provinces, territories, municipalities, etc.), now require some level of compensation or ‘no net loss’ proposal if wetlands will be impacted during construction or operation activities of a given land use, and as such, requests for wetland construction and restoration designers has grown. When an environmental designer embarks on a wetland design, they must first articulate the design objective(s). Design objective selection is usually guided by regulations[3], as well as the landscape functions desired from the wetland, such as those functions within the catchment or surrounding lands that may have been previously lost or are anticipated to be lost in the future by a proposed development/land use activity.
SALMTEC's Top Six Wetland Design Objectives
Here are SALMTEC’s picks for the top six most common wetland function design objectives. Although not an exhaustive list, these tend to be recurring objectives in different jurisdictions throughout Western Canada, given the comparable pressures of water use, wastewater disposal/treatment and stormwater management.
- The ability for flood control/attenuation/stormwater management, where wetlands can:
- Delay water by reducing velocity of the water as it moves through the wetland,
- Store water through additional holding capacity, and
- Reduce downslope flood potential.
- The ability to supply late-season water to downslope/downstream aquatic environments
- The ability to improve water quality, where wetlands can:
- Delay and retain inorganic sediments.
- Retain Phosphorus through chemical adsorption and plant translocation.
- Retain and remove Nitrogen through chemical and biological interactions.
4. The ability to provide labile carbon and other nutrients to downstream aquatic environments.
5. The ability to provide habitat to a variety of wildlife groups, including wetland-obligate waterfowl, shorebirds, raptors, mammal, songbirds, plants, pollinators, amphibians, fish, invertebrates, such as insects and snails.
6. The ability to provide sustainable enjoyment to humans, including aesthetics and accessibility.
When designing a wetland, different types of design elements are considered, and, often, design requires contributions from different subject matter experts. For example, wetland ecologists may provide contributions focusing on wetland habitat design objectives, whereas stormwater engineers may provide more contributions such as stormwater management design objectives. Once a designer has identified wetland design objectives, often through collaboration with other subject matter experts, they must then explore the interplay between design elements, where they are placed, and how they affect each other. To illustrate this, let’s focus on a single wetland design element and explore multiple different considerations when applying it to a single wetland design objective.
In this example, let’s consider woody plants (trees and shrubs), a design element often recommended by a wetland ecologist to provide structural habitat diversity. In this example, let’s examine the application of the woody plant element to the flood attenuation/stormwater management wetland design objective. In general, adding woody plants increase the evapotranspiration potential, increasing water outputs in that wetland’s water balance equation.
Water Storage Potential
=
Water Inputs < Water Outputs
where…
Water Inputs = precipitation, surface inflows, groundwater discharge
Water Outputs = evaporation, evapotranspiration, surface outflow, groundwater recharge
When designing a stormwater wetland, the designer, often a stormwater engineer, examines how the woody plant element affects the ability of the wetland to hold or remove water within its catchment and downstream/downslope catchments. To do this, the stormwater engineer looks at the entire water balance equation and adjusts parameters based on the designed, anticipated, values of water inputs and outputs. Water inputs include precipitation, surface run-off/overland flow and groundwater discharge. Water outputs include evaporation, evapotranspiration, surface drainage (outflow), and groundwater recharge. In this example, the designer must understand how the addition of woody plants affects all expressions in the water balance equation. It is at this point that a designer may want to consult with the ecologist to understand the types of trees and shrubs and their prescribed distribution within the wetland.
Although primarily designing for stormwater management, the designer must consider how woody plants affect a wetland’s water balance by examining how the woody plant element interacts with water inputs and outputs (see below for a list of references):
- Precipitation, where rain and snow interception increases, resulting in removal through evaporation from leaves;
- Surface-run off/overland flow, where stems of woody plants intercept surface run-off, potentially reducing the overland flow by increasing ability for infiltration or evaporation;
- Groundwater discharge, where deep roots of trees and shrubs translocate water from the ground to their leaves, where it evaporates, potentially reducing the elevation of the groundwater table, locally;
- Evaporative potential, where woody plants can act as a wind barrier if planted on the side of the wetland where wind usually prevails, plus tall woody plants can shade surface waters, reducing surface temperatures
- Evapotranspiration, where woody plants remove water from the inputs through translocation from the ground to the leaves, where it evaporates;
- Surface outflow, where woody plant stems in an outflow can delay water from leaving the wetland; and
- Recharge ability, where trees and shrubs that shade the ground tend to increase the number of days that the ground is frozen, thus reducing the value of water outputs, as surface water cannot infiltrate into the ground.
For stormwater engineers, this level of analysis between all recommended design elements would likely need to be considered within each sub-catchment when using a modeling application such as the Storm Water Management Model (SWMM). In this example, a single design element, woody plants, was used to illustrate the complexity of wetland design, and how designers must consider how each element influences another and how collaboration between subject matter experts is needed throughout the process. If we were to explore considerations for the woody plant design element with a different design objective, there would be an equal or greater number of things to consider. However, if the wetland designer can articulate the desired objectives to all subject matter experts and regulators involved in wetland design planning, and understands that it is an iterative and collaborative process, the desired wetland functions are more likely to reach the desired levels as anticipated.
Footnotes
[1] https://peatlands.org/assets/uploads/2019/06/ipc16p248-252a171locky.pdf
[2] Some exceptions include raised bogs and abandoned oxbow wetlands.
[3] Regulations depend on where the wetland is and what the land use activity. Most provinces and territories in Canada have some type of water regulation that may apply to both private and public lands. The Federal Policy on Wetland Conservation (GoC 1991) applies to federal lands (no net loss prevailing), but often the more stringent provincial/territorial policies are applied
Links to References
- General Design Guidelines for a Constructed ‘Habitat’ Wetland – Grasslands Natural Region of Alberta
- Guidelines for the Approval and Design of Natural and Constructed Treatment Wetlands for Water Quality Improvement
- Alberta Guide to Wetland Construction in Stormwater Management Facilities
- Directive for Permittee-responsible Wetland Construction in Alberta
- Alberta Wetland Restoration Directive
- Green Stormwater Infrastructure Design Guidelines for the Capital Region
- Interim Guidelines for Wetland Protection and Conservation in British Columbia
- The Federal Policy on Wetland Conservation
- Wetland Design Guidelines – City of Saskatoon
- Alberta Wetland Rapid Evaluation Tool – Actual (ABWRET-A) Guide
- US EPA Constructed Wetland Manual