Contributor: Karolyn Sweeting
Location: Vancouver Island, British Columbia
Land Use: Other
Habitat Type: Wetlands
Project Timeline:
Highlights
Permanent engineered wetlands, wet ponds, infiltration basins and grassed treatment swales have been constructed on the Inland Island Highway to improve the quality of highway runoff entering fish-bearing streams. These water quality treatment systems capture contaminants from highway runoff before it is released into nearby watercourses. An added benefit is that engineered wetlands provide additional habitat for various animal species including amphibians, aquatic mammals, and birds. This report provides the rationale for undertaking this work and summarizes the facilities and the considerations for maintaining their function.
Engineered wetlands are water quality treatment facilities. They are constructed on highway right-of-ways to intercept and detain runoff water, to trap and filter sediments, and to capture contaminants before they enter nearby streams. An engineered wetland is a vegetated, shallow pond in which the water level is controlled by an outlet structure. Wetland ponds store runoff and rainfall, reduce flooding and soil erosion, and purify water by filtering wastes, sediments, and toxic compounds. A number of mechanisms that include physical filtering and sedimentation, chemical reactions between soil and water, adsorption to vegetation, and biological uptake and conversion by plants and bacteria are believed to account for the improvements in water quality. A vast array of species relies on wetlands because of the presence of water, the rich nutrients, and the diversity of habitat. The converted ponds provide habitat to a variety of animals including newts, frogs, and birds.
This study and the associated designs for drainage facilities were initiated in response to the Ministry of Transportation’s (MoT) desire to protect the quality of runoff exiting from highway corridors during the construction and operation of the new sections of the Inland Island Highway. The entire highway crossed about 150 fish-bearing streams and consequently could have had a large impact on the fishery resource and downstream water quality. For instance, runoff from the highway would have the potential to impair water quality as a result of vehicle traffic. Site erosion and subsequent downstream sedimentation during construction was also a concern as it also had the potential for negative impacts on water quality and aquatic habitat. As a result, water quality treatment systems, such as sediment ponds, were implemented to improve the quality of the stormwater. Eventually, some of these sediment ponds were converted into permanent engineered wetlands where they function throughout the operational phase of the new sections of the Vancouver Island Highway. The goal of the engineered wetlands is to improve water quality by removing contaminants and sediment from highway runoff.
Project Objectives
The Ministry of Transportation’s program objectives are to- ensure that the water treatment systems maintain and/or improve the quality of highway runoff entering sensitive watercourses during construction
- control soil erosion and highway runoff during construction by limiting disturbances to the area using measures that include grassed treatment swales and sediment ponds
- encourage the settlement and filtration of suspended particles, and the capture of contaminants before runoff is released into downstream watercourses.
- continue developing and refining effective designs for water quality treatment systems
- maintain the water quality treatment systems and to ensure downstream water quality is not impaired.
The MoT identified and mitigated water quality concerns for the Inland Island Highway in three stages: Pre-Construction, Construction, and Post Construction.
The highway was designed to ensure all runoff was treated prior to it entering a sensitive watercourse. In the pre-construction phase, water quality treatment systems sites were selected and identified during the design phase. The Vancouver Island Highway Project Guidelines for Environmental Design of Highway Drainage1 provided additional strategies to select sites for constructing water quality treatment systems based on 1) hydrologic design information; 2) methods for erosion control; and, 3) recommendations for permanent stormwater treatment facilities.
During construction, Environmental Monitors ensured that the water quality treatment systems were receiving highway runoff. Environmental agencies were in charge of ordering plant material for the ponds, and meeting with Environmental Monitors to identify which sediment ponds could be converted into engineered wetlands.
The sediment ponds that were not converted to engineered wetlands were instead converted to wet ponds and cleaned-out of sediments accumulated during construction. They were also planted around the perimeter and/or complexed to provide habitat for wildlife and/or fish.
Water quality treatment systems were not monitored in the post construction phase due to insufficient funding. These treatment systems are expected to be self-maintaining.
Pre-construction
For the expected daily traffic volumes, the Vancouver Island Highway Project Guidelines for Environmental Design of Highway Drainage1 recommended that sensitive receiving waters should be protected from contamination by highway runoff. According to the guidelines, at a particular daily traffic volume, drainage designs should incorporate some combination of grassed medians, grassed shoulder strips, grassed ditches, grassed treatment swales at the end of highway ditches, or constructed wetlands to remove contaminants from highway runoff.The sediment ponds that satisfied all four of the following requirements were converted to constructed wetlands:
- receiving waters were sensitive
- highway runoff catchment area was significant
- ponds were expected to hold water all year
- pond was large enough (larger than 125 m2) to accommodate a raised platform suitable for wetland plants.
Some sediment ponds that did not meet this criteria were converted to wet ponds by removing accumulated sediments and installing wetland plants around the perimeter, thus providing some water quality and enhancing wildlife habitat. Planting platforms were not installed in these ponds.
Converting the sediment ponds to constructed wetlands included removing accumulated sediments, constructing planting platforms, modifying drainage works when necessary, and installing wetland plants.
Construction
Engineered wetlands and drainage works
Discharges and pond flows were designed so they did not cause erosion of the natural drainage system. Common pond outlets were either swales or pipes. Each outlet pipe was equipped with a vertically oriented “T” to prevent floating oil from exiting the pond2. An emergency spillway lined with riprap or filter cloth was implemented to allow only clarified surface water to leave the pond. Most pipe outlets also had an overflow notch in the bank to direct overflow if the pipe was blocked.Total suspended solids (TSS) were monitored during the construction phase to ensure the relative effectiveness of the sediment ponds. TSS were measured by a grab sample at both the inlet and outlet during rainy periods to confirm that the ponds were displaying acceptable water quality.
Planting platform construction
Prior to plant installation, accumulated sediments were removed if required and a planting platform was constructed. The platform occupied 65% of the pond area. The deeper water upstream of the platform, located near the inlet, was the forebay. The forebay, one-meter deep and 25% of the total pond area, was designed to improve the settling of suspended solids. The deeper water downstream of the platform near the outlet, the aftbay, covered the remaining 10% of the pond.The platform was sloped to provide planting sites of varying depths to better suit each species. The top of the planting platform ranged from 0.1 m at the upstream end to 0.5 m at the downstream end. The top 0.3 m depth of the platform was soil. The rest of the platform could be soil or some other inert fill material. The tops of slopes or berms around sediment control ponds were wide enough to provide a safe and stable work area for the operation of maintenance equipment and personnel. The area was covered with crushed stone and/or turf to prevent damage to the structure, and loosening of the soil, which could wash into the pond.
The stability of the side slopes was confirmed by a professional engineer to ensure they were structurally stable under all loads and hydraulic conditions. Where the natural substrate was coarse textured and/or highly permeable, the sediment pond was lined with swelling clay to hold water. In one situation, a strong, impermeable, synthetic membrane was used to prevent rapid infiltration of water entering the system. If the water treatment system did not retain water, they were considered as infiltration systems instead. They were not planted and were not converted into engineered wetlands.
Plant Installation
Year-round water levels in the pond’s wetland area were known for the selection of wetland vegetation species. If the hydrology of the area was an unknown factor at the time of pond installation, it was recommended that vegetation establishment be initially attempted with a variety of species adapted to a range of possible moisture conditions. Plant species included Cattail and broadcasting the seed of Red Canary Grass (Phalaris arundinacea; wet soils) and Red Fescue (Festuca rubra; drier soils). A number of other wetland plants were used on the Inland Island Highway. There were Hard-stemmed Bulrush (Schoenoplectus acutus), Small-flowered Bulrush (Scirpus microcarpus), wapato arrowhead, burreed, Common Rush (Juncus), Slough Sedge (Carex obnupta), Henderson’s Checker-Mallow (Sidalcea hendersonii), Hardhack (Spiraea douglasii), Sweet Gale (Myrica gale), Marsh Cinquefoil (Comarum palustre), and Palmate Coltsfoot (Petasites frigidus var. palmatus). Some plant species were more effective than others in removing soluble metals and organics. It was not necessary to remove grass cuttings from the grassed treatment swales, as well, harvesting and disposing of wetland vegetation was viewed as an unnecessary practice.The wetland was then planted with native species for nutrient and pollutant uptake. Mineral topsoil containing a high content of organic matter covered the area. The vegetation was planted on the wetland site as soon as the platform was built and before water refilled the pond2. Individual species were planted in the wetland according to locations recommended in the Plant Layout2. Vegetation was planted in an appropriate depth of water, (0.6 m on-center apart) and blended to create a more naturalized appearance.
Wet Ponds
Wet ponds do not have a planting platform. Instead, vegetation is also planted at 0.6 m on-center spacing intervals along the perimeter of the normal water’s edge. Spacing can be adjusted as required to take advantage of the specific conditions of each site.Sediment Removal
It will be years before the water quality treatment systems need to be cleared of sediment build-up. The best way to remove sediments out of an engineered wetland is with a pumper truck. When sediments are excavated, care must be taken to ensure that the soil and granular material do not enter streams when they are emptied from the pumper truck. The Environmental Monitor is consulted when selecting the location to dump the sediment. Also, if the filter fabric decomposes or is ineffective, it is replaced or repaired.Post Construction
Post construction monitoring programs of the engineered wetlands, undertaken by the Maintenance Contractor, are completed to ensure that sediment and drainage control measures function as designed and that optimum plant growth and pollutant uptake is achieved. The frequency of such inspections and monitoring efforts correspond to the sensitivity and location of the construction activities, and the environmental conditions.The Vancouver Island Highway Project (VIHP) created engineered wetlands along the Inland Island Highway to preserve water quality by effectively removing pollutants. Constructed wetlands and wet ponds like those being installed along VIHP are used throughout North America to treat highway runoff. The engineered wetlands improved the quality of the runoff water by providing both spill containment and a clean-up opportunity. Maintaining access roads to engineered wetlands, as well as removing sediments and contaminants ensured the function of these systems. In addition to improving the quality of the runoff water, both the engineered wetlands and the converted ponds provided habitat to a variety of animals including newts, frogs and birds.
Highway runoff pollutants originated from a variety of sources including pavement wear, tire and brake wear, motor oil additives, emissions, petroleum products used in vehicles, spills, vehicle corrosion, rust, right-of-way maintenance activities, and litter. Properly designed and maintained grassed filter areas, sediment ponds, and engineered wetlands removed a majority of suspended particulates. Water quality treatment systems, such as engineered wetlands, contained and isolated deleterious substances from reaching nearby water bodies.
The majority of particulates settle out in the engineering wetlands. Those soluble pollutants remaining flow to the wetland chamber, which functioned as a physical, chemical, and biological treatment system. Biological activity, the primary mechanism for the removal of soluble pollutants, occurred primarily during the summer growing season.
In order to ensure that the water quality of the engineered wetlands is effective and that fish-bearing streams are protected, the Vancouver Island Region needs to implement appropriate maintenance standards for the grassed treatment swales, the sediment ponds, and the engineered wetlands. These standards should include appropriate methods for removing trash and debris, maintaining access roads, and implementing regular system inspections. Periodic sampling of the substrate and the vegetation should be conducted to ensure the contaminants that are being absorbed by wetland vegetation are not detrimental to water quality. However, there is no funding for post construction monitoring or periodic sampling of substrate accumulation or vegetation decomposition.
References
- Vancouver Island Highway Project Guidelines for Environmental Design of Highway Drainage. KPA Engineering Ltd. Victoria, B.C. 1992.
- Conversion of Sediment Ponds to Water Quality Treatment System: Cumberland Interchange to Headquarters Creek; Headquarters Creek to the Jubilee Parkway. Par Terre Environmental Consulting Services Ltd. Victoria, B.C. May 29, 2001.
Partners
Ministry of Transportation
Environmental Services, Engineering Branch
Al Planiden
Manager, Roadside De
940 Blanshard Street
Victoria, BC Canada
V8W 9T5
Phone: (250) 387-3736
Email: Al.Planiden@gems3.gov.bc.ca
Par Terre Environmental Consulting Services Ltd.
Isobel Doyle
7266 Kristin Place
Brentwood Bay, BC Canada
V8M 1J4
Phone: (250) 652-4345
Email: parterre@islandnet.com