Bioswale in a parking lot
A bioswale has been implemented at a shopping centre parking lot to manage stormwater runoff form the parking lot.
Components installed in the solution
The parking lot for more than 200 vehicles at the shopping centre has been designed to capture stormwater and direct its flow towards a bioswale from the area of around 5 400 m2. The parking lot’s surface is sloped at 1.5 % towards the bioswale. The bioswale has sloping banks with vegetation groundcover providing protection from erosion (in addition, the areas where groundcover protection is insufficient, geotextile fabric provides additional erosion control). A 1 m wide pebble ribbon separates the lot from the bioswale. From there it is via D100 pipe connected to the collecting well containing oil separator (model Techneau YHO0506) with the maximum flow of 6 l/s, finally being discharged into the sewer network. Uni-directional valve is installed at the discharge into D700 municipal sewer, to avoid the backflow.
Water level in the bioswale may reach up to 30 cm in extreme rain. The volume of the bioswale is ~150 m3. Depending on the real runoff rate from the concrete block pavement, it can withstand the storm event with the precipitation from 50 mm (ψ = 0.6) to 140 mm (ψ = 0.2). After the rain event on 18.09.2017 with the cumulative depth of up to 44 mm in 24 h, the bioswale was less than half-full.
Operational mode
The conventional collection and conveyance of stormwater runoff as a result of the shopping centre development was not possible due to limited capacity of the current urban drainage system – a network of drainage ditches and storm sewers. Technical conditions issued by the city road maintenance department, responsible for the storm sewers, specified 5 l/s maximum discharge. Alternatives included costly expansion of the current network or underground attenuation tanks (grey infrastructure). However, the green infrastructure approach had been tried out and proved to be successful. In the green part of the development plot, downstream from the parking lot, a bioswale was constructed. It offers multiple functions/benefits: stormwater runoff attenuation before restricted discharge into sewer network, groundwater recharge, stormwater treatment by plants and soil, landscaping function, space for snow storage during winter. Respecting regulation requirements, oil separator has been built at the discharge from the bioswale. However, filter media has not been changed since constructed as very little amount of oil products reach the oil separator – most runoff being treated by the green infrastructure. Therefore, in future projects like this, oil separator is considered redundant.
Maintenance activities include:
Once every 3 months: check backwater valve at the facility discharge into the city sewer;
Once every 6 months: oil separator check and oil filter change if full;
As needed: leaves and debris removal, grass cutting, plants maintenance and replacement, watering in the dry periods
Results
Prior to construction, the area was undeveloped, therefore no stormwater management solutions had been necessary; the construction of a new parking lot introduced the need for a runoff stormwater management. By implementing a bioswale, an attractive and functional solution for the rainwater problem has been created. The runoff from the bioswale barely reaches the city sewer system, providing almost completely decentralised stormwater management.