Soakage Testing and Soak Pit Design: Meeting New Stormwater Requirements

At Cook Costello, we are committed to providing innovative and effective engineering solutions to meet the evolving needs of our clients and communities. Recently, our team in Wellington has undertaken numerous projects to address the new stormwater full hydrological control requirements introduced by the Greater Wellington Regional Council (GWRC) as part of their Section 1 Plan Change. This initiative led us to perform soakage testing and design soak pits to manage stormwater for various infill housing projects.

Understanding Plan Change 1 to the Natural Resource Plan

Based on the rules introduced with Plan Change 1 to the Natural Resources Plan, GWRC mandates that all new developments must incorporate comprehensive stormwater management systems to control runoff on-site. This change aims to mitigate the adverse effects of urbanisation on natural waterways by ensuring that stormwater is adequately managed and treated before entering the environment. For our team, this has involved conducting detailed site analyses and designing solutions tailored to specific site conditions.

The Soakage Testing Process

Soakage testing is a crucial first step in designing an effective stormwater management system. This process involves:

1. Site Assessment: We begin each project by thoroughly assessing the site to understand its topography, soil composition, and existing drainage patterns. This information is vital in determining the site's suitability for a soak pit and other stormwater management solutions.

2. Soil Percolation Tests: To assess the soil's ability to absorb and transmit water, we perform percolation tests at various locations across the site. These tests measure the rate at which water infiltrates the soil, providing data on its permeability and drainage capacity.

3. Data Analysis: The results from the percolation tests are analysed to determine the infiltration rate. This rate is critical in designing a soak pit that can handle the expected volume of stormwater runoff without causing overflow or flooding.

Designing the Soak Pits

Based on the soakage testing results, our team designs soak pits tailored to each site's specific conditions. The key considerations in our designs include:

1. Capacity: Each soak pit is designed to accommodate the peak stormwater runoff expected during heavy rainfall events. This involves calculating the total catchment area and the corresponding runoff volume.

2. Location: Choosing the optimal location for each soak pit is essential to ensure efficient drainage and prevent waterlogging. We select sites with the highest infiltration rates and sufficient distance from buildings and other infrastructure.

3. Materials: We use rock-filled pits or modular units for the soak pits to facilitate water infiltration and distribution.

4. Maintenance: To ensure the long-term functionality of the soak pits, we incorporate features that allow for easy maintenance and inspection. Regular maintenance is crucial to prevent clogging and maintain the system's efficiency.

The Benefits

Implementing these soak pit designs has provided several benefits for our projects and the wider community:

• Environmental Protection: By managing stormwater on-site, we reduce the risk of flooding and minimize the impact on local waterways. This contributes to healthier ecosystems and improved water quality.

• Regulatory Compliance: Our designs meet the stringent requirements set by the GWRC, ensuring that our projects comply with local regulations and standards.

• Sustainable Development: The soak pit designs support sustainable development practices by promoting natural infiltration and groundwater recharge, reducing the need for artificial drainage systems.

The numerous soakage testing and soak pit design projects undertaken by Cook Costello exemplify our commitment to innovative and sustainable engineering solutions. 

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