Impermeable surfaces such as concrete and asphalt are becoming more prevalent as urban areas grow. These surfaces prevent water from naturally infiltrating the ground, which leads to a set of environmental and infrastructural problems. Dual purpose water tanks offer an effective response, providing efficient site coverage and water management on the same footprint.
Understanding impermeable surface issues
Impermeable surfaces are non-porous materials that do not allow water to pass through them. Common examples include roads, parking lots and rooftops. Individually none of these seems consequential. The problem is cumulative: a catchment that once absorbed most of what fell on it is progressively resurfaced until almost nothing does, and the water still has to go somewhere. Every roof and driveway added to a developed area removes a little more of the ground's ability to take water in, and the volume that used to soak away arrives at the drain instead.
Three problems they cause
The proliferation of these surfaces leads to several issues:
- Increased runoff: water that cannot infiltrate the ground becomes surface runoff, which can overwhelm drainage systems and lead to flooding.
- Reduced groundwater recharge: impermeable surfaces prevent the natural replenishment of groundwater, leading to lower water tables and potential water shortages.
- Pollution: runoff from impermeable surfaces can carry pollutants such as oil, chemicals and debris into water bodies, degrading water quality.
Note that the first two pull in opposite directions and are the same fault. There is too much water at once where you do not want it, and too little where you do.
Dual purpose tanks do two jobs at once
One of the most innovative and sustainable solutions is the dual purpose water tank. These tanks are designed to serve as both stormwater detention and rainwater retention devices, allowing homeowners to capture and reuse water for purposes such as laundry, toilet flushing or irrigation. Dual purpose water tanks not only reduce the amount of runoff entering the stormwater network, they also decrease demand for potable water from the mains supply. In doing both, they conserve water resources, ease the pressure on infrastructure, and lower water bills. Our article on dual purpose tanks covers how they are put together.
Detention and retention are different jobs
The terms get used loosely, so they are worth separating. Detention means the tank temporarily stores excess stormwater and releases it at a controlled rate, flattening the peak that would otherwise hit the network all at once. Retention means the tank permanently stores a portion of rainwater for reuse, keeping it out of the network entirely and putting it to work on the property. Detention manages timing. Retention manages volume. A dual purpose tank provides both functions in a single tank, which is why it can answer a stormwater obligation and a water supply want with one piece of equipment. Our explainer on detention versus retention tanks goes deeper.
What SMAF controls require
For developers in areas with SMAF controls, which stands for Stormwater Management Area Flow, dual purpose water tanks are an ideal option for complying with stormwater mitigation requirements. SMAF controls aim to reduce both the peak flow and the total volume of runoff from new developments or redevelopments, in order to protect downstream properties and waterways from flooding and erosion. Those are two separate targets, and that detail is what makes the control harder to satisfy than it first appears. Our summary of the SMAF stormwater rules sets out the requirements.
Why one tank can satisfy both targets
Peak flow and total volume need different mechanisms. Slowing the release rate addresses the peak but does not reduce how much water eventually reaches the network, since the same volume arrives, just later. Reducing volume requires water to leave the system altogether, by being used. A dual purpose tank can meet SMAF controls precisely because it provides detention and retention functions together: the detention portion controls the rate, the retention portion removes volume by supplying the property. By installing one, developers can satisfy council regulations with confidence. The SMAF calculator will put numbers against your own site.
Getting the specification right
Promax is a leading provider of dual purpose water tanks in New Zealand, offering products that meet the specifications and requirements of different councils and regions. That regional variation is the reason this is worth a conversation rather than a catalogue pick, since the control that applies to your site determines the split between detention and retention volume, and that split determines the tank. Promax can help you design a dual purpose water system that suits your needs and budget. Get in touch with our team for a custom water solution.