Stormwater Modular Tank Systems Overview
How Modular Tank Systems Work
Structure and Function: Stormwater modular tank systems consist of engineered, interlocking modules (often made of high-strength plastic) that form underground storage tanks. They are assembled within an excavation and wrapped in geotextile fabric or an impermeable liner, depending on the application.
During a storm, runoff is directed (via pipes or catch basins) into the subsurface modules, where water is temporarily stored in the void space between and within the modular units. The system then either slowly releases the water through an outlet control to mimic natural (pre-development) flow rates, or allows it to infiltrate into the surrounding soil if designed as an infiltration system. In effect, these tanks perform the same function as traditional above-ground detention ponds – capturing, holding, and gradually releasing stormwater, but they do so below ground.
Typical Applications: Modular tank systems are used where space is at a premium or where open water ponds are not feasible. Common applications include beneath parking lots, roadways, athletic fields, and plazas in urban or high-density developments. They are often employed to meet stormwater requirements for new subdivisions, commercial developments, and industrial sites where local regulations require on-site stormwater detention to prevent flooding and downstream impacts.
These systems are versatile: they can be configured for detention (slow release to a storm sewer), retention (water storage for reuse, using an impermeable liner), or infiltration (no liner, allowing water to percolate into the ground). In retention or rainwater harvesting mode, stored runoff can be reused for irrigation or other non-potable demands. In infiltration mode, the modules often have open bottoms or perforated sides and may sit on a stone base, enabling groundwater recharge.
Modular systems provide the structural strength to support traffic loads at the surface while offering a high storage volume underground.
Design Features: Each module typically has a high void ratio (often 95% or greater empty. The modular nature means the tank’s footprint and depth can be tailored to the site’s shape and volume requirements by connecting or stacking units. A geotextile wrap is used when infiltration is desired to keep soil from entering while letting water out, whereas an impermeable geomembrane liner is used around the modules if a fully enclosed tank is needed (for water reuse or when protecting groundwater from contaminants).
An inlet manifold or header pipe distributes incoming flow into the modules, and an outlet control (such as an orifice or flow control chamber) regulates discharge rates. Many designs also incorporate pretreatment devices like sediment traps or filter chambers to remove debris and sediment before water enters the tank, which is important for water quality and to prevent clogging if the system infiltrates.
Example Systems: GRAF EcoBloc Inspect and EcoBloc Inspect Smart are advanced modular stormwater management solutions designed for underground installation. Each system consists of high-strength polypropylene blocks that interlock to create customizable underground tanks for detention, retention, or infiltration.
These systems can be built to any size or shape and are strong enough to support traffic loads, including heavy vehicles. Typical applications include:
- Parking lots, commercial properties, and roadways
- Industrial and municipal stormwater management
- Green infrastructure and rainwater harvesting projects
Their modular design allows for easy inspection and cleaning through integrated access channels, setting GRAF apart from traditional plastic chamber systems.
Key Benefits of Modular Tank Systems
Stormwater modular tank systems offer several advantages over conventional surface ponds or concrete vaults, making them attractive for modern stormwater management:
Efficient Land Use: Because they are buried, modular tanks free up surface land for parking, landscaping, or even buildings. This is critical in urban areas where land is costly. Using an underground system can recapture about 15% of a site that would otherwise be occupied by a surface pond, allowing additional development – often the added revenue from this land can offset the tank cost. In high-density projects, underground detention may be the only viable option to meet runoff control requirements.
High Storage Capacity in Small Footprint: Modular systems have void ratios up to ~95%, meaning they store a large volume of water in a compact space. Compared to gravel beds or pipe-and-stone systems, they require less excavation for the same storage volume. This reduces the amount of soil to haul off-site and the quantity of aggregate to import, yielding cost savings and less site disruption.
Rapid and Flexible Installation: The modular units are lightweight and designed for easy assembly – often a single unit can be assembled in minutes and an entire tank field in a matter of days (depending on size). Installation does not require heavy lifting cranes (unlike large precast concrete structures), and contractors can adjust the layout (length, width, depth) by adding or removing modules to fine-tune capacity on site. Prefabricated components and simplified connections mean faster construction and less project downtime. This can be especially beneficial for retrofits or tight construction schedules.
Structural Strength and Durability: Quality modular tank products are engineered to support substantial loads (e.g. HS-20/HS-25 truck loading) when properly installed. Many systems boast design lives of 50+ years, using materials like polypropylene or PVC that are corrosion-resistant. Unlike corrugated metal pipes, they won’t rust, and unlike concrete, they aren’t prone to cracking from minor settlement. The uniformly distributed support (columns or lattice within modules) helps distribute loads and prevents collapse. Manufacturers typically test modules to ASTM and AASHTO standards for structural performance under traffic.
Reduced Safety and Liability Concerns: By eliminating open water basins, underground tanks remove hazards associated with ponds – there’s no risk of drowning and no stagnant water for mosquito breeding or odor issues. For the community, this means improved safety and aesthetics. The system is essentially “out of sight, out of mind”, which can be important for developments where a pond might be seen as an eyesore or take away from usable land (e.g. play fields or parking lots can exist right above the tank).
Environmental Benefits: When designed for infiltration, modular tanks can help recharge groundwater and support low-impact development goals by treating runoff on-site. The soil beneath the system can filter pollutants as water percolates, improving water quality (though by itself an underground tank provides minimal treatment, it can be combined with filters or bio-media). By controlling peak flows, these systems reduce downstream erosion and protect stream health. In retention configurations, they enable rainwater harvesting which can supplement water supply and reduce potable water use for irrigation or industrial processes. Also, keeping runoff on-site helps maintain the natural hydrologic cycle and can mitigate urban flooding.
Lower Maintenance than Ponds: While underground systems do require maintenance (periodic inspection and clean-out of sediments via manholes or inspection ports), they often incur lower lifetime maintenance costs compared to open ponds. Surface ponds can require dredging of sediment every 5–10 years, which is costly and disruptive (often hundreds of thousands of dollars each decade). Underground tanks, by contrast, typically only need vacuuming or flushing of accumulated sediment, which can be performed with standard vacuum trucks. They are also less prone to algae growth, weed issues, or thermal heating of water – problems that plague surface ponds and necessitate additional management. Moreover, because underground systems are not exposed to sunlight, there is less evaporation loss, preserving more water for potential reuse or sustained slow release.
In summary, modular tank systems enable developers to meet strict stormwater requirements without sacrificing valuable land, all while providing effective flood control and the flexibility to integrate with green infrastructure plans.
Cost Considerations
While EcoBloc systems typically cost more upfront than surface ponds or concrete vaults, they deliver significant long-term value:
- Land optimization: By replacing surface ponds, developers can reclaim up to 15–20% more usable land.
- Faster installation: Lightweight components assemble quickly without cranes, reducing labor time.
- Reduced excavation volume: The system’s high void ratio minimizes soil removal and imported aggregate.
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Lower lifetime costs: Underground systems avoid expensive dredging and maintenance associated with open ponds.
For high-value or space-constrained sites, the return on investment often justifies the initial cost.
Installation Methods and Considerations
Typical Installation Process: Deploying a modular tank system involves several key steps, generally straightforward for civil contractors:
Installing a modular stormwater tank system: Workers assemble plastic tank modules in an excavated pit lined with geotextile fabric. The lightweight modules are placed by hand and locked together to form a continuous underground storage structure (shown mid-installation above).
- Excavation & Base Preparation: Level and compact the base, add a sand or gravel bedding layer.
- Lining: Wrap the pit with geotextile for infiltration or install an impermeable liner for retention.
- Assembly: Snap EcoBloc modules together by hand to form the designed footprint and height.
- Integration: Connect inlet/outlet pipes, flow control devices, and inspection shafts.
- Wrapping & Backfilling: Enclose the structure with geotextile or liner, then backfill with clean granular material.
- Surface Restoration: Finish the surface as pavement, grass, or landscape.
Installation can take only a few days for medium-sized systems. Proper quality control ensures structural integrity and performance.
Typical Equipment and Timeline: The installation requires standard earth-moving equipment (excavator, loader), compaction equipment, and hand tools for assembling modules and cutting fabrics. No large crane is needed for modular systems, unlike for precast concrete vault installation. For a moderately sized tank (say 100–500 m³ storage), the excavation and base prep might take a day or two, module assembly another few days, and backfilling another day, often the entire system can be constructed in under a week or two by a crew, not counting final surface work. Larger installations (thousands of cubic meters) might stretch to a few weeks.
The modular nature means crews can work on different sections in parallel if space allows. Speed should be balanced with quality: improper leveling or rushed fabric placement can cause issues (e.g. misaligned modules or torn liners), so contractors are encouraged to follow best practices and even involve manufacturer technicians or experienced installers for guidance on the first projects.
Quality Control: During installation, attention is paid to maintaining a clean site (to avoid sediment filling the system before it’s even operational) and protecting the modules and liner. Many manufacturers have checklists, for example, ensuring geotextile overlaps are correct, no gaps between modules, and that the modules aren’t displaced during backfill compaction.
Before covering up, some projects require an inspection of the system by an engineer or official to verify it’s built per plan. After construction, the as-built is usually documented (with photographs or even a video inspection of internal voids via access ports) to have a record for maintenance.
Regulations and Standards
GRAF EcoBloc systems comply with CSA B184 - Polymeric Subsurface Stormwater Management Structures, the Canadian standard governing modular underground storage. This certification ensures safety, durability, and load performance.
Municipalities across Canada reference CSA B184 when approving underground stormwater systems. Local bylaws typically require:
- Professional engineering design and stamped drawings
- Compliance with infiltration or detention targets (e.g., 24–48 hour drawdown)
- Pretreatment for sediment and hydrocarbons before infiltration
In the U.S., systems follow AASHTO HL-93 load and relevant ASTM standards for structural testing.
