High Groundwater Considerations for Detention and Infiltration Systems

High groundwater can significantly influence the design of underground detention and infiltration systems. It can limit available storage depth, affect infiltration feasibility, create buoyancy concerns, and complicate installation.

Identifying groundwater conditions early in the design process can help engineers avoid costly redesigns and determine whether detention, infiltration, or a combination of the two is appropriate for the site.

Understand the Groundwater Conditions

A geotechnical investigation is an important first step when groundwater may be present near the proposed system. Designers should understand not only the groundwater elevation observed during the investigation, but also how that elevation may fluctuate seasonally.
Groundwater conditions can vary significantly throughout the year. A reading taken during a dry period may not represent the seasonal high groundwater table.
Important information to consider includes:

• Seasonal high groundwater elevation
• Soil type and permeability
• Soil bearing capacity
• Settlement potential
• Available depth from finished grade to groundwater
• Local groundwater separation requirements

Understanding these conditions early helps determine how much vertical space is available and whether infiltration is feasible.

Determine Whether Infiltration Is Appropriate

Infiltration can reduce runoff volume and promote groundwater recharge, but it is not appropriate for every site.

When groundwater is too close to the bottom of an infiltration system, there may not be enough unsaturated soil to provide adequate infiltration or meet regulatory requirements. High groundwater can also reduce the effective storage volume available in the stone backfill surrounding an infiltration system.

Designers should evaluate applicable state and local requirements for minimum separation between the bottom of the infiltration system and seasonal high groundwater.

If adequate separation cannot be maintained, a detention system with a controlled discharge may be a more appropriate approach.

Consider Available Depth and System Profile

High groundwater often reduces the vertical space available for underground detention and infiltration systems.

On sites with limited depth, a shallower system with a larger footprint may be preferable to a deeper system. Designers should evaluate the relationship between:

• Required storage volume
• Available footprint
• System depth
• Minimum cover
• Inlet and outlet elevations
• Groundwater elevation

The goal is to provide the required storage while keeping the system within the usable vertical space available on the site.

Account for Buoyancy

Underground detention systems installed below or near the groundwater table may be subject to buoyant forces.

When groundwater surrounds an underground system, the upward force created by displaced water can become significant, particularly when the system is empty or only partially full.

A buoyancy analysis may be necessary to determine whether the system has adequate resistance from its own weight, overburden, backfill, or other design measures.

If the buoyant forces are greater than the resisting forces, countermeasures are necessary to overcome flotation. Here are some methods used to overcome buoyancy: 

• Increase soil cover- an increase in soil cover improves the net resisting forces.
• Raise the system invert out of the groundwater table using Variable Diameter CMP Systems, thus removing buoyancy concerns.
• Specify anti-flotation anchors - the underground detention system is anchored to concrete deadman via hold-down straps.  

Understand the Impact on Infiltration Storage

For infiltration systems, groundwater can affect both the available storage volume and the system's ability to drain.

Many underground infiltration systems rely on the void space within the surrounding stone backfill for additional storage. If groundwater rises into that stone, some of the available storage volume is already occupied and cannot be counted toward stormwater storage.

High groundwater can also reduce the hydraulic gradient available to move water into the surrounding soil, potentially slowing drawdown.

For this reason, seasonal groundwater conditions should be considered when determining both storage volume and expected infiltration performance.

Plan for Construction Conditions

Groundwater can also complicate installation.

Excavations may require temporary dewatering to maintain a stable working area and allow proper placement and compaction of bedding and backfill. Contractors should understand anticipated groundwater conditions before construction begins so appropriate excavation and dewatering procedures can be planned.

Poor groundwater management during installation can contribute to unstable subgrades, difficult backfill placement, and settlement concerns.

Early coordination between the design engineer, geotechnical engineer, manufacturer, and contractor can help reduce these risks.

Choose the Right Detention or Infiltration Approach

High groundwater does not necessarily eliminate underground stormwater storage, but it can influence the type and configuration of the system.

Depending on site conditions, designers may consider:

• Shallow underground detention systems
• Low-profile storage configurations
• Larger-footprint systems that reduce required excavation depth
• Detention with a controlled outlet when infiltration is not feasible
• Partial infiltration with controlled overflow or discharge
• Fully infiltration-based systems where adequate groundwater separation and suitable soils are available

The appropriate approach depends on required storage volume, available footprint, groundwater elevation, soil conditions, site grades, and applicable regulatory requirements.

Design Around Groundwater From the Start

Groundwater should be treated as a fundamental design constraint for underground detention and infiltration systems.

Understanding seasonal groundwater elevations, evaluating infiltration feasibility, selecting an appropriate system profile, accounting for buoyancy, and planning for construction conditions can help prevent significant problems later in the project.

By incorporating groundwater conditions into system selection and layout from the beginning, engineers can develop detention and infiltration systems that provide the required storage while remaining practical to install and capable of performing as intended.