What are infiltration trenches and how do they work?

As the world’s drainage challenges grow, designers are constantly looking for new ways to handle stormwater. Commonly known as stormwater controls (SWCs), a subset of sustainable drainage systems, known as SuDS (LIDs/BMPs/WSUDs), have grown popular globally. SuDS work with nature, rather than against, and can be seen as more sustainable means of handling stormwater inflow on a site.

One of the most popular SuDS globally are infiltration trenches, which are designed to manage stormwater runoff, improve water quality, and reduce the risk of flooding. These trenches are essentially shallow excavations filled with coarse aggregate, allowing rainwater to infiltrate into the ground rather than flowing directly into drainage systems.

By creating a highly permeable surface, water can be drained from a site relatively quickly and redirected to other SWCs, like ponds or natural streams and rivers. Let’s take a closer look at the components of infiltration trenches.

What are the main parts of infiltration trenches?

An infiltration trench typically consists of five main components:

• Trench excavation: A trench is dug to a specified trench depth and width (shown in illustration) and is usually lined with a geotextile fabric to prevent the surrounding soil from clogging the system.
• Aggregate fill: The trench is filled with coarse aggregate, such as gravel, which creates void space between the rocks for water storage and infiltration. There are certain types of infiltration trenches which use plastic chambers or grids to hold water while it infiltrates.
• Inlet structures: These are designed to direct stormwater into the trench, which can include pipes, swales, or surface channels.
• Overflow systems: To prevent overflow during heavy rainfall events where inflow rate exceeds the infiltration rate into the ground, overflow outlets are integrated, ensuring excess water is safely diverted to other drainage systems or watercourses.
• Vegetation cover: In some designs, a layer of vegetation or turf is added on top of the trench to enhance aesthetics and further improve infiltration.

So then, how do all these parts come together to deliver a more sustainable stormwater control system?

How infiltration trenches work

The operation of an infiltration trench can be broken down into four stages:

• Collection: Stormwater is collected from impervious surfaces like roofs, roads, and pavements and directed into the infiltration trench through pipes and channels
• Conveyance: The collected water enters the trench through inlet structures, where it is temporarily stored in the void spaces within the aggregate (or plastic chamber).
• Infiltration: The stored water gradually infiltrates into the surrounding soil, facilitated by the large surface area of the aggregate on the infiltration region of the ground. The larger the surface area of the infiltration zone, the more water in the trench will be able to infiltrate the surrounding soil. This process mimics natural groundwater recharge, reducing runoff and enhancing water quality through natural filtration. Infiltration trenches, like all stormwater controls, do have design lives, meaning that every so often they will need to be cleaned out or flushed to remove filtered sediment and contaminants.
• Overflow: During extreme rainfall events, any excess water that exceeds the trench’s capacity is safely diverted through overflow systems, preventing surface flooding.

The processes of managing infiltration trenches in the drainage design process transcends just theory. Modern drainage design tools like InfoDrainage can accurately model the hydrologic behavior of these stormwater controls. With this level of insight, engineers and hydraulic modelers can know exactly how their SuDS will perform in the real world – before they get built.

These SuDS features inside our software are a big selling point for InfoDrainage because they make it easy to model exactly how they will work over time based on regional rainfall standards, either as standalone SuDS or as a linked system of SuDS. These kinds of sustainable implementations are most popular in the UK, where they are known as SuDS, but Americans may know them as LIDs or simply BMPs. Aussies call ’em WSUDs. No matter what acronym you choose to assign to them, they will become an increasingly important tool to help prevent flooding in cities as the planet warms and rainfall increases.

There is another very popular option for conveying stormwater: culverts. However, culverts do not have many benefits beyond moving water from one place to another around an obstacle – usually underneath a road.

Benefits of infiltration trenches

Why use these? Lots of reasons:

• Flood mitigation: By promoting groundwater infiltration, infiltration trenches reduce the volume and peak flow of stormwater runoff, decreasing the risk of urban flooding.
• Water quality improvement: As stormwater percolates through the aggregate and soil, pollutants like sediment, heavy metals, and nutrients are filtered out, enhancing the quality of the infiltrated water.
• Groundwater recharge: Infiltration trenches aid in replenishing groundwater levels, which is crucial for maintaining base flows in rivers and supporting ecosystems.
• Eco-friendly solution: Incorporating infiltration trenches into urban landscapes contributes to sustainable and environmentally friendly stormwater management practices.
• Flexible: Can easily be incorporated into a site’s landscaping and can work well when placed beside roads, as well as public recreation areas.

As drainage designers know, more impermeable surfaces mean more runoff that a drainage design must handle. By designing with sustainable drainage systems, we’re able to deliver a drainage system that doesn’t simply create additional impermeability on the site. Rather, by working with nature to handle the flow, our drainage designs can be more efficient and effective at handling the ebbs and flows of increasing stormwater events.

It should be noted though that one of the downsides of infiltration trenches arises when dealing with highly contaminated stormwater flow. In most infiltration trench designs, designers will use vegetation strips or swales to slowly filter out larger contaminants that would clog up the infiltration trench aggregate. Frequent instances of high, turbulent flow in a region may reduce the design life of infiltration trenches as they collect and filter out more sediment that makes it past pre-treatment controls. However, when properly designed and implemented, these downsides can be mitigated.

Now that we have a good understanding of how infiltration trenches work, let’s explore how we can design with them inside of InfoDrainage.

Infiltration trenches are especially important alongside highways, which is why some of our customers take advantage of the Integrated highway design with the Civil 3D + InfoDrainage end-to-end workflow.

Designing with infiltration trenches in InfoDrainage

InfoDrainage, a comprehensive site drainage design tool, has SuDS integrated into the application to facilitate the design and implementation of sustainable drainage solutions. This allows engineers and planners to model, analyze, and optimize infiltration trenches, including connecting them with other SuDS components if necessary to create an extremely accurate representation of reality and help ensure your site has effective stormwater management.

With InfoDrainage, you’re able to:

• Simulate infiltration trenches: Model the performance of infiltration trenches under various rainfall events, assessing their capacity to manage runoff and prevent flooding.
• Optimize the design: Optimize trench dimensions, aggregate specifications, and inlet/outlet configurations to enhance efficiency and sustainability.
• Integrate with other SuDS: Seamlessly integrate infiltration trenches with other SuDS features such as green roofs, permeable pavement, and rain gardens for a holistic approach to stormwater management.
• Comply with local regulations: Ensure that designs meet local and national regulations for sustainable drainage, promoting best practices in urban planning and development.

Because InfoDrainage has integrated SuDS, you can simply select infiltration trench as a SWC and add in the necessary dimensions and hydrology figures. You can adjust the dimensions, inlet and outlet details, infiltration rates, and pollution details. This feature can also be used to design different types of trenches like Trench Dry Wells or French Drains, with or without under drain pipes.

Want to know more about how they work?

• The InfoDrainage documentation covers all of the details, and these details are also covered in our free Understanding SuDS design course.
• Download our free Guide to Representing SuDS in InfoDrainage in accordance with the SuDS Manual Ciria 753.
• Want to try InoDrainage? We offer a 30-day free trial with no credit card required. Are you a student or educator? If so, we have some very good news for you.