Infiltration Trench Sizing Calculator

From rainfall to recharge

Infiltration trenches are narrow excavations filled with coarse aggregate or engineered media designed to intercept runoff and allow it to percolate into the subsoil. They are commonly deployed along parking lots, building perimeters, or downslope of compacted yards where natural infiltration has been reduced. Unlike surface basins, trenches occupy little land area while providing subsurface storage. This calculator assists designers and homeowners in estimating how long a trench must be and how quickly it will drain after a storm event. By inputting the contributing drainage area, expected storm depth, trench geometry, aggregate porosity, and soil infiltration rate, users can evaluate whether a proposed trench will capture and infiltrate a given rainfall.

The volume of runoff generated by a storm is approximated as the product of the drainage area and rainfall depth. Although real systems consider runoff coefficients to account for infiltration and interception on the surface, this tool assumes impervious contributing areas for simplicity. The storage volume provided by the trench equals its geometric volume multiplied by the void ratio of the aggregate, since only the voids between stones hold water. To contain the runoff, trench storage must be at least as large as the incoming volume. Length is therefore derived from $L=\frac{\mathrm{A\_d}\times \mathrm{D\_r}}{B\times Z\times n}$, where $\mathrm{A\_d}$ is drainage area in square meters, $\mathrm{D\_r}$ is rainfall depth in meters, $B$ is trench width, $Z$ is depth, and $n$ is void ratio. Converting rainfall from millimeters to meters allows consistent units.

Storing water is only half the task; the trench must also empty in a reasonable time to be ready for the next storm. The infiltration rate of the native soil governs how quickly water seeps from the bottom. With $f$ representing soil infiltration rate in meters per hour, the drainage time $\mathrm{t\_d}$ is $\mathrm{t\_d}=\frac{B\times Z\times L\times n}{B\times L\times f}$ or simply $\mathrm{t\_d}=\frac{Z}{f\times n}$, assuming infiltration occurs uniformly across the bottom. This relationship shows that deeper trenches or media with low void ratios drain more slowly, while high soil permeability shortens the emptying period.

The table below lists representative void ratios for common aggregates and infiltration rates for various soil textures. These values can guide initial selections when site data are unavailable. Local codes may stipulate minimum drain‑down times, often 24 to 48 hours, to avoid prolonged saturation that could compromise soil structure or lead to mosquito breeding.

Consider an example: a 200 m² rooftop drains to a proposed trench. A design storm of 25 mm yields 5 m³ of runoff. With a width of 0.6 m, depth of 1 m, and void ratio of 0.4, each meter of trench stores 0.24 m³. Dividing the runoff by storage per meter gives a required length of about 20.8 m. If the soil infiltration rate is 10 mm/hr (0.01 m/hr), the drain‑down time is $1/0.01\times 0.4=250$ hours, or roughly ten days. Such a slow drain indicates either the soil is too tight or the trench should be shallower and wider to expose more bottom area. Adjusting the width to 1.2 m doubles bottom area and cuts the drain time in half.

Trench design must also account for pretreatment to remove sediment and prevent clogging. Typical installations include a vegetated swale, filter strip, or catch basin upstream. Geotextile fabric lining the trench can stop fines from entering, though it may itself clog over time. Regular maintenance and inspection are crucial to maintain performance. The calculator assumes clean aggregate and unobstructed infiltration; in practice, safety factors or additional length may be warranted to compensate for degradation.

Soil testing provides more accurate infiltration rates than generic texture tables. Double‑ring infiltrometer tests or percolation tests measure site‑specific permeability. If tests reveal rates below 2 mm/hr, infiltration trenches may be unsuitable, and alternative practices such as rain gardens or detention basins should be considered. Conversely, excessively high rates might raise concerns about groundwater contamination if runoff carries pollutants. Designers must balance hydrologic function with water quality protection.

Compared to subsurface pipes or surface basins, infiltration trenches are relatively unobtrusive and can be retrofit along existing development edges. They work best for small drainage areas (less than a few thousand square meters) and shallower flows. For larger catchments or where sediments are heavy, sediment forebays or other pre‑treatment devices are advisable. Many jurisdictions limit trench depth to maintain separation from groundwater or bedrock; always consult local regulations and call utility locate services before excavating.

Climate variations influence design depth and spacing. In cold regions, trenches must be placed below the frost line to remain functional during winter thaws. In arid climates, infrequent but intense storms may warrant deeper trenches to capture large events. Vegetation above the trench can enhance evapotranspiration and provide aesthetic benefits, but deep‑rooted trees should be avoided to prevent structural disruption.

In summary, the infiltration trench sizing calculator offers a first‑order estimate of length and drain time for stormwater management. By recognizing the relationships among runoff volume, trench geometry, media properties, and soil permeability, users can tailor trenches to specific sites. While simplified, the tool underscores critical design tradeoffs and highlights when additional analysis or professional consultation is necessary. Properly implemented, infiltration trenches restore a measure of natural hydrology, reducing peak flows, improving water quality, and replenishing groundwater in developed landscapes.