stormwatercenter.net · printed Jun 12, 2026
Infiltration Trench Design Example
This worked example sizes an infiltration trench for a 3-acre community-center site that must provide groundwater recharge and treat the full water quality volume. It follows the same sequence a reviewer expects in a stormwater submittal: feasibility screening, volume computation, trench geometry, flow diversion and pretreatment. The companion infiltration trench fact sheet covers applicability and maintenance; the Simple Method calculator produces the loading estimates used in planning.
Design objectives
- Provide the full recharge volume (REv)
- Treat the water quality volume (WQv)
- Bypass larger storms safely around the trench
Step 1 — Screen infiltration feasibility
Infiltration lives or dies on the site investigation. Before any sizing, confirm each criterion with soil borings and infiltration tests taken at the location and depth of the proposed practice:
| Criterion | Typical requirement | This site |
|---|---|---|
| Soil infiltration rate | ≥ 0.5 in/hr measured | 1.0 in/hr — pass |
| Soil texture | No silt/clay lenses in trench zone | Loamy sand — pass |
| Water table / bedrock separation | ≥ 3–4 ft below trench bottom | Pass, limits depth to 5 ft |
| Contributing drainage area | ≤ 5 ac per trench | 3.0 ac — pass |
| Hotspot runoff | Not permitted | None — pass |
| Trench width | ≤ 25 ft (maintenance access) | Governs layout |
Step 2 — Water quality volume
WQv for the site was computed from the impervious cover and local rainfall depth as 6,752 cubic feet. The derivation is shown in the sizing criteria guide, and the pond design calculator performs the same computation interactively.
Step 3 — Recharge volume
REv was computed as 1,688 cubic feet. Because an open-bottom trench infiltrates the entire WQv — which is larger than REv — the recharge requirement is met automatically. This is one of the structural advantages of infiltration practices: recharge is a by-product of treatment rather than a separate facility.
Step 4 — Water quality peak discharge
The peak discharge associated with the water quality storm (qp) is needed to size the diversion structures. Using the modified curve-number method for a 1-inch storm on this site, qp = 2.6 cfs. The same computation appears in Step 4 of the sand filter design example, where it is worked in full.
Step 5 — Size the trench
Trench surface area follows the storage equation used in the Maryland standards for infiltration practices:
Step 6 — Size the flow diversions
Each trench entrance gets a diversion that admits the water quality flow and bypasses larger storms. With two entrances, two diversions are required.
| Structure | Design flow | Equation | Result |
|---|---|---|---|
| Point A low-flow orifice | 0.9 cfs @ 1.5 ft head | Q = C·A·√(2gh), C = 0.6 | d = 5.3 in → use 6 in |
| Point A 10-yr overflow weir | 4.7 cfs @ 1.0 ft head | Q = C·L·H1.5, C = 3.1 | L = 1.2 ft → use 1.5 ft |
| Point B low-flow orifice | 1.7 cfs @ 1.5 ft head | same | d = 7.3 in → use 8 in |
| Point B 10-yr overflow weir | 9.3 cfs @ 1.0 ft head | same | L = 2.4 ft → use 2.5 ft |
Step 7 — Size pretreatment
Infiltration practices clog without serious pretreatment. The target here is treatment of at least 25% of WQv — 1,688 cubic feet — before water reaches the stone reservoir, provided by three components in series:
- Pea gravel filter layer: a 2-inch layer over the full trench footprint stores (0.32)(2/12)(4,000) ≈ 213 cf within its voids.
- Plunge pools: a 5×10 ft pool at Point A and a 10×10 ft pool at Point B, average depth 2 ft, together ≈ 300 cf.
- Grass channel: the remainder (1,688 − 213 − 300 ≈ 1,175 cf) is provided as treatment in the grass channels leading to each entrance, sized with the channel geometry from the dry swale calculator.
Trenches fail from sediment, not from hydraulics. An observation well (a capped perforated standpipe to the trench bottom) costs almost nothing at construction and is the only honest way to confirm drawdown years later.
Design summary
| Element | Value |
|---|---|
| Water quality volume | 6,752 cf |
| Recharge volume | 1,688 cf (met within WQv) |
| Trench footprint | ≈ 3,800 sf (L-shape, ≤ 25 ft wide, 5 ft deep) |
| Stone reservoir porosity | 0.32 |
| Pretreatment volume | ≥ 1,688 cf (pea gravel + plunge pools + grass channel) |
| Low-flow orifices | 6 in / 8 in |
| Overflow weirs | 1.5 ft / 2.5 ft |
Frequently asked questions
What soil infiltration rate is required for an infiltration trench?
Most manuals require a measured rate of at least 0.5 in/hr at the depth of the proposed trench bottom. Rates above about 8 in/hr can also be restricted because very rapid drainage provides little treatment before runoff reaches groundwater.
How deep can an infiltration trench be?
Depth is governed by the separation requirement to the seasonally high water table or bedrock — usually 3 to 4 feet below the trench bottom. In this example that limited the trench to 5 feet.
Why does the trench meet the recharge requirement automatically?
An open-bottom trench infiltrates everything it captures. Since the captured water quality volume exceeds the computed recharge volume, the recharge requirement is satisfied without a separate facility.
What porosity should be used for the stone reservoir?
Clean, uniformly graded coarse aggregate is typically assigned a porosity of 0.32 to 0.40. The example uses 0.32, the conservative end.
How much pretreatment does an infiltration trench need?
A common target is at least 25% of the water quality volume treated upstream of the stone reservoir, combined from components such as plunge pools, grass channels and a pea gravel surface layer.
How is the trench protected during construction?
Keep the trench offline until the contributing area is fully stabilized. A trench that receives construction sediment is usually clogged before it ever operates.
What maintenance does an infiltration trench require?
Inspect the observation well after major storms in the first months and at least annually after that. If standing water persists beyond the design drawdown time, the surface layer needs to be stripped and replaced.
Can infiltration trenches serve stormwater hotspots?
No. Sites with high pollutant potential — fueling areas, vehicle maintenance, outdoor loading — must not infiltrate untreated runoff because of the groundwater contamination risk.