Infiltration-based best management practices, or BMPs, are designed to capture, store, and gradually infiltrate stormwater into the ground. These systems, which include bioretention basins, infiltration trenches, dry wells, and porous pavement systems, are particularly vulnerable during and immediately after major storm events. When rainfall exceeds design capacity or introduces large volumes of sediment and debris, these systems can become compromised in ways that are not always obvious at first glance.
The first 24 hours after a significant storm event provide a critical window for inspection. Conditions are still fresh, problems are easier to identify, and corrective actions can often be taken before minor issues become long-term failures. A structured inspection approach helps ensure that nothing is overlooked.
The first step is a general walk-through of the BMP and its surrounding drainage area. This is not yet a detailed inspection, but rather a high-level assessment of how the system performed during the storm. Look for signs of overtopping, bypassing, or unexpected flow paths. Water that has carved new channels or flowed around the BMP instead of into it is a strong indicator that something is wrong upstream or that the system capacity was exceeded.
Pay close attention to adjacent slopes and contributing areas. Erosion, sediment plumes, or displaced mulch often point to upstream instability that will continue to impact the BMP if not addressed. Any obvious structural damage, such as displaced stones, cracked concrete, or shifted piping, should be noted immediately.
One of the most important performance indicators for any infiltration BMP is how quickly it drains. Most systems are designed to fully dewater within 24 to 72 hours. During the first day after a storm, inspectors should document the extent and depth of any standing water.
If significant ponding remains after 24 hours, it may indicate clogging from fine sediments, compaction of the soil media, or a high groundwater condition. Even if the system eventually drains, slow drawdown reduces capacity for subsequent storms and can lead to vegetation stress or mosquito concerns. Measuring and recording water depth at consistent reference points helps establish trends over time.
Inlets and pretreatment components take the brunt of stormwater flows, and they are often where the most visible problems occur. Check curb cuts, catch basin inlets, forebays, and sediment traps for accumulated debris and sediment.
Sediment that has bypassed pretreatment and entered the main infiltration area is a red flag. It suggests that maintenance intervals are too long or that the pretreatment design is inadequate. Remove accumulated materials as soon as possible to prevent them from being transported deeper into the system where removal becomes much more difficult.
The surface of an infiltration BMP tells a detailed story about its condition. Inspect for crusting, which can form when fine sediments seal the soil surface and dramatically reduce infiltration rates. Areas of bare soil, mulch washout, or sediment deposition should be clearly documented.
In systems with engineered soil media, check for signs of compaction or uneven settling. Infiltration trenches and dry wells should be examined for sediment accumulation within stone void spaces, which reduces storage capacity. If the system includes geotextiles, note any signs of exposure or failure.
Healthy vegetation is not just aesthetic, it plays a critical role in infiltration, pollutant removal, and soil stability. After a major storm, look for uprooted plants, buried vegetation, or areas where sediment has smothered plantings.
Extended ponding can stress or kill vegetation not suited to saturated conditions. Conversely, erosion may expose root systems and reduce plant stability. Replanting, soil amendment, or species selection adjustments may be necessary if recurring issues are observed.
If the BMP includes an underdrain system, inspect observation wells and outlet structures for flow. A continuously flowing underdrain long after the storm may indicate that the system is not infiltrating as intended, while a lack of flow in a saturated system could suggest clogging.
Overflow structures should be checked for debris blockage and signs that they were activated during the storm. Frequent overflow activation may indicate undersizing or reduced infiltration capacity.
The final step in the 24-hour window is thorough documentation. Photographs, water depth measurements, sediment estimates, and notes on observed damage all contribute to a valuable maintenance record. Over time, these records help identify patterns, such as recurring sediment loading or seasonal groundwater influences.
Any deficiencies identified during the inspection should be prioritized based on their potential impact on system performance. Immediate maintenance, such as debris removal or minor repairs, should be completed quickly, while larger issues may require scheduling more extensive rehabilitation.
Infiltration BMPs are highly effective when they function as designed, but they are also sensitive systems that depend on proper maintenance and timely inspection. A disciplined 24-hour post-storm checklist not only preserves performance but also extends the life of the infrastructure. For municipalities managing large inventories of stormwater assets, this kind of proactive approach is essential to maintaining compliance, protecting water quality, and avoiding costly failures.