J.L. Nieber, A.J. Erickson, P.T. Weiss, J.S. Gulliver, R.M. Hozalski
Visual inspection (level 1) of an infiltration practice involves investigation of the practice for indications of inadequate infiltration capacity. The scope depends on the specific inspection objectives and the type of stormwater treatment practice involved. Visual inspection of infiltration practices should be conducted at least annually.
Visual inspection is useful for identifying obvious problems with an infiltration basin. Visual indicators that the basin may be failing include: standing water for hours after a storm event, the presence of a visible layer of fine material (i.e., mud) on the surface of the basin, and the presence of wetland vegetation. Standing water in an infiltration basin 48 hours after a design storm event indicates that stormwater runoff is not infiltrating at a rate recommended by design (Minnesota Stormwater Manual). A layer of fine material on the surface of the basin is an indication that stormwater was held for an extended period of time, such that fine material was allowed to settle out of the stormwater. The presence of wetland vegetation suggests that the stored stormwater drains so poorly that wetland vegetation can develop.
Infiltration trenches typically do not promote the growth of vegetation over the trench itself. One of the main indications of poor infiltration capacity for a trench is a crust or layer of fine sediment that lies over the surface of the trench, indicating that the water was standing for a substantial time period. If a crust is present, even if it shows signs of desiccation cracking, the crust could easily become a barrier to infiltration upon rewetting.
While it might not be obvious during visual inspection, the pores of the trench material could be clogged below the surface of the trench, even if the surface is clear of sediments. Closer examination by poking just beneath the surface of the trench with a trowel or shovel might reveal this clogging.
Although no vegetation will grow in the trench material, an indication of poor infiltration in the trench could be poor vegetative growth in the area surrounding the trench. If infiltration is small through the trench, water will pond around the trench for a relatively long period of time, increasing the chances that resident vegetation will suffer.
Checklists have been developed to simplify visual inspection for infiltration basins and trenches, which are available for download here:
The exact procedure for application of visual inspection of permeable pavements depends on the type of permeable pavement. If the pavement is a vegetated permeable pavement, then the approaches used for other vegetated stormwater treatment practices also apply to the permeable pavement. Some indicators of inadequate infiltration capacity include dead or unhealthy vegetation during the growing season, or standing water or saturated surface soil for hours following a significant runoff event.
More involved observations include examining the soil profile for signs of persistent wet conditions in the surface soil or shallow subsurface soil. Such wet conditions indicate poor drainage conditions, which mean that infiltration capacities are lower than designed. Signs of persistent wet conditions in the soil are discoloration of the soil to a grayish tone and soil mottling. Mottling is an indication of anaerobic conditions resulting from persistent saturated or very wet conditions.
For asphalt or concrete pavements, indicators of poor infiltration performance are persistent standing water on the pavements following rainfall or evidence of sediment deposition on the surface.
Checklists have been developed to simplify visual inspection for permeable pavements, which are available for download here:
Once visual inspection is complete, the appropriate maintenance can be selected and scheduled.