Visual Inspection for Biologically Enhanced Practices

A.J. Erickson, B.C. Asleson, J.S. Gulliver, R.M. Hozalski

Visual inspection (level 1) of biologically enhanced practices focuses on the vegetation (species, condition, abundance, etc.) and the condition of the soil. The species found in the practice and their condition and abundance can provide visual clues as to functionality. For example, abundant terrestrial vegetation in a rain garden indicates adequate soil moisture and quick drainage of stored runoff. Conversely, standing water and wetland vegetation (cattails, water lilies, etc.), or no vegetation at all in a bioretention practice shows that stormwater runoff does not infiltrate in the amount of time for which the practice was designed. It is therefore recommended that visual inspection and any associated maintenance be performed at least once per year. 

Bioretention practices (rain gardens)

According to the Minnesota Stormwater Manual, bioretention practices are required to drain within 48 hours. This requirement should be considered along with the time of the last rain event when conducting a visual inspection of the practice. When standing water is observed more than 48 hours after the last large rain event, further evaluation of the practice should be conducted to determine potential causes of failure.

Vegetation specified in the design of the bioretention practice should be documented in the original plant design plan with photographs or videos, if possible. After the vegetation has become established, it should cover most of the basin. Annual photo or video records of the vegetation can be used to keep track of changes in health and migration of the plant species with respect to time. Annual photo or video records can also be used to document effects of channelization, sedimentation, and erosion. Plant guides are available that identify vegetation appropriate for stormwater practices, such as “Plants for Stormwater Design: Species Selection for the Upper Midwest” (Shaw and Schmidt 2003). The vegetation present in the rain garden should be evaluated based on health, density, abundance, and location. Observations should also be made about the presence of weeds or invasive plants and wetland plant species, as these plants are undesirable for rain gardens.

If the bioretention practice does not successfully sustain the desired vegetation, the reason may be that the soils are retaining water for excessive periods of time, the vegetation is not receiving sufficient amounts of water, the soil has been compacted and is limiting root growth, or that the plants were killed by introduction of toxic substances (e.g., road salt, herbicides). When the evaluation of the vegetation indicates unsuccessful vegetation, the ideal conditions for each species should be examined and the soil properties (i.e., texture, compaction, sediment accumulation) should be investigated to determine the cause. If, after completion of the visual inspection, the potential cause(s) of failure are not determined, consideration of further assessment (i.e., level 2, 3, or 4) is recommended.

Adverse sediment accumulation is evidenced by “sandbars,” areas in which sediment deposition covers or chokes out established or developing vegetation. Sediment accumulation can reduce infiltration rates and stormwater storage capacity. The practice should also be examined for erosion occurring near inlets or along the side slopes, as this material will likely be deposited in the bioretention practice. An example of visual inspection applied to bioretention stormwater treatment practices is provided in Asleson et al. (2009). Checklists have been developed to simplify visual inspection for bioretention (rain gardens), which are available for download here:

Visual Inspection Checklist

Constructed wetlands

Vegetation in constructed wetlands should be well established and diverse unless the wetland has been operating for less than two growing seasons. Visual inspection of constructed wetlands should include the areas around the inlet and outlet structures to ensure that vegetation is not being damaged by large velocity flows or by clogging. If there is a lack of vegetation or if vegetation is “washed over” at the inlet, installing an energy dissipation device may allow vegetation to become established near the inlet, which will increase the treatment area within the wetland. Additionally, if solids are rapidly accumulating in the wetland, maintenance should be scheduled to remove the solids to maintain the desired stormwater residence time and corresponding pollutant removal efficiency. Alternatively, capacity testing for sedimentation (level 2) or synthetic runoff testing with a conservative tracer (level 3) assessment could be used to assess sediment accumulation in a constructed wetland. It is recommended that visual inspection and any associated maintenance be performed at least once per year. Checklists have been developed to simplify visual inspection for constructed wetlands, which are available for download here:

Visual Inspection Checklist

Filter strips and swales

Vegetation in filter strips and swales should be well established and consistent throughout the filter strip or swale. The flow will find the path of least resistance, which will likely occur at gaps in the vegetation, if present. Erosion and channelization are visual clues that the filter strip or swale is not functioning properly and requires maintenance or repair. Additionally, excess solids deposited in filter strips or swales can choke out vegetation, prevent new growth, and become resuspended during large storm events. If excess deposition occurs, pretreatment systems can be installed to remove sands before entering the filter strip/swale, or a periodic maintenance schedule could be established to remove excess solids. Checklists have been developed to simplify visual inspection for filter strips and swales, which are available for download here:

Visual Inspection Checklist

Once visual inspection is complete, the appropriate maintenance can be selected and scheduled. 

Continue to Scheduling Maintenance or to Capacity Testing.

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