The most frequently cited maintenance concern for stormwater step pools is surface soil/media and underdrain clogging caused by organic matter, fine silts, hydrocarbons, and algal matter. Common operational problems include
Implicit in the design guidance is the fact that many design elements of infiltration and filtration systems can minimize the maintenance burden and maintain pollutant removal efficiency. Key examples include
Proper construction methods and sequencing play a significant role in reducing problems with operation and maintenance (O&M). In particular, with construction of filtration and infiltration practices the most important action for preventing operation and maintenance difficulties is to ensure that the contributing drainage area has been fully stabilized prior to bringing the practice on line (this is a REQUIRED practice).
Inspections during construction are needed to ensure that the filtration or infiltration practice is built in accordance with the approved design standards and specifications. Detailed inspection checklists should be used that include sign-offs by qualified individuals at critical stages of construction, to ensure that the contractor’s interpretation of the plan is acceptable to the professional designer. An example construction phase inspection checklist is provided below.
|Construction Sequence||Satisfactory / Unsatisfactory||Comments|
|Runoff diverted (Note type of bypass)|
|Facility area cleared|
|Soil tested for permeability|
|Soil tested for phosphorus content (include test method)|
|Verify site was not overdug|
|Project benchmark near site|
|Facility location staked out|
|Temporary erosion and sediment protection properly installed|
|Lateral slopes completely level|
|Soils not compacted during excavation|
|Longitudinal slopes within design range|
|Stockpile location not adjacent to excavation area and stabilized with vegetation and/ or silt fence|
|Verify stockpile is not causing compaction and that it is not eroding|
|Was underlying soil ripped or loosened|
|Size, location, and inverts per plans|
|Side slopes stable|
|Groundwater / bedrock verified|
|3. Check dams|
|Dimensions per plans|
|Spacing and grade installed per plans|
|Materials per specifications|
|4. Structural Components|
|Outlets installed pre plans|
|Underdrain installed to grade|
|Pretreatment devices installed per plans|
|Soil bed composition and texture conforms to specifications|
|Inlets installed per plans|
|Underdrain installed per plans|
|For native stormwater step pools, plants and materials ordered 6 months prior to construction|
|For native stormwater step pools, construction planned to allow for adequate planting and establishment of plant community|
|Complies with planting specs|
|Topsoil complies with specs in composition and placement|
|Soil properly stabilized for permanent erosion control|
|6. Final Inspection|
|Dimensions per plans|
|Check dams operational|
|Soil/filter bed permeability verified|
|Effective stand of vegetation stabilized|
|Construction generated sediments removed|
|Contributing watershed stabilized before flow is diverted to the practice|
|Actions to be taken:|
Proper maintenance is critical to the successful operation of a filtration/infiltration practice. Without regular maintenance, the soil or media of filtration and infiltration systems can become clogged, losing its ability to conduct and infiltrate water at the designed rate. This can lead to stagnant water, mosquito breeding habitat, and reduction or elimination of pollutant removal capacity.
A maintenance plan clarifying maintenance responsibilities is REQUIRED. Effective long-term operation of filtration/infiltration practices necessitates a dedicated and routine maintenance schedule with clear guidelines and schedules. Proper maintenance will not only increase the expected lifespan of the facility but will improve aesthetics and property value.
Some important post-construction considerations are provided below along with RECOMMENDED maintenance standards.
|Maintenance Item||Satisfactory / Unsatisfactory||Comments|
|1. Debris Cleanout (Monthly)|
|Contributing areas clean of litter and vegetative debris|
|Filtration or infiltration facility clean|
|Inlets and outlets clear|
|2. Check Dams or Energy Dissipators (Annual and After Major Storms)|
|No evidence of flow going around structures|
|No evidence of erosion at downstream toe|
|3. Vegetation (Monthly)|
|Vegetation maintenance complies with O&M plan|
|Vegetation meets performance standards (including control of specified invasive species)|
|Minimum mowing depth not exceeded|
|No evidence of erosion|
|4. Dewatering (monthly)|
|Dewaters between storms within 48 hours|
|5. Sediment Deposition (Annual)|
|Area clean of sediment|
|Contributing drainage area stabilized and free of erosion|
|Winter accumulation of sand removed each spring|
|6. Outlet/Overflow Spillway (Annual, After Major Storms)|
|Good condition, no need for repair|
|No evidence of erosion|
|No evidence of any blockages|
|No evidence of structural deterioration|
|7. Other (Monthly)|
|Encroachment on easement area (if applicable)|
|Complaints from residents (if applicable)|
|Any public hazards (specify)|
|Actions to be taken:|
The list below highlights the assumed maintenance regime for a stormwater step pool.
All estimated hours listed below would be to perform maintenance on a stormwater step pool system approximately 1,000 square feet in size that has adequate pretreatment and where seed and/or live plants have been installed appropriately. Travel time is not included.
Regular inspection of not only the BMP but also the immediate surrounding catchment area is necessary to ensure a long lifespan of the water quality improvement feature. Erosion should be identified as soon as possible to avoid the contribution of significant sediment to the BMP.
Pretreatment devices need to be maintained for long-term functionality of the entire BMP. Accumulated sediment in filter strips, rock diaphragms, water quality sump catch basins, or any pretreatment features will need to be inspected yearly. Timing of cleaning of these features is dependent on their design and sediment storage capabilities. In watersheds with erosion or high sediment loadings, the frequency of clean out will likely be increased. A vacuum truck is typically used for sediment removal. It is possible that any sediment removed from pretreatment devices or from the bottom of a step pool system may contain high levels of pollutants. All sediments, similar to those retrieved from a stormwater pond during dredging, may be subjected to the MPCA’s guidance for reuse and disposal.
If a grassed filter strip is used as pretreatment, they should be mowed as frequently as a typical lawn. Native vegetated filter strips can be maintained less frequently, such as once per year (e.g., mow and remove cut material or prescribed burn). Depending on the contributing watershed, grassed BMPs may also need to be swept before mowing. All grassed BMPs should be swept annually with a stiff bristle broom or equal to remove thatch and winter sand. The University of Minnesota’s Sustainable Urban Landscape Series website provides guidance for turf maintenance, including mowing heights.
Sediment loading can potentially lead to a drop in infiltration or filtration rates. It is recommended that infiltration performance evaluations follow the four level assessment systems in Stormwater Treatment: Assessment and Maintenance (Gulliver et al., 2010). See the section on assessment for more information.
Plant selection during the design process is essential to limit the amount of maintenance required. It is also critical to identify who will be maintaining the BMP in perpetuity and to design the plantings or seedings accordingly. The decision to install containerized plants or to seed will dictate the appearance of the BMP for years to come. If the BMP is designed to be seeded with an appropriate native plant based seed mix, it is essential the owner have trained staff or the ability to hire specialized management professionals. Seedings can provide plant diversity and dense coverage that helps maintain drawdown rates, but landscape management professionals that have not been trained to identify and appropriately manage weeds within the seeding may inadvertently allow the BMP to become infested and the designed plant diversity be lost. The following are minimum requirements for seed establishment and plant coverage.
For proper nutrient control, stormwater step pools must not be fertilized unless a soil test from a certified lab indicates nutrient deficiency. If this is the case, apply the minimum rate of appropriate nutrients to provide a suitable environment for vegetation establishment while also minimizing the mobilization (and loss) of nutrients to downstream receiving waters. Irrigation may be needed during establishment, depending on soils, precipitation, and if stormwater flows are kept off-line during establishment.
Weeding is especially important during the plant establishment period, when vegetation cover is not 100 percent yet. Some weeding will always be needed. It is also important to budget for some plant replacement (at least 5 to 10 percent of the original plantings or seedings) during the first few years in case some of the plants or seed that were originally installed don’t become vigorous. It is highly recommended that the install contractor be responsible for a plant warranty period.
Typically, plant warranty periods can be 60 days or up to one year from preliminary acceptance through final inspections. If budget allows, installing larger plants (#1 container vs. 4” pot) during construction can decrease replacement rates if properly cared for during the establishment period.
Weeding in years after initial establishment should be targeted and thorough. Total eradication of aggressive weeds at each maintenance visit will ultimately reduce the overall effort required to keep the BMP weed free. Mulch is generally not recommended for use in stormwater step pools since flowing water typically washes it downstream; however, mulch may be appropriate in planting beds or around individual trees on upper sideslopes and adjacent areas.
Rubbish and trash removal will likely be needed more frequently than in the adjacent landscape. Trash removal is important for prevention of mosquitoes and for the overall appearance of the BMP.
The service life of stormwater step pools depends upon the pollutant of concern.
Infiltration rates in stormwater step pools have not been studied as closely as many other BMPs (e.g., infiltration basins and bioretention systems). However, it is known that plant roots are essential in macropore formation, which helps to maintain the infiltration rate. If proper pretreatment is present, service life for infiltration should be unlimited. However, if construction site runoff (or another source of fines) is not kept from entering the stormwater step pool, clogging will occur, limiting or eliminating the infiltration function of the system, thus requiring restorative maintenance or repair (Brown and Hunt, 2010).
Nitrogen removal is not a primary function of stormwater step pools.
Phosphorus removal in stormwater step pools is achieved primarily through infiltration and sorption of phosphorus to trapped sediments. Sediment bound phosphorus is removed through sedimentation, while removal of soluble phosphorus depends on the type of soil/media used. If the soil/media is already saturated with P (i.e., its P binding sites are full), it will not be able to retain additional dissolved P and the P in stormwater will tend to leach from the soil/media as it passes through the biofilter (Hunt et al., 2006). It is highly recommended that the P-index of the media at installation be below 30, which equates to less than 36 milligrams per kilogram P, to ensure P removal capacity. Laboratory research has suggested an oxalate extractable P concentration of 20 to 40 milligrams per liter will provide consistent removal of P (O’Neill and Davis, 2012). For information on phosphorus leaching from bioretention media, link here.
Metals are typically retained in infiltration systems (including stormwater step pools) through sedimentation and adsorption processes. Since there are a finite amount of sorption sites for metals in a particular soil/media, there will be a finite service life for the removal of dissolved metals. Morgan et al. (2011) investigated cadmium, copper, and zinc removal and retention with batch and column experiments. Using synthetic stormwater at typical stormwater concentrations, they found that 6 inches of filter media composed of 30 percent compost and 70 percent sand will last 95 years until breakthrough (i.e., when the effluent concentration is 10 percent of the influent concentration). They also found that increasing compost from 0 percent to 10 percent more than doubles the expected lifespan for 10 percent breakthrough in 6 inches of filter media for retainage of cadmium and zinc. Using accelerated dosing laboratory experiments, Hatt et al. (2011) found that breakthrough of Zn was observed after 2000 pore volumes, but did not observe breakthrough for Cd, Cu, and Pb after 15 years of synthetic stormwater passed through the media. However, concentrations of Cd, Cu, and Pb on soil/media particles exceeded human and/or ecological health levels, which could have an impact on disposal if the media needed replacement. Since the majority of metals retainage occurs in the upper 2 to 4 inches of the soil/media (Li and Davis, 2008), long-term metals capture may only require rejuvenation of the upper portion of the media.
Accumulation of polycyclic aromatic hydrocarbons (PAHs) in sediments has been found to be so high in some stormwater retention ponds that disposal costs for the dredging spoils were prohibitively high. Research has shown that rain gardens, on the other hand, are “a viable solution for sustainable petroleum hydrocarbon removal from stormwater, and that vegetation can enhance overall performance and stimulate biodegradation.” (Lefevre et al., 2012). Stormwater step pools provide some of the same functions as rain gardens, and therefore would be expected to provide some PAH management. However, stormwater step pools performance in PAH management has not been the focus of any identified studies.
The following table summarizes common maintenance concerns, suggested actions, and recommended maintenance schedule.
Typical maintenance problems and activities for stormwater step pools
Link to this table
|Inspection Focus||Common Maintenance Problems||Maintenance Activity||Recommended Maintenance Schedule|
|Drainage Area and Drawdown Time||Clogging, sediment deposition||Ensure that contributing catchment areas to practice and inlets are clear of debris||Monthly|
|Erosion of catchment area contributing significant amount of sediment||In case of severely reduced drawdown time, scrape bottom of basin and remove sediment. Disc or otherwise aerate/scarify basin bottom. De-thatch if basin bottom is turf grass. Restore original design cross section or revise section to increase infiltration rate and restore with vegetation as necessary.||Upon identification of drawdown times longer than 48 hours or upon complete failure|
|Site Erosion||Scouring at inlets||Correct earthwork to promote non‐erosive flows that are evenly distributed||As necessary|
|Unexpected flow paths into practice||Correct earthwork to eliminate unexpected drainage or created additional stable inlets as necessary||As necessary|
|Vegetation||Reduced drawdown time damaging plants||Correct drainage issues as described above||Replace with appropriate plants after correction of drainage issues|
|Severe weed establishment||Limit the ability for noxious weed establishment by properly mowing, mulching or timely herbicide or hand weeding. Refer to the MDA Noxious Weed List||Bi‐monthly April through October|
|Vegetative cover||Add seed/plants to maintain ≥95% vegetative cover.||Bi‐monthly April through October|
|Pretreatment||Pretreatment screens or sumps reach capacity||Remove sediment and oil/grease from pretreatment devices/structures||Minimum yearly or as per manufacturer's recommendations|
|Vegetative filter strip failure||Reduce height of vegetative filter strip that may be limiting in‐flow. Re‐establish vegetation to prevent erosion. Leave practice off‐line until full reestablishment.||Mow grass filter strips monthly. Restore as necessary|
A Maintenance Agreement is a legally binding agreement between two parties, and is defined as ”a nonpossessory right to use and/or enter onto the real property of another without possessing it.“ Maintenance Agreements are often required for the issuance of a permit for construction of a stormwater management feature and are written and approved by legal counsel. Maintenance Agreements are often similar to Construction Easements. A Maintenance Agreement is required for one party to define and enforce maintenance by another party. The Agreement also defines site access and maintenance of any features or infrastructure if the property owner fails to perform the required maintenance.
Maintenance Agreements are commonly established for a defined period such as five years for a residential site or 10 to 20 years for a commercial/governmental site after construction of the infiltration/filtration practice. Maintenance agreements often define the types of inspection and maintenance that would be required for that infiltration/filtration practice and what the timing and duration of the inspections and maintenance may be. Essential inspection and maintenance activities include but are not limited to drawdown time, sediment removal, erosion monitoring and correction, and vegetative maintenance and weeding. If maintenance is required to be performed due to failure of the site owner to properly maintain the infiltration/filtration practices, payment or reimbursement terms of the maintenance work are defined in the Agreement. Below is an example list of maintenance standards from an actual Maintenance Agreement.
In some project areas, a drainage easement may be required. Having an easement provides a mechanism for enforcement of maintenance agreements to help ensure stormwater step pools are maintained and functioning. Drainage easements also require that the land use not be altered in the future. Drainage easements exist in perpetuity and are required property deed amendment to be passed down to all future property owners.
As defined by the Maintenance Agreement, the landowner should agree to provide notification immediately upon any change of the legal status or ownership of the property. Copies of all duly executed property transfer documents should be submitted as soon as a property transfer is made final.
To link to the maintenance inspection report, click here. The contents of the inspection form are provided below. For another source of information on visual indicators, see Chesapeake Stormwater visual indicators form.
Maintenance Inspection Report for Dry Swale with Check Dams and Stormwater Step Pool. Can be used for wet swales with exceptions, as noted in footnotes.
Inspector Name/Address/Phone Number: _______________________________________
Site Address: ______________________________________________________________
Owner Name/Address/Phone Number: _________________________________________
Drainage Area Stabilization (Inspect after large storms for first two years, Inspect yearly in spring or after large storms after first two years)
Inlets & Pretreatment Structures (Inspect in Spring and Fall)
Swale (Inspect after large storms for first two years, Inspect yearly in spring or after large storms after first two years)
Outlet/Emergency Overflow (Inspect in Spring and Fall)
1For wet swale, check condition of inundated area
2For wet swale with check dam, drawdown applies to the water elevation at the botton of weir
3Not applicable for wet swale
Stormwater step pools are currently not included as a BMP in the MIDS calculator. The swale main channel BMP can be used, but the maximum allowable slope is 4 percent. To dtermine volume retention for slopes greater than 4 percent, you will need to develop a relationship between the slope and volume retained. To do this, determine volume retention at 0.5 percent slope increments for your site at slopes ranging from 0.5 to 4 percent. Determine the appropriate regression for volume retention and slope and calculate the volume retained at the slope for your site. The relationship is not linear. Links to MIDS calculator information are provided below.