Two of the most common and simple stormwater control measures are swales and filter strips. However, an overly simplistic one-size-fits-all design approach typically is used for these practices. To provide more flexibility in design, a coupled hydraulics and particle-settling model was created to predict swale and filter strip total suspended solids (TSS) reduction as a function of catchment area, longitudinal slope, side slope, cross section type (triangular swale, trapezoidal swale, or filter strip), and length. The hydraulics and hydrology models were based on Manning’s equation and the rational method, respectively, with the underlying requirement that the water quality design storm does not exceed the height of the grass. The particle-settling model was underpinned by the Aberdeen equation. The model predicts that triangular swales produce the least and filter strips the most TSS removal because of increased hydraulic retention time; trapezoidal swales had on average 10% greater TSS removal than triangular swales with similar design characteristics. Grass filter stormwater controls performed better with decreasing slope, increasing length, smaller catchment area, and shallower side slopes. Filter strips registered the majority of their TSS reduction within the first meter of their width and were insensitive to increases in longitudinal slope. This water quality design storm-abating modeling approach could be coupled with modeling for infrequent-return-interval storms to obtain dual benefits of sediment reduction and conveyance. This water quality model may provide regulatory agencies a tool to provide variable TSS credit to swales and filter strips as a function of design.

Two stormwater control measures (SCMs) installed in series were monitored for their individual impact on the hydrology and water quality of stormwater runoff from a 0.08-hectare watershed in Fayetteville, North Carolina, for 22 months. Runoff was first treated by permeable interlocking concrete pavement (PICP), the underdrain of which discharged into a proprietary box filter (Filterra^® biofiltration) which combined high-flow-engineered media with modest biological treatment from a planted tree. Due to a deteriorating contributing drainage area and high ratio of impervious area to permeable pavement area (2.6:1), clogging of the permeable pavement surface caused an estimated 38% of stormwater to bypass as surface runoff. Fifty-six percent of runoff volume infiltrated underlying soils, and the remaining 6% exited the Filterra^® as treated effluent; the hydrologic benefit of the Filterra^® was minimal, as expected. Primary treatment through the PICP significantly reduced event mean concentrations (EMCs) of total suspended solids (TSS), total phosphorus (TP), total nitrogen (TN), and total Kjeldahl nitrogen (TKN) but contributed to a significant increase in nitrate/nitrite (NO_2,3–N) concentrations. Secondary treatment by the Filterra^® further reduced TSS and TP concentrations and supplemented nitrogen removal such that treatment provided by the overall system was as follows: TSS (removal efficiency (RE): 96%), TP (RE: 75%), TN (RE: 42%), and TKN (RE: 51%). EMCs remained unchanged for NO_2,3–N. Despite EMC reductions, additional load reduction due to the Filterra^® was modest (less than 2%). This was because (1) a majority of pollutant load was removed via PICP exfiltration losses, and (2) nearly all of the export load was from untreated surface runoff, which bypassed the Filterra^®, and therefore the manufactured device never had the opportunity to treat it. Cumulative load reductions (based only upon events with samples collected at each sampling location) were 69%, 60%, and 41% for TSS, TP, and TN, respectively. When surface runoff was excluded, load reductions increased to over 96%; lower run-on ratios (which would reduce clogging rate) and/or increased maintenance frequency might have improved pollutant load removal.

Manufactured treatment devices (MTDs) are increasingly installed to treat pollutants from stormwater in urban areas, but few peer-reviewed studies have assessed their field-scale performance. The Filterra biofiltration unit is a box filter combining high-flow engineered media with potential for biological treatment from a planted tree. A Filterra device was monitored for its impact on hydrology and water quality of runoff from a 0.10-ha impervious watershed in Fayetteville, North Carolina, for 22 months. Undersizing of the system and clogging of the media bed likely caused 22% of runoff to bypass the system. Although peak flows were significantly reduced ($p<0.0001$ ) by a median 57% (likely owing to parking lot detention), postdevelopment peak flows were not consistently reduced to predevelopment flows. The Filterra provided stormwater treatment by significantly reducing event mean concentrations (EMCs) and total pollutant loadings for sediment (total suspended solids and suspended sediment concentration), nutrients (total phosphorus, total nitrogen, total ammoniacal nitrogen, and total Kjeldahl nitrogen), and zinc. Median EMC removal efficiency ranged from 35 to 97% and median loading efficiency was 39–80% for these pollutants. The device did not reduce nitrate/nitrite, total dissolved phosphorus, or copper. Nutrient concentrations of stormwater discharged from the Filterra generally met “good” water-quality thresholds established for the state of North Carolina and EMC targets established by the state of Washington for emergent MTDs. Clogging susceptibility and frequent maintenance of the Filterra unit were notable. Overall, results suggest the Filterra is an effective MTD for treating stormwater from small impervious watersheds.

Optimizing stormwater capture for urban green infrastructure is contingent on the placement and performance of inlets. It is crucial to thoroughl |