Investigation of Coarse Organic Matter and Gross Solids Loading in Minnesota Urban Stormwater

Abstract

This study collected the gross solids, or largest particles in stormwater, using several different methods then characterized the nutrient content, settling velocity, and particle sizes of the material collected. This work can be summarized in four main components: Full Storm Capture; Settling Velocity; Annual Loadings; and P8 Modeling. The Full Storm Capture investigated different methods of storm runoff collection to better understand how collection methods may bias the amounts of the gross solids collected. The most reliable method was found to be with the use of 55-gallon barrels with modified lids that can be plumbed together and attached to a watershed outflow location. The first barrel in the series was fitted with an internal filter bag to capture most solids with additional barrels to collect flow. The last barrel lid includes a discharge weir that is monitored in cases where the storm event exceeds the barrel capacity. Use of other methods, such as automated samplers, sumps, regional collection facilities, all created some bias in the solids collected. The full storm capture method results in as much as 11 times more solids mass than other techniques. Settling Velocity investigated how large organic matter with internal air voids will saturate and settle in flow. Saturation of the voids occurred in 3 to 5 days resulting in specific gravity values just over 1.0 allowing particles to settle. The settling rates were measured and new coefficients were determined to model settling using the Ferguson and Church settling equation. The Annual Loading investigated how the gross solids are generated in urban watersheds and relates the accumulation to the watershed parameter of canopy area over pavement. The amount of phosphorous in the material was also related to the organic content. The organic content of the gross solids varied seasonally but did show strong relationship with the Normalized Difference Vegetation Index (NDVI) or “greenness” of the satellite imagery. The P8 Modeling effort created new particle files for the P8 software using the accumulation rates and settling velocities determined in the prior sections. This allows a P8 model to estimate the gross solids generated in an urban watershed and model removal with control measures such as ponds, sumps, or biofiltration systems. The P8 Model does not have a mechanism to calculate biomass generation, which limits the accuracy of the prediction of gross solids from vegetation.