THE FLOW PATTERN, SOLIDS SETTLING, AND CHARACTERISTICS OF A SASKATOON, SASKATCHEWAN STORMWATER MANAGEMENT POND

Abstract

The work described herein took place during 2014 (June 23 to September 30) to investigate the removal efficiency of solids, the settling of solids, and the hydraulics (flow pattern, velocity) within a conventional stormwater pond located in John Avant Park, Saskatoon, Saskatchewan, Canada. The stormwater pond in John Avant Park has a total contributing catchment area of 114.62 ha, from three inlets, one outlet, a man-made aeration stream, and an aeration fountain. Programmable ISCO 6700 series automated stormwater samplers equipped with ISCO 750 acoustic Doppler area velocity flow modules were installed in the inlet and outlet manholes to measure the water level and velocity. Stormwater samples were collected within 24 hours for concentration and particle size distribution analysis, following each of 14 rain events that occurred during the measurement period. A hand corer and a water depth to sediment surfaces measurement apparatus (telescopic pole with foot) were used to investigate spatial variances in the particle size distribution and accumulation of settled solids. A robotic total station and a three dimensional prism were used to track drogues to investigate pond hydraulics. For comparison, camera equipment was used and calibration points were surveyed to evaluate the movement of drogues using concepts from large scale particle tracking velocimetry (LSPTV) along with visual observations. In total, 82 mm of rainfall was recorded over the field season, resulting in an inflow to the pond of approximately 66,517 m3, as measured from the inlets. The total outflow from the pond was approximately 154,150 m3, where 86,943 m3 of the outflow was not accounted for as inflow measured in the inlets. It was found that the stormwater pond had a total solids removal efficiency of -67%, indicating a higher concentration of solids were leaving the pond than entering. Solids sampled from the pond’s outflow had a higher concentration of finer particles than the inflow, suggesting the pond was more effective at removing larger particle sizes. From the negative removal efficiency and the higher concentration of finer particles in the outflow, it was determined that resuspension of finer grained solids (clay, silt size) potentially occurs. From 51 sediment depth measurements taken throughout the pond it was estimated that the pond is accumulating approximately 1 cm of solids per year, based on when the pond went into service in the late 1980s. It was found that over the summer season, from June 24 to August 25, the spatial distribution of sediment thickness slightly differed. The thickness of core samples varied from approximately 10 cm to just over 30 cm, where the clay liner of the pond was captured in some cores and not others such that the cores themselves could not be used to indicate depth. Core samples were predominately comprised of sand, with coarser particles (sand, gravel) located closer to the inlets and the periphery. Finer particles (silt, clay) were more predominant further away from the inlets. Drogue tracking was performed on five different occasions throughout the field season to evaluate pond hydraulics (flow pattern, velocities) using surveying with a robotic total station, concepts from Large Scale Particle Tracking Velocimetry (LSPTV), and visual observations. The flow path and velocity of drogues were found to be predominately related to the wind speed and direction. Results from this field investigation suggest that short-circuiting could occur when the wind direction is from the inlets to the outlet during rain events or baseflow conditions as drogues were seen to travel in this direction under those conditions.