Performance Evaluations of Sediment Barrier Practices and Unmanned Aircraft System Mapping Technologies

Abstract

Sediment barriers are sediment control practices typically installed along the perimeter of construction sites that are intended to intercept, capture, and temporarily contain sheet to shallow concentrated flows prior to site discharge. In doing such, transported sediment is retained within the limits of the site, thus preventing polluted stormwater runoff from adversely affecting the surrounding environment. This dissertation explores improvements made in the design, selection, and application of sediment control technologies through full-scale performance testing of sediment barrier practices (e.g., silt fence, manufactured silt fence systems, sediment retention barriers, and manufactured sediment barrier products) and small-scale performance testing of geotextile fabrics (e.g., nonwoven and woven) used as the filtering component of silt fence installations. Full-scale performance evaluations of various nonwoven, wire-backed silt fence installation designs suggested that an offset 24 in. (61.0 cm) fence with 1.25 lb./ft (1.9 kg/m) T-post spaced 5 ft (1.5 m) on-center resulted in the best overall improvement, retaining an average of 93% of sediment and deflecting only 0.18 ft (0.05 m) over the course of three 2-yr, 24-hr simulated storm events. Additionally, the implementation of a dewatering mechanism within a silt fence installation proved to be an effective means for controlled dewatering. Effluent flow rates calculated from the dewatering board installation were 6.2 times greater than flows calculated for installations without a dewatering board. Increased effluent flows reduced dewatering time from +24 hours (w/o dewatering board) to 4 hours (w/ dewatering board). Water quality analyses indicated that the inclusion of the dewatering board had little to no effect on water turbidity. Full-scale performance evaluations of innovative and manufactured sediment barrier practices indicated that a major failure mode was undermining. Performance based comparisons of sediment retention rates, maximum impoundment heights, effluent flow rates, and treatment efficiencies were determined for each practice. Longevity tests were conducted to evaluate how each of these parameters changed over multiple storm events. Overall performance evaluations indicated practices which achieve impoundment depths between 1 and 1.5 ft (0.3 and 0.46 m) achieved sediment capture rates of at least 90% and reduced impoundment surface turbidity up to 60% when compared to turbidity along the bottom of the impoundment. Small-scale performance evaluations of geotextiles used in silt fence applications indicated that effluent flow rates observed during the 30-minute test period for nonwoven geotextiles were on average 43% lower than woven materials. Sediment retention evaluations indicated that on average nonwoven geotextiles (97% retention) outperformed woven geotextiles (91% retention). Water quality analyses suggested that the primary means for turbidity reduction was sedimentation during the 30-minute test periods (46% reduction) and geotextile filtration during the 90-minute dewatering periods (19% reduction). Stormwater pollution prevention plans (SWPPP) are used to communicate erosion and sediment control (ESC) designs and installation instructions to construction personnel. Accurate topographic depictions are critical to insure proper functionality of ESC practices. Thus, an evaluation of unmanned aircraft system (UAS) mapping technologies used in the development of triangular irregular network (TIN) surface models was completed. Vetical accuracy analyses indicated elevation differences between 9 TIN surface models and known ground surface points from a single test site were within the range of -0.62 ft (-0.19 m) and +0.72 ft (+0.22 m) for data collected at flight altitudes of 100 feet above ground level (AGL) and -0.65 ft (-0.20 m) and +1.57 ft (+0.48 m) for flight altitudes of 300 feet AGL.