The effects of dredging on dissolved oxygen in agricultural waterways in King County, Washington

by Milburn, Elizabeth Anne

by Elizabeth A Milburn, M.S. Washington State University May 2007 Chair: Michael E Barber Oxygen levels in streams, rivers, and lakes play an important role in habitat availability and are a good indicator of overall water system health. The purpose of this experimental study was to investigate the alterations to dissolved oxygen levels in agricultural waterways as a result of vegetation removal. This was part of a comprehensive water quality study in a series of lowflow low-gradient agricultural drainage ditches in King County, Washington. This investigation was accomplished through water quality monitoring and modeling. The monitoring portion of the study involved routine stab readings and lab analysis including diel measurements with data sondes. The EPA water quality model Qual2kw was used to model dissolved oxygen in the study reaches. In conjunction with the modeling process several parameters hypothesized to be altered as a result of dredging operations were examined in detail including channel roughness, energy gradients, sediment oxygen demand, reaeration, sedimentation, and photosynthesis. Hydraulic parameters such as channel roughness, wetted perimeter, and energy gradients were modeled using HEC-RAS software. Sediment oxygen demand was measured ex-situ in the Washington State University lab using sealed Plexiglas iv chambers and dissolved oxygen probes. The water system’s dependence on reaeration, sediment oxygen demand, and photosynthesis was investigated by a sensitivity analysis in Qual2kw. The study results indicated that there are significant differences in key channel characteristics as a result of maintenance activities. Manning’s roughness was found to be between 0.41 and 1.9 in pre-dredged and 0.08 in post-dredged reaches respectively. The energy grade slopes in pre- and post-dredged segments were found to be approximately .0029 and .000021. Dissolved oxygen levels rose approximately 3.4 mg/l (over 250%) following vegetation removal. Sediment oxygen demand rose after maintenance activities from approximately 0.67 to 3.17 g/m2/d. The vegetation removal was performed by hand which left a large amount of loose, highly organic decaying matter on the channel floor. Instead, the primary contributor to increased dissolved oxygen was found to be increased reaeration and photosynthesis as a result of vegetation removal. v