Los Angeles and Long Beach Harbors Model Enhancement Program. Numerical Water Quality Model Study of Harbor Enhancements.

摘要

Plans for enhancing these harbors include dredging and new landfill which may affect water quality through changes in water circulation and flushing. The major water quality concern was possible depression of dissolved oxygen (DO) in poorly flushed, deepened navigation channels. Comparison of flushing and DO between existing (Base) and an enhanced plan (Scheme B) was conducted. A 3-D numerical WQM was adapted to process the output from the 3-D hydrodynamic model CH3D to transport the water quality constituents. The overlaid WQM grid required that the hydrodynamic data be spatially averaged, and the WQM kinetic interactions allowed the data to be temporally averaged. Averaging resulted in 92% fewer computational cells in the WQM and allowed a 1,400% increase in the time step while maintaining computational stability. The WQM formulation included the state variables DO, carbonaceous biochemical oxygen demand, ammonia nitrogen, nitrite plus nitrate nitrogen, algal biomass, and orthophosphate. Processes directly affecting DO included tidal circulation; phytoplankton oxygen production and growth as a function of light and nutrients, respiration, and nonpredatory mortality; sediment oxygen demand; biochemical oxidation of organics; nitrification; and reaeration. The predicted DO was nearly equivalent in Scheme B compared with existing conditions.