Data Requirements for Load Estimation in Well-Mixed Tidal Channels

Load estimation is an important component of watershed analysis and management, particularly in light of the Total Maximum Daily Load program set forth by the Clean Water Act. This paper addresses load estimation in tidally driven channels that are dominated by advection and well mixed over the channel cross section. Such systems are common in Southern California, and represent an important link between watersheds and the coastal ocean. The oscillating flow field of tidal channels calls for a time series of data (volumetric flow rate and scalar concentrations) sampled at a sufficiently high frequency to resolve the net load of each scalar. Results of an experiment conducted in the Talbert Channel in Huntington Beach, California, to quantify freshwater loads suggest that sampling should be conducted at a minimum frequency of 0.5/h. In this system, traces of freshwater were detected in the channel by salinity depressions that ranged from 1-7 ppt and lasted 4-6 h. The freshwater load was computed as the integral of the volumetric flow rate multiplied by the freshwater fraction of the water column. Based on the duration of these events and the 0.5/h sampling frequency, these results suggest that loads will be resolved to a reasonable level of accuracy when each event is captured by at least three or four data points. Strategies to estimate the flow rate and scalar concentrations are discussed.