Numerical modeling of thermal plume and residual chlorine fate in Qatar's coastal waters

Abstract

A 3-dimensional hydrodynamic model was developed and calibrated to simulate the thermal mixing zone and predict the fate of residual chlorine and chlorination by-products (CBP) from industrial cooling water discharged from Ras Laffan City. This model is novel since the individual and cumulative effects of cooling water discharges from several outfalls are estimated in a single simulation instead of requiring separate model runs for each outfall. The concept of incremental temperature rise (increase over the natural water temperature in the absence of outfall) was used to evaluate mixing zone plume dimensions. A probability-based computational methodology was developed to define the thermal plume mixing zone for the shallow coastal environment, which is influenced by strong tides and winds. A new boundary condition was introduced to address existing, ongoing and proposed breakwaters and structures without altering the model grid system while preserving the same boundary conditions for various management scenarios. In comparison to freshwaters, little is known about the formation of CBP in saline waters. Therefore, a comprehensive study was devised that included laboratory experiments to quantify the kinetics of residual chlorine loss and subsequent formation of CBP in seawater as well as extensive field data collection and testing of Arabian Gulf seawater samples for chlorine and CBP in the vicinity of the discharges. Equations to describe site-specific chlorine reactions were developed to replicate observations. This empirical approach takes into account the complexity of the reactions between organic precursors and chlorine, which usually involve several parallel pathways leading to a great variety of CBP formation products. This complexity makes it difficult to develop more generic models for simulating CBP formation. Analysis of lab and field data obtained in this study have enabled calibration of a site-specific numerical modeling tool that can be used to study transport and fate of various constituents in the coastal area.