K.W. Choi

The extended Kalman filter for forecast of algal bloom dynamics.

A deterministic ecosystem model is combined with an extended Kalman filter (EKF) to produce short term forecasts of algal bloom and dissolved oxygen dynamics in a marine fish culture zone (FCZ). The weakly flushed FCZ is modelled as a well-mixed system; the tidal exchange with the outer bay is lumped into a flushing rate that is numerically determined from a three-dimensional hydrodynamic model. The ecosystem model incorporates phytoplankton growth kinetics, nutrient uptake, photosynthetic production, nutrient sources from organic fish farm loads, and nutrient exchange with a sediment bed layer. High frequency field observations of chlorophyll, dissolved oxygen (DO) and hydro-meteorological parameters (sampling interval Deltat=1 day, 2h, 1h, respectively) and bi-weekly nutrient data are assimilated into the model to produce the combined state estimate accounting for the uncertainties. In addition to the water quality state variables, the EKF incorporates dynamic estimation of algal growth rate and settling velocity. The effectiveness of the EKF data assimilation is studied for a wide range of sampling intervals and prediction lead-times. The chlorophyll and dissolved oxygen estimated by the EKF are compared with field data of seven algal bloom events observed at Lamma Island, Hong Kong. The results show that the EKF estimate well captures the nonlinear error evolution in time; the chlorophyll level can be satisfactorily predicted by the filtered model estimate with a mean absolute error of around 1-2 microg/L. Predictions with 1-2 day lead-time are highly correlated with the observations (r=0.7-0.9); the correlation stays at a high level for a lead-time of 3 days (r=0.6-0.7). Estimated algal growth and settling rates are in accord with field observations; the more frequent DO data can compensate for less frequent algal biomass measurements. The present study is the first time the EKF is successfully applied to forecast an entire algal bloom cycle, suggesting the possibility of using EKF for real time forecast of algal bloom dynamics.

Mixing in the Intermediate Field of Dense Jets in Cross Currents

AbstractThis study presents an experimental and numerical study for an inclined (60° to horizontal) dense jet discharged into a coflowing current. The mixing and transport of the density current arising from the jet impingement on a horizontal bottom boundary is investigated. A light attenuation technique is employed to measure the layer-averaged concentration field over a region that extends 20FrD downstream and 9FrD laterally (where F = jet densimetric Froude number, and D = jet diameter). A comprehensive characterization of the resulting buoyant spread in both steady and unsteady cases is obtained. The concentration field is also computed using a three-dimensional (3D) shallow water equation model via the distributed entrainment sink approach that incorporates a near-field Lagrangian integral jet model (JETLAG) into the 3D model for dynamic simulation of the near-far field transition. The results show the occurrence of bifurcation in the gravitational spreading layer when the impinging dense jet is ben...

Field Experiments of Chlorine Demand in Disinfection of Treated Primary Effluent

AbstractIn the Hong Kong Harbour Area Treatment Scheme (HATS), a concentrated 10% chlorine solution (in sodium hypochlorite with a specific gravity of 1.2) is used directly to disinfect sewage that...

Modelling of Dense Jet in Co-Flow and Counter Flow

Wastewater effluents with a density higher than that of the environment are often discharged into coastal waters in the form of submerged dense (negatively buoyant) jets. Examples include brine discharge from desalination plants, cooled water from liquefied natural gas plants and gypsum waste from fertilizer factories. It is necessary to design dense jet discharges to achieve rapid mixing and minimize the environmental impact. Currently, however, there is no generally accepted predictive model for the mixing of a dense jet in a current. In the present study, numerical modelling of mixing and near-intermediate field interactions of upward discharged dense jet is carried out. The near field behaviour of the jet is computed by the Lagrangian jet model JETLAG. The mixing and transport in the intermediate field is predicted by dynamically coupling a three-dimensional (3D) shallow water circulation model with JETLAG using a Distributed Entrainment Sink Approach (DESA). Simulation of inclined dense jet in stagnant water and flowing current has been undertaken. The 3D model is able to simulate the initial rise and fall of the jet as well as the spreading layer along the bottom. The computed dilutions agree well with those obtained from the laboratory experiments, which demonstrate the feasibility of using the DESA method to compute the mixing of dense jet in a current.

The Extended Kalman Filter for Short Term Prediction of Algal Bloom Dynamics

An Extended Kalman Filter (EKF) is incorporated into a water quality model to assimilate high frequency field observations of algal bloom and dissolved oxygen dynamics in a marine fish culture zone (FCZ). The weakly-flushed fish culture zone is modeled as a well-mixed system with numerically determined flushing rate. The ecosystem model incorporates phytoplankton growth kinetics, nutrient uptake, photosynthetic production, nutrient sources from organic fish farm loads, and nutrient exchange with a sediment bed layer. Supported by the high frequency observations (sampling interval Δt = 1 day, 1 hour, and 2 weeks for chlorophyll and dissolved oxygen, hydro- meteorological parameters, and nutrient, respectively), a number of algal blooms observed at Lamma Island of Hong Kong are used to assess the performance of the EKF. Daily chlorophyll levels estimated by the EKF are compared with field observations and the unfiltered deterministic model prediction for different algal bloom events. The data assimilation with different observation lead-times is also studied. It is found that the EKF estimate well captures the nonlinear error evolution in time and gives good predictions of short term algal bloom dynamics up to a lead-time of 2 or 3 days. The present study is the first time the Extended Kalman Filter is successfully applied to forecast an entire algal bloom cycle, suggesting the possibility of using EKF for real time forecast of algal bloom dynamics.

Tracking major endocrine disruptors in coastal waters using an integrative approach coupling field-based study and hydrodynamic modeling

Many of the worlds large coastal cities discharge partially treated wastewater effluents containing various endocrine disrupting chemicals (EDCs) to coastal environments. Nonylphenols (NP) and bisphenol A (BPA) were found to be the most abundant EDCs in sewage effluents in Hong Kong. The environmental fate and ecological risk of these two EDCs remains largely unknown, particular for coastal systems with complex hydrodynamic flows. Based on a validated three-dimensional (3D) multiple-scale hydrodynamic model, a field-based study was conducted to track the two EDCs from potential sources to the only marine reserve in Hong Kong. The two compounds were detected in all seawater, suspended particle, and sediment samples, with higher aqueous concentrations in wet season than in dry season. High concentrations in sediments suggest sediment is a sink, posing an ecological risk to the benthos. The fate and transport of the two EDCs was predicted using a 3D near-field Lagrangian jet model seamlessly coupled with a 3D shallow water circulation model. The results suggested the NP and BPA in the marine reserve cannot be solely attributed to the nearby submarine sewage outfall, but likely concurrently contributed by other sources. This study calls for more effective measures of reducing the use and release of these EDCs, and research to investigate their impacts on the marine benthos.

Internal Hydraulics of a Chlorine Jet Diffuser

AbstractMultiport diffusers are used in many environmental applications to distribute a discharge flow uniformly through a number of jet nozzles into the same or another fluid. For example, a sewag...

Modeling of Mixing and Rapid Chlorine Demand in Sewage Disinfection with Dense Chlorine Jets

AbstractA concentrated 10% chlorine solution (in sodium hypochlorite) with a specific gravity of 1.2 is used for the disinfection of treated primary effluent in the Stonecutters Island Sewage Treat...

Postoperation Performance of the Tai Hang Tung Storage Scheme in Storm Events: 2D Hydraulic Analysis and Field Verification

AbstractThe Tai Hang Tung Storage Scheme (THTSS) is an essential part of the West Kowloon Drainage Improvement Project that aims to solve the flooding problem in Mongkok, a congested and densely populated district of the Hong Kong Special Administration Region of China. The heart of the flood detention scheme is a 100,000-m3 underground storage tank beneath the Tai Hang Tung Recreation Ground, a triple side-weir system, and transitions to the upstream inlets and downstream channels. During heavy storms, part of the flood flow will spill over the side weirs into the underground storage tank and be temporarily stored; the flow diversion serves to attenuate the flood peaks and prevent downstream flooding. The THTSS was commissioned in 2005, and flow monitoring has been conducted since 2007. The operation of THTSS has essentially resolved the historic flooding problem. Nevertheless, the THTSS performance was put to an extreme test on June 7, 2008, when the highest hourly rainfall on record occurred. The heavy...