Modelling surface temperature of granite seawalls in Singapore

Abstract

Temperature is a critical abiotic factor structuring intertidal life on hard-bottom habitats, including manmade coastal defense structures. To quantitatively describe the daily surface temperature variations of a seawall in Singapore, a physically based mathematical model is developed, which considers key thermal processes (i.e., solar radiation absorption, long wave emission, and heat convection). This model is nonlinear because of the nonlinear boundary conditions and the moving seawater level boundary. It is therefore solved numerically with appropriate initial conditions. Good model - data comparison is confirmed with field measurements. Results show that the maximum seawall temperature reached as high as about 48 C∘. Temperature variation was illustrated using contour plots. A high temperature index on the seawall, T95%, is defined as the threshold temperature that 95% of all temperatures at a given elevation do not exceed. T95% will be useful for investigating the effects of temperature on biodiversity in intertidal systems. The effect of seawall slopes and monsoons on T95% is further examined. For Singapore’s seawalls, it is found that the extreme temperature decreases with increasing slope, and the T95% during the southwest monsoon season is approximately 4 C∘ higher than that during the northeast monsoon. Keywords: Granite seawall, Intertidal zone, Heat conduction equation, Numerical model, Ecological engineering, Coastal environment