This work investigated the removal efficiencies of the turbidity and organic content from high-turbidity stormwater induced by tropical storm Nari using polyaluminum chloride (PACl) as the coagulant. The resulting floc size and compactness (fractal dimension) were monitored using small-angle light scattering technique. The response surface method along with the Box-Behnken design of experiments was adopted to investigate the effects of pH, turbidity, and alkalinity of the suspension, the PACl dosage, and the dosed amount of humic acid. The PACl dosage was the most significant variable for turbidity removal, while that for humic acid removal was the suspension pH. The floes corresponding to the removal efficiencies tend to show a loose interior structure for effective removal of both turbidity and humic acid. The acidic suspension with medium PACl dosage and alkalinity level were favorable to producing loose floes. The results indicated that humic acid concentration should be kept low if possible. The optimal conditions needed to generate large floes include the neutral condition and a high PACl dosage. A compromise is needed to produce both large and loose floes. The charge neutralization mechanism does not correspond to turbidity/humic acid removal from high turbidity stormwater.