The bit allocation problem for hierarchical B-pictures in H.264/SVC is studied with a GOP-based dependent distortion model in this work. Inter-dependency between temporal layers of H.264/SVC is often neglected because of the complexity involved, which often leads to poorer rate control performance. To address this shortcoming, we propose a distortion model that takes inter-dependency into consideration while preserving the low complexity of the encoding process. It is demonstrated by experimental results that the new distortion model results in a highly efficient bit allocation scheme, which outperforms the rate control algorithm in the JSVM 9.12 reference codec by a significant margin.