The study takes driftwoods intercepted in Da-jia River basin from De-ji Reservior down to outfall as samples after Typhoon Mindulle. We established numerical models to evaluate the relationships of driftwoods, such as length vs. accumulative volumes and length vs. accumulative average diameters. Finally we find out two models, one of driftwood length to fit detention pond and the other to check the capacity of detention ponds. The study also provides with the analytic results of driftwood length, diameter by statistical techniques to some departments like road-designing as references, and it is anticipated that the study is able to reduce damages from driftwoods.