To initiate the motion of a solid particle in a conduit, the velocity of the carrier fluid needs to exceed the threshold velocity for particle transport. Several models have been developed that predict such velocity. However, none of these models provide information regarding the confidence of these predictions. In this paper, a new semi-mechanistic model is introduced using force balances on a particle in the horizontal and vertical directions, and torque balance on a particle. A unique contribution of this paper is the use of the bootstrap method to quantify the uncertainty of the model's prediction. This information is used to compute the envelope of the threshold velocity predictions to within a predetermined confidence level. We compared the performance of our semi-mechanistic model to existing models using statistical analysis and parity plots. The comparison suggests that our semi-mechanistic model is accurate, and is capable of explaining the variation in the experimental data. The threshold velocity envelopes suggested by our model cover the experimentally-observed values for 92%, 90%, and 93% of the experimental data for hydraulic transport from a bed of solids, pneumatic transport from a bed of solids, and pneumatic transport from the bottom of the conduit, respectively.