  We model the effect of ground movement, based on empirical experience, on the
transport properties of long neutron guides by ray-tracing simulations. Our
results reproduce the large losses found by an earlier study for a simple
model, while for a more realistic engineering model of guide mounting, we find
the losses to be significantly smaller than earlier predicted. A detailed study
of the guide for the cold neutron spectrometer BIFROST at the European
Spallation Source shows that the loss is 7.0(5) % for wavelengths of 2.3-4.0
{\AA}; the typical operational wavelength range of the instrument. This amount
of loss does not call for mitigation by overillumination as suggested in the
previous work. Our work serves to quantify the robustness of the transport
properties of long neutron guides, in construction or planning at neutron
facilities worldwide.
