New siltation measurements should be taken if it has been a considerable length of time since the last measurement, or if there is reason to suspect a significant change in siltation, such as after a severe flooding event.
(New siltation measurements; should be taken; ) (it; has been; a considerable length of time; T:since the last measurement)
New siltation measurements it
should be taken has been
a considerable length of time T:since the last measurement