Highway drainage Flashcards
why do we need drainage?
prevent flooding and aqua planning
prevent spray from vehicles creating visibility issues
prevent water ingress into pavements
What are some typical relationships in an IDF chart?
intensity is inversely proportional to length of a storm
for a given duration and site, intensity can vary with the least rainfall occurring the most and the most rainfall occurring the least.
What are some limitations?
Large areas are not accurate, typically overestimated
ares 5-50 km are suitable
What are some assumptions ?
Design storm produces uniform rainfall intensity over the entire catchment, valid because its a small area
relationship between rainfall intensity and rate of runoff is constant for a particular catchment,related to the C coefficient. Typically 1 for buildings and roads
What is the difference between wet surface and conditions for aqua planning?
Aqua planning occurs when vehicle speeds cause the contact time between the Tyre and the road to decrease such that there is not enough time to remove water and make contact with the road surface
what is the mechanism for aqua planning?
water is dragged by the tyre into a wedge shape, causing hydrodynamic pressure to build up in front of the tyre
sufficient pressure build up will cause the tyre to be separated from the road surface by a thin film of water
full aquaplaning is when the tyre loses all grip and slippage is unopposed
Describe macrotexture from a drainage persepctive
General shape of aggregates and spaces between particles in a road
positive such as chipseal
negative such as SMA (sand mixed asphalt)
