My suggestion is to have one of the above every 25 metres minimum.  Both are used for inspection of drains and routine cleaning.

Pits can be placed in a drive or pathway, with the surface graded to the opening.  This pit can then be used as an inspection/clean out point, as well as pick up any surface water.  Pits can also be placed in a garden or lawn area and fitted with a solid lid.

Inspection Openings are tees cut into the storm water drain and bought to the surface, with either a grate or screwed cap for easy access.

Storm Water drainage has evolved over time.  Many years ago, storm water drains were layed like sewers, in earthenware with a string line.

Today in domestic properties, the majority of drains are layed in 90mm PVC, but some plumbers do prefer to use 100mm PVC, the same as sewers.

Although there are no hard and fast rules, storm water drains should be layed to a minimum fall off 1 in 60, this allows for proper discharge of water to barrel drains or gutters.

Storm water drainage in domestic properties start at the bottom of your 100mm x 50mm downpipes.  Start your drain off with a 100 x 50 x 90mm storm water adaptor, then the appropriate bends as required.  When branching into another line, I have always believed that you should use a 45degree junction, and a 45 degree bend to straighten the pipe up.  This allows the water to merge, and not hit at 90 degrees.