Flow Rates and Capacity
Any drainage system should have sufficient capacity to carry the expected flows of water at any point in the system. The actual flow in the system depends upon the area to be drained, the rainfall intensity and the position of the outlets.
GUTTERS LAID TO FALLS
BS EN 12056-3:2000 refers to gutters having a fall “not steeper than 3mm/m (ie. it is nominally level)”. Gutters laid to a fall will have a somewhat higher capacity, but this increase should be viewed as an additional factor of safety. Our flow rates have, therefore, been based on level gutters.
Gutters laid to a fall of 4mm/m (1/250) will generally have a higher flow rate than when laid level, which can be used to offset friction losses in longer gutters.
GUTTER CAPACITY
The profile and dimensions of a gutter determines the maximum flow rate (capacity), currently given in litres/second. The height of gutter profile determines the maximum length of gutter run before resultant overflow at the ‘high point’ (i.e. stop-end or mid-way between outlets), calculated per BS EN 12056-3:2000 as 50 x height.
It is most important to consider the position and number of outlets as well as the size of gutter.
RAINFALL INTENSITY
Rainfall and storm intensities vary between differing locations within the UK and a number of pages are devoted in BS EN 12056-3:2000 to variations and safety factors, relating to various types of buildings and gutter locations. Eaves gutters should be designed using a one year storm event, and the intensity found from maps in BS EN 12056-3:2000.
CATCHMENT AREA
In accordance with BS EN 12056-3:2000 the drainable catchment area of a pitched roof is calculated by adding half the height to the plan width and multiplying by the length as diagram left.
CAPACITY AND FLOW CHARTS
Large, complicated and industrial projects are best treated individually with reference to BS EN 12056-3:2000.
Design flow rates
| End Outlet in Gutter System | Outlet size
(mm) | Max flow rate (l/s) to
BS EN 12056 | Maximum distance*
To stop end (m) | between
outlets (m) | | Half Round | 68 dia | 0.66 | 2.5 | 4.87 | | Square Line | 65 sq. | 1.18 | 2.8 | 5.62 | | Universal XL | 65sq. | 1.29 | 3.5 | 6.9 | | Universal Plus | 65 sq. | 1.66 | 3.7 | 7.3 |
| Centre Outlet in Gutter System | Outlet size (mm) | Max flow rate (l/s) to BS EN 12056 | Max flow rate (l/s) to BS6367 | Maximum distance*
To stop
end (m) | between outlets (m) | | Half Round | 68 dia | 1.32 | 1.4 | 2.5 | 4.87 | | Square Line | 65 sq. | 1.79 | 2.13 | 2.8 | 5.62 | | Universal XL | 65 sq. | 2.59 | 3.37 | 3.5 | 6.9 | | Universal Plus | 65 sq. | 2.60 | 3.5 | 3.7 | 7.3 |
* If longer gutters are to be used, the effects of the longer length can in some ways be counteracted by fitting the gutter at a nominal fall of 1 in 250 (4mm/m).
Due to increases in the safety factors applied, the design flow rate based on BS EN 12056 is lower than that for BS6367. This does not mean that the gutter flow is actually any lower. It is purely the figure used for design purposes. For this reason we have included the flow rates for BS6367 so that comparisons can be made.
Gutter/outlet capacities with BSEN12056-3:2000 are lower than with BS6367:1983, which it replaced, as freeflow must now be used for eaves gutters.
| |
| |