Guidelines for Return Periods

Ideally, the choice of a design return period should be based on an economic evaluation in which the costs of providing the drainage works are compared with the benefits derived. However, comprehensive local flood damage data are normally not available to the degree of precision required for cost-benefit analysis. For this reason, a general policy decision based on such considerations as land use, hazard to public safety and community expectations is more appropriate. Admittedly, for new drainage systems or drainage upgrading in some existing areas, particularly low lying ones or those in congested urban locations, the recommended standards may not be suitable or achievable. A pragmatic approach should be considered. In a case in Hong Kong the return periods recommended in urban drainage situations ranged from 50 to 200 years (Drainage Services Division, 2014). For the City of Dublin, Ireland a 100 year return period is applied for protection of flooding within properties (Greater Dublin Strategic Drainage Study, 2005). These values may not be appropriate when climate change is considered.


Guidelines for the selection of return period

No.

Type of project or feature

Return period (yr)

1

Urban drainage [low risk] (up to 100 ha)

5 to 10

2

Urban drainage [medium risk] (more than 100 ha)

25 to 50

3

Road drainage

25 to 50

4

Principal spillways (dams)

25 to 100

5

Highway drainage

50 to 100

6

Levees [medium risk]

50 to 100

7

Urban drainage [high risk] (more than 1,000 ha)

50 to 100

8

Flood plain development

100

9

Bridge design (piers)

100 to 500

10

Levees [high risk]

200 to 1000

11

Emergency spillways (dams)

100 to 10,000 (PMP)

12

Freeboard hydrograph [for a class (c) dam]

10,000 (PMP)


Source: Ponce, V.M. Q & A on the return period to be used for design. Sourced 20 May 2016. http://returnperiod.sdsu.edu/