Update of ARC TP108 Run-Off Calculation Guideline

This paper focuses on the update of the Guidelines for Stormwater Runoff Modelling in the Auckland Region Technical Publication (TP108) to provide technical procedures to calculate event based stormwater discharges to design stormwater drainage facilities, treatment/detention devices and delineate flood inundation within the Auckland region.

The guideline provides rainfall depths for design storms from 3-month to 500-year Average Recurrence Interval (ARI). Maps with spatial distribution of rainfall depths for the relevant recurrence intervals are provided. For both approaches to calculate runoff hydrographs (SCS and Initial Loss and Constant Infiltration) it is necessary to run the full 24 hour storm duration.

For various methods to calculate runoff (Rational Method, SCS method and Initial Loss and Constant Infiltration) rain losses are provided.

The empirical based parameters curve numbers for the SCS method and runoff coefficients for the Rational Method are provided in relation to initial loss and constant infiltration values.

Two methods to calculate the transformation from rainfall excess to runoff are provided in this guideline. The first employs the unit hydrograph approach which is based on the assumption that a catchment, in converting rainfall excess to runoff, acts as a linear, time invariant system. To take the large diversity of flow characteristics in imperviousconnected and pervious-unconnected catchments into consideration, two unit hydrographs are provided.

The alternative methodologies can be picked in accordance with there limitations (eg lag time not more than ten minutes for Rational Method) and the simulation purpose (eg peak flow for flood estimation).