Stormwater Catchpits Capacity in New Zealand - Between Theory and Practice

New Zealand Territorial Authorities (TA’s) infrastructure design standards adopt different designs for catchpit inlets, all based on the same well known flow concept of kerb back entry (kerb opening), channel grates or a combination of both.

During road design, catchpit inlets are modelled, designed, and located to allow all road runoff to be diverted to the stormwater network system. Most designs are based on a theoretical prediction of the specific catchpit capacity, if available, or conservative practice values stated in the standards. In the theoretical calculation of catchpit inlet capacities, designers have to assume values of a number of important input parameters, hence, the consistency of the results are often variable. A number of capacity calculations of various catchpit inlet types, using the HEC 22 method with variable input values, are presented in this paper. This highlights that designers may end up with a wide range of “suspect” capacity values if well defined inlet component design parameters and TA specific design limitations are not available.

In construction, many channel slopes and catchpit openings are not built to the standard design. Calculations of “as built” capacities using the HEC 22 method compared to “as designed” capacities, indicate that changes to original design features have a significant effect on the capacity of the catchpit inlet and hence may result in unpredicted overland runoff during a storm event.

A field survey of a number of poorly constructed catchpits in the Auckland area has lead to a better understanding of the reasons behind many construction problems.

Conclusions and recommendations from this work provide a good starting point for the industry to provide innovative solutions to the problems.