Tauranga City Council Stormwater Pond 11

Background

Due to the expansion of land development within the area Tauranga City Council required an additional stormwater retention pond in order to accommodate increased stormwater flows. The brief set forth by the Council was to provide geotechnical inputs and design of the attenuation pond within a narrow gully feature surrounded by existing semi- rural residential properties.

Scope

CMW carried out ground investigations, detailed design of the pond and downstream embankment structures in order to formalise the final design.

Several machine and hand auger boreholes, together with cone penetrometer tests (CPT’s), were drilled in order to confirm soil strength parameters and develop a detailed ground model for the site. From this model various issues were considered for the design including:

Quantifying the risk imposed from a debris- flow induced wave over topping the downstream embankment and embankment stability;
Design of overflow spillway dimensions and erosion protection measures to consider a peak flood event (Q100);
Consideration of differential settlements across the design footprint;
Assessment and design of specific under- drainage and piping control measures beneath the dam embankment

Findings/Challenges

The nature of the landform and associated risks resulted in a challenge in terms of completing the design within the required timeframes. CMW were able to assist with providing a satisfactory design and efficient construction.