DocumentS & Evidence

At the "Pinehaven Stream Improvements" hearing, held 3 - 5 August 2020, the Commissioners ruled 'Save Our Hills' (SOH) evidence on the impact of future development on the hills as "out of scope" for the proposed streamworks. Hydrology and flood engineering expert Bob Hall of R J Hall and Associates Ltd disagrees.

The Pinehaven flood model, flood maps and proposed stream improvements are all based on this hydrology (rainfall-runoff) report by MWH for Greater Wellington Regional Council, which hydrology experts Graeme Horrell Consultancy Ltd and R J Hall & Associates Ltd find flawed and unreliable.

Expert hydrologist Graeme Horrell reviewed MWH’s Pinehaven hydrology and concludes:
“this rainfall-runoff modelling … [has] large uncertainties … at least ± 100% … This highlights the risk of using just one single event to calibrate the rainfall runoff model, which then feeds inputs into the hydraulic model. … Any further use such as … [for flood mapping] will result in large errors …[and] in unreliable design values for the Pinehaven stream works upgrade.”

SKM (now Jacobs) fed the ‘flow’ outputs from the flawed and unreliable MWH ‘rainfall-runoff’ model into SKM’s flood model, and prepared flood maps (2010) showing where they predict flooding will occur. A major error later became apparent in SKM’s ‘future case scenario’ (Vol1 pp14-16, 31-32).

In 2016 Jacobs re-worked the flood model to fix the error described as a “major issue” in the Beca audit (2015). The Beca auditor stated (in 2017) that Jacobs had corrected the error. However, R J Hall & Associates Ltd (2019) found that Jacobs had not actually addressed the error.

Hydrologist and flood engineering expert Bob Hall, of R J Hall & Associates Ltd, calculated that SKM’s and Jacobs’ ‘future development scenarios’ will increase flooding by about 500%, not 1% as asserted in the work by SKM, Jacobs and Beca.

Graham Macky, of Macky Fluvial Consulting Ltd, peer-reviewed Bob Hall’s Pinehaven Hydrology in which Bob Hall finds that future development on the Pinehaven hills will increase runoff five-fold. Macky states: “I concur with Mr Hall’s conclusions … The general hydrological method adopted in Mr Hall’s report is sound” and agreeing that Jacobs model has “assumed an exceptionally impervious catchment.” Bob Hall relied on on-site infiltration tests by Alex Ross which differ significantly from desktop infiltration rates in Cardno’s map for the Pinehaven area.

In his calculations for Pinehaven hydrology, Bob Hall relied on on-site infiltration tests by Alex Ross which differ significantly from desktop infiltration rates in Cardno’s map for the Pinehaven area. For comparison, Hall re-did his calculations (see R J Hall - ADDENDUM A) using Cardno’s questionable rates yet still found a doubling of runoff from the SKM / Jacobs ‘future development scenarios’ on the Pinehaven hills, not 1% as asserted in the work by SKM, Jacobs and Beca.

‘Time of Concentration’ is the estimated time it takes for rain that falls on the highest, most distant part of the catchment (the top of sub-catchment B on the Pinehaven hills) to arrive at the point of interest.

When rain falls on the land the resulting runoff depends on catchment characteristics, on land usage, on the degree of urbanisation etc. These factors also influence the amount of rainwater infiltration into the ground. Infiltration is a significant component of hydrologic processes.

By back-calculating Jacobs’ figures, RJ Hall and Associates Ltd find a CN (rainfall loss) value of 96 for the pre-development hydrology, which means the existing forested hills are treated by Jacobs as being more or less impermeable, and the runoff characteristics between pre- and post-development are almost indistinguishable.