Memorandum by R J Hall and Associates Ltd, dated 02 Dec 2020
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 attached memorandum by R J Hall & Associates Ltd, dated 02 December 2020, explains why (because of Council's incorrect baseline hydrology model) future development on the Pinehaven hills is not out of scope but is in fact critically relevant to the streamworks project.
Because of the flawed baseline hydrology, all the subsequent work by the Councils that is based on this hydrology is incorrect. This includes the Pinehaven flood extent maps, Council's 'future case' assessment of the impact of future [Guildford] development on the Pinehaven hills on flooding, and the design of the proposed Pinehaven streamworks - it is all incorrect because it all relies on Council's flawed underlying hydrological model (the rainfall-runoff model).
R J Hall and Associates Ltd have carried out extensive investigations of Council's baseline flood hydrology for the Pinehaven catchment, and provided several peer reviewed reports to SOH (listed under References at the end of the Memorandum) which identify major flaws in the hydrological and hydraulic modelling. Because of the flawed baseline hydrology, R J Hall and Associates Ltd conclude that Council's Plan Change 42 'hydraulic neutrality' rules will fail, allowing large volumes of unmitigated stormwater runoff from future development on the hills to have harmful and damaging impacts on the catchment.
The memorandum explains why, for the safety of residents in the catchment, for protection of the environment and for the correct design of the streamworks, it is imperative that Council's flawed flood hydrology for the Pinehaven catchment be fixed. R J Hall and Associates Ltd explain how the hydrology must be fixed:
"The hydrology must first be transparently corrected by using infiltration rates that are truly representative of the catchment, and by ensuring proper account is had of the rainfall interception effects of the catchment's heavy vegetation cover."
The memorandum also points out that although it is customary to use detention dams (artificial ponds or lakes) to capture and regulate additional stormwater runoff from urban developments on greenfield sites, detention dams would not be an appropriate method for attenuating runoff from future development on the Pinehaven hills. The reason is that although detention dams can be used to reduce peak outflow to be equivalent to pre-development peak flow, this peak flow would have to be sustained for a considerable period of time during and after the storm. The sustained flow of a greater-than-normal bulk of water for a number of hours will erode the stream bed and sides of the channel, carrying soil, rock, sediment and riparian vegetation downstream, quickly removing any benefits of the proposed stream improvements in the lower reaches of the catchment.
R J Hall and Associates Ltd advise in this memorandum that stormwater neutrality studies for future development on the Pinehaven hills must do more than just attenuate peak flow. They must also address morphological impacts on the stream channel and the potential increase in hillslope landslides that could result from additional stormwater runoff in the steep upper reaches of this catchment.
R J Hall and Associates Ltd suggest that a more appropriate way to deal with additional stormwater runoff from future development on the Pinehaven hills would be to pipe it out to the Hutt River rather than discharge it into the main channel and tributaries of the Pinehaven Stream.