Plan model is based on one flood event 35 years ago
The Woy Woy Peninsula Floodplain Management Study has admitted its plan has been created "in the absence of any gauged water level or discharge data".
Instead, it is modelled on just one flood event 35 years ago using just 20 locations across the Peninsula and using rainfall data from Peats Ridge.
Despite the lack local rainfall, flood level or discharge data, the plan has not recommended that these be monitored.
The study said that "the parameters related to soil and groundwater were already calibrated against the long-term groundwater level observations".
"Calibration of the model in this study focused on parameters affecting surface water."
This was to be carried out against the estimated peak water depths from flooding events identified in a 2010 study.
Candidate events were in August 1972, May 1974, 1984, March 1986, April 1988, January 1989, February 1990, March 1991, February 1992, May 1998, April 1999 and June 2007.
However the study said: "The April 1988 flood event was the only event with enough data collected during the community consultation (DHI, 2010) for model calibration."
"It should be noted that the temporal patterns were taken from the Peats Ridge gauge which was the nearest available for this event
"The six-minute interval temporal pattern of Peats Ridge (Bureau of Meteorology gauge 61351) was scaled by the daily rainfall measured at Everglades Golf Course (BOM Gauge 61318) to produce this timeseries."
It said that 311mm of rain was recorded over 72 hours between April 27and 30, 1988.
"This corresponds to a six to seven per cent rainfall event.
"The sea boundary condition was taken from two of MHL's tidal gauge stations: Ettalong
212423 Station and Koolewong (Decommissioned in 2016) 212422 Station.
"The water levels along the coast between the two stations were linearly interpolated.
"There were no corresponding reported water depths for the April 1988 calibration event at the Kahibah Creek system.
"The Kahibah Creek Flood Study (Willing and Partners, 1991) considered two large flood events January 1989 and 7 February 1990 in the Kahibah Creek System as calibration events.
"The Kahibah Creek system has been modified largely after the study in 1991, including widening and lining of the channel paths, replacement of major structures and clearance of weeds.
"These changes are so significant that a calibration of the model against either of these events would not be relevant for the current study."
SOURCE:
Website, 18 Jan 2023
Floodplain Management Study, Central Coast Council