Narellan Creek Overland Flood Study (Vol 1) - Flipbook - Page 74
6
SENSITIVITY AND CLIMATE CHANGE ANALYSIS
6.1
Overview
Computer flood models require the adoption of parameters that are not necessarily known
with a high degree of certainty or are subject to variability. Each of these parameters can
impact on the results generated by the model.
As outlined in Chapter 4, computer models are typically calibrated using recorded rainfall,
stream flow and/or flood mark information. Calibration is achieved by adjusting the
parameters that are not known with a high degree of certainty until the computer model is
able to reproduce the recorded flood information. Calibration is completed in an attempt to
ensure the adopted model parameters are generating realistic estimates of flood behaviour.
As discussed in Chapter 4, the WBNM and TUFLOW models were validated against recorded
and observed flood information for three historic events. In general, this information
confirmed that the models were providing realistic descriptions of flood behaviour across
the catchment.
Nevertheless, it is important to understand how any uncertainties and variability in model
input parameters may impact on the results produced by the model. Therefore, a sensitivity
analysis was undertaken to establish the sensitivity of the results generated by the
computer model to changes in model input parameter values. The outcomes of the
sensitivity analysis are presented below.
6.2
Hydrologic Model Inputs
6.2.1
Initial / Burst Loss
An analysis was undertaken to assess the sensitivity of the results to variations in
antecedent wetness conditions (i.e., the dryness or wetness of the catchment prior to the
design storm event). A catchment that has been saturated prior to a major storm will have
less capacity to absorb rainfall. Therefore, under wet antecedent conditions, there will be
less
