FLOOD01798

• County: Statewide
• Title: Australian Rainfall and Runoff 1998, Revision of Book VI - Estimation of Large to Extreme Floods
• Date: 11/28/1998
• Prepared By: Rory Nathan, Sinclair Knight Merz
• Floodplain - Doc Type: Educational/Technical/Reference Information

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model to include the facility for separate representation of
the hillslope and stream network phases of the hydrograph
formation process (Kemp, 1998).

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(Ii) Separation of routing elements with different
non~Jinearities

Different catchment elements (e.g. overland flow,
streamflow, channel flow, concentrated storage elements)
may be characterised by different non-linearities in their
routing response. A model structure that allows the
separate representation of routing elements with different
non-linearily characteristics (e.g. Kemp, 1998) offers
distinct advantages, as extrapolation of the routing
characteristics for individual elements can be achieved in a
more controlled fashion than for the lumped response of a
combination of different elements.

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4.3,3 Catchment Representation in Model

This section provides recommendations on how the
functionalities of a selected model should be used to
ensure adequate representation of the real catchment's
runoff response.

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(a) General considerations

Most hydrograph models are highly conceptual in
nature; in setting up a model representation of the
catchment, the modeller should therefore try to define
conceptual model elements that match the routing
response of the main components of the real catchment,
without necessarily attempting to exactly match physical
catchment features (e.g. individual drainage lines, drainage
divides). How this can best be achieved will depend on the
specific features of the selected model. However, the most
important factor determining the quality of the modelling
results is the modeller's understanding of the routing
functions incorporated in the modelling package and
hisiher appreciation of the catchment response under flood
conditions. More specific guidance on selected model
configuration issues is provided in the next section.

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(b) Specific issues

(i) Degree of catchment homogeneity

The model should be subdivided into as many sub-
catchments as required to represent the broad variation in
Large to Extreme flood response resulting from differences
in topographic, drainage system, land cover and land use
attributes (see recommendations on the minimum number
of sub-catchments provided in Book V Section 3). In many
cases, the variation in parameter sets for the different sub-
catchments' can be directly related to differences in
measurable catchment characteristics. However, where
different parameter sets for the sub-catchments have to be
determined by calibration, the specification of variation in
parameter values should be restricted to those sub-
catchments for which calibration information is available.

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(iI) Representation of important catchment
features

Major catchment features may have a significant
influence on <:etohment flood response, and may "xhlbit
significantly different routing characteristics compared to
the rest of the catchment, particularly when extrapolated to
extreme events. All the significant natural storage areas
(e.g. swamps, extensive flood plains, off-channel storage
areas) and distribotary or effluent channel systems should
be identified and adequately represented. Consideration
should also be given to the modelling of anthropogenic
features, such as the specification of diversion channel

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capacities, or road/rair crossings that may act as retarding
basins during extreme events.

(iii) Representation of catchment areas close to a
reservoir

In the vicinity of a reservoir, the routing response varies
from near zero delay for rainfall on the inundated areas, to
significant delays for rainfall excess from the less directly
connected areas draining to the storage. The modelled
hydrographs and the calibrated modei parameters can be
quite sensitive to the representation of these areas,
particularly when the inundated area constitutes a large
part of the total catchment. Considerable care should be
exercised in ensuring that the routing characteristics of the
inundated parts of the catchment and the areas close t6 it
have been realistically represented.

(iv) Modelling of changed catchment conditions

The effects of likely changes to the catchment during
the design life of the structure need to be considered.
Uribanisation and destruction of vegetation by clearing or
fire may reduce the response time of the catchment and
increase the peak flow, while soil conservation measures
over a large portion of a catchment may have the opposite
effect. Generally, a rather aribitrary allowance must be
made for these effects. Construction of a reservoir may
inundate appreciable lengths of streams in the catchment
and can lead to large decreases in travel time and
increases in flood peaks, despite the attenuation resulting
from the routing effect of the reservoir. This effect is
discussed and examples are given by Weeks and Stewart
(1982), Brown (1982) and Watson (1982). The last two
references give examples where the inflow flood peak is
increased by 85% by the construction of a dam. It is
therefore important to consider this effect when using a
model to derive design floods for a dam if it has been
calibrated to pre-dam conditions.
In general, allowance for different catchment conditions
can be made more easily by runoff routing than by unit
hydrograph models. In runoff-routing models the different
routing characteristics of existing or future catchment
conditions can be incorporated by the judicious selection of
parameters and the types of routing elements. With the unit
hydrograph approach it is more difficult to relate differences
in specific catchment conditions to changes in model
parameters, particularly with unit hydrograph models that
are lumped rather than distributed in nature.

4.3.4 Determination of Model Parameter Values

(aJ Genera/considerations

Calibration of a flood event model for application to
design flood estimation is traditionally restricted to the
selection of parameter values to achieve a fit between
observed and estimated hydrographs. Attention is focused
on the collation of concurrent streamflow and rainfall data
corresponding to the largest events on record, where
considerable effort is required to ensure that the temporal
and spatial distribution of the rainfall data is representative
of the actual processes.
The ability of a model to reproduce historic events
certainly gives some confidence to the validity of
subsequent design estimates. However it is possibie .
perhaps even probable - that the available historic
information is not relevant to the design floods of interest.
In a well gauged catchment the AEPs of the calibration
floods ara Hkely to range between 1 in 5 tc;> 1 in 20. While it
would be expected that floods of this magnitude will
activate some floodplain storage, the non-linear nature of
catchment flood response is such that the catchment