Catchment Modeling Report: Analysis of Pluvial and Fluvial Flows

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Added on 2022/09/18

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This report provides a comprehensive analysis of catchment modeling, specifically focusing on the Blackwattle Bay area in Sydney, Australia. The study investigates the impacts of pluvial and fluvial flood flows, emphasizing the importance of rainfall estimation, runoff estimation, and hydraulic modeling. The report delves into the complexities of pluvial flooding, which results from heavy precipitation, and differentiates it from fluvial flooding. It also highlights the various factors influencing runoff, including continuing losses (evaporation and infiltration) and initial losses (depression storage and interception). The report details the use of TUFLOW, a hydrodynamic modeling software, for simulating overland flow and discusses the considerations for sewer flow modeling. Furthermore, the report examines hydraulic modeling processes, including flow obstructions in floodplains, hydraulic roughness, and the impact of bridges and culverts. The methodology includes data collection from pluviometers, data analysis, and the application of modeling techniques to assess flood risks. The report is designed for peer review by urban catchment modeling specialists and provides detailed insights into the design and implementation of a catchment modeling system.

CATCHMENT MODELLING
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CATCHMENT DETAILS:
The Balckwattle Bay catchment is located in the inner suburb of the city of Sydney. This
catchment area is about 315 ha which drains its water into the catchment for Blackwattle Bay.
Another section of this catchment area is draining Sydney water´s main stalk drainage system
that is used to make the water flow in the upper region of the catchment. This flood´s reliability
is linked to the sloping nature of the land. Blackwattle is steeper at the upper section of the
catchment, it undulant in the middle part of the catchment but it is also flat at the lower section.

Table of content
Contents
CATCHMENT DETAILS:.......................................................................................................................2
Table of content.......................................................................................................................................... 3
Contents......................................................................................................................................................3
Introduction:................................................................................................................................................ 3
Rainfall estimation (Generation).................................................................................................................5
Runoff estimation........................................................................................................................................ 6
Continuing losses:................................................................................................................................... 6
Evaporation......................................................................................................................................... 6
Infiltration:...........................................................................................................................................6
Initial losses............................................................................................................................................. 7
Depression storage and surface wetting...............................................................................................7
Interception:.........................................................................................................................................7
MODEL BUILD..........................................................................................................................................7
Overland flow modelling:........................................................................................................................ 8
Sewer Flow Modeling:............................................................................................................................ 8
Hydraulic Modelling (Transport).............................................................................................................9
Modelling of Flow Obstructions in the Floodplain.............................................................................. 9
Hydraulic Roughness......................................................................................................................... 10
Bridges and Culverts.........................................................................................................................11
Bibliography...............................................................................................................................................12

Introduction:
For this research paper, we are looking into the effect of the pluvial and fluvial flows
within the Blackwattle Bay regions. Two key problems must be checked keenly in the process
of the modelling conceptual and this simulation ought to be carried out with respects to two
cases. Pluvial is a flooding type that's as a result of heavy precipitation. In most scenarios it
happens due to heavy rainfall, this heavy rainfall can lead to surface water runoff whose volume
is more than that of the city´s seepage framework. But in some few cases, this type of flooding
can result when the rainfall is not that heavy (Australia, 2011). It is exceptionally difficult to
predict pluvial than other flooding types such as fluvial because it happens in a brief period and
it can too pond before it forms surface runoff. Fundamentally pluvial flooding connects to direct
ponding or runoff before moving into a framework of drainage in this place.
In this project, it is possible to use the two scenarios that are waterway flooding, runoff
additionally surcharge that can be utilized as components of pluvial flooding (Kaufman, 2011). As
previously mentioned pluvial flooding because heavy precipitation can fall in regions such as
impervious and gardens areas such as pathways (Trun, 2011). In the area of pervious some
volume of water are prevented from infiltrating through the vegetation while others manage to
infiltrate but some will evaporate into the atmosphere and the remaining part forms the surface
runoff. A summary of the whole process discussed above is shown in the diagram below;

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Figure 1: Showing the processing of the movement of water after rainfall has occurred (Clarkson,
2017)
But as for the impervious area after the rainfall, all the water which is as a result of the
precipitation becomes surface runoff where there are only very few volumes of water which do
not form the surface runoff because they are blocked by the building as some are harvested
through gutters. And this is only applicable to the areas where there are buildings in this area.
The water volume which flows to the system of drainage will greatly be dependent on the
drainage system´s gutter intake volume (Grayson, 2013). But on occasions where there is heavy
rainfall, there is a high probability that a higher amount of water won´t flow to the drainage
system even though the underground is not saturated yet. The flood duration time in this
catchment area will fully depend on the slope of the catchment, the amount of rainfall which falls
and also the saturation of the soil before the rainfall.
Rainfall estimation (Generation)
As illustrated in this work pluvial flooding is essential because overwhelming rainfall and
we use pluviometer to collect the amount of water which are shown in Table 1 below. The
occasions of precipitation that leads to flooding within the catchment of Blackwattle Bay is

enlisted through a proximate gauge. These gages are located about 1 km within the coastal region
in this catchment area catchment. This gage offers a diverse sum of precipitation profundity and
intensity within the catchment area (Argue, 2012). To get the data from the Blackwattle Bay the
pluviometer was employed and the collected data helped in making several analysis and also
used for modelling as will be discussed below in other chapters. Even though the pluviometer
data provided will not be able to cover the entire period since there were some non- operational
times whose data were taken using the same gauge, but still, they will be very vital in our
modelling. The table 1 below illustrates the data and even of flooding taken from the Blackwattle
Bay catchment in Sydney city.
Table 1: Showing data of Events Identified from Annandale Gauge in the Blackwattle Bay
catchment.
The data collected from the table 1 above were collected from four pluviometer gauges. The use
of this gauge aids in the collection of very correct data which will be used in modelling. The
modelling done by the use of these data were very vital for these two types of flooding in this
catchment part of Sydney city in Australia.
Runoff estimation
For this part we will basically discuss the rainfall mount estimation which makes the
significant surface runoff and then flows to the impervious area, the surface runoff will hence
move to the drainage system as this is true as per the design of this catchment. But still, there are
ways which prevent the rainfall from ending up as surface runoff in the impervious and pervious

areas in this catchment (Burlando, 2009). And these are rainfall blocked by the plants, splits,
holding in depression etc. These losses are hence grouped into two groups like continuing losses
and initial losses. Some of the factors that are linked to these two types of losses include the type
of the soil, topography of the land, and the use of the catchment land and also the saturation of
the soil. These two types of losses can either be fully or partially distributed and they are
elaborated below;
Continuing losses:
This is a type of loss which involves a slow but continuous process after the rainfall and
it includes evaporation and infiltration. To approximate the significant runoff or rainfall this type
of loss is always subtracted at every stage from the hyetograph rainfall and this is done after the
initial loss has been deducted.
Evaporation
This is a continuous loss where water is lost from the pond, the water collected in the
pond after the rainfall heats up when then is solar radiation and /or moving wind which
transforms water ( liquid ) into water vapour ( air). And it is measured as mm/day, the
evaporation for the pluvial flooding is always ignored during the modelling and analysis.
Infiltration:
In this type of continuous loss, the rainwater passes / sips to the soil after a rainfall. For
this catchment, surface infiltration will be dependent on some factor such as the conductivity of
hydraulic, the capacity of the water on the surface after rainfall, type of the soil and the
saturation of the soil. This type of process commenced in the upper part of the soil due to two
factors; the upper soil is not saturated and due to the gravity, therefore, the upper part must have
water first. The process then proceeds to the lower part of the soil which is saturated as
compared to the upper part of the soil (David, 2012). And for obvious reasons, we will only use
the upper part filtration for modelling and ignore the lower (saturated region). The upper part
will witness a lot of infiltration hence it is easier to do modelling using this part as opposed to the
saturated part where the amount of infiltration is less.

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Initial losses
This is a type of loss which occurs at the early stages and it is in most times assumed that no
runoff is developed because there is lost in 2 main processes which are elaborated below;
Depression storage and surface wetting
This is mainly the volume in the depression on the surface of the ground and also the
inside surface layers of materials which are impervious which is saturated surface. The water
sored will hence get out through spillage, evaporation and infiltration (Wong, 2014). The storage
depression size is dependent on antecedent precipitation, surface type and surface incline. The
model has higher storage conceivable which is got via using an equation known as regression.
Interception:
Interception is the process of water collection and retention after rainfall, and it is
occurring through plants, higher buildings and higher mountain. This initial loss is unpopular
which records just about 20% of the entire rainfall every time. However, it is less for the
impervious area and in most occasions joint together with the depression stage for the drainage
modelled in the Blackwattle Bay (Wanielista, 2012).
MODEL BUILD
For this project we will employ the use of hydrodynamic modelling software named
TUFLOW, this software is applied for both hydrology and hydraulic. This software is better as it
can offer conversion of 1D to 2 D for the modelling of flow. The TUFLOW permits many
modelling as will be seen in the below chapters. The below modelling linkages of the processes
of generic which will be modelled from the data obtained above will be clearly seen. TUFLOW
program that is a linkage program is used in the below processes of generic modelling (Clarkson,
2017). Below are some of the 2D modelling screenshots from the data given above. The results
of modelling are obtained from the fluvial and pluvial flooding in the Blackwattle catchment in
Sydney Australia;
Overland flow modelling:
The overland flow analysis which is the city´s surface runoff after a rainfall can be
modelled using the TUFLOW program. Actually, this is a flow before the surface runoff moves

to the sewer systems (Wainwright, 2013). The first stage overland is known as runoff
concentration or simply conveyance. The conveyance stage is where the runoff is generated
before it enters the sewer system. While the second part is the sewer flooding where the surface
runoff moves from surcharged. During the modelling these two stages are treated differently
even though they are highly related. And its modelled form from the TUFLOW program can be
illustrated using the diagram below;’
Sewer Flow Modeling:
This is a serious part of modelling which is done in some parts of the world but it is not
conducted in any area in Australia and being that Blackwattle Bay is part of Australia it will not
be done in this catchment (Hromadka, 2017). This modelling is not conducted in Australia
because there are different systems of the sewer and the drainage system. The surface runoff
moves into the sewer system through gullies manhole. These two systems are isolated in a way
where the wastewater moves minus the stormwater or the dry-weather flow. Dry weather flow
can be realized through the analysis of the flow record and it can result in additional input to the
model of sewer combined system.

Figure 2: Showing sewer flow modelling (Australia, 2011)
Hydraulic Modelling (Transport)
This modelling process includes other three processes which are Bridges and culverts, modelling
of flow obstruction in the floodplains and Hydraulic Roughness.
Modelling of Flow Obstructions in the Floodplain
In this modelling process, the boundary fences property is viewed to be colour bond type
of fencing which would always fall in the hydrostatic load since the floodwater build up against
the fence thus it does not very clear during modelling (Begum, 2017). Although the creation of the
ballast on the railway is porous so their capability to convey the flow of flood through the
formation is seen as low. For this reason, they are treated as an obstruction to the floodwater in
the overland that is flowing in the rail corridor in the orientation of west to east. And this is done
in the TUFLOW and the result is given in the diagram below;

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Figure 3: Showing Modelling of Flow Obstructions in the Floodplain (Mannina, 2018)
Hydraulic Roughness
This is a very significant process to be modelled for the Blackwattle Bay catchment area.
This process of modelling is conducted in 1D which is then transformed into 2 D as that
mechanism is fully supported by the TUFLOW program as discussed earlier. 2 D model is
employed for values of roughness that are based on land use (Mannina, 2018). While the 1 D
model is required to be estimated based on the observation done in the channel and also channel
type knowledge. But all these modelling before they are completed are converted to 2 D for
easier visualization. The representative concept of the average roughness on every block for the
obstruction to a flow which is very smooth like other structures like buildings and fences can be
illustrated. Diagram below shows parts of this modelling using TUFLOW;

Figure 4: Showing Hydraulic Roughness modelling (Mannina, 2018)
Bridges and Culverts
The modelling of the bridges and culverts are also done in 1 D reach object containing
the parallel 1 D weird object with paths of overflow during higher flow. The handrails and
footbridges are assumed to be fully blocked if the space in bars is about 150 mm. When the
modelling in 1 D is complete it is transformed into 2 D through the TUFLOW model. And this
model can be seen from the diagram below;
¨
Figure 5: Showing Bridges and Culverts (Mannina, 2018)
Bibliography

Argue, J., 2012. Storm Drainage Design in Small Urban Catchments: A Handbook for Australian Practice.
1st ed. Sydney: Australian Road Research Board.
Australia, I. o. E. A., 2011. Australian Rainfall and Runoff: A Guide to Flood Estimation. 2nd ed. Sydney:
Institution of Engineers, Australia.
Begum, S., 2017. Flood Risk Management in Europe: Innovation in Policy and Practice. 2nd ed. Brisbane:
Springer Science & Business Media.
Burlando, P., 2009. Rainfall in Urban Areas: Selected Proceedings of the 5th International Workshop on
Rainfall in Urban Areas. 2nd ed. Melbourne: IWA Pub.
Clarkson, C., 2017. Lithics in the Land of the Lightning Brothers: The Archaeology of Wardaman Country,
Northern Territory. 2nd ed. Perth: ANU E Press.
David, B., 2012. Peopled Landscapes: Archaeological and Biogeographic Approaches to Landscapes. 1st
ed. Perth: ANU E Press.
Grayson, R., 2013. Spatial Patterns in Catchment Hydrology: Observations and Modelling. 2nd ed.
Wollongong: CUP Archive.
Hromadka, T., 2017. Flood Risk Management. 2nd ed. Sydney: BoD – Books on Demand.
Kaufman, M. M., 2011. Urban Watersheds: Geology, Contamination, and Sustainable Development. 1st
ed. Perth: CRC Press.
Mannina, G., 2018. New Trends in Urban Drainage Modelling. 2nd ed. Sydney: Springer.
Trun, J., 2011. Pluvial flooding analysis. 2nd ed. Sydney: Springer .
Wainwright, J., 2013. Environmental Modelling: Finding Simplicity in Complexity. 2nd ed. Perth: John
Wiley & Sons.
Wanielista, M., 2012. Stormwater Management. 1st ed. Sydney: John Wiley & Sons.
Wong, F., 2014. Australian Runoff Quality: A Guide to Water Sensitive Urban Design. 2nd ed. Sydney:
Engineers Media.