4 M ODELLING A PPROACH
4.3 Network Representation
1- Model Selection
2- Software selection
3 – Network Representation
Project Definition
Skeletonization
4 – Data Assimilation &
Data Handling
Drawings &
Schematics
Documentation
Site Visits
5- Model Construction
Variable Assignment
& Data Input
Regression
Hydraulic Calculations
6-Model Calibration, Verification &
Application
7- Analysis & Display of Results
4.3.1 Project Definition
The project definition process was initiated by a data surveying phase. Data surveying was conducted in order to define the boundaries of the scope of the project. During this phase, the necessary and available data, for use within the model, was identified and compared. Data acquisition was subsequently initiated using a top-down approach, thereby decomposing the entire system into more specific subsystems. The macroscopic data, which includes the Western Aqueduct (WA) layout and topography were initially collected in order to completely understand the extents of the WA system, and its effect on the projected trajectory of the study.
The acquisition of the microscopic data involved meetings with the EWS consultants that were tasked with the design, implementation and construction of the system, in which the system together with other related background material were discussed. This was seen as necessary in
63 order to ensure that no misunderstandings would undermine the quality of the study, thus avoiding time-consuming errors. These meetings were also essential to ensure that the expectations of EWS was aligned to the projected outcomes of the study.
4.3.2 -Skeletonization
Umlaas Road Reservoir
Ashley Road BPT (20Ml)
Wyebank Road BPT (20Ml)
Emberton Reservoir Tshelmny
ama Reservoir
Haygarth
Reservoir Abelia
(Kloof) Reservoir Jerome
Drive Reservoir
Mount Moriah Reservoir Wyebank
Reservoir
KwaDabe ka1 Reservoir
Ntuzuma 5 (NR5) Reservoir
Lumped Demand
0.53 km 1400 mm 3.30 km 1400 mm
1.06 km 1400 mm
1.14 km 1400 mm
3.26 km 1400 mm
0.24 km 1400 mm 0.43 km 1400 mm
2.06 km 1400 mm
8.00 km 1400 mm 20.01 km 20.01 km
Figure 21 - Skeletonized representation of the WA.
The skeletonization was carried out according to the procedure outlined in Section 2.4.1.3.a.
The aim of the skeletonization procedure was to simplify the modelling and calculation procedure without significantly affecting the accuracy of the model results.
With a complete understanding of the requisite outcomes of the project, it was decided that the pressure profile of the trunk main ‘central spine’ (Figure 21) would be necessary, yet in order to ensure that the accuracy of the pressure profile, the mass balance of the system would have
64 to be conserved. This was achieved by ‘fixing’ the value and location of offtake draws, allowing for the dynamic losses within the trunk mains to be accurately calculated while accounting for the change in flowrate due to each offtake draw.
The ‘fixing’ of offtake flows was achieved through the assumption that the offtake flows (to reservoirs) can be accurately represented by the ‘characteristic flow’ – the flow that occurs in a reservoir intake line when its valve or pump is activated. The characteristic flow was calculated by the procedure outlined in Section 4.4.3, by examining level-time graphs for each reservoir, allowing for simultaneous outflow to consumers. The reservoir offtakes after the Ashley Road BPT were considered individually, as the impact of their drawing schedules, which is determined by their inlets switching at upper and lower levels, in response to varying consumer demands, were projected to have an appreciable impact on the operation of the BPTs.
In the case of reservoirs supplied from the aqueduct between Umlaas Road and the Ashley Drive BPT, these were lumped into a single, static demand draw at the midpoint (see Figure 21). The value of the static (constant) demand draw was calculated as the sum total of the average consumer draws on each of the included reservoirs, for the specified year. This was deemed as pragmatic, as these early draws are relatively small and the focus of the study leans more heavily towards the performance of the BPTs.
By eliminating the consideration of pressure profiles within the offtake branches and fixing the value of offtake flows as each reservoir draws, the trunk main’s pressure profile is thus decoupled, allowing for its determination directly through the hydraulic calculations and a trunk mains and offtake mass balance, rather than through the solution of a highly-coupled, network-wide pressure balance that is expected to only marginally improve calculation accuracy for the desired pressures.
The system was then inspected to confirm that the fundamental structure of the system remained unaltered, and that no significant portions of the distribution system were unaccounted for. This was followed by the construction of a flowchart of the skeletonized system, in order to understand the outstanding data requirements, and formulate a basic mathematical strategy to represent the system. The skeleton flowchart also served the purpose of providing the foundation to begin the evaluation of solution strategies and the assessment of the impact of the accuracy of information and approximations on the results of the model. The outstanding data requirements were then requested from the relevant parties (data types and sources are presented in Table 6).
65 Table 5 - Data sources, their estimated impact on the model, their estimated accuracy, and recommendations on their usage.
Data Source Impact
on model Estimated
accuracy Recommendation(s) System
(macroscopic) Layout
EWS/Consultants High +++++ Adjustment only necessary if
changes to infrastructure are planned.
System Topology EPANET Model Results High +++++ - System
(microscopic) Layout
EWS/Consultants &
Construction Supervisor (Site Inspection)
High +++++ Adjustment only necessary in case of maintenance or changes to infrastructure are planned.
BPT Dimensions and Construction Plans
Construction Supervisor (Site Inspection)
High +++++ Adjustment only necessary in case of maintenance or changes to infrastructure are planned.
Pipe Lengths EWS/Consultants High +++++ -
Pipe Diameters EWS/Consultants High +++++ -
Pipe Materials EWS/Consultants Medium +++++ Factors based on this can be adjusted during model calibration or to accommodate for
scaling/corrosion.
Equipment (Valve No., Types etc.)
Construction Supervisor
(Site Inspection) High +++++ Changes may be necessary if valve types differ from initial plans.
Valve
Characteristics (Sleeve)
Manufacturer High +++++ -
Valve
Characteristics (Globe)
Competitor Manufacturer Medium +++ It would be prudent to update this if the information becomes available.
BPT Control
System EWS/Consultants High +++++ Can be adjusted for
optimization/planning purposes.
Reservoir Control Scheme
Inferences Based on Visit to EWS Pinetown Control Centre
High ++ Must be adjusted according to
planned control scheme.
Recommend increasing monitoring, automation and telemetry.
Reservoir Characteristic Draws
Calculations Based on Data Accumulated from Visit to EWS Pinetown Control Centre
High ++ Must be accurately measured and
updated. The system is not operational and accuracy may thus be compromised.
Reservoir Mean Demands and Peak Factors
EWS/Consultants High +++++ Only amend if more accurate
information is attained.
Area
Classifications for hydrograph development
Based on observed patterns vs those from Stephenson (2012)
Medium +++ Change if more accurate information is attained.
Reservoir Diurnal Consumption patterns
Calculated based on mean demands and peak factors, together with hydrographs from Stephenson (2012) and area classifications
Medium +++ Only amend if necessary.
66