Confluence node - SRG

Overview

Description and rationale

This node is used to represent a stream confluence where two streams join to become one. Typically, this represents the junction of two tributaries or a tributary joining the main river channel. The streams could be regulated or unregulated. The outflow from a confluence node is equal to the sum of the inflows.

Scale

Model operates at the site scale and at any time step Source is capable of using (e.g. daily).

Principal developer

eWater CRC.

Scientific Provenance

Established practice common to all link-node river models, of which Source is one of many.

Version

Source v4.3

Dependencies

Confluences require an upstream node and link for each inflow and a downstream node and link for the outflow.

Availability

Automatically installed with Source.

Structure & processes

In Source, a confluence node combines flows from two incoming links and outputs the result to a single outflow link. Inflowing streams can be:

1. both unregulated,
2. one regulated and the other unregulated or
3. both regulated.

Order phase

When modelling regulated systems the treatment of water orders at each confluence node needs to be specified for evaluation during the order phase (i.e. not relevant for unregulated systems). Water orders are propagated upstream and order propagation at confluences differs depending on whether none, one or both of the incoming branches are regulated. The two approaches available for modelling order propagation are heuristic (rules based) mode and optimised ordering (NetLP). The SRG entries on ordering (Rules-Based Ordering - SRG and NetLP - SRG) contain details on these approaches. However, this section summarises the options and parameters available.

• If neither of the upstream branches are regulated, then no orders are propagated past the confluence (for both rules based and optimised ordering).
• If only one of the upstream branches is regulated and the other is unregulated, the unregulated branch is treated an inflow for the purposes of estimating volume contributing to order fulfilment. Remaining order volumes are passed up the single regulated branch (for both rules based and optimised ordering). Only regulated branches are included in the network in the NetLP method. 
• If both upstream branches are regulated, a decision may be required about which branch to pass orders up. This will depend on where an order is directed, and whether the upstream storage can be reached by one path or by multiple paths. If orders are directed to a particular storage along a particular branch, the confluence node will just pass these orders up that branch. If orders can be supplied along multiple paths or by multiple storages, the confluence node needs to decide how to split the orders. When rules based ordering is being used, the methods of order propagation available are constraint based and harmony rules. Optimised ordering (NetLP) uses constraints. If NetLP is used and both branches are regulated, then the network is solved without regard to harmony rules or priorities - the network is solved according to the usual costs.
  
  

When both upstream branches are regulated in Rules based ordering:

• Orders are first passed up each upstream branch up to the amount of that branch’s projected minimum flow,
• Remaining order volumes are then assigned to each branch either using outright priorities (Option A) or harmony rules (Option B)
• Option A: Constraint Based Ordering
  • User specifies priorities on each upstream branch.
  • Remaining order volumes are then assigned to each upstream branch in priority order, up to its projected maximum operating flow.
• Option B: Harmony Rules
  • User specifies volume split as either a lookup table or as functions.
  • Remaining order volumes are assigned to each upstream branch based on the harmony split ratio, until all the order volumes are assigned or until the order assigned to one of the branches equals its projected maximum operating flow.
  • Order volumes that remain after harmony allocation are allocated to the other branch up to that branch’s projected maximum operating flow.
• If order volumes remain after the order volume assigned to each upstream branch equals its projected maximum operating flow then the orders are constrained, and there may be a supply shortfall (see Rules-Based Ordering - SRG and NetLP - SRG for details on how ordering constraints are handled).
  • For constraint based ordering, the remaining order volumes are assigned to each upstream branch based proportionally on that branch’s share of the maximum upstream flow.
  • For harmony based ordering, the remaining order volumes are assigned to each upstream branch based on the harmony split ratio.

For information on ownership at confluences, please see Ownership - SRG

For information on constituent behaviour at confluences please see Constituent Routing - SRG

Data

Input data

Details on data are provided in the Source User Guide.

Parameters

Parameters are as follow:

• Both upstream branches unregulated: No parameters
• One upstream branch regulated:
  • K: Recession parameter on unregulated upstream branch
  • Option to forecast unregulated branch from previous time step or current time step
• Both upstream branches regulated:
  • Option to split orders based on Priorities (i.e. Constraint Based) or Harmony Rules
  • When Constraint Based splits, Priority parameter on each branch
  • When Harmony Based splits, Lookup table for distribution OR function for distribution