Storage Node - SRG

You can change between the Backward Euler reservoir routing methods (default) and the Piecewise-linear Integral in Scenario Options » Storages.

Storages are used to hold water for varying periods. They include dams and other reservoirs; weir pools; urban detention, retention or retarding basins; and natural lakes. In regulated river systems, storages control the supply of water to consumptive and non-consumptive users, and may also provide flood mitigation and environmental services. Typically, inflows to storages include stream flow from upstream catchments, rainfall over the storage surface area, recharge from groundwater, and runoff from the local catchment surrounding the storage. Outflows from storages include controlled releases and spills. Losses from storages include evaporation from the storage surface area and seepage to groundwater.

Controlled releases from a storage include discharge via regulated outlet structures such as gated spillways, valves, pumps and gates. The amount of water released is dependent upon downstream demands, storage operating rules and maximum and minimum release constraints. In river systems with ownership, releases are also influenced by owners’ shares within the storage and the ownership of the outlet capacity.

Spills via gated spillways are modelled by specifying a minimum release for the gated spillway as a function of reservoir level. Pre-releases for flood control may be modelled using either the minimum release functionality of the gated spillway or a minimum flow node, for more complex pre-release rules such as seasonal targets.

Uncontrolled spills occur when a storage fills above the minimum level of an un-gated spillway, or the capacity of the gates on a gated spillway to control outflows is exceeded. Uncontrolled outflow may also occur through an uncontrolled outlet such as an ungated pipe culvert and via leakage through the dam wall.

The modelling of the physical operation of storages in Source is described below. Other functionalities related to storages are described in other SRG sections; these functionalities include:

• Resource assessment and water allocation

• Ownership

  • of storage volume, inputs, losses, spills and outlet capacities

  • Internal spilling between owners

  • Internal ceding (transfer from one owner to another based on agreed protocols)

  • Local borrow and payback systems.

• Weir operation (where the routing of flows in the headwater is significant - ie long flat weir pools)

• Ordering

  • Storages in series (orders passed upstream and transfer between storages),

  • Storages in parallel (choice of supply storage, optimisation of order delivery).

• Water quality in storages.

The ability to model the physical behaviour of storages is essential for fulfilling one of the primary purposes of Source, which is to model regulated river systems.

Scale

This node, in common with all others, is treated as a point location even though the storage represented may have large dimensions. It can therefore be considered to be site scale. It is used at every model time-step.

Principal developer

eWater Solutions, Department of Primary Industries, New South Wales

Scientific provenance

The level pool approximation for simulating the attenuation and delay caused by a reservoir has been used from the earliest days of engineering hydrology (beginning of the 20th Century).

Version

Source version 4.1.1

Structure and processes

Assumptions:

• The solution technique used assumes inflows, loss and gain fluxes, and outflows are averaged over a model time-step. This is consistent with the approach used in other parts of Source, such as link routing;

• The storage reservoir is assumed to have a level pool; and

• Relationships between storage water level, volume, surface area, and outflows are defined in terms of piecewise linear, monotonically increasing coordinate sets.

Theory

Approach

Source uses an implicit (backward) Eulerian approach for link storage routing. It assumes that the flux averages over a time step are a function of the state at the end of the time step:

Equation 1

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We also use the same approach for reservoir routing. Source assumes that all reservoir variables are a piecewise linear function of the reservoir's volume. For a time step the usual applies:

Equation 2

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Where Out consists of a number of components:

• Nett Evaporation

• Seepage

• Release

• Spill

And where ε is the mass-balance error.

Release and spill are the sum of the water flowing downstream in all of the outflow paths. For each path we have two relationships:

Equation 3	Open
Equation 4	Open

So the flow down each path will be:

Equation 5

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The nett evaporation is:

Equation 6	Open
Equation 7	Open