Reservoir

Updated on 16 Aug 2022
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The Reservoir is used primarily for unsteady flows and relates the rate of water level rise to the net discharge. The Reservoir makes the assumption that the water levels are identical in all nodes attached to it in the same way as the Junction.In unsteady mode, it will ensure conservation of mass so that, for example, the overbank spills from a channel are accounted for and may drain back as the flood subsides

Data

Field in Data Entry Form	Description	Name in Datafile
n/a	Number of elevation/area pairs	n
Elevation	Elevation (mAD)	h
Plan Area	Surface area of reservoir at level h (m²)	A
Easting	Easting coordinate of reservoir reference point (not used in computations)	easting
Northing	Northing coordinate of reservoir reference point (not used in computations)	northing
Runoff Factor	Rainfall runoff factor	runoff

Theory and Guidance

The Reservoir is used primarily for unsteady flows and relates the rate of water level rise to the net discharge.

The Reservoir makes the assumption that the water levels are identical in all nodes attached to it in the same way as the Junction . In unsteady mode, it will ensure conservation of mass so that, for example, the overbank spills from a channel are accounted for and may drain back as the flood subsides.

When a steady solution is achieved, clearly there should be no outflow from or inflow to the reservoir if it is attached to the network by a single unit. Otherwise it will behave in the same way as a Junction where all the flows sum to zero.

Equations

The conservation of mass equation:

reservoir1

(1)

where:

reservoir2

(2)

and:

h = water surface elevation

Dt = timestep

N = number of inflows

qi = flow at node i

A = surface area of the reservoir

To ensure a level water surface in the reservoir:

h1 = h2 = ... = h

(3)

General

There is no constraint on the maximum number of nodes attached to a Reservoir. Additionally, up to four Lateral Inflow nodes can be attached. These may take the form of inflows or direct rainfall/evaporation. A rainfall/evaporation boundary (REBDY) may be connected either directly to the lateral inflow node or via a Lateral Inflow unit, which is in turn connected to such a node. A runoff factor is available to enable rainfall impacting on a dry area of the reservoirs to contribute to its volume. The volume will be calculated using the formula:

dV = rainfall × [(wet area) + (runoff factor) × (dry area)]

A reservoir must always have at least one regular (i.e. non-lateral inflow) node attached, which may be a dummy unit (e.g. a QTBDY with zero flow) if the only 'real' inflow/outflow is via Lateral Inflow / REBDY units.

If large reservoirs are used in steady computations, the model can take a long time to converge. Small reservoirs can be prone to instabilities at large timestep sizes. These can often be avoided by reducing the timestep.

If boundaries are attached to reservoirs, the convention is adopted that outflows have a negative sign and inflows have a positive one. It is not possible to attach a reservoir to a junction directly.