Reference & User Manual
|Last modified on: Tuesday, 17-Aug-10 12:40 CEST|
WRM web version: Reaches and the Reach EditorA well formed WRM network scenario (describing the topology, the individual reaches, and links to the nodes connected) connects all nodes defined in the scenario specific set.
Several of the nodes, and the reaches connecting them, can also be linked to one or more underliying aquifer to describe conjunctive use and interactions of surface and groundwater.
A reach connects two nodes hydraulically; it represents unsteady open channel flow in natural or man made channels. Reaches are assumed to be hydraulically homogeneous, their length should be less than the flow travel distance within one time step (i.e., reach length shoud nor exceed between 20 and a maximum of 40 km). If a length is too long or changes its characteristics without any structural (confluence, diversion) element to break it, a geometry or auxiliary node can be used to start a new reach with new geometric and hydraulic properties.
Reaches are abstract constructs: while they may represent real sections of natural or man made channels, they can also be of zero length to facilitate the combination of several nodes at the same location to obtain more complex behaviour.
The reaches are used to route the unsteady open channel flow using the Muskingum method; resistance, slope, length of a reach as well as a weight factor to describe the respective importance of inflow and storage within the reach are used.
Reach editorThe entry to the reach editor is again a listing of all reaches defined; any one can be selected for editing. A new reach can be generated for editing with the corresponding new reach button.
Once a reach is selected, the corresponding editor page shows its location in the network graph, and its properties that can be edited:
Reach propertiesA reach is defined by:
Speed of open channel flowFor the routing of flow through the river basin, we require estimates of speed. These are dynamically obtained from the reach geometry (length, slope), the cross-section (flow specific depth), obtained from the rating curve or in the simplest approximation, using the simple trapezoidal cross-section data, and Manning N as a coefficient describing resistence, in turn estimated from reach shape and bank material, vegetation, etc.
The simple estimator used is:
V = 1/M * Rh**0.67 * SQRT(SLOPE)where V is velocity in m/s, M is manning's n, Rh is Hydraulic Radius, approximated by hydraulic mean depth in m (good enough wherever width exceeds depth considerably like in most natural rivers), and SLOPE is the "energy gradient line" or head loss, approxinated by the slope of the channel bottom.
In terms of frictional head losses, the perimeter is important. Hydraulic radius, Rh,
Rh = A/Pwis defined as the area of the flow section divided by the wetted perimeter.
Reach GeometryTo estimate the magnitude of (optional) reach related processes, reach geometry has to be defined. This consists, alternatively, of:
Rating CurvesRating Curves constitutte again a separate Object Class, primarily associated with flow monitoring STATIONS.
A RATING TABLE or RATING CURVE defines the relationship between flow (in m3/s) and a depth reading for the segment - while normally used to derive flow data from level monitoring, in WRM they are used to obtain estimates of LEVEL from the flow calculated by the model. Together with the DEM and a georeferencing of reaches and their Cross-Sections, this can be used for the prediction and monitoring of flood conditions.
The values can either be tabulated as pairs of L (level or stage) and Q (flow, discharge pairs), or expressed by an exponential curve of the form:
Q = aL**bwhere Q is flow, L is level, and a and b are coefficients to be estimated from sets of measurements. Please note that L can either be absolute, i.e., in masl, or relative starting at 0; in the latter case, a reference elevation for the rating curve in masl (meters above sea level) must be specified.
Quoting the NOAA definition:
A rating table or curve is a relationship between stage and discharge at a cross section of a river. In most cases, data from stream gages are collected as stage data. In order to model the streams and rivers, the data needs to be expressed as stream flow using rating tables. Conversely, the output from a hydrologic model is a flow, which can then be expressed as stage for dissemination to the public.