Tutorial 2 - Basics of XPRafts

XPRafts models are built to represent the channel/pipe network in an urban, rural, or partly urban catchment. The network is made up of links with a node at each end. Each node is at the downstream end of a sub-catchment that collects local inflows.

The minimum number of nodes required to represent a catchment network is one, and is located at the catchment outlet. In this case, the area of the node sub-catchment is equal to the complete catchment. Sub-catchments may be optionally split into two portions, usually representing separate pervious and impervious areas in an urban sub-catchment.

Storm infiltration can be represented by simple initial continuous loss estimates or infiltration determined using a comprehensive soil water balance module.

Nodes can contain the definition of a retarding basin. Individual sub-catchments can also contain multiple water sensitive urban design storage structures including on-site detention and retention facilities.

Computed hydrographs, based on an input storm of any duration, are estimated at each defined node. Additionally, channel routing estimates velocity, normal depth, and storage hydrograph attenuation.

Network Terminology

Each sub-catchment is divided into 10 sub-areas (called isochronal sub areas). These areas are based on equal times of travel to the sub-catchment outlet. Any sub-catchments may contain distributed water sensitive urban design structures, such as on-site detention tanks/ponds, roof water tanks, and/or infiltration/evaporation structures.

Network Terminology

A link, by XPRafts definition, includes an upstream node and a downstream node. The upstream node has an optional local sub-catchment attached. The upstream node can also include an optional retarding basin. Links routing is carried out between the upstream and downstream nodes. The flow entering the upstream node can consist of that from the local attached sub-catchment, any upstream channel flow (from upstream link), and any flow diverted from any upstream node.

The slope of the sub-catchment is described in the following figure, depending on whether the sub-catchment is at the top or at mid-way of the network. The channel slope in the channel routing module is the average bed slope of the channel between upstream and downstream nodes.

Link Node Network

XPRafts further divides the sub-catchments for the pervious and impervious portions into 10 isochronal sub-areas. In this way, large sub-catchments may be represented via the time/ area (slope) characteristics. By default, XPRafts adopts 10 equal sub-areas as shown in the following figure.

Isochrones

Retarding basins/detention basins are represented in XPRafts as defined stage/storage and stage/discharge curves. A retarding basin can optionally occur in any node in the network. The stage/discharge curve can be internally estimated using defined outlet structure characteristics.

When two storage structures interact hydraulically during a storm event, XPRafts automatically accounts for the varying stage discharge characteristics based on the relative levels in each basin.

Diagrammatic Representation of a Retarding Basin

A range of spillway conditions are available within a basin including standard weirs, collapsible weirs and direct stage/ discharge routing, and curves as indicated in the following figure.

Multiple Orifices

Inlets to basins can include multiple level entrances to provide hydrograph attenuation at various return periods.

Diagrammatic Representation of a Floodway

Channel Routing may be based on any of the following:

• Lagging of hydrographs between the link’s upstream and downstream node
• Muskingum routing directly inputting K and X values
• Muskingum-Cunge routing that automatically computes K and X values based on input channel x-sections and longitudinal characteristics