Ponds, Detention and Sedimentation Basins

Ponds and Sedimentation Basins are simply basins or waterbodies (eg. ponds, retarding basins) reliant on the physical settling of suspended solids as the principal treatment mechanism and can reduce peak flows through detaining a volume of the runoff and allowing for slower release during and after the event. Other mechanisms associated with phytoplankton assimilation of soluble nutrients and ultra-violet disinfection are currently not included in the modelling algorithm.

MUSIC provides separate Treatment Nodes for Ponds, Detention Basins and Sedimentation Basins, which are modelled in exactly the same way as each other, but have different default k, C* and CSTR values (refer to Appendix G: Selecting Appropriate k and C* Values for advice on selecting appropriate k, C* and CSTR values for particular treatment measures). Neither the Pond or the Sedimentation Basin treatment nodes include provision to enter an inlet pond volume. The Pond node is similar to the macrophyte cell in a wetland, but with different default k, C* and CSTR values to represent their different water quality treatment behaviour. 

Pond, Detention and Sedimentation Basin Properties

The initial  properties dialogue box contains the parameters that describe the basic physical characteristics of the pond, detention basin or sedimentation basin. A description of each of the parameters is given below.

A conceptual diagram of the pond, detention basin and sedimentation basin properties in MUSIC is presented below:

Conceptual diagram of pond, detention basin and sedimentation basin properties. 

Inlet Properties

The Inlet Properties define the physical characteristics of the inlet section of the pond, detention basin or sedimentation basin. 

Flow is hydrologically routed through the pond, detention basin or sedimentation basin, based on the characteristics defined by the user. 

Low Flow Bypass

All of the stormwater that approaches the pond below the user-defined Low Flow Bypass amount (in units of m3/s) will bypass the pond. Any flow above the Low Flow Bypass (subject to the presence of a High Flow Bypass) will enter and be treated by the pond. 

High Flow Bypass

When the stormwater inflow rate exceeds the user-defined High Flow Bypass amount (in units of m3/s), only a flow rate equal to the High Flow Bypass (less that specified in any Low Flow Bypass) will enter and be treated by the pond. All of the stormwater flow in excess of the High Flow Bypass amount will bypass the pond and will not be treated.

Storage Properties

The Storage Properties define the physical characteristics of the main pond, detention basin or sedimentation basin.

Surface Area

Defines the surface area of the pond, detention basin or sedimentation basin in m2. The hydrologic routing analysis calculates the volume of water in storage during a storm event by multiplying the depth of water above the permanent pool in the pond or sedimentation basin by this surface area. 

Extended Detention Depth

Defines the depth of water above the permanent pool that must be reached in the pond, detention basin or sedimentation basin before flow starts to discharge over the outflow weir. The Extended Detention Depth is defined as a depth in metres. 

Permanent Pool Volume

The pond, detention basin or sedimentation basin has a permanent volume of water which does not affect the hydrologic routing of a storm event through the system. However, this permanent pool does affect the hydraulic detention time during a storm event, and so affects the treatment of contaminants that flow into the pond, detention basin or sedimentation basin. The Permanent Pool Volume is defined in m3 and is available for stormwater reuse abstraction. 

Initial Volume

This parameter is only relevant to the pond and sedimentation basin and specifies the volume of the pond or sedimentation basin at time-step zero (or prior to the model run). 

Exfiltration Rate

Exfiltration from the permanent pool of a pond, detention basin or sedimentation basin into the underlying soil can be modelled by defining the exfiltration rate (mm/hr). Representative exfiltration rates for different soil types are provided in the table below. The water that seeps from the permanent pool of the pond, detention basin or sedimentation basin is lost from the catchment, and cannot re-enter the system downstream. Contaminants in the water that is lost to exfiltration are removed from permanent pool, along with the exfiltrated water and are also lost from the catchment. Representative exfiltration rates for different soil types are shown in the following table. 

Evaporative Loss

Evaporation from the permanent pool of a pond, detention basin or sedimentation basin can be modelled by defining the evaporative loss rate, defined as a percentage of the daily Potential Evapotranspiration data contained in the Meteorological Template used to create the model. The water that evaporates from the permanent pool of the pond or sedimentation basin is lost from the catchment. Contaminants in the water that is lost to evaporation remain within the permanent pool.

The  button allows you to estimate the surface area of the sedimentation basin. Clicking it opens the Estimate Storage Properties dialogue box, which is explained further below. 

Outlet Properties

The physical characteristics of the outlet pipe and weir are defined in this section of the dialogue box.

Equivalent Pipe Diameter

Defines the equivalent diameter of the outlet pipe (this may not be the diameter of the pipe itself, but the equivalent diameter of its orifices). The outlet pipe is notionally set with an invert at the standing water level of the permanent pool. The Equivalent Pipe Diameter is defined in mm. The pond, detention basin or sedimentation basin dialogue box will display the notional detention time, based on the pond or basin volume, and the Equivalent Pipe Diameter.

Overflow Weir Width

Defines the width of the overflow weir for the pond, detention basin or sedimentatin basin. The overflow weir will only start to carry a discharge of water once the depth in the pond reaches the Extended Detention Depth defined above. The Overflow Weir Width is defined as a length in metres. 

Custom Storage-Discharge-Height Relationship

A custom pipe flow, weir flow and storage relationship can be specified to represent custom outlet and storage configurations for ponds, detention basins and sedimentation basins. The outflow relationships can either replace or add to the standard MUSIC outflows. More information on how to use the custom outflow and storage facility is available in this section.  

This button opens the Estimate Storage Properties dialogue box:

The parameters are the same as those for wetlands (refer to this page), with one exception:

Permanent Pool Volume

This value is calculated based on two parameters that were specified in the Properties for Sedimentation dialogue box - Permanent Pool Depth and Initial Surface Area.

There is an option to re-use water from the pond, detention basin or sediment basin, by specifying a demand. Refer to Water Re-use from Treatment Nodes for further information about this.

You can use the node water balance report to obtain basic information on the overall water balance at the pond, detention basin or sediment basin, including basic information on reuse at the node.  To do this, select node water balance from the list of available reporting boxes by right clicking on the node.

A reporting window will then be presented showing the water balances for various inflows and outflows at the pond, detention basin or sediment basin as shown below.

The  button can also be used to save data on the water re-use demand and supply at every time-step should that be required (see below).

It is possible to record flux data for the pond or sedimentation basin:

Refer to Fluxes for more information about fluxes.

The Notes button allows you to record any important details or assumptions for the pond or sedimentation basin (for example, you may provide an explanation of how the volume was calculated, or how the notional detention time was selected). It is good practice to provide notes of any important assumptions, for future reference by others using the model.

Advanced Pond, Detention Basin or Sedimentation Basin Properties

The advanced properties section (opened using the ‘More’ button) of the pond, detention basin or sedimentation basin displays the parameters that describe the hydraulic characteristics for the outflow structures, and the parameters that describe the treatment process in the pond, detention basin or sediment basin as shown below.

Orifice Discharge Coefficient

The low flow outlet from the pond, detention basin or sedimentation basin is modelled as a circular orifice with an invert at the notional standing water level of the permanent pool. Although a default value of 0.6 is adopted in the model, you can set any appropriate value between 0 and 1. More information is available from most hydraulics text books.

Weir Coefficient

The overflow weir carries a discharge when the water level in the pond or sedimentatin basin exceeds the Extended Detention Depth. The overflow weir is modelled as a sharp crested weir whose discharge equation is given by:

where:

Q         Discharge over the weir

Cw       Weir Coefficient

L         Overflow weir width

H         Height of pond above the Extended Detention Depth. 

The default value of 1.7 may be altered if desired.

Number of CSTR Cells

An infinite number of CSTRs would replicate the effects of plug flow through the pond, detention basin or sediment basin. MUSIC defaults to two CSTR cells for pond and one CSTR cell for a detention basin or sediment basin, however, as the shape of the system can vary markedly dependent on design, the number of CSTR cells that is required to represent the hydraulic efficiency of the design is dependent on that shape. Refer to Treatment Devices for more information. The follows parameters are used for a pond, detention or sedimentation basin:

• k and C* Values - The rate at which each contaminant is treated, and the background concentration for each contaminant will be different within a pond or sedimentation basin and different values should be adopted for each contaminant; and
  
• C** Values - Normally C** is less than C*. 

Note that for a sedimentation basin, the default values for k, C* and C** for each of the constituents are as shown in the diagram above. 

Threshold Hydraulic Loading for C**

Defines the threshold hydraulic loading below which the C** concentration is adopted in the first order kinetic model. The threshold is defined in m/year.