Visual OTTHYMO

3.5.4.2 Discharge

Manual Input

  • When users choose Unknown as the shape and Manual Input as discharge type, users need to give the table of Discharge-Storage Curve (Rating Curve).

It has pairs of discharge-storage values to describe the Discharge-Storage relationship of the reservoir (m³/s vs ha.m or ft³/s vs ac.ft). A maximum of 20 co-ordinates can be entered. The first set of co-ordinates must be 0,0. If the outlet to the pond is not at the bottom of the facility, the curve can have multiple 0 values for discharge as shown in the example below. In this example the reservoir would fill up to a volume of 0.100 ha-m before discharging any run-off.

  • When users choose a particular shape (Truncated Triangular Pyramid, Truncated Rectangular Pyramid, Truncated Wedge, Truncated Cone, or Elliptical Paraboloid) and uses Manual Input as discharge type, users need to give the table of Height-Flow Curve. Then, the software will calculate the Rating Curve table, where the discharge is read from the Height-Flow Curve and the storage volume at each height is calculated based on the shape parameters.

Discharge Type – Orifice

There are 2 types of Orifice to choose: Bottom and Side. Users also need to give Area and Height of the Unknown shape of reservoir. Based on the parameters of reservoir shape and orifice, the software calculates the Rating Curve table, where at different height of the shape the volume is calculated by using shape parameters and the discharge flow rate is calculated by orifice equations. Note: when the Shape is Unknown, the constant horizontal area is used for the shape volume calculation.

Bottom Orifice:

where, Co is the orifice discharge coefficient, O_Area is the orifice area, g is the gravity constant, and is the height above the bottom of the reservoir.

Side Orifice:

  • When the is not above Bottom Elev. Offset (i.e. the distance from the bottom of the side orifice to the bottom of the reservoir), the flow is 0.
  • When the is above Bottom Elev. Offset but does not exceed the side orifice crown:

    where BottomElevOffset is the Bottom Elev. Offset, O_Height is the orifice height, Co is the orifice discharge coefficient, O_Area is the orifice area, g is the gravity constant, and is the height above the bottom of the reservoir.
  • When the is above the side orifice crown:

    where BottomElevOffset is the Bottom Elev. Offset, O_Height is the orifice height, Co is the orifice discharge coefficient, O_Area is the orifice area, g is the gravity constant, and is the height above the bottom of the reservoir.

Discharge Type – Weir

Users need to give Area and Height of the Unknown shape of reservoir, when discharge type is Weir. Based on the parameters of reservoir shape and weir, the software calculates the Rating Curve table, where at different height of the shape the volume is calculated by using shape parameters and the discharge flow rate is calculated by weir equations. Note: when the Shape is Unknown, the constant horizontal area is used for the shape volume calculation.

Rectangular Weir

  • When the is not above the weir bottom (i.e. <= reservoir height – weir height), the flow is 0.
  • When the is above the weir bottom:

    where W_Width is the weir width, W_Height is the weir height, Cw is the weir discharge coefficient, R_Height is the reservoir height, and is the height above the bottom of the reservoir.

Triangular Weir

  • When the is not above the weir bottom (i.e. <= reservoir height – weir height), the flow is 0.
  • When the is above the weir bottom:

    where W_Slope is the weir slope, W_Height is the weir height, Cw is the weir discharge coefficient, R_Height is the reservoir height, and is the height above the bottom of the reservoir.

Trapezoidal Weir

  • When the is not above the weir bottom (i.e. <= reservoir height – weir height), the flow is 0.
  • When the is above the weir bottom:

    where W_Slope is the weir slope, W_Width is the weir width, W_Height is the weir height, Cwr is the weir discharge coefficient for the rectangular part, Cwt is the weir discharge coefficient for the triangular part, R_Height is the reservoir height, and is the height above the bottom of the reservoir.