The RTK method is used to generate a hydrograph based on precipitation data. It forms the hydrograph by combining triangular hydrographs from three components of flow:

• Rapid inflow
• Moderate infiltration
• Slow infiltration

A typical RTK hydrograph is shown below. Q1, Q2 and Q3 refer to the three components of flow which must be summed to determine the flow.

A Typical RTK Hydrograph

For information on the theory behind this method, see "RTK Methods" . RTK methods are described further in the wet weather flow chapter of "Wastewater Collection System Modeling and Design," available from Bentley Institute Press.

The RTK method is most appropriate for determining RDII (Rainfall Derived Infiltration and Inflow) to sanitary sewers. It treats the system between the rainfall and flow in the sewer as a black box which can be represented by the three parameters R, T and K. In storm sewer systems, most of the flow moves over the surface and enters the sewer at known locations; a method such as the SCS hydrograph method is more appropriate in these systems.

The RTK method requires that the storm data be specified as a hyetograph, not simply a peak intensity. The resulting hydrograph from the catchment will have time steps equal to the time step size in the hyetograph. For example if precipitation is specified in 0.25 hr increments, flow will be calculated in those increments.

For more information about RTK methods, see:

• "Assembling RTK Parameters"
• "Creating an RTK Table and Assigning it to a Catchment"
• "RTK Tables Dialog Box"
• "RTK Methods"-448