Assembling RTK Parameters

The RTK parameters are:

R - fraction of precipitation that enters the collection system for that component
T - the time from the precipitation pulse to the peak of that component of the hydrograph
K- the ratio of the time to peak to time to end of hydrograph for that component.

There is no theoretical method to determine R, T and K. They must be determined empirically for each system based on a comparison of a measured rainfall hyetograph with measured wet weather sewer flow.

In general, R will be higher for systems that have significant infiltration and inflow problems than tight systems. T will be larger for larger catchments and will be larger for slow infiltration than rapid inflow. K is usually on the order of 1.5 to 2.5. The sum of the R values for all components should be positive but less than 1. The default units for T is hours while R and K are dimensionless.

Determining R, T and K for a particular catchment (or group of catchments) and event involve constructing a model, then trying different values of R, T and K that, when combined with sanitary dry weather flow, match the measured system hydrograph.

In some cases, the RTK parameters may have been determined for a large area, say several square kilometers (square miles) but the model is being loaded based on catchments on the order of a few hectares (acres). In this case, the R and K values are likely to be valid but the T values may need to be decreased to reflect the shorter time of concentration of these smaller catchments.

It is best to calibrate the RTK method using several storm data.

Note: The word "infiltration" is used in two different ways in wet weather flow monitoring:1. It is the precipitation that seeps into the ground.2. It is the precipitation that seeps through the groundwater into collection systems. In Bentley SewerGEMS , generally the first definition is used. In the RTK method, the second definition is typically used.