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 Drainage and Utilities, generally the first definition is used. In the RTK method, the second definition is typically used.