Any advice on how to use the hydrograph?

August 26th 2024

Problem: What data do I need to model a debrisflow using a hydrograph?

Answer: You only need the hydrograph for the debris flow itself, not for the river flow from before or after the event. RAMMS can only predict the runout of debris flows (not the flood flows before or after the debris flow).

1. The easiest way to run RAMMS using an input hydrograph is to start with an estimate of the total volume of the debris flow, which you may be able to estimate using other engineering methods or based on estimates from previous events in the area. When you enter this into the 3-point hydrograph table in the “Release Tab”, RAMMS will then propose a maximum discharge based on a semi-analytical relationship in the literature (as described in the handbook, from Figure 2 in Rickenmann (1999, Empirical Relationships for Debris Flows).
2. If you only have an estimate of the maximum discharge, but not the total event volume, you could use the Rickenmann (1999) curve (mentioned above) and your maximum-expected discharge value to estimate the total debris-flow volume. You could do this either with the graph in Rickenmann (1999) or you could do this iteratively in RAMMS by adjusting the total volume until it matches your predicted maximum discharge value. This will give you the standard three-point hydrograph.
3. If you have information that you trust regarding the expected duration of the peak discharge, you could also manually edit the hydrograph table.

As a reminder, the hydrograph volume in RAMMS (as well as the block release volume) is the sum of both the water and the sediment in the debris flow, not the pure-water discharge, so depending on the methods you are using, you may need to increase the volume of the flow to include the sediment in the flow, depending on your expected sediment concentration. To a first approximation, debris flows are roughly half water and half sediment (by volume), but you may already have a better estimate of the expected properties of the debris flows you are modelling.