In Ras help it's just said "The Hydrograph Output Interval is used to define at what interval the computed stage and flow hydrographs will be written to HEC-DSS. This interval should be selected to give an adequate number of points to define the shape of the computed hydrographs without losing information about the peak or volume of the hydrographs. This interval must be equal to or larger than the selected computation interval."
But,in fact i don't get the point.would you plz explain it easier or whit an example?
e.g. for computation interval, base on what "ras-help" has said, i choose 5min.
Is it ok to choose 20min for Hydrogragh output interval & 1hour for Detailed output interval?or these 2 components have specific way to choose?
The hydrograph output interval has several things you should consider to determine. First is that the increase in output interval and computation interval will be more resource intensive for the PC and therefore will increase the overall model run time. Second, typically an output interval is not less than the estimated time of concentration for the contributing drainage shed. Third, as in all modeling programs that include the dimension of time you need to ensure you have captured what you want to see. If you are looking for the peak discharge you need to ensure that your timestep is sufficient to capture this data and not skip it on either side and giving you a falsely lower value. All of these factors need to weighed by the modeler with the specific goal of the model in mind and a decision made on a case by case basis.
When looking at the time cycles of RAS computations and subsequent output, we should separate the computational aspects (solving the equations) from the output aspects (viewing the results).
The RAS computational time step (Computation Interval) considerations that do indeed affect the simulation results are governed by:
1. The stability of the computations.
2. The numerical accuracy of the computations.
3. The resolution of the input hydrographs.
Computational stability governs the computational time step for many applications. There are some text book situations where stability may be satisfied, but the numerical accuracy of the solution requires a smaller time step. Also, there are situations where numerical stability and accuracy may be satisfied by a relatively large time step, but the time scale of the inflow hydrograph (perhaps a dam break), may require a smaller computational time step so that the peaks will be captured by the computations. On the other side of applications, a tidal forcing function has a period of 12 to 24 hours. Such an input stage hydrograph can be adequately resolved at a one hour time step; indeed the stable computational time step for tidal situations often can be on the order of 10 to 30 min. However, resolution of inflow hydrogaphs and details of velocity and stages may require smaller computational time steps so that the output can be obtained at more frequent intervals.
RAS Output time steps controls. The concept here is that an unsteady flow model such as HEC-RAS can generate voluminous output, so the user gets to control what gets saved – where and how often. The output selections do not affect the unsteady flow computations.
As a first step in testing the operation of the unsteady flow application of HEC-RAS, one should just look at the hydrograph plots. The information sampled for these plots (Hydrograph Output Interval) should be equal to an integer multiple of the computational time step. The post-processing computations (Detailed Output Interval) may be done at a different (usually longer) time step than the hydrograph output. Again, integer multiples of the hydrograph output interval are necessary. Post-processing of a large unsteady flow simulation can take some time; that is why the hydrographs are available after the unsteady flow run. These hydrographs can be used for simulation diagnostics immediately after the run is finished.