How can we determine the design storm hyetograph

A hyetograph is a graphical representation of the distribution of rainfall intensity over time. For instance, in the 24-hour rainfall distributions as developed by the Soil Conservation Service (now the NRCS or National Resources Conservation Service), rainfall intensity progressively increases until it reaches a maximum and then gradually decreases. Where this maximum occurs and how fast the maximum is reached is what differentiates one distribution from another. One important aspect to understand is that the distributions are for design storms, not necessarily actual storms. In other words, a real storm may not behave in this same fashion. The maximum intensity may not be reached as uniformly as shown in the SCS hyetographs.

The frequency of the design storm could be assigned to one or more characteristic entities that define a hyetograph, namely:

  • maximum intensity over a time interval (Imax)
  • total depth (htot)
  • duration (D)
  • the couple depth-duration that gives the average intensity across the duration D. This value of the average intensity results from
  • the IDF curves.

A variety of methods able to generate design storm hyetographs exist in the literature. Veneziano and Villani (1999) suggest that most methods can be classified into one of the following categories:

  1. Specification of simple geometrical shapes anchored to a single point of the intensity duration frequency (IDF) curve
  2. Use of the entire IDF curve
  3. Use of standardized profiles obtained directly from rainfall records
  4. Simulation from stochastic models.
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