The difference between Continuous and Single-Event Hydrology.

This article will describe the differences between Continuous and Single-Event hydrology, while giving examples of these methods and the software packages that utilizes them.

Overview

There has been significant development in stormwater design and hydrology methodologies over the best 50 years. Before this, hydrology calculations had to be performed on slide rule calculators, in an era where high-powered computer processors simply did not exist. Engineers got by with the best tools available at the time, however, these methods continue into the present day. To learn more about the evolution of “Slide-Rule Hydrology”, you can read the article here:

Single-Event Hydrology

The most well-known and widely used methods for calculating runoff, determining peak flows and designing facilities utilize single-event hydrology methods. Single-event hydrology methods make a few basic assumptions when determining these factors of flow and runoff. Examples of these methods includes the SCS Runoff Method, The Rational Method, and Hydrograph Methods.

1. Rainfall event always returns the same flood period.

2. Cannot account for long term infiltration.

3. The soil moisture conditions are assumed to be the same before every storm.

4. There is no account for back-to-back storm events.

As stated above, these were necessary assumptions that hydrologists and engineers needed to make in order to have calculations possible at all! Some of the basic equations include the following:

1. Q = CIA

2. Q = (P-0.2S)^2/(P + 0.8S)

3. I = 0.2S

4. S = 1000/CN-1

Basic Hydrographs can also be generated from these results.

Continuous Simulation Hydrology

Despite these methods being used for hundreds of years, hydrologists eventually developed superior methods utilizing the advancing technology of the times. Simply put, slide rules were no longer going to cut it!

In the 70’s, software developers began to develop more advanced methods utilizing the FORTRAN programming language. This method, known as HSPF (Hydrological Simulation Program - FORTRAN). HSPF is able to account for more factors in the hydrologic process than single-event methods. The improvements include:

1. Models the entire Hydrologic Cycle; all the water is tracked all the time.

2. Routes all three components of runoff; Surface Runoff, Interflow, and Groundwater

3. Accounts for soil type, moisture, and storage.

4. HSPF is a software engine that uses continuous simulation, and is built into all CCS software package.

Continuous simulation utilizes historic rainfall data to account for storms likely to occur in the future. The model runs for the entire historic period, noting changes in soil moisture conditions, saturation, porosity and flow between layers. An example of a continuous simulation software modeling project can be seen below.

We will use the software package WWHM-SWMM. Double click the icon to open the software.

Select a project location, which is required to utilize the 65+ years of historical rainfall data.

Enter the project landuse for the predevelop and mitigated scenarios.

Continuous simulation software reports a range of flows and flow frequencies for a project, as opposed to returning a synthetic hydrograph or peak flow. Hydrographs can however be constructed using continuous simulation, but the results are not synthetically generated based on an assumed peak flow and storm return.

Want to learn more? Watch the video down below.