Dumbing Down Hydrology - Part 1
Written by Doug Beyerlein, P.E., PH, D. WRE is with Clear Creek Solutions in Mill Creek, WA.
Why are we dumbing down hydrology for engineers? When I say “dumbing down”, what I mean is that we (the stormwater profession) are telling engineers to use pre-computer, simple methods and techniques to size stormwater facilities. I recently asked a noted, national stormwater expert this question, and his response was (to paraphrase him), “We need to keep things simple because 99% of stormwater design is being done by a general civil engineer using somewhat dated models, or simple automations of older methods, for whom the stormwater design is about 10% of what he does.” That may be true, but it is hardly an appropriate excuse. Today in the United States we spend billions of dollars on stormwater management. For that much money, isn’t it important that we use the most accurate tools and methods available? So, why in most parts of the country are we still using pre-computer, slide rule hydrology to do our calculations?
Slide Rule Hydrology
What is slide rule hydrology? Slide rule hydrology is a set of simplified engineering methods to compute runoff from rainfall. This includes such as the Rational Method (Q = CIA) and SCS curve numbers. These methods were developed when the only computational tool we had to use to computer runoff was the slide rule of the electro-mechanical calculator. (An aside here for anyone under the age of 50 who doesn’t know what a slide rule is: A slide rule is an ingenious analog set of sliding scales that was invented prior to electronic calculators and personal computers to make complex engineering and scientific calculators. I used a slide rule in my engineering classes all the way through graduate school (1973). Today slide rules can be found only in museums and the desk drawers of old engineers.)
More than 100 years ago, hydrologists and engineers understood that runoff is a function of land cover, soil, precipitation and drainage area size. They knew that soil moisture changes between storm events and even during storm events as soils alternately get wetter and drier. The more saturated the soil, the greater the runoff. Precipitation is also highly variable. No two storms are the same in their volume, intensity, or duration. But when our only tool was the slide rule to compute runoff, we had to keep hydrology simple. We only had the computational power to calculate the runoff from a single event, and even then we had to make major assumptions about the physical mechanisms that convert rainfall to runoff.
These major assumptions include assuming that a specific storm return period produces the same return period flood (e.g., a two-year storm always produces a two-year flood). They assume a representative storm of a standard shape and volume (e.g., Type 1A storm) to compute runoff. They also include the assumption of a certain average soil moisture condition at the start of the rain event. All of these assumptions are built into single-event hydrologic modeling. These assumptions are bundled into the standard runoff coefficients for the Rational Method and standardized curve numbers for SCS-based methods.
These single-event methods, by their very nature, have no mechanism to simulate back-to-back storms, variable soil moisture conditions, or the effects of long-term infiltration and evapotranspiration. In the real world, a two-year runoff event. Real storms do not have a standard shape, volume, or length. And the soil moisture varies from one event to the next and even during a single event, depending on the climate, season, storm and soil type. Today no one uses a slide rule. So why are we still using hydrologic modeling methods designed for the slide rule? What is the alternative?
References
Booth, D.B., and C.R. Jackson. 1997, Urbanization of Aquatic Systems, Degradation Thresholds, Stormwater Detention, and the Limits of Mitigation. University of Washington. Published on the Journal of American Water Resources Association, Volume 33, Issue 5, pp. 1077-1090
Ecology. 2005, Stormwater Management Manual for Western Washington, Washington State Department of Ecology, Olympia, WA.
Jackson, C.R., S.J. Burges, X Liang, K.M. Leytham, K.R. Whiting, D.M. Hartley, C.W. Crawford, B.N. Johnson, and R.R. Horner. 2001. “Development and Application of Simplified Continuous Hydrologic Modeling for Drainage Design and Analysis.” King County Department of Natural Resources, Water and Land Resources Division. Seattle, WA. Published by the American Geophysical Union in Land Use and Watersheds: Human Influence on Hydrology and Geomorphology in Urban and Forest Areas, Water Science and Application, Volume 2, pp. 39-58.
