Dumbing Down Hydrology - Part 2
Written by Doug Beyerlein, P.E., PH, D. WRE is with Clear Creek Solutions in Mill Creek, WA.
Continuous Simulation Hydrology
Continuous simulation models the entire hydrologic cycle. All of the water in the hydrologic cycle is tracked everywhere all of the time. This type of hydrologic modeling can’t be done with a slide rule. A computer is required for the needed calculations.
With the use of a computer the continuous simulation model represents all of the processes observed in the hydrologic cycle with the appropriate algorithms. The model routes the rainfall through the various storage components. of the hydrologic cycle: interception storage, shallow and deep soil moisture storage, and groundwater storage. Evaporation and transpiration return water back to the atmosphere from each of the storages at different rates as a function of vegetation and soil type. Continuous simulation models can track and route each of the three different components of runoff: surface runoff, interflow (shallow, subsurface runoff), and groundwater (or baseflow).
Continuous simulation modeling is not a new concept. This year is the 50th anniversary of the publication of the Stanford Watershed Model by Dr. Norman H. Crawford and Professor Ray K. Linsley Jr. The Stanford Watershed Model went through a number of iterations and refinements before becoming EPA’s HSPF (Hydrological Simulation Program - Fortran), which was first released in 1980 (36 years ago). Today HSPF is included in a number of continuous simulation hydrology modeling packages, including EPA BASINS, HEC-HMS, and WWHM. Some single-event hydrology models, such as SWMM, have been rewritten to do continuous simulation modeling.
Continuous simulation models provide more accurate hydrologic estimates that the pre-computer, slide rule, single-event models because fewer major assumptions are required.
No longer do we have to assume some standard soil moisture condition at the start of a storm event. Continuous simulation models track soil moisture changes, both between storm events, and how these changes vary the rainfall-runoff relationship with time.
With the use of continuous simulation models, no longer do we have to assume a standard storm shape, intensity, volume, and duration. Continuous simulation models use long-term, measured historic precipitation data, which include big storms, little storms, extended dry periods, and back-to-back major storm events. While we know that we will not see these exact storms in the future, we can be fairly certain that we will see the same general storm patterns and climatic conditions. If we think that in the future that climatic conditions and storm patterns will change intensity, volume, and/or duration, we have the computational tools alter the historic record to reflect these expected changes.
No longer do we have to assume that a specific storm return period produces the same return period produces the same return period flood. We know, just through observation, that if a two-year storm occurs when the watershed is already fully saturated then the resulting runoff will be much greater than if the storm occurs when the watershed is fully saturated then the resulting runoff will be much greater than if the storm occurs at the end of a long dry spell. This means that sometimes a two-year produces a 1.1-year flood (which has a 90% change of occurring in any individual year), and sometimes it produces a five-year flood (20% chance). Rather than making an assumption about frequency or return period, with continuous simulation models we can produce a long-term continuous flow record that we then statistically analyze to independently determine the appropriate flow frequency.
These are some of the advantages of using continuous simulation:
We gain more accurate hydrologic results and a better understanding of the important hydrologic processes that control local, regional, and national water issues.
We gain the ability to reproduce historic flood events and compare our modeling results with observed flow data, where such data are available.
We gain the ability to evaluate how flow control facilities behave over a full range of actual hydrologic conditions, not just a single hypothetical event.
And we gain the ability to produce multiple-year, long-term records to statistically evaluate runoff and streamflow in terms of magnitude, frequency, and duration.
Magnitude and frequency do not tell the whole picture. Duration (the percent of time that a particular value is exceeded) is now recognized as an important flow statistic. Duration tells us how long the runoff is at or above a specific value. When it comes to stream-defining mechanisms such as erosive flows, the number of hours (or percent of time) that the flows are large enough to cause erosion is critical to the management of the riparian system. If the flow control facility maintains the existing flood frequency (in other words, the two-year flood does not increase), but the duration of erosive flows increases then the facility is not doing its job.
The common complaint against the use of continuous simulation hydrology is that it is too data intensive and too complicated for the average stormwater engineer to use. I don’t see other professions saying “Don’t include complex computer calculations in the design of our product because they will be too difficult to understand".” Structural engineers are smart enough to know how to include dynamic loads in building and bridge designs. And automotive engineers are smart enough to know how to make use of computer-controlled fuel injection in today’s cars. From what I have observed in my 40+ years on the profession is that stormwater engineers are just as smart.
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.
