Dumbing Down Hydrology - Part 3
Written by Doug Beyerlein, P.E., PH, D. WRE is with Clear Creek Solutions in Mill Creek, WA.
What Is Needed
What the stormwater engineering profession is lacking is not intelligence; it is education. I have met very few stormwater engineers who have taken a hydrology course in college. It is not the engineers’ fault; it is the fault of our engineering colleges and universities. While many teach drainage design, few teach continuous simulation hydrology modeling. There is a difference. Drainage design typically focuses in the pre-computer single-event runoff calculation methods that I rail against above. That is hardly what is needed in today’s complex stormwater world.
Many engineers first learn about continuous simulation hydrology modeling through workshops and training seminars. While typically these are good venues for learning continuous simulation modeling, they are too brief and too infrequent. As a result, many engineers simply try to wing it. They assume that because they know how to do single-event modeling that they can use the same techniques and knowledge to do continuous simulation modeling. That is like a car mechanic assuming that because he knows how to adjust a carburetor that he can also fix a fuel injection system. Nice idea, but not realistic.
The argument that we should stay with old methods because most stormwater design is being done by a general civil engineer, for whom the stormwater design is only about 10% of what he does, is a poor argument. If this work is outside of the general civil engineer’s area of expertise then that engineer is violating his or her professional code of ethics. This violation is specifically stated in ASCE Code of Ethics Canon 2: Engineers shall perform service only in areas of their competence.
This solutions to this potential professional code of ethics violation is to hire someone who specializes in this continuous simulation hydrology analysis and knows how to use the latest, most accurate techniques and modeling tools. A general civil engineer certainly does this when dealing with complex structural design problems. Complex stormwater design problems are no different. (Although it should be noted that when a structural design fails it is pretty obvious, when a stormwater design fails we claim that the facility was not designed for that size of event - a convenient excuse, but one that will eventually fail; see my lawyer comment below.)
The other missing piece in what is needed to advance stormwater design and management into the 21st century is updating the stormwater engineering standards and regulations. Many; if not most, of these standards and regulations were written in the 1950’s and ‘60s when the focus was on flood control and the computational tools were limited to slide rules and nomographs (“a graphical calculating device in the form of a two-dimensional diagram designed to allow the approximate graphical computation of a function,” in case you have never seen one). We have come a long way since then, but our standards and regulations have not always kept up.
During my 40+ years in the profession I have met many federal, state, and local regulators and technical staff. And for five years I was one of them in the Seattle metropolitan area. I know only too well how overworked and underappreciated they are. Typically stormwater regulations and standards are only one of their many responsibilities, and usually they do not have the technical background and education to make technical decisions that, in the end, drive the regulations and standards. They often rely upon us consultants to provide them with the technical expertise. And all too often they have received misguided and/or just bad advice all in the name of keeping things simple.
An example of this “keep it simple” approach is the idea of containing onsite the runoff from small storms up to some percentile (85th or 95th). This is based on the idea that for predevelopment conditions there is no stormwater runoff from these smaller storms. That assumption depends on a number of factors - the most important being the soil type and antecedent soil conditions (infiltration) and vegetation (evapotranspiration). The goal is then to provide for a developed land-use condition sufficient storage onsite to prevent runoff from these small storms. It is an admirable goal, but how does it really work?
With land-use development comes impervious surfaces and the loss of both infiltration and evapotranspiration opportunities. Now the engineer is tasked with providing sufficient storage to compensate for these losses. The question is: Once you provide this storage, where does the water go? If you leave it sitting in a pond or a vault with little chance to infiltrate into the soil or evaporate into the atmosphere, what happens when the next storm arrives? If you discharge it into the stormwater system, what rate of discharge is acceptable, and who decides? And if you discharge it into the stormwater system, then you are not really mimicking the predevelopment runoff conditions.
You discover these problems only when you are forced to account for all of the water all of the time, and that is what continuous simulation hydrology modeling requires you to do.
So what is the solution when it comes to setting stormwater standards and regulations? Just as there is a saying at cross-walks, “Look before crossing,” there should be an equivalent saying in stormwater: “Model before regulating”. Spend the time and money to find out what the real-world impacts are going to be before setting a standard that may sound good on paper but makes little or no sense in the real world. Think of it as regulatory insurance. You spend a little up front to save you from a world of pain later on.
Some may argue that there is no harm in using these “tired and true” old methods used by our grandfathers. They argue that these methods worked then and they work now. But have they ever really worked?
In the Puget Sound region, King County (the Seattle metropolitan area) found that stormwater control facilities designed using SCS curve number methods consistently failed in urban and suburban communities. The SCS-based facilities were undersized and discharged too-large flows that caused down-stream erosion and property damage (Booth and Jackson 1997). King County’s solution was to change to a standard (flow duration) and method (continuous simulation hydrology) that provided the actual protection specified by the stormwater regulations (Jackson et al. 2001).
This standard (flow duration) and method (continuous simulation hydrology) were incorporated into the Washington State Department of Ecology’s regulations for the 19 counties of Western Washington (Ecology 2005) and the development of the Western Washington Hydrology Model (WWHM). (In the spirit of full disclosure I should note that I, as an advocate for continuous simulation hydrology and a practitioner of it for over 40 years, strongly encouraged the Department of Ecology to convert from a single-event standard to a continuous simulation standard for its stormwater regulations and later assisted in the development of WWHM.)
And that is the final problem with dumbing down hydrology for engineers. What if, by doing so, you are not getting the actual protection specified by the stormwater regulations? Can you hide behind the law and claim that failure was an Act of God? Maybe. But, knowing lawyers, it is my prediction that it is only a matter of time before some young, savvy environmental attorneys realize the potential liability caused by state and local municipalities not fully implementing the newest and best technology and standards to meet the Clean Water Act NPDES permit requirements and file class action lawsuits on the behalf of flood-damaged homeowners. Win or lose, it is going to be a costly and unnecessary battle.
Conclusions
It is time we stop dumbing down hydrology for engineers. Years ago we traded in our slide rules for computers. It is time to trade our slide rule-based, single event hydrology methods for computer-based continuous simulation hydrology models and join the 21st century.
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.
