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Continuous Simulation: Make Stormwater Assets Great Again


by Tony Kuch, MSc
VP Client Services, xpswmm Product Owner

More than twenty years ago I wrote a Master’s Thesis about software tools that could be put together with EPA SWMM to create a toolbox for very long term continuous simulation for stormwater and watershed simulations.  I was inspired at the time by Dr. William James who was my advisor for that research.  At that time typical stormwater design and modeling (analysis) employed the rational method or a regional design storm approach.  Continuous simulation was not typically used even though we had such computational capabilities for about 20 years.

Fast forward to today and there is still too much use of the Rational method and design storms.  Perhaps they have their place in sizing a culvert based on conveyance or some onsite detention in the case of a design storm.  However, there is simply too much misuse since these methods ignore the physical processes that are occurring.  It is simply not appropriate to use such a method to design stormwater systems when storage is a significant component of the system or when there is sensitive downstream receiving waters to name just a couple cases.  The rational method produces a peak flow and using that flow value in design ignores the natural attenuation that would occur to storage effects in the system and can even result in under design.

I am happy to see in many locations the analysis and design of stormwater systems using continuous simulation.  For example, in Portland Oregon it is commonplace to use continuous simulation for analyzing the performance of Low Impact Development (LID) and combined sewer overflow mitigation.  This allows a much better representation of the physical system such as a changing infiltration rate and recovery of infiltration during dry periods. 

A few years ago where I live we hit flood stage in a few of our city’s creeks and small rivers.  But it was not any single day’s rainfall accumulation but rather a really wet month.  The back-to-back storms in the winter when the evaporation was low, the infiltration low and lack of surface storage is what lead to damaging flows.  Only a continuous simulation can capture this, since continuous simulation tools based on SWMM or HSPF model what happens during the dry periods.  This ability removes the agonizing antecedent moisture condition that must be approximated since it simulates it based on continuous meteorological input.  Want to really understand system performance for a 10% or 5% chance storm?  Just run a 50 year rainfall record or at a minimum run a “wet” year that contains a significant storm.  The excuses of no rainfall data or it takes too long to run or setup are no longer reasonable.  A second example is the performance of LID structures.  How much benefit is realized if a bio retention facility can manage the 6 month return period runoff volume but not return to an acceptable dry condition in time for the next storm?

In the case of sensitive downstream receiving waters it is becoming common to perform a “Hydromodification Analysis” using continuous simulation and a statistical assessment of the flow durations.  If your design simply addresses the addition of storage to reduce the peak flow to a predevelopment peak flow then your engineered system will produce much longer durations of flows that can impact downstream.  For example along the west coast and great lakes areas of North America it is becoming a requirement to perform a Hydromodification study to protect the cold water fisheries.  To simply “peak shave” to the predevelopment flow may produce long durations of flows that will cause stream bank erosion and habitat change.

My employment allows me to travel to many parts of the world and teach Hydrologic and Hydraulic modeling.  We interact with many engineers and learn about their methods and standards.  Unfortunately, too many places are requiring or mandating simple methods for Hydrology which is leaving us with significant downstream and receiving water impairment.  Consulting engineers seem required to follow very dated stormwater manuals from counties and cities; and while their designs may be compliant we still have poor quality lakes, creeks and rivers.  There is a lot of inertia to overcome to get the requirements to the state of the art.  We can and should do better.  Continuous simulation and Hydromodification are two keys for us to up our game and make our stormwater assets great again.

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