Continuous Simulation Modeling
Posted on 17
Upgrading the Design Standard from Single-Event Hydrology to Continuous Simulation Modeling
If we put the calculator down ages ago and shifted to continuous simulation modeling, then why are pre-computer hydrologic models still used to estimate stormwater runoff in urban settings?
Long before we entered the 21st century, experts agreed that you need to factor in the entire hydrologic cycle to accurately calculate runoff from rainfall. Since then, there have been thousands of continuous simulation models developed in urban hydrology. So then why are so many stormwater designs still based on single-event hydrologic models like the Rational and SCS method?
Whether the goal is to prevent flooding and design failures, or to protect urban watershed and stream health, shouldn’t engineers avoid failed single-event methods that are inundated with assumptions and unify under continuous simulation modeling?
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Assumptions of Single-Event Hydrology
Single-event hydrology uses simplified engineering methods that were developed when we only had slide rule hydrology or electro-mechanical calculator to compute runoff and include a variety of inaccurate or inappropriate assumptions.
So what are those inaccurate assumptions?
- Runoff frequency is the same as rainfall frequency (i.e. the 100-year storm causes the 100-year flood).
- Soil moisture is always the same before a storm.
- Rainfall events are never back-to-back.
- Storm shape and rainfall distribution are always the same
But, as we know in the real-world, “always” and “never” situations are virtually non-existent; so let’s get back to reality, shall we?
Continuous Simulation Modeling
Continuous simulation modelers and engineers approach hydrology like a historian approaches history: using the past (and present) to predict the future. Continuous simulation modeling is based on data from real-world rainfall events, the entire hydrologic cycle, drainage size, and every aspect of land use. The most accurate models out there today can predict pretty much anything that happens before, in between, and after rainfall events, helping us design mitigation facilities and stormwater BMPs that actually work for a specific site.
By simulating flow and duration using multiple years of hourly (or-shorter) runoff data, continuous simulation modeling, when done correctly, can determine changes in soil moisture, evapotranspiration, and the three types of runoff: surface runoff, interflow, and groundwater flow.
By accounting for all the water all the time, continuous simulation modeling leads to fewer design failures and better solutions to stormwater problems.
Continuous Simulation Modeling vs. Single-Event Hydrology
So, how is continuous simulation modeling different from the Rational Method (Q = CIA), SCS curve numbers, and other single-event hydrologic methods? For one, continuous simulation modeling accounts for all of the major components of the hydrologic cycle based on the historical record. As such, fewer assumptions are required, providing more accurate hydrologic estimates.
Most notably, continuous simulation modeling can answer stormwater questions that single-event methods simply aren’t designed to handle. By simulating past rainfall events and present circumstances to predict runoff, continuous simulation is the future of stormwater modeling.
For more information on continuous simulation modeling, the use of time series data, and how to develop a simulation plan using the most accurate model, check out stormwater modeling luminary Doug Beyerlein’s webinar,
Talk the Talk! Introduction to HSPF Continuous Simulation