Two-Dimensional (2D) Approach to Hydrologic Modeling

With advances in computing power, two-dimensional (2D) unsteady flow models are increasingly being used for open channel hydraulic analyses. However, less common is the use of 2D techniques to simulate a watershed’s (losses, runoff, and flow transformation) response to an extreme rainfall event. 2D hydrologic modeling has advantages over conventional (lumped and semi-distributed) hydrologic models (e.g., HEC-HMS). 2D models are physically-based, making it more flexible in modeling hydrologic and hydraulic responses to rainfall events of various magnitudes, intensities, spatial patterns, and temporal patterns. While calibration is important, the modeling approach provides greater accuracy in simulating hydrologic and hydraulic responses throughout the watershed. Conventional hydrologic models commonly use a representative lumped response (Unit Hydrographs) to transform runoff volume to a flow hydrograph, which assumes flow is proportional to runoff volume and hydrograph shape is constant for varying rainfall magnitudes. This linearity assumption is not entirely true when a Unit Hydrograph, even when calibrated at a gaged location, is applied to a storm with significantly different characteristics (e.g., magnitude, temporal patterns, etc.) than the calibration storm. In practice, generic non-linearity adjustments are made to the Unit Hydrograph ordinates (specifically peak timing and magnitude) that introduce an unknown level of inaccuracy. The 2D approach reduces concerns over the accuracy of generic non-linearity Unit Hydrograph adjustments. This paper presents a sample application of the 2D hydrologic modeling technique and how it can supplement and inform conventional models for improved accuracy, particularly for extreme events (e.g., Probable Maximum Flood).

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