Stormwater Master Plan Development Employing an Integrated 12/2D model for the Redwood Basin in Josephine County, Oregon

Ryan Retzlaff and Anthony W. Kuch


The use of two-dimensional (2D) stormwater models has increased over the years primarily due to advances in software, hardware and the availability of surface data. These advances allow for robust and diverse applications including basin wide stormwater assessments. Stormwater master planning is a critical element for the development of state and local agency comprehensive plans, capital improvement projects, and associated budgets.

This study presents an application of an integrated one-dimensional/two-dimensional (1D/2D) model used for stormwater master plan assessment. When a stormwater conveyance system is under capacity, a one-dimensional (1D) schematization alone cannot accurately model flow once it surcharges out of the 1D elements. The linkage of 1D and 2D models allows water to move out of a 1D element then return back to the same or another 1D element. This complete description eliminates model losses due to surcharging and inaccurate 1D description of surface storage. For example, flows leaving an irrigation canal, defined with cross sections in 1D, can flow overland via the 2D grid cells to a downstream irrigation canal or stream.

The 1D/2D model developed for this study consists of 979,627 square grid cells each representing a surface area of 100ft2 (9.29m2). The 2D grid is integrated with 1D elements which include 5.8mi (9.3km) of large irrigation canals, 4.22mi (6.8km) of creeks and 5.44mi (8.8km) of open channels. These integrated 1D elements exchange water with the 2D model via 1D/2D Interface Boundaries placed within the model approximately along the top of bank.

The results of this comprehensive hydrologic and hydraulic analysis include the discovery of overland flow routs between open channel elements during surcharge and 2D map results which include depth, volume, and the spatial extent of flooding. This paper discusses the integrated 1D/2D model built for Josephine County which provides a powerful assessment of the stormwater infrastructure and subsequent Capital Improvements Plan (CIP) developed, based on the model results.

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