Storm Water Master Plan development employing an integrated 1D/2D model for the Redwood Basin in Josephine County, Oregon - Case Study
The use of two-dimensional (2D) storm water models has increased in recent years primarily due to advances in software, hardware and the availability of surface data. These advances allow for robust and diverse applications including basinwide storm water assessments. Storm water master planning is a critical element for the development of state and local agency comprehensive plans, capital improvement projects, and associated budgets.
The Josephine County Public Works Department in Josephine County, Oregon, used an integrated onedimensional/two-dimensional (1D/2D) model for storm water master plan assessment. When a storm water conveyance system is under capacity, a 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 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 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.
Josephine County’s Model
The integrated 1D/2D model built for Josephine County was xpswmm, a hydrologic and hydraulic modeling software package provided by XP Solutions.
The initial model build for this study included only 1D elements. The catchment was divided into 177 sub-catchments, runoff from these catchments was described using the Runoff method and the GreenAmpt infiltration method. The runoff method is an industry standard routing method utilizing the catchment width and slope to characterize the catchment and determine the runoff from there. The Green Ampt Infiltration Method is utilized to predict the infiltration. The average capillary suction, initial moisture deficit and saturated hydraulic conductivity are the primary input elements for the Green Ampt Method.
The 1D hydraulics included all significant elements such as drainage canals, open channels, all pipes larger than 12 inches, streams, roadside ditches, and ponds. The 1D model was generated as a standalone model not accounting for 2D linkage and was calibrated to gage data collected during the 2010 winter. The results of the 1D model revealed significant difficulties in capturing an accurate representation of what was occurring in the catchment during larger events. Over 30% of the total runoff generated was lost due to flooding at 55 nodes across the catchment. These results were unacceptable and it was determined the best course of action would be a fully integrated 1D/2D model.
The resulting 1D/2D model developed for this study consists of 979,627 square grid cells each representing a surface area of 100 square feet (9.29 square meters). The 2D grid is integrated with 1D elements which include 5.8 miles (9.3 kilometer) of large irrigation canals, 4.22miles (6.8 kilometer) 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 along the top of bank for all the open channels.
This area the model was based on presented a challenge due to the long established irrigation canals which flow from east to west across the catchment. Two large canals, roughly 25 feet across and 4 feet deep bisect the catchment, intercept all runoff from the catchment and moves it towards the west side of the catchment where in makes its way back to the Rogue River which forms the northern boundary of the basin. The development of these canals forever altered the hydrology and hydraulics of the catchment creating a unique modeling problem.
During larger events these canals flood, as they were not designed for the conveyance of storm water. The result is extensive flooding across the basin and the transfer of significant storm water from the east side of the catchment to the west side. For the 25 year 24 hour Soil Conservation Service Type 1A design storm for this location, the ‘fingers’ of flooding would all start from one of the main canals. Once these overland or 2D flow paths are started they would followed to the Rogue River where this basin terminates. In multiple locations there are instances where one of these flow paths will move in and out of five or six unique 1D elements connected by the linkage with the 2D domain. This situation required a large and complex fully integrated 1D/2D model to appropriately describe the entire nature of the flooding in the catchment.
Using modeling allowed the project team to appropriately classify the flood-prone areas with a detailed description of the depth, spatial extent and velocity associated with the flooding. Subsequent projects proposed to alleviate flooding were tested within the model, which lead to the development of a capital improvement project (CIP) list.