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Olsson undertakes South Platte River hydraulic modeling project

Saturday, April 13, 2013

Deb Ohlinger, PE, CFM, Water Resources

Along portions of the South Platte River, several projects are under development that will increase habitat, stabilize river banks, and improve recreational opportunities. Olsson is helping with the development efforts for these projects by providing hydraulic modeling expertise.

To build a project along or adjacent to the South Platte River, a municipality, developer, or other entity must obtain approval from the Federal Emergency Management Agency (FEMA), since the river has a FEMA-designated floodplain. This process is typically accomplished by obtaining a Conditional Letter of Map Revision (CLOMR) before construction and a Letter of Map Revision (LOMR) that changes the FEMA-designated floodplain after construction. 

To start the process, the effective hydraulic model on which the floodplain is based must be obtained and modified to reflect floodplain and topography changes or flood control measure updates. Often, it is difficult to find these effective models. When they are located, the models are often outdated as compared to the actual conditions along a drainageway.

Because of the numerous projects under development along the South Platte River, several hydraulic modeling efforts are under way. The Urban Drainage and Flood Control District (UDFCD) wanted to assemble one continuous floodplain model that would facilitate easier and more efficient floodplain determinations. 

As a result, the UDFCD contracted Olsson Associates to develop a single HEC-RAS model for the South Platte River between the Chatfield Reservoir in Littleton and the UDFCD boundary in Adams County, a distance of approximately 39 miles. Olsson researched and obtained 12 different existing models. Some were from large studies, such as Flood Hazard Area Delineations, and others were from approved LOMRs. 

One of the first challenges was figuring out what area of the river each study or model covered. One Olsson team member put together a colorful chart, affectionately called the “Skittles Chart,” along with maps to illustrate how all of the pieces fit together. The models were also on different vertical datums, which required research and modifications to put everything on the same datum. In addition, some of the effective models were still using old software, HEC-2. Those models were updated to HEC-RAS and were subjected to extensive error checking.

The models were combined into two large models with a break point at Confluence Park in downtown Denver. A physical model was created in the Colorado State University hydraulics lab decades ago to simulate this complicated area. The results from the physical model are being used to set the downstream water surface elevation for the upstream model.

The different pieces did not always blend together smoothly, and combining the models caused differences in the results. However, significant differences have been investigated and explained. Some reaches needed to be updated to account for newer bridges and topography to represent current conditions. On Olsson’s recommendation, information will be collected on bridges that were not included in the existing models, and new topography will be used in some locations. Olsson is coordinating with other consultants on current modeling efforts for proposed projects. Olsson is also collaborating closely with the UDFCD to determine the path the effort should take and which efforts should be completed this year.

When starting a project like this, it’s hard to know in advance what the exact challenges will be and whether the end product will be useful. However, the Olsson team is making significant progress. The end goal is to use this model as the basis for future floodplain maps and to simplify the process of approving future projects. Olsson is well on its way to accomplishing the major first step of assembling the continuous model, which will greatly benefit the exciting new construction projects along the South Platte River.  For more information, please contact me at 303.237.2072 ordohlinger@olssonassociates.com.

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