Apple Gate

Steamboat Lake, Photo by Richard Stenzel
Steamboat Lake, Photo by Richard Stenzel

Hydrology & Floodplain Studies

Our engineers are experienced in large and small basin hydrology studies as well as floodplain modeling. We frequently evaluate extreme precipitation events and the impacts of the resulting flood to dams, diversion headgates, gravel mining operations, buildings, culverts, bridges and other structures located in the floodway. Our staff is familiar with the rules and regulations governing development in and near a floodplain and we assist our clients in the preparation of Conditional Letters Of Map Revision (CLOMR’s) and Letters Of Map Revision (LOMR).

Specific Project Experience

Parkdale—Tallahassee Creek Floodplain Revision

Applegate Group, Inc. was retained by Front Range Aggregates, LLC to revise the 100-year floodplain of Tallahassee Creek just upstream of its confluence with the Arkansas River. The floodplain revision required the preparation and submittal of a Letter of Map Revision (LOMR) to the Federal Emergency Management Agency (FEMA) and obtaining a county floodplain permit. In support of the submitted LOMR, Applegate Group, Inc. performed a detailed hydrologic analysis on the 296-square mile watershed upstream of the project site, which incorporated the application of a composite design storm based upon a statistical analysis of historic rainfall records near the basin. A hydrologic model was created in HEC-HMS to convert design storm precipitation into a modeled estimate for runoff volume at the basin outlet. A hydraulic model was implemented to route the modeled runoff peak flowrate through the reach of Tallahassee Creek bisecting the project site, which included multiple bridges, hydraulic regime shifts, and split flow analysis. The completed study produced more site specific hydrologic information for the basin. The revised floodplain limits allowed Front Range Aggregates, LLC to have a better understanding of the floodplain and consequently optimize their operation layout and minimize their risk from flooding. 

Poudre River Floodway Study

Applegate Group conducted a detailed floodplain study on the Poudre River near the Larimer-Weld County line. The study, performed for a private client, assessed potential impacts on adjacent properties of floodway excavation, and temporary fill and grading activities. Among other considerations the study area featured split flows, divided flows, upstream and downstream bridges, proposed off-site development, road overtoppings in two directions, and a ¾ mile-wide floodway. The study included sensitivity analysis by both HEC-2 and HEC-RAS to satisfy regulatory and accuracy requirements.  The study was accepted by Larimer County, and will be used for communication with area property owners on coordination of upcoming floodplain activities in three jurisdictions.