Flooding Sizes and Scenarios Calculator 

Document Context

Upstream Sizing:

This section is used to determine the flooding potential of our detention basins. The five scenarios are described and each correlates to a column. To determine the flood sizes, the rows 8-13 are filled out with estimated basin sizing information, such as side slope, length, width and a reasonable scale. Using these inputs, rows 14-21 are calculated providing valuable flooding information such as volume and runoff information. 

For the curve number and precipitation, values relevant for Lexington, Kentucky were used. According to the United States Department of Agriculture Web Soil Survey, Lexington is primarily made up of soil types B and C (United States Department of Agriculture, 2025). Using the Hydrologic Engineering Center’s Curve Number Tables, it was determined that the appropriate curve number for agricultural lands with soil types B/C would be 82.5 (USACE Hydrologic Engineering Center, 2025). For the Precipitation, in inches, for Lexington, KY, the NOAA’s National Weather Service Precipitation Estimates were used to determine a precipitation of 4.36 inches for the 10-year 24 hour storm (NOAA's National Weather Service, 2025).

Downstream Sizing:

This section calculates area information about the downstream drainage area based on local lot sizing information. The minimum lot size for the relevant area is inputted into cell B25. Based on this lot size, the number of houses present and the size of the houses can be determined for the model. The standard lot sizing and setback information is from the Lexington-Fayette Urban County Government Code of Ordinances (Lexington-Fayette Urban County Government, 2025)

Flooding:

This section is used to determine when flooding will occur by calculating how much water is needed. The first row is checked based on if the scenario is planning to flood the drainage area or not. Then, based on the weir depth, the volume of water and time required to flood the drainage area is calculated.

Weir:

This section calculates many parameters based on weir calculations. The dimensions of the weir are inputted, which then allows the spreadsheet to calculate the discharge rate and velocity which are important to calibrating the system.

Formulas:

This section shows what formulas are used and is purely for reference.

References

Lexington-Fayette Urban County Government. (2025). Lexington-Fayette Urban County Government Code of Ordinances. https://codelibrary.amlegal.com/codes/lexingtonfayettecoky/latest/lexingtonfayettecoky_code/0-0-0-10066

NOAA's National Weather Service. (2025). NOAA Atlas 14 Point Precipitation Frequency Estimates: KY. Hydrometeorological Design Studies Center Precipitation Frequency Data Server (PFDS). https://hdsc.nws.noaa.gov/pfds/pfds_map_cont.html?bkmrk=ky

United States Department of Agriculture. (2025). Web Soil Survey. Natural Resources Conservation Service. https://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx

USACE Hydrologic Engineering Center. (2025). CN Tables. HEC-HMS Technical Reference Manual. Retrieved October 9, 2025, from https://www.hec.usace.army.mil/confluence/hmsdocs/hmstrm/cn-tables