Floodplain Maps and Analysis for the Big Laurel Creek – Whitetop Laurel Creek Watershed
The determination of the flood hazard within a watershed is
typically defined in the United States as the maximum flooding extant that
would be caused by a "100-year design storm”, or an annual exceedance
percentage (AEP) of 1 percent (which equals the inverse of the 100-year event,
i.e. AEP = 1 / rate of recurrence). The
AEP means that in any given year, there is a 1% chance that the AEP 1% flood
will occur.
Many people use the Federal Emergency Management Agency’s (FEMA’s) National Flood Hazard Layer (NFHL) as the definitive delineation of the extreme flood hazard to be considered in planning and mitigation of watershed management and improvement projects.
Taken from the webpage FEMA's National Flood Hazard Layer (NFHL) Viewer
Based on my experience, relying on the FEMA NFHL alone is insufficient for design. A better method to determine the floodplain delineation and flooding hazard is to perform an independent hydrologic and hydraulic (H&H) model analysis of the watershed using free and publicly available software such as the U.S. Army Corps of Engineers Hydrologic Engineering Center's (CEIWR-HEC) River Analysis System (HEC-RAS).
For a H&H analysis, the “design storm” is modeled as a
maximum expected maximum flow rate in the watershed stream system, in units of either
cubic feet per second, cfs (US units) or cubic meters per second, cms (SI
units). Using the results from the H&H analysis, you can assess the
floodplain delineation and flooding hazard based on your own best judgement,
while using the FEMA NFHL as a guide to ensure that your model is accurately reflecting
the watershed behavior during a “100 year” design flood event.
As described in the earlier blog post, Flood
and Drought Data for the Big Laurel-Whitetop Laurel Creek Watershed
(10/12/2025), I have previously performed a one-dimensional (1-D) HEC-RAS model of the Whitetop Laurel Creek watershed from the
confluence of Whitetop Laurel Creek and Green Cove Creek (near Creek Junction, VA) to the confluence of
Whitetop Laurel Creek and Laurel Creek (upstream of Damascus, VA). The modeled maximum flow was the Annual
Exceedance Probability (AEP) of 1%, corresponding to the “100-year design
flood”.
In this blogpost, I will conduct a “side-by-side” comparison
of the results from the 1-D HEC-RAS model with the FEMA NFHL to illustrate that
in my opinion, an analysis based on independent H&H modeling produces a more valid flood assessment than simply
relying on the FEMA's
National Flood Hazard Layer (NFHL) Viewer.
(Additional information on obtaining the information to set
up the 1-D HEC-RAS model can be found at the previous blog post of Mapping
and Delineating the Big Laurel Creek – Whitetop Laurel Creek Watershed and Equipment
Research and Useful Modeling Technologies for Watershed Analysis. )
The full-scale results for the 1-D HECRAS model and the FEMA NFHL are shown below. A side-by-side comparison of the 1-D HECRAS model and the FEMA NFHL is shown below. The 1-D HECRAS model results shown above the FEMA NFHL.
Eleven (11) panes were created to provide a finer resolution visualization of the comparison between the 1-D HECRAS model results in comparison to the FEMA NFHL. The eleven panes are overlaid on the full-size results plot, and each pair of panes is shown afterwards with comments when appropriate.
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The 1-D HECRAS model was also used to determine the depth and flow velocity of the Whitetop Laurel Creek flow for the bank full flow, which is assumed to be the AEP 50% (the two-year flood) flow. The AEP 50% (the two-year flood) flow condition is indicated by the “PF 1” in the upper right of the plot.
The Federal Emergency Management Agency’s (FEMA's) National Flood Hazard Layer (NFHL) for the same section of Whitetop Laurel Creek adjacent to the section of the Virginia Creeper Trail (VCT) from mile 18 to mile 19. is provided for comparison.
Assessing the stream conditions during different flow conditions can be very important for determining the ecological health of a watershed. The ability to model the geomorphic changes in a watershed for planned watershed restoration and improvement project is very helpful to a water resources management professional.
For this introduction to H&H modeling, a 1-D HECRAS model is sufficient for modeling flow conditions in the lower section of the the Whitetop Laurel Creek watershed.
For actual projects, I would recommend that a two-dimensional (2-D) H&H analysis be budgeted for and completed by a qualified professional whenever possible.
A complete, calibrated and validated two-dimensional (2-D)
H&H analysis will greatly assist watershed managers and water resource
managers in understanding and best managing flow conditions in a complex watershed H&H system.
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