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HEC-RAS Modeling Series: Modeling a Steady-State Floodway in HEC-RAS

This article is the first in our blog series on HEC-RAS modeling. Be sure to check back for subsequent series articles.

Background

The Federal Emergency Management Agency (FEMA) regulates development within the 100-year floodplain by administrating the National Flood Insurance Program (NFIP). Communities that participate in the NFIP must prevent excessive development within the floodplain that could cause increases in potential flood damage. FEMA also does not allow development to occur within the floodway, which has a maximum allowable surcharge of 1.0 ft. FEMA defines the regulatory floodway as “the channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than a desired height.” Local agencies and municipalities have the option to specify a smaller surcharge, but they can go no larger than FEMA’s limit of 1.0 ft.

Keep in mind that there may already be a regulatory floodway for the study area. If there is an updated model for the floodway, encroachment stations may not need to be changed; however, if the project involves drastic changes to the existing model or it is a completely new model for the area, developing a floodway may be necessary. If a floodway exists for an area, it is typically shown on a FIRM (Flood Insurance Rate Map) panel as a hatched area, as shown in the figure below.

Methodology

The first step to develop a floodway is to establish the 100-year (or base flood) using USACE’s HEC-RAS software. After the HEC-RAS geometry has been finalized and the 100-year storm event has been executed, the modeler can then start the process of creating a floodway.

Starting this process includes creating a floodway flow data file and setting boundary conditions. The flow data file should include at least two profiles – the base flood (100-year) and the floodway. More than two profiles may be used depending on if the modeler chooses to run several floodway runs at one time, but all profiles should have the same flows. For the boundary conditions, all floodway profiles should start with a downstream boundary condition equivalent to the starting water surface elevation from the unencroached profile plus the target surcharge (up to 1.0 ft.).

There are five encroachment methods within HEC-RAS, but using Method 4 initially and then switching to Method 1 can be the most efficient in our experience. With Method 4, HEC-RAS computes the encroachment stations by keeping the conveyance of the encroached cross section equal to the conveyance of the original cross section. The modeler specifies a target increase in water surface elevation (surcharge), and the encroachments are determined so that an equal loss of conveyance occurs on each bank. After creating a “rough” floodway with Method 4, the modeler can switch to Method 1, which allows the user to fine-tune encroachment stations. Method 1 allows the user to adjust the stations of the encroachments, but requires some trial and error iterations. It is important to remember that when developing a steady state floodway model, it is generally better to work from downstream to upstream to develop the encroachment stations.

Another way to increase the efficiency in creating a floodway is to start with multiple profiles in Method 4, where the target surcharge is close to 1.0 ft. After running a few scenarios, the modeler can work from downstream to upstream in selecting the target surcharge that worked the best in resulting in a surcharge of 1.0 ft. (or very close to it). Using that surcharge, the modeler updates the “Target WS Change” for the downstream cross sections.

Floodway Results

The results of running the floodway can be shown in cross section plots, the X-Y-Z perspective plot, or with encroachment tables. Cross section plots are great for making sure your encroachments are not within channel banks and the X-Y-Z perspective can be helpful in checking for gradual transitions. The encroachment tables are good for comparing top widths from section to section. It is important to remember that floodways should not change drastically between cross sections unless there is some type of flow restriction, such as a bridge or culvert.

Common Problems

Below is a list of common problems in modifying or developing a floodway.

1. Top Widths

The floodway should not lie outside of the 100-year floodplain or within the channel banks at any point. The top width values in the output table may not provide a good indication of whether either previously mentioned scenario is occurring, so always check your encroachment stations. It is easy to visually inspect a cross section in the cross section layout view with the 100-year (base) and floodway profiles turned on.

2. Excessive Surcharge Value

The floodway surcharge should not be greater than 1.0 ft. based on FEMA requirements. However, it’s important to check with your local agency as they may specify a stricter requirement. Typical surcharge values range between 0.8 and 1.0 ft. in order to keep the floodway smooth and reasonable. It is also not necessary that all cross sections have the maximum surcharge. Check the encroachment tables in HEC-RAS to see the surcharge depths.

3. Negative Surcharge Value

The floodway surcharge can never be less than zero. Often, if the floodway is too narrow, then there will be an increase in velocity which drops the water surface elevation. In these cases, widening the floodway and checking for consistency in the velocities may help in correcting this error. It can also be beneficial to look at the energy grade lines. If the energy grade line for the floodway profile is higher than the unencroached profile, negative surcharges can occur if the floodway is too narrow in comparison to the unencroached top width or there are errors with the bridges (See Common Problem 4 below). If the energy grade line for the floodway profile is equal to or lower than the unencroached profile, negative surcharges can occur if the floodway is almost as wide as the unencroached top width or if there is too large of a difference in conveyance ratio, depth ratio, or top width ratio. Additionally, if flow exists in one overbank area and then shifts to the other overbank between cross sections or the distance between cross sections is greater than 500 ft, then a negative surcharge can occur.

4. Bridges

Because bridges are often an abrupt change, this can make defining the floodway around them tricky. Focusing on smooth transitions in top width should help. Additionally, if cross sections are not close enough to the bridge, this can cause issues with surcharge amounts, so adjusting internal cross sections of the bridge may help with the transition. Additionally, it is important to make sure the same bridge modeling method is being used for the unencroached profile and the floodway profile.

5. Output

Look for unrealistic (drastic changes in top width, large jumps in surcharge, etc.) output in the tables and plotted profiles. The floodway is considered a “conveyance zone” so it should be smooth and have gradual changes.

Steady-State HEC-RAS Floodways vs. Unsteady-State HEC-RAS Floodways

Modeling a floodway in steady-state HEC-RAS is much easier than modeling a floodway in unsteady-state HEC-RAS because flow is held constant in a steady-state model. When applying encroachments to an unsteady HEC-RAS model, the modeler must balance flow increases and surcharges as shown in the hydrographs in the graphic below. While in steady state you can generally work from downstream to upstream to achieve the desired surcharge depths, in unsteady state, it may be necessary to go back through the model from upstream to downstream.

FEMA LOMR/CLOMR Requirements for Floodway Revisions

Keep in mind that if you are requesting a floodway change as a part of a LOMR or CLOMR submittal to FEMA, you must adhere to the NFIP regulation as listed in 44 CFR Section 65.7. This includes submitting a copy of the public notice stating the intent to revise the floodway along with proof of publication or providing a statement by the community that it has notified all affected property owners, in compliance with NFIP regulation Subparagraph 65.7 (b)(1). The notice should include the extent of revision, the changes to the floodway, and contact information for any interested parties.

References

  1. ASCE HEC-RAS Computer Workshop Workbook. West Consultants, Inc. (2012).
  2. Floodplain Modeling Using HEC-RAS. Haestad (2003).
  3. Harris County Flood Control District Hydrology and Hydraulics Guidance Manual. HCFCD (2009).

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TagsHEC-RAS, Floodway, floodplain, 100-year flood, FEMA,

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