At the end of this lesson, you will be able to:

• Describe the difference between hydrologic and hydraulic studies
• Recognize four Riverine Hydrologic methods of analyses
• Recognize three sources of data used in Riverine Hydraulic analyses
• Recognize how the Base Flood Elevation (BFE) is calculated for Coastal Areas
• Recognize information that would be included in the Coastal Hydraulic Analyses
• Indicate why the FIS identifies the vertical datum used in the study

This section includes five sub-sections.

3.1 – Riverine Hydrologic Analyses

3.2 – Riverine Hydraulic Analyses

3.3 – Coastal Hydrologic & Hydraulic Analyses

3.4 − Flood Protection Measures

3.5 − Vertical Datum

Some of the sub-sections have additional layers of detailed information.

Identification and explanation of flood frequencies

Section 3.0 provides a brief explanation of probability and recurrence intervals for floods.

It explains how a 1-percent annual chance, or more severe flood event, can occur more than once over a short time interval.

Important terms used in this section:

• Hydrologic = amount of water
• Hydraulic = flood height or elevation

Hydrologic analyses are studies of the amount of water flowing in a stream during flood events.

Generally, FISs are concerned with the peak rates of flow or discharges in streams for the 10-percent (10 year), 4-percent (25 year), 2-percent (50 year), 1-percent (100 year), and 0.2-percent (500 year) annual-chance exceedance flood events. Peak discharges are typically measured in cubic feet per second (cfs).

The 4-percent annual chance (25-year) flood event may not be calculated in older Flood Insurance Studies.

In newer Flood Insurance Studies a 1-percent-plus annual chance profile may also be included to communicate uncertainty in estimated discharge due to factors such as saturated ground conditions. It can also reflect uncertainty for other parameters or future hydrologic conditions.

Example: The 1-percent annual chance flow is assumed to be under typical conditions, however the soil in the watershed may be saturated because of higher than average rainfall. The 1-percent-plus profile reflects the potential higher base flood elevation under those conditions. It can also provide more information for communities to predict future flooding conditions in areas under heavy development or when considering higher standards like freeboard.

The major items addressed in this section are:

• Sources of Data
• Methods of Analysis (Riverine)
• Summary of Discharges Table

This section identifies the data used to determine peak discharges or the agency from which the discharges were obtained.

The data used in determining discharges may include:

• Topographical maps
• Gage data
• Land use maps
• Soil information

This section provides detailed explanations on the methods used to determine the peak discharges, and why that methodology is appropriate for the watershed.

Typical methodologies are:

• Drainage Area-Discharge Curves
• Gage Data Analysis
• Regression Equations
• Rainfall-runoff Models

Drainage Discharge Curves are graphs relating peak discharges to the drainage areas. They are developed from known peak discharges and drainage areas for other streams in the area.

Using the graphs, the peak discharges at any location on the stream can be determined by simply knowing the drainage area to that location.

A separate curve is used for each flood frequency.

Gage data analysis involves statistical computations performed using a historical record of flood data at a stream gage to determine the peak discharge on the stream for a flood event of a given probability (e.g., a flood that has a 1-percent annual chance).

For gage data analysis, this section provides information about the stream gage, which includes the:

• Location of the gage
• Name of the agency or organization that owns, operates, and maintains the gage
• Length of historical record used in the analysis

Any changes in the watershed that could influence the peak discharges recorded at the gage may also be discussed in this section.

These are mathematical equations based on statistical analysis that calculate the peak discharge based on watershed characteristics.

This section indicates:

• From what publication the equation was obtained,
• Who developed the equation, and
• What variables are required for the equation.

Typical variables used in regression equations include:

• Drainage area
• Rainfall
• Watershed slope.

Any limitations on the use of the equation, such as size of watershed or region, are also included in this section.

The United States Geological Survey (USGS) is a major source of stream flow data from gages. This data is then used to develop regression equations. Other agencies and researchers may develop their own peak discharge analyses. The FIS describes the techniques, agencies, and contractors that developed the peak discharge data used in the study.

Rainfall-runoff models are computer models that calculate the peak discharges for given rainfall events and watershed characteristics.

For the hydrologic model(s) used, the information in this section includes the:

• Name of the computer program,
• Name of the agency that created the program, and
• Major parameters of the program.

Rainfall events that may have been used to calibrate the model are also identified in this section.

This table briefly summarizes the drainage areas and peak discharges at locations along the streams. The locations chosen for the table are generally at physical features shown on the maps.

Not all discharges used in the analyses are shown on the table. Typically, discharges for the 10-percent (10 year), 4-percent (25 year), 2-percent (50 year), 1-percent (100 year), and 0.2-percent (500 year) floods are shown in the tables.

Note: Peak discharges for 25-year flood events are shown on newer studies

Hydraulic analyses are studies that determine the flood height or water surface elevations (WSELs) on streams or rivers.

FISs are primarily concerned with the 1-percent annual chance (100-year) flood WSELs. These are known as Base Flood Elevations (BFEs).

However, the water surface elevations for the 10-percent (10 year), 4-percent (25 year), 2-percent (50 year), 1-percent-plus (100 year), and 0.2-percent (500 year) flood events may also be determined.

The major items addressed in this section are:

• Sources of Data
• Methodologies

This section identifies the sources of data used in the analysis to calculate the flood elevations, which include:

• Cross Sections
• Roughness Coefficients
• Starting Water Surface Elevations for riverine studies

A cross section is an elevation view of the floodplain taken perpendicular to the flow at a given point. Cross sections are typically determined using aerial and field survey information, topographic maps or Digital Elevation Models (DEMs), or some combination of the two.

This section will contain pertinent information about the cross sections, such as how they were determined; the date of any field survey; and the scale, contour interval, and date of topographic maps or accuracy of DEMs that may have been used.

Exact BFEs are determined at each cross section. The flood elevation is then interpolated between cross sections to develop the flood profiles.

Cross sections are commonly located at regular intervals, additional cross sections are typically incorporated into the model upstream and downstream of bridges, culverts, weirs and other stream crossings or impediments to flow or at locations where floodplain characteristics change.

Some of the locations of the cross sections used in the analyses will be shown on the flood map in detailed study riverine areas.

Commonly referred to as Manning's “n” coefficients, these coefficients are used in the hydraulic calculations to reflect the resistance to flow in the channel and overbanks. Resistance to flow is typically due to the composition of surfaces (cement vs boulders, etc.) and type of vegetation that is present in these areas. This section lists the range of Manning's “n” coefficients used in the study for the channel and overbanks.

Example of how the Manning's “n” coefficients may be used in a FIS

Channel and overbank roughness coefficients (Manning's “n”) were assigned by field inspection, photographs, and textbook resources (References 6 and 7).

In Sampleville, the “n” coefficients for the Mud River and Mud River East Channel range from 0.025 to 0.028 for the channel and 0.038 to 0.055 for the overbanks. For Sandy Creek, the “n” coefficients range from 0.035 to 0.040 for the channel and from 0.040 to 0.065 for the overbanks. For Southside Road Drainage Ditch, the channel “n” coefficient is 0.035 and the overbank “n” coefficient is 0.04.

Select USGS Surface-Water Field Techniques, available at https://wwwrcamnl.wr.usgs.gov/sws/fieldmethods/Indirects/nvalues/index.htm to see examples of some typical channels whose roughness coefficients are known.

This section describes how the starting water surface elevations (WSELs) were determined in a study.

Starting WSELs are the flood elevations used at the first downstream cross section in the step-backwater computations for riverine studies.

For Starting WSELs, absent established downstream elevations or a control cross section, FEMA generally uses normal depths, which are computed using the channel slopes and cross-sectional areas (also known as slope-area method).

This section lists the methodologies used to compute the flood elevations and the various components used in the calculations. The most common methodology used to calculate flood elevations for a stream is a step-backwater computer program, such as HEC-2 or HEC-RAS. For more complex flooding situations, a computer program that models two-dimensional flow may be used.

HEC indicates the model was developed by the US Army Corps of Engineers Hydrologic Engineering Center (HEC). RAS is an acronym for their “River Analysis System”.

When coastal flood hazard areas (V Zones) have been determined, subsection 3.1 and 3.2 of the FIS explains how they were determined. In addition to coastal areas along the Atlantic, Pacific, and Gulf coasts, some areas of the Great Lakes are also considered to be at risk of coastal flooding.

Stillwater elevations are calculated for lakes, ponds and similar bodies of water. Stillwater elevations for Large bodies of water such as the oceans or the great lakes typically include the effects of storm surge.

The FIS typically includes the Summary of Stillwater Elevations that reflect coastal hydrology and coastal transect data information used in coastal hydraulic analyses. Transects for coastal analyses are similar to cross sections for Riverine analyses.

In coastal areas, BFEs are calculated by taking into account the:

• storm surge Stillwater Elevation,
• wave height above the storm surge Stillwater Elevation, 
• amount of wave setup, and
• wave runup (where present).

The factors involved in a typical coastal Hydrologic and Hydraulic analysis include:

Coastal Storm Surge Analysis—Storm surge is the amount of water, combined with the effect of normal tides, that is pushed towards the shore during a storm. The height of the surge is driven by many variables, including the strength and size of the storm, and the speed and direction in which the storm moves. A storm surge Stillwater Elevation (SWEL) model is used in a coastal analysis when supported by the history of storms in the area.

Wave Setup Analysis—Is the increase in water level caused by waves breaking ashore during a storm. This is affected by the height of the waves, the speed at which waves approach the shore, and the slope of the underwater ground near the shore.

Wave Runup - Overland Wave Modeling —Typically, the Wave Height Analysis for Flood Insurance Studies (WHAFIS) model is used which considers water depth, wind speed, vegetative cover, building density, and other factors to predict how the waves run up on the shore and help determine accurate coastal BFEs and flood zone boundaries.

This section includes a general explanation of how tropical and extra-tropical cyclones, such as hurricanes, nor’easters and gales or “Freshwater Furies” on the Great Lakes are reflected in coastal flood studies.

Storm Surge Analyses are used when historical data indicate that storm surges should be incorporated into the Stillwater Elevation (SWEL) for a coastal area.

The “forcing functions” of these large storms (wind speed, central pressure depression, radius to maximum winds, forward speed, and direction of approach to the shoreline) are discussed.

The type and specific name or version of the computer program used to establish the storm surge Stillwater Elevation (SWEL) as well as the sources of data used in the storm surge program that generated the model are included.

Finally, an explanation of the storm surge model and identification of the storm(s) that were used to calibrate the model are included. Storm surge analyses and parameters may include:

• Storm intensity (central pressure depression)
• Radius from storm center to maximum winds
• Forward speed of storm
• Direction of storm path approaching shoreline
• Frequency of the storm occurrence
• Astronomic tide effects
• Joint probability analysis
• Determination of Stillwater Elevation (SWEL)

This table reflects coastal hydrology. The Stillwater Elevation table will also include lacustrine (lake) flooding sources.

The Summary of Stillwater Elevations Table lists stillwater elevations (without waves) for selected recurrence intervals at certain locations.

The general term Stillwater Elevation (SWEL) is used in this table and in this course to include typical Stillwater Elevation and storm surge Stillwater Elevation when used in coastal analyses.

This section provides details of the Coastal Hydraulic studies and findings.

Topics in this section include:

• Wave setup, runup and height analysis
• Storm erosion and effects of shoreline profiles
• Identification of computer programs, field surveys, and topographic maps used during the study
• Transect descriptions

This section discusses how erosion was considered in the coastal flood hazard analysis. Generally, erosion is considered using geographically appropriate methods in determining the equilibrium profile on which the nearshore hydraulics were analyzed.

For example, FEMA’s approach in some east coast areas is to remove 540 square feet of the dune area above the stillwater elevation and adjust the transect profile accordingly. The 540 square-foot criteria is based on the national average.

In order to be considered as a topographic feature during the base flood, the primary frontal dune must have at least 540 square feet of area in cross section above the stillwater elevation.

In order to be considered intact, the eroded profile must be included into the wave height and wave runup analysis.

This section identifies the maps used to delineate the flood zones, explains how transects were used, and lists the date(s) the transects were surveyed.

A transect is a line taken perpendicular to the shoreline. They are similar to the cross sections used in Riverine Hydraulic Analyses and represent a specific portion of the shoreline in which ground cover and ground elevations will have similar hydraulic characteristics during a storm event.

Map data includes:

• Location of Transects
• Numbering of Transects

Note: Newer studies will show transect locations on the FIRM panels as well as on a transect location map in the FIS.

The Transect Descriptions table includes:

• Transect Number
• Description of Transect
• Stillwater Elevation at each Transect
• Maximum Wave or Runup Elevation at each Transect

The transect data table includes:

• Flood Source
• Transect Number
• Stillwater Elevations for All Recurrence Intervals
•  Range of Wave Height and Runup Elevations along the transect

All of the flood elevations, including those listed in the tables in the FIS, are referenced to a specific vertical datum

A datum is an abstract coordinate system with a reference surface that serves to provide known elevations to begin surveys. If a report says that a flood will rise 100 feet, and the datum being used is sea level, it means that the flood will rise 100 feet above the sea level reference surface. Over time, technology has enabled ever more accurate ways to establish a datum that accounts for factors like gravitational pull.

This section indicates the vertical datum used for the information in the hydraulic analysis and presented in the FIS. The vertical datum used in the FIS should not be confused with local vertical datum historically used for navigation, etc. in many areas.

Vertical datum is important to ensure that like values are being used when the information in the FIS, such as the BFE, is being compared to other vertical data. There is a potential for error if the datums representing the height of the flood and height of the grade (ground) are mixed.

As our National Spatial Reference Systems are improved over time, the datum FEMA uses when developing FISs and FIRMs changes with it. For example, FEMA had primarily used the National Geodetic Vertical Datum of 1929 (NGVD 29) in the original FIRMs and FISs for most communities, but began using the North American Vertical Datum of 1988 (NAVD 88) after that new datum was established. In the future, the FISs and FIRMs will likely continue to utilize improvements in this and other aspects of flood hazard mapping

For more information on conversion factors and datums, see NGVD -> NAVD available at https://pubs.usgs.gov/sir/2010/5040/section.html.

Is sea level rise included?

• No, sea level rise is not included because it considers a future condition. The NFIP is not currently permitted to base flood insurance rates on future conditions. Communities are officially encouraged to consider higher regulatory standards. When changes to existing flood hazards occur, the area is re-studied.

Is erosion included?

• Some erosion is included. Where sandy dunes exist, erosion is evaluated to determine how the dune will be affected by a storm event.

Is bluff failure included?

• No, bluff failure and long-term erosional processes are not included. Bluff erosion is usually the result of many factors including, but not limited to, precipitation, irrigation, and undercutting by wave action. Because bluff failure is episodic, a constant rate of change cannot be assumed or applied in the analysis. Assumptions would have to be made to estimate a future condition. When changes to existing flood hazards occur, the area is re-studied.

You have completed Lesson 3.

It covered Section 3.0: Engineering Methods and its five sub-sections:

3.1 – Riverine Hydrologic Analyses

3.2 – Riverine Hydraulic Analyses

3.1 and 3.2 – Coastal Hydrologic & Hydraulic Analyses

3.3 − Vertical Datum

In this lesson, you learned to:

• Describe the difference between hydrologic and hydraulic studies
• Recognize four Riverine Hydrologic methods of analyses
• Recognize three sources of data used in Riverine Hydraulic analysis
• Recognize how the Base Flood Elevation (BFE) is calculated for Coastal Areas
• Recognize information that would be included in the Coastal Hydraulic Analyses
• Indicate why the FIS identifies the vertical datum used in the study