Week 2: Understanding the 1% Chance Flood and Base Flood Elevation¶

• Video Reference

Base Flood Elevation (BFE) and Floodplain Concepts¶

• Base Flood Elevation (BFE) is defined as the flood having a 1% chance of being equaled or exceeded in any given year.
• This is also known as the 100-year floodplain or Special Flood Hazard Area (SFHA). All three terms describe the same concept.

Understanding the 1% Chance¶

Concept	Explanation	Common Misunderstanding
1% Chance Flood	There is a 1% chance each year that a flood will reach or exceed the BFE	It does not mean a flood happens strictly once every 100 years
Marble Metaphor	If you have 100 marbles, 1 red and 99 white, drawing the red marble is a 1% chance	People mistakenly think flooding happens exactly once every 100 years, which is incorrect

Key point: Flooding is a probabilistic event, meaning it can occur multiple times within 100 years or not at all.

Freeboard Concept¶

Freeboard is an additional height above the BFE to provide a safety margin against flood uncertainties.

Location	Typical Freeboard Requirement	Explanation
Hillsborough County	6 inches	Local ordinance requires building 6 inches above the BFE
Some Panhandle Communities	4 feet	Higher freeboard requirements due to local risk assessments
Putnam Island	3 feet	Unique local freeboard based on specific flood risk

• Freeboard varies by local ordinance and can cause confusion when comparing different areas.
• Communities often adopt higher freeboard requirements to reduce flood insurance costs through the Community Rating System (CRS).

Construction Standards in Flood Zones¶

Zone	Construction Requirements	Notes
A Zone	Standard flood-resistant construction practices, elevation per BFE	Elevation certificate required, lowest floor elevation measured
V (Velocity) Zone	More stringent construction standards due to wave action and velocity	Coastal areas; requires stronger foundation and impact-resistant materials

• The Pima Coastal Construction Manual is a recommended reference for construction practices in coastal and flood-prone areas.
• Elevation certificates document the base flood elevation, finished floor, and lowest horizontal structural member height for regulatory compliance.

Flood Insurance Study (FIS) and Flood Insurance Rate Maps (FIRM)¶

Document	Purpose	Details
Flood Insurance Study (FIS)	Provides detailed flood profiles and data	Contains precise flood elevation profiles (not rounded), hydraulic analysis, flood history, and geographic info
Flood Insurance Rate Map (FIRM)	Displays general floodplain boundaries	Shows rounded BFE numbers, flood zones, floodplain limits

• The FIS is more detailed and accurate than the FIRM.
• The BFE on FIRMs is typically rounded to the nearest foot.
• When asked for regulatory BFE, specify whether it is the FIRM rounded value or the FIS detailed value.

Hydraulic Engineering Concepts in Flood Studies¶

Manning's n Value¶

• Describes resistance to water flow in channels and overbank areas.

• Varies depending on the surface type:

• Engineers use Manning's n to calculate flow velocities and water surface profiles.

• You do not need to calculate Manning's n but understand its role in determining flood elevations.

• The value of Manning's n can range from $0.010$ for smooth, new concrete to $0.100$ for extremely rough, weedy channels.

  | Surface Type | Resistance to Flow (Manning's n) | |-------------|----------------------------------| | Concrete | Low resistance (smooth, fast flow) | | Tall Grass | High resistance (slower flow) | | Rocky Bottom | Moderate resistance | | Mud | Moderate to high resistance |

  

Floodplain Components and Terminology¶

• Floodplain: The area adjoining a river or stream that is subject to flooding during a specific flood event, such as the 100-year flood.

  | Term | Definition | |------|------------| | Floodway | The channel of a river or stream and the adjacent land areas that must be reserved to discharge the base flood without increasing the water surface elevation more than a designated height. | | Fringe | The area of the floodplain outside the floodway where floodwaters may stand but do not flow rapidly. | | 100-year Floodplain | The combined area of the floodway and the fringe; represents the area subject to flooding in a 100-year (1% annual chance) event. | | Stream Channel Bottom (Thalweg) | The lowest point along the length of the stream channel, where the deepest water flows. |

  

• Flood fringes are often not symmetrical; this reflects natural river characteristics such as cut slopes and slip-off slopes.

• Floodplain widths vary greatly in scale: a small local river and a massive river like the Mississippi can have similar schematic representations but vastly different real-world sizes.

Floodway and Regulatory Floodway¶

Floodway is the channel of a river or water body and adjacent land areas reserved to discharge the base flood without increasing flood heights significantly.

• The floodway is a regulated zone where construction and fill are often restricted to prevent increasing flood risk elsewhere.
• It is defined as causing no more than a designated rise in water surface elevation (typically 1 foot).

Building in the Floodway and the No-Rise Concept¶

Can you build in the floodway?¶

• Technically yes, but it is strongly discouraged by the National Flood Insurance Program (NFIP).
• Building in the floodway requires a no-rise certification.

No-Rise Certification¶

A professional engineering study that demonstrates that any construction in the floodway will not increase the base flood water surface elevation beyond a certain allowable rise (usually 1 foot or less, sometimes zero).

• This study is required to prove the building won't cause increased flooding risks to other properties.

• The term "no-rise" means the allowed surcharge (rise in water surface) is minimal or zero.

• Surcharge is the maximum allowable increase in water surface elevation caused by construction in the floodway.

  | Parameter | Description | |-----------|-------------| | Surcharge | The increase in water surface elevation due to floodway encroachment or construction. | | Allowed Surcharge | Typically ≤ 1 foot; some areas may require zero rise (no increase allowed). |

Strategies to Comply with No-Rise¶

• Structures may need to be elevated on piers or pilings rather than slab-on-grade to avoid displacing floodwater.

• Buoyancy calculations are important to ensure buildings don't float away during floods.

  

Floodway Mapping and Data Tables¶

Hydraulic Parameters in Floodplain Analysis¶

• Regulatory BFE on maps is often a rounded number (e.g., 10 feet).

• Precise BFE from the FIS data table can be more exact (e.g., 9.9 feet), which may be important for construction decisions to minimize elevation requirements and costs.

  | Parameter | Definition | |-----------|------------| | Distance/Station | Position along the river or stream where data is collected. | | Cross-Section Area | Area of the river cross-section at the given station. | | Mean Velocity | Average water velocity at the cross-section. | | Slope (S) | Energy gradient or bed slope at the cross-section. | | Base Flood Elevation (BFE) | Water surface elevation during the base flood event at the cross-section. | | Surcharge | Increase in water surface elevation caused by floodway encroachment. |

Using Flood Insurance Rate Maps (FIRMs) and Flood Insurance Studies (FIS)¶

• The FIS data tables give detailed measurements for each cross-section: station distance, area, velocity, slope (S), base flood elevation (BFE), and surcharge.

• These tables should be used together with the FIRM for accurate floodplain management.

• Each river or water body has its own data table (e.g., c table for Rocky River floodplain).

  | Resource | Purpose | |----------|---------| | FIRM (Flood Insurance Rate Map) | Shows floodplain boundaries including floodways and flood fringes on a geographic map. | | FIS (Flood Insurance Study) Data Table | Provides detailed hydraulic data per cross-section (e.g., velocity, cross-sectional area). |

Reading Flood Insurance Rate Maps (FIRMs)¶

• Floodway and Fringe Areas are marked with different shading and cross-hatching.

• Zone designations such as Zone X shaded and Zone X unshaded indicate areas outside the floodplain with different flood risks.

• The legend on the map clarifies what each symbol and shading means.

• The black boundary lines on maps can sometimes represent detailed flood zone limits.

  

Key Points About Cross Sections¶

Aspect	Explanation
Location	Cross sections represent a specific line perpendicular to the river/stream flow.
Measurement Lines	When measuring floodway width, measure between the red floodway boundary lines, not between cross-section hexagons or other symbols.
Data Correspondence	Each cross section shown on the map corresponds to data in a floodway data table, including width and elevation.
Distance Along River	Distance values in data tables represent distance downstream from a reference point, possibly in feet or miles. Always check units.

Important Considerations for Measurements and Data Interpretation¶

Topic	Explanation
Measuring Floodway Width	Measure between the red boundary lines on the map, not across hexagons or other map symbols.
Width Data Source	The exact width of a cross section is found in the floodway data table, not by measuring on the map.
Distance Units	Distance downstream in flood data tables may be in feet or miles. Always verify the units.
Reference Points	Distances are measured from a known station, confluence, or benchmark along the river.
Zone Differences	Differences between shaded and unshaded Zones X relate to topography and historical flood data.
Potential Errors	Sometimes units or labels in data tables may be inconsistent or unclear; always cross-check.

Practical Usage Example¶

If asked: "What is the width of cross section A?"

• Do not measure the map directly.

• Consult the floodway data table in the Flood Insurance Study; it will list the width explicitly.

• For example: width = 115 feet.

  

Map and Data Table Integration¶

Map Element	Corresponding Data Table Element
Cross-section hexagon (e.g., A)	Cross-section data row labeled with the same letter (e.g., A)
Floodway boundary red lines	Floodway width and elevation data

| Distance downstream | Distance values in the floodway data table (check for units!)

Additional Notes¶

• The term "thalweg" refers to the lowest point in the river channel where the deepest flow occurs.
• Rivers are naturally asymmetrical, so floodplain fringes will often be uneven, reflecting natural features like cut banks and depositional bars.
• Always use the legend and data tables together for accurate interpretation.

Week 2: Understanding the 1% Chance Flood Quiz¶

1. The Base Flood Elevation (BFE) is defined as:
   a) The highest recorded flood in a community's history
   b) The flood that has a 1% chance of being equaled or exceeded in any given year
   c) The average flood level over a 100-year period
   d) The flood that occurs exactly once every 100 years

2. Which of the following is NOT a term used to describe the same concept as the "100-year floodplain"?
   a) Base Flood Elevation (BFE)
   b) Special Flood Hazard Area (SFHA)
   c) 1% annual chance floodplain
   d) 500-year floodplain

3. The "marble metaphor" for understanding the 1% chance flood means:
   a) There is exactly one flood every 100 years
   b) The chance of flooding increases over time
   c) If you have 100 marbles with 1 red, drawing the red marble is like the 1% chance of flooding in any given year
   d) The odds of flooding are random and cannot be predicted

4. Modern methods to determine BFE include which advanced tools?
   a) Satellite imagery and GPS only
   b) Field surveys and topographic sheets
   c) LiDAR and radar
   d) Flood gauges and stream monitoring

5. What is "freeboard" in floodplain management?
   a) The highest level a flood has historically reached
   b) The minimum elevation for constructing buildings in a coastal zone
   c) An additional height above the BFE to provide a safety margin
   d) The distance between the water surface and the bottom of a bridge

6. What percentage of flood insurance claims occur in Floodplain X (outside the SFHA)?
   a) 5%
   b) 10%
   c) 25%
   d) 50%

7. What is required when building in a floodway?
   a) Special flood insurance
   b) Approval from FEMA
   c) No-rise certification
   d) Concrete foundations

8. What does the term "no-rise" mean in floodplain development?
   a) Buildings must be elevated above the BFE
   b) The allowed surcharge (rise in water surface) is minimal or zero
   c) New buildings cannot be constructed in the floodplain
   d) Water cannot enter the building during a flood event

9. On Flood Insurance Rate Maps (FIRMs), cross sections are always shown as:
   a) Squares
   b) Lines
   c) Hexagons
   d) Triangles

10. When measuring floodway width on a map, you should measure between:
    a) Cross section hexagons
    b) River banks
    c) Red floodway boundary lines
    d) Zone designation labels

Answer Key¶

1. b) The flood that has a 1% chance of being equaled or exceeded in any given year
2. d) 500-year floodplain
3. c) If you have 100 marbles with 1 red, drawing the red marble is like the 1% chance of flooding in any given year
4. c) LiDAR and radar
5. c) An additional height above the BFE to provide a safety margin
6. c) 25%
7. c) No-rise certification
8. b) The allowed surcharge (rise in water surface) is minimal or zero
9. c) Hexagons
10. c) Red floodway boundary lines