1. V. Flood Hydrology And Floodplain Hydraulics
Introduction
A. Hydrology
Definitions
Methods for Estimating Discharges
Hydrologic Models
B. Hydraulics
Types of Floodplains
Methods for Delineating Floodplains
Hydraulic Models

2. V. Flood Hydrology And Floodplain Hydraulics
A. Hydrology
1. Definitions
2. Methods for Estimating Discharges
3. Hydrologic Models

3. V. Flood Hydrology And Floodplain Hydraulics
A.1. Hydrology: Definitions
a) Recurrence Interval
How often (statistically) does the event occur? Once every 10 years, once every 50 years?
Small floods happen more frequently, large floods happen less frequently
Storm Based: x-year storm causes x-year flood
It’s a statistical estimate; it may not happen that often or it may happen more often

4. V. Flood Hydrology And Floodplain Hydraulics
A.1. Hydrology: Definitions
b) The Base (100-year) Flood:
The Base Flood is FEMA’s standard for flood insurance mapping
The Base Flood has a recurrence interval of 100-years.
It is the flood having a one percent chance of being equalled or exceeded in any given year.

5. V. Flood Hydrology And Floodplain Hydraulics
A.1. Hydrology: Definitions
c) Hydrograph
The hydrograph for a given flood event is a plot of time vs. discharge.
The “Peak Flow Rate” is the maximum amount of flow (in unit volume per unit time).

6. V. Flood Hydrology And Floodplain Hydraulics
A.1. Hydrology: Definitions
Example Flood Hydrograph
Peak Flow = 90 Time = 15 minutes

7. V. Flood Hydrology And Floodplain Hydraulics
A.2. Hydrology: Methods for Estimating Discharge
a) Existing Information
Flood Insurance Studies
Other state or local floodplain studies
Subdivision reports
Highway/roadway reports
Check to make sure existing data includes evaluation of needed return interval

8. V. Flood Hydrology And Floodplain Hydraulics
A.2. Hydrology: Methods for Estimating Discharge
b) Flood Frequency Estimates from Gage Data
USGS and others maintain gages that measure stream flow
Many gages have established flood frequency relations
USGS Water Resources Investigations Report documents USGS gage data for Arizona through 1996

9. V. Flood Hydrology And Floodplain Hydraulics
A.2. Hydrology: Methods for Estimating Discharge
b) Flood Frequency Estimates from Gage Data
Typical Stream Gage

10. V. Flood Hydrology And Floodplain Hydraulics
A.2. Hydrology: Methods for Estimating Discharge
c) Regional Regression Equations
Based on gaged data from actual watersheds within the region of application
Recent (1997) equations developed by USGS
Should not be used for:
Urbanized watersheds, and
Watersheds with characteristics that vary from those of watersheds used to develop the equations

11. V. Flood Hydrology And Floodplain Hydraulics
A.2. Hydrology: Methods for Estimating Discharge
c) Regional Regression Equations
Cover and Figure 41 from USGS WSP 2433

12. V. Flood Hydrology And Floodplain Hydraulics
A.2. Hydrology: Methods for Estimating Discharge
d) Watershed Modeling
Mathmatical approach based on specific watershed characteristics including;
Catchment subarea delineation
Rainfall relationship input
Rainfall-runoff relation description
Flood routing
Lacks the benefit of calibration with gaged data

13. V. Flood Hydrology And Floodplain Hydraulics
A.2. Hydrology: Methods for Estimating Discharge
Example Watershed from HEC-1 Manual

14. V. Flood Hydrology And Floodplain Hydraulics
A.2. Hydrology: Methods for Estimating Discharge
e) State Standard 2-96
Outlines three level approach
Level 1 – Most conservative – applies envelope curve from USGS WSP 2433
Level 2 – Applies individual regional equations from USGS WSP 2433
Level 3 – Recommends:
Computer Models HEC-1, TR-20, TR-55, others
Approved Local Methodologies
Flood Frequency from Gage Data

15. V. Flood Hydrology And Floodplain Hydraulics
A.2. Hydrology: Methods for Estimating Discharge
f) Issues
Selection of Method; Agency requirements, availability of existing information and/or gage data
Design Frequency; Selecting the proper return interval
Calibration; Comparison of method results with real world data such as gage data or high water marks.
Arid Region Environments; Application of methods to distributary flow areas, alluvial fans, high-permeability surfaces, etc.

16. V. Flood Hydrology And Floodplain Hydraulics
A.3. Hydrology: Hydrologic Models
a) FEMA Requirements
b) ADOT Manual; Rational Method and HEC-1
c) Local Models/Procedures
Maricopa County; Rational Method and HEC-1
Pima County; Modified Rational Method
d) Computer Models
HEC-1/HEC-HMS most commonly used
See FEMA requirements list for others

17. V. Flood Hydrology And Floodplain Hydraulics
A.3. Hydrology: Hydrologic Models
d) Computer Models
FEMA
Accepted
Hydrologic
Models

18. V. Flood Hydrology And Floodplain Hydraulics
B. Hydraulics
1. Definitions
2. Types of Floodplains
3. Methods for Floodplain Delineation
4. Hydraulic Models

19. V. Flood Hydrology And Floodplain Hydraulics
B.1. Hydraulics: Definitions
Open Channel Flow – Flow with a free surface
Energy – The total energy (ft-lbs/ft) of the flow, equal to the flow depth (d) plus the velocity head (V2/2g), or E = d + V2/2g
Energy Gradeline – Slope of the energy of the flow measured along the direction of flow.
Uniform Flow – Flow at constant or gradually changing depth along the channel.
Normal Depth – Open channel flow depth under uniform flow conditions.

20. Definition Sketch for Hydraulic Grade Line & Energy Grade Line

21. V. Flood Hydrology And Floodplain Hydraulics
B.1. Hydraulics: Definitions (continued)
Backwater – Flow condition accounting for obstructions and other non-uniformities within the floodplain.
Critical Depth – A condition where the energy is at a minimum for a given discharge.
Subcritical Flow – A condition where the flow depth is greater than critical depth and flow is influenced by downstream conditions.
Supercritical Flow – A condition where the flow depth is less than critical depth and flow is influenced by upstream conditions.

22. V. Flood Hydrology And Floodplain Hydraulics
B.2. Hydraulics: Types of Floodplains
a) Riverine
Includes any flow of runoff in a well-defined, tributary flow pattern
Evaluation generally assumes one- dimensional flow
Includes all major and most smaller streams in Arizona

23. Aerial View of Typical Riverine Condition

24. V. Flood Hydrology And Floodplain Hydraulics
B.2. Hydraulics: Types of Floodplains
b) Sheet Flooding
Low topographic relief across flow path.
Very poorly defined channels (or none).
No channel banks readily identified from aerial.
Very uniform vegetative characteristics over area affected by sheet flow.
Soil characteristics may not be visible on aerials due to vegetation density.
Soils characteristics are usually very uniform within the sheet flow area.
Soil units mapped by the Natural Resources Conservation Service (NRCS, formerly the Soil Conservation Service) as floodplain soils.

25. Aerial View of Sheet Flow Condition

26. V. Flood Hydrology And Floodplain Hydraulics
B.2. Hydraulics: Types of Floodplains
c) Ponding
Flooding condition that occurs behind floodplain obstructions such as roadways, railroads, agricultural berms, etc.
Little or no flow velocity.
Can occur at isolated locations along other flow systems (e.g, riverine, sheet flow, etc.)
Often shown as Zone AH on flood insurance maps

27. V. Flood Hydrology And Floodplain Hydraulics
B.2. Hydraulics: Types of Floodplains
d) Uncertain Flow Path; includes
Distributary flow
Braided flow
Anastomosing Flow
Alluvial Fans (discussed in Section VI)

28. V. Flood Hydrology And Floodplain Hydraulics
B.2. Hydraulics: Types of Floodplains
d) Uncertain Flow Path
Distributary Flow
Low, but distinguishable topographic relief across flow path.
Topographic relief is sufficient to create isolated islands during flooding.
Channels divide in the downstream direction so that the number of flow paths conveying floodwaters increases in the downstream direction.
Higher vegetative density along flow paths with upland vegetation between.
Soils units mapped by the NRCS as alluvial fan terraces, inactive alluvial fans, or alluvial fans.

29. Aerial View of Distributary Flow Condition

30. V. Flood Hydrology And Floodplain Hydraulics
B.2. Hydraulics: Types of Floodplains
d) Uncertain Flow Path
Braided Flow; occurs where flow within a well-defined channel or floodplain is divided into separate flow paths created by shifting patterns of sediment deposition.

31. V. Flood Hydrology And Floodplain Hydraulics
B.2. Hydraulics: Types of Floodplains
d) Uncertain Flow Path
Anastomosing Flow
Branching, interlacing, and interconnecting flow paths, which produce a net-like or braided appearance.
Slight topographic relief across flow path.
Anastomosing flow areas have poorly defined channels downstream of a relatively large drainage area.
Channel banks may not be visible on aerial photos for large portions of the anastomosing alluvial surface.
May occur on the lowest portion of alluvial fans.
Higher vegetative density may occur along flow lines, with uniform vegetative characteristics between flow lines.
Soils mapped by the Soil Conservation Service as floodplain soils.

32. Aerial View of Anastomosing Flow Condition

33. V. Flood Hydrology And Floodplain Hydraulics
B.2. Hydraulics: Types of Floodplains
d) Uncertain Flow Path
Alluvial Fans; Per FEMA (Feb. 2002) an active alluvial fan flood hazard is indicated by the following three related criteria.
Flow path uncertainty below the hydrographic apex;
Abrupt deposition and ensuing erosion of sediment as a stream or debris flow lose its ability to carry material eroded from a steeper, upstream source area; and
An environment where the combination of sediment availability, slope, and topography creates an ultrahazardous condition for which elevation on fill will not reliably mitigate the risk.

34. Alluvial Fans Illustration
Figure G-1 of FEMA Guidelines and Specifications for Flood Hazard Mapping Partners (Feb. 2002)

35. V. Flood Hydrology And Floodplain Hydraulics
B.3. Hydraulics: Methods for Floodplain Delineation
a) Approximate Methods
b) Backwater Modeling
c) Two-Dimensional Modeling
d) State Standards
e) Information Required
f) Issues
g) Assumptions
h) Encroachment
i) Floodways

36. V. Flood Hydrology And Floodplain Hydraulics
B.3. Hydraulics: Methods for Floodplain Delineation
a) Approximate Methods
Manning’s equation for uniform flow commonly used for approximate delineations:
V = 1.49 Rh2/3 S1/2, Q = VA
Generally limited to areas where flood profile slope is equal to channel bed slope (i.e., no backwater).
Guidelines for application available in State Standard SS 2-96, Level 2 procedures n

37. V. Flood Hydrology And Floodplain Hydraulics
B.3. Hydraulics: Methods for Floodplain Delineation
b) Backwater Modeling
Based on measurement of energy losses along floodplain reach
One-dimensional model approach, i.e., flow profile varies in one direction only.
Generally limited to riverine flooding conditions.
Assumes steady flow (i.e., no hydrograph)
HEC-2 and HEC-RAS are most common computer applications
State Standard SS 9-02 provides application guidelines

38. V. Flood Hydrology And Floodplain Hydraulics
B.3. Hydraulics: Methods for Floodplain Delineation
c) Two-dimensional modeling
Based on assumption of variation in flow in two directions (upstream to downstream and perpendicular to flow path)
Generally based on routing of complete event hydrograph rather than peak flow rate
Accounts for floodplain storage through hydrograph routing
Generally more applicable to broad shallow flow, distributary or alluvial fan flow conditions.

39. V. Flood Hydrology And Floodplain Hydraulics
B.3. Hydraulics: Methods for Floodplain Delineation
d) State Standards
SS 2-96 for riverine systems using approximate methods
SS 9-02 for riverine systems using one- dimensional backwater methods.
SS 4-95 for sheet flow areas.
SS 3-94 for supercritical flow conditions

40. V. Flood Hydrology And Floodplain Hydraulics
B.3. Hydraulics: Methods for Floodplain Delineation
e) Information Required
Hydrology; Peak flow rate and/or event hydrograph needed.
Topography; ground elevation data needed for cross-section development
Field Data; information on channel and floodplain roughness, structures (bridges, culverts, levees, etc.)

41. V. Flood Hydrology And Floodplain Hydraulics
B.3. Hydraulics: Methods for Floodplain Delineation
f) Issues
Roughness; References for guidance include USGS and Maricopa County.
Flow Regime; Subcritical vs. Supercritical, FEMA requires subcritical for flood hazard delineation, but supercritical can predict higher velocities and associated scour.
Event Selection; Roadway design level vs. flood hazard delineation.
Multiple Events; Flood Insurance Studies (FIS) generally evaluate multiple events.

42. V. Flood Hydrology And Floodplain Hydraulics
B.3. Hydraulics: Methods for Floodplain Delineation
g) Assumptions
Horizontal Water Surface; not always true in bend reaches
Divided Flow; may require separate analysis of each flow path
Composite Flow; most models assume flow within section is entirely subcritical or supercritical
Steady vs. Unsteady Flow; most models are a “snap-shot” and to not measure change in flow over time.
Gradually vs. Rapidly Varied Flow; most models assume either uniform flow depth or gradually varied depth over short distances.
Stable Bed Geometry; most models assume the channel and floodplain ground surface does not change during the flood.

43. V. Flood Hydrology And Floodplain Hydraulics
B.3. Hydraulics: Methods for Floodplain Delineation
h) Encroachment
Placement of fill or obstructions within the floodplain is an encroachment
Hydraulic Response
Flood depths usually increase (if depths decrease check flow regime)
Flow velocities usually increase
Flow quantity distribution can be shifted from one part of floodplain to another
FEMA/Local Requirements
FEMA prohibits increased flood depths within the “floodway”
Local jurisdictions can have stricter encroachment limits

44. V. Flood Hydrology And Floodplain Hydraulics
B.4. Hydraulics: Hydraulic Models
a) Approximate Methods
b) Backwater (one-dimensional) Models
c) Two-Dimensional Models
d) FEMA Requirements

45. V. Flood Hydrology And Floodplain Hydraulics
B.4. Hydraulics: Hydraulic Models
a) Approximate Method (* = not formally accepted by FEMA)
Check model applicability to design situation
Quick 2 (FEMA)
XSPRO (US Forest Service)*
Simplified Floodway Determination (SFD), (U.S. Army Corps of Engineers)
Customized Spreadsheet Applications*

46. V. Flood Hydrology And Floodplain Hydraulics
B.4. Hydraulics: Hydraulic Models
b) Backwater (1-D) Models
HEC-2 (USACOE, 1991)
HEC-RAS (USACOE, 2002), replaces HEC-2
WSPRO (USGS, 1988)

47. V. Flood Hydrology And Floodplain Hydraulics
B.4. Hydraulics: Hydraulic Models
c) Two-Dimensional Models
TABS RMA4 (USACOE, 2000)
FESWMS 2DH (USGS, 1995)
FLO-2D (Jimmy S. O’Brien, 2000)

48. V. Flood Hydrology And Floodplain Hydraulics
B.4. Hydraulics: Hydraulic Models
Bridge/Culvert Models
HY-8 (FHWA, 1992)
WSPGW (Joseph E. Bonadiman & Associates, Inc., 2000)
Culvert Master (Haestad Methods Inc., 2000)

49. V. Flood Hydrology And Floodplain Hydraulics
B.4. Hydraulics: Hydraulic Models
d) FEMA Accepted Models
All of the previous models (except as noted)
Others as listed in the reference CD