# V. Flood Hydrology And Floodplain Hydraulics

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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

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

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

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.

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).

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

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

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

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

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

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

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

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

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

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.

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

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

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

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.

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.

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

Aerial View of Typical Riverine Condition

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.

Aerial View of Sheet Flow Condition

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

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)

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.

Aerial View of Distributary Flow Condition

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.

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.

Aerial View of Anastomosing Flow Condition

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.

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

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

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

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

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.

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

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.)

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.

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.

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

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

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*

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)

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)

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)

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