introduction to fema hydrology 01-31-2012

7/23/2019 Introduction to Fema Hydrology 01-31-2012

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Hydrology for Floodplain Analysis

Presented by

Z. John Licsko, PE, CFM


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Conservation of Mass: Inflow (I) Outflow (O) = Change in storage (S)

Distribution and movement of water on, above and below earth surface in liquid, vapor, &

solid states Hydrologic Cycle

Global Water Cycle Hydrological Analysis


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Watershed Scale Hydrologic Processes

Initial Abstractions (up to 4 inches under

healthy forest stands)

Infiltration - Function of soil texture, thickness

and structure highest rates usually foundunder forested areas

Depression Storageability to store water on

the soil surface again forested soils Interception/Evapo-Transpiration

Baseflow perennial flow

Interflow shallow subsurface flows

Surface Runoff - water that is not infiltrated or

or

otherwise stored.


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Floodplain Analysis Methodology

Hydrology Collect Stream Gage Data (annual peak)

Gage Analysis

Regression Analysis( (ungaged sites)

Rainfall Runoff Models (calibrated to gage

data, discharges based on rainfall)

Analyze Data Probability, Statistics


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Watershed Scale Flood Study

Hydrology, Ungaged Sites


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USGS New York StreamStats


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USGS New York StreamStats Results


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USGS Gage Locations


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Discharge Measurements


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Flooding in Natural Streams

Unsteady Flow

Mixed flow regime (sub-critical, critical

and super critical)

Non homogeneous fluid (air and

sediment mixes with water)

Floating debris


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Hydraulics

Floodplain

Hydrology

Hydrology Hydraulics and

Floodplain Mapping Process


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Hydrology used in Hydraulic Analysis

Recurrence Interval - actual number of years between floods

100-year flood 1% chance in a year

500-year flood 0.2% chance in a year

10, 25 and 50 year profiles

Purely Statistical Designation

there can be multiple 100-year floods in a given year


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Limitations on the use of

Regression Equations

Regulated Watersheds

Overbank Storage Swamps/Wetlands

Drainage Area Outside Recommended Limits

Recently Developed Watersheds Need for in-depth Hydrologic Analysis (i.e. urban

areas)

Ignores difference in watershed shapes


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Applicable FEMA Publications

FEMA Guidelines and Standards for Flood HazardMapping Partners

Volume 1: Flood Studies and Mapping

Appendix I: Discovery

Appendix C: Guidance for Riverine Flooding Analyses and

Mapping

FEMA Procedure Memorandum 59, Guidance forImplementation of Watershed-Based Studies,


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Selection of suitable hydrologic methods

Guidelines for Riverine Flooding Analysis & Mapping - Appendix C, FEMA, Nov, 2009

Methodology Suitability

Gage Analysis At gage locations/ sufficient length of

recordRegression

Equations Ungauged locations, unregulated

watersheds

Watershed

Models

Regulated/ Urbanized watersheds.

Calibration is needed


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Approved Methodologies

Gage Analysis (Preferred)

Uses observed annual peak discharge from USGS and other gage locations

Data is analyzed using statistical procedures outlined in Bulletin 17B also

referred to as a LPIII analysis (PeakFQ program is used)

Need a minimum of 10 years of homogeneous data

Can be transferred upstream and downstream of the gage location

Regression

Ungaged locations

Multiple regression analysis

Usually published by USGS (For NY this SIR 2006-5122, includes data to 1999)


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Approved Methodologies Continued

Rainfall Runoff Modeling

Usually HEC-HMS or HEC-1

FEMAs accepted models list includes a number of others

Esopus Watershed has two examples of Effective Rainfall Models

Stony Clove in Greene County -TR20

Esopus HEC-1

Both have use old TP-40 rainfall totals


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Esopus Rainfall Runoff Model (HEC-HMS)


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Reasons to Revise Exist ing Hydrologic Analysis

Reflect Longer periods of record or data revisions

Effective hydrology on part of the Esopus (calibrated HEC-1- 1 gage, new model

use 6 gages ) and Stony Clove in Green County (TR-20 1982, probably un-

calibrated since little or no gage data (approximate in Ulster County also gaged.

( HEC-HMS rainfall change (Cornell data updates to TP-40), more gages,

Reflect changed physical conditions

For example, construction of a dam

To take advantage of improved hydrologic analysis

HEC-HMS

To correct an error in the effective hydrologic analysis


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100-YEAR FLOODPLAIN- area subject inundation during a 100-

year flood

FLOODWAY- channel and adjacent areas reserved to discharge

the 100-year flood without cumulatively increasing water surface

elevation more than a foot

ENCROACHMENT- man-made obstruction to natural conveyance

of flood waters

SURCHARGE - increase in flood elevation due to destruction of

floodplain conveyance capacity

Important Riverine Floodplain Terms


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Riverine Flood Plain, Floodway Schematic

Runoff Runoff


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Questions?

introduction to fema hydrology 01-31-2012

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