7/23/2019 Introduction to Fema Hydrology 01-31-2012
1/22
Hydrology for Floodplain Analysis
Presented by
Z. John Licsko, PE, CFM
7/23/2019 Introduction to Fema Hydrology 01-31-2012
2/22
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
7/23/2019 Introduction to Fema Hydrology 01-31-2012
3/22
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.
7/23/2019 Introduction to Fema Hydrology 01-31-2012
4/22
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
7/23/2019 Introduction to Fema Hydrology 01-31-2012
5/22
Watershed Scale Flood Study
Hydrology, Ungaged Sites
7/23/2019 Introduction to Fema Hydrology 01-31-2012
6/22
USGS New York StreamStats
7/23/2019 Introduction to Fema Hydrology 01-31-2012
7/22
USGS New York StreamStats Results
7/23/2019 Introduction to Fema Hydrology 01-31-2012
8/22
USGS Gage Locations
7/23/2019 Introduction to Fema Hydrology 01-31-2012
9/22
Discharge Measurements
7/23/2019 Introduction to Fema Hydrology 01-31-2012
10/22
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
7/23/2019 Introduction to Fema Hydrology 01-31-2012
11/22
Hydraulics
Floodplain
Hydrology
Hydrology Hydraulics and
Floodplain Mapping Process
7/23/2019 Introduction to Fema Hydrology 01-31-2012
12/22
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
7/23/2019 Introduction to Fema Hydrology 01-31-2012
13/22
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
7/23/2019 Introduction to Fema Hydrology 01-31-2012
14/22
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,
7/23/2019 Introduction to Fema Hydrology 01-31-2012
15/22
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
7/23/2019 Introduction to Fema Hydrology 01-31-2012
16/22
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)
7/23/2019 Introduction to Fema Hydrology 01-31-2012
17/22
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
7/23/2019 Introduction to Fema Hydrology 01-31-2012
18/22
Esopus Rainfall Runoff Model (HEC-HMS)
7/23/2019 Introduction to Fema Hydrology 01-31-2012
19/22
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
7/23/2019 Introduction to Fema Hydrology 01-31-2012
20/22
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
7/23/2019 Introduction to Fema Hydrology 01-31-2012
21/22
Riverine Flood Plain, Floodway Schematic
Runoff Runoff
7/23/2019 Introduction to Fema Hydrology 01-31-2012
22/22
Questions?