Introduction to Hydrology and Hydraulics Basics of Floodplain ...

Hydrology for Floodplain Analysis

Presented by Z. John Licsko, PE, CFM

Global Water Cycle – Hydrological Analysis Distribution and movement of water on, above and below earth surface in liquid, vapor, & solid states – Hydrologic Cycle Conservation of Mass: Inflow (I) – Outflow (O) = ± Change in storage (S)

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 found under forested areas Depression Storage – ability 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.

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

Watershed Scale – Flood Study Hydrology, Ungaged Sites

USGS – New York StreamStats

USGS – New York StreamStats Results

USGS Gage Locations

Discharge Measurements

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

Hydrology – Hydraulics and Floodplain Mapping Process

Hydrology

Hydraulics

Floodplain

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…

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

Applicable FEMA Publications FEMA Guidelines and Standards for Flood Hazard Mapping Partners  Volume 1: Flood Studies and Mapping  Appendix I: Discovery  Appendix C: Guidance for Riverine Flooding Analyses and Mapping

FEMA Procedure Memorandum 59, Guidance for Implementation of Watershed-Based Studies,

Selection of suitable hydrologic methods

Methodology

Suitability

Gage Analysis

At gage locations/ sufficient length of record

Regression Equations

Watershed Models

Ungauged locations, unregulated watersheds Regulated/ Urbanized watersheds. Calibration is needed

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

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)

Approved Methodologies Continued  Rainfall Runoff Modeling  Usually HEC-HMS or HEC-1  FEMA’s 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

Esopus Rainfall Runoff Model (HEC-HMS)

Reasons to Revise Existing 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 uncalibrated 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

Important Riverine Floodplain Terms  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

Riverine Flood Plain, Floodway Schematic

Runoff

Runoff

Questions?