Hydrology for Floodplain Analysis

Presented by

Z. John Licsko, PE, CFM

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

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

Hydraulics

Floodplain

Hydrology

Hydrology – Hydraulics and Floodplain Mapping Process

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

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

Methodology Suitability

Gage Analysis At gage locations/ sufficient length of record

Regression Equations Ungauged locations, unregulated

watersheds

Watershed Models

Regulated/ Urbanized watersheds. Calibration is needed

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 revisionsEffective 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

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

Riverine Flood Plain, Floodway Schematic

Runoff Runoff

Questions?