Aching Joint Probability? – Diagnosing Your Options for Interior Drainage Behind Levees for Compliance with 65.10 (A Reviewer’s Perspective) William Zung, PMP, CFM Anish Pradhananga, PE, CFM Rick Nusz, PG, PH, CFM ASFPM 2013 Conference June 12, 2013
Aching Joint Probability? – Diagnosing Your Options for Interior Drainage Behind Levees for Compliance with 65.10 (A Reviewer’s Perspective) William Zung, PMP, CFM Anish Pradhananga, PE, CFM Rick Nusz, PG, PH, CFM ASFPM 2013 Conference June 12, 2013
Agenda
Levees in NFIP
65.10 Overview
Interior Drainage Analysis
Diagnosis
Levees in NFIP
• Levee – A man-made structure, usually an earthen embankment, designed and constructed in accordance with sound engineering practices to contain, control, or divert the flow of water so as to provide protection from temporary flooding.
• 44 CFR 65.10 establishes the criteria for levee systems to meet to be accredited on FIRMs
• Accredited Levee – A levee shown on the FIRM as providing protection from the 1-percent-annual-chance flood.
Levees in NFIP
• Interior Area – USACE EM 1110-2-1413 defines as “the area protected from direct riverine, lake, or tidal flooding by levees, floodwalls, or seawalls and low depressions or natural sinks.”
• Coincident Events – two events occurring together
• Joint Probability – The probability of two events occurring together.
Levees in NFIP Coincident Events
Source: HEC SSP User’s Manual
• Exterior needs, – Variable B : Stream Gage data or Hydraulic Study
• Interior needs, – Variable A : Stream Gage data or Rainfall Runoff
models with synthetic storms – Variable C : Hydraulic model with varying outlet
conditions
Levees in NFIP
Initiation
• Study initiated and identify levees • Levee ownership and background research • PAL eligible / Accredit / Not Accredit
PAL
• PAL Meeting • Progress Reports • 65.10
Mapping
• Accredit on FIRM • Not Accredit on FIRM (Levee Analysis and
Mapping Process)
65.10 Overview
65.10 - Structural (b1) Freeboard
(b2) Closures
(b3) Embankment Protection
(b4) Foundation Stability
(b5) Settlement
(b6) Interior Drainage
(b7) Other
65.10 – Non Structural
(a) General
(c) Operation
(d) Maintenance
(e) Other
65.10 Overview
www.fema.gov/living-levees-its-shared-responsibility/fema-levee-resources-library
65.10 Overview Reviewer’s Perspective
• PM 63 • Completeness Check
– Submittal Includes all Elements
– Signed by a PE – Checklist breaking down
each element • Tab Submittal • Detailed Review of
Interior Drainage Analysis
65.10 Overview Reviewer’s Perspective • The submitting P.E. is responsible
for the technical content – Old can still be good
• Check submittal for completeness to comply with all 65.10 criteria
• Friendly coordination with submitter to resolve questions
• We will check for consistency…but make it easy to find
65.10 Overview Reviewer’s Perspective • Digital submittals are preferred over
volumes of hard copies • Rely heavily on the summary report
to tell us the answer
65.10 Overview Reviewer’s Perspective
• 65.10(b)(6) Interior Flooding – Interior Flooding Analysis submitted? – Coincidental Peak Analysis done? – Pump station operation included in the
analysis? – Data certified by a Professional
Engineer?
65.10 Overview Reviewer’s Perspective • 65.10(b)(6) Interior Flooding
– Areas of Interior Flooding Identified?
– Site map showing areas of interior flooding mapped?
– Base Flood Elevations shown on areas with greater than 1 foot of flooding?
65.10 Overview Reviewer’s Perspective • 65.10(c) Operation Plans and Criteria
– Operation Plan submitted? – Are closure devices addressed in
Operation Plan? – Is the interior drainage system (including
flood warning system) addressed in Operation Plan?
– Periodic Inspection and operation stated?
– Operations Plan formally adopted as per criteria in 65.10(c)?
65.10 Overview Reviewer’s Perspective • Common Errors
– Not using Joint Probability Analysis – Documentation is inconsistent
• Pumps in model do not have the same parameters as documented in the Operation Plan.
• Number of closures in Interior Analysis do not match the number on the As-Builts and do not match the number in the O&M plans
– Not using the 1% annual chance • Many times the design event for interior flooding is more
frequent than the 1% (e.g. 4% [25yr])
Interior Drainage Analysis What are you symptoms?
• Rainfall Runoff analysis of interior drainage area • Data availability – Interior and Exterior measurements
– Stream gage data – Precipitation gages – Existing detailed flood study on exterior or interior
• Multiple uses of this study than just 65.10 – Capital Improvement Planning
• Engineering Judgment • Risk justify the analysis
Simple Complex
Interior Drainage Analysis What are you symptoms?
• Risk justify the analysis Heavy Industrial with storm water pipe network Agricultural with open channels
Interior Drainage Analysis What are my options?
A. USACE EM 1110-2-1413 has two general methods to perform the hydrologic analysis in interior areas 1. Continuous Record Analysis Method
a. Period of record b. Discrete events of historic record c. Stochastically generated continuous record
2. Coincident Frequency Method: Weighted frequency relationship developed from probabilities of exterior and coincident interior stage condition
B. American Association of State Highway and Transportation Officials (AASHTO)
C. The Worst Case Scenario
Interior Drainage Analysis A.1.a. Period of record method • Estimates the interior runoff hydrograph for a
period of historic precipitation. • Period of record exterior stage hydrograph at
desired location. • Need stream data as well as interior rainfall
data. • The process preserves the, seasonality,
persistence and dependence or independence of exterior and interior events.
Key Features: Gages Historic Interior
and Exterior Lots of Data Preserves
Seasonality
Simple Complex
Interior Drainage Analysis A.1.b. Multiple Discrete Event method
• Generates composite stage- frequency function by analyzing two conditions. – 1. Selected high exterior events that have on
effect on interior flooding with coincident historic interior rainfall.
– 2. Selected low exterior events associated with interior flood generated by coincident historic interior rainfall or hypothetical frequency storms. The joint probability theorem is applied to combine these two functions to generate a composite function.
• Joint Probability Theorem P= P(A)+ P(B) For Partial Duration series P= P(A)+P(B)-P(A)xP(B) For annual Series
Key Features: Gages Interior Flooding
with High Exterior Interior Flooding
with Low Exterior Joint Probability
Theorem
Simple Complex
Interior Drainage Analysis A.1.c. Stochastic Method
• Generate synthetic sequences from the historic record
• Accounts for the randomness of events • Overcome the limitation due to short period of
records.
Key Features: Can be analyzed
with synthetic sequences
Recorded Data limitation is avoided
Randomness of events are represented
Simple Complex
Interior Drainage Analysis B: Coincident Frequency Method
• Hydrologic and Hydraulic modeling – Uses synthetic events – Frequency event is known
• Applies Total Probability Theorem to generate stage-frequency function
– P(A) = probability of exceeding a given interior ponding elevation
– P(Bi) = probability river is at specific stage interval (i) – P(A/Bi) = probability of exceeding a given pond
elevation if the river stage is at the stage interval (i)
P(A)= � P ABi
X P(Bi)𝑛
𝑖=1
Key Features: Can be analyzed
with synthetic sequences
Recorded Data limitation is avoided
Randomness of events are represented
Simple Complex
Interior Drainage Analysis Coincident Frequency Method cont.
HEC-SSP
Interior stages (ft) P(A/Bi)
Exterior stages (ft) P(Bi)
1-percent-annual-chance joint probability water surface elevation P(A)
Interior Drainage Analysis A.2. Coincident Frequency Method – HEC SSP
• Coincident Frequency Plot • HEC SSP will perform the
Coincident frequency Analysis and generate a frequency curve
474.44
Interior Drainage Analysis B. American Association of State Highway and Transportation Officials (AASHTO) • Joint probabilities of design
coincident stream flows at confluences.
• Coincident frequencies based on watershed Area Ratio
• Alternative approach for interior areas with very limited data
• Use the higher result of the two scenarios
Simple Complex
Interior Drainage Analysis C. Assume the Worst Case
• All gates closed • Pump Stations operate per O&M Plan • 1-percent-annual-chance rainfall on watershed • Duration of 24 hours
Pros
• Easy to Calculate
Cons
• The event likely will have less than 1-percent-annual-chance in any given year
• The inundation area may be larger than 1-percent-annual-chance event
Simple Complex
Diagnosis
• Consider your symptoms – What is at Risk – Agricultural fields, industrial, homes, etc. – What impacts of the interior flooding extents on the FIRM – What is your data availability
• Stream gages, • precipitation gages • H&H modeling
– What is your PE comfortable with • Document your assumptions and clearly summarize your analysis
in the report – Include backup data as needed – Make sure it is consistent throughout the report
Diagnosis
Prescription: Perform the Coincident Frequency Method • The method most applicable in Joint Probability
Analysis for Interior Drainage Analysis for 65.10 certification
• Applicable with limited gage data • Hydrologic and Hydraulic modeling using FEMA
approved models • HEC-SSP used to perform the statistical
calculations • Repeatable and can be modified for future map
revisions • Detailed analysis for BFE placement on FIRMs
Key Features: Can be analyzed
with synthetic sequences
Recorded Data limitation is avoided
Randomness of events are represented
Simple Complex
Questions?
William Zung, PMP, CFM [email protected]
913-498-0500 x 1
Anish Pradhananda, PE, CFM [email protected]
913-498-0500 x 5
Rick Nusz, RG, PH, CFM [email protected]
816-283-7907