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 will.zung@stantec.com

913-498-0500 x 1

Anish Pradhananda, PE, CFM anish.pradhananga@stantec.com

913-498-0500 x 5

Rick Nusz, RG, PH, CFM rick.nusz@fema.dhs.gov

816-283-7907