Session G: Floodplain Workshop

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Session G Floodplain Workshop
Level 1 Floodplain Items Needed 1) Drainage
Area from Hydrology Workshop (1-B) 2) Level 1
Floodplain Limits Worksheet 3) Level 1
Floodplain/Floodway Limits (map)
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Session G Floodplain Workshop
Level 1 Floodplain Limits
Worksheet
STEP 1

Determine Flood Depth Region on Figure E-1.
Location
Pima County - T13S, R14E, Sec. 22
Region
_____________________________
STEP 2

Select appropriate Flood depth (Y) equation.
0.213
Region I-
D Y5.47 A
0.132
Region II-
D Y9.89 A
0.118
Region III-
D Y7.62 A
STEP 3

Compute flood depth (Y) from selected equation.
Drainage Area A (
sq.mi.)
2.13
Flood Depth Y (feet) ___________________
STEP 4
Estimate Base Flood Elevation (BFE).
Convert Y to local water surface elevation
( channel invert
elevation Y BFE.)
X-SEC_________
_ BFE (ft) _________________________
X-SEC_________
_ BFE (ft) _________________________
X-SEC_________
_ BFE (ft) _________________________
X-SEC_________
_ BFE (ft) _________________________
X-SEC_________
_ BFE (ft) _________________________
X-SEC_________
_ BFE (ft) _________________________
X-SEC_________
_ BFE (ft) _________________________
STEP 5

Plot floodplain limits.
Date
___________ By____________
Checked_____________
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Session G Floodplain Workshop
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Session G Floodplain Workshop
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Session G Floodplain Workshop
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Session G Floodplain Workshop
Level 2 Floodplain Items Needed
1) 100-Year Peak Discharge from Hydrology
Workshop (1-C) Q2,000 cfs (Level 3) or 2,200
cfs (Level 2).
2) Level 2 Cross-Section Data Worksheets 3) Level
2 Floodplain/Floodway Limits (map)
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Session G Floodplain Workshop
FLOWMASTER EXAMPLE
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Session G Floodplain Workshop
solution
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Session G Floodplain Workshop
Level 3 Example HEC-RAS Floodplain
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solution
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Session H Floodway Delineation
1. Introduction a. Floodway Discussion b.
Delineation of Floodways c. SS 9-02
Requirements for Floodplain Modeling
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Session H Floodway Delineation

• Floodway Discussion
• Why Necessary?
• Reserve area for flow conveyance
• Development within floodplain versus outside
  floodplain
• Floodway Encroachment Varies
• 0 increase in elevation to 1-foot increase

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Session H Floodway Delineation
Floodway Discussion

• Level 2
• Equal conveyance (area) versus non-equal area
  versus
• Conveyance K - function of n, s, A
• (n Mannings roughness, s channel slope, A
  cross section area)

• State Standard Equal area only based on
  existing
• water surface elevation

• Floodway Sketch
• Administrative floodway reminder
• Regulate development not a FEMA Floodway
• Fill /bank stabilization and/or erosion setback
  (SS 5-96, SS 7-98)
• Floodway fringe encroachment add 1 foot for
  FFE (lowest)

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Session H Floodway Delineation
Floodway Fringe The floodway fringe is the area
between the floodway and the edges of the
100-year (1) floodplain, some of which may be
reclaimed from the area inundated by the
100-year (1) flood.
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Session H Floodway Delineation
State Standard Select Pages FEMA Guidelines and
Specifications for Flood Hazard Mapping Partners
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Session H Floodway Delineation
Floodway Boundaries. The floodway is the area
along a watercourse which must be reserved for
the conveyance of flood waters. The floodway
width is estimated using the 100-year peak
discharge and appropriate local encroachment
criteria. Encroachment means placing fill,
structures, developments, or other material
within the floodplain limits. This document
describes specific procedures for estimating
administrative floodway boundaries. An
administrative floodway is a floodway delineated
by the procedures described in this document, as
opposed to floodway delineation procedures
mandated by the NFIP.
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Session H Floodway Delineation
Administrative Floodway Boundaries. The
administrative floodway is an area reserved for
the conveyance of flood flows. Figure 1
illustrates a typical riverine 100-year
floodplain, administrative floodway, and floodway
fringe. The area between the edge of the
100-year floodplain and the administrative
floodway is commonly referred to as the floodway
fringe. Encroachment into the floodway fringe
area shall be allowed for development as long as
the structure(s) is protected from the 100-year
flood. No encroachment of any kind shall be
allowed within the area determined as an
administrative floodway using a Level 2
procedure.   ________________________ 1 These
criteria do not apply to structures constructed
on engineered fill material elevated above the
regulatory water surface elevation.
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Session H Floodway Delineation

• The administrative floodway procedures may be
  used if all of the following criteria are met
•  
• A detailed FEMA Flood Insurance Study does not
  exist for the watercourse.
• The local floodplain administrator has approved
  the use of the Level 2 floodway procedure.
• The procedure is being used for a single lot
  residential or single lot commercial development.
• The watercourse consists of an identified
  flooding area which may or may not have overbank
  flooding and has a subcritical flow regime.
• The flood conveyance area of the watercourse is
  generally uniform, without significant changes in
  cross-sectional geometry or longitudinal slope.
•  
•  
• The procedure is not intended for use if a
  detailed FEMA Flood Insurance Study exists or for
  the following conditions
•  
• I Multi-lot development.
• II Alluvial fan, distributary, or bifurcated
  flood hazard areas.
• III Watercourses having a supercritical flow
  regime.
• IV Watercourses which have large changes in
  cross-sectional geometry or longitudinal slope
  within close proximity of the study area.
• V Watercourses where significant hydraulic
  structures are present that would create
  backwater effects, such as large roadway
  crossings with culverts or bridges, dams, and
  detention/retention structures.
•  
• Level 3 floodway delineation procedures are
  required for the situation listed above.
  Administrative floodway procedures are described
  in Appendix I.

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Session H Floodway Delineation
2. How to Delineate Floodways a. Existing
Floodway Delineations 1) FEMA Studies 2) County
or Municipal Flood Control Departments 3)
Engineering Reports b. Floodway Delineation SS
2-96 1) SS 2-96 Examples
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Session H Floodway Delineation
SS 2-96 Level I Floodway Delineation 1) Level I
Methodology a) Regional Approach b) Regression
Equations
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Session H Floodway Delineation
Items Needed 1) Watershed Delineation 2)
Example F1
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Session H Floodway Delineation
SS 2-96 Level 2 Floodway Delineation 1) Level 2
Methodology a) Mannings Equation b)
Limitations (see pages 7, 8, 9 and 10)
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Session H Floodway Delineation
SS 2-96 Level 2 Floodway Delineation
2) Level 2 Required Information a) Level 2,
100-Year Peak Discharge b) Level 2, Floodplain
Delineation c) Example I1
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Session H Floodway Delineation
SS 2-96 Level 3 Floodway Delineation 1) Level 3
Methodology a) Hydraulic Modeling b) HEC-2,
HEC-RAS, Other Hydraulic Models
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Session I Floodway Delineation Workshop
Level 1 Floodway Items Needed 1) Level 1
Floodway Limits Worksheet 2) Level 1
Floodplain/Floodway Limits Map
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Session I Floodway Delineation Workshop
Level 1 Floodway Limits
Worksheet
STEP 1
Determine Floodway Region on Figure F-1.
Location Pima County - T13S, R14E, Sec. 22
Region_____________________________
STEP 2
Select appropriate Floodway width (FW) equation.
0.449
Region I-W
FW 105A
0.407
Region II-W FW 157 A
0.261
Region III-W FW 218 A
0.289
Region IV-W FW 377A
STEP 3
Compute Floodway width from selected equation.
Drainage Area A (sq.mi.)_2.13__________
Floodway Width FW (feet) ______________
STEP 4
Estimate floodway setback at ½ the estimated
floodway width.
Setback (ft)___________________________
STEP 5
Plot setback limits.
Date___________ By____________
Checked_____________
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Session I Floodway Delineation Workshop
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Session I Floodway Delineation Workshop
Level 1 Floodway Limits
Worksheet
STEP 1
Determine Floodway Region on Figure F-1.
Location Pima County T13S, R14E, SEC 22
Region IV - W
STEP 2
Select appropriate Floodway width (FW) equation.
0.449
Region I-W FW 105A
0.407
Region II-W FW 157 A
0.261
Region III-W FW 218 A
0.289
Region IV-W FW 377A
STEP 3
Compute Floodway width from selected equation.
Drainage Area A (sq.mi.) 2.13
Floodway Width FW (feet) 469
STEP 4
Estimate floodway setback at ½ the estimated
floodway width.
Setback (ft) 235
STEP 5
Plot setback limits.
SOLUTION
Date___________ By____________
Checked_____________
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Session I Floodway Delineation Workshop
Level 2 Floodway Items Needed 1) Level 2
Floodway Cross-Section Data Worksheets 2) Level
2 Floodplain/Floodway Map
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FLOWMASTER EXAMPLE
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LEVEL 3 EXAMPLE HEC-RAS FLOODWAY
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Simplified Floodway Determination Model
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QUICk 2 model example
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Session J Open Discussion/ Comments/Questions
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State Standard Work Group CD
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Errata
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Errata
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FEMA and HEC Material