RIVERINE EROSION HAZARD AREAS Mapping Feasibility Study

F

E

M

A

EDERAL MERGENCY ANAGEMENT GENCY

TECHNICAL SERVICES DIVISION HAZARDS STUDY BRANCH

RIVERINE EROSION HAZARD AREAS

Mapping Feasibility Study

September 1999

F

E

M

A

EDERAL MERGENCY ANAGEMENT GENCY

TECHNICAL SERVICES DIVISION HAZARDS STUDY BRANCH

RIVERINE EROSION HAZARD AREAS

Mapping Feasibility Study

September 1999

Cover:House hanging 18 feetoverthe Clark Fork RiverinSanders County, Montana, afterthe rivereroded its bank inMay 1997. PhotographbyMichaelGallacher.

Table of Contents

Report Preparation........................................................................................xi Acknowledgments.........................................................................................xii Executive Summary......................................................................................xiv

1.

Introduction........................................................................................1

1.1. Description of the Problem...........................................................................................................1 1.2. Legislative History.........................................................................................................................1

1.2.1. 1.2.2. 1.2.3. 1.2.4.
National Flood Insurance Act (NFIA), 1968 .......................................................................3 Flood Disaster Act of 1973 ...............................................................................................4 Upton-Jones Amendment, 1988........................................................................................4 National Flood Insurance Reform Act (NFIRA), 1994........................................................7

1.3. Extent of the Problem...................................................................................................................7 1.4. Purpose and Scope of the Study...............................................................................................10

1.4.1. 1.4.2.
Purpose..........................................................................................................................10 Scope.............................................................................................................................11

1.5. Mapping of Riverine Erosion Hazard Areas..............................................................................12

2.

Background of Riverine Erosion......................................................14

2.1. Fluvial Systems ...........................................................................................................................14 2.2. Factors Affecting Alluvial Channels...........................................................................................15 2.3. Channel Types ............................................................................................................................16 2.4. Channel Form and Processes....................................................................................................19

2.4.1. 2.4.2. 2.4.3. 2.4.4. 2.4.5. 2.4.6. 2.4.7.
Definitions........................................................................................................................19 Geomorphic Characteristics............................................................................................20 Planforms........................................................................................................................22 Cross Section.................................................................................................................25 Slope ..............................................................................................................................26 Roughness and Bed Configuration..................................................................................27 Mechanics of Sediment Transport...................................................................................29

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2.5. Evaluation Methods for Channel Response to Imposed Changes.........................................30

2.5.1. 2.5.2. 2.5.3.
Equilibrium Approaches...................................................................................................31 Fluvial Hydraulics.............................................................................................................32 Streambank Stability .......................................................................................................34

2.6. Computational Models................................................................................................................36

2.6.1. 2.6.2. 2.6.3. 2.6.4. 2.6.5.
Modeling Approach.........................................................................................................37 Hydraulics .......................................................................................................................38 Sediment Transport ........................................................................................................39 Stability of Streambanks .................................................................................................39 Future Directions.............................................................................................................41

3. 4.
Literature Review..............................................................................43

3.1. Research Sources and Search..................................................................................................43 3.2. Literature Summary....................................................................................................................43 3.3. Selected Documents...................................................................................................................43

Analysis of Case Studies..................................................................44

4.1. AMAFCASediment and Erosion Design Guide........................................................................46

4.1.1. 4.1.2.
Overview.........................................................................................................................46 Detailed Description........................................................................................................46
4.1.2.1. Level 1 -- Geomorphic and Other Qualitative Analysis..........................................47 4.1.2.2. Level 2 – Quantitative Geomorphic and Basic Engineering Analysis .....................48 4.1.2.3. Level 3 – Quantitative Analysis Using Numerical Models.......................................51 4.1.2.4. Prudent Line Analysis ...........................................................................................51
4.1.3.

4.1.4. 4.1.5. 4.1.6. 4.1.7. 4.1.8. 4.1.9.
Applicability .....................................................................................................................52 Limitations.......................................................................................................................53 Relevant References.......................................................................................................53 Mappability......................................................................................................................54 Cost................................................................................................................................54 Regulatory Potential........................................................................................................54 Summary........................................................................................................................55

4.2. Inventory and Analysis of Stream Meander Problems in Minnesota......................................56

4.2.1. 4.2.2.
4.2.2.1. Data Collection and Preparation...........................................................................57 4.2.2.2. Shift Measurement...............................................................................................57 4.2.2.3. Results Summary.................................................................................................58 4.2.2.4. Relationships Between Stream Parameters and Measured Results.....................58

4.2.3.

Applicability .....................................................................................................................58

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4.2.4. 4.2.5. 4.2.6. 4.2.7. 4.2.8. 4.2.9.
Limitations.......................................................................................................................58 Relevant References.......................................................................................................58 Mappability......................................................................................................................59 Cost................................................................................................................................59 Regulatory Potential........................................................................................................59 Summary........................................................................................................................59

4.3. Probabilistic Spatial Assessment of River Channel Instability.................................................60

4.3.1. 4.3.2.
4.3.2.1. Methodology ........................................................................................................60 4.3.2.2. Case Study ..........................................................................................................62
4.3.3.

4.3.4. 4.3.5. 4.3.6. 4.3.7. 4.3.8. 4.3.9.

4.4. Geomorphology and Hydrology of the Santa Cruz River, Southeastern Arizona.................64

4.4.1. 4.4.2. 4.4.3. 4.4.4. 4.4.5. 4.4.6. 4.4.7. 4.4.8. 4.4.9.
Overview.........................................................................................................................64 Detailed Description........................................................................................................64 Applicability .....................................................................................................................66 Limitations.......................................................................................................................66 Relevant References.......................................................................................................66 Mappability......................................................................................................................67 Cost................................................................................................................................67 Regulatory Potential........................................................................................................67 Summary........................................................................................................................67

4.5. San Diego County Alluvial Studies.............................................................................................69

4.5.1. 4.5.2.
4.5.2.1. Computer Modeling..............................................................................................69 4.5.2.2. Definition of Hazard Areas....................................................................................70
4.5.3.

4.5.4. 4.5.5. 4.5.6.
Applicability .....................................................................................................................70 Limitations.......................................................................................................................71 Relevant References.......................................................................................................71 Mappability......................................................................................................................71

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4.5.7. 4.5.8. 4.5.9.
Cost................................................................................................................................72 Regulatory Potential........................................................................................................72 Summary........................................................................................................................72

4.6. City of Austin Technical Procedures for Watershed Erosion Assessments .........................73

4.6.1. 4.6.2.
4.6.2.1. Introductory Material.............................................................................................73 4.6.2.2. Impervious Cover Computation.............................................................................74 4.6.2.3. Watershed "Vital Statistics" ..................................................................................74 4.6.2.4. Urbanization Effects..............................................................................................75 4.6.2.5. Stream Inventory..................................................................................................75 4.6.2.6. Enlargement Potential...........................................................................................77 4.6.2.7. Nick Point and Meander Management..................................................................79 4.6.2.8. Prioritization of Erosion Problems .........................................................................79 4.6.2.9. Stream Management Recommendations.............................................................80
4.6.3.

4.6.4. 4.6.5. 4.6.6. 4.6.7. 4.6.8. 4.6.9.

4.7. River Stability Studies for Virgin River, Santa Clara River, and Fort Pierce Wash, Utah ......83

4.7.1. 4.7.2.
4.7.2.1. Historical Evaluation, Virgin River ..........................................................................83 4.7.2.2. Geomorphic Evaluation, Virgin River.....................................................................84 4.7.2.3. Conclusion............................................................................................................86 4.7.2.4. Santa Clara River.................................................................................................87 4.7.2.5. Fort Pierce Wash.................................................................................................87 4.7.2.6. Recommended Management Plans.....................................................................88
4.7.3.

4.7.4. 4.7.5. 4.7.6. 4.7.7. 4.7.8. 4.7.9.

4.8. Hydrologic and Geomorphic Studies of the Platte River Basin, Nebraska............................91

4.8.1. 4.8.2.

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