Municipality of Anchorage DRAINAGE DESIGN GUIDELINES

Municipality of Anchorage

DRAINAGE DESIGN GUIDELINES

Document Number WMP CPg09001

March 2007 ver 4.09

Project Management & Engineering Department

Municipality of Anchorage

DRAINAGE DESIGN CRITERIA REVISION LOG

Drainage Design Guidelines March 2007

Rev. Date No. Executed

Description

Sections Revised or Added

1. 1/26/2007 Removed "DRAFT"; Inserted Section 7 ? Sizing &Designing Storm Water Controls

Section 7

2. 3/2008

Break out discrete sections; added and rearranged appendices

All

3

4/2009

Add WMS Document number and update version number

Title page

4

4/2009

Revise zoning for single lot residential

Page 2-1

5

4/2009

Revise Figure 2-1 for project categorization, but not design requirements

Page 2-2

6

4/2009

Add Table 3-1, showing what design criteria are required for each project type

Page 3-1

7

4/2009

Revise water quality protection criteria to cover single-lot residential if more than 2

dwelling units

Page 3-1

8 4/2009 9 4/2009 10 4/2009 11 4/2009 12 4/2009 13 4/2009 14 4/2009 15 4/2009

16 4/2009

Clarify preliminary and final drainage report requirements Clarify preliminary and final drainage report requirements Clarify reporting requirements Orographic factor to nearest one-hundredth; major drainageway volume 1.77 inch CN for B soils, lawn, flat slopes Corrected Tt equations Design infiltration rate versus field infiltration rate Clarification and renumbering of outline, including requirement to submit computer model input and output electronically Drainage Project Notification Form

Page 4-2 Page 4-4 Page 4-5 Page 6-3 Table 7-1 Page 7-13 Page 9-7 Appendix A

Appendix C

17 4/2009

Drainage Certification - rearranged sections to clarify what is being certified

Appendix C

18 4/2009

Drainage Statement of Compliance ? removed penalty of law clauses and references to those who manage the system

Appendix C

New/*Revised* Template

Municipality of Anchorage

Drainage Design Guidelines March 2007

TABLE OF CONTENTS

Abbreviations and acronyms............................................................................................................v

Glossary ........................................................................................................................................ vii

1 GENERAL PROVISIONS..................................................................................................... 1-1 1.1 Jurisdiction....................................................................................................................... 1-1 1.2 Enforcement Responsibility............................................................................................. 1-1 1.3 Document Revision.......................................................................................................... 1-1 1.4 Using This Document ...................................................................................................... 1-2 1.5 Review and Approval ...................................................................................................... 1-2 1.6 Interpretation.................................................................................................................... 1-2 1.7 Guidelines Limitations..................................................................................................... 1-3 1.8 Relationship to Other Standards, Permits, and Plans....................................................... 1-3 1.9 Design Variances ............................................................................................................. 1-3

2 PROJECT CATEGORIZATION AND DESIGN REQUIREMENTS ............................. 2-1 2.1 Project Category............................................................................................................... 2-1 2.2 Threshold Runoff Rates or Minimum Infiltration Provision ........................................... 2-3 2.3 Receiving Waters ............................................................................................................. 2-3

3 DESIGN CRITERIA AND EXEMPTIONS......................................................................... 3-1 3.1 Conveyance Design ......................................................................................................... 3-1 3.2 Wetland Retention ........................................................................................................... 3-1 3.3 Water Quality Protection ................................................................................................. 3-1 3.4 Extended Detention.......................................................................................................... 3-2 3.5 Flood Hazard Protection .................................................................................................. 3-2 3.6 Project Flood Bypass ....................................................................................................... 3-2 3.7 Downstream Impact Analysis .......................................................................................... 3-3

4 REPORTS AND REVIEW SUBMITTALS ......................................................................... 4-1 4.1 Required Methods and Parameters .................................................................................. 4-1 4.2 Required Submittals......................................................................................................... 4-1 4.2.1 Drainage Project Notification .............................................................................. 4-2 4.2.2 Preliminary Drainage Report ............................................................................... 4-4 4.2.3 Drainage Report ................................................................................................... 4-4 4.3 Project Reporting Requirements ...................................................................................... 4-5 4.3.1 Crossing Project ................................................................................................... 4-5 4.3.2 Single-Lot Residential Project ............................................................................. 4-5 4.3.3 Small Project........................................................................................................ 4-5 4.3.4 Large Project........................................................................................................ 4-6

5 BASIN CHARACTERIZATION .......................................................................................... 5-1 5.1 Required Basin Feature Reporting................................................................................... 5-1 5.2 Information Sources......................................................................................................... 5-1 5.3 Conveyance and Streams ................................................................................................. 5-2

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Municipality of Anchorage

Drainage Design Guidelines March 2009

5.4 Contributing Areas (Drainage Basins)............................................................................. 5-3 5.5 Landcover ........................................................................................................................ 5-4

5.5.1 Pre-Development Landcover ............................................................................... 5-4 5.5.2 Post-Development Landcover.............................................................................. 5-4 5.5.3 Use of Empirical Landcover Coefficients............................................................ 5-4 5.5.4 Required Landcover Characterization ................................................................. 5-4 5.5.5 Landcover Archetypes ......................................................................................... 5-6 5.5.6 Community Management Landcover Adjustments ............................................. 5-8 5.6 Soil ................................................................................................................................... 5-9 5.7 Slope ................................................................................................................................ 5-9 5.8 Surface Roughness......................................................................................................... 5-10 5.9 Basin Mapping Procedures ............................................................................................ 5-10 5.9.1 Base Mapping .................................................................................................... 5-10 5.9.2 Downstream Mapping........................................................................................ 5-11

6 DESIGN STORMS ................................................................................................................. 6-1 6.1 Updated IDF Curves ........................................................................................................ 6-1 6.2 Storm Volume ? Base and Adjusted................................................................................ 6-1 6.3 Storm Duration................................................................................................................. 6-4 6.4 Storm Distribution ........................................................................................................... 6-4 6.5 Storm Frequency.............................................................................................................. 6-5

7 RUNOFF RESPONSE............................................................................................................ 7-1 7.1 Precipitation Losses ......................................................................................................... 7-2 7.1.1 Infiltration and Depression Storage ..................................................................... 7-2 7.1.2 NRCS/SCS Methods............................................................................................ 7-3 7.1.3 Rational Method Runoff Coefficients.................................................................. 7-7 7.1.4 Adjusting for Drainage Connectivity................................................................... 7-9 7.2 Times of Concentration.................................................................................................. 7-10 7.2.1 Overland and Sheet Flow................................................................................... 7-11 7.2.2 Shallow Concentrated Flow............................................................................... 7-12 7.2.3 Channel Flow ..................................................................................................... 7-13 7.3 Lag and Computation Times.......................................................................................... 7-15 7.3.1 Lag Time............................................................................................................ 7-15 7.3.2 Computation Time Interval and Hyetograph Time Step.................................... 7-15 7.4 Estimating Runoff Flows ............................................................................................... 7-16 7.4.1 Rational Method for Peak Flow......................................................................... 7-17 7.4.2 Unit Hydrograph Method................................................................................... 7-17 7.4.3 Kinematic Wave Method (Using Conceptual Methods).................................... 7-18 7.4.4 Time-Area Method (Using Flow Travel Times)................................................ 7-19 7.5 Runoff Models and Programs ........................................................................................ 7-20

8 ROUTING STORM WATER RUNOFF .............................................................................. 8-1 8.1 Routing Analysis Applications and Tools ....................................................................... 8-1 8.2 10% Method Routing Analyses ....................................................................................... 8-2 8.2.1 Considerations for Storage................................................................................... 8-3 8.2.2 Step-Wise Analysis.............................................................................................. 8-3

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Municipality of Anchorage

Drainage Design Guidelines March 2009

8.3 10% Routing Reporting Requirements ............................................................................ 8-7

9 SIZING AND DESIGNING STORMWATER CONTROLS............................................. 9-1 9.1 Detention Facilities .......................................................................................................... 9-2 9.1.1 Storm Detention Design....................................................................................... 9-2 9.1.2 Extended Detention Design ................................................................................. 9-2 9.2 Infiltration Controls ......................................................................................................... 9-3 9.2.1 Testing Infiltration Capacity ................................................................................ 9-4 9.2.2 Infiltration Control Design................................................................................... 9-6

10 CHANNEL EROSION AND ICING CONTROLS ......................................................... 10-1 10.1 Channel Erosion Control Design ................................................................................. 10-1 10.2 Icing Control Design.................................................................................................... 10-1

11 SELECTED REFERENCES ............................................................................................. 11-1

APPENDICES

Appendix A.................................................................................Drainage Report Contents Outline Appendix B ...................................................................................... Threshold Runoff Calculations Appendix C ................................................................................................................General Forms Appendix D............................................................................Type 1 24-Hour Rainfall Distribution Appendix E .................................................................................................... Example Calculations Appendix F .......................................................................................................Information Sources Appendix G.............................................................................. 10% Method Downstream Analysis

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