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DRN-006 Drainage Report Required Contents

Permit and Resource Management Department Banner 750

Version: 02/01/2024

Drainage reports shall be prepared with sufficient clarity to indicate the nature and extent of proposed drainage improvements. Drainage improvements must be designed according to the Sonoma County Water Agency’s Flood Management Design Manual and to Sonoma County Code. Drainage improvements must also demonstrate no adverse impacts to existing and proposed structures and to adjacent properties.

Drainage reports for engineered grading may only be prepared by a civil engineer. Drainage reports for regular grading and all other permit applications may be prepared by the property owner, a civil engineer or an architect (if that architect also designed and prepared the grading or site plans for the same development). The cover sheet of drainage reports prepared by property owners shall include the property owner’s signature and a note stating acceptance and responsibility of the drainage report by the property owner.

Drainage reports shall include the following items, where applicable:

A. Cover Sheet

  1. Name of project and site address
  2. Assessor’s Parcel Number(s)
  3. Permit number
  4. Property owner name and contact information
  5. Report preparer name and contact information
  6. For engineered construction grading, seal and signature of civil engineer
  7. For regular construction grading signature of property owner or licensed professional acting within the scope of their license
  8. Date prepared

B. Project Narrative 

  1. Description/scope of project
  2. Methodologies used to prepare drainage report and analysis (IRM, Mannings, SR LID, BASMAA, etc.)
  3. Assumptions pertinent to drainage design (runoff coefficient, curve number, time of concentration, Manning’s roughness coefficient, etc.)
  4. Disclosure of existing on-site and off-site drainage conditions to nearest natural water body that runoff releases to
  5. Impacts of proposed drainage design
  6. Conclusion statement addressing adequacy of design

C. Hydrology and Hydraulic Calculations

  1. Pre- and post-development analysis
  2. Hydrology calculations for all pertinent drainage areas
  3. Hydraulic calculations for proposed drainage improvements such as swales, culverts, inlets, curbs, gutters, and pipes
  4. Hydraulic calculations for existing drainage improvements affected by drainage design
  5. Hydraulic Grade Line (HGL) calculations for storm drain pipe network systems
  6. Explanation of established or determined starting HGL
  7. Post-development stormwater levels and pollutant discharges shall be limited in compliance with Permit Sonoma best management practices guide

D. Hydrology Maps

  1. All items listed under cover sheet requirements
  2. Drawn to an engineering scale no less than 1 inch = 100 feet
  3. North arrow and scale (written and graphic)
  4. Property lines, right-of-ways, and easements
  5. Contours and elevations labeled as existing and proposed
  6. Structures labeled as existing and proposed
  7. Finished floor elevations of all relevant structures
  8. Pre- and post-development drainage areas for on-site and off-site hydrology analysis clearly delineated and labeled
  9. Points of concentration clearly shown and labeled
  10. All storm water capture, treatment & mitigation features shall be shown & appropriately detailed on the plans & hydrology maps
  11. Drainage improvements clearly labeled as existing and proposed
  12. Elevations for inverts, flowlines, top of grates, and high points
  13. Drainage specifications (e.g. size, material, length, slope)
  14. Structure list for large networks

E. Other Required Documentation

  1. The 100-year overland release route/map including areas of existing and anticipated inundation
  2. Projects within the Flood Prone Urban Area shall include documentation demonstrating the proposed project will not adversely affect drainage
  3. Projects within Special Flood Hazard Areas shall include documentation demonstrating the proposed project will not cause a reduction in flood carrying capacities (no net fill)
  4. Any other supporting or supplemental data, documentation or calculations relevant to the proposed project

For questions regarding drainage report required contents, please visit or call Permit Sonoma's Grading, Drainage, and Storm Water section.