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DRN-006 Drainage Report Required Contents

Version: 06/07/2021

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 §11.16.040 and §11.16.050. 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:

Cover Sheet

  • Name of project & site address 
  • Assessor’s Parcel Number(s) 
  • Property owner name & contact information
  • Report preparer name & contact information 
  • Seal & signature of civil engineer or architect 
  • Date prepared

Project Narrative

  • Description/scope of project
  • Methodologies used to prepare drainage report & analysis
  • Assumptions pertinent to drainage design
  • Disclosure of existing on-site & off-site drainage conditions
  • Impacts of proposed drainage design

Hydrology & Hydraulic Calculations

  • Pre- & post-development analysis
  • Hydrology calculations for all pertinent drainage areas
  • Hydraulic calculations for proposed drainage improvements such as swales, culverts, inlets, curbs, gutters, & pipes
  • Hydraulic calculations for existing drainage improvements affect by drainage design
  • Hydraulic Grade Line (HGL) calculations for storm drain pipe network systems
  • Explanation of established or determined starting HGL
  • Post-development storm water levels & pollutant discharges shall be limited in compliance with Permit Sonoma best management practices guide

Hydrology Maps

  • All items listed under cover sheet requirements
  • Drawn to an engineering scale no less than 1 inch = 100 feet
  • North arrow & scale (written & graphic)
  • Property lines, right-of-ways, & easements
  • Contours & elevations labeled as existing & proposed
  • Structures labeled as existing & proposed
  • Finished floor elevations of all relevant structures
  • Pre- & post-development drainage areas for on-site & off-site hydrology analysis clearly delineated & labeled
  • Points of concentration clearly shown & labeled
  • Drainage improvements clearly labeled as existing & proposed
  • Elevations for inverts, flowlines, top of grates, high points
  • Drainage specifications (e.g. size, material, length, slope)
  • Structure list for large networks

Other Required Documentation

  • The 100-year overland release route/map including areas of existing & anticipated inundation
  • Projects within the Flood Prone Urban Area shall include documentation demonstrating the proposed project will not adversely affect drainage
  • 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)
  • 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 & Storm Water customer service cubicle.

Contact Information

Contact us by Phone
Monday – Tuesday
Thursday – Friday
8:00 AM – 4:00 PM
10:30 AM – 4:00 PM
Permit Sonoma
2550 Ventura Avenue
Santa Rosa, CA 95403
38.465074, -122.723705


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