Sustainability for all the places between the buildings.

Supplemental Materials for Workshop Participants from Green Girl LDS

Calculating average field infiltration rates (excel)

Cost estimating form (excel)

Cost estimate supplemental (word)

Contact Maria Cahill for more info on these files

Raingarden Sizing Worksheets from Derek Godwin

The following is an email/disclaimer about the files below.

Folks,

Please remember that we haven’t completed our peer-review process so these items may contain some errors.

• The Calc. Imp. Surf. document and Example site pdf are used together when we present this during a workshop. This was created by the East Multnomah Soil and Water Conservation District (Candace Stoughton – candace@emswcd.org).  

• The soil test methodology and data sheet are used together in the field. These methods are based on EPA’s falling head test. I could dig around for the reference if you need it. Remember, the Simple method is only meant for homeowners designing their own rain garden in areas where their local jurisdiction does not require a more elaborate test.  

• The sizing table is ONLY for Type 1A rainfall hydrographs and the Modeling approach to the infiltration test. Note – as a safety factor, the City of Portland requires the tested infiltration rate to be divided by 2 for a design infiltration rate. I haven’t found this to be consistent among jurisdictions and consultants.

• I have attached the spreadsheet model in case you want to see the calculations that were used. It uses a Type 1A hydrograph and adjusts for rain gardens with 3:1 side slopes. I made a few notes on which numbers to change in order to run the model. This is a common method used in many jurisdictions. This spreadsheet was developed by Steve Fancher with the City of Gresham in the Portland Metro area.

Design Storms - I have found that larger cities have completed rainfall frequency analysis and specific storm sizes for design (i.e. 80-90% rainfall amount and 2, 5, 10 and 25 year events). Since the frequency analysis (80-90% annual rainfall) has not been completed for smaller cities, but our Oregon Climate Service has rainfall estimates for various return intervals, I have found that the 6-month 24-hour return interval rainfall is a conservative estimate for a design storm for water quality (estimate of the 80-90% rainfall amount). I provide a rainfall map for this size event for people to use when choosing a design storm if their local jurisdiction doesn’t specify a storm size.

Let me know if you have questions.  

Derek Godwin
Staff Chair and Watershed Management Specialist
OSU Extension Service, Marion County
3180 Center Street NE Room 1361
Salem, Oregon 97301
phone: 503-566-2909
fax: 503-585-4940
email: derek.godwin@oregonstate.edu

Download Files

Calculate Impervious Surface 6_10_09.doc

Testing Your Soil and Drainage for Rain Gardens.doc

Calculate Impervious Surface example site NO LOGO.pdf

Infiltration Test data sheet.doc

Rain Garden Sizing Practice sheet.xls

Sizing Table for Rain Gardens in Western Oregon.doc

Testing Your Soil and Drainage for Rain Gardens.doc