Risk Level Determination

The California General Permit places all non-exempt construction sites and linear underground project sites encompassing more than one acre in size into one of three risk levels associated with polluting the waterways.

Risk Level 1 has the least amount of risk to the waterways and the least amount of restrictions.
Risk Level 2 has increased restrictions including extensive sampling requirements for stormwater runoff.
Risk Level 3 has the most restrictions including extensive sampling requirements for stormwater runon and runoff.

While the calculation to determining the risk level of your construction site may seem complicated, it is actually rather simple. You need to know two values:

1. Site Sediment Risk Factor
2. Receiving Water Risk Factor

Site Sediment Risk Factor
This is the formula to get the Site Sediment Risk value.

A = (R)(K)(LS)(C)(P)

A = tons per acre site sediment risk factor
R = rainfall-runoff erosivity factor
K = soil erodibility factor
LS = length-slope factor
C = cover factor (erosion controls)
P = management operations and support practices (sediment controls)
The C and P factors are given values of 1.0 to simulate bare ground conditions.

With C and P equal to one, we can revise the formula to be as follows:

A = (R)(K)(LS)

R Value
To determine the erosivity factor, go to the calculator provided by the EPA at Erosivity Calculator. It will tell you your erosivity factor based on your geographic location such as the physical address or latitude and longitude numbers.

K Value
The soil erodibility factor value is taken from the value given on this map:
Soil Erodibility Map

LS Value
The length-slope factor value is taken from the value given on this map:
Length / Slope Map

By multiplying the three values together, you get the A value otherwise know as your Site Sediment Risk value which can then be broken down to one of three risk levels.

Low Site Sediment Risk: A < 15
Medium Site Sediment Risk: A >= 15 and A < 75
High Site Sediment Risk: A >= 75

Receiving Water Risk Factor
The risk factor is a Yes or No (High or Low) question. Does the disturbed area discharge (directly or indirectly) into either a 303(d) listed water impaired by sediment waterbody or a waterbody with designated beneficial uses of spawn and cold and migratory?

An easy to read map to determine if you are at risk can be found here - 303(d) Listed, Spawn, Cold, and Migratory Waterbodies.

California’s comprehensive list of 303(d) listed waterbodies is found on this PDF document found on the California State Water Resources Control Board website -
http://www.waterboards.ca.gov/water_issues/programs/stormwater/docs/constpermits/wqo_2009_0009_app_1_303d.pdf.

More detailed listing of Spawn, Cold and Migratory waterbodies can be found on these three web pages maintained by UC Davis.
Spawn - http://www.ice.ucdavis.edu/geowbs/asp/wbusequery.asp?use=SP
Cold - http://www.ice.ucdavis.edu/geowbs/asp/wbusequery.asp?use=CO
Migratory - http://www.ice.ucdavis.edu/geowbs/asp/wbusequery.asp?use=MI

So if your disturbed area discharges to any of the waterbodies listed here, the value is “High”; otherwise, the value is “Low”.

Final Determination
Now apply your known values to the following chart.