Drainage Rate Structure FAQ
Why was the rate structure changed in 2008?
The drainage rate structure was implemented in 2008 to improve the equity of drainage charges. The rate structure, which introduced additional rate tiers, more accurately reflects the differences in customers’ property characteristics, and therefore, in their impact on the drainage system.
How was the drainage rate structure changed in 2008?
In 2007, all single family residential and duplex property owners paid the same drainage flat fee regardless of parcel size. Commercial customers paid varied rates depending on the percentage of their total parcel which was covered by impervious surface. These rates were multiplied by the number of acres on the parcel to calculate the total drainage fee. In 2008, the following changes took effect:
- Single family residential and duplex parcels smaller than 10,000 square feet are now divided into four categories, each associated with a range of parcel sizes (e.g., 3,000 to 4,999 square feet). Each rate category is assigned a flat fee which increases as the parcel size range increases. So properties assigned to the 3,000 to 4,999 square feet rate category are charged less than properties assigned to the 5,000 to 6,999 square feet rate category. However, all properties assigned to the same rate category will be charged the same fee, regardless of variances in lot size.
- Large single family residential/duplex parcels (10,000 square feet or greater) are no longer charged a flat fee. Rather, these properties are charged in the same manner as commercial properties.
- The fee structure for large residential and commercial properties remains similar to the 2007 commercial fee structure, with the following exceptions:
- Properties with significant amounts of highly pervious (absorbent) surfaces on their properties may qualify for new low impact drainage rates.
- Unit rates will be charged per 1,000 square feet and not per acre.
What is the drainage fee based on?
Drainage rates are based on a property’s impact on the drainage system. This impact is measured by the estimated amount of stormwater runoff which flows off of a property. Stormwater runoff is a function of the size of a parcel and what is covering it. Different types of cover absorb more water than other types. For example, an undeveloped landscaped piece of land will infiltrate more stormwater than a paved parking lot of the same size. Likewise, a 20,000 square feet parking lot will generate more runoff than a 10,000 square feet parking lot. To capture these differences, properties are assigned to rate categories which group properties with similar sizes and/or cover types.
How did SPU determine what the rate categories are?
SPU’s drainage rate categories group parcels with similar property characteristics which will, in turn, contribute similar amounts of stormwater runoff. SPU estimates the fraction of rainfall that becomes “runoff” from a parcel using standard runoff coefficients utilized in hydrological modeling. These runoff coefficients vary depending on how pervious (permeable) a surface is. More stormwater will run off of hard or impervious surfaces while pervious or permeable surfaces will absorb more water.
The amount of stormwater that will run off of a property depends on the type of surface covering the property and its size. The SPU rate categories incorporate both of these factors in the development of their respective rates. The fee structure for some rate categories relies on category averages for surface type while the structure for other categories is based on parcel specific surface type information and size. However, the assumptions regarding how much stormwater will flow off of a given surface are consistent across all rate categories, regardless of fee structure
Small residential customers with billable areas less than 10,000 square feet are charged a flat rate based on the surface type profile for a typical customer in the specified rate category. These properties, which account for approximately 93% of all single family and duplex parcels, are fairly homogeneous in terms of surface cover, which makes property size the key determinant of parcel stormwater flow contribution. Small residential customers are assigned to one of four size-based categories, each representing a range of total area (e.g., 3,000 to 4,999 square feet). SPU used sample data collected City-wide from aerial photographs for small residential parcels to estimate the typical surface types for each parcel size range. This surface type information combined with the parcel sizes within category was used to develop the respective flat rate by category.
Large single family and duplex parcels 10,000 square feet or greater (“large residential”) and commercial parcels (all sizes), pay a unit rate (per 1,000 square feet of billable parcel area) based on their actual property characteristics (percent impervious and parcel size) rather than category averages. There is too much variation between these properties in terms of parcel size and surface characteristics to be fairly captured by a flat rate structure like that applied to small residential customers. SPU has five impervious surface-based rate categories. Each category represents a range of impervious surface (e.g., 66-85% impervious). SPU used commercial and large residential parcel data collected from aerial photos to analyze the surface area for each parcel which was then used to develop a rate per 1,000 square feet of parcel area for each rate category. In addition, beginning in 2008 low impact rates are available for commercial and large residential parcels which contain significant amounts of highly pervious (absorbent) area and which are composed of no more than 65% impervious area.
What is impervious surface?
Impervious surface is any hard or impermeable surface such as blacktop, rooftops, parking lots, patios, hardpan, and hard packed athletic fields. This type of surface absorbs much less rainwater than pervious surfaces covered with grass, trees, or other vegetation.
What parcels may qualify for low impact drainage rates?
Low impact rates apply to large residential and commercial parcels with significant amounts of highly pervious surface, such as forested land or other unmanaged vegetated areas such as pasturelands and meadows. This highly pervious surface must cover a continuous area of at least one-half an acre, although this coverage may span more than one parcel. Low impact rates are available for the Undeveloped (0-15 percent impervious), Light (16-35 percent impervious) and Medium (36-65 percent impervious) rate categories.
Why are low impact rates lower than rates for other properties with the same percentage of impervious surface?
Parcels containing large amounts of highly pervious area will generate significantly less stormwater run-off than parcels with similar amounts of impervious surface but whose pervious area is less absorbent (e.g., a highly managed lawn).
How do I know if my parcel contains enough highly pervious surface to qualify for a low impact rate?
The total estimated stormwater runoff for a parcel is calculated based on specific stormwater runoff factors for each surface type. If the total run-off for a given property falls below specific thresholds for each rate category, the parcel will be billed the low impact rate. SPU customer service can verify if your property meets a low impact threshold if the parcel area, impervious area, and highly pervious area are known.
To be considered for these rates, the area which is considered highly pervious must span an area of at least one-half an acre. This area may include continuous coverage on an adjacent parcel or parcels. Secondly, the area must be in a natural, unmanaged state (e.g., golf courses, landscaped parks or lawns, etc. would not qualify as highly pervious).
Why do small residential properties pay a flat rate and no one else does?
Drainage rates are structured so that each property is assigned a cost which is proportionate to its impact on the drainage system. Properties assigned to small residential rate classes are relatively homogenous both in terms of size (limited square feet ranges for each rate category) and the type/size of structures on their properties. Therefore, using actual impervious surface information for a sampling of these properties allows us to develop a good estimate of stormwater runoff from properties in a narrow lot size band. In addition, collecting and maintaining property data for a sample of these properties, rather than the entire population, holds down administrative costs as these properties account for 74% of all parcels.
There is a much greater variance in lot size and property composition on residential properties 10,000 square feet and greater. Likewise, non-residential properties vary considerably in terms of lot size and the type/amount of surface covering these parcels. Therefore, the fairest way to bill these properties is to use their actual lot size and percent impervious.
Is my drainage rate the same thing as my drainage bill?
That depends on the size and type of your property. For small residential and duplex properties smaller than 10,000 square feet, the drainage rate, which is a flat fee, is equal to the total drainage bill. For all other properties, the drainage rate for the assigned category must be multiplied by the lot area (in 1,000’s of square feet) to calculate the total drainage bill.