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1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />1 <br />predominates. As an example, the table 2 compares the percentage of total <br />runoff contributions for a residential curve number of 83 (CN 83) versus <br />commercial /industrial curve number of 94 (CN 94) considering different <br />rainfall depths. <br />Greater runoff, and its associated pollutant load, is generally related to the <br />more intensely developed properties (commercial/industrial). Therefore, <br />using the basic premise of the utility (contributors pay), a rainfall amount that <br />will result in highly developed properties paying more than residential <br />properties should be used. <br />4.5 Typical Rainfall <br />A review of utility programs in other communities shows that a two -inch <br />rainfall has typically been used in determining utility. As illustrated in Table <br />2, a two -inch rainfall results in almost four times the runoff from a <br />commercial/industrial acre then from a residential acre. Therefore, <br />commercial/industrial properties would pay up to four times as much as a <br />residential parcel of equal size. <br />Table 2 <br />Rainfall /Runoff Ratios Example <br />Rain Depth <br />Runoff <br />Commercial/ <br />Industrial <br />Residential <br />Ratio* <br />1" <br />0.32" <br />0.01" <br />32.00 <br />2" <br />1.10" <br />0.29" <br />3.79 <br />3" <br />1.99" <br />0.81" <br />2.46 <br />4" <br />2.92" <br />1.46" <br />2.00 <br />5" <br />3.88" <br />2.19" <br />1.77 <br />2" Rainfall <br />Type C Soils <br />1.40" <br />0.70" <br />2.0 <br />1" Rainfall <br />Type C Soils <br />0.5" <br />0.13" <br />3.85 <br />* Ratio equals commercial runoff divided by residential runoff. <br />* This table uses typical curve number utilized in Minnesota Storm Water <br />Utilities. <br />There is also a statistical basis for the two -inch rainfall. A two -inch rain can <br />be expected to be equaled or exceeded once annually in any given 24 -hour <br />period; i.e., a one -year event (Oberts, 1984). While it is true that rainfall <br />amounts of less than two inches occur more frequently, smaller rainfall <br />events (one inch, for example) create the majority of the annual runoff <br />volume and pollutant load. <br />Feasibility Study <br />City of Lino Lakes, Minnesota Page 16 <br />A- LINOL0602.00 <br />