320A.10  STREAM CHANNEL AND FLOODPLAIN EROSION DESIGN CRITERIA.
   (a)   Runoff Rate.   The peak runoff rate from the development area shall not be greater after development than it was before development. The applicant shall provide calculations proving no increase in the runoff rates from the one (1), two (2), five (5), ten (10), twenty-five (25), fifty (50) and one hundred (100) year storms.
   (b)   Runoff Volume.  Increases in the runoff volume shall be offset by further restricting runoff rates. Based on the increase in runoff volume, the applicant shall determine the critical storm for the development area. The runoff rate from the critical storm shall be restricted to the one (1) year predevelopment storm runoff rate. The critical storm shall be calculated as follows:
      (1)   Determine the total volume of runoff from a one-year frequency, twenty-four hour storm, occurring on the development area before and after development.
      (2)   From the volumes in paragraph (1) determine the percent of increase in volume of runoff due to development according to the equation (Q after divided by the Q before-1) X 100 and, using this percentage, select the critical storm from this table:
Table 4: Critical Storm Selection
 
The Percentage Increase in Volume Of Runoff Is:
Equal To Or
Greater Than
And Less
Than
The 24-Hour “Critical Storm”
For Discharge Will Be
0
10
1 Year
10
20
2 Years
20
50
5 Years
50
100
10 Years
100
250
25 Years
250
500
50 Years
500
---
100 Years
 
   (c)   Detention Or Retention Basin Exemption For Redevelopment Or For Expansion Of Existing Facilities:
      (1)   For any development regulated by this chapter, the construction of a detention or retention basin may not be required for the development if the post-development peak discharge for a 100 year frequency 24 hour storm increases the existing peak discharge by one (1) cubic foot per second or less using the TR-55 method of calculation or other method approved by the City Engineer. The City Engineer can waive this requirement if existing storm sewers and drainage structures can safely handle the expected increase in flow.
      (2)   Only one (1) exemption will be allowed per parcel. Any subsequent expansion must provide for detention or retention and must include the previously exempted area.
   (d)   Where the City Engineer determines that site constraints exist in a manner that compromises the intent of this chapter to improve the management of storm water runoff as established in this section, practical alternatives may be used to result in an improvement of water quality and/or a reduction of storm water runoff. Such alternatives must be in keeping with the intent and likely cost of those measures that would otherwise be required to meet the objectives of this section. When possible, all practical alternatives shall be implemented within the drainage area of the proposed development project. Practical alternatives can include, but are not limited to:
      (1)   Fees shall be paid in an amount specified by the City Engineer. These fees shall be applied by the Community to storm water management practices that reduce existing storm water runoff.
      (2)   Implementation of off-site storm water management practices.
      (3)   Watershed or stream restoration.
      (4)   Retrofitting of an existing storm water management practice.
      (5)   Other practices approved by the City Engineer in keeping with the intent of this section.
         (Ord. 2011-4.  Passed 1-3-11.)