The Stormwater Design Standards can be found in Chapter 2 of the Riverton City Standards Specifications and Plans.
To comply with the Clean Water Act the City encourages development to reduce their runoff impact by retaining all or a portion of runoff.  Development that conforms to this approach can receive up to a 45% reduction of their Stormwater Utility Fee.  Stormwater Utility Fee Adjustments may be apply for by application.  
The Utah Department of Water Quality provides a manual; A Guide to Low Impact Development within Utah for reference, which can aid designing infrastructure that can qualify for the Stormwater Utility Fee reduction.