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MS4s are required to develop and implement a post-construction program to prevent or reduce pollutant runoff from new development and redevelopment sites. These sites disturb one or more acres or are part of a larger common development plan or sale. The developers must provide best management practices, such as vegetated swales, extended detention basins, or other water quality treatment measures, that must be included in the construction of those projects. Post-construction runoff controls, and best management practices, help reduce pollutant loads from being transported off a developed site and decrease the quantity of water being delivered to water bodies during storm events. 

Control Measure Selection and Design

The City of Thornton must ensure the Selection and Design of Permanent Stormwater Control Measures on all applicable developments. Each development must meet one of several Design Standards for Water Quality through the use of one or more Control Measures. The process for choosing which Design Standard will be used and which Control Measures built to meet said Standard is largely performed by Engineers. Mile High Flood District has created a Criteria Manual, for which the City of Thornton has subscribed and utilized, which includes many useful tools for the selection and design process. Also, for documentation a Base Design Standards Worksheet is collected to fulfill and document these requirements for the City’s MS4 permit.

Control Measure Installation

The City of Thornton must ensure the proper Installation of Permanent Stormwater Control Measures on all applicable developments. This is achieved through field inspections of each Control Measure as they are installed and requires close coordination with builders.

O&M/Non Structural Control Measures

The City of Thornton must ensure the Operation and Maintenance of Permanent Stormwater Control Measures used to fulfill the City’s MS4 permit. The City also makes a good faith effort to ensure the O&M of Control Measures built before the MS4 permit requirement existed or those without full water quality functionality. This is achieved through systematic field inspections of all said Control Measures, or as complaints arise. When a Control Measure is found to have need for maintenance the City reaches out to the owner, offers guidance, and if needed escalates to enforcement. Owners of such Control Measures can help in this process by maintaining the facilities regularly to avoid expensive repairs. Many of the ways to keep a Control Measure in good order involves the use of Non Structural Control Measures that prevent or reduce pollutants getting into water and/or the generation of illicit discharges. These include trash and debris removal, mowing, limitations on use, snow removal procedures, etc.

Extended Detention Basin Maintenance (training videos intended for owners and maintenance workers)


Control Measure Educational Signage

The City of Thornton has a campaign to improve citizen awareness of permanent stormwater quality treatment features around the City through the use of educational signage. Signage is located near or in the Control Measures and includes information about the feature’s design and function. The City’s hope is that the signs will bring greater understanding of the importance of stormwater quality and how these often forgotten or misunderstood Control Measures work. Details for installation as well as artwork for builders are available in the Links section of this page.