Stormwater runoff from construction activities can have a significant impact on water quality. As stormwater flows over a construction site, it picks up pollutants like sediment, construction debris, and chemicals such as gas, fertilizer, and paint. Polluted stormwater runoff can harm or kill fish and other wildlife. Sedimentation can destroy aquatic habitats, and high runoff can cause stream bank erosion.
Before obtaining a City of Thornton construction permit, the owner/contractor must obtain a Colorado Department of Public Health and Environment (CDPHE) Stormwater Discharges Associated with Construction Activity Permit Application. Under current regulations, this permit application covers construction sites disturbing one or more acres that include construction activities such as clearing, grading, excavation, and other ground disturbance activities.
Control Measures shall be installed before any earth disturbing activities commence.
The Owner/Contractor shall notify the Thornton inspector once all initial control measures have been installed for an initial inspection at least forty eight (48) hours prior to the inspection. Construction activity cannot begin until a passing initial inspection has occurred.
Stormwater discharges from construction activities shall not cause, have the reasonable potential to cause, or measurably contribute to exceed any water quality standard.
Construction shall be phased in a manner to limit earth disturbing activities (i.e. the entire project site should not be disturbed if construction will only be occurring in one particular section).
Sediment caused by accelerated soil erosion shall be removed from runoff water before it leaves the construction site.
Bulk storage structures for petroleum products and any other chemicals shall have secondary containment or equivalent protection to contain all spills and prevent any spilled material from entering the MS4 or State waters.
A copy of the SWMP and Erosion and Sediment Control (ESC) Plans must be available at all times on the construction site unless otherwise approved by CDPHE or Thornton.
The SWMP and EC plan shall be continuously updated to reflect new or revised Control Measures (CM) due to changes in design, construction, operation, or maintenance of the construction site. Updates must be made within 72-hours following the change in Control Measures.
The Owner/Contractor shall inspect the construction site (including all Control Measures, storage containers, and construction equipment) at a minimum of every 7 calendar days or every 14 calendar days. If on the 14 day frequency a 24-hour post storm inspection must be conducted after a precipitation event or snow melt. Inspections shall continue until an Inactivation Notice is filed with CDPHE.
The Owner/Contractor shall keep a record of all inspections on site and available for review by CDPHE or City staff. Inspection reports must identify any incidents of non-compliance with the terms and conditions of the Permit.
Control Measures requiring maintenance or adjustment shall be repaired immediately after observation of the failing Control Measure.
Silt fence patching: patching is only allowed on the top half of the fence. Not more than two (2) patches per section of fence. Silt fence with holes or deterioration on the lower half of the fence must be replaced. Repair typically involves replacing the silt fence to maintain the CMs effectiveness to drain slowly and function as originally designed.
For all instances of noncompliance based on environmental hazards and chemical spills and releases, all needed information must be provided orally to CDPHE spill reporting line (24-hour number for environmental hazards and chemical spills and releases: 1-877-518-5608) within 24-hours from the time the Owner/Contractor becomes aware of the circumstances.
Straw bales shall not be used for primary erosion or sediment control (i.e. straw bales may be used for reinforcement behind another BMP such as silt fence).
Control measures referred to as “Cutback Curb” are not allowed. The cutback curb may become ineffective and may also compromise the integrity of the curb and in most cases does not provide any water quality benefit for filtering out sediment.
Inlet Protection and Vegetative Buffer Control Measures shall not be used as standalone CMs. These methods must be utilized with at least one additional CM.
Control Measures intended for sheet flow sediment runoff shall be placed parallel to the slope.
All Control Measures shall be cleaned when sediment levels accumulate to half the design of the CM unless otherwise specified.
A Vehicle Tracking Control (VTC) shall be placed at all entrances/exits from the site as well as any egress from exposed dirt to paved areas to prevent track-out onto streets. If track-out does occur, the Owner/Contractor shall immediately sweep the street of debris. Recycled crushed concrete or asphalt shall not be used for vehicle tracking pads.
For residential projects, back of curb protection is required along all interior lots.
All sediment collected in Control Measures shall be removed upon initial acceptance.
Wind Erosion and Dust Control Measures must be utilized to minimize airborne particulate dust. Control Measures may include minimizing disturbed areas, watering, and/or providing temporary stabilization.
Permanent erosion control measures for slopes, channels, ditches, or any disturbed land area shall be completed within 14 calendar days after final grading or the final earth disturbance has been completed. When it is not possible to permanently stabilize a disturbed area after an earth disturbance has been completed or where significant earth disturbance activity ceases, temporary soil erosion control measures shall be implemented within 14 calendar days. Temporary erosion control measures shall be maintained until permanent soil erosion measures are implemented.
Final stabilization has been achieved when all earth disturbing activities at the site have been completed, and uniform vegetative cover has been established with an individual plant density of at least 70 percent of pre-disturbance levels, or equivalent permanent, physical erosion reduction methods have been employed.
All temporary Control Measures shall be removed from the site upon submitting the Inactivation Notice.
All site wastes (including trash and building materials) must be properly managed to prevent potential pollution discharges to the MS4 or State waters.
Street repair operations such as rotor milling, slurry seal and chip seal. The minimum CMs required are; inlet protection, curb socks and street sweeping.
Permanent Stormwater Treatment Feature Construction Notes:
All Permanent Stormwater Treatment Features are to be protected and maintained once installed throughout the duration of Construction.
Information regarding ease of access and Maintenance Access agreements can be found in the Plat as recorded by the County.
This list is not intended to be all‐inclusive, but is intended to identify the general notes identified by the City to be included as part of the ESC plans/SWMP for compliance with the City’s stormwater management requirements for construction activities.
The MHFD Resource Library contains great resources for Construction and Post-Construction reference. Check out the MHFD Criteria Manual Volume 3: Stormwater Quality!
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.
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.
The City of Thornton must ensure the proper installation of all Permanent Stormwater Control Measures altered or added through the MS4. This is ensured by both our engineering inspectors (via their acceptance process) and MS4 inspection staff who view the features as they are installed. Some features are more complex than others so, as a result, close coordination with builders is needed to ensure key points of feature construction is not missed. One example of such key points of inspection are components which will be buried after properly installed. To increase the chances of builder awareness and cooperation outside of the acceptance process, the City has developed a Stormwater Quality Feature Builder Checklist which highlights key steps and points of inspection with MS4 staff.
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.
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.
