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NS754: Development of inlet protection BMP phasing performance and implementation criteria for all stages of construction

Problem

Curb and field Inlet protection is typically the last line of defense for the discharge of stormwater from trackout, gutter/conveyance, stockpile placement, loss at down gradient perimeter controls and other construction actions. A stormwater upset (illicit discharge) is considered when the device is overtopped or bypass of sediment into emergency overflow holes to receiving waters. A properly selected, properly installed, and properly maintained inlet should be able to perform properly, as intended, or to manufacturer recommendations to a 2-yr type storm (2023 NPDES MPCA Permit Definition).

Due to the near impossibility of phased inlet protection design over the life of the project, MnDOT commonly designs by a simple tabulation of number of proposed inlets and pays for inlet protection as “Each.” There are multiple inlet device protection variations (riser, surface, drop-in, rings). As the inlet protection device functions relative to soil disturbance contributing area, soil types and turbidity reduction performance is unspecified in the contract, the least cost BMP is bid. Furthermore, due to low bidding, there is no incentive to properly select and update appropriate inlet protection based on multiple factors (time of year, intensity of rain, acres of soil disturbance, project access, sub vs topsoils, adjacent construction operations, likelihood of spills) for more than one type of inlet protection Best Management Practice (BMP) over the life of the contract.

Why is this a priority and what are the benefits?

Functional inlet controls are a leading NPDES stormwater issue with construction permit compliance. Inlets are one of the most difficult for implementation while keeping roads safe to operate. A one size fits all approach for inlets does not work for either public safety nor illicit turbid water minimization. What is known about some inlets devices that function to limit discharges of sediment use the area around them as a sediment trap (for example, the road surface). In this case, approximately 90% of the stormwater may stored on pavement, grade, or ditch as the water is passed through the inlet. If too fast, not enough residence time to allow control of sediments. If too slow, then flooding of pavements or transfer down gradient of excessive water. On pavements that are open to public utilization this means standing water that could be as deep as the curb back with spread into the drive lanes.

The priority for inlet devices must utilize the correct device for each stage of construction with the shortest period possible. Inlet devices must be phased based on sediment and public safety risk and removed appropriately. This will also enable inspectors, regulators, and others to properly evaluate functional and non-functional inlets during inspections. Safety is a high priority. Sediment capture for permit compliance is a high priority. The ultimate benefit to construction is having properly selected and phased implementation of inlet devices as the work progresses. Following research will develop the implementation factors, standard procedures and contract language.

Objectives

  1. Perform literature search of previous inlet studies (start with Auburn, AL).
  2. Research how inlet devices commonly used for field and curb connected structures control sediment release (e.g. filtration, sedimentation, capture, barrier) to stormwater conveyances.
  3. Research how to predict and measure maximum flow rate of clean water until overtopped or bypass flow and how this rate changes with simulated turbid stormwater. Determine what criteria leads to a stormwater failure event (inches per hour, total accumulated rain necessary to indicate maintenance, or limits to a design storm event). Run models to show how disturbed area and soil type relate to inlet function and performance.

Previous research

Research has examined performance of individual non-propriety and propriety inlet BMPs. ASTM standards have been developed using target soil types, sediment concentration, flow rate, and test duration. This has led to development of a silt loam as the target textural class. What has not been studied is how the exposed soil areas and soil types exposed by grading and excavation relate to specific inlet types, how combined rain on rain events influence inlet protection BMPs and how performance must be adaptable as the field condition changes.

  • Bhattarai et al. 2016. Evaluation of curb and gutter inlet protection products for sediment retention. A report of the findings of ICT-R27-102. Illinois Center for Transportation Series No 16-004. Research Report No. FHWA-ICT-16-004.
  • Grimm et al., 2024. Holistic evaluation of inlet protection devices for sediment control on construction sites. J. Env. Management, V364 July 2024, 121256.
    • Champion comments: This paper defined inlet devices by Type: 1. Above grate sheet covers; 2. Above grate enclosures; 3. Curb inserts; 4. Under grate baskets with curb inlet attachments; 5. Wattles (what MnDOT calls sediment control logs); and 6. Non-proprietary devices. I have developed an inlet guidance document and naming regimen to aid in staged deployment.
  • Hathaway et al., 2024. A synthesis of climate change impacts on stormwater control measures in the U.S.: designing for resiliency and future challenges. J. Soil Water Conserv., 60(4)(2005), pp. 193-199.
  • McCaleb and McLaughlin, 2008. Sediment trapping by five different sediment detention devices on construction sites. Trans. ASABE 51(5) (2008), pp. 1613-1621.
  • McFalls, Jett. 2024. Developing a performance evaluation facility for sediment control devices. Texas A&M Transportation Institute, 0-7100-R1. https://tti.tamu.edu/documents/0-7100-R1.pdf.
  • Perez et al., 2016-07-01. Evaluation of inlet protection practices using large-scale testing techniques. ALDOT 930-853R.
  • Renna et al., 2010. Inlet Protection devices and their effectiveness. Florida Dept. of Transportation, University of Central Florida.
  • Strecker et al., 2001. Determining urban stormwater BMP effectiveness. J. Water Resour. Res., 127 (3) (2001), pp. 144-149.
  • Stenlund et al. 2016. Biosystems and Bioproducts Engineering Capstone Project.
  • Winston et al., 2023. Measuring sediment loads and particle size distribution in road runoff: implications for sediment removal by stormwater control measures. Sci. Total Environ., 902 (2023) Article 166071.

Expected outcomes

  • New or improved tool or equipment

Expected benefits

The numbers 1 and 2 indicate whether the source of the benefit measurement is from: 

  1. A specific research task in your project that will be measuring this particular benefit, or
  2. A separate effort to analyze data provided by the state or local agency involved in this project.
  • Environmental Aspect: (1)
  • A specific research task in your project that will show statistically how performance based inlet protection reduces both sediment transport loss and permit violations.
  • Operation and Maintenance Saving: (2)
    • Demonstrate cost savings using sediment loss prevention approach relative to cleaning, retrieval, and disposal of underground stormwater conveyances prior to permit termination.
  • Safety: (2)
    • Estimate reduction of inlet and street flooding due to routinely amending inlet devices to the intended purpose, season, and operation.

Technical advisory panel