Volume 2-05, Chapter 3 - City of Wichita
Volume 2-05, Chapter 3 - City of Wichita Volume 2-05, Chapter 3 - City of Wichita
Section 3.2.6 - Infiltration Trench 3.2.6.1 General Description Infiltration trenches are excavations typically filled with stone to create a temporary underground reservoir for stormwater runoff (see Figure 3-24). This runoff volume gradually filtrates through the bottom and sides of the trench into the subsoil and eventually reaches the water table. By diverting runoff into the soil, an infiltration trench not only treats the WQ V , but also helps to preserve the natural water balance on a site and can recharge groundwater and preserve baseflow. Due to this fact, infiltration systems are limited to areas with highly porous soils (infiltration rate of > 0.5 in/hr) where the water table and/or bedrock are located well below the bottom of the trench (5’ or more below). In addition, infiltration trenches must be carefully sited to avoid the potential of groundwater contamination. Infiltration trenches are not intended to trap coarse sediment and must always be designed with a sediment forebay and grass channel or filter strip, or other appropriate pretreatment measures to prevent clogging and failure. Due to their potential for failure, these facilities must only be considered for sites where upstream sediment control can be ensured. 3.2.6.2 Stormwater Management Suitability Figure 3-24 Infiltration Trench Example (Knox Co., TN, 2008) Infiltration trenches are designed primarily for stormwater quality, i.e. the removal of stormwater pollutants. However, they can provide limited runoff quantity control, particularly for smaller storm events. For some smaller sites, trenches can be designed to capture and infiltrate the CP v in addition to WQ v . An infiltration trench will need to be used in conjunction with another structural control to provide flood control, if required. Water Quality Protection Using the natural filtering properties of soil, infiltration trenches can remove a wide variety of pollutants from stormwater through sorption, precipitation, filtering, and bacterial and chemical Page 3 - 82 Volume 2, Technical Guidance
Section 3.2.6 - Infiltration Trench degradation. Sediment load and other suspended solids are removed from runoff by pretreatment measures in the facility that treats flows before they reach the trench surface. Section 3.2.6.3 provides pollutant removal efficiencies that can be used for planning and design purposes. Channel Protection For smaller sites, an infiltration trench may be designed to capture and infiltrate the entire CP v in either an off- or on-line configuration. For larger sites, or where only the WQ v is diverted to the trench, another structural control must be used to provide CP v extended detention. Flood Control Infiltration trench facilities must provide flow diversion and/or be designed to safely pass design flood events and protect the filter bed and facility. The volume of runoff removed and treated by the infiltration trench may be included in the onsite and/or downstream flood control calculations (see Chapter 3). 3.2.6.3 Pollutant Removal Capabilities An infiltration trench is presumed to be able to remove 90% of the total suspended solids load in typical urban post-development runoff when sized, designed, constructed, and maintained in accordance with the recommended specifications. Undersized or poorly designed infiltration trenches can reduce TSS removal performance. The following design pollutant removal rates are average pollutant reduction percentages for design purposes derived from sampling data, modeling, and professional judgment. In a situation where a removal rate is not deemed sufficient, additional controls may be put in place at the given site in a series or “treatment train” approach. • Total Suspended Solids – 90% • Total Phosphorus – 60% • Total Nitrogen – 60% • Heavy Metals – 90% • Fecal Coliform – 90% For additional information and data on pollutant removal capabilities for infiltration trenches, see the National Pollutant Removal Performance Database (2nd Edition) available at www.cwp.org and the National Stormwater Best Management Practices (BMP) Database at www.bmpdatabase.org. Volume 2, Technical Guidance Page 3 - 83
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Section 3.2.6 - Infiltration Trench<br />
degradation. Sediment load and other suspended solids are removed from run<strong>of</strong>f by<br />
pretreatment measures in the facility that treats flows before they reach the trench surface.<br />
Section 3.2.6.3 provides pollutant removal efficiencies that can be used for planning and<br />
design purposes.<br />
Channel Protection<br />
For smaller sites, an infiltration trench may be designed to capture and infiltrate the entire CP v<br />
in either an <strong>of</strong>f- or on-line configuration. For larger sites, or where only the WQ v is diverted to<br />
the trench, another structural control must be used to provide CP v extended detention.<br />
Flood Control<br />
Infiltration trench facilities must provide flow diversion and/or be designed to safely pass<br />
design flood events and protect the filter bed and facility.<br />
The volume <strong>of</strong> run<strong>of</strong>f removed and treated by the infiltration trench may be included in the onsite<br />
and/or downstream flood control calculations (see <strong>Chapter</strong> 3).<br />
3.2.6.3 Pollutant Removal Capabilities<br />
An infiltration trench is presumed to be able to remove 90% <strong>of</strong> the total suspended solids load<br />
in typical urban post-development run<strong>of</strong>f when sized, designed, constructed, and maintained<br />
in accordance with the recommended specifications. Undersized or poorly designed<br />
infiltration trenches can reduce TSS removal performance.<br />
The following design pollutant removal rates are average pollutant reduction percentages for<br />
design purposes derived from sampling data, modeling, and pr<strong>of</strong>essional judgment. In a<br />
situation where a removal rate is not deemed sufficient, additional controls may be put in<br />
place at the given site in a series or “treatment train” approach.<br />
• Total Suspended Solids – 90%<br />
• Total Phosphorus – 60%<br />
• Total Nitrogen – 60%<br />
• Heavy Metals – 90%<br />
• Fecal Coliform – 90%<br />
For additional information and data on pollutant removal capabilities for infiltration trenches,<br />
see the National Pollutant Removal Performance Database (2nd Edition) available at<br />
www.cwp.org and the National Stormwater Best Management Practices (BMP) Database at<br />
www.bmpdatabase.org.<br />
<strong>Volume</strong> 2, Technical Guidance Page 3 - 83