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      "context_excerpt": "# 11.5 SUBSURFACE GRAVEL WETLANDS\n\n[OCR image]\n\nSubsurface gravel wetlands are a stormwater management system designed to maximize the removal of pollutants from stormwater runoff; the system is a combination of a surface marsh and a subsurface gravel bed. Pollutants are treated through settling, by both uptake and filtration by vegetation and by chemical transformation in the subsurface bed, specifically denitrification. Both the total suspended so",
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      "context_excerpt": "ter runoff through the gravel wetland system. For more information on each component, refer to the Subsurface Gravel Wetland Components section of this chapter, beginning on Page 10.\n\n## Initial Equilibrium\n\n[OCR image]\n\nBetween storm events, runoff from the previous storm is only detained in the gravel cells; subsurface gravel wetlands are not intended to store stormwater runoff in any surface component for longer than 72 hours after the end of a storm event.\n\n## Stage 1 - Onset of Runoff\n\n[OCR image]\n\nRunoff enters the pretreatment zone, in this case, a forebay.",
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      "context_excerpt": "ed in the gravel cells; subsurface gravel wetlands are not intended to store stormwater runoff in any surface component for longer than 72 hours after the end of a storm event.\n\n## Stage 1 - Onset of Runoff\n\n[OCR image]\n\nRunoff enters the pretreatment zone, in this case, a forebay.",
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      "context_excerpt": "[OCR image]\nStage 2 – Runoff Enters Wetland Cell #1\n\nRunoff fills the forebay and begins to flow into Wetland Cell #1. As Wetland Cell #1 fills, runoff is discharged through the perforated riser pipe into Gravel Cell #1. The runoff from the previous storm event only begins to discharge through the outlet structure when it is displaced by the new flow.\n\n[OCR image]\nStage 3 – Wetland Cell #1 Fills\n\nAs t",
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      "context_excerpt": "noff is discharged through the perforated riser pipe into Gravel Cell #1. The runoff from the previous storm event only begins to discharge through the outlet structure when it is displaced by the new flow.\n\n[OCR image]\nStage 3 – Wetland Cell #1 Fills\n\nAs the rain event continues, Gravel Cell #1 begins to discharge into Gravel Cell #2 through the subsurface cross drain. The continuous inflow of runoff into the system forces the flow from Gravel Cell #2 to discharge through the outlet structure.",
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      "context_excerpt": "[OCR image]\nStage 4 - Runoff Enters Wetland Cell #2\n\nAs runoff continues to enter the system, it discharges from Wetland Cell #1 to Wetland Cell #2 through the surface cross drain, and it continues to discharge from Gravel Cell #1 to Gravel Cell #2 through the subsurface cross drain. As Wetland Cell #2 fills, runoff begins to discharge into Gravel Cell #2 through the perforated riser pipe in Wetland Cell #2. This r",
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      "context_excerpt": "rface wetland cell, two subsurface gravel cells connected by a subsurface cross drain, two perforated riser pipes connecting each of the wetland cells to the gravel cells below them and an outlet structure.\n\n[OCR image]\n\nProfile View – General Components\n\nEach surface wetland cell must be connected by a cross drain and sized to store 50% of the WQDS volume. The strategy (forebay, vegetative filter strip or MTD) selected for use in the pretreatment zone must be sized separately from the gravel wetland system.\n\nThe transition layer between the wetland soil and the gravel cells is required to prevent the finer portion of",
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      "context_excerpt": "where the native soils are sufficiently impermeable, the liner may be omitted provided that the system has the required separation from the SHWT.\n\n[OCR image]\nDetailed Profile of Wetland Cell #1 and Gravel Cell #1\n\nThe illustration above shows a closer view of the first half of the subsurface gravel wetland system. Runoff enters the system through the forebay; from there, it enters Wetland Cell #1. The riser pipe in the wetland cell has perforations up to the elevation of the WQDS. Flows up to this elevation are slowly discharged into Gravel Cell #1; flows i",
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      "context_excerpt": "# Detailed Profile of Wetland Cell #2 and Gravel Cell #2\n\n[OCR image]\n\nThis illustration shows a closer view of the second half of the subsurface gravel wetland system. As shown in the previous illustration, runoff up to the elevation of $\\frac{1}{2}$ the WQDS volume is temporarily stored in Wetland Cell #1. Flows in excess of this elevation enter Wetland Cell #2 through the cross drain. As with Wetland Cell #1, there is another riser pipe in Wetland Cell #2, which also",
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      "context_excerpt": "[OCR image]\nGuide to Sections in Plan View\n\nThe above illustration depicts the same gravel wetland system divided into three horizontal sections, each at different elevations. The following three illustrations each focus on a specific section of the gravel wetland system, as shown above.\n\n[OCR image]\nSection 1 Plan View – Gravel Cells #1 and #2\n\nThe above illustration depicts a plan view of the gra",
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      "context_excerpt": "gravel wetland system divided into three horizontal sections, each at different elevations. The following three illustrations each focus on a specific section of the gravel wetland system, as shown above.\n\n[OCR image]\nSection 1 Plan View – Gravel Cells #1 and #2\n\nThe above illustration depicts a plan view of the gravel cells, which shows an optional horizontal perforated manifold; the manifold is intended to help distribute the flow through the gravel cells in systems where the width of the gravel cells is wide enough to impede dispersal.\n\nThe above illustration also shows the required 15-foot minimum length in each",
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      "context_excerpt": "um inflow rate; this is necessary to address potential clogging and to ensure that the pipes do not act as a hydraulic control. The system must be designed to drain the WQDS volume between 24 and 48 hours.\n\n[OCR image]\nSection 2 Plan View – Wetland Soil Layer in Both Cells\n\nThis illustration shows the wetland soil layer at a distance of 4 inches above the transition layer. This elevation is important because it is the elevation at which the invert of the primary outlet must be located. At this elevation, the primary outlet maintains the water level at 4 inches above the bottom of the wetland soil; continuous inundati",
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      "context_excerpt": "# Section 3 Plan View – Wetland Cells at the Surface Cross Drain\n\n[OCR image]\n\nThis illustration depicts the top of the gravel wetland system looking down through the wetland cells at the elevation of the surface cross drain; the vegetation has been omitted to better show the other components. As previously shown, runoff enters the system through the forebay and then flows into Wetland Cell #1. All flows up to the elevation of the WQDS volume are discharged into Gravel Cell #1 t",
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