{"metadata": {"source_document": "NJ BMP", "version_year": "2026", "chapter_subchapter": "Chapter 11.4", "citation": "BMP 2026, Ch 11.4, p. 1", "topic": "Table 1"}, "programmatic_checks": {}, "content_markdown": "# Sand Filters With Underdrain (Non-GI)\n\n## Table 1\n\n|  N.J.A.C. 7:8 Stormwater Management Rules – Applicable Design and Performance Standards  |   |   |\n| --- | --- | --- |\n|   | Green Infrastructure | Not Allowed  |\n|   | Stormwater Runoff Quantity | Only with a waiver or variance from N.J.A.C. 7:8-5.3  |\n|  GR | Groundwater Recharge | Not Allowed  |\n|  % | Stormwater Runoff Quality | Only with a waiver or variance from N.J.A.C. 7:8-5.3, 80% TSS  |\n|  Stormwater Runoff Quality Mechanisms and Corresponding Criteria  |   |\n| --- | --- |\n|  Settling  |   |\n|  Storage Volume | Entire Water Quality Design Storm Volume  |\n|  Filtration  |   |\n|  Sand bed minimum thickness | 18 inches  |\n|  Maximum storage above sand bed | 24 inches  |\n|  Maximum design permeability rate of sand bed | 2 inches/hour  |\n|  Minimum topsoil permeability rate, if using optional vegetative cover | ≥ 200% the hydraulic head loss of the underdrain  |\n\nSource context and notes:\n\nSource revision: January 2026.\n\n# 11.4 SAND FILTERS WITH UNDERDRAINS (NON-GI)\n\n[Figure: img-0.jpeg (original PDF, p. 1)](SOURCE.pdf#page=1)\n\n![OCR image: img-0.jpeg](images/page-0001-img-01.jpg)\n\nA sand filter with underdrain is a stormwater management system designed to maximize the removal of pollutants from stormwater. It consists of a pre-treatment zone and a treatment zone, which includes the sand bed and the underlying components. Pollutants are treated through settling, filtration, and adsorption by the sand bed. The total suspended solids (TSS) removal rate is 80%. However, sand filters with underdrains can only be used to address the quantity control aspects of development and stormwater runoff water quality when a waiver or variance from N.J.A.C. 7:8-5.3 is granted as they are not considered to be green infrastructure.\n\n\n|  N.J.A.C. 7:8 Stormwater Management Rules – Applicable Design and Performance Standards  |   |   |\n| --- | --- | --- |\n|   | Green Infrastructure | Not Allowed  |\n|   | Stormwater Runoff Quantity | Only with a waiver or variance from N.J.A.C. 7:8-5.3  |\n|  GR | Groundwater Recharge | Not Allowed  |\n|  % | Stormwater Runoff Quality | Only with a waiver or variance from N.J.A.C. 7:8-5.3, 80% TSS  |\n|  Stormwater Runoff Quality Mechanisms and Corresponding Criteria  |   |\n| --- | --- |\n|  Settling  |   |\n|  Storage Volume | Entire Water Quality Design Storm Volume  |\n|  Filtration  |   |\n|  Sand bed minimum thickness | 18 inches  |\n|  Maximum storage above sand bed | 24 inches  |\n|  Maximum design permeability rate of sand bed | 2 inches/hour  |\n|  Minimum topsoil permeability rate, if using optional vegetative cover | ≥ 200% the hydraulic head loss of the underdrain  |\n\n\n\n[Source PDF](SOURCE.pdf#page=1)\n\nOCR images on the cited PDF pages (page association only): [p. 1, image 1](images/page-0001-img-01.jpg)"}
{"metadata": {"source_document": "NJ BMP", "version_year": "2026", "chapter_subchapter": "Chapter 11.4", "citation": "BMP 2026, Ch 11.4, p. 11", "topic": "Table 2"}, "programmatic_checks": {}, "content_markdown": "# Sand Filters With Underdrain (Non-GI)\n\n## Table 2\n\n|  Inflow Drainage Area = | 50,000 sf of motor vehicle surface  |\n| --- | --- |\n|  Pavement NRCS Curve Number (CN) = | 98  |\n|  Sand Bed Depth = | 18 in  |\n|  Maximum Design Permeability of Sand Bed = | 2 in/hr  |\n|  WQDS Depth = | 24 in = 2 ft  |\n\nSource context and notes:\n\nSource revision: January 2026.\n\n- Once both the sand filter with underdrain and its drainage area are stabilized, the infiltration rate of the sand bed must be retested to ensure that the design permeability rate is the same as the as-built permeability rate.\n\n# Designing A Sand Filter with Underdrain\n\nThe example below illustrates how to design a sand filter with underdrain to treat the runoff generated by the Water Quality Design Storm. The following parameters apply:\n\n\n|  Inflow Drainage Area = | 50,000 sf of motor vehicle surface  |\n| --- | --- |\n|  Pavement NRCS Curve Number (CN) = | 98  |\n|  Sand Bed Depth = | 18 in  |\n|  Maximum Design Permeability of Sand Bed = | 2 in/hr  |\n|  WQDS Depth = | 24 in = 2 ft  |\n\n\n# Step 1: Runoff Calculations\n\nUsing the NRCS method described in National Engineering Handbook, Part 630 (NEH) and discussed in Chapter 5, the volume of runoff produced by the WQDS was calculated to be 4,310 cf based upon an NRCS Curve Number (CN) of 98 for asphalt.\n\n# Step 2: Forebay Sizing\n\n[Source PDF](SOURCE.pdf#page=11)"}