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Higher or Lower Density? What Gives the Best Bang for the Buck for Mitigating the Effects of Development?

Higher or Lower Density? What Gives the Best Bang for the Buck for Mitigating the Effects of Development? John Jacob, Ph.d. Ricardo López, M.S. Low Impact Development Vs. Urban Sprawl Low Impact Development (LID) Increase the amount of perviousness in developed areas Smart Growth

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Higher or Lower Density? What Gives the Best Bang for the Buck for Mitigating the Effects of Development?

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  1. Higher or Lower Density? What Gives the Best Bang for the Buck for Mitigating the Effects of Development? John Jacob, Ph.d. Ricardo López, M.S. The AWRA 2006 Conference on GIS & Water Resources IV

  2. Low Impact DevelopmentVs.Urban Sprawl • Low Impact Development (LID) • Increase the amount of perviousness in developed areas • Smart Growth • Increase density (and therefore imperviousness), but save more open space overall The AWRA 2006 Conference on GIS & Water Resources IV

  3. Goal Report the results of a modeling effort comparing the two approaches in a hypothetical watershed The AWRA 2006 Conference on GIS & Water Resources IV

  4. Impervious land cover • Impervious surface features are those that prohibit water from naturally infiltrating the ground (concrete, pavement, etc) • Urban Landscapes: • Concrete, pavement, rooftops • Swimming Pools • Impervious Land Cover: essentially the area that is not “green” The AWRA 2006 Conference on GIS & Water Resources IV

  5. Total Population: 100,000 5,000 Pop/Sq. Mile 10,000 Pop/Sq. Mile Total Study Area: 50 Sq. Miles Total Study Area: 50 Sq. Miles Developed Area: 20 Sq. Miles Developed Area: 10 Sq. Miles Pop. Density Vs. Impervious Land Cover The AWRA 2006 Conference on GIS & Water Resources IV

  6. Impervious Cover Model Good Sensitive Fair Stream Quality Impacted Urban Drainage Poor Non-Supporting 10% 25% 40% 60% 100% Watershed Impervious Cover Center for Watershed Protection The AWRA 2006 Conference on GIS & Water Resources IV

  7. City of League City, TX2002 Aerial Photo The AWRA 2006 Conference on GIS & Water Resources IV

  8. The AWRA 2006 Conference on GIS & Water Resources IV

  9. 100,000 new residents The AWRA 2006 Conference on GIS & Water Resources IV

  10. 8.3 sq mi @ 12,000 pop / sq mi 100,000 new residents The AWRA 2006 Conference on GIS & Water Resources IV

  11. 3.4 sq mi @ 30,000 pop / sq mi 100,000 new residents The AWRA 2006 Conference on GIS & Water Resources IV

  12. BOGOTA, COL160 Sq. mi. @ 40,000 pop/sq mi League CityArea: 53 Sq. MilesPopulation: 50,000 (2000) BogotaArea: 160 Sq. MilePopulation: 6,500,000 (2000) The AWRA 2006 Conference on GIS & Water Resources IV

  13. Pollutant Load Calculations The AWRA 2006 Conference on GIS & Water Resources IV

  14. Pollutant Load Model (1)Input: LULC Vector formatArcGIS 9 Model Builder The AWRA 2006 Conference on GIS & Water Resources IV

  15. Pollutant Load Model (2)Input: LULC Raster formatArcGIS 9 Model Builder The AWRA 2006 Conference on GIS & Water Resources IV

  16. Pollutant Load Model • Simplified, GIS-based application • Developed in ArcGIS 9.x (Model Builder) • Raster data model • Estimates total pollutant loads (NPS) in lbs / yr, for any user-specified pollutant • Based on the empirical Simple Method developed by Schueler (1987) for estimating pollutant export from small urban watersheds The AWRA 2006 Conference on GIS & Water Resources IV

  17. Geo-spatial Data Watershed Boundaries Land-Use/Land-Cover (LULC) - Vector or raster Tabular Data Event Mean Concentration (EMC) table - Text (csv) Imperviousness factors table - Text (csv) Model Input Data The AWRA 2006 Conference on GIS & Water Resources IV

  18. Runoff Coefficient RVU * RVU = 0.05 + (0.009 * IU) Where: • RVU = Runoff Coefficient for land use type u, inches(runoff) / inches(rainfall) • IU = Percent Imperviousness * Schueler 1987 (Washington D.C.) The AWRA 2006 Conference on GIS & Water Resources IV

  19. IMP - Imperviousness values(%) The AWRA 2006 Conference on GIS & Water Resources IV

  20. Total Pollutant Load (lb/yr) * LP = Σ U (P * PJ* RVU * CU* AU * 2.72 / 12) Where: • LP = Pollutant load, lb/yr • P = Precipitation, in/yr (assumed 46 for study area) • PJ = Ratio of storms producing runoff (default = 0.9) • RVU= Runoff Coefficient for land use type u, inches(runoff)/inches(rainfall) • CU = EMC for land use type u, mg/l • AU = Area of land use type u, in acres * Schueler 1987 (Washington D.C.) The AWRA 2006 Conference on GIS & Water Resources IV

  21. Event Mean Concentration (EMC) • An EMC is defined as the total constituent mass discharge divided by the total runoff volume (EPA 1983) • EMCs were developed by the EPA’s Nationwide Urban Runoff Program (NURP) (1983) to serve as a national measure of the magnitude of urban runoff, specifically pollutant loadings The AWRA 2006 Conference on GIS & Water Resources IV

  22. EMC - Event Mean Concentration values for TSS, BOD, TN and TP in mg/l(Houston Area EMC Database) GBNEP – 15March 1992 The AWRA 2006 Conference on GIS & Water Resources IV

  23. Scenarios • No development • Full buildout at 4000 people/mi2 (Status Quo) • Same population as full buildout but at 12,000 people/mi2 • Same population as full buildout but at 30,000 people/mi2 • Full-buildout scenario at 4000 people/mi2 with the addition of best management practices (BMPs or “LID”) treating 20% of the area with a 65% effectiveness. The AWRA 2006 Conference on GIS & Water Resources IV

  24. Results The AWRA 2006 Conference on GIS & Water Resources IV

  25. The AWRA 2006 Conference on GIS & Water Resources IV

  26. Total Nitrogen Total Phosphorus Biochemical Oxygen Demand Total Suspended Solids Pollutant Loads - Comparison Chart(lbs / year) The AWRA 2006 Conference on GIS & Water Resources IV

  27. Conclusions • The BMP scenario reduced pollutant loadings from the conventional-density, full-buildout scenario by 13-15%. • Holding the population constant and increasing density 3-fold to 12,000 people/mi2, decreased pollutant load overall by 50-75%. • Clearly, density should be considered as a BMP in its own right when considering development scenarios. • But the LID approach remains valid in the low density developments that are destined to remain with us. The AWRA 2006 Conference on GIS & Water Resources IV

  28. Texas Sea Grant / Tx. Coop. ExtensionTexas A&M University www.urban-nature.org Ricardo A Lopez M.S. 17000 El Camino Real, Suite 301 Houston, TX 77058 (281) 218 0570 E-mail: rilopez@tamu.edu The AWRA 2006 Conference on GIS & Water Resources IV

  29. Appendix The AWRA 2006 Conference on GIS & Water Resources IV

  30. What is Urban Sprawl? • To spread out in a way that is not organized.www.inhs.uiuc.edu/chf/pub/virtualbird/glossary.html • Haphazard growth or outward extension of a city resulting from uncontrolled or poorly managed development.www.co.monterey.ca.us/gpu/glossary2.htm • Current development patterns, where rural land is converted to urban uses more quickly than needed to house new residents and support new businesses, and people become more dependent on automobiles. www.smartgrowth.org/bibliographies/greenlit_search/glossary.html The AWRA 2006 Conference on GIS & Water Resources IV

  31. BASINS • BASINS: Better Assessment Science Integrating Point and Nonpoint Sources – U.S. Environmental Protection Agency (EPA) • Multipurpose environmental analysis system designed to perform watershed and water quality-based studies • Makes it possible to quickly assess large amounts of point source and non-point source data • Geographic Information System (GIS) tool developed as an extension to ArcView software program (Environmental System Research Institute – ESRI) The AWRA 2006 Conference on GIS & Water Resources IV

  32. Watershed Boundary • Watersheds define the areas for which the pollutant loads are calculated • Available from local government agencies in most moderate- to high-density urban areas • May be derived using standard GIS or BASINS tools and the United States Geological Survey (USGS) Digital Elevation Model (DEM) data files (See appendix) • Dataset must have a code field containing unique identifiers for each watershed • Vector format, projected CS, same projection and datum, stored in meters (map units) The AWRA 2006 Conference on GIS & Water Resources IV

  33. LULC Dataset • Defines Land-Use/Land-Cover types for the study area • Dataset must encompass the entire watershed • Essential for calculating the pollutant loads. • Available from local government agencies in most moderate- to high-density urban areas • If available in raster format (grid of cells), must be converted to vector format (polygon spatial features) The AWRA 2006 Conference on GIS & Water Resources IV

  34. Imperviousness & EMCTables DATASET:imp_csv: Imperviousness table Attributes: LUCODE: Land use unique identifier Imperv: Imperviousness factor (Percentage in percent fraction format) DATASET: emc_csv: Event mean concentrations table Attributes: LUCODE: Land use unique identifier TN: Total Nitrogen (mg/l) BOD: Biochemical Oxygen Demand (mg/l) TSS: Total Suspended Solids (mg/l) TP: Total Phosphorus (mg/l) The AWRA 2006 Conference on GIS & Water Resources IV

  35. Software References P-LOAD (ArcView extension to BASINS (Better Assessment Science Integrating Point & Nonpoint Sources – U.S. Environmental Protection Agency - EPA): BASINS 3.1 Description: http://www.epa.gov/waterscience/ftp/basins/system/BASINS3/areadb3.htm Download BASINS 3.1 program and data: http://www.epa.gov/waterscience/basins/index.html The AWRA 2006 Conference on GIS & Water Resources IV

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