Houston, TX

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.

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

3
Goal

• Report the results of a modeling effort comparing
  the two approaches in a hypothetical watershed

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

5
Pop. Density Vs. Impervious Land Cover
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
7
City of League City, TX 2002 Aerial Photo
8
(No Transcript)
9
100,000 new residents
10
8.3 sq mi _at_ 12,000 pop / sq mi
100,000 new residents
11
3.4 sq mi _at_ 30,000 pop / sq mi
100,000 new residents
12
BOGOTA, COL160 Sq. mi. _at_ 40,000 pop/sq mi
League City Area 53 Sq. MilesPopulation
50,000 (2000) BogotaArea 160 Sq.
MilePopulation 6,500,000 (2000)
13
Pollutant Load Calculations
14
Pollutant Load Model (1)Input LULC Vector
formatArcGIS 9 Model Builder
15
Pollutant Load Model (2)Input LULC Raster
formatArcGIS 9 Model Builder
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

17
Model Input Data

• 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)

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.)
19
IMP - Imperviousness values()
20
Total Pollutant Load (lb/yr)

• LP S 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.)
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 EPAs Nationwide Urban
  Runoff Program (NURP) (1983) to serve as a
  national measure of the magnitude of urban
  runoff, specifically pollutant loadings

22
EMC - Event Mean Concentration values for TSS,
BOD, TN and TP in mg/l(Houston Area EMC Database)
GBNEP 15March 1992
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.

24
Results
25
(No Transcript)
26
Pollutant Loads - Comparison Chart(lbs / year)
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.

28
Texas Sea Grant / Tx. Coop. ExtensionTexas AM
University
www.urban-nature.org
Ricardo A Lopez M.S. 17000 El Camino Real, Suite
301 Houston, TX 77058 [phone] E-mail
rilopez_at_tamu.edu
29
Appendix
30
What is Urban Sprawl?

• To spread out in a way that is not
  organized.www.inhs.uiuc.edu/chf/pub/virtualbird/g
  lossary.html
• Haphazard growth or outward extension of a city
  resulting from uncontrolled or poorly managed
  development.www.co.monterey.ca.us/gpu/glossary2.h
  tm
• 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/g
  reenlit_search/glossary.html

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)

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)

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)

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)
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