Overview & Purpose
This spreadsheet tool enables a user to explore the environmental impacts of numerous redevelopment scenarios in the City of Austin's portion of the Barton
Springs Zone. Two separate properties may be evaluated to estimate the ability to achieve the pollutant loading and runoff (flow) control goals of the SOS
Ordinance. The first property is the Project Site to be redeveloped. The second property is the Mitigation Site that might be considered for purchase (land or
development rights) or modification in order to reduce the collective impact of the Existing and Mitigation sites. For example, a project might retain a higher
level of impervious cover and a lower level of structural controls on the project site and buy enough mitigation land off-site to collectively lower a key indicator
pollutant to an impact level or an impervious cover level approaching or achieving SOS Ordinance levels.
User Entry Inputs
Fill in the Yellow Entry Cells to explore scenarios. The grey cells are automatically populated based on the entry cells and should only be modified with
caution. You may want to save a copy of the file as a backup of the file in its original form. User inputs are as follows:
Acreage: Total acreage of the existing project site. Final acreage will automatically be filled in with the same acreage. The acreage for the mitigation site is
automatically calculated based on mitigation target choices.
Impervious Cover: Percentage IC of site. Must be a percentage between 1% and 100%. Note: For existing mitigation site, this is the POTENTIAL
development level of the site. The default for the existing mitigation site assumes land outside of the City of Austin ETJ with TCEQ-level controls. The default
for the final mitigation site assumes protected land with undeveloped impervious cover levels (1%). For further explanation of these assumptions, see the
Default Settings for Existing Mitigation below.
Control: Choose among typical BSZ "pond" options from a list (see Structural Controls below).
Capture Depth: Automatically entered by Worksheet. Advanced users can select from a menu, but this is not advised (the results will not greatly vary with the
Max. Irrigation?: Select Yes or No for installing rainwater harvesting (or retention-irrigation if final IC is low enough). Selecting Yes will assume that a
rainwater harvesting or retention-irrigation system will be constructed on-site to collect runoff and will use the maximum amount of pervious space available
for irrigation. The pollutant loads will be calculated for this portion of the site using retention-irrigation removal rates.
Porous Sidewalks?: Select Yes or No for using porous pavement for sidewalks. Selecting Yes will assume that sidewalks will comprise 5% of the site.
Porous pavement sidewalks will not be given a water quality credit, but will lower the effective impervious cover of the site by 5%.
Aquifer: Specify whether the project site is within the Recharge or Contributing Zone of the Edward's Aquifer.
Watershed: Specify watershed where the project site is located. This field has a drop-down list of the six watersheds within the Barton Springs Zone. The
Little Barton Watershed is included as part of the larger Barton Watershed. The only impact of this entry is to show the SOS target impervious cover level.
Mitigation Type: Can either be set to (1) Impervious Cover Reduction or (2) Pollutant Load Reduction. Impervious Cover Reduction will calculate the
number of mitigation acres needed to reduce the overall impervious cover of both the project site and the mitigation site to a specified percentage (IC Target).
Pollutant Load Reduction will calculate the number of mitigation acres needed to achieve a specified reduction (Pct. Reduction) in the pollutant load of a
specified pollutant type (Pollutant Type).
IC Target: The overall percentage of impervious cover that the final project and final mitigation sites will average out to, given the calculated mitigation
acreage is purchased or put under conservation easement.
Pollutant Type: The pollutant that will be targeted for a specified reduction (Pct. Reduction). User can also choose 'Most Restrictive', which will pick the
maximum amount of mitigation acres required among Flow, TSS, TN, and Lead.
Pct. Reduction: The target percentage reduction for a specified pollutant (Pollutant Type).
Cost per Acre: The assumed cost per acre for mitigation land. User can vary this number depending on assumptions such as location, fee simple vs.
conservation easement, and whether the mitigation land is part of a larger City of Austin mitigation bank. This number is multiplied by the number of mitigation
acres to determine the total cost of purchasing mitigation land.
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BSZ: Barton Springs Zone. The geographical area (watershed) that drains to the Barton Springs portion of the Edwards Aquifer recharge zone, including the
recharge zone itself.
Capture Depth: The inches of runoff across the entire drainage area of the control required to be captured under regulation. This number is multiplied by the
site's drainage area to determine the volume of the water quality control. Varies depending on the regulations under which the control was built.
Contributing: Contributing Zone of the BSZ. All lands NOT over the Edwards Aquifer limestone outcrop that drain TO the outcrop, supplying it with water that
recharges to supply the springs and Edwatrds Aquifer well users with water.
Flood: Flood Control. Structural control designed to temporarily detain stormwater events to help prevent downstream flooding. These controls generally are
not designed to help with water quality.
(continued from previous)
Flow: Runoff flow from storm events not captured by structural controls. Uncontrolled runoff can widen and downcut streams, cause erosion, and threaten
lives and property.
IC: Impervious Cover. Surfaces such as rooftops, parking lots, and roads that do not allow water to infiltrate into the soil, causing high levels of runoff during
Pb: Total Lead. Heavy metal correlated with impervious cover and development that can impair the health of aquatic species.
Porous Pavement: Porous pavement uses permeable paved surfaces (i.e. porous asphalt, pervious concrete, grass pavers) that allow stormwater to drain
into a underlying layer of coarse gravel and small stones.
Rainwater Harvesting: Rainwater harvesting collects stormwater runoff from rooftops during rain events and stores it in a large cistern. The collected
rainwater can then be used for landscape irrigation.
Recharge: Recharge Zone of the BSZ. All lands comprising the Edwards Aquifer limestone outcrop; caves, sinkholes, and other karst features enable water
to recharge the Edwards Aquifer below.
SOS: Save Our Springs Water Quality Rules codified in the Land Development Code and Environmental Criteria Manual.
TCEQ: Texas Comission on Environmental Quality; state agency which requires stormwater controls in areas which do not have their own stormwater
requirements (e.g., in areas beyond city limits and extraterritorial jurisdictions, such as Hays County lands).
TDR: Transfer of Development Rights. A buyer can purchase the development rights on a property such that the existing owner can retain the property (for
ranching, homestead, etc.) but not be allowed to develop it beyond an extent agreed upon by the two parties.
TN: Total Nitrogen. Dissolved nutrients that can lower dissolved oxygen levels in water. Not effectively treated by sand filtration systems.
TSS: Total Suspended Solids. Sediments suspended by runoff that enter creeks or come from creek bank erosion that can clog aquifer recharge features,
carry adsorbed pollutants, destroy aquatic habitat, and impair waterways for swimming, fishing, and suitability for drinking water.
WQ: Water Quality Control. Structural control designed to improve water quality of stormwater runoff.
Flood Only/No WQ: Flood detention pond that does not provide any level of water quality treatment. No assumed WQ Capture Depth.
No Flood or WQ: No structural controls located on the site. No assumed WQ Capture Depth.
Retention-Irrigation: SOS-level water quality control that retains stormwater runoff in a basin and then irrigates it onto the site within 3 days. Classified as a
non-degradation control, meaning that any runoff from a site will not increase pollutant loads. Requires enough vegetated area on the site for irrigation
(approximately a 1:1 ratio to the amount of impervious cover, depending upon soil conditions). Assumed SOS Capture Depth.
Sand Filter Only: Older water quality control where stormwater runoff filters through a sand bed. The potential for clogging increases because the filtration
basin is not accompanied by an initial sedimentation basin that prevents some of the larger particles from entering the sand filter. Assumed constant Half-Inch
Capture Depth regardless of impervious cover since these controls were built under older regulations.
Sed/Sand Filter: Pre-SOS water quality control where larger particles settle out of stormwater runoff in the sedimentation basin and then finer pollutants are
captured in the filtration basin as stormwater soaks down through a layer of sand. Assumed Half-Inch-Plus Capture Depth, which increases volume with
Sedim. Only: Older water quality control where larger particles are settled out in a sedimentation basin. Sedimentation basins are not considered equivalent
to sedimentation/filtration controls in terms of water quality treatment because finer pollutants are not captured by a sand filter. Assumed constant Half-Inch
Capture Depth regardless of impervious cover since these controls were built under older regulations.
TCEQ Sed/Sand Filter: Same as Sed/Sand Filter, except follows the TCEQ Edwards Aquifer Rules to calculate the capture depth. Assumed TCEQ Capture
Undev.: No structural controls on the site and only 1% impervious cover. No assumed WQ Capture Depth.
Default Settings for Existing Mitigation
Acreage: Automatically calculated based on mitigation target choices (Impervious Cover Reduction or Pollutant Load Reduction)
Impervious Cover: 25% - Typical single-family residential scale of development, esp. where dependent upon on-site wastewater disposal (e.g., via septic
Control: TCEQ Sed/Sand Filter - TCEQ Edwards Aquifer Rules allow a variety of structural controls to be used. Among the most common are
sedimentation/sand filtration systems. Others (e.g., vegetated swales) may be less expensive, but are less effective at removing pollutants. Sed-Sand filters
were selected as a typical yardstick of comparison.
Capture Depth: TCEQ - TCEQ Edwards Aquifer Rules spell out a system to calculate structural WQ controls; the formulas in this spreasheet use the
methods required to calculate a sed-sand filter size in Travis County. Hays County would be almost indistinguishable.
Aquifer: Contributing - Land to be purchased assumed to be in the Contributing Zone. Land might also be in the recharge zone, but this is generally more
expensive (due to closer proximity to Austin).
Watershed: Onion - Most of the BSZ is in the Onion Creek watershed. (Does not have an impact on calculations.)
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Existing Project Site Final Project Site
Acreage: 16.84 Acreage: 16.84
Imper. Cover: 83% Imper. Cover: 83%
Control: No Flood or WQ Control: Sed/Sand Filter
Max. Irrigation? No
Porous Sidewalks? No
Aquifer: Contributing Aquifer: Contributing
Watershed: Williamson Watershed: Williamson
Mitigation Information Impervious Cover Reduction
Mitigation Type: Pollutant Load Reduction IC Target Pct: 25%
Enter input into yellow cells. See
Mitigation Land Cost Pollutant Load Reduction
attached instructions and
Cost per Acre: $15,000 Pollutant Type: TN glossary for further information.
Total Cost: $783,462 Pct. Reduction: 100%
Per Foot Cost $1.07
Existing Mitigation Site Final Mitigation Site
(Actual or Potential)
Acreage: 52.23 Acreage: 52.23
Imper. Cover: 25% Imper. Cover: 1%
Overall IC 21%
Control: TCEQ Sed/Sand Filter Control: Undev.
Aquifer: Contributing Aquifer: Contributing
Watershed: Onion Watershed: Onion
SOS IC 25%
Flow & Pollutant Load Results Percent Toward SOS Goal
Existing On-Site: Pct. Toward Goal
0% Uncaptured Runoff (Flow)
0% 20% 40% 60% 80% 100%
0% Total Susp. Solids (TSS) Load
0% Total Nitrogen (TN) Load
0% Total Lead (Pb) Load
Flow 85% 5% 90%
Proposed On-Site: Pct. Toward Goal
85% Uncaptured Flow
77% TSS Load
23% TN Load TSS 77% 14% 90%
64% Pb Load
Mitigation Site: Pct. Toward Goal
5% Uncaptured Flow TN 23% 77% 100%
14% TSS Load
77% TN Load
14% Pb Load
Lead 64% 14% 78%
Pct. Toward SOS Goal
90% Uncaptured Flow
90% TSS Load
100% TN Load No Controls SOS Level
78% Pb Load
Mitigation/TDR Worksheet Calculation Worksheet
100% 0% Pct. Site area to control
Subject Tract Mitigation Land Totals Comparison
Undev. Undev. SOS
Existing Control 1 Control 2 Post-Project Existing Post-Project Existing Post-Project (Subject (Both (Both
Tract) Tracts) Tracts)
Area (ac.) 16.8 16.84 0.00 16.8 52.2 52.2 69.1 69.1 16.8 69.1 69.1
IC pct. 83% 83% 0% 83% 25% 1% 39% 21% 1% 5% 25%
No Flood or Sed/Sand No Flood or Sed/Sand Retention-
Control Type Sed/Sand Undev. Undev. Undev.
WQ Filter WQ Filter Irrigation
Capture Depth None None TCEQ None None None SOS
Recharge or Contributing Contributing Contributing Contributing Contributing Contributing Contributing
Watershed Williamson Williamson Williamson Williamson Onion Onion 0 Williamson Williamson
Average annual rainfall (in) 33.2 33.2 33.2 33.2 33.2 33.2 33.2 33.2 33.2
Rv 0.65 0.65 0.03 0.65 0.22 0.04 0.04 0.07 0.22
C TSS (mg/l) 153.7 153.7 153.7 153.7 153.7 153.7 153.7 153.7 153.7
C TN (mg/l) 1.78 1.78 1.78 1.78 1.78 0.90 0.90 0.90 1.78
C Pb (ug/l) 29.85 29.85 2.26 29.85 10.57 2.59 2.59 3.92 10.57
Pond WQ Depth (in) 0.00 1.13 0.00 1.13 0.30 0.00 0.00 0.00 1.05
BMP treatment, TSS 0% 90% 0% 90% 90% 0% 0% 0% 100%
BMP treatment, TN 0% 27% 0% 27% 27% 0% 0% 0% 100%
BMP treatment, Pb 0% 78% 0% 78% 78% 0% 0% 0% 100%
Annual BMP capture pct. 0% 92% 0% 92% 87% 0% 0% 0% 97%
O&M: Pct Functioning 0% 88% 0% 88% 88% 0% 0% 0% 88%
Flow Volume & Loading Rates Before treatment
Average annual runoff (in/yr) 21.6 21.6 1.2 21.6 7.3 1.4 1.4 2.4 7.3
TSS load (lb/ac/yr) 751.4 751.4 40.1 751.4 254.4 48.7 48.7 83.0 254.4
TN Load (lb/ac/yr) 8.70 8.70 0.46 8.70 2.95 0.29 0.29 0.49 2.95
Pb Load (lb/ac/yr) 0.1459 0.1459 0.0006 0.1459 0.0175 0.0008 0.0008 0.0021 0.0175
Flow Volume & Loading Rates with Treatment
Uncaptured Flow (in/yr) 21.6 4.1 1.2 4.1 1.7 1.4 6.6 2.0 1.4 2.4 1.0
TSS Load (lb/ac/yr) 751.4 202.7 40.1 202.7 79.9 48.7 243.6 86.2 48.7 83.0 36.6
TN Load (lb/ac/yr) 8.70 6.80 0.46 6.80 2.34 0.29 3.89 1.87 0.29 0.49 0.42
Pb Load (lb/ac/yr) 0.146 0.054 0.001 0.054 0.007 0.001 0.041 0.014 0.0008 0.002 0.003
Final Annual Flow Volumes & Loads Before Treatment
Uncaptured Flow (ac-ft/yr) 30.28 30.28 0.00 30.28 31.79 6.09 62.06 36.36 1.96 13.71 42.04
TSS Load (lb/yr) 12,654 12,654 0 12,654 13,286 2,543 25,940 15,197 820 5,731 17,570
TN Load (lb/yr) 147 147 0 147 154 15 300 161 5 34 203
Pb Load (lb/yr) 2.46 2.46 0.00 2.46 0.91 0.04 3.37 2.50 0.01 0.15 1.21
Final Annual Flow Volumes & Loads After Treatment (where applicable)
Uncaptured Flow (ac-ft/yr) 30.28 5.71 0.00 5.71 7.56 6.09 37.84 11.80 1.96 13.71 6.04
TSS Load (lb/yr) 12,654 3,414 0 3,414 4,173 2,543 16,827 5,957 820 5,731 2,526
TN Load (lb/yr) 147 114 0 114 122 15 269 129 5 34 29
Pb Load (lb/yr) 2.46 0.90 0.00 0.90 0.37 0.04 2.83 0.95 0.01 0.15 0.17
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