1. Bernie Engel, Larry Theller, James Hunter
Web-based Low Impact Development Decision Support and Planning Tool
Bernie Engel, Larry Theller, James Hunter
Purdue University

2. L-THIA Model Long-Term Hydrologic Impact Assessment
Average annual runoff
NPS pollution
An overview / screening model
User friendly tool
Does not require detailed data input
Identifies need for more detailed modeling
Provides "What-If" alternatives evaluation scenarios

3. User supplied information
L-THIA Model
Data requirements and components for analysis in L-THIA
Soils
Land use
Curve Number values
Daily precipitation
Location
Runoff depths
NPS pollutants
Runoff volumes
User supplied information
Performed by L-THIA

4. Assumptions Water flows across the surface to flowshed
no storm drains
Water equally spread across landscape
No routing
Average antecedent moisture
soil is not saturated or frozen
Rainfall is evenly spread in local area

5. Limitations Accuracy of landuse and soil data
Big limitation
Accuracy of runoff curve number
Hydrological impacts are understated
LID not reflected
Accuracy of published NPS relationship
Lead in runoff based on 1990’s models

6. Curve Number Basics: rainfall to runoff ratio for different surfaces
Direct
Runoff
Rainfall

7. Averaged mass of NPS contaminant from each landuse
EMC Table

8. Take-Home points DEM is only used to calculate flowshed
DEM is not used for erosion – or runoff
Runoff is based on surface type – not slope
won’t account for frozen ground
30 year average rainfall
Chemistry is based on landuse averages

9. Databases provide input
Weather data (30 years of real rainfall)
Soil data (SSURGO ) or STATSGO
Elevation data 30m or 10m DEM (for contributing area)

10. L-THIA LID

11. Calculating a Custom LID CN
Determine the land use combination and soil type
Calculate composite CN without considering disconnection of imperviousness (Watershed and site level)
Where impervious area is < 30% of site, adjust CN for % disconnected impervious area (Site level)
Calculating a Custom LID CN

12. Purdue University is an Equal Opportunity/Equal Access institution.
LTHIA-Web with Online Watershed Delineation
Region 5 States, visible on Google Maps ™.
Flowshed is calculated from pour point then landuse and soil characteristics are loaded into model.
Purdue University is an Equal Opportunity/Equal Access institution.

13. Purdue University is an Equal Opportunity/Equal Access institution.

14. One Product of MSDSS partnership

16. Purdue University is an Equal Opportunity/Equal Access institution.

17. Purdue University is an Equal Opportunity/Equal Access institution.

18. Purdue University is an Equal Opportunity/Equal Access institution.
Landuse and BMP Editor.
Draw polygon by clicking corners, then right-click inside it to select landuse or BMP.
Purdue University is an Equal Opportunity/Equal Access institution.

19. Purdue University is an Equal Opportunity/Equal Access institution.
Landuse and Soil are automatically entered into spreadsheet
Purdue University is an Equal Opportunity/Equal Access institution.

20. Tabular and Graphic results
Purdue University is an Equal Opportunity/Equal Access institution.

22. Low-Impact Development (LID)
An approach to land development to mimic the pre-development site hydrology to:
Reduce volume of runoff
Decentralize runoff, diffusing flows into smaller retention/detention areas
Improve water quality
Encourage groundwater infiltration
Reduce runoff = managing stormwater volume and quality, reduce flooding

23. Adapted from Prince George's County, MD, LID IMP Guidance Document, 2002. www.lid-stormwater.net

24. Adapted from Prince George's County, MD, LID IMP Guidance Document, 2002. www.lid-stormwater.net

25. LID Benefits Ecologically Sensible
Provides Added Values / Ecosystem Services
Economically Sustainable
Lower Costs (Construction, Maintenance & Operation) vs. Conventional
Multifunctional Practices
Ideal for Urban Retrofit

26. 1. Conservation (Watershed and Site Level)
LID Major Components
1. Conservation (Watershed and Site Level)
2. Minimization (Watershed and Site Level)
3. Integrated Management Practices (Site Level)
Storage, Detention & Filtration
Rain gardens
Drainage swales
Green roofs
Porous Pavement
Conservation
Forest/Woods
Infiltrable Soils
Minimization
Reduce imperviousness
Soil Compaction

27. LID Hydrologic Analysis - CN
Curve number & consequential runoff / NPS is affected by:
Soil Type
Landuse / Site Envelope
Site imperviousness
Vegetation
Disconnection of site imperviousness

28. LID Hydrologic Analysis - CN
Prince George’s County, Department of Environmental Resources Low--Impact Development Hydrologic Analysis, July 1999

29. TR-55 Hydrologic Analysis – CN
Hydrologic soil type
% Impervious
A B C D
Standard table from TR-55 assumes a certain % impervious cover. Engineers typically choose the CN corresponding to the lot size or urban type without calculating actual impervious %.
If different % is different from the assumed value, TR-55 gives the option of using Figure Must chose one pervious CN corresponding to one land cover and soil type.

30. L-THIA LID Basic Application: Target preliminary goals at the
watershed and site level
Reduce imperviousness
Conserve infiltratable soils
Conserve functional / sensitive landscape
Minimize land disturbances
Anticipate need for other LID practices to reduce NPS and stormwater volume

31. Reduce volume of runoff
Decentralize runoff, diffusing flows into smaller retention/detention areas
Improve water quality

39. L-THIA LID: Lot Level Screening Tool
Application: Target preliminary goals by adjusting lot level features
Site Design & Development preparation
Narrowing impervious areas (sidewalks, driveways, roads)
Natural resource preservation
Heavy equipment use  compaction
Permeable paving materials
Vegetative roof systems
Bioretention cells
Vegetated swales /Filter strips
Rain barrels
Disconnect impervious areas

45. For more information and a list of tools:
L-THIA
L-THIA LID
Web-GIS tools (Online LTHIA) for Spatial Hydrologic Analysis

46. Preliminary Evaluation of LID for Pendleton, IN

47. Using web-based tools to delineate and capture watershed, land use, and soils data for L-THIA input

48. Current Landuse (from L-THIA)
Acres
Percent
Agriculture
1278
73%
Forest 70 4%
Grass/Pasture
105 6%
Commercial 53 3%
Industrial 17 1%
HD-Residential 87 5%
LD-Residential
Water 35 2%
Current Curve Number = 77

49. Assumption For Max. Development Scenario
983 Acres: Commercial / Mixed Residential
Most Agriculture 55% B Soils, 45% C Soils
Current CN = 78
Post –Developed w/o LID CN = 84
Post – Developed w/ LID CN  76-82

50. Using L-THIA LID Basic – Reducing Impervious Surface by 10%
Reduces runoff by 23%

51. Using L-THIA LID Lot Level
Reduce street width from 26ft. to 18ft.
Rain barrels for Residential
Green Roofs for Commercial
Bioretention/Raingardens
Reduces Post-developed runoff by 46%
Purdue University is an Equal Opportunity/Equal Access institution.

52. Summary
L-THIA LID is a screening tool to evaluate the benefits of LID practices
L-THIA LID provides an easy to use interface
Will enable decision makers to formulate watershed management plans to meet goals
Along with other tools, allows stakeholders to understand impacts of water quantity and quality resulting from land use change