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Low-Impact Development Hydrologic Analysis

Low-Impact Development Hydrologic Analysis. Santa Barbara County Flood Control November 2007. Hydrologic Effects of Development. Low-Impact Development.

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Low-Impact Development Hydrologic Analysis

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  1. Low-Impact Development Hydrologic Analysis Santa Barbara County Flood ControlNovember 2007

  2. Hydrologic Effects of Development

  3. Low-Impact Development Goal: To mimic the pre-development site hydrology by using site design techniques that store, infiltrate, evaporate and detain runoff. Use of these techniques helps to reduce runoff and encourage groundwater recharge.

  4. Low-Impact Development

  5. Hydrologic Analysis - SBUH • Estimates excess precipitation (runoff depth) using SCS Curve Number approach TR-55 and NEH

  6. Hydrologic Analysis - SBUH 2. Computes the instantaneous hydrograph as a function of runoff depth, area and the computational time interval 3. Computes the runoff hydrograph by routing the instantaneous hydrograph through a reservoir with a time delay equal to the time of concentration of the basin

  7. TR-55 Hydrologic Analysis – CN TR-55 flowchart for determining site CN

  8. TR-55 Hydrologic Analysis – CN % Impervious Hydrologic soil type

  9. TR-55 Hydrologic Analysis – CN

  10. LID Hydrologic Analysis - CN Source: Low-Impact Development Hydrologic Analysis, Prince George’s County, MD

  11. LID Hydrologic Analysis - CN • Curve numbers may be affected by: • Preservation of infiltratable soils • Preservation of existing vegetation • Minimization of site imperviousness • Disconnection of site imperviousness • Creation of transition zones and bioretention

  12. Calculating a Custom LID CN • Determine percentage of each combination of land use and soil type • Calculate composite custom CN without considering disconnection of imperviousness • Where impervious area is < 30% of site, adjust CN for % disconnected impervious area

  13. LID Concept:Maintaining Pre-development Tc • Maintain pre-development flow path length • Increase roughness • Detain flows • Minimize disturbance (compaction, etc.) • Flattening grades • Disconnecting impervious areas • Directing flow over pervious areas Note: Computational procedure is the same as that used in TR-55

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