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Module # 4 Basic Hydrologic/Hydraulic Calculations & Impact of Urbanization

Module # 4 Basic Hydrologic/Hydraulic Calculations & Impact of Urbanization. Iowa’s River Restoration Toolbox Level 1 / Base Training. Stream Hydrology. Streams Need Water… Channels Form from Runoff and Snowmelt Channels Dissect Landscape and Intercept Groundwater Water Flows Downhill…

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Module # 4 Basic Hydrologic/Hydraulic Calculations & Impact of Urbanization

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  1. Module # 4Basic Hydrologic/Hydraulic Calculations & Impact of Urbanization Iowa’s River Restoration Toolbox Level 1 / Base Training

  2. Stream Hydrology • Streams Need Water… • Channels Form from Runoff and Snowmelt • Channels Dissect Landscape and Intercept Groundwater • Water Flows Downhill… • Higher to Lower Elevations w/in Watersheds

  3. Types of Channels • Perennial • Flows Year Round • Groundwater • Intermittent • Flows Part of the Year • Groundwater • Ephemeral • Flows Limited Amount of Time • Response to Rain Events

  4. Stream Hydrology • Volume – Channel and Floodplain Capacity (Discharge) • Velocity • Peak Discharge – • Slope • Imperviousness

  5. Rationale Method Q=CiA • Q = Peak Discharge (cfs) • C = Runoff Coefficient • i = Average Rainfall Intensity (inches per hour) • A = Drainage Area (Acres) • NRCS – TR55 • Applicable for Small Watershed

  6. Hydrologic Modeling System (HEC-HMS) • Computer Model Developed by USACE • Computes Range of Discharges • Applicable for Most Watersheds

  7. Varying Stages in Streams

  8. Impacts of Urbanization on Runoff • Urbanization Dramatically Increases Imperviousness • Increased Imperviousness = Decreased Infiltration + Increased Stormwater Runoff • Increased Runoff = Increased Flooding + Increased Stream Channel Erosion + Loss of Aquatic Habitat

  9. Impacts of Detention • Volume Control (Retention) • Peak Flow Rate Control (Detention) • Single Event • Multiple Events (2-, 10-, 25-, 100-Year Events)

  10. Impacts of Detention Peak Discharge Post-Development • Slightly higher flow rate/volume; Much longer duration Uncontrolled Post-Development - Much higher flow rate/volume Pre- Development Discharge Rate Time

  11. Stream Hydraulics • Energy Dissipation • Transport • Water • Sediment • Dynamic Equilibrium

  12. Continuity Equation Q=VA • Q = Discharge (cfs) • V = Velocity (fps) • A = Cross Section Area (sq ft)

  13. Manning’s Equation * 2/3 1/2 R S ______ V = 1.49 n Where: V = Velocity (fps) R = Hydraulic Radius (ft) S = Slope n = Roughness

  14. Hydraulic Radius A R = P Width = 43’ Area (A) = Mean Depth (D)* Width (W) Depth 1.8’ Depth 2.1’ Depth 3.8’ Depth 4.2’ Depth 3.1’ Depth 4.5’ Depth 4.1’ Depth 4.3’ Wetted Perimeter(P) is the portion of the channel (in feet) exposed to channel roughness at a give stage. Depth 3.1’ Depth 4.4’ Depth 4.4’ __ P=63.2

  15. Manning’s n Roughness is the amount of drag a channel or floodplain puts on flowing water.

  16. Manning’s n by Stream Type

  17. Other Velocity Equations Chezy Darcy-Weisbach Relative Roughness

  18. R S = Shear Stress  Where: = Shear Stress (psf) = Unit Weight of Water (pcf) (62.4pcf) R = Hydraulic Radius (ft) S = Slope

  19. Permissible Shear Stress (lb/sq ft) • Source: Fischenich (2001) • Suitable Shear Stress for Various Boundary Conditions

  20. = V Stream Power Stream Power = Shear * Velocity Where: = Stream Power (lbs/ft*sec) = Shear Stress (psf) V = Velocity (fps)

  21. Bankfull Discharge • Controls Channel Form • Corresponds to the Discharge at Which Channel Maintenance is Most Effective • Recurrence Interval on Order of 1.1 to 1.6 Years • Higher Recurrence Interval in Urban Watersheds

  22. Bankfull Q vs Effective Q

  23. Regional Curves • Relate a Variety of Bankfull Variables Versus Drainage Area • Discharge • Area • Width • Depth • Used to Help Validate Bankfull Calls

  24. Bankfull Discharge Region Curves

  25. Development of Regional Curves • Sites with Good Bankfull Indicators • Range of Drainage Areas • Need Half of Points at Gage Sites • Need to Validate Discharge/Velocity

  26. Regional Curves Be Weary of Width and Depth Curves – Do Not Take into Account Width to Depth Ratio!!

  27. MN BKF Area Region Curves

  28. Flood Frequency Analysis from USGS Gages • Developed by USGS • Estimate of Various Discharges • 2, 5, 10, 25, 50, 100 • Regression Equations • Gumbel Plot • Log Pearson Type 2

  29. Flood Frequency Plots Return Interval

  30. Flood Frequency Plots Probability of Annual Exceedance – Inverse of Return Interval A 100-Year Storm Has a 1% Chance of Annual Exceedance

  31. Module # 4Basic Hydrologic/Hydraulic Calculations & Impact of Urbanization Iowa’s River Restoration Toolbox Level 1 / Base Training

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