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Introduction to Urban Hydrology

Introduction to Urban Hydrology. Philip B. Bedient Rice University June, 2005. Major Urban Hydrologic Processes. Precipitation (measured by radar or rain gage) Evaporation or ET (loss to atmosphere) Infiltration (loss to subsurface soils) Overland flow (sheet flow toward nearest stream)

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Introduction to Urban Hydrology

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  1. Introduction to Urban Hydrology Philip B. Bedient Rice University June, 2005

  2. Major Urban Hydrologic Processes • Precipitation (measured by radar or rain gage) • Evaporation or ET (loss to atmosphere) • Infiltration (loss to subsurface soils) • Overland flow (sheet flow toward nearest stream) • Streamflow (measured flow at stream gage) • Reservoir storage to decrease flow rate • Channelization to increase flow rate

  3. The Hydrologic Cycle

  4. The Watershed or Basin • Area of land that drains to a single outlet and is separated from other watersheds by a drainage divide. • Rainfall that falls in a watershed will generate runoff to that watershed outlet. • Topographic elevation is used to define a watershed boundary (land survey or LIDAR)

  5. Major Causes of Flooding(Excess Water that Inundates) • Highly Developed (urbanized) Area • Intensity and Duration of Rainfall • Flat Topography with Little Storage • Poor Building Practices in floodprone areas • Major floods in 1989, 1992, 1994, 1998, 2001

  6. Brays Bayou Watershed Harris Gully Area: 4.5 sq. mi. Brays Bayou Area: 129 sq. mi. Rice/TMC Area Watershed Boundary

  7. T S ALLISON Southwest Freeway (US 59) Detention storage between Mandell and Hazard Looking West Looking East

  8. The Watershed Response - Hydrograph • As rain falls over a watershed area, a certain portion will infiltrate the soil. Some water will evaporate to atmosphere. • Rainfall that does not infiltrate or evaporate is available as overland flow and runs off to the nearest stream. • Smaller tributaries or streams then begin to flow and contribute their load to the main channel at confluences. • As accumulation continues, the streamflow rises to a maximum (peak flow) and a flood wave moves downstream through the main channel. • The flow eventually recedes or subsides as areas drain out.

  9. Design Rainfalls • Design Storm from HCFCD and NWS • Based on Statistical Analysis of Data • 5, 10, 25, 50, 100 Year Events • Various Durations

  10. Hydrologic Theory • One of the principal objectives in hydrology is to transform rainfall that has fallen over a watershed area into flows to be expected in the receiving stream. • Losses must be considered such as infiltration or evaporation (long-term) • Watershed characteristics are important

  11. Unit Hydrograph Theory • The unit hydrograph represents the basin response to 1 inch (1 cm) of uniform net rainfall for a specified duration. • Linear method originally devised in 1932. • Works best for relatively small subareas - in the range of 1 to 10 sq miles. • Several computational methods exist.

  12. . 3 0 , 0 0 0 2 5 , 0 0 0 J u n 7 6 A p r 7 9 2 0 , 0 0 0 S e p 8 3 Flow, cfs M a r 9 2 1 5 , 0 0 0 M a r 9 7 1 0 , 0 0 0 5 , 0 0 0 2 1 8 1 4 3 6 9 1 5 1 2 2 T i m e , h r s Urban Hydrology - As a watershed develops the peaks get larger and time response is faster Effect of Urban Development

  13. Problems in Hydrology • Extreme weather and rainfall variation • Streamflow and major flood devastation • River routing and hydraulic conditions • Overall water supply - local and global scales • Flow and hydraulics in pipes, streams and channels • Flood control and drought measures • Watershed management for urban development

  14. Flood Control Systems in the Field of Hydrology • Concrete or earthen channels - increase flows • Detention or retention reservoirs - storage • Minimize increase in flow as development occurs • Older downstream areas can be impacted • Large projects are underway in several areas • Advanced forecasting tools for severe weather and flood Alert

  15. RADAR Rainfall Estimates • NEXRAD provides real-time data on a ~16 km2 (6 mi2) grid • Equivalent to about 21 rain gages in Brays Bayou watershed • Each estimate represents an average rainfall amount over the entire 4 x 4 km2 area • NEXRAD rainfall estimates compare well with point rain gage measurements (r2 ~ 0.9)

  16. FAS – NEXRAD Midnight 1 a.m.

  17. Harris Gully Drains to Brays Bayou Low Flow Box Culvert During Tropical Storm Frances

  18. Texas Medical Center - Moursund Westbound 6/10/01 - 6:44 AM

  19. Flood Warning SystemsDowntown Houston Emergency Response • Flood Doors • Flood Gates • Facility Entrances • Communications • Operations • Training

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