introduction to urban hydrology
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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) - PowerPoint PPT PresentationTRANSCRIPT
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Introduction to Urban Hydrology
Philip B. BedientRice University
June, 2005
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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
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The Hydrologic Cycle
100Precipitation on land
InfiltrationWater table
Groundwater flow
1 Groundwaterdischarge
38 Surface discharge
61Evaporation from land
39Moisture over land
385Precipitation
on ocean
424Evaporationfrom ocean
Surfacerunoff
Imperviousstrata
GroundwaterRecharge
Precipitation
Snowmelt
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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)
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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
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Rice/TMC Area
Brays Bayou WatershedHarris Gully Area: 4.5 sq. mi.Brays Bayou Area: 129 sq. mi.
Watershed Boundary
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T S ALLISONSouthwest Freeway (US 59)
Detention storage between Mandell and Hazard
Looking East
Looking West
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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.
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Design Rainfalls
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
0:30 1:00 1:30 2:00 2:30 3:00 3:30 4:00 4:30 5:00 5:30 6:00
Time
Rainfall (in)
100 Year Storm8.4 inches
10 Year Storm5.6 inches
25 Year Storm6.6 inches
5 Year Storm4.8 inches
Design Storm from HCFCD and NWS
Based on Statistical Analysis of Data
5, 10, 25, 50, 100 Year Events
Various Durations
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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
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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.
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Urban Hydrology - As a watershed develops the peaks get larger and time response is faster
Sep 83
Jun 76Apr 79
Mar 92Mar 97
25,000
30,000
5,000
10,000
15,000
20,000
3 6 9 12 15 18 21 24Time, hrs
Flow
, cfs
Effect of Urban Development
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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
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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
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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)
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FAS – NEXRAD
Midnight 1 a.m.
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Harris Gully Drains to Brays Bayou
Low Flow Box Culvert During Tropical Storm Frances
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Texas Medical Center - Moursund Westbound6/10/01 - 6:44 AM
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Flood Warning SystemsDowntown Houston
EmergencyResponse
Flood Doors Flood Gates Facility Entrances Communications Operations Training