Introduction to Surface Water Hydrology Philip B Bedient

Introduction to Surface Water Hydrology Philip B. Bedient Rice University 2006

The Hydrologic Cycle

Major 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) Ground water flow and well mechanics Water quality and contaminant transport (S & GW)

Recent History of Hydrology Early 1900 s saw great expansion of water supply and flood control dams in the western U. S. - in response to Dust Bowl and the Economic Depression of the 1930 s U. S. Dept of Agriculture began many hydrologic studies Sherman UH and Horton infiltration theory 1930 s Theis well mechanics (1935) Horton theory of infiltration (1940 s) Penman (1948) - complete theory of evaporation

Recent History of Hydrology Great urban expansion in 1950 s and 60 s - led to demand for better water supply and prediction (after WW II) EPA formed in 1970 with a mission to clean up the rivers and lakes of America - beginning of environmental eng. as we know it today USGS and EPA actively involved in large-scale sampling programs at the national level - The Woodlands, TX EPA funded development of computer models to address water quality issues in streams and lakes, and estuarine bays (1970 s).

Guadalupe River Map

The Woodlands planners wanted to design the ultimate comm In doing this, they attempted to minimize any changes to the e

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)

Brays Bayou Watershed Harris Gully Area: Brays Bayou Area: 4. 5 sq. mi. 129 sq. mi. Rice/TMC Area Watershed Boundary

Watershed Characteristics Divide v Size v Slope v Shape Reservoir Natural stream Urban v Soil type - LU v Storage capacity Concrete channel

Urban runoff near Brays Bayou - moderate flow

Major Causes of Flooding (Excess Water that Inundates) Highly Developed (urbanized) Area Intensity Flat Topography with Little Storage Poor No and Duration of Rainfall Building Practices in floodprone areas replacement of lost storage as area grows

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

The Watershed Response As rain falls over a watershed area, a certain portion will infiltrate the soil. Some water will evaporate back. Net Rainfall 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 all areas drain out.

Measured Flow for Brays Bayou 30, 000 29, 000 cfs Flow, cfs 25, 000 Jun 76 Apr 79 20, 000 Sep 83 Mar 92 15, 000 Mar 97 10, 000 5, 000 3 6 9 15 12 Time, hrs 18 21 24

Problems in Hydrology: Extreme weather and rainfall Streamflow and runoff predictions River routing and hydraulic conditions Overall water balances - local and global scales Flood control and drought measures Water supply for growing communities Watershed management for agric/urban development

Applications in Hydrology: Surface water supply and delivery systems (sewers) Ground water for supply, wells, and springs Contamination and environmental quality issue – – Lake and Coastal Bay quality studies River quality for drinking and recreation Hazardous waste studies for GW contamination Waste sources from urban/industrial runoff Land use impacts from urban development Disaster mitigation and flood control

Technology has Revolutionized the Field of Hydrology High Speed Digital Computation Geographical Information Systems (GIS) Large Hydrologic and Meteorologic Databases GPS and LIDAR methods for ground surveys RADAR rainfall estimates from NEXRAD Advanced forecasting tools for severe weather and flood Alert

A Note on Units Rainfall volume is normally measured in inches or cm Rainfall rate or intensity in inches/hr or cm/hr Infiltration is measured in inches/hr or cm/hr Evaporation is measured in inches or in/hr (cm/hr) Streamflow is measured in cfs or m 3/s One acre-ft of volume is 43, 560 ft 3 of water 1 ac-inch/hr is approx. equal to 1. 008 cfs Ground water flows are measured as ft 3/day or m 3/day

Rainfall and Conversion to Runoff Use either design rainfalls or historical events Spread uniformly over a given basin area Use Rational Method to compute peak flow for small basin area - few hundred acres Use Unit hydrograph to compute response for larger basins - 10 to 100 sq miles.

Rainfall and Conversion to Runoff Rational Qp Method predicts peak flow = C I A in cfs – C = runoff coefficient - fcn of land use – I = rainfall intensity at time of concentration Tc – A = watershed area in acres – Tc = time for water to travel from most distant pt to the outlet of a watershed

Rational Method uses IDF Curves

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

T. S. Allison vs. the 100 -year (Inches of rainfall) 1 -hr 3 -hr 6 -hr 12 -hr 1976 TMC 3. 8 7. 5 9. 8 10. 4 Allison (a) 4. 3 10. 3 12. 1 14. 7 100 -yr (b) 4. 6 6. 8 8. 5 10. 5 Diff (a–b) – 0. 3 3. 5 3. 6 4. 2 Note: Allison dropped 8. 5 inches in 2 hours

RADAR Rainfall Estimates NEXRAD provides real-time data on a ~16 km 2 (6 mi 2) grid Equivalent to about 21 rain gages in Brays Bayou watershed Each estimate represents an average rainfall amount over the entire 4 x 4 km 2 area NEXRAD rainfall estimates compare well with point rain gage measurements (r 2 ~ 0. 9)

FAS – NEXRAD Midnight 1 a. m.

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

Loss Rate Method: Initial and Uniform Loss Rate Method Amount Lost to Infiltration (in) Inches/Hour Initial Uniform Loss at a Constant Rate (in/hr) Example: Initial Loss = 0. 5 in, Uniform Loss = 0. 05 in/hr

Unit Hydrograph Theory The unit hydrograph represents the basin response to 1 inch (1 cm) of uniform net rainfall for a specified duration, D. 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.

Synthetic UH Methods • Snyder’s Method (1938) • Clark Method (1945) • Nash (1958) • SCS (1964, 1975) • Espey-Winslow (1968) • Kinematic Wave (1970 s)

SCS Triangular UH Example 1 inch of Net Rain over D = 1. 6 hr

SCS Triangular UH Example Volume = Qp. Tr /2 + Qp. B /2 Qp Qp = 2 Vol/(TR + B) TR B B = 1. 67 TR Qp = 484 A / TR tp = lag time L = length to divide in ft Y = Avg basin slope in % S = 1000/CN - 10 (ins) TR = D/2 + tp tp = L 0. 8 (S + 1)/ 1900 (Y)0. 5

SCS Methods Triangular UH CN = curve number for various soil/LU See SCS Table 2. 1 Dimensionless UH

Hydrograph Convolution 1 2 Add and Lag Method 3 1 Q 2 FINAL STORM HYDROG 3 T Add up the ordinates of all three to produce storm hydrograph This add and lag procedure can be extended to large basins

Flow in Pipes and Channels Rain falls over watershed A portion becomes pipe flow (storm water). The remaining portion becomes overland flow in streets and yards. The total runoff reaches a stream and is the sum of both components URBAN RUNOFF Overland Flow Total Hydrograph Outflow Pipe Flow (SWWM) Pipe Flow Time

Hydraulic Calculation - Pipes (z + P/g + V 2/2 g)1 = (z + P/g + V 2/2 g)2 + h. L E = total energy = z + P/g + V 2/2 g at pts 1 and 2 Energy G P 1 rade Line Flow h. L P 2 z 1 Datum (MSL) z 2

Manning’s Equation Open Channels A P = Wetted Perimeter A Pipe P = Circum. A Natural Channel

Small Watershed Response lvd B e Ric Ha rr is G ull y Brays Bayou

Digital Elevation Model Based on 1999 Aerial Survey DEM Used to Determine Overland Flow Connectivity and Storage

Existing Pipe Network 66” 96 72 66” 1. 5 ’x Hermann 2 Park 1 11 2 -1 Rice 4” ” 15’ TMC 5’ x 1 5’ 84” 7. 5’x 11’ NEW PIPES 54” ” 60” 72” 60” 6. 5’x 10’ 72” 90” 60”

Bayou Camera - June 8 -9, 2001 Provided valuable data on water levels and timing 10 p. m. 12 p. m. 11: 00 a. m

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

Fannin at Holcombe Overpass - TS Allison 6/9/01 - 5: 58 AM

T. S. Allison - Houston, June 9, 2001 Rice Blvd at Entrance 16 looking west Jeep indicating high water mark - inlet to Harris Gully

Southwest Freeway (US 59) Detention Storage between Mandell and Hazard Looking West Looking East

Flood Warning Systems Downtown Houston Emergency Response § § § Flood Doors Flood Gates Facility Entrances Communications Operations Training

The Woodlands - a Totally Planned Community The community was designed as if it were fully develop Strict requirements were made about land use and natu

Mountain runoff - steep and dependent on snowmelt

Hoover Dam

Hoover Dam Facts Hoover Dam supplied farmers with dependable supply of water in Nevada, California and Arizona. Because of the Hoover Dam, the Colorado River was controlled for the first time in history.

Mansfield Dam Facts Mansfield Dam sits across a canyon at Marshall Ford on the Colorado River west of Austin, Texas Built from 1937 to 1941 Named in 1941 in honor of U. S. Representative J. J. Mansfield Created a 50 mile long lake that is hundreds of feet deep in lower end

Mansfield Dam Facts Mansfield Dam, owned by LCRA Mansfield Dam and Lake Trav Variable level lake Cleanest in all of Texas

Agricultural runoff in California - source of chemicals