Surface Water Hydrology - Civil and Environmental Engineering - Lecture Slides, Slides of Civil Engineering

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Introduction to Surface Water Hydrology Docsity.com The Hydrologic Cycle 1 0 0 P r e c i p i t a t i o n o n l a nd Infiltration Water table Groundwater flow 1 Groundwater discharge 38 Surface discharge 61 Evaporation from land 39 Moisture over land 385 Precipitation on ocean 424 Evaporation from ocean Surface runoff Impervious strata Groundwater Recharge Precipitation Snow melt Docsity.com Recent History of Hydrology • Great urban expansion in 1950s and 60s - 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 (1970s). Docsity.com The Woodlands planners wanted to design the ultimate community to handle a 100-year storm. In doing this, they attempted to minimize any changes to the existing, undeveloped floodplain. The Woodlands Docsity.com 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) Docsity.com 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. Docsity.com S e p 8 3 J u n 7 6 A p r 7 9 M a r 9 2 M a r 9 7 2 5 , 0 0 0 3 0 , 0 0 0 5 , 0 0 0 1 0 , 0 0 0 1 5 , 0 0 0 2 0 , 0 0 0 3 6 9 1 2 1 5 1 8 2 1 2 4 T i m e , h r s Fl ow , c fs Measured Flow for Brays Bayou 29,000 cfs Docsity.com 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 Docsity.com 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 m3/s • One acre-ft of volume is 43,560 ft3 of water • 1 ac-inch/hr is approx. equal to 1.008 cfs • Ground water flows are measured as ft3/day or m3/day Docsity.com 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. Docsity.com Rainfall and Conversion to Runoff • Rational Method predicts peak flow • Qp = 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 Docsity.com 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) Docsity.com 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 Docsity.com Loss Rate Method: Initial and Uniform Loss Rate Method • Initial Amount Lost to Infiltration (in) • Uniform Loss at a Constant Rate (in/hr) 0 0.2 0.4 0.6 0.8 1 1.2 1 2 3 4 5 6 7 8 9 10 11 12 13 Ra in fa ll (in /h r) Time (hours) Rainfall Excess Losses Example: Initial Loss = 0.5 in, Uniform Loss = 0.05 in/hr In ch es /H ou r Docsity.com SCS Triangular UH Example 1 inch of Net Rain over D = 1.6 hr Docsity.com SCS Triangular UH Example Volume = QpTr /2 + QpB /2 Qp = 2Vol/(TR + B) B = 1.67 TR Qp = 484 A / TR TR = D/2 + tp tp = L0.8 (S + 1)/ 1900 (Y)0.5 B TR Qp tp = lag time L = length to divide in ft Y = Avg basin slope in % S = 1000/CN - 10 (ins) Docsity.com SCS Methods Triangular UH CN = curve number for various soil/LU See SCS Table 2.1 Dimensionless UH Docsity.com Hydraulic Calculation - Pipes Datum (MSL) z1 P1 z2 P2 hL (z + P/γ + V2/2g)1 = (z + P/γ + V2/2g)2 + hL E = total energy = z + P/γ + V2/2g at pts 1 and 2 Docsity.com Manning’s Equation Open Channels P = Wetted Perimeter Pipe P = Circum. Natural Channel Q = 1.49 n AR 2 3 S A A A Docsity.com Small Watershed Response Ric e Blvd . S h e p h e r d Gr ee nb ri ar M ain St . F an ni n M on tr os Kir b y Harris Gully Richmond Westheimer Bissonnet Holcombe SW Freeway Rice University Texas Medical Center Brays Bayou Docsity.com The Woodlands - a Totally Planned Community The community was designed as if it were fully developed with minimal impacts on water. Strict requirements were made about land use and natural drainage concepts were used throughout. Docsity.com 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. Docsity.com 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 Docsity.com