WRE-1 BY MOHD ABDUL AQUIL CIVIL ENGINEERING

Hydrology Meteorology Surface water hydrology Hydrogeology Study of the atmosphere including weather and climate Surface water hydrology Flow and occurrence of water on the surface of the earth Hydrogeology Flow and occurrence of ground water Watersheds

Intersection of Hydrology Water supplies Drinking water Industry Irrigation Power generation Hydropower Cooling water Dams Reservoirs Levees

Engineering Uses of Surface Water Hydrology Average events (average annual rainfall, evaporation, infiltration...) Expected average performance of a system Potential water supply using reservoirs Frequent extreme events (10 year flood, 10 year low flow) Levees Wastewater dilution Rare extreme events (100 to PMF) Dam failure Power plant flooding Probable maximum flood

Flood Design Techniques Use stream flow records Limited data Can be used for high probability events Use precipitation records Use rain gauges rather than stream gauges Determine flood magnitude based on precipitation, runoff, streamflow Create a synthetic storm Based on record of storms

Sources of Data Stream flows Precipitation US geological survey Http://water.usgs.gov/public/realtime.Html Http://www-atlas.usgs.gov National weather service Http://www.nws.noaa.gov/er/nerfc/ Precipitation Local rain gage records Atlas of US national weather service maps Global extreme events www.cdc.noaa.gov/usclimate/states.gast.Html Sixmile Creek

(Daily Discharge) Snow melt events! Calendar year vs Water year?

Fall Creek Above Beebe Lake (Peak Annual Discharge) 7/8/1935 10/28/1981

Local Rain Gage Records (Point Rainfall) Spatial variation Maximum point rainfall intensity tends to be greater than maximum rainfall intensity over a large area! Rain gage considered accurate up to 10 square miles Correction factor (next slide) Various methods to compute average rainfall based on several gages Rain gage size

Rain Gage Area Correction Factor Storm duration Technical Paper 40 NOAA

Synthetic Storm Design Total precipitation is a function of: Frequency: f(risk assessment) Duration: f(time of concentration) Area: watershed area Time distribution of rainfall Small dam or other minor structures Uniform for duration of storm Large watershed or region Must account for storm structure Can construct synthetic storm sequence How often are you willing to have conditions that exceed your design specifications?

Flood Design Process Create a synthetic storm Estimate the infiltration, depression storage, and runoff Estimate the stream flow We need models!

Engineering (Empirical) Hydrology Based on observations and experience Overall description without attempt to describe details Mostly concerned with various methods of estimating or predicting precipitation and streamflow Largely probabilistic, but with trend to more deterministic models

“Rational Formula” Qp = CIA QP = peak runoff C is a dimensionless coefficient C=f(land use, slope) Http://www.Cee.Cornell.Edu/cee332/scs_cn/runoff_coefficients.Htm I = rainfall intensity [L/T] A = drainage area [L2] Example

“Rational Formula” - Method to Choose Rainfall Intensity Intensity = f(storm duration) Expectation of stream flow vs. Time during storm of constant intensity Q Qp Outflow point t Watershed divide tc Classic Watershed

“Rational Method” Limitations Reasonable for small watersheds The runoff coefficient is not constant during a storm No ability to predict flow as a function of time (only peak flow) Only applicable for storms with duration longer than the time of concentration

Flood Design Process (Review) Create a synthetic storm Estimate infiltration and runoff Soil-cover complex Estimate the streamflow “Rational method” Hydrographs

Runoff As a Function of Rainfall Not stream flow! Runoff As a Function of Rainfall Exercise: plot cumulative runoff vs. Cumulative precipitation for a parking lot and for the engineering quad. Assume a rainfall of 1/2” per hour for 10 hours. Parking lot ? Engineering Quad Accumulated runoff Accumulated rainfall

Infiltration Water filling soil pores and moving down through soil Depends on - soil type and grain size, land use and soil cover, and antecedent moisture conditions (prior to rainfall) Usually maximum at beginning of storm (dry soils, large pores) and decreases as moisture content increases Vegetation (soil cover) prevents soil compaction by rainfall and increases infiltration

Stream Flow  Runoff vs. Time ___ stream flow vs. Time Water from different points will arrive at gage station at different times Need a method to convert runoff into stream flow

Hydrographs Graph of stream flow vs. time Obtained by means of a continuous recorder which indicates stage vs. time (stage hydrograph) Transformed to a discharge hydrograph by application of a rating curve Typically are complex multiple peak curves Available on the web Real Hydrographs

* Required for linearity Hydrographs Introduction There are many types of hydrographs I will present one type as an example This is a science with lots of art! Assumptions Linearity - hydrographs can be superimposed Peak discharge is proportional to runoff rate* * Required for linearity

Hydrograph Nomenclature storm of Duration D Precipitation P tl tp peak flow Discharge baseflow Q new baseflow w/o rainfall Time

SCS* Dimensionless Unit Hydrograph Unit = 1 inch of runoff (not rainfall) in 1 hour Can be scaled to other depths and times Based on unit hydrographs from many watersheds 0.000 0.200 0.400 0.600 0.800 1.000 1 2 3 4 5 t/tp Q/Qp * Soil Conservation Service now Natural Resources Conservation Service

Storm Hydrograph Calculate incremental runoff for each hour during storm using soil-cover complex method Scale SCS dimensionless unit hydrograph by Peak flow Time to peak Runoff depth for each hour (relative to 1 inch) Add unit hydrographs for each hour of the storm (shifted in time) to get storm hydrograph

Addition of Hydrographs

Storm Hydrograph Wynoochee River Near Montesano in Washington Flow (m3/s)

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