1. WRE-1
BY MOHD ABDUL AQUIL CIVIL ENGINEERING

2. 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

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

4. 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

5. 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

6. Sources of Data Stream flows Precipitation US geological survey
National weather service
Precipitation
Local rain gage records
Atlas of US national weather service maps
Global extreme events
Sixmile Creek

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

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

9. 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

10. Rain Gage Area Correction Factor
Storm duration
Technical Paper 40 NOAA

11. 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?

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

13. 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

14. “Rational Formula” Qp = CIA QP = peak runoff
C is a dimensionless coefficient
C=f(land use, slope)
I = rainfall intensity [L/T]
A = drainage area [L2]
Example

15. “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

16. “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

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

18. 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

19. 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

20. 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

21. 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

22. * 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

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

24. 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

25. 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

26. Addition of Hydrographs

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

28. THANK YOU