EX1 Flashcards
How to Manually Delineate a Watershed
- Get map-Overlay the map
- Locate outlet on map
- Identify existing drainage network
4.Identify high ground
5.Starting at the outlet, visualize flow paths - Perform water drop test
Delta Storage Equation
deltaS=P-(G+F+E+R+T)
1 Acre Conversion
43560 ft^2
Types of Rainfall
Cyclonic
Orographic
Convective
Cyclonic
Long storm durations
Cold or warm fronts
Cold fronts often form tornadoes
Orographic
Intense storms
westerly sloped mountains
Common in Denver, SW monsoonal moisture
Convective
Heavy thunderstorms
heavy rain from slow moving storms
Rainfall Measurement
Physical measurement on ground (Gages)
Radar Rainfall
Gauge Adjusted Rainfall (GARR)
Tipping Bucket Rain Gauge
Measures time and depth
Approaches to interpolate rainfall
Areal Average
Thiessen Polygon
Isohyetal
Doppler Radar
Arithmetic Mean
gauges in watershed
simple
least accurate
ok with uniformly spread gages
p=1/n summation Pi
Thiessen Polygon
Gages in or near watershed
a weight technique
most widely used
Isohyetal
contours of consistent precipitation
need extensive gage network
Doppler Radar
Most accurate
Thiessen Polygon Method
- Connect each rain gage use dashed lines
- Create a bisect line to each dashed line
- Extent bisect lines to the watershed
- Connect bisect lines
p=1/A summation Ai*Pi
A=summation Ai
Importance of Streamflow
Municipal water supply
Water rights allocation
Reservoir operations
John Wesley Powell
Expedition to survey the west
Professor in Illinois
Went down Grand Canyon/Lake Powell
Flow measurement
Streamflow
Discharge, Q
Rate at which volume of water passes through cross section of area
Q=VA
Streamflow Devices/Methods
Gaging rod
Dye tracer test
Float method
ADCP (Acoustic Doppler Current Profiler)
USGS Method
Subsection widths
Q=summation Vidiwi
Stream gage technologies
Stilling Well
Pressure transducer
Rating Curve
Represents sate vs discharge ( gage H vs Q)
Convert water level readings into flow rate
Create it
Use it
Adjust
Evaporation
water transformed to vapor from liquid
Evaporation affected by
soil type, soil moisture content, temp …..
Transpiration
Water moves through plants and evaporates through leaves
Transpiration affected by
Type of vegetation, Growth stage
Pan evaporation
pan designed to measure evaporation by monitoring loss of water over time
Why probability and frequency analysis?
- Hydrologic processes are random
- Use stats to interpret and predict
Random variables
Discrete
Continuous
Discrete RV
A countable # of distinct values
# of children in family
# of students in classroom
Continuous RV
An infinite # of possible values
height, weight …..
Return Period
The average time between events
p=1/T
Reliability
The probability that a T-year storm event will not occure in n years
R=(1-P)^n
Risk
Probability that a T-year storm will occur at least once in n years
Risk=1-R
Binomial Distribution
Probability of n successes in N trails independent from each other
Infiltration
Water that goes into the ground
Interception
Water that does not hit the ground
(hits trees)
Infiltration Capacity
Max infiltration rate
Green-Ampt Model
Best for soil that exhibit a sharp wetting front
Darcy’s Law (Ifiltarion is proportional to gradient)
Horton’s Method
Used to describe potential infiltration rate vs time
Decay graph
Phi index method
- Calc volume of direct runoff
- Assume # of intervals
- Sum of rainfall in each interval
- Calc phi
- check phi vs excess precipitation
- Iterate if necessary
- Check Pe=rd
Model Phi Index
An index only
Intensity can be taken uniformly
Non-linearly depending on index
Model Hortons
Empirical
Nonlinear
Represented by a first order math equation
Model Green-Ampt
Analytical basis on soil properties
Nonlinear
Wetting front travels through soil
Account for unsteady rainfall
Hydrologic Design
Assessing impact of hyrdologic events
Determining values for key variables
Design system to perform adequately
Hydrologic Design process
- Select runoff
- Predict runoff
- Route flow
- Evaluate flow at points of interest
- Design system
- Iterate
r
Estimated limiting value
largest possible for a hydrologic event at given location (based on the best hydro info)
Probable Max Precipitation (PMP)
estimated greatest depth of precipitation for a given duration that is physically possible and depth of rain
Probable max storm (PMS)
rain over time
Probable max flood (PMF)
Greatest possible flood assuming complete coincidence of all factors that would produce the heaviest rainfall and max runoff
Intensity-Duration-Frequency (IDF)
Curves representing a localized relationship amoung intensity and duration
Standard Project Storm (SPS)
Greatest storm that may be reasonably expected
Standard project flood
design flood, estimated using rainfall-runoff modeling
TP-40
Rainfall frequency, depth of rainfall
NOAA
Rainfall frequency, yields depth of rainfall
Rational Method
Q=kCiA
Time of concentration
time for an entire water shed to contribute to runoff
Sheetflow
Less shallow, cannot exceed 300ft
Shallow
shallow, can exceed 300 ft
Rational method steps
1.find c and n values
2.Areas of each land type
3.tc for each flowpath
P2 from TP-40
V from TR-55
tc from TR-55 ws
4. choose longest time
5. IDF to find i
6. plug into Q=kCiA
Factors affecting hydrographs
Intensity
rainfall duration
watershed size
etc
Characteristics on flood hyrdrograph
slope
roughness
storage
drainage intensity
channel length
above all to runoff
UHG
Unit Hyrdrograph
depth of 1 in or 1 cm of Pe
DRH
Direct Runoff hydrograph
CN
Curve Number
land use
soil type
antecedent runoff conditions
ARC
arc 2 = normal
arc 1= dry
arc 3 =wet
