Test 1 Flashcards
what are the five steps to delineate a water shed
- get a map
- Locate outlet
3.Identify drainage network - Identify high ground
5.Visualize flow paths - drop of water test
Continuity equation
Storage =P-(G+F+E+R+T)
Precipitation
Ground water
Infiltration
Evaporation
Runoff
Transpiration
1 acre
43560 cft
How is most precipitation formed
by moving moist air to higher elevations
Convection
Heating below quickly
Stratiform
Gentle motions
Orographic
Air being physically lifted (like over a mountain)
Lifting Condensation Level (LCL)
the height at which air reaches it’s dew point when cooled by dry adiabatic lifting
Types of precipitation
Drizzle – very light small droplet size (.1 - .5 mm)
•Rain – water drops larger than 0.5 mm
•Snow – branched hexagonal crystals of ice
•Sleet – pellets of ice, usually rain that freezes as it falls
•Glaze – freezing rain or drizzle that freezes when lands
•Snow Pellets – pellets of ice 0.5 to 5mm in diameter
•Small hail – ice pellets from 2 to 5 mm
•Soft hail – hail that is melting faster
•Hail – larger pellets (5 to 50 mm), can have concentric structure
•Dew – water on the ground surface due to condensation of water vapor
•Hoar frost – same as dew, but freezes on surfaces
Cold front
Advancing Mass of cold air steep interface
Precipitation intensifies along front
thunder storms common
Warm Front
Advancing mass of warm air
Less steep
Stability determines precipitation
day long drizzle common
what side of a mountain will be wetter
windward side
What are the categories of precipitation
Point and areal
Point Precipitation
Precipitation measured at a specific location
Areal Precipitation
Collective use of point precipitation data to evaluate areal variability
Types of rain gages
Standard- Depth only
Tipping Bucket- Depth and time
temporal analysis
Precip Recorded and reported over differing averaging periods
Hyetograph
Record of intencity
Cumulative Mass Curve
Accumulated precipitation over time
Approaches to Interpolating rain gage values
Areal Average
Thiessen Polygon
Isohyetal
Doppler Radar
Areal Average
-Gages in water shed
-Simple but least accurate
-ok with uniformly spread gages
Thiessen Polygon
-Gages in on near water shed
-a weighting technique
-most widely used
Isohyetal
-Contours of consistent precipitation
-need extensive gage network
-typically very accurate
Doppler
-most accurate
What are the steps for creating a thiessen
- Connect Each Raingage use dashed lines.
- Create a bisect line perpendicular to each dashed line.
- Extend our bisect lines to the watershed boundary.
- Connect bisect lines.
What is the importance of stream flow?
• Municipal water supply
• Water rights allocation
• Reservoir operations
• Hydropower
• Agriculture/Irrigation
• Planning/Drought
• Recreation
• Water Supply
• Designing hydraulic structures
• National Weather Service – flood forecasting!
Who is John Wesley Powell
- Union Major
- Lost arm at Battle of Shiloh
- 1890’s
- Professor at Illinois
- Expedition to survey the west
- Went down the Grand Canyon/Lake Powell
- Became 2nd Director of USGS
Where was the earliest stream flow gage
1889 Embudo NM of the Rio Grande
How many gages does the USGS have? How many flow measurements do they make
7,000 gages and 50,000 flow measurements a year
What is stream flow?
rate at which a volume of water passes through a cross-sectional area
What are the ways to measure streamflow?
Gaging rod+ Current meter
Dye/tracer test
Float method
ADCP (acoustic doppler current profiling) (Most accurate)
Is stream flow constant through a rivers cross section
NO
Lag time
Difference between peak rainfall and peak discharge
When and where was the earliest flow measurement
Samaria in 5,000 BC
What is a rating curve
Represents stage discharge relationship
Used to convert water level readings into flow rates
Evapotranspiration
Water transformed from
liquid phase to vapor phase
How much of the rain in the US evaporates
70%
transpiration
Water moves through plants
and evaporates through
leaves.
What is evaporation affected by
Soil Type, Soil Moisture Content, Temp, Humidity, Wind, Cloud Cover
What is transpiration affected by
type of Vegetation,
Growth Stage
What is Pan Evaporation/Transpiration (ET)
Pan designed to measure evaporation by
monitoring loss of water over time (1 day).
Why do we do probability and frequency analysis?
1) Hydrologic processes are random.
2) Use statistics to interpret data, Describe random variables using Probability Distribution
3) Identify best fitting distribution that is used to Predict future events.
Give and example of a discrete variable
A countable # of distinct values.
Ex: # of children in a family
# of flow probes in a lab
# of students in a class
Give an example of a continuous variable
- An infinite # of possible values.
Ex: Measurements: height, weight, etc.
Streamflow
Return Period
The average time between events [storms, floods,
etc.] that equal or exceed a certain magnitude
Reliability
The probability that a T-year storm event
will not occur in n years.
Risk
Probability that a T-year storm will occur at
least once in n years
Ia
Initial Abstraction before runoff can occur such as surface/ depression storage and interception
Infiltration
Entry of water into soil.
Impacts the amount of runoff
What factors influence runoff?
1.Soil Type
2. Soil Moisture
3. Surface Condition
4. Vegetation
5. Rainfall Intensity
6. Slope
Infiltration capacity
maximum rate of infiltration happens when rainfall is heavy
Rainfall infiltration
all of the water seeps into the ground
Green ampt method
Physically based, semi-
empirical.
• One of the most realistic
models of infiltration
available to engineers.
• Originally derived in 1911
by Green and Ampt and
then Phillip in 1954
improved it giving it a
more physical basis.
-Analytical model that has a physical basis to soil properties
-non linear can be solved in a spreadsheet format
-wetting front travels through soil
-can account of unsteady rainfall (reality)
What kind of soil is the green ampt method best for?
• Best for soils that exhibit
a sharp wetting front, ie.
course soils w/uniform
pore shapes
Horton Equation
Horton (1940) observed the infiltration begins at a rate and decays to a final rate
-Empirical
-nonlinear and can be solved in a spreadsheet format
-infiltration in measured by a first order mathematical expression
What are the assumptions of hortons equation
Assumes rate of infiltration is a function of time and
follows a first order decay.
• Must have field derived data.
Phi index
Used when both flow data & corresponding rainfall
hyetograph are available.
-An index only
-Intensity can be taken uniformly over the storm duration
-can provide nonlinearly depending on index
What are the steps to use the phi index
1.Calculate volume of direct runoff (rd)
2. Assume number of intervals (M) that contribute to runoff.
3. Calculate the sum of observed rainfall (Rm) in each interval.
(∑𝑅𝑅𝑚𝑚)
4. Calculate Phi(∅): ∑𝑅𝑅𝑚𝑚−𝑟𝑟𝑑𝑑
𝑀𝑀(∆𝑡𝑡)
5. Check Phi versus each interval for excess precipitation (Pe)
6. Iterate if necessary
7. Check Pe = rd
Hydrologic design
– Assessing impact of hydrologic events
– Determining values for key variables (Qpeak,
stage, precipitation, etc.)
– Design system to perform adequately
Hydrologic design process
Hydrologic Design Process-
1. Select Design Storm
2. Predict Runoff
3. Route Flow
4. Evaluate flow at Point(s) of Interest
5. Design System (e.g. structures)
6. ITERATE!
Estimated limiting value
Estimated Limiting Value - largest
magnitude possible for a hydrologic event
at a given location based on the best
available hydrologic information
Probable Maximum Precipitation (PMP)
estimated
greatest depth of precipitation for a given duration that is
physically possible and reasonable characteristic over a particular geographic location at a certain time of year; depth of rain.
Probable Maximum Storm (PMS)
spatial and
temporal distribution of the PMP; rain over time.
Probable Maximum Flood (PMF)
greatest possible
flood assuming complete coincidence of all factors that
would produce the heaviest rainfall and maximum runoff;
derived from PMS (and therefore PMP); frequency
(return period) cannot be determined- some apply
arbitrarily high values but they have no physical basis.
Hydrologic design scale.
helps to understand, predict, and manage water resources by looking at the limiting value of the design
Intensity-Duration-Frequency (IDF) Curves
curves representing a localized relationship among intensity, duration, and frequency (return period).
Standard Project Storm (SPS)
greatest storm that may be reasonably expected.
Standard Project Flood (SPF)
design flood; estimated using rainfall-runoff modeling by applying unit hydrograph methods to the SPS
•Technical Paper No. 40 (TP-40)
rainfall frequency atlas of the eastern United States for duration for 30
minutes to 24 hours and return periods from 1 to 100
years; yields depth of rainfall.
NOAA Atlas 14
rainfall frequency atlas of the United States for duration for 5 minutes to 60 day and return
periods from 1 to 1000 years; yields depth of rainfall.
rational method
Most widely used peak runoff method; been around since the 1800’s
What are the rational method assumptions
1.Entire catchment is contributing to runoff
2. Rainfall is distributed uniformly over the catchment area
3. All hydrologic losses in the catchment are
homogeneous and uniform
Represented in Runoff Coefficient, C.
what are the rational method’s limitations
- Used on catchments smaller than ~100 Acres
- Provides only 1 point on the Runoff Hydrograph (Peak)
When using in complex subcatchments it overestimates the actual flow…results in oversized infrastructure.
No means to generate or route hydrographs through detention facilties. - Routing must be incorporated as we design from point to point.
Time of concentration
Time for entire watershed to contribute to
runoff.(Single drop of water from the most remote point)
sheet flow
shallow, unconcentrated and irregular flow down a slope does not exceed 300 meters
Shallow concentrated flow
flow that gains speed and increases depth, forming small channels
rational method steps
- Obtain runoff coefficient (C) and Manning’s n
values for each land cover type (from tables) - Determine areas of each land use type
- Determine time of concentration for each
possible flow path
a. Determine P2 from TP-40
b. Determine V from TR-55 Figure 3-1
c. Use TR-55 worksheet to:
i. Calculate tc’ for sheetflow (first 300 feet of flow path)
ii. Calculate t0 for shallow concentrated flow (remaining
distance of the flow path) - Choose the longest time of concentration (it
might not be the flow path you expect it to be!) - Use that time on the regionalized IDF curve
to find the rainfall intensity (i) - Plug all values into Q = kCiA
NRCS Design storm
1) Select Return Period
1 ≤t ≤ 100-Yrs
2) Select precipitation duration
[flood depth]
3) Apply NRCS Temporal Distribution
pgs. 425-426
30 min →24 hour →Tp 40
4) Multiply your rain depth by
distribution.
What are the factors Affecting Hydrographs
• Rainfall Intensity
• Rainfall Duration
• Watershed size
• Watershed slope
• Watershed shape
• Watershed storage
• Watershed morphology
• Channel type
• Land Use/Cover
• Soil Type
• Percent Impervious
Unit Hydrograph
Idea introduced by Sherman in 1932.
– Most widely used method of estimating runoff
hydrographs
• A direct runoff hydrograph resulting from a unit
depth (1” or 1cm) of excess precipitation (Pe)
over a drainage area at a constant rate.
• A linear model that transforms excess
precipitation into direct runoff.
What are the unit hydrograph assumptions
Assumptions:
1. Pe constant intensity
2. Pe uniform over watershed
3. Base time resulting from Pe is constant
4. Ordinates of all DRH of common base time are
proportional
5. Hydrograph from Pe reflects unchanging
characteristics of watershed
UHG Derivation
Integration: storm hydrograph, baseflow,
and watershed area are known.
- Scale DRH
Deconvolution: storm hydrograph, baseflow,
and precipitation data are known.
- Matrix methods
Synthetic Methods: several options, most
used NRCS.
NRCS Curve Number(CN)
The Curve Number quantifies the watershed
response to rainfall.
What is the CN a function of?
Land Use
• Soil Type
• Antecedent Runoff Conditions (ARC)
Antecedent Runoff Conditions (ARC)
Wetness of the soil (corrects for moisture)
ARC II
Normal (No correction)
ARC I
Dry
ARC III
Wet
Phi Index (compared to NRCS)
• Simple
• Need Hyetograph
• Need Streamflow Data
(Vol of Direct Runoff)
• Need Watershed Area
• CONSTANT Loss
• No Ia
NRCS (Compared to phi)
Not as simple
• Need Hyetograph
• Need CN
– Land Use
– Soils
– Watershed Area
– Moisture Content
• VARIABLE Loss
• Ia
NRCS Dimensionless UGH
NRCS developed this method from the
average shape of a large number of UHGs
from small agricultural watersheds
throughout the U.S.
• Allows us to create a UHG for anywhere in
the U.S. with minimal data.
• Convert NRCS Dimensionless UHG to
actual hydrograph by determining Qpeak
and Tpeak and multiplying.
