Test 1 Flashcards
What are civil and environmental systems/infrastructure designed to do?
-Designed to manage hydrologic processes (culverts, stormwater pipes, detention ponds, treatment wetlands)
-Potentially impacted by hydrologic processes (bridges, buildings, fish habitat, power infrastructure, water supplies)
What is engineering hydrology?
-Hydrology is a field that is studied/practiced by several disciplines (engineers, earth scientists, biologists, planners)
-Engineering Hydrology is focused on quantitative prediction of hydrologic phenomena that is relevant to designing/analyzing natural or engineered systems.
Engineering hydrology applications
-culvert/bridge design
-floodplain mapping
-urban stormwater systems
-sustainable water allocation
Watershed (catchment)
-The area of land that would drain surface water towards a specific point on the landscape or on a stream/watercourse
-The watershed is the fundamental spatial unit, or control volume, that we use to conduct hydrologic analyses
-Watershed sizes can range from very small (10’s of square meters, i.e. a parking lot) to very large (millions of square kilometers, i.e. the Mackenzie River Watershed).
How to Manually Define or
“Delineate” a Watershed
-Water flows downhill!
-Need a topographic map or digital elevation model
-Pick a watershed “outlet”
-Identify stream channel/drainage network (valleys)
-Identify adjacent stream channels/drainage networks
-Identify high points (hills, ridges) that are between the stream network you are delineating and adjacent stream networks
-Starting at watershed outlet, draw a line connecting high points, making sure to cross contour lines at right angles (perpendicular).
Water Balances
Formulation and components will depend on how the control volume is defined and the time step
Water Balance Components: Storage (S): watershed vs lake
-For Watersheds S represents soil moisture storage and surface water impoundments:
* Typically assume S = 0 if conducting water balances on an annual time step
-For Lakes S is dependent on the Stage-Storage and Stage-Discharge relationships for the system.
Stage-Discharge Relationships
-Defines relationship between water level in system and outflow rate
* Need to identify the type of control structure (Engineered structure (weir, pipe, drop inlet), Natural control (channel).
Water Balance Components: Precipitation (P) (annual time step)
-Rainfall and Snowfall
-When operating on an annual time step need to consider what an appropriate “Water Year” should be:
* All snowfall should have melted (e.g. October – September).
Water Balance Components:
Evaporation and Transpiration (ET)
-Critical parameter in long term water budgets
-Combination of evaporation and transpiration
Evaporation
Phase change of liquid water to vapour from open water surfaces
Transpiration
-Phase of change of liquid water to vapour and movement into atmosphere through plant stomata
Water Balance Components: Groundwater (G): Watershed vs. lake
-Watersheds: Represents water recharging deeper geologic systems not connected to surface water
-Lakes: Represents water entering or leaving lake through groundwater system (Quantification requires field measurements (piezometers, seepage meters, and/or numerical modeling).
Water Balance Components:
Runoff (R) or Water Yield (Y): Watershed vs. lake
-Watersheds: Water leaving watershed at outlet as surface water flow
-Lakes: Water entering lake through a surface water feature
Baseflow
-Baseflow is “Dry weather flow”, typically originating from groundwater or lakes/reservoirs
-Significant flow mechanism in perennial streams
-Baseflow Index (BFI) = Baseflow/Total Streamflow
Interflow
Interflow is lateral flow of water through the unsaturated zone to stream
Stream Order (or Strahler Number)
-Stream with no tributaries are designated 1st Order
-Confluence of two 1st order tributaries initiates a 2nd order stream and so on…
-Stream Order is correlated with watershed characteristics
Drainage Density
Total Length of Streams / Catchment Area
Bifurcation Ratio
Related to stream order, ratio of number of streams within successive stream order categories (e.g. # of 1st order streams / # of 2nd order streams)
Hydrographs vs. Hyetographs
-Hyetograph is a time series of precipitation amounts
-Hydrograph is a time series of streamflow/discharge
2 Types of Hydrologic Data
-Climate (Precipitation, Temperature, Evaporation, Solar Radiation, Wind Speed, Relative Humidity)
-Hydrometric (Discharge, Level (Stage))
Pro-rating Discharge (Qug)
-Common practice to transfer discharge records from gauged to un-gauged watersheds
-Should ensure the watersheds are hydrologically similar: Area, Climate, Topography, Surface water storage, Soils/geology
Statistical Hydrology
-Precipitation, streamflow and other quantities of importance can be treated as random variables, with associated measures of frequency that represent likelihood, percentage of time, or probability
Random variable
-A random variable is a variable described by a probability distribution
-A probability distribution is a function representing the frequency of occurrence of a random variable
-A set of observations from (e.g. x1, x2, x3) from the random variable is called a sample
Return Period and Annual Exceedance Probability (AEP)
-The concept of Return Period is commonly employed in hydrology:
-The return period (T) is the number of years, on average, between events of equal or larger magnitude. It is also equal to the inverse of the annual exceedance probability (AEP) of occurrence of an event of equal or larger magnitude in any given year
-T = 1/AEP
-e.g. The 1 in 100 yr flood has an AEP of 1/100 or 1% chance of occurrence in any given year
Frequency Analysis
-A variety of methods can be used to assign return periods, or probabilities of occurrence to specific hydrologic phenomena (Empirical or Graphical Methods, Analytical Probability Distributions)
Analytical Probability Distributions
-The Cumulative Distribution Function (CDF) provides the probability of being below a particular value: Therefore for flood frequency analysis (Annual Exceedance Probability): T = 1/AEP = 1 / (1-F(x))
-We fit a sample set of hydrologic data to a probability distribution and then use the fitted distribution to estimate exceedance probability (e.g. return period) for various hydrologic quantities
Applying Flood Frequency Analysis in Practice
-In general, the return period of the estimate should be not be more than double the length of the data record (e.g. a 50 yr discharge record is required to reliably estimate a 100 yr return period discharge)
-It is common practice to compute the flood flows using several probability distributions. You can compare the fit to each distribution, see how well the estimates agree, and potentially use the highest estimated discharge to be conservative
-The Skew Coefficient is quite sensitive to the sample size. Can confirm by examining longer flow records in the same geographic area
-Should check for outliers: always be cautious about removing data from the record…need to have strong evidence that the data point is a true outlier. This usually involves examining flow information and flood information at nearby sites.
Flow-Duration (F-D) Relationships
-F-D relationships show the frequency, or percentage of time, the streamflow falls within various ranges
-A typical F-D relationship is shown with the percent of time flow is equaled or exceeded on the x-axis and the magnitude of flow on the y-axis.
Precipitation Formation
-Precipitation includes rainfall, snowfall, hail and sleet
-Formed by condensation of water vapour on condensation nuclei
Storm Types
Orographic, convective, cyclonic (frontal)
Spatial Variability
-For large watersheds there may be several precipitation gauges
-Various methods could be used to develop an average precipitation for the watershed (Arithmetic means, Thiessen Polygons, Isoheyetal method)
Rainfall events have 3 primary characteristics
Intensity, duration, frequency
Return period
Return period is defined as the average period of time in years expected between rainfalls of a specific intensity
Characterizing Rainfall
We treat rainfall events as random occurrences and use probability theory to describe the likelihood of occurrence of certain rainfall events. These probabilities are typically presented in the form of Intensity-Duration-Frequency (IDF) curves
How are IDF relationships developed?
-Step 1: Long term rainfall records are analyzed to identify the maximum rainfall depth in each year for various durations
-Step 2: For each duration, the maximum depth data series is fit to a probability distribution. Environment and Climate Change Canada
typically apply the Gumbel distribution.
-Step 3: The required return period depth estimates are then estimated using frequency analysis methods (the same methods we used for flood frequency analysis)
Explain Design Storms
-For some engineering analysis and design we need to develop a synthetic storm (i.e. A temporal sequence of rainfall amounts/intensities)
-Actual rainfall events would rarely, if ever, have a constant intensity
-Most storms having varying intensities throughout the duration
Balanced Storm Approach
-The rainfall depth at any duration in the storm is equal to the depth predicted from an IDF relationship
-The incremental depths are rearranged so that the greatest incremental depth is located at the middle of the storm duration
-The next highest incremental depth is placed just before the middle increment, the next highest is placed just after…
Chicago Storm Distribution
-Used widely for designing urban stormwater systems
-Analytical expressions that are derived from an IDF equation
What is a design return period influenced by?
Cost, safety
The Estimated Limiting Value (ELV)
-Is the largest magnitude possible for a hydrologic event at a given location
-The Probable Maximum Precipitation (PMP) is an example of an ELV
Explain Design Storm Duration
-We want to make sure we are designing for worst case scenarios (higher rainfall intensity → higher runoff rates → larger structures)
-as storm duration increases, rainfall intensity decreases
-The other key factor that governs peak runoff response is the “time of concentration, Tc” of the watershed
Time of concentration
-The Time of Concentration of a catchment represents the travel time of a wave to move from the hydraulically most distant point in the catchment to the outlet
-If the storm duration is equal to or greater than Tc the whole watershed will be contributing flow to the outlet
-Once the “time of concentration” of the watershed has been reached, we can assume an equilibrium condition in which the runoff rate is equal to rate of effective rainfall.
Probable Maximum Precipitation
-Greatest amount of precipitation that would be physically possible in a given location at a specific time of year
-Represents the maximum amount of moisture that could be held in the atmosphere
Snowwater Equivalent (SWE)
Liquid water generated from melted snow
Snowfall and Snowpack Measurement
- Total Precipitation Gages: Standard Melting Gages (Nipher Gage), Universal Gages
- Snowfall Depth Measurements: Snowboards, ultrasonic sensors
- Snowpack Measurements: Snow stakes (Depth), Snowpack Surveys (Core samples to determine depth and SWE)
Structure of Natural Soils
-Soil formation results in the natural development of layers within a profile. Each layer is composed of structural units consisting of various fractions of sand, silt, clay and organic matter. Each layer is called a HORIZON
-Each horizon has differing: colour, particle size, density, hydraulic conductivity
Primary vs. Secondary Soil Structure
-Primary soil structure refers to the size distribution of individual particles (sand, silt, clay)
-Secondary structure refers to arrangement of particles into larger aggregates
-Secondary structure can a have large influence on water movement
When would Darcy’s Law not be valid for unsaturated flow?
-Strong soil-water bonding
-Macropores
Volumetric Water Content (Θ)
Volume of water/Volume of total soil (air + water + solids)
Gravimetric Water Content, (u)
Mass of water/mass of dry soil
Degree of Saturation (S)
Fraction of pore space filled with water (ranges from 0-1)
Matric Potential, Soil Suction, Soil Tension (Ψ)
-Pressure required to remove water from soil
-Cohesive attractive force between water and soil particle
HYDRAULIC GRADIENT
In the saturated zone, flow is driven by a gradient in positive pore pressure, whereas in the unsaturated zone, flow is driven by a gradient in negative matric potentials
Soil Water Characteristic Curve
MEASUREMENT OF MATRIC POTENTIAL AND SOIL WATER FLOW DIRECTION
Matric potential and hydraulic gradients in the unsaturated zone are measured using tensiometers
Infiltration Capacity (fp)
Theoretical maximum rate of water transmission into soil as influenced by soil factors
Infiltration Rate (f)
Rate of water transmission into soil as influenced by soil factors and the water availability at the soil surface
GREEN-AMPT MODEL
Based on application of Darcy’s Law and the assumption of a Sharp Wetting Front moving vertically downward under for forces of gravity and capillary suction
HYDROLOGIC SOIL GROUPS
-Group A soils have high infiltration rates and consist of coarse sands and gravels
-Group B soils are moderately well drained and consist of moderately fine to moderately coarse textures
-Group C soils have slow infiltration rates. They consist of fine grained soils, or soils which possess a constricting layer at depth
-Group D soils have very slow infiltration rates and generally posses high clay contents
