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(7)
Hydrology (7)
| Question | Answer |
|---|---|
| Rainfall Statistics | A method of analysis that involves a scientific and biased description of the characteristics of storms that have a high probability of occurring |
| Intensity-Duration-Frequency (IDF) | Most common way to show rainfall statistics |
| Hydrologic Cycle | The full life cycle of water: precipitation, runoff, evaporation, transpiration, and condensation. |
| Initial Abstraction | The total amount of water that is intercepted and absorbed into groundwater before runoff begins. |
| Infiltered Water | After runoff begins, the soil continues to absorb some water |
| Evaporation | The process in which liquid water turns to gas as it warms up |
| Transpiration | Precipitation absorbed by plants will return to the atmosphere |
| Condensation | The process in which the vapor cools and turns back into liquid, and precipitation will occur again. |
| Interception | The process by which rainfall is captured by aboveground objects before it reaches the soil surface |
| Depletions (Abstractions) | When evaporation, infiltration, and interception are collectively combined. |
| Water Balance Equation (Water Budget Equation) | Used to find the change in water storage in a surface water system |
| Storm Distribution | Refers to the statistical representation and analysis of the characteristics of storm events. |
| Statistical Distributions | Describes the probability of occurrence of different data points within a dataset. |
| Binominal Distribution | Used for situations where there are two possible outcomes, success or failure. |
| Normal Distribution | Used for variables that tend to cluster around a mean value; precipitation rarely follows this. |
| Log Normal Distribution | In which values are log-transformed to approximate a normal distribution; used for positively skewed data such as precipitation amounts. |
| Exponential Distribution | Is used to model the time between events in a Poisson process (in which events occur randomly and independently at a constant average rate), such as the time between storm intervals. |
| Generalized Extreme Value (GEV) Distribution | Commonly used to model the distribution of extreme events, such as maximum annual precipitation |
| Isohyet | A line on a map joining regions of an area (e.g., a catchment or watershed) that receive similar amounts of precipitation in a given duration. |
| Long-Term Normal Precipitation | The average amount of precipitation that a specific location receives over a standard period, typically 30-years. |
| Distance-Weighting Method | Finds precipitation at a station where the rain data is missing by using precipitation data from other sections, but assumes that the influence of a given station's data on the station of interest (sti)decreases with that station's distance from the sti. |
| Regression Method (Double Mass Curve Method) | Used to analyze the consistency of a rain gauge station's precipitation record by comparing its accumulated annual or seasonal precipitation with the concurrent accumulated values of mean precipitation from surrounding stations. |
| Break Point (In Regression Method Curve) | The point at which the slope changes |
| Favorable Outcomes | The number of possible outcomes of that event that are acceptable, desired, needed, or wanted |
| Annual Exceedance Probability (AEP) | The chance that a flood of a certain magnitude will occur or be exceeded in a given year. |
| Return Period (T) | The average interval of time between floods of a certain intensity and duration |
| Risk (Probability of Exceedance) | The probability of a given storm or flood will happen at least once over a period of "n" years. |
| Reliability (Probability of Nonexceedance) | The probability that a flood of a certain magnitude or greater will not occur in any given year or series of years |
| Time of Concentration | The time it takes for runoff to travel from the most distant point of a watershed to outlet during a storm event. |
| Empirical Models | Are based on statistical relationships between rainfall and runoff |
| Conceptual Models | Use simplified representations of the hydrological system to estimate runoff and are useful for predicting the overall response of a watershed to rainfall events. |
| Physical-Based Models | Use complex mathematical equations to stimulate physical processes of rainfall and runoff. |
| Hydrograph | A chart that depicts runoff versus time for a storm event over a watershed |
| Base Flow | The flow measured before a storm event |
| Total Flow | The runoff measured when precipitation begins |
| Delayed Flow | Water from a previous precipitation event that is still flowing after that event is over and before the new storm |
| Overland Flow (Direct Runoff) | The portion of the total flow that is not the base flow, which causes flooding, erosion, or other problems. |
| Rising Limb (Concentration Curve) | The curve representing the increase in total flow as the storm event begins |
| Peak | Is the point at which the total flow is at its max |
| Falling Limb (Recession Curve) | The curve representing the decrease in total flow as the storm event ends |
| Time Base | Is the amount of time over which all flow on the hydrograph occurs (this is not the same as the duration of a storm) |
| Time to Peak | The time from the beginning of the storm event to the peak |
| Hydrograph Separation (Hydrograph Analysis) | A procedure used to separate the base flow from a total hydrograph, resulting in a hydrograph that only shows the overland flow, which is referred to as a direct runoff hydrograph or overland flow hydrograph |
| Straight-Line Method | Line is drawn connecting the point where the rising limb of the hydrograph starts to the point where the receding limb ends |
| Two-point Method | Averages the velocities measured at 20% and 8-% of the depth of the stream |
| Six-Tenths Depth Method | Assumes the average velocity of a stream is equal to the velocity measured at 60% of the stream depth from the surface |
| Three-Point Method | Averages the velocities measured at 20%, 60%, and 80% of the stream depth |
| Five-Point Method | The velocity is averaged from velocities measured at the surface of the stream, 20%, 60%, and 80% of the depth of the stream, and the bed of the stream |
| Six-Point Method | Similar to the five-point method but includes an additional velocity measurement at 40% of the stream depth for greater accuracy |
| Two-Tenths Depth Method | The average velocity is assumed to be equal to the velocity measured at 20% of the stream depth |
| Surface Velocity Method | Uses the surface velocity and a correction factor to find the average velocity |
| Rainfall Intensity | The rate at which rain falls, typically measured in terms of depth per unit of time (e.g., inches per hour) |
| Unit Hydrograph | A direct runoff hydrograph of a storm event that produces a single unit of effective rain in a given watershed |
| Synthetic Unit Hydrograph | An artificially created form of a unit hydrograph. Represents the amount of runoff generated in a given watershed by one inch of rainfall over a specified duration. |
| SCS (NRCS) Unit Hydrograph | Developed as a dimensionless unit hydrograph to represent the average runoff response of small agricultural catchments across the United States |
| Synder Synthetic | Unit hydrograph method is an empirical approach that was developed based on studies of 20 catchments located primarily in the Appalachian Highlands in the United States. |
| Lagging Storm Method | A technique used in hydrology to create a unit hydrograph for a storm based on a known unit hydrograph for a storm of a different duration. |
| Probable Maximum Flood | Is a hypothetical flood that can be expected to occur as a result of the most severe possible combination of critical meteorological and hydrologic conditions. |
| Probable Maximum Precipitation | The greatest amount of precipitation that is meteorologically possible in a region |
| Design Flood/Design Basis Flood | The flood is adopted as the basis for design of a particular project and is dependent of the project site |
| Standard Flood/Standard Project Flood | A flood selected from the most severe combinations of a meteorological and hydrological conditions reasonably characteristic of a given region |
| Hydraulic Path | The travel path of water from the most hydraulically remote point of the watershed to the outlet |
| Sheet Flow | Is the flow of water over land prior to concentration |
| Shallow Concentrated Flow | The flow after sheet flow, where water has gained momentum and creates gullies/shallow channels along its path. |
| Channelized Flow | The flow of water through a channel, either a defined natural channel such as a stream or an artificial channel as a pipe. |
| Rain Gauges | Are instruments used to measure the amount of rainfall at a specific location, usually managed by the government agencies or research organizations. |
| Stream Discharge | A measure of the volume of water that flows through or out of a body of water over time. |
| Horton Model | Describes infiltration as an exponential decay process, where the rate decreases over time as the soil becomes saturated |
| Ponding Time | The time required for surface ponding to occur after the onset of rainfall |
| Depression Storage | The portion of precipitation from a rainfall event that accumulates in surface depressions on uneven ground |
| Stormwater Attenuation | Underlying philosophy of stormwater management which involves reducing and delaying the peak flow of stormwater, thereby reducing the risk of flooding and stream erosion downstream. |
| Detention Pond | Storage facilities design to temporarily impound stormwater runoff and release it slowly through an outlet structure |
| Retention Pond | Designed to hold water for a longer period, functioning as a final storage destination for stormwater. |
| Routing | Process of measuring flow rate, measured in volume per unit time, at a downstream section of a channel based on the temporal distribution of flow rate at an upstream section. |
| Storage | Refers to precipitation retained in surface depressions |
| Interception | The portion of precipitation that adheres to plant foliage, buildings, and other aboveground objects until its returned to the atmosphere through evaporation |
| Evaporimeter | An instrument used to measure evaporation |
| Infiltration Rate | A measure of how fast water can enter the soil at the surface and is typically reported as a depth per hour |
| Phi Index | A method used in watershed hydrology for estimating flood magnitudes due to severe storms |
| Infiltrometer | A device used to measure the infiltration rate of water into soil. |
| NRCS Peak Flow Method | Calculates peak flow as a function of drainage area, potential watershed storage, and time of concentration |
| Rural Peak Flow | Equations based on watershed and climatic characteristics within specific regions of each state that can be obtained from topographic maps, rainfall reports, and atlases. |
| Urban Peak Flows | Rural peak flows are converted with the Nationwide Urban regression equations developed by USGS |
| Rising Limb | Known as the concentration curve, this is the ascending portion of the hydrograph due to gradual accumulation of storage in channels and over the basin surface |
| Peak or Crest Segment | Includes the part of the hydrograph from the inflection point on the rising limb to an inflection point on the recession limb. |
| Falling Limb | Known as the recession limb, descending portion of the hydrograph due to the withdrawal of water from the storage built up in the basin during the earlier phase of the hydrograph when the surface runoff was occurring. |
| Gauging Stations | Are small data collection and transmission centers located next to rivers and streams. |
| Unit Hydrograph | A tool that can be developed for a gauged watershed to predict the effect that precipitation events will have on streamflow. |
| Excess Precipitation | Surface runoff that discharges directly into the stream. |
| Streamflow Hydrograph | Depict contributions from both baseflow and direct runoff. |
| Triangular Unit Hydrograph | May be used to represent the curvilinear unit hydrograph |
| Reservoir Mass Diagram (Rippl Method) | A plot of the cumulative net volume,. A simultaneous plot of the cumulative demand (known as draft) and cumulative inflow. |
| Topsoil | The upper, outermost layer of soil (usually the top 2 to 8 in) containing a mix of decaying organic matter and deposited eroded material. |
| Macropores | Small openings in soil, provide a mechanism for water to move through the soil. |
| Basins | Are primarily used for flood control and channel protection and, to a lesser degree, pollutant removal. |
| Swales/Strips | Designed to slow down velocities to cause sedimentation of pollutants. |
| Filters | To reduce effluent pollutants by temporarily holding runoff until it is processed through the filter media. |
| Infiltrators | A design capture volume infiltrates the soil and percolates to shallow aquifers or is removed by plants via evaporation. |
| Gross Pollutant Traps | Primarily utilize screening and skimming at storm drain inlets to remove litter, organic debris, and coarse sediments. |
| Swales | Parallel channels that collect and convey stormwater runoff from the roadway. |
| Biofiltration Swale | A modification of a conventual swale designed to increase hydraulic residence for removal of solids |
| Surface Infiltration Basin | Graded surface depression is located in soil with high infiltration capacity to allow for a portion of the stormwater inflow to infiltrate through the bottom of the basin |
| Underground Infiltration Basis | An excavated pit filled with crushed stone and pipes or vaults, which are often built under parking lots and playing fields to maximize the buildable footprint of the site. |
| Infiltration Trenches | Narrow, linear, and relatively shallow excavations backfilled with clean stone to provide structure and temporary storage for infiltration |
| Bioinfiltration System | Known as a rain garden, is a depressed landscaped area designed to temporarily store water for use by vegetation and filter it through a soil media to the underlying soil. |
| Permeable Pavement | Porous alternative to traditional pavement that can be used in low traffic areas for stormwater treatment. |
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