Hydrology/Hydraulics V2

Hydrology/Hydraulics V2

Chicago Storm / AES / SCS Type II

Standard temporal distribution patterns

Antecedent Moisture Condition (AMC)

Soil wetness before storm

Curve Number (CN)

Represents runoff potential

Initial Abstraction (Ia)

Rainfall lost before runoff begins

Green-Ampt

Physically based infiltration method

Unit Hydrograph (UH)

A conceptual and empirical tool used to simulate how a watershed responds to effective rainfall (runoff) by converting a unit depth of excess precipitation into a direct runoff flow rate over time

NASHYD

Used to simulate runoff flows with Nash instantaneous unit hydrograph. This hydro-graph is made of cascade of “N” linear reservoirs.

Nashyd rainfall loss computation methods

SCS Modified CN Procedure or Proportional Loss Coefficient.

Muskingum Routing

Storage-based hydrologic stream routing method used to predict the downstream movement and attenuation of a flood wave through a river reach

Dynamic / Hydrodynamic Routing

Solves flow wave propagation

Lag / Translation

Time delay of peak

Peak Flow

Maximum discharge at outlet

Baseflow

Portion of flow from groundwater

STANDHYD loss methods

The losses over the pervious surfaces are calculated by one of three methods: 1) Horton’s soil infiltration equation; 2) SCS modified CN procedure; or 3) Proportional Loss Coefficient.

STANDHYD

Used to simulate runoff flows from urban watersheds. Two parallel standard instantaneous unit hydrographs are used to convolute the effective rainfall intensity over the pervious and impervious surfaces.

Steady Flow

Flow does not vary with time at a location (∂Q/∂t = 0) - no change in storage (flow is constant along reach)

Unsteady Flow

Flow varies over time (∂Q/∂t ≠ 0) - storage can change (flow, depth and area can vary over time)

Quasi-Steady Flow

Flow is slowly varying in time, changes negligible over a short timestep

Subcritical Flow (Fr < 1)

Slow, deep; controlled by downstream conditions

Supercritical Flow (Fr > 1)

Fast, shallow; controlled by upstream conditions

Critical Flow (Fr = 1)

Flow at minimum specific energy

Hydraulic Jump

Super to sub transition with energy dissipation

Discharge Formula

Q = V·A

Velocity Formula

V = Q / A

Hydraulic Radius Formula

R = A / P

Manning’s Equation

Q = (1/n)·A·R^(2/3)·S^(1/2)

Uniform Flow

Flow where depth, velocity, slope constant along reach

Normal Depth

Depth at uniform flow for given Q and slope

Specific Energy Formula

E = d + V²/(2g)

Energy Grade Line (EGL)

Total energy = pressure + velocity + elevation

Hydraulic Grade Line (HGL)

Pressure + elevation head

Momentum Equation

Conserves momentum / accounts for forces, What forces are acting on it?

Continuity Equation

Conservation of mass, Where does the water go? Inflow - Outflow = change in storage

Ineffective Flow Area

Flow present but not contributing to conveyance

CN value range 30–40

very permeable, heavily vegetated areas

CN value range 60–80

moderate runoff

CN value range 90–100

highly impervious areas with very high runoff

SCS soil group A

Sandy/gravelly, high infiltration, Low Runoff Potential

SCS soil group B

Moderate infiltration, Moderate Runoff Potential

SCS soil group C

Fine textured soils, High Runoff Potential

SCS soil group D

Clay/high groundwater/shallow bedrock, very high runoff potential

Chicago Storm

Based on 25 years of rainfall in Chicago. Generally time step should not be less than 10 minutes.

SCS Type II Storm

Rural areas. Developed by Soil Conservation Service of America in 1973

AES Storm

The AES storm refers to a set of synthetic design storm distributions developed by the Atmospheric Environment Service (AES) of Environment Canada in the 1970s and 1980s.

AES Storm research

AES researchers looked at hundreds of actual storm events across southern Canada. They normalized the storms by duration and depth, then grouped them into four types (AES I–IV) representing different temporal rainfall patterns.

Initial abstraction

Rainfall occurring before direct runoff begins; commonly represents interception, depression storage, and initial infiltration.

Continuing loss

Rainfall loss that continues after runoff begins, such as infiltration and evapotranspiration during an event.

Interception storage

Precipitation temporarily retained by vegetation, litter, or other surfaces before reaching the ground.

Depression storage

Water temporarily stored in surface depressions before infiltrating or becoming runoff.

Infiltration-excess runoff

Runoff generated when rainfall intensity exceeds the soil's infiltration capacity.

Saturation-excess runoff

Runoff generated when the soil profile becomes saturated and additional rainfall cannot infiltrate.

Soil-water deficit

Amount of additional water required to bring the soil profile to a specified saturation or moisture condition.

Field capacity

Approximate soil-water content remaining after excess gravitational drainage has largely ceased.

Permanent wilting point

Soil-water condition at which plants can no longer extract enough water to avoid wilting.

Available water capacity

Water held between field capacity and permanent wilting point that is potentially available to plants.

Actual evapotranspiration

ET that actually occurs under prevailing water, energy, and atmospheric conditions.

Potential evapotranspiration

ET that would occur if adequate water were continuously available.

Reference evapotranspiration

Standardized estimate of atmospheric evaporative demand from a reference surface.

Groundwater recharge

Water that moves downward from the surface or unsaturated zone to replenish groundwater.

Groundwater discharge

Groundwater leaving an aquifer through springs, streams, wetlands, pumping, or other pathways.

Interflow

Relatively rapid subsurface lateral movement of water through shallow soil layers toward a stream.

Throughflow

Lateral subsurface movement of water through the soil profile toward a drainage outlet.

Baseflow recession

Decline in streamflow following a storm as delayed groundwater and subsurface contributions diminish.

Recession constant

Parameter describing the rate at which flow declines during a recession period; often represented by Q(t)=Q0 k^t.

Hydrograph separation

Method of estimating baseflow and direct runoff components from an observed hydrograph.

Direct runoff hydrograph

Portion of the storm hydrograph attributed primarily to rainfall excess.

Hyetograph

Graph showing rainfall intensity or rainfall depth versus time.

Mass curve

Cumulative plot of rainfall depth or another hydrologic quantity versus time.

Temporal rainfall distribution

Pattern describing how storm rainfall is distributed through time.

Spatial rainfall distribution

Variation of rainfall depth or intensity across a watershed.

Areal reduction factor

Factor used to reduce point rainfall estimates to represent average rainfall over a larger area.

Rainfall temporal pattern

Specified sequence of rainfall intensities or depths through a design storm.

Rainfall-runoff transformation

Hydrologic process converting precipitation into runoff while accounting for losses and watershed storage.

Watershed response

Timing, magnitude, and shape of runoff generated by a watershed in response to precipitation.

Hydrologic connectivity

Degree to which surface and subsurface pathways allow water to move between landscape units and receiving waters.

Runoff coefficient

Ratio or parameter representing the fraction of rainfall becoming runoff under a specified method and conditions.

Initial conditions

State of soil moisture, groundwater, storage, and other model variables at the beginning of a simulation.

Continuous simulation

Hydrologic simulation over an extended period that explicitly represents changing soil moisture, storage, ET, and other state variables.

Event-based simulation

Hydrologic simulation focused on an individual rainfall-runoff event using specified initial conditions.

Flux

Rate of movement of water into, out of, or through a hydrologic system.

Water balance

Accounting relationship among inflows, outflows, storage changes, and losses.

Continuity equation for storage

Mass conservation relationship: dS/dt = I - O, where S is storage, I is inflow and O is outflow.

Unit hydrograph

Direct-runoff hydrograph resulting from one unit of effective rainfall of specified duration uniformly distributed over a watershed.

Unit hydrograph assumptions

Common assumptions include linearity, time invariance, and uniform effective rainfall over the watershed.

S-curve

Cumulative response constructed by continuously applying a unit hydrograph at regular intervals.

Convolution

Mathematical process used to combine a rainfall-excess hyetograph with a unit hydrograph to obtain a runoff hydrograph.

Superposition

Principle that individual linear hydrologic responses can be added to obtain a combined response.

Translation

Shift of a hydrograph or rainfall-response signal in time without necessarily changing its shape.

Attenuation

Reduction and spreading of a hydrograph peak caused by storage and routing.

Peak attenuation

Reduction in peak discharge as a hydrograph is routed through storage or a channel reach.

Hydrograph volume

Area under a discharge-time hydrograph; represents runoff volume.

Runoff depth

Runoff volume divided by watershed area, commonly expressed in mm.

Baseflow index

Proportion of total streamflow attributed to baseflow over a defined period or event.

Double-mass curve

Graphical method used to assess consistency of hydrologic records by comparing cumulative values against a reference series.

Rating curve

Empirical relationship between stage and discharge at a streamflow measurement section.

Stage-discharge relation

Relationship used to estimate discharge from observed water level at a gauging station.

Frequency analysis

Statistical analysis relating the magnitude of hydrologic events to their probability of occurrence or exceedance.

Exceedance probability

Probability that a random event will equal or exceed a specified magnitude.

Non-exceedance probability

Probability that an event will be less than or equal to a specified magnitude.

Probability distribution

Mathematical function describing the probability behavior of a random variable.