SCE PE Water Resources & Environmental flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Rational method

Peak discharge equals runoff coefficient times rainfall intensity times drainage area; Q = CiA/360 for cubic feet per second, inches per hour, and acres.

Time of concentration

Minimum time for rainfall at the farthest point to contribute to outlet flow; used to select rainfall intensity for rational method design.

Return period

Average interval between events exceeding a given magnitude; reciprocal of annual exceedance probability, so T = 1/p.

SCS curve number

Empirical index for direct runoff; runoff Q = (P - 0.2S)^2/(P + 0.8S) when P exceeds 0.2S, with S from curve number.

Unit hydrograph

Direct runoff hydrograph from one unit depth of excess rainfall uniformly distributed over a watershed in a specified duration.

Horton infiltration

Infiltration rate decays exponentially from initial to final capacity: f(t) = fc + (f0 - fc)e^(-kt), where k controls decay rate.

IDF curve

Graph or equation giving rainfall intensity for a specified duration and return period; intensity decreases with duration and increases with return period.

Darcy-Weisbach equation

Head loss in pipe flow: hf = f(L/D)(V^2/2g), where f is friction factor, L length, D diameter, V velocity, g gravity.

Reynolds number pipe

Re = VD/ν; flow is laminar below about 2000, transitional between 2000 and 4000, and turbulent above 4000.

Hazen-Williams formula

Empirical pipe head loss: hf = 10.67 L Q^1.852/(C^1.852 D^4.87) in SI units, with C depending on pipe roughness.

Minor loss

Local head loss from fittings, valves, bends, or transitions: hL = K(V^2/2g), where K is the loss coefficient.

Pump power

Power required: P = ρgQH/η, where ρ is density, Q flow, H pump head, and η overall efficiency.

Water hammer

Pressure surge caused by rapid velocity change; magnitude approximated by Δp = ρaΔV, where a is pressure wave celerity.

Manning equation

Open-channel velocity: V = (1/n)R^(2/3)S^(1/2) in SI units, where n is roughness, R hydraulic radius, S slope.

Hydraulic radius

Flow area divided by wetted perimeter, R = A/P; used in Manning and Chezy equations for open-channel flow.

Froude number

Fr = V/sqrt(gD), with D hydraulic depth A/T; subcritical when Fr < 1, critical at 1, supercritical when > 1.

Critical depth

Depth at minimum specific energy for a given discharge; occurs when Froude number equals one in open-channel flow.

Hydraulic jump

Rapid transition from supercritical to subcritical flow; dissipates energy and raises water surface, with sequent depths related by momentum.

Best hydraulic section

Most efficient open-channel cross section for a given area and roughness; trapezoidal best section has side slope 60 degrees and R = y/2.

BOD5

Five-day biochemical oxygen demand at 20°C; approximately 68 percent of ultimate BOD when k = 0.23 per day.

F/M ratio

Food-to-microorganism ratio: F/M = Q S0/(V X), where Q influent flow, S0 influent substrate, V aeration volume, X biomass.

Solids retention time

SRT = VX/(QwXr + QeXe); average time biomass remains in activated sludge system, controlling treatment performance.

Overflow rate

Surface settling rate in clarifier: Q/A; particles with settling velocity greater than overflow rate are removed.

Stokes law

Settling velocity of small spherical particle: vs = g(ρp - ρw)d^2/(18μ) for laminar settling in water.

CT disinfection

CT = C × T, product of disinfectant residual concentration and contact time; used with tables to verify pathogen inactivation.

Nitrification

Biological oxidation of ammonia to nitrite then nitrate by autotrophic bacteria; consumes oxygen and alkalinity in wastewater treatment.

Benefit-cost ratio

Ratio of present worth of benefits to present worth of costs; project acceptable when ratio exceeds one.

Present worth factor

P/F = 1/(1+i)^n; converts a future single payment to present value at interest rate i over n periods.

Design life risk

Probability of at least one exceedance in n years: R = 1 - (1 - 1/T)^n for return period T.

Detention storage

Required stormwater storage approximated by inflow hydrograph volume minus allowable outflow volume; often estimated with modified rational method.

Sensitivity analysis

Tests how changes in input parameters affect model outputs; identifies which uncertainties most influence design decisions.

Life-cycle cost

Total present worth of capital, operation, maintenance, replacement, and salvage costs over project life; used to compare alternatives.