PE Water Resources flashcards

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

Manning's equation

Equation for steady uniform open-channel flow: Q = (1.486/n) A R^(2/3) S^(1/2) in US customary units, where R is hydraulic radius (area divided by wetted perimeter).

Rational method

Peak runoff formula Q = CiA for small watersheds, typically under 200 acres; C is a runoff coefficient, i the rainfall intensity for the time of concentration, and A the area.

Time of concentration

Travel time for runoff from the hydraulically most remote point to the outlet; it sets the storm duration whose rainfall intensity is used in the rational method.

Curve number method

NRCS method for direct runoff depth: Q = (P − 0.2S)²/(P + 0.8S) with S = 1000/CN − 10 inches; CN depends on hydrologic soil group, cover, and antecedent moisture.

Unit hydrograph

Direct runoff hydrograph produced by one inch of rainfall over a specified duration; it assumes linearity, so larger storms are modeled by scaling and lagging it.

Froude number

Dimensionless ratio Fr = V/√(gD) classifying open-channel flow: below 1 is subcritical, 1 is critical, and above 1 is supercritical, where standing waves can form.

Hardy Cross method

Iterative procedure for pipe networks that repeatedly corrects assumed loop flows by ΔQ = ΣhL/(n Σ|hL/Q|) until head losses around every loop sum to zero.

Muskingum routing

Flood routing method using storage S = K[XI + (1 − X)Q], where K approximates flood wave travel time and X, usually 0 to 0.5, weights inflow versus outflow.

Five-day BOD

Oxygen consumed by microbes degrading organic matter in five days at 20 °C; with k = 0.23 per day it equals about 68 percent of ultimate carbonaceous BOD.

Solids retention time

Mass of solids in the aeration basin divided by the mass wasted plus lost in effluent each day; it governs sludge age, effluent quality, and nitrification.

Food-to-microorganism ratio

Daily BOD or COD mass entering the aeration tank divided by the MLVSS mass in the tank; low ratios mean long sludge age and extended aeration.

Surface overflow rate

Clarifier flow divided by surface area, expressed in gallons per day per square foot; it must stay below the settling velocity of the design floc.

CT value

Disinfectant residual concentration multiplied by contact time; the required CT increases with target log inactivation and pH and decreases as water temperature rises.

Langelier saturation index

LSI = pH − pHs computed from calcium hardness, alkalinity, temperature, and dissolved solids; positive values scale, negative values corrode, and near zero is balanced.

Sludge volume index

Settled sludge volume in milliliters per liter after 30 minutes, times 1000, divided by MLSS in mg/L; values near 100 indicate good settling, above 200 bulking.

Chlorine demand

Difference between the chlorine dose applied and the residual remaining after a contact period; it is exerted by reducing agents, organics, and ammonia.

Mass curve

Plot of cumulative reservoir inflow against time; the storage needed for a given constant demand equals the maximum vertical gap between the demand line and the curve.

Log-Pearson Type III

Standard US flood-frequency model that fits the base-10 logarithms of annual peak flows to a three-parameter skewed distribution to estimate design floods.

Darcy's law

Groundwater flow equation Q = KIA, where discharge is proportional to hydraulic conductivity, cross-sectional area, and hydraulic gradient; valid for laminar flow in porous media.

Theis equation

Transient well solution, s = Q W(u)/(4πT) with u = r²S/(4Tt), used to derive aquifer transmissivity and storativity from pumping-test drawdown data.

Return period

Average interval between events of a given magnitude; the annual exceedance probability is 1/T, so a 100-year flood has a 1 percent chance each year.

Freeboard

Vertical distance between a design water surface and the top of a dam or levee; it covers wave runup, wind setup, settlement, and a safety margin.

Benefit-cost ratio

Present worth of project benefits divided by present worth of project costs, both discounted at the same rate; a ratio at or above 1.0 justifies construction.

Probable maximum flood

Flood resulting from probable maximum precipitation; it is the design spillway event for high-hazard dams whose failure would endanger lives downstream.

Critical path

Longest continuous chain of dependent activities through the network; it sets the shortest possible project duration and its activities carry zero total float.

Total float

Time an activity can slip without delaying project completion; computed as late start minus early start, or late finish minus early finish.

Cost performance index

Earned value divided by actual cost; a value below 1.0 means work is costing more than planned, and budget divided by CPI estimates final cost.

Schedule performance index

Earned value divided by planned value; a value below 1.0 indicates less work completed than scheduled, and it predicts duration overrun at completion.

Liquidated damages

Predetermined dollar amount per day of delay deducted from payment when the contractor finishes after the contract date; it must be a reasonable forecast, not a penalty.

Substantial completion

Stage at which the work is usable for its intended purpose although minor punch-list items remain; it typically starts the warranty and stops liquidated damages.

Crashing

Compressing the schedule by adding labor, equipment, or overtime to critical-path activities, accepting higher direct cost in exchange for shorter duration.

Change order

Written modification signed by the owner that changes contract scope, price, or time after award; it establishes the basis for an equitable adjustment.