PE Water Resources key terms

An empirical formula that estimates average flow velocity in an open channel from channel slope, hydraulic radius, and a roughness coefficient that reflects the lining material and channel condition.

The cross-sectional area of flowing water divided by the wetted perimeter. It measures channel efficiency, since a channel with less wetted perimeter per unit of area carries flow with less friction.

A peak-discharge estimate for small drainage areas found by multiplying rainfall intensity for the time of concentration by a runoff coefficient and the drainage area, written as Q = CiA.

The travel time for water to move from the most hydraulically remote point of a watershed to its outlet; it sets the rainfall duration and intensity used in peak-flow calculations.

A dimensionless index based on hydrologic soil group, land cover, and antecedent moisture that estimates how much of a rainfall total becomes direct runoff after initial abstractions are satisfied.

The direct-runoff hydrograph resulting from one unit of rainfall (commonly one inch) falling uniformly over a watershed in a stated duration, used as the basis for scaling to larger storms.

The average number of years between events equal to or exceeding a given magnitude. Its reciprocal is the annual exceedance probability, so a 100-year event has a 1 percent chance each year.

The flow depth at which specific energy is at a minimum for a given discharge. Flow shallower than this is supercritical, deeper flow is subcritical, and the depth acts as a control for water-surface profiles.

A dimensionless ratio of inertial to gravitational forces in free-surface flow. Values below one indicate subcritical flow, values above one indicate supercritical flow, and values near one accompany a hydraulic jump.

A line plotted along a conduit showing total head, the sum of pressure head, velocity head, and elevation above a datum; its downward slope represents energy lost to friction and fittings.

A pressure surge created by a rapid change in flow velocity, such as sudden valve closure or a pump trip, that travels as a wave through the piping and can damage pipes and appurtenances.

An empirical head-loss formula for water flowing full in pipes that relates discharge, pipe diameter, pipe length, and a roughness coefficient C; it is widely used in distribution system design.

A hydrologic flood-routing technique that moves a hydrograph downstream while attenuating its peak, using storage coefficients that represent wedge storage and prism storage within a channel reach.

The mass of oxygen consumed by microorganisms decomposing organic matter in a sample incubated at 20 degrees Celsius for five days; a standard indicator of organic strength in wastewater.

The mass of particulate matter captured on a filter after a sample is filtered and dried, reported as a concentration; it gauges solids loading and the performance of treatment processes.

The volume in milliliters occupied by one gram of activated sludge solids after settling for 30 minutes, used to judge whether sludge settles and compacts well or is prone to bulking.

The average time solids stay in an activated sludge system, calculated as the solids inventory in the system divided by the daily solids leaving; it governs sludge age and effluent quality.

The daily mass of substrate entering an aeration basin divided by the mass of microorganisms maintained there; low ratios mean longer sludge age and more complete treatment.

The product of disinfectant residual concentration and contact time, used to judge disinfection effectiveness; the required value rises with higher pH, lower temperature, and more resistant target organisms.

The flow passing through a clarifier divided by its surface area, expressed as volume per unit area per day; it is the primary design basis for removing settleable solids.

A federal permit issued under the Clean Water Act that establishes effluent limits, monitoring, and reporting requirements for point-source discharges of pollutants to surface waters.

A calculated number comparing a water's actual pH with the pH at which it would be saturated with calcium carbonate; negative values suggest corrosive tendencies and positive values suggest scaling tendencies.

A hypothetical rainfall event defined by depth, duration, and frequency of occurrence, selected to size drainage, detention, and flood-control facilities for a chosen level of protection.

A graph giving rainfall intensity for a range of durations and return periods; once the time of concentration is known, it supplies the design intensity for peak-flow estimates.

The probability distribution commonly used for flood-frequency analysis in the United States; logarithms of annual peak flows are fitted and solved for flows at a chosen return period.

A step-by-step calculation tracking inflow, storage, and outflow through a reservoir to determine peak outflow and how much storage reduces downstream flood peaks.

The maximum dependable supply a water source can deliver through a critical drought without failing to meet demands, usually estimated using the drought of record.

A storage facility that temporarily holds stormwater and releases it slowly through a controlled outlet, reducing downstream peak flows, flooding, and channel erosion.

The water-surface elevation of the flood that has a 1 percent chance of occurring in any year; it anchors floodplain regulation and minimum lowest-floor requirements.

The volume of water an aquifer releases from storage per unit surface area per unit decline in hydraulic head; it is small for confined aquifers and near specific yield for unconfined aquifers.

The nonequilibrium well equation predicting drawdown in a confined aquifer from pumping rate, transmissivity, storage coefficient, and the time since pumping started.

A scheduling technique that identifies the longest chain of dependent activities, which sets the shortest possible project duration; delay on this chain delays project completion.

The time an activity can be delayed without delaying the project finish date, computed as the difference between its late finish and early finish dates.

A cost and schedule control measure that compares the budgeted cost of work performed with actual cost and planned value to forecast final cost and completion date.

A written modification to the contract scope, price, or schedule signed by the owner and contractor; without one, performed extra work may not be compensable.

A formal document a contractor sends to the designer to clarify drawings, specifications, or apparent conflicts; the written response becomes part of the contract record.

A sum agreed in the contract that the contractor pays for each day of delay past the required completion date, intended to approximate the owner's actual losses from late completion.

The stage at which the work is finished enough that the owner can use it for its intended purpose, which starts warranty periods and typically reduces retained amounts.

A site-specific plan required by construction stormwater permits that describes erosion and sediment controls, inspection schedules, and housekeeping practices to keep sediment and pollutants out of receiving waters.