PE Water Resources common mistakes
PE Water Resources · Understand the traps and how to avoid them.
Plugs numbers into a formula without carrying units, mixing cfs with MGD, inches with feet, or acre-feet with cubic feet inside the same calculation, then selects the answer choice closest to the resulting (unit-mismatched) number.
The exam deliberately mixes the US customary units that appear in the NCEES handbook (Q in cfs, i in in/hr, A in acres, loading rates in gpd/ft2) with the units in the problem statement, and the distractor answers are built from the unconverted result.
What to do instead
Before substituting, write the conversion as a fraction chain in the work space (e.g., cfs x 0.6463 = MGD; acres x 43,560 = ft2) and strike the cancelled units. Only after every term is in the same system, compute a numeric value and perform a quick order-of-magnitude sanity check against a known reference flow (1 MGD ~ 1.55 cfs).
Uses the full-flow Manning capacity and hydraulic radius D/4 for a pipe that the problem states flows partially full, or sizes a storm sewer by setting Q_full equal to the design flow without computing normal depth.
Manning's equation with R = D/4 is the most heavily memorized form, so it gets applied reflexively even when the problem's depth, slope, or 'flowing 0.6 full' language signals partial flow.
What to do instead
Always compute R = A/P from the actual cross-section. For partial flow, tabulate the circular-segment area and wetted perimeter at the stated depth ratio, or use the proportionality Q/Q_full versus y/D curve, and confirm the resulting velocity is within the problem's stated minimum and maximum.
Writes the energy equation between two points with a wrong sign on a head-loss or pump-head term, omits the velocity head at a free discharge, or forgets minor losses when the problem gives a loss coefficient K.
Candidates try to hold the equation in their head instead of drawing it, so 'downstream minus upstream' becomes arbitrary and the v-squared/2g term at the outlet is dropped.
What to do instead
Sketch the system and draw both the energy grade line and hydraulic grade line before writing anything. Label each term (p/gamma, z, v-squared/2g, h_pump, h_loss) at both stations; on the sketch the EGL must drop in the direction of flow and drop by exactly the sum of losses, which catches nearly every sign error.
Uses the rational method with an intensity pulled from the wrong duration or return period, or neglects that the time of concentration must be recomputed for the developed condition rather than the pre-development condition.
IDF curves in the handbook are multi-panel and it is easy to read the 10-year panel while solving a 25-year problem, and the tc from the problem narrative (overland plus channel travel) is often stated in pieces across several sentences.
What to do instead
Write the return period and duration at the top of the solution before touching the IDF curve, and label them again beside the intensity value. Compute tc by the method the problem specifies (Kirpich, NRCS segment method, etc.), and confirm tc equals the storm duration used, since the rational method is only valid at that duration.
Selects average annual or average daily flow as the design basis when the question asks for peak hourly, peak day, or a peaking factor applied to a specific base flow, or applies a peaking factor to a flow that is already a peak.
Problems often give several flows (average day, max day, peak hour) in one table, and the question stem buries the required basis in a phrase like 'for sizing the force main' or 'for the receiving grit chamber.'
What to do instead
Underline the facility being sized in the question, then state in one line which design basis that facility uses (force main and sewers: peak hourly; treatment process units: max month or average day depending on unit; chemical feed: average day with peak feed capacity). Write the basis beside every number you extract from the table.
Computes chemical feed as lb/day = mg/L x MGD x 8.34 and stops there, ignoring the stated solution strength, commercial purity, or specific gravity, or reports a volumetric dose in gallons per day without dividing by the solution density.
The 8.34 factor is the only piece most candidates rehearse, so purity and specific gravity, which appear as short parenthetical notes in the problem, get dropped.
What to do instead
Set up a two-step chain: mass of pure chemical per day from the 8.34 relationship, then divide by the decimal purity to get commercial product mass, then convert to volume using specific gravity and 8.34 lb/gal. If the problem gives a feed rate in mL/min, close the loop by checking the resulting dose in mg/L comes back to the stated target.
Confuses hydraulic loading rate with solids loading rate when sizing clarifiers or thickeners, and reports an answer in gpm/ft2 when the question asks for lb/day/ft2 (or vice versa).
Both rates are called 'loading' in practice, and the units in the answer choices are frequently in the other basis, so a numerically reasonable but dimensionally wrong answer looks correct.
What to do instead
Write the required unit of the answer first, then build the ratio that produces it. For solids loading, convert flow x concentration to lb/day before dividing by area; for overflow rate, convert gpm to gpd before dividing. Check the result against typical design ranges (primary clarifier overflow roughly 800 to 1200 gpd/ft2, solids loading roughly 25 to 35 lb/day/ft2) as a plausibility gate.
Sets up a reservoir or detention-basin routing without converting acre-feet of storage to cfs-days, or takes a time step for the storage-indication routing that is inconsistent with the inflow hydrograph interval.
Storage volumes from elevation-storage curves come out in acre-feet while routings are done in cfs, and the conversion factor (1 cfs-day is about 1.98 acre-feet) is easy to drop when the numbers are round.
What to do instead
Convert all storage to cfs-hours or cfs-days before building the routing table, and keep the time step fixed and identical to the hydrograph ordinate interval. Verify the mass balance at the end: total inflow volume minus total outflow volume must equal the change in storage, and that residual check catches the unit slip.
Reads an annual exceedance probability as a recurrence interval (or the reverse) and reports the probability of at least one occurrence over a multi-year period as simply n/T.
'100-year flood' and '1 percent chance' feel interchangeable, and the simple n/T shortcut is intuitive but wrong for anything beyond very small n/T.
What to do instead
Convert explicitly at the start: T = 1/p and p = 1/T, writing both symbols with units. For risk over a project life use R = 1 - (1 - 1/T)^n, and sanity-check that R is always less than n/T and can never exceed 1. If the question asks for the exceedance probability in a single year, never apply the life-of-project formula.
Determines the critical path by visually tracing the path with the most activities rather than the path with the longest total duration, or computes total float but reports it as free float (or reports float without accounting for lags on finish-to-start relationships).
Network diagrams with more nodes on one branch invite the 'longest chain of boxes' shortcut, and the two float definitions differ only by whether the successor's early start is used.
What to do instead
Forward-pass early starts and early finishes for every node, then backward-pass late finishes and late starts, writing all four values at each node. Total float = LS - ES = LF - EF; free float = early start of successor minus early finish of the activity. The critical path is the chain of zero-total-float activities and must equal the project duration from the forward pass.
Applies contractor markups, overhead, and contingency by adding the percentages together and applying them once to the direct cost, instead of applying each sequentially to the running subtotal.
Bid schedules list several percentages in a column, and summing them is faster and produces an answer that is close enough to appear among the choices.
What to do instead
Build a vertical stack: direct cost, then each markup applied to the accumulated subtotal in the order the contract states, carrying the running total forward. Note that contingency and escalation are typically applied to the base before markup, not after, and check whether the question asks for total bid price or for the owner's construction cost.