SCE PE Water Resources & Environmental common mistakes
SCE PE Water Resources & Environmental · Understand the traps and how to avoid them.
In time-of-concentration work, the candidate measures a sheet-flow path of 400–800 ft along the ground surface and plugs that length into the kinematic-wave or paved-area sheet-flow equation before switching to shallow concentrated flow.
The flow-path map is traced all the way from the drainage divide to the first defined swale or gutter, and nothing on the map signals where sheet flow physically breaks down into concentrated flow.
What to do instead
Break the path into segments first. Limit the sheet-flow segment to about 300 ft (100 ft on paved or disturbed ground) plus a small elevation drop, then compute shallow concentrated flow and channel travel time separately and sum the segment times.
For the rational method, the candidate looks up rainfall intensity for the full design storm duration (e.g., the 24-hour depth) or for a convenient 10- or 60-minute duration, rather than for the computed time of concentration.
The IDF curve is read from whatever duration is printed on the axis, and the candidate forgets that the method requires the intensity whose duration equals Tc so the entire catchment is contributing at the peak.
What to do instead
Compute Tc first, then enter the IDF or regression equation at exactly that duration. As a sanity check, compare Tc against the 5- to 20-minute minimum used by local criteria, and if the catchment has subareas, use the Tc of the entire contributing area, not of the most remote subarea alone.
The candidate uses the nominal pipe size (12 in, 24 in) as the flow diameter in the continuity and head-loss equations for ductile iron, C900 PVC, or concrete pressure pipe.
Nominal trade size is the number on the drawings and the submittal, and cement mortar lining plus wall thickness reduce the true inside diameter by amounts that are easy to overlook.
What to do instead
Pull the actual inside diameter from the manufacturer's dimension table for the specified class and lining before computing area, velocity, and head loss, and carry that same ID through the Hazen-Williams or Darcy-Weisbach calculation so the friction slope and velocity head stay consistent.
NPSH available is computed as atmospheric pressure head minus static suction lift minus suction-line friction, and the vapor pressure head term is left out entirely (or entered in psi while the other terms are in feet).
Vapor pressure feels like a fluid property rather than a length, and at ordinary water temperatures it is a small number that is easy to treat as negligible.
What to do instead
Write the full expression every time: absolute pressure head at the suction vessel plus static elevation to the pump centerline minus suction friction and fitting losses minus vapor pressure head converted to feet (psi times 2.31 divided by specific gravity). Then subtract NPSH required and confirm a positive margin against the stated criterion.
For a circular conduit flowing partly full, the candidate computes the hydraulic radius as the diameter divided by four, or as the flow depth, instead of using the wetted area divided by the wetted perimeter at that depth.
The full-flow result R = D/4 is memorized and applied to partial-flow conditions, where the wetted perimeter includes the free surface location only as an arc and the maximum R occurs near, but not at, half depth.
What to do instead
For each flow depth, compute the circular segment area and the arc length from the central angle, then take R = A/P. Remember that conveyance peaks near 0.94 depth and discharge near full depth, so partial-full capacity can exceed full-flow capacity - do not cap the answer at the full-flow value.
The candidate assumes normal depth prevails at every cross section of a channel, including immediately upstream of a culvert inlet, a weir, or a confluence, and reports the uniform-flow depth as the design water surface.
Manning's equation is the only open-channel tool that feels familiar, so it is applied everywhere without asking whether a downstream control fixes the water surface instead.
What to do instead
Before applying Manning's equation, identify the control: compute critical depth and normal depth, classify the reach slope as mild, steep, or critical, and draw the water-surface profile. Where backwater exists, compute the profile by standard step or direct step rather than assuming uniform flow.
Mass loading in the plant calculation is written as flow in MGD multiplied by concentration in mg/L and the product is used directly as pounds per day, so the result comes out roughly 8.34 times too small.
The unit conversion hides inside a single constant that is easy to drop when the equation is written from memory rather than from a dimensioned setup.
What to do instead
Carry units explicitly: MGD times mg/L times 8.34 gives lb/day, and the same conversion feeds F/M, solids loading, and chemical dose. Verify the magnitude against a rough check - a few MGD at a few hundred mg/L should land in the thousands of lb/day, not the hundreds.
Aeration basin sizing and solids calculations use the hydraulic detention time (volume divided by influent flow) wherever the problem actually requires solids retention time or mean cell residence time.
Both parameters have units of time, and detention time is the easier one to compute because the flow is printed on the problem statement while the waste sludge flow must be derived.
What to do instead
Decide first which clock the question is asking about. If it is SRT, build the solids inventory (aeration tank volume times MLSS, plus secondary clarifier solids if the criteria include them) and divide by the solids leaving in waste sludge plus effluent suspended solids; use detention time only for hydraulic questions such as basin volume for a target contact time.
The candidate finishes a hydraulic computation with an answer that matches a listed option and moves on without checking the order of magnitude, dimension, or reasonableness of the result against the physical system described.
Under time pressure, finding an option that matches to three significant figures feels like confirmation, and the distractor options are often built from precisely the unit or conversion error the candidate made.
What to do instead
Before selecting, restate the answer in the units requested and test it: a 10-ft-wide channel at 2 ft depth does not carry 0.05 cfs, a municipal plant does not treat 0.05 MGD per 1,000 capita equivalent. Do a one-line dimensional check and a rough mental estimate, and if they disagree, re-examine the conversion rather than the arithmetic.
When the problem states specific design values, the candidate substitutes a memorized default - Manning n of 0.013, runoff coefficient of 0.9, per-capita flow of 100 gpcd, or the 2-year storm - instead of the values given or required by the cited criteria.
The defaults feel safe because they appear in so many practice problems, and the problem's own data are buried in a sentence or a short table that gets skimmed.
What to do instead
Inventory the given data before computing: underline every numeric value, stated criterion, and condition in the problem statement, and use a default only when the problem is truly silent and the default is standard practice. If a stated criterion conflicts with a remembered one, the stated criterion governs.