Hydrologic & Hydraulic Analysis Flashcards

A detention basin is designed to reduce the post-development peak discharge to pre-development levels. Which routing method is most commonly used in HEC-HMS for this purpose?

Answer: Level pool (modified Puls) reservoir routing

Level pool routing (modified Puls method) is the standard approach for detention basins, using the storage-outflow relationship and continuity equation.

The concept of 'hydrologic routing' differs from 'hydraulic routing' primarily in that hydrologic routing:

Answer: Uses the continuity equation and a storage-discharge relationship without solving the full Saint-Venant equations

Hydrologic routing uses simplified storage-discharge relationships and the continuity equation, avoiding the complex momentum equation used in hydraulic (Saint-Venant) routing.

In a LOMR (Letter of Map Revision) flood study, a surveyor must demonstrate that a proposed fill project does not cause more than how much rise in the BFE?

Answer: 1.0 foot (or the community's floodway encroachment standard)

NFIP regulations generally allow up to 1.0 foot of rise outside the floodway (in the fringe) when supported by a no-rise certification or equal-degree-of-encroachment analysis, subject to community standards.

Which type of boundary condition is required at the upstream end of a HEC-RAS model running in subcritical flow?

Answer: No boundary condition is needed at the upstream end for subcritical flow

For subcritical flow, the downstream boundary condition controls the profile; no upstream boundary condition is required because information propagates upstream from the downstream end.

A watershed's time of concentration (Tc) is important in hydrology primarily because it:

Answer: Defines the storm duration at which the entire watershed is contributing to peak flow at the outlet

Time of concentration is the travel time from the hydraulically farthest point to the outlet; when storm duration equals Tc, the entire watershed contributes, producing the peak runoff rate.

The Muskingum routing method uses two parameters, K and X. What does the parameter X represent?

Answer: A weighting factor between 0 and 0.5 that describes the relative influence of inflow versus outflow on storage

X is a dimensionless weighting factor (0 ≤ X ≤ 0.5) that weights inflow versus outflow in the storage equation; X=0 is pure reservoir routing, X=0.5 is pure translation.

When modeling a split flow situation in HEC-RAS where a river bifurcates around an island, the primary hydraulic challenge is:

Answer: Satisfying energy and continuity at the flow split junction where flow divides between two channels

At a flow split, HEC-RAS must iteratively solve for the discharge distribution between the two channels such that the water surface elevation at the junction is consistent with energy and continuity.