Question

The hydrologic design of the drop inlet structure consists of knowing which of the following parameters?

• A. Rainfall depth expected
• B. Peak rate of runoff expected
• C. The inflow hydrograph
• D. Both peak rate of runoff expected and the inflow hydrograph.

Hint:

Storage/retardation structures require the full Inflow Hydrograph (volume) + Peak Runoff for flood routing ($I - O = \Delta S / \Delta t$).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Hydrologic sizing of detention structures with conduit spillways requires flood routing through temporary storage capacity.
Key Formula or Approach:
\[ I - O = \frac{dS}{dt} \quad \text{(Hydrologic Storage Routing Equation)} \]

Step 2: Detailed Explanation:
A drop inlet (pipe spillway) structure acts in conjunction with an earthen embankment to provide temporary flood detention storage.
Sizing the riser crest, barrel conduit, and emergency spillway requires flood routing:
1. Peak Rate of Runoff (\(Q_p\)): Determines maximum emergency spillway capacity.
2. Inflow Hydrograph (\(I(t)\)): Provides the total flood volume and time distribution necessary to calculate temporary stage-storage surcharge and conduit discharge attenuation.
Therefore, both parameters are essential.

Step 3: Final Answer:
Thus, the design requires Both peak rate of runoff expected and the inflow hydrograph, matching option (D).