Which one of the following methods is generally not considered suitable for urban rainwater harvesting projects?

Introduction / Context:
This question tests environmental and geographic knowledge related to rainwater harvesting techniques, with a specific focus on urban areas. Different methods are suited to different terrains, and understanding this helps in planning sustainable water management.

Given Data / Assumptions:

• The focus is on urban rainwater harvesting methods.
• Four techniques are mentioned: rooftop recharge pits, recharge wells, gully plugs and recharge trenches.
• We must identify which technique is not typically appropriate for urban settings.

Concept / Approach:
Urban rainwater harvesting usually deals with rooftop runoff, paved areas and limited open spaces. Structures such as rooftop recharge pits, recharge wells and recharge trenches are commonly used in cities. Gully plugs, on the other hand, are small embankments constructed across gullies or small seasonal streams in hilly or rural areas to check surface runoff and promote percolation.

Step-by-Step Solution:
Step 1: Consider rooftop recharge pits. These collect water from roofs and channel it into pits where it percolates into the ground, a typical urban technique.Step 2: Recharge wells are also used in cities to inject roof or surface runoff directly into deeper aquifers.Step 3: Recharge trenches along roads or open areas help capture runoff in narrow strips and are compatible with urban layouts.Step 4: Gully plugs are usually built in hilly or rural terrains where natural gullies carry seasonal runoff. Such structures are not common in dense urban environments where gullies and natural streams are rare or heavily modified.Step 5: Therefore, gully plugs are not generally considered suitable for typical urban rainwater harvesting.

Verification / Alternative check:
Government manuals and city level guidelines for rainwater harvesting emphasise rooftop systems and underground recharge structures. They rarely mention gully plugs for cities, reinforcing that this method belongs more to watershed development in rural or hilly regions.

Why Other Options Are Wrong:
Option A is wrong as the incorrect choice because rooftop recharge pits are specifically promoted in urban building byelaws.
Option B is wrong because recharge wells are widely used where space is limited but depth is available.
Option D is wrong because recharge trenches can be fitted along compound walls and roadsides in cities.

Common Pitfalls:
Students may not distinguish between watershed development methods used in rural catchments and those designed for dense cities with more concrete surfaces. Recognising that gully plugs require natural gullies helps avoid confusion.

Final Answer:
The method that is generally not suitable for urban rainwater harvesting is gully plug structures across small hill streams or gullies.