Dholera Soak Pit Design Calculations: DSIRDA Sizing Formulas

1. The Mathematical Sizing Formula

Calculating the required capacity of a rainwater soak pit is based on the peak runoff volume generated by the building’s roof footprint:

The sizing formula is defined as: \[V = A \times I \times C \times 1000\]

Where: * \(V\) = Required volume of the soak pit (in Liters). * \(A\) = Roof Catchment Area (in square meters). * \(I\) = Design hourly rainfall intensity (taken as 0.05 meters/hour or 50 mm/hour for the Dholera region based on historical monsoon data). * \(C\) = Runoff coefficient (taken as 0.90 for concrete roof slabs and tiled surfaces, representing the percentage of water that flows off the roof). * \(1000\) = Conversion factor to convert cubic meters to liters.

Calculation Example:

For a standard building with a roof area of 200 square meters: \[V = 200 \times 0.05 \times 0.90 \times 1000 = 9,000 \text{ Liters}\] Thus, the developer must construct a soak pit with a minimum capacity of 9,000 Liters (or 9.0 cubic meters of open space/void volume).

2. Filter Media Layer Configuration

Because Dholera’s marine clay has a low percolation rate (0.5 to 1.5 liters/hour/m²), the pit must be backfilled with graded filter media. This allows the pit to store water during heavy downpours while it drains slowly into the ground:

• Top Silt Filter (Geotextile Fabric): A layer of non-woven geotextile fabric is laid over the top of the filter bed to trap fine silt and prevent it from clogging the aggregate layers below.
• Upper Sand Layer (300mm Depth): Coarse, clean river sand (grain size 0.5mm to 2.0mm) to filter out organic debris.
• Middle Gravel Layer (400mm Depth): Medium aggregate gravel (size 10mm to 30mm) to support the sand bed and provide initial storage volume.
• Base Aggregate Layer (400mm Depth): Coarse stone aggregate (size 30mm to 60mm) placed at the bottom of the pit, creating a high-void storage chamber where water collects before draining into the soil.

The table below outlines soak pit sizing by roof area:

3. Maintenance and Desilting Guidelines

• Silt Pit Cleaning: The brick-lined silt trap located before the soak pit inlet must be cleared of sand and leaves before the start of the monsoon season.
• Sand Replacement: The top 100mm of the river sand layer should be scraped off and replaced with clean sand every two years to maintain filtration rates.
• Perimeter Protection: The ground surrounding the soak pit must be graded to slope away from the pit opening, preventing dirty surface runoff from washing into the filtered system.

Frequently Asked Questions (FAQ)

#### How do you calculate the size of a soak pit in Dholera?

The volume is calculated using the formula \(V = A \times I \times C \times 1000\). For example, a 150 sq.m roof area requires a soak pit capacity of 6,750 Liters under DSIRDA’s design guidelines.

#### What filter layers are required inside a Dholera soak pit?

The pit must contain a 300mm coarse sand layer on top, a 400mm medium gravel layer in the middle, and a 400mm aggregate base layer at the bottom, topped with a geotextile silt filter.

#### Why is geotextile fabric used in soak pits?

The geotextile fabric acts as a filter layer that allows water to pass through while preventing fine soil and silt from washing down and clogging the gravel bed, maintaining the pit’s drainage capacity.