Rainwater Runoff & Recharge Structures Calculator

Project Code Project Name Location / Site Client Prepared By Date

Climate Parameters IMD / Site Records

Annual Rainfall (mm) Average Rainy Days (days/yr) Design Return Period Design Event Duration

15-min Intensity (mm in 15 min) 30-min Intensity (mm in 30 min) 60-min Intensity (mm in 60 min)

Reference: Intensity values to be read from IMD IDF curves for the project locality and selected return period. Default values are typical for humid Indian conditions; verify against site IDF.

Land Use Distribution CPWD Rational Method

Sl.	Land Use Type	Area (m²)	Coefficient of Runoff (C)	First-flush?	⊘

CPWD C-values: Roof 0.85 · Paved 0.65 · Green 0.15 · Open 0.20 · Gravel 0.30 (see Constants tab for full reference). First-flush typically applies to rooftop and paved surfaces (IS-15797).

Storage & Domestic Demand (Optional)

Sump / Tank Capacity (KL = m³) Sump Cycles per Rainy Day Sump Surface Area (m², for evaporation) Domestic Water Demand (KLD)

Recycle / reuse parameters and evaporation rates are managed in the Constants tab.

Recharge-Rate Source CGWB / GEC-2015

Unit Recharge Rate Mode:

Theoretical Unit Recharge Rate (m³/hr per structure)

Use when site infiltration / pump-in test data is available directly.

Empirical Constants & Design Parameters

Clogging / Sustainability Factor CGWB Operating Hours per Day (hr) First-flush Depth per Event (mm) IS-15797 Open-Storage Evaporation (m/yr) Recycle / Reuse Recovery Factor

References: Clogging factor 0.5–0.7 (CGWB Manual on Artificial Recharge, 2007); first-flush 1–2 mm/event (IS-15797: 2008); operating hours typically 6–12 hr per rainy event; evaporation 1.5–2.5 m/yr semi-arid Indian conditions; recycle recovery 0.60–0.70 for greywater treatment (kitchen + flushing reuse).

CPWD Runoff-Coefficient Reference Table

Sl.	Surface Type	C (Coefficient of Runoff)	First-flush typically applied?
1	Roof / terrace (RCC, GI sheet)	0.85	Yes
2	Paved area (concrete, bitumen, paver-block)	0.65	Yes
3	Gravel / kankar	0.30	No
4	Open / unpaved / lawn (compacted)	0.20	No
5	Green / landscaped (well-drained)	0.15	No

Source: CPWD Plumbing Manual (Rainwater Drainage, Chapter 9) and IS-15797 first-flush guidance.

GEC-2015 / NAQUIM Aquifer Parameter Reference

Sl.	Principal Aquifer / Formation	K (m/day)	Sy (%)

Source: GEC-2015 (CGWB), NAQUIM aquifer reports, with cross-reference to Domenico & Schwartz (1998) and FAO Irrigation & Drainage Paper 64. Values are representative; site-specific verification through pump-test interpretation is recommended for design.

Select Structure Type & Dimensions

Recharge Structure Type

Wetted-Area Multiplier (wf) Storage Volume per Structure (m³) Active Recharge Area per Structure (m²)

Per-Structure Capacity (computed)

Theoretical Unit Recharge q_theor (m³/hr) Effective Unit Recharge q_eff (m³/hr) Daily Capacity per Structure (m³/day) Annual Capacity per Structure (m³/yr)

Recommendation — Number of Structures (Back-Calculated)

Sl.	Sizing Basis	Demand	Capacity per Structure	No. of Structures Required	Basis / Formula

Sizing logic (back-calculated from recharge volume target): (1) Annual recharge target — overflow runoff diverted to recharge structures must be percolated within the rainy-season operating window. (2) Daily-sustained basis — structures must clear average daily runoff during the monsoon. (3) Peak-event basis — structures must collectively detain & infiltrate the design-duration peak storm. The governing number is MAX of the three. Theoretical recharge is derated by the clogging factor (CGWB) over the design life.

Total Catchment Area

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m²

Net Annual Runoff

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m³/yr

Design-Event Peak Runoff

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m³

Recommended Structures

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nos.

Table A — Annual Runoff Computation (with First-Flush Deduction)

Sl. No.	Land Use Type	Area (m²)	C	Annual P (m)	Gross Runoff (m³/yr)	First-flush Loss (m³/yr)	Net Annual Runoff (m³/yr)

Table B — Peak Runoff at Multiple Durations (IDF-based)

Sl. No.	Land Use Type	Area (m²)	C	15-min Q (m³)	30-min Q (m³)	60-min Q (m³)

Table D — Annual Saving / Recharge Summary

Sl. No.	Component	Volume (m³/yr)	Volume (KL/yr)	Remarks

Design Notes: (1) Tank/structure capacity is sized to hold runoff during the design event duration as per CPWD Rational Method. (2) Number of structures = MAX of annual-target, daily-sustained and peak-event bases (Structure tab). (3) Theoretical recharge rate is derated by the clogging factor (CGWB) over the design life (≥ 10 years). (4) Aquifer-derived rates assume unit hydraulic gradient at the saturated bottom; verification via site infiltration / pump-in test is recommended. (5) For hard-rock terrain (basalt, granite-gneiss), recharge shafts / borewells often outperform pits — pre-select on the Structure tab.