Rainwater-Harvesting Sizing Study for a College Campus from Measured Catchment Data

Campus RWH Sizing is a field-measurement study that turns rainwater harvesting from a compliance checkbox into measured engineering. The team surveys every harvestable rooftop on their own college campus — plan areas, surface types, downpipe locations — installs a graduated rain gauge, and runs a barrel-catch rig: roof downpipes divert into calibrated drums so that, storm by storm, real runoff volumes against rainfall depth and catchment area yield experimentally measured runoff coefficients per surface type (GI sheet, RCC slab, tiled roof).

Those measurements feed the sizing study. Using rainfall statistics from the IMD Data Supply Portal and KSNDMC, the team computes dependable monsoon rainfall, applies its measured coefficients instead of textbook ones, and sizes storage tanks and a recharge structure per the CGWB Manual on Artificial Recharge and the BWSSB rules that make such systems mandatory for new buildings in Bengaluru. A first-flush diverter study and basic water-quality tests (pH, turbidity, TDS) on the harvested water complete the engineering picture, and a cost-per-kilolitre comparison against the campus's tanker purchases states the case in rupees.

The honesty of the project is its method: every formula in the sizing report is fed by a number the team measured on their own campus, and every assumption — dependable rainfall, filter efficiency, percolation rate — is stated with its source. It is buildable within one monsoon season on consumables under ₹3,500, uses laboratories the department already owns, and ends with a recommendation the college administration can act on.

Bengaluru and much of Karnataka face recurring water stress: groundwater falls, tankers get costlier every summer, and the BWSSB mandates rainwater harvesting for new buildings above threshold plot sizes — yet the harvesting systems that do exist are frequently sized by rule of thumb. Student-scale studies and many real installations use textbook runoff coefficients that no one measured on the actual roof, an annual-average rainfall figure that hides the monsoon's concentration, and tank sizes inherited from a contractor's habit rather than computed from demand and dependable supply.

The result is hardware that underperforms: tanks that overflow unused for months, recharge pits sized without percolation data, and no economic argument that survives scrutiny. There is a need for a measured campus rainwater-harvesting sizing study — surveyed catchments, on-site rain gauge, experimentally determined runoff coefficients and percolation rate, first-flush and quality checks, and storage and recharge structures sized per CGWB and BWSSB guidance — ending in a costed recommendation the college can actually implement.

Abstract

Rainwater harvesting is mandated in Bengaluru for new buildings above threshold plot sizes, yet student projects usually size systems from textbook runoff coefficients and a single annual-rainfall figure. This study measures first: ten years of local rainfall statistics from IMD/KSNDMC data, a full rooftop catchment survey of the campus, and storm-by-storm runoff coefficients measured with a barrel-catch rig on two surface types. Those inputs size storage tanks and a recharge structure per CGWB manual guidance and BWSSB rules, with first-flush and water-quality checks, and a cost-per-kL comparison against the campus's tanker water purchases. Consumables cost under ₹3,500; the deliverable is a defensible sizing report the administration can implement.

Introduction

Bengaluru's water reality — falling groundwater, costly tankers, and a BWSSB mandate for harvesting in new construction — makes the campus an ideal live laboratory. The engineering content is genuinely hydrological: how much rain falls, how much runs off each roof type, how much must be stored to bridge dry spells versus recharged, and what it costs per kilolitre compared with buying water.

Existing System

• Sizing in student work and many real proposals uses literature coefficients and annual averages, with no measurement on the actual catchment.
• The campus has scattered harvesting hardware with no sizing documentation and no measured performance.
• Administration decisions are driven by compliance forms rather than by yield and payback numbers.
• Limitations: unmeasured catchments, borrowed coefficients, and no defensible answer to 'is this tank the right size?'

Proposed System

• Rainfall analysis: ten-plus years of monthly data from IMD and KSNDMC, plus on-campus gauge readings; averages, monsoon concentration and a stated dependable rainfall for design.
• Catchment survey: every rooftop measured and mapped (tape/total station), classified by surface type and drainage, with a catchment table and a scaled sketch plan.
• Measured runoff: a barrel-catch rig on two contrasting roof sections; per significant storm, volume against depth and area gives experimental coefficients, compared with literature.
• First flush and quality: diverted first-flush volumes recorded; pH, turbidity and TDS tested in the environmental lab on pre- and post-filter samples.
• Sizing and economics: storage by demand-buffer and supply methods, recharge sized per CGWB guidance with a measured percolation test, and a cost-per-kL comparison against tanker water, with the recommended storage-recharge split.

Feasibility Study

• Technical: the chain uses standard surveying and lab instruments plus calibrated drums; rainfall data comes from IMD at modest cost, supplemented by KSNDMC public data. The monsoon calendar drives the schedule — the rig must run by June — handled by front-loading preparation.
• Economic: gauge, drums, diverter piping, filter media and test consumables total roughly ₹3,000–3,500; IMD data for one station is inexpensive; no equipment purchase is needed.
• Operational: measurements happen on the team's own campus with maintenance-section cooperation. Fieldwork is outdoors in daylight, under a written two-person safety note — wet surfaces avoided, ladders handled by maintenance staff only. The deliverable doubles as a real recommendation to the college.

In scope

• The team's own college campus: its rooftops, downpipes and one proposed recharge site.
• One study season of on-campus rainfall and storm-runoff measurement, plus ten-plus years of procured rainfall statistics.
• Barrel-catch coefficient measurement on two roof types; first-flush quantification; basic water-quality tests (pH, turbidity, TDS).
• Sizing of storage tanks and one recharge structure per CGWB/BWSSB guidance, with an economic comparison to tanker purchases.

Out of scope (v1)

• Construction or installation of the recommended structures — the deliverable is the sizing and recommendation pack, executed by the college later.
• Drinking-water treatment design and potable certification of harvested water; the quality study stops at basic parameters.
• Sewage-treatment plant effluent recycling, greywater systems and landscape demand modelling beyond a stated demand figure.
• Groundwater-model simulation of aquifer response to recharge, and any off-campus catchments.

The study uses a sequential field-research design: hydrological data first, catchment survey second, measurement campaign third, sizing and economics last — each stage gating the next, with the monsoon calendar explicitly planned around.

Quality control: the gauge is read at fixed times daily and after every significant storm by a logged rota; drum readings are taken at level-eye with a graduated dipstick and cross-checked once per month against a second drum; every storm gets a one-page data sheet (start, depth, volumes, anomalies). Coefficients from storms under 5 mm are excluded as unreliable, and the report states how many storms survived that filter — examiners probe exactly this measurement-honesty.

This is a field study, so the architecture is the measurement-to-sizing data chain, drawn as the study design.

flowchart TD
  IMD["IMD / KSNDMC: 10+ years monthly rainfall"] --> RA["Rainfall statistics: averages, monsoon share, dependable design rainfall"]
  GAUGE["On-campus rain gauge (study season)"] --> RA
  SURVEY["Rooftop survey: areas, surface types, downpipes"] --> CT["Catchment table (m2 by type)"]
  RIG["Barrel-catch rig on two roof sections"] --> COEF["Measured runoff coefficients per surface type"]
  GAUGE --> COEF
  FF["First-flush volumes + filter media test"] --> WQ["pH, turbidity, TDS: pre- and post-filter"]
  RA --> SIZE["Sizing: storage by demand-buffer and supply methods"]
  CT --> SIZE
  COEF --> SIZE
  PERC["Percolation test at recharge site"] --> RE["Recharge pit/trench sizing per CGWB manual"]
  SIZE --> ECO["Economics: kL harvested, cost per kL vs tanker records"]
  RE --> ECO
  ECO --> REC["Recommendation pack: drawings, calculations, costs"]

Three decisions carry the design. First, measured coefficients replace textbook ones in every sizing calculation, with the comparison to literature values reported — that substitution is the study's core contribution. Second, the dependable design rainfall is stated explicitly (with its return logic) rather than using the annual average, because sizing on averages systematically under-provisions storage in a monsoon climate. Third, the economics use the campus's own tanker-purchase records — real rupees the college already spends — so the recommendation lands as a business case, not a slogan.

• Post-implementation monitoring: after the college builds the recommended structures, instrument the tank level and recharge pit and compare actual yield against this study's prediction — a ready-made Phase-II for a successor team.
• Automated logging: replace manual gauge and drum readings with a tipping-bucket logger and ultrasonic tank sensor for continuous data.
• Demand-side modelling: extend the study to metered hostel and lab water demand patterns, matching harvested supply against a real demand curve rather than a single figure.
• Multi-source campus water balance: combine rainfall harvesting, borewell and tanker sources into one campus water-balance model with reuse scenarios.
• Neighbourhood extension: apply the same measured method to a neighbouring government school or Anganwadi as a community-outreach extension of the recommendation pack.