Rainwater Reuse Modeling

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Rainwater Reuse Modeling

• Houston Land/Water Sustainability Forum
• June 17, 2009

Jay Morris
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Introduction

• Goals
• Questions to Obtain Goals
• Rainwater Runoff Quantity
• Storage Volume
• Selecting Areas to Capture
• Rainfall Data Supply
• Usage Demand
• How Supply and Demand Affect a Model
• Return on Investment (ROI)
• Recap

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Goals

• To provide an economically viable alternative to
  municipal water for non-potable uses.
• Achieve a LEED point for SS Credit 6.1. Case 1
  (site less than 50 impervious)
• prevent the post-development peak discharge
  rate and quantity from exceeding the
  pre-development peak discharge rate and quantity

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Questions to Obtain Goals

• What is the difference in runoff quantities from
  pre-developed to post-developed conditions?
• How much water should be stored?
• From where should water be collected?
• What can the water be used for?
• How can supply and demand be balanced?
• What is the ROI?

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Rainwater Runoff Quantity
The change in the runoff volume is the volume to
be infiltrated into the soil or reused onsite to
achieve LEED SS Cr. 6.1. In Houston
infiltration is limited due the soils low
percolation rate.
?V Aimp (?CRunoff )(d2year)

• ?V Change in Volume of Runoff (CF)
• AImp Total Impervious Area (SF)
• ?CRunoff Change in Runoff Coefficient (SF)
• d2year Depth of 2-Year 24-Hour Rainfall (FT)

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Rainwater Runoff QuantityExample
?V Aimp (?CRunoff )(d2year)

• 10,000 SF of impervious area is proposed for a
  development in Fort Bend County. The
  pre-developed runoff coefficient is assumed to be
  0.35 and the impervious area runoff coefficient
  is assumed to be 0.95. The Fort Bend County
  Drainage Criteria Manual References TP-40 which
  states that the point rainfall amount (depth) of
  the 2-year 24-hour storm is 4.90 inches for Fort
  Bend County.
• AImp 10,000 SF, ?CRunoff 0.95-0.35 0.60,
  d2year 4.90 IN
• ?V 10,000 SF x 0.60 x (4.90 IN/ 12IN/FT)
  2,450 CF
• 2,450 CF (7.48 Gal/CF) 18,326 Gal

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Storage Volume

• Calculating the runoff volume for the 2-year
  24-hour storm establishes the minimum volume
  needed to meet LEED SS Credit 6.1.
• The volume needed can also be driven by upfront
  cost, desired drawdown time, and available site
  area.
• If the difference in pre and post-developed
  2-year 24-hour runoff volumes is not stored and
  reused, LEED SS Credit 6.1 will be difficult to
  achieve.

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Selecting Areas to Capture

• This process will be site specific and will
  likely involve collection from roof drains in an
  urban area and more geared towards overland flow,
  swales, etc. for suburban or rural areas.
• Natural drainage patterns will also affect what
  areas can be captured.
• The sites outfall location(s) shall also be
  considered.

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Selecting Areas to Capture
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Rainfall Data Supply

• Actual Daily Data vs. Monthly Average Data
• Daily data will more accurately depict how often
  the storage volume will go dry.
• Rainfall data for at least 10 years is preferred.
• Daily rainfall data is available at
  www.srh.noaa.gov for most major airports and a
  number of other monitoring locations.

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Usage - Demand

• Potential uses include irrigation, greywater,
  condensers, fire suppression, livestock,
  fountains and pools.
• Usage estimates can be determined from a variety
  of different sources including water bills,
  manufacturer specifications and many resources
  available online.
• Consider seasonal usage when appropriate.
• Account for evaporation from ponds and pools.

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How Supply and Demand Affect a Model

• EXCEL MODEL

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Return on Investment

• The most basic estimate
• (1- Empty)(Yearly Usage)(Water Rate) Yearly
  Savings
• Yearly Savings/Upfront Cost Years to Payback
• This does not account for storm sewer savings,
  the rising cost of water, maintenance costs, etc.

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Recap

• Determined Storage Volume
• Delineated Areas to Capture
• Quantified Available Rain
• Quantified Usage
• Combined All Variables to Determine the Systems
  Functionality

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References

• Technical Paper No. 40
• http//www.nws.noaa.gov/oh/hdsc/PF_documents/Techn
  icalPaper_No40.pdf
• Southeast Texas Climate Data
• http//www.srh.noaa.gov/hgx/climate.htm
• Texas Manual on Rainwater Harvesting
• http//www.twdb.state.tx.us/publications/reports/R
  ainwaterHarvestingManual_3rdedition.pdf
• City of Houston Discharge Criteria for
  estimating usage
• http//documents.publicworks.houstontx.gov/documen
  t-center/planning-and-development-services-divisio
  n/dicharge-criteria-sheet/details.htm