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John Walton University of Texas at El Paso.

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Presentation on theme: "John Walton University of Texas at El Paso."— Presentation transcript:

1 John Walton University of Texas at El Paso

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15  Evolution of Hydraulic Engineering  Development leads to greater runoff and shorter time of concentration  Phase I: don’t worry, be happy, new development dumps on downstream land owners (e.g., most of 2006 flood damage)  Phase II: Retention ponds hold all water  Phase III: Smaller detention ponds route water by shaving peak discharge  Phase LID: Bioretention and infiltration areas use stormwater to lower stormwater runoff and make a greener environment

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17 Note that setting duration to 24 hours leads to under design of facilities unless NRSCS synthetic hydrograph is used (it embeds shorter duration peaks inside 24 hour hydrograph)

18  Rain gardens, rain barrels, green roofs don’t work in desert environment  Conditions harsher, rain more sporadic  What does work?  How can we use LID to maintain a lush green environment in the Chihuahuan Desert?  How can we put it in local streets, subdivisions, and industrial areas to make El Paso a greener and less flood prone area?

19  Development replaces desert with impermeable surfaces: roads, roofs, sidewalks, driveways  Harvesting water from these areas multiplies the available moisture above the climatic norm  If only 20% of the lot has plants, El Paso is as wet as Atlanta

20  A watershed is the area of land where all of the water that is under it or drains off of it goes into the same place. (EPA definition)  Think of a house and yard (or subdivision) as a series of mini watersheds  Where does each portion of roof drain?  How can the water from roof, sidewalks, driveways, yards be infiltrated into the soil whenever it rains?

21 native plants Impermeable areas concentrate water in vegetated areas Consider that if rainfall is increased by 5X, El Paso has a lot of water capture area capture area/plant area

22  Break development into a series of microwatersheds  Where does every portion of roof, sidewalk, road, driveway drain?  Build bioretention areas, properly sized, in each microwatershed  Carefully balance flood control, water storage, and plant evapotranspiration in each microwatershed  No sprinklers needed, little maintenance, more knowledge applied, less money

23  Nature’s place to store water is in the soil  Two years ago we had a wet winter followed by a dry spring  Everything in the desert bloomed because the winter precipitation was stored in the soil  This natural process can be enhanced to store the moisture in the soil beneath the yard  Native species are very drought resistant, most just go dormant

24 Water storage = V soil * (field capacity – wilting point)

25 The soil can store the equivalent of 1-2 feet deep of water over the entire yard Tanks store much less water and are expensive In desert climate tanks are only useful for watering small flower or herb gardens

26  Active rainwater harvesting stores water in a tank; passive rainwater harvesting stores water in the soil – nature’s way of storing water during dry periods  Most hydrological methods are designed for non- desert locations & don’t work well here, the time period between precipitation events in El Paso and the hot climate mean very large tanks are required for active systems  The cost of active rainwater systems is dominated by the cost of the storage tank  Passive systems always payback financially, active systems generally do not in this climate  Passive systems simply enhance natural processes – design with nature

27 Mulch (usually rock) Landscape cloth (screen) Must block weeds and let water into soil, storage is in the soil bioretention Stormwater diverted to gravel filled trenches and depressions, moisture moves into soil where it is stored indefinitely, mulch and landscape cloth stop weeds and evaporation (sources of water loss)

28  Bioretention volume = depth*area*porosity  Sized to hold size of storm desired  Sized to hold enough water to transfer to soil for plant growth during dry periods (we have a lot of them)  Soil moisture holding capacity  Sufficient area and depth of soil to hold moisture to support plant growth without external watering  Soil moisture holding capacity can be increased by adding diatomaceous earth, fines, and organic matter

29 Plants can be located some distance from bioretention areas

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32  Root depths > 5 meters (16.4 ft) (mesquite)  Root span > 12 m (39 ft) (mesquite)  Volume > 565 m 3 (20,000 cubic feet)  Assume field capacity, 0.3, wilting point 0.1  Soil moisture storage:  113 cubic meters, 4,000 cubic feet, 30,000 gallons  How much would a rain barrel that size cost?  Rain barrels are not practical in the desert except for small gardens

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35 water from parking lots

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37  Divide development into watersheds  Think of where every portion of the roof/sidewalk/driveway drains  Make shallow rock filled depressions – bioretention areas  Match bioretention volume to desired retention (e.g, 2 inch rain)  Use landscape cloth to prevent weed growth, water cannot be stored if it is robbed  Use spreadsheet to estimate: plant density, groundwater recharge, bioretention volume  Plant native vegetation with density related to capture area/ growth area  Plants will need watering for about a year, until roots are established, about once every two weeks during growth periods

38 Impermeable area (roof, parking lot) French drains/depressions/trenches filled with sorted gravel

39 The Model House

40 Runoff Paths.Lot Only Subdivision water neutral

41 Locations of LID Practices and Flow Path.

42  Native Vegetation Scientific NameCommon NameType Height Ft Width Ft Evergreen Or Deciduous Water Requirements Ceratoides LanataWinterfatShrub32EvergreenLow Larrea TridentataCreosote BushShrub86EvergreenLow Koberlinia Spinosa Crucifixion Thorn Shrub57EvergreenLow Atriplex Canescens Four Wing Saltbush Shrub68 Simi- Evergreen Low Leucophyllum Frutescens Texas Sage/Ranger Shrub4-8 EvergreenLow Acacia BerlandieraGuajilloShrub12 DeciduousLow Prosopis GlandulosaHoney MesquiteTree30 DeciduousLow Chiloposo LinearisDesert WillowTree2520DeciduousLow Fraxinus greggiiGregg’s AshTree158 Semi- Evergreen Low Quercus ArizonicaArizona White Oak Tree3530EvergreenLow

43  El Paso  Native species (e.g., mesquite, desert willow, acacia)  30 years of historical temperature and rainfall data  Plant area = total crown area of plants in looking from above

44 Change in soil moisture storage = runoff in – evapotranspiration loss Concept is to design system so we never reach wilting point Alternatively can design so plants need watering once per year (or during extreme droughts)

45  Roof and carport water exit carport corner  Cobbles allow subsurface ponding and infiltration into soil  Soil stores water between rains

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48  Passive rainwater harvesting works in El Paso  Capture/green area ratio from 10-25%  Saves money  Saves water  Reduces flooding  Provides a green, shaded lot, not xeriscaping with a bunch of hot rocks  Active systems generally not appropriate for Southwest


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