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ADAPTING LOW IMPACT DEVELOPMENT TO THE CHIHUAHUAN DESERT
John WaltonUniversity of Texas at El Paso
Low Impact Development (LID)
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
El Paso
El Paso
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)
LID Documents and Training Oriented to Different Climates
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?
Basic Concept
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 Atlanta Austin El Paso 5X El Paso
0
10
20
30
40
50
60
Annual Precipitation
Inches/y
rea
Watershed
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?
Capture Zone
nativeplants
Impermeable areas concentrate water in vegetated areasConsider that if rainfall is increased by 5X, El Paso has a lot of water
capture area
capture area/plant area
General Scheme
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
But it doesn’t rain very often, where do we store the moisture? 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
Soil Moisture Storage
Water storage = Vsoil * (field capacity – wilting point)
Soil stores more water than tanks at lower cost (free)
0 2000 4000 6000 8000 10000 120000
50000
100000
150000
200000
Lawn Area, ft2
Gallons o
f W
ate
r Sto
red
5,000 gallon water tankcosts $2,000
• 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
Why Passive?
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
Storage in Soil
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)
Everything is Sized
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
Mesquite Root System
Plants can be located some distance from bioretention areas
Mesquite Roots
Soil Moisture Storage Beneath One Mesquite Tree
Root depths > 5 meters (16.4 ft) (mesquite) Root span > 12 m (39 ft) (mesquite) Volume > 565 m3 (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
BUT YOU CAN’T GROW TREES IN EL PASO
WITHOUT WATERING!
Accidental Example Near UTEP
Second UTEP Example
water from parking lots
Accidental case 3
How is it done?
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
Distribute water to soil and have sufficient storage for flood control and plant growth
Impermeable area (roof, parking lot)
French drains/depressions/trenches filled with sorted gravel
LID Design
The Model House
LID Design Con. Mini-Watersheds Options.
Runoff Paths. Lot OnlySubdivision water neutral
LID Design: LID practices – Entire subdivision or just the lot can be hydraulically neutral (same pre and
post development)
Locations of LID Practices and Flow Path.
Passive landscape
Native VegetationScientific Name Common Name Type Height
FtWidth
FtEvergreen
Or Deciduous
Water Requirements
Ceratoides Lanata Winterfat Shrub 3 2 Evergreen LowLarrea Tridentata Creosote Bush Shrub 8 6 Evergreen Low
Koberlinia Spinosa Crucifixion Thorn
Shrub 5 7 Evergreen Low
Atriplex Canescens Four Wing Saltbush
Shrub 6 8 Simi-Evergreen
Low
Leucophyllum Frutescens
Texas Sage/Ranger
Shrub 4-8 4-8 Evergreen Low
Acacia Berlandiera Guajillo Shrub 12 12 Deciduous LowProsopis Glandulosa Honey Mesquite Tree 30 30 Deciduous LowChiloposo Linearis Desert Willow Tree 25 20 Deciduous Low
Fraxinus greggii Gregg’s Ash Tree 15 8 Semi-Evergreen
Low
Quercus Arizonica Arizona White Oak
Tree 35 30 Evergreen Low
Simulations
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
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)
Walton Household Example
Roof and carport water exit carport corner
Cobbles allow subsurface ponding and infiltration into soil
Soil stores water between rains
Design Spreadsheet
0 5 10 15 20 25 300%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%Soil Moisture Storage Relative to Capacity)
Time of Simulation, year
Wat
er S
tora
ge S
tatu
s, %
ful
l)
Monitoring
0 5 10 15 20 25 30 35 40 450
10
20
30
40
50
60
70
Soil Suction (centibars)Water Content (VWC%)
Day
rain rain
Summary
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