Urban Drainage Field Trip
Hydraulic Engineering
John Walton
Stormwater runoff and urban landscaping are two of the challenges for Civil Engineers. Periodically large
storms occur, even in desert areas such as El Paso/Juarez. Urban development replaces natural landscapes
that slow runoff waters and allow them to infiltrate with roofs, driveways, parking lots, and streets that
stop the infiltration of water and speed its passage. These flash floods move downstream with great force.
Without the careful consideration of Civil Engineers urban development causes downstream erosion and
flooding.
A second challenge is how to procure the water for urban landscape watering. In the Western US around
40% of urban water use is to water landscaping. In order to make urban water use more sustainable we
must find other sources of water for the landscaping.
The urban water budget in arid and semi-arid regions is changing rapidly in response to population
growth, over pumping of groundwater, and over allocation of most renewable water sources. 50+ years
ago large cities in arid climates supplied water through increasing capture of surface water resources and
over pumping (mining) of groundwater. As new water sources became more scarce and expensive water
utilities moved towards lowering water use per capita (e.g., low flush toilets, low flow shower heads,
higher prices for water). This was an excellent but temporary solution.
The current challenge for sustainable water supply in arid and semi-arid regions is to close the loop by
fully recycling all waters entering sewage treatment plants. This works well except for the approximately
1/3 of water that is used for landscape watering (Figure 1). Landscape water predominantly exits the
urban watershed through evaporation and transpiration. It does not go down a sewer and cannot be
recycled within the context of the urban water system. However, urbanization has a side effect of
increasing stormwater runoff. This occurs as undeveloped areas are replaced with impermeable surfaces
such as roads, parking lots, and roofs. The resulting problem of increased urban stormwater runoff caused
by urbanization can be turned into a resource if the stormwater is used to replace municipal water that is
currently used for landscape watering. This is the argument for passive stormwater harvesting. The
benefits are that a) the remaining urban water system can be recycled to form a nearly closed loop, and b)
passive stormwater harvesting which benefits from the enhanced urban runoff is adequate to provide a
lush urban landscape that does not require additional watering.
Low Impact Development (LID) is intended to help with both of these problems. The idea is that we catch
the water near where it falls and store it in the ground for plant growth. This lowers flooding + erosion,
reduces the need for and size of urban stormwater channels, and lowers urban water use by using the
stormwater to water yard landscaping. Instead of running over the natural world - we design with nature.
We substitute more holistic adaptive thinking and planning for the large hammer used in the past. LID,
especially when applied to desert regions such as El Paso/Juarez requires more knowledge but less money
than traditional development methods.
What are the traditional methods? They are what you will see on the field trip & what a mess they have
made! Prior to the floods of 2006 there was no comprehensive stormwater planning in El Paso. Some
developments implemented stormwater features and others did not. Enforcement was uneven and often
absent. Stormwater features from one development did not connect in a proper method with other
developments. The 2006 floods caused extensive property damage that could have been prevented by
proper engineering.
The next step above doing relatively little to protect downstream property owners from the increased
storm runoff intensity characteristic of urban development is to put in a series of detention and retention
ponds to slow and stop the stormwater. A retention pond permanently captures water where it is stored
until it infiltrates into the ground or evaporates. A detention pond temporarily stores water, thereby
spreading the flood water flow over time in a manner that keeps the peak flow rate within appropriate
limits for downstream drainage structures. Imagine you design a new subdivision. The roofs, driveways,
and roads of the subdivision result in an increase in stormwater and a lowering of the time of
concentration. The time of concentration is the time it takes a drop of water from the top of the watershed
to reach the point of interest. This peak could be attenuated by a well-designed detention pond that lowers
the peak discharge by spreading the water release over time. Alternatively a much larger retention pond
could store the entire storm discharge. This is the current state of the art in El Paso. It is expensive, ugly,
and wastes precious water.
LID methods, in contrast, put small bioretention depressions around each and every house and parking
lot. Curb cuts periodically take stormwater off the streets and infiltrate it in bioretention depressions. The
captured water is stored in the soil (nature’s way of storing water for plants) and used to water native
vegetation, including large shade trees, during drought periods. LID is NOT the same as xeriscaping.
First stop: Mission Hills Park:
From UTEP go north on Mesa, turn right on Mesita Drive (across from Sun Bowl Drive) then turn left on
Okeefe Drive. Stop at Mission Hills Park near the intersection of Okeefe and Bancroft Drive.
Features: Trapezoidal Channel flows through culvert across street into unlined drainage. This water is
collected from the urban area above that was built without and retention/detention facilities. This water
later pours into an arroyo that drains down to I-10 and the old ASARCO property.
Notice how much vegetation – a forest – grows in this arroyo. These trees were never planted, watered, or
maintained by anyone. They grow there because the excess urban runoff that pours down this arroyo
dramatically increases the amount of water available for plant growth. They jumped on the opportunity
and grew.
The idea behind LID when properly applied to desert climates is multiplication factor and storage. The
impermeable surfaces in the urban landscape capture water that is then diverted to the remaining soil
covered areas. The water available for plant growth on the remaining soil areas has now been multiplied
by the ration of (total developed land area)/(remaining soil area). For new homes and subdivisions (large
houses on little lots) the multiplication factor is around 5:1. When you multiply the amount of
precipitation in El Paso by a factor of five – we have a lot of water. Properly designed LID provides
shady yards that never require artificial watering. Sprinkler systems are not needed. No city water is
required. The second part is storage, the water must be stored in the soil layer. This is accomplished by
amending the soil if necessary and putting a layer of landscape cloth on the surface to prevent week
growth. The landscape cloth (plastic screen) is covered with a thin layer of rocks or pecan shells.
Second Stop: Go along Bancroft to Waymore. Turn left on Waymore and right on Mesa. Turn left at
Executive Center Boulevard. In about 100 yards turn right into the empty lot(s) just past the Baker Glass
sign. Drive along the power lines until you find Walton (Silver FJ Cruiser). Be very careful as some of
the parking lot is undercut by erosion and unsafe to drive or walk on.
Features: drainage drop box, erosion damage, urban runoff
Third stop: Go back to Executive and turn right. Almost immediately turn right at Brock and Bustillos,
Consulting Engineers. Drive to the back of the building.
Features:
a) Look back at where we were previously and note the erosion. See the structures that failed.
Consider the power of erosion and how it can be controlled. The best way is to capture the water
at the source and put it to beneficial use growing trees and recharging groundwater. Less
stormwater means fewer expensive structures that eventually fail. Down in the drainage is an old
tent camp. To your right are successful UTEP trained engineers, they passed the Fundamentals of
Engineering Exam (FE) became successful registered Professional Engineers (PE), and started
their own company. Down in the tent lives Samuel, he failed his FE Exam. He used to be an
alcoholic until he got too poor to buy liquor. Perhaps you’ve seen him on the street corners?
b) Go back to the parking lot. How does it drain? How much water is available? Does it drain off
Executive? Or only this lot? Does it flood the back of the building? Notice the vegetation growing
at the edge of the parking lot. It is taking advantage of the excess water from the parking lot
stormwater runoff.
Design a LID structure at the end of the parking lot. Get the approximate area of the lot from
Google Maps (or Bing Maps). Design a rock filled depression (bioretention structure) at the
lower end of the lot (where the plants are already growing) that captures all the stormwater from a
one inch runoff generating storm. The depression is filled with gravel having a porosity of 0.30.
What are the dimensions of your basin? The depression holds all the water from the on inch storm
and slowly infiltrates it into the soil. Native vegetation then extracts the water from the soil
during drought periods in order to grow. The barren area in turned into a lush, shaded park area
for employees to come and eat lunch.
Fourth Stop: Go back on Executive and turn right. Make a U-turn. Before reaching Mesa turn
right on Westcity Court. Park on the right just before the orange+black right turn sign.
Features: drainage structures, some are failing, some have failed, arroyo with lush plant growth
because of the excess urban runoff
Notice the lush plant growth, a grove of trees, that have never been planted, attended, or watered.
They are here because the excess stormwater runoff caused by improper urban development
increased the normal amount of water available in El Paso to the point where trees will grow.
Imagine if this water were captured in all the homes, commercial buildings, and apartments above
and used to grow trees there. Urban water use would drop and we would have a greener, shady,
environment.
Fifth Stop: Work your way back out to Mesa, turn right towards UTEP. Just past the Carnitas
Queretaro restaurant turn right into the Westside Pediatric Night Clinic and drive to the back of
the parking lot behind the building.
Features:
a) To the north behind the Pocket Billiards building there is a retention pond that was designed
to take stormwater from the strip mall. It has an emergency overflow that dumps wter into the
parking lot below (Planning? Permitting? What is that?). The drain pipe from the
retention/detention pond (it’s not at all clear what the designer intended, clearly not a UTEP
engineer) presumably dumps the water into the arroyo (not sure).
b) To the west notice the forest of trees supported by urban runoff. Those trees and more could
have been grown in the developments above if LID concepts had been applied.
c) To the south notice the erosion caused by stormwater from urban development that is
diverted into the arroyo.