resort in your own backyard
more than the vanishing
edge. The undisrupted sightlines and
soothing sounds can immediately
transport you to a restful oasis,
without actually having to 
home.
For pool builders and designers,
however, creating the proper
vanishing-edge design can be a real
challenge. In order to create the pool
of your clients’ dreams, it is important
to understand the fundamentals of
vanishing-edge design, construction
and engineering.
GENESIS, a company of the Pool
& Hot Tub Alliance (PHTA), has
been teaching courses internationally
about vanishing-edge construction
and design for more than 25 years.
This article will walk you through
three of the more important topics
and protocols to follow if you want
your projects to end with a successful
design and execution.
This water feature was modified to ensure minimal water was necessary to create the desired effect. Designer/Builder: Skip Phillips
The pool seen here was new but unfinished, with a 5-foot wall on the beam to support the waterfall conclusion. Simply using a short drop vanishing edge into the landscape solved the problem. Designer: Skip Phillips
SURGE TANKS
Surge tank capacity is one of the
most common problems with
vanishing-edge designs. There is a
fallacy that one can properly calculate
the necessary size of a surge tank
simply by using individual weight displacement as the criteria. But when
was the last time you saw people using
a water-in-transit vessel who were
slowly lowered in by crane without
splashing? Probably never. How often
do you know in advance how many
bathers will be in the pool at any given
time? Probably not very often.
There are anticipated losses
in vanishing-edge pools, such as
evaporation and splash-out described
previously, as well as unanticipated
losses, such as leaks. It is just as
important to design for unanticipated losses. When discussing how to
prevent leaks, some pool construction
courses recommend waterproofing
both sides of the vanishing-edge wall
— but this would only prevent leaks
where the leaks would be found. What
about the rest of the vessel?
Compensating for a known
problematic concern only in the areas
that are visible is not a valid criteria in
any vessel. GENESIS courses explain
where and how to waterproof all parts
of a vanishing-edge vessel.
While using body displacement
values as the sole reference for surge
tank displacement may be appropriate
for spas, it is obviously flawed when
considering the equation for pools.
It is important to consider all of the
variables, including atmospheric
conditions that can heavily influence
the surge tank and water-in-transit
details. For example, wind can
commonly shift 2 inches of water into
the surge system.
A preferable “rule of thumb” is
using a displacement value based
on square footage — 2 inches of the
vessel’s surface area, as an example.
Even after correctly sizing the
“net” capacity of the surge tank, there
can still be issues. It is important to
look at the full picture. The minimum
operating level, the autofill sensor
location, and the feed not returning
to the appropriate point are all
problematic. Remember, having
filtered discharge on dedicated water-in-transit systems with redundant
backflow protection (such as a
vacuum breaker or check valves) are
paramount.
This was the first new Lautner edge pool ever built. It had a significant deflection issue, as well as the challenge of the site being on the San Andreas Fault. Designers: Helena Arahuete, Lautner & Associates / Skip Phillips; Builder: Skip Phillips
FLOW RATES
When defining flow rates, there
are three factors to consider first:
tolerance, soil conditions and
deflection, as a minimum.
You cannot have a reasonable
conversation about flow rates without
first establishing the anticipated
edge tolerance. While it is true
some materials can be refined in
place to improve the level of the
vessel, and Variable Frequency Drive
(VFD) systems make adjustments
to the operational design variables a
possibility, you still have to know your
tolerance. As an example, GENESIS
often suggests plus or minus 1/16
inch.
Some vanishing-edge pools also
employ a slot overflow. If the vessel
is elevated, target flow rates typically
parallel vanishing-edge flow rates.
Otherwise, use lower flows (1 gallon
per minute per foot, as an example)
and set slot elevation 1/8 inch higher
than vanishing-edge elevation. Also, if
an elevated pool is flowing into a slot,
remember to pitch the deck back to
the slot a minimum of 5 feet, to help
recover surge.
Although GENESIS always
recommends a soils report prior
to beginning construction, it is
especially important when building a
pool with a vanishing edge. You will
want to ensure that site conditions
do not preempt the ability to meet
and maintain edge tolerance, as
aforementioned. A soils report could
indicate that a water-in-transit design is potentially predetermined to fail.
The quality of the concrete, its
application, and the scope of the
site-specific engineering are also
key. Some of the greatest adverse
variables we have found were due to
deflection. GENESIS recommends that
the shotcrete meet or exceed the ACI/
ASA minimum compressive criteria of
4,000 psi. While deflection is always a
challenge, it is especially problematic
for a vanishing-edge design because
the smallest shift in the structure can
impact where and how the water flows
over the edge, either creating gaps and
dry spots or causing more water to
flow in one area. Improper foundation
design can also eventually lead to the
collapse of the entire structure.
This vanishing-edge pool was completely rethought during a renovation, both mechanically and cosmetically. Designer: Skip Phillips; Builder: Mike Fuller, Poolworks
DESIGN CONSIDERATIONS
Two of the most important design
considerations for a pool with a
vanishing edge — and, indeed, any
water vessel — are visual and audible.
The primary cosmetic responsibility
of a water vessel is its ability to reflect.
With a vanishing-edge pool, enhancing
the reflective quality is paramount.
This feature can be defeated in a multitude of ways, including poor
hydraulics or water feature designs,
but turbulence is a key factor.
The hydraulics decisions can create
turbulence that could have easily been
mitigated. GENESIS recommends
separate primary and vanishing-edge
systems. You cannot properly protect
from equalization with a common
vanishing edge and primary system.
There may be different flow rates for
the primary system, the vanishingedge
system, and the slot overflow.
Both the primary and vanishing-edge
systems should have the line sized at
4 1/2 feet per second suction and 6 1/2
feet per second discharge at maximum
velocities.
In addition to having a pool that
is visually appealing, your clients will
also want a pool that sounds nice. If
the water flowing over the weir sounds
more like Niagara Falls, in my view that
means something somewhere went
terribly wrong. Consider the target
audible qualities you want to achieve.
Normal conversation is approximately
65 decibels. A reasonable target for
your vessel may be 55 decibels, or half
as noisy as 65 decibels.
The information in this short article
is not intended to cover all aspects
of this design detail. The vanishing-edge
courses led by GENESIS do a
much better job of expanding your
knowledge of water-in-transit options.
But hopefully this information will
prompt discovery as pool builders
and designers rethink the details that
create a successful vanishing-edge or
water-in-transit design, with a focus
on the intent of the design and the
possible repercussions.
This pool, with a 160-foot edge, has no turbulence. The spa was used to diffuse the potential turbulence. Designer: Bruce Rudd, Island Architects / Skip Phillips; Builder: Skip Phillips
