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\technote{$\aleph_0$}{Dan Ports}{2004/05/16}{drkp@mit.edu}
\tntitle{Flowing Water Effects}

\stepcounter{footnote}

\footnotetext{This is Tech Note $\aleph_0$ (read: ``tech note
  aleph-null''), in keeping with the policy of using non-numeric tech
  note numbers I started in Tech Note $\sqrt{2}$. In the field of
  transfinite set theory, $\aleph_0$ is the smallest possible infinite
  size (``the first transfinite cardinal''). So it seems like an
  appropriate choice for this, my final tech note.}

\section{The problem}
\label{sec:problem}

Many plays have outdoor scenes that call for some sort of water in the
environment --- a stream, pool, or waterfall. Sometimes these can be
simulated using lighting or sound effects to give the impression that
water is nearby. For maximum realism, or if the actors need to
interact with the water, it is necessary to construct have actual
water on the stage. This tech note examines the problem of having a
stream on stage that flows down a waterfall and into a pool, and
presents some notes for solving the problem, based on a design by
Michael Powers \cite{powers-water}.


\section{Building a pool}
\label{sec:building}

A pool is simply a reservoir that holds water. This is not complicated
to make, but introduces a few new concerns not usually present in most
theatrical problems.

The first step is to construct a base for the pool. This is a
three-dimensional scenery problem, since generally we are interested
in natural-looking pools, which have that rather inconvenient property
of not being in regular, easy-to-build geometric shapes. (Of course,
sometimes a simple cubic basin is what is called for --- this makes
things that much easier.) This calls for building a frame for the
pool, which may be irregularly shaped.

An important and easy-to-neglect issue in building the frame is the
weight of the water it will need to support. One cubic foot of water
contains about $7 \nicefrac{1}{2}$ gallons and weighs approximately 63
pounds \cite{backstage-handbook}. For a pool of any reasonable size,
this quickly adds up; for large volumes of water, careful structural
analysis will be important to ensure that the frame will be able to
hold the weight, especially if the pool is being recessed into a
platform. One simple way to minimize the amount of water used and thus
the amount of weight that must be supported is to make the pool less
deep; this is often a very reasonable option since the actual depth of
the pool will always not be obvious to the audience.

The frame should then be covered with plywood (given the loads it will
be supporting, something substantial like $\nicefrac{3}{4}''$ plywood
should be used. It then needs to be covered with a pond liner so that
it can be filled with water. These are readily available from garden
stores as waterproof plastic sheets, which can be cut to size and fit
into the pool.

The pool can then be filled with water. Once filled, it should be
emptied or changed regularly --- the last thing anyone needs on the
stage is a place for bacteria and other unpleasant things to grow, and
a stagnant pool of water fits that bill all too well. A pool treatment
can also be useful here, though ones containing chlorine are not ideal
due to their distinctive smell.


\section{Building the waterfall}
\label{sec:waterfall}

Many of the same techniques from building a pool of water apply to
building a waterfall. Again, we will need to use a waterproof pond
liner around a frame and support structure. The difference, of course,
is that the waterfall will be a stream (albeit one that sometimes
flows horizontally and sometimes flows vertically), so it will not
have as much depth. This allows the support structure to be more
lightweight. A common means for building streams is to construct a set
of curved ribs, then lay a thin layer of plywood or lauan over it that
conforms to the curve of the ribs. This should then be covered with
the pond liner.

The water should flow through the stream and down the waterfall into
the pool, which serves as a reservoir. It then needs to be returned to
the top of the waterfall; for this, a pump is required. A household
sump pump, available from the Home Depot or similar hardware store in
the \$100 range, is probably the easiest option. For especially large
installations, a common pump may not suffice. One option is to obtain
a larger pump; McMaster-Carr carries sump pumps up to
$\nicefrac{3}{4}$ hp and effluent pumps up to 2 hp, though the latter
may be a bit expensive for this purpose at over \$700 \cite{mcmaster}.
A simple alternative is to use multiple smaller pumps. Submersible
pumps are best for this application: other types of pumps may allow
the pump to be placed further away from the pool, reducing noise, but
adding a drain to the pool requires creating and sealing a hole in the
liner, which only adds complexity. Sump pumps may pump into a standard
garden hose, which is certainly easy to find and easy to work with.
However, the pump will be able to operate more efficiently if PVC pipe
(usually $1 \nicefrac{1}{4}''$ or $1 \nicefrac{1}{2}''$, though it
depends on the pump) is used.

The pressure created by a pump, especially a large one, may be rather
high. This could lead to the unusual effect of the water spewing forcefully
upward or in a powerful stream --- which, while useful for a fountain,
isn't very realistic in a stream effect. The solution here is to
increase the diameter of the pipe by using couplers with successively
larger diameters. This causes the same amount of water to flow through
a larger area, and thus with less force.

Waterfalls and streams are generally rather noisy. The sound of water
flowing down the stream and into the pool can be a rather pleasant
one, and can be a helpful part of the sound design for creating the
outdoor atmosphere. If, however, that doesn't fit with the sound
designer's vision, it becomes the technical designer's problem to fix.
Reducing the velocity of the flowing water is an obvious solution with
an obvious drawback. Adding rocks and similar objects to the stream
will help reduce the volume of water that needs to flow, reducing the
noise created (and also allowing a smaller, quieter pump to be
used). It is also quieter to not have water fall directly from the
waterfall into the pool; instead, it should flow onto a surface that
slopes into the pool.

\nocite{backstage-handbook}

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