Forces of Nature

How natural elements such as rain, flood, freeze, and snow impact the integrity of the pool shell, pipes and structures.

By Jeff Nodorft, PE LEED AP and Michelle Schwartz | 2007
World Waterpark

Man’s understanding of science and nature began with natural interactive experiences, and consequently was expanded with the basic forces—earth, water, sun, and wind. While engineers and designers today plan aquatic projects to appear to be part of Earth’s basic elements, natural phenomena must be addressed in the outdoor swimming facilities’ function and design. Waterparks with their twisting rivers, tidal wave pools, waterslides, lagoons, grottos, sprays and fountains must endure the real forces of nature on a daily basis.

The Earth revolves around the sun triggering weather patterns, storm systems and geologic movements. These natural cycles of events—rain, flood, heat, freeze, snow, melt—create natural forces that work against the integrity of structures, pipes, materials and pool shells. A typical aquatic project begins with a site investigation of soil and bedrock to determine how the site will interact with aquatic features that must have stabile and uniform bearing capacities. Specific soil conditions can affect below grade construction, thus soils must be tested for geotechnical properties and soil characteristics of the earth supporting the pool. Any inconsistencies in the soil conditions across the site must be corrected, while the presence of shallow rock may require a change in the program or removal of stone for pool and piping installation.

Water, an ever increasing precious natural resource, is the backbone of the waterpark industry. As we continue to understand water as a resource, we do know that water can create challenges for designers. The elevation of groundwater, or high water table, creates a design condition where it is required to de-water the site for construction, essentially pump down the groundwater. In this situation, groundwater must be considered in the structural design of the pool vessel to address hydraulic pressures, including buoyancy force on the pool vessel structure. These forces can be mitigated adding mass to the structure to prevent buoyancy or by adding hydrostatic relief valves to equalize the pressure, internal and external, to the pool structure. The addition of water to expansive soils where soils can shift with expansion and contraction as a result of the presence of water also creates external structural loads on the pool structure. This condition can be mitigated by the removal of expansive soils near the pool structure and designing water removal systems that surround the pool.

The structural design of the pool vessel is critical for the daily forces placed upon the pool as well as non-typical forces. The structural design of pool vessels follows the design requirements of the building code for structural concrete (ACI 318) for the strength and materials of the vessel. Building code ACI 350 is followed for further durability and serviceability considerations for watertight structures. These design criteria make pool structures strong reinforced frameworks that withstand the typical and non-typical forces of the earth.

The mere presence of an intense rainfall may cause challenges for outdoor aquatic facilities and waterparks, creating an overflow condition for all of the pools. Overflow systems must be designed to handle significant rainfall events without eroding adjacent soils or overflowing and discharging chlorinated water into undesirable areas such as landscaping or waterways. Rainfall events may cause a rise in elevation of adjacent storm detention ponds or waterways, which may result in flooding of the pool and pool deck. The elevation of the pools and aquatic features relative to adjacent elevations is critical to thwart potential flooding issues, as well as mud or debris that may wash into the pool area from higher elevations.

Winter weather can cause freeze/thaw cycles in water retained by the soil around the pool and must be addressed through the backfill around the pool and the structural strength of the pool wall. All pools requiring winterization must have piping installed in a manner that allows for draining water from the piping to prevent freezing water in the piping system. Winterization procedures must also ensure that all forms of water, including melted snow and ice, do not enter the system throughout the winter season.

Once the pool is in place and operating, the earth acts as a heat sink for the pool. Water temperature of the pool is typically elevated above the surrounding temperature of the earth, inducing a continual conductive heat loss from the pool water to the earth. Outdoor pools in colder climates may have insulation placed around the pool vessel to combat the conductive heat loss from the pool to the earth. Moreover, bits of earth that enter the pools in the form of dirt, sand, pebbles, etc. attached to guests’ bodies can be an issue for water clarity and operations. Well designed pathway layouts and landscaping design can minimize the amount of earth that enters the pools.

A sunny day is the catalyst that brings bathers to waterparks in record numbers. The sun also has other indirect effects. Prolonged exposure to UV rays has a direct effect on water chemistry, construction materials and even the swimmers. The direct effect the sun has on the destruction of chlorine is well documented. When chlorine is exposed to the ultraviolet rays of the sun, the free chlorine in the pool water will break down and escape. Chlorine, the disinfectant of choice throughout most of the industry, must be protected from UV rays through stabilized chlorine for all outdoor pools to proactively manage water chemistry.

As the sun shines and temperatures heat up, a great amount of active and passive heat gain can be added to the pools and decking, creating an uncomfortable situation for guests. In some areas the water is cooled (typically through aeration) to remove excess unwanted heat from the water. The sun has the greatest and longest lasting impact on guests. Sun worshippers love the bronzed, sporty suntan that reveals an active, healthy lifestyle, but sunburns and overexposure can cause serious skin cancers that are a health threat in general to society. Shady areas at outdoor aquatic centers and waterparks are critical for relief against the sun. Plentiful pavilions and large umbrellas provide festive, colorful areas for congregating, eating and relaxing out of harmful rays. Shade trees and natural landscaping provide canopies over outlying picnic areas and at various areas throughout the park.

Over time, the continual exposure of equipment to powerful UV rays changes the properties of materials, causing brittleness and fading of various color pigments as the equipment ages. Direct sunlight also causes significant movement and stress in structures and waterslides with the cyclical expansion and contraction of this equipment, leading to ongoing operational issues with sealants between joints of the equipment. Selection of equipment materials and colors will help reduce the effects of UV exposure and thermal expansion.

Subtle breezes keep swimmers content on warm, humid days; however, there are several design considerations for wind. Prevailing winds are a continual source for blowing debris from adjacent sources into the pool requiring continual skimming and cleaning. The wind is also the source of evaporation of water from pools and a source of convective heat loss from the pool. Convective losses may or may not be desirable depending on the season and location of the pool. In certain coastal areas, structures need to be designed to withstand gale and hurricane force winds as well as consideration to materials that are resistant to the effects of exposure to salt water.

The aquatics market is continuing to realize and respond to the need for conservation of water as a natural resource. New technology and products continue to evolve and offer designers choices such as pool covers and regenerative filtration systems, which minimize evaporation losses and water consumption through filtration.

While waterparks may appear to be created from Earth’s natural elements, engineers and aquatic designers continually improve durability and safety provisions against Mother Nature’s impact as she seeks to destroy what has been so carefully designed. As forces of nature impact waterpark structures, mankind has identified and worked with the sources and effects of nature, creating ways to protect against the many challenges that will continue to evolve. As waterpark guests celebrate the warmth of the sun and the soothing and exciting experiences that water offers, communities and families will come together to partake in nature’s best exercise, share life stories and recreational experiences, and . . . talk about the weather.