California has earthquakes; we in the south and east coast have hurricanes. One thing we all share is power outages. Ours are usually caused by storms. Most power outages last only a few hours and only happen a few times a year. A hurricane is always a threat that hopefully will never happen or have only a minimal consequence. Flooding may be more common then a power interruption and both can happen at the same time.
A well built well thought out well designed pond will survive a big storm with a long power outage, lots of rain and flooding. This pond also will be less worrisome during the numerous smaller but strong storms. The most obvious and the most popular is the garden pond. It has few fish so it does not need extra filter or aeration. There is enough surface area for oxygen levels to be sufficient. The surfaces of the pond and plants handle the filtration. A koi pond should be able to stand-alone for hours without danger of quick suffocation or toxic waste build up. The key is in the number of fish. Conditions in an over stocked over fed pond will quickly deteriorate. A lesser or under stocked pond will always be healthier, have less maintenance, and survive periods of no power.
Heavy rain can make a pond quickly overflow or be flooded out. Heavy rains can cause a rapid change in the pH and the temperature of your pond water. Check your inventory of chemicals such as pH adjustors, Amquel or Ammo Lock. Heavy rains can cause a change in the pH and the temperature of your pond water. Water with higher alkalinity will have a stable pH. Buffers can be used like baking soda will help keep ph stable. If it is possible use a tarpaulin a few feet above the surface to permit airflow yet keep rains out.
A pond should be built with the top above grade. Six inches is minimum and twelve inches is better. This can be a raised wall or lip. It also can be graded out and still have an in ground look. This will make rainwater drain away from the pond instead of into it. Choosing a spot that is not the low spot that always floods should be avoided or built higher. ? If at ground level consider surrounding the perimeter with a wall of sand bags 1 or 2 bags high. Drastic, but it has been done. This may protect your fish from being washed away and never to be seen again. Is it possible to set up a portable show tank, maybe in a garage or other protected area? We know that with changes to their environment, Koi will often jump. Murphy says that they will land in the worst possible location. A net surrounding the edge or covering the pond is therefore advisable.
An overflow drain is easy to build and may be the most used and important design of a pond. Without an overflow the water level will never be constant, be hard to maintain and get too high during a storm flowing over the top. An overflow drain should be large enough to handle large amounts of water I like four inch pipe. It can be above the waterline or stubbed up from below. It needs to drain to an area that can handle a lot of water. A storm drain, a dry well or a drain field, which is a large hole with gravel in it, then covered work well. Making a bog area where the 7.62×39 hunting ammo overflow water goes also works. Just take care that it does not flood someone else. Having the waterline above grade makes any overflow drain simple.
Lower stocking and overflow drains are simple ways of making a pond storm proof but there is more we can do. Most of us like to be heavily stocked with more fish then we should have. We may not want our fish not to have periods of low oxygen or risk an ammonia build up. One solution is to have an emergency generator that can be started automatically when the power goes out. They may not be too expensive for the piece of mind and convenience they provide. A portable generator can be used for some or all your equipment but will have to be started and switched usually during a storm. Both of these solutions work but will need fuel on hand and tested regularly for events that may only happen once or twice a year or a big event that may never happen.