Calculating Available Water Supply From a Dry Hydrant


CarolinaFireJournal - Bob Twomey
Bob Twomey
07/27/2018 -

Over many years I have been asked to calculate available water supplies from dry hydrants for numerous fire departments for their rating inspections and certifications. I did this as a service when I worked for the Natural Resources Conservation Service (NRCS), since water calculations were a routine part of the job for engineered conservation practices. It is a relatively simple process for ponds, but is a bit more complex for hydrants placed in streams or rivers.

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Ponds are the easiest of the calculations to figure. In general, when a department installs a dry hydrant in an existing pond, hydrants are usually placed at least 3.0 feet below the normal permanent water level. This is done for several reasons. First is to allow a minimal sufficient depth such that when the pumper or tanker is hooked to the hydrant, drawing water quickly will produce the least chance of “vortexing” or pulling air into the hydrant, thus cavitating the pump. Second, is to allow some usable depth of water during dry or drought years, when the normal water level is likely to drop below long-term normal. However, for both of the reasons above, it is always best to have as much depth as possible, keeping in mind the maximum suggested lift of your pump. I have always known this to be 15 feet or less in our area, and most departments in my area prefer to lift no higher than 12 feet. Your situations may be somewhat different.

Calculating Water in Dry Hydrant

The purpose of a dry hydrant is to provide a certifiable volume of water available for emergency situations. Thus, a hydrant placed four feet under the normal water level will provide for you a “lens” or “layer” of water available to you instantly. But how do you know how much water this is? Well, this has to be calculated, and here is how it’s done.

You must first determine the size, or surface area, of the pond. This is easily done using a program like Terrain Navigator or Google Earth, using the measuring capabilities of your mapping program. Sometimes, county tax websites will have these measuring features available to you. You can usually go to your local Soil and Water Conservation District Office or NRCS office and see if they will calculate pond surface areas for you. If not, a surveyor or engineer can do this for you.

So, you have determined that the pond you want to use is, say, 0.6 of an acre. Remember now that an acre of anything, water or land, is 43,560 square feet. So 0.6 of an acre is calculated as such: 0.6 acre x 43,560 square feet/acre = 26,136 square feet. Next, you have placed the top invert of your hydrant 4.0 deep into the water, based upon the long-term normal pond water level. So now you need to calculate the volume of this 0.6 acre pond at 4.0 feet deep. This is easily done as such: 26,136 square feet x 4.0 feet deep = 104,544 cubic feet of water (the volume, at 4.0 feet deep over 0.6 acres of water).

Alright, now we know we have 104,544 cubic feet of water available to us, but to most folks, this doesn’t mean a lot. We need to know how many gallons of water this is. Again, easily calculated: since 1.0 cubic foot of water contains 7.48 gallons of water, just multiply the cubic feet of available water by 7.48: 104,544 cubic feet x 7.48 gallons/cubic foot = 781,989 gallons of water.

A way to cross-check your calculations is as follows. One “acre-foot” of water is 1.0 foot of water over 1.0 acres of area, Therefore, an acre-foot of water contains approximately 325,851 gallons. As such, a 0.6 acre pond at a hydrant depth of 4.0 feet is: 0.6 acres x 4.0 feet deep = 2.4 acre-feet of water. Thus, 2.4 acre-feet of water x 325,851 gallons/acre-foot = 782,042 gallon of water available to you. The difference in calculated water volumes is 782,042 – 781,989 = 53 gallons! I think this is close enough to meet water volume calculations for you ratings.

Stream Installed Dry Hydrant

So, you have seen two different ways to figure water volumes available to you with your installed dry hydrant. But when you install a dry hydrant into a stream, the calculations are far different.

In each county in the various regions of the country, the U.S. Geological Survey (USGS) office has long-term data that will tell you what the 10 year-seven-day average low flow is for your county. Sometimes, the NRCS or Soil and Water District Office will have this data as well. The 10 year-seven-day low flow is that flow, which exists in a given stream, as measured over a seven-day time period during a 10-year drought event. Now I don’t mean to complicate things, but there are drought measurements just like there are flood measurements, such as the one-year flood, five-year flood, 10-year flood, and so on, up to the 100 year and larger flood events. So it is with droughts, obviously the higher the number drought, the dryer it is. You get the picture now.

For our county, the average 10 year-seven-day low flow is measured as the water coming from a one square mile area of land, measured in cubic feet per second per square mile. Our county average is one cubic foot/second/square mile. Said another way, you could reasonably expect a stream flow of 7.48 gallons/second coming off of one square mile (640 acres) of land. As you can see, it is VERY important to have a large enough contributing drainage area to supply your water needs during times of drought.

So now you are looking to install a dry hydrant in a stream, knowing that on some smaller streams, you may need to use a ladder dyke and tarp or canvas, to “pool” water deep enough to avoid pump cavitation/vortexing as you pump water from the channel. IF your 10-year-seven-day low flow is similar to ours in Transylvania County, here is how you would figure the water volume in a stream channel:

You measure the drainage area ABOVE the point of hydrant placement, and say you measured the drainage area to be 8,845 acres. Not a bad little watershed. But you need to know have many square miles 8,845 acres is. Well, you know that one square mile contains 640 acres. Therefore, 8,845 acres divided by 640 acres/square mile = 13.82 square miles surface area. If the 10 year-seven-day low flow volume is one cubic foot/second/square mile, you calculate the following: 13.82 square miles x 1 cubic foot/second/square mile = 13.82 cubic feet/second. But again, what is this in something we can use, like in gallons? Easily figured: 13.82 cubic feet/second x 7.48 gallons/cubic foot = 103.4 gallons per second at the hydrant site. There are other factors that could affect this flow rate at the point of the hydrant installation, but in general, under normal watershed ground covers (i.e., not urban or excessively developed or extremely steep terrain) this is the expected water yield one could expect, at this given 10 year-seven-day drought watershed yield of one cubic foot/second/square mile. Your data will likely be somewhat different based upon your area’s 10 year-seven-day drought index.

I hope that this data will be of help to you when you are siting dry hydrants. I urge you to ascertain your drought index watershed yield rates for your area if you plan to install hydrants in streams. I know of no other reliable way to know how much water yield your watersheds will provide without this data.

You should place hydrants in streams in straight sections of the stream. If placed on the inside of a curve, sediment tends to build up over the hydrant intake. If placed on the outside of stream curves, creek bank erosion and bank scour can wash out or damage a dry hydrant. Inspect these hydrants regularly, especially after hard rains and/or flooding. Remember that the closer the hydrant is to the stream bottom, the more likely sand and sediments are to be sucked into the intake during drafting. Have sound, firm access to your hydrants for use in all weather conditions. 

Bob Twomey is the founder and past chief of the North Carolina High Level Extraction Rescue Team, Inc. a volunteer helicopter search and rescue support team based in Transylvania County, North Carolina. He is the senior helicopter pilot for Wolf Tree Aviation, LLC operating out of Transylvania Community Airport. He has served in numerous officer and training positions in the Transylvania County Rescue Squad. He has been active in SAR for 41 years. He can be reached at 828-884-7174 or [email protected].
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Issue 32.4 | Fall 2018

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