Name ______
OCN 201 Lab
Rates of Water Flushing & Water Residency Times
Waikalua Loko Fish Pond
What to Bring:
• Covered shoes or tabis
• Work clothes
• Sunscreen
• Hat (optional)
• Water bottle
• Change of clothes and towel (optional)
• Clipboard
Cautions… Wear boots to go into the water- this is a wet exercise. It is not recommended that you go into the water without boots or some kind of foot protection due to sharp debris, broken shell fragments, sharp rocks, broken concrete bits, metal objects, crabs, and such that can result in cuts and bruises.
Instructor Notes:
- Waiver forms
- Schedule trip
- Meter sticks and tape measures
- Stop watches
- Calculators
LIABILITY WAIVER FORM:
PLEASE DOWNLOAD AND COMPLETE
FOR SCHEDULING :
Please contact Kaohua Lucas
E-mail: /
Phone: / (808) 843-1217
Objective: To determine and understand the flushing rate by seawater in Waikalua Loko, then approximate residency time for water in the fish pond, both critical factors in fish pond health and productivity as an aquaculture facility.
If the water becomes stagnant, most biological activity diminished, and the fish pond does not function for food productivity. Water input is critical for replenishing nutrients and dissolved oxygen, among other things; water discharge is critical for removing wastes and other contaminants. A healthy pond environment must have an exchange balance where input = discharge at some rate of water exchange.
But what are these rates? And how can these be studied, designed for, and controlled? These are the appropriate questions:
- What are the necessary or existing rates of water exchange?
- How long does the water reside in the pond before it is recycled with the ocean- its residency time?
- Can thus occur in one, two, three, or more, tidal cycles?
- How many makaha’s are needed to see effective change?
- How big should a makaha be to be effective?
These are questions pursued in this laboratory exercise. They were pondered in ancient times when the fishponds were built, some 300-600 years ago, and remain questions today. Notice that we assumed that the fish pond is a loko i’a type (as is Waikalualoko), and that the ocean is our primary water source exchange.
It must be emphasized that these same questions can be – and are- asked about the global ocean. Thermohaline circulation is the exchange mechanism, while the makaha’s might be various straights, channels, and passageways that connect oceans. The pond thus is both a miniature scale ocean and a proxy for the global situation.
Another question that comes up is why did the ancient Hawaiians design three mahaka’s (gates) for this fish pond? And why did they place them where they did in terms of controlling or responding to circulation within the pond? Clearly this number of makaha’s and their placement were perfect to maintain a healthy pond.
Note- items below in bold require answers.
Field measurement:
Step 1. Measure makaha dimensions and cross-sectional area.
- Measure the width and height of your makaha (use the meter stick or the tape measure): note the dimensions on the attached data sheet
- Measure the water depth within and on the pond side of the makaha- make three measurements with one in the middle and one at each side (recommend using the meter stick for these measurements, holding the stick so that its narrow dimension faces current flow simply to make it easier to hold the stick still and vertical in the current; if the current forms a “bow wave” over the stick, please estimate what you think the actual depth might be); record these depths on the attached data sheet.
Step 2. Measure flow rates through the makaha. Make these measurements at least three times.
In advance, obtain some small sticks, or leaves, or something that floats, for use in measuring flow rates by simply dropping them into the flow and letting them flow away.
Person 1: Hold the meter stick within the makaha, at or near water level, with the zero mark at the pond edge and the 100 cm end towards the ocean.
Person 2: Have a floating object (wood fragment, leaf, etc.) ready to drop in the flow through the makaha. Drop the object into the flow- announce/yell “START” or “GO”, or something , so person #3 can start the stopwatch. When the floating object passes the one-meter (100 cm) mark at the end of the meter stick, announce/yell “STOP” or something so that person #3 can stop the stopwatch.
Person 3: Have the stop watch in hand that is at zero and set to count in seconds, finger poised on the start button, ready to do timing. This person can be in the water, on the bank, or on the bridge over the makaha- your choice. You will be told when to start and stop by person #2. Record the time on the data sheet.
Make these measurements at least three times.
If the values from these measurements vary widely, do additional measurements until three values are somewhat close to the same value.
This measurement can be difficult to do using this technique. It is recommended that all the values be recorded should you later need to finds three somewhat similar values from these series of measurements.
Laboratory Procedures:
Step 1: Calculate the cross-sectional area of the makaha through which the water is flowing- be sure your units are all the same (cm or m, or in, or ft).
Cross-sectional area (cm2) = width x average water depth
Thus the cross-sectional area of water entering your makaha = ______cm2
This, then, is the cross-sectional area of water flow through your makaha.
Step 2: But we need to convert this to a volume measurement to use in calculating and comparing to the water volume in the pond. To convert to a volume of water flow through the makaha, simply multiply by 1 cm:
Volume (cm3) = area (cm2) x 1 cm
Thus the volume flow entering your makaha = ______cm3
This, then, is the volume of water flowing through your makaha at any one-time interval.
Step 3: Calculate the water flow rates (the velocity) through the makaha. Velocity is measured as cm/sec (your cars velocity is measured as miles/hr, or mph = distance/unit time). For this calculation, use the average of the three most consistent values measure during the field measurements. Note that what was measured was a flow rate for 100 cm (or a meter) during x number of seconds – convert this to cm/sec.
Average velocity of water flow through your makaha = ______cm/sec.
This, then, is the rate at which water is flowing through your makaha, every second, at the time you made the measurements.
Step 4: Calculate the volume of water passing through the makaha per second, or cm3/sec.This is easy- simply multiply the cross-sectional area of water flow (step 1), times, the flow rate (step 3), or:
Cross-sectional area (cm2) x flow rate (cm/sec) = volume of flow per second (cm3/sec).
Thus the volume of water passing through your makaha = ______cm3/sec.
This, then, is the volume of water flowing through the makaha every second during the time interval you made measurements.
Step 5: Compile the data from the other makaha’s, and sum this for an approximation of the total volume of water that is passing through these three makaha’s.
Makaha A = ______cm3/sec
Makaha B = ______cm3/sec
Makaha C = ______cm3/sec
Total water volume = ______cm3/sec
This, then, is the total volume of water flowing out of the pond during the time we were at the pond making measurements.
Note that our assumption is that this is the only exit (or entry) of water out of (or into) the pond. How valid is this assumption—that is, can you think of another way that water may be leaking out of (or coming into) the pond from the ocean, and if so, how effective do you think this might be? It is an important consideration because if it is effective, then the next few steps in this exercise may, or may not, be very significant.
Step 6: Calculate the total volume of water in the fishpond. Use these estimates.
The total area of the pond is about 10 acres.
One acre = 4047 m2 = 4047 x 104 cm2 = 40, 470,000 cm2
Thus the pond has an area of ______cm2
The average depth of the pond (as determined by OCN 201 last years lab), considering all the mangrove and shallow areas along the mauka side = roughly 1 foot, or about 30 cm.
Thus the pond volume = area (cm2) x depth (cm) = ______cm3.
This, then, is the approximate volume of water in the fishpond.
Step 7: Our measurements represent a typical outgoing current through the makaha’s for one part of the tidal cycle. Using the attached tidal chart-
The duration of the outgoing tide was ______sec/min/hrs (pick one)
Step 8: Given this amount of time for the water to have been flowing out through all three makaha’s (step 7), and the volume of water in that flow (step 5), calculate the total volume of water that flowed out of the pond during that one tidal cycle:
Thus total volume of water that flowed out of the pond during that one tidal cycle:
Thus the total volume of water that flowed out of the pond during this tidal cycle, through all three makaha’s was about ______cm3
Step 9: And, finally, given this volume of water flushed out of the pond into Kaneohe Bay during this part of the tidal cycle, what percent of the total pond volume was removed (and presumably replaced during the next incoming tide)?
One tidal flushing of pond water = ______% of the total water volume in the fishpond.
Clearly this was adequate to maintain a healthy pond. Realize that in ancient times, the fishpond:
1)Was deeper (it had seriously silted in during historic times)
2)Had one, perhaps two, other makaha’s that influenced the water input from adjacent streams, perhaps also allowing flushing out as much as a freshwater input
3)May have had more porous fishpond wall towards both the bay and both streams, thus allowing more water exchange than that provided by the makaha’s
4)Was significantly more productive than today with, for example, more fish, and thus more wastes and oxygen consumption that had to be managed.
You can see how our calculations today merely suggest a situation in the past.
LOCATION (
Waikalua Loko is located on the Southern shores of Kane'ohe Bay on the island of O'ahu, State of Hawaii, USA. It is also situated between the two waterways known as Kane'ohe Stream and Kawa Stream. Both of these streams drain approximately 5,000 plus acres of urbanized land extending from the Ko'olau mountain range to the bay.
It has a pond water surface area of approximately 11+ acres and has a fishpond wall (kuapa) extending from the land about 1,400 linear feet. Within the wall are three sluice gates (makaha) which currently are the primary sources of water to the pond based on tidal movements in the bay.
The current gates were last modified in 1930 utilizing concrete mortar and iron clad gates to control the entry and exit of fish. Current historical information to date indicate that the pond has been in existence for at least 150 years. Analysis of the pond walls and sediment floor will help to further determine a more accurate age of the pond.
Tidal Chart: