Understanding Detention Time and Flushing Rate

The average length of time which water spends in a lake is quite simply the volume of the lake divided by the inflow of water from all sources. The flushing rate is the inverse of detention time, or the amount of time it takes to replace the volume of a lake. This is best understood as an example.

A 100 acre lake with an average depth of 10 feet holds 1000 acre-feet of water, or 43.56 million cubic feet. If surface water inflows average 10 cubic feet per second, that adds 864,000 cubic feet per day. If direct precipitation is 46 inches per year, that adds another 383 acre-feet of water per year, or about 46,000 cubic feet per day. If groundwater in-seepage contributes another 20,000 cubic feet per day, the grand total of water inputs would be 930,000 cubic feet per day, with the surface inflow being dominant in this case (which is not always the case, so adding in direct precipitation and groundwater is important). The volume of 43,560,000 cubic feet divided by an average inflow of 930,000 cubic feet per day yields a detention time of just under 47 days. The flushing rate would be just under 8 times per year.

In the example above, if the inflow was less, the detention time would increase and the flushing rate would decrease. If more water was somehow put in the lake, the detention time would decrease and the flushing rate would increase. One could use sources of outflow to get detention time and flushing rate, with surface outflow, evaporation, and out-seepage of groundwater as the primary outflows (unless there is a major withdrawal of some kind), but the inflow and outflow have to match over the long run for the lake to remain stable, and it is easier to get values for inflow sources than outflows.

A key point is that it is only the volume of the lake and the total inflow or outflow that matter to detention time and flushing rate. One cannot change these values by altering an outlet structure, dredging a channel, or other physical manipulations within the lake. If no more water is added or subtracted from a lake with the same volume, the detention time and flushing rate do not change. Dredging and other lake manipulations may alter the circulation pattern, and if the lake is subdivided into parts, this may affect detention and flushing of one part, but the overall detention time and flushing rate cannot change without a change in lake volume or inflow quantity.

Detention and flushing rate are important to lake function and many management options. A lake with a detention time of less than about two weeks is unlikely to develop algae blooms, as the water does not stay around long enough to let blooms form. Lakes with very long detention times, more than a year, are less subject to watershed influences on a day to day or even season to season basis; there is simply not enough inflow to alter water quality over a short space of time. And most water quality models that allow us to predict changes with specified management actions depend on detention time or flushing rate as an important term in the mathematical model; being off by even a small amount affects calculations and reliability.

Detention time and flushing rate are not constants, however, and vary over time with changing inflows. For lake with long average detention times, this is not a major influence, but for lakes with average detention times of days to a few months, the variation within a year can be meaningful. A lake with an average detention time of a month could experience much lower summer inflows and have the same water present for 3 months, while spring thaw and related snowmelt and rain reduce the detention time to a matter of days in April.

Detention time and flushing rate can vary over space as well. A “dead end” part of a lake may have a much longer detention time and be flushed much less than some area in the main path of inflows, leading to stagnation and possible water quality problems in that dead end cove or segment of the lake. Rerouting water through the dead end may improve circulation and reduced detention time for that area but unless this is new inflow to the system, it will not change the average detention time for the whole lake.

One other related concept is important to understand when managing lakes. If a lake is long and linear, with most inputs at one end and the outlet at the other end, as with certain human-made reservoirs, the movement of water is called “plug-flow” in engineering terms, and is similar to water moving through a garden hose. Each increment of inflow pushes the next increment forward, and while there will be some mixing, water entering the lake at any time will have more or less similar detention time to water entering at any other time, with variation due mainly to changes in inflow rate. An algae bloom that forms in such a lake will be removed after about one flushing, or replacement of the equivalent of about one lake bottom.

But for the more typical lake configuration of a bowl with various input sources and an outlet somewhere along shore, the engineering model is called a “continuously stirred tank reactor”. In this model, each increment of water added is completely mixed with all other water in the lake, and while average detention time is calculable, the actual detention time for any increment of water can be highly variable. Because of the mixing, it takes much longer to get any undesirable water mass flushed out of the system. Just replacing the water volume of the lake one time (one flushing) will not get rid of all of an algae bloom or a spike of phosphorus or an oil spill; it takes 3 to 5 flushings to clear the lake.

Sources of Plankton Blooms

For many years it was generally thought that planktonic algae blooms involved a few resting stages germinating from the bottom or a few cells hanging on from some previous time in the water column encountering adequate nutrients under sufficient light, resulting in enough growth to discolor the water and be called a “bloom” (Note: there is no technical definition of a bloom; it is in the eye of the beholder, although there is general agreement that there should be reduced clarity and increased color involved). This mode of bloom formation is certainly possible and does occur, but for the harmful algal blooms currently getting so much media attention, there are two other modes of bloom formation that seem to be dominant.

One mode is metalimnetic accumulation, which means that algae accumulate in the boundary layer between upper, well-lit, warmer waters (the epilimnion) and lower, darker, colder waters (the hypolimnion). This can only happen where the lake is deep enough to develop a thermal gradient, usually more than 20 feet in New England, but thermal stratification can develop in shallower lakes under the right circumstances. Algae in this zone get enough light from above and enough nutrients from below to grow into a dense layer which may be disrupted by wind or may synchronously rise in response to lower light availability or changes in temperature that signal breakdown of thermal stratification. These blooms are most often evident in late summer or early fall, but can be moved into upper waters by summer storms. Several types of golden algae that produce taste and odor use this mode of growth, and water supplies with intakes in this boundary layer have to deal with them. Several blue-green algae known for toxicity also utilize this mode of growth and are a threat to both human health and lake ecology.

The other mode involves growth at the sediment-water interface, much like algae mats, with an eventual rise in the water column, again like the mats. But we think of these algae as planktonic forms, and never really considered their origin. It appears that these algae, mostly blue-greens and often potentially toxic forms, gain most nutrients from decay or other releases from sediment, accumulate excess amounts in cells, grow to full “maturity” on the sediment, then produce gas pockets in cells and rise over just a few days to form a ready-made bloom at the surface.

These bottom-formed blooms are particularly problematic in lakes known for being “clean” for many decades. The inputs from the watershed over many years were assimilated into the bottom sediments and at some point the balance tipped, such that the accumulated nutrients, especially phosphorus, became available for uptake at the sediment-water interface. It appears that this is related to oxygen loss in the surficial sediment, largely a function of organic build up. The water column may still have low nutrient levels, but the rising blue-greens already have enough extra nutrients to survive for days to a few weeks, and the resulting blooms seems to come out of nowhere to impair lake use and threaten ecological functions. Worse yet, this appears to bring nutrients into surface waters, allowing follow on blooms when the initial algae die off. This mode of bloom formation is both a harbinger of eutrophication and a vector of it.

Planktothix sp., a metalimnetic bloom former

Dolichospermum lemmermannii, a bottom bloom former

NALMS 2017 International Symposium Registration Open

The 2017 NALMS conference will be held in Westminster, CO, just outside Denver. This annual gathering showcases the best in lake management through presentations, exhibits and networking. This is a gathering of researchers, consultants, governmental representatives and lake enthusiasts. If you work on lakes, you really can’t afford to miss it. But if you just love your lake and want to know more about how to keep it in its best condition, this is a golden opportunity. The best experts in lake management are readily accessible and enjoy interacting at this gathering. It is always a great event, starting with workshops and progressing through 3 days of interactive sessions. This year it all starts on Monday, November 6th and ends on Thursday, November 9th. Check out details at the NALMS website at https://www.nalms.org/.

Your Lake and You Publication Available

For many years NALMS has had this newspaper-style publication available, one that has a lot of basic info on lakes and can be customized for any particular lake, association, or state. Your Lake and You has recently been updated and is provided as an electronic file that may be very useful to your lake group. As explained on the NALMS website, the 2017 online edition of the Your Lake & You! booklet is an updated version of the 8-page newspaper and helps explain to homeowners the steps they can take to protect the lakes they live on and love. This wonderful resource is loaded with basic lake information, strategies for taking better care of lakes, and descriptions of resource publications. Find it online at https://www.nalms.org/nalms-publications/.

Lakes Appreciation Month Wraps Up

In case you missed it, July was officially “Appreciate Your Lake” month. For members of NALMS, July represents a time to reflect on how we value lakes, usually with a focus on individual favorites. A number of people were interviewed on National Public Radio’s program Here and Now, segments of which can be found online hereOf course, you can appreciate your lake any time, and some of us like our lakes even better at other times of year besides summer. But summer is the key season to most, and July marks the annual kick off of the Secchi Dip In, an annual collaborative data collection effort that has resulted in a magnificent data base that allows us to know what the distribution of water clarity is for all regions of the USA. If you measured Secchi transparency in July 2017, please submit your data to this valuable data base. Check out the NALMS website for more on Lakes Appreciation Month and what you can do for your lake!

 

Algae Mats

Two groups of algae form almost all the mats: green and blue-green algae. These mats mostly form on the bottom, utilizing nutrients at the sediment-water interface, then move upward as they trap their own photosynthetic gases or accumulate gas released from the sediment under thick tangles of filaments. These mats may continue to grow for a time at or near the surface as a function of stored nutrients from their time on the bottom or from additional nutrients in the water column. Yet ultimately they tend to wind up on the surface, often blown to the edges by wind, in large decaying masses that turn various colors from yellow to blue and may be quite malodorous. In great quantities, they can really detract from the lake experience.

One partial exception includes the “cotton candy” or “cloud” growths of certain filamentous greens, mostly in the Spirogyra group. These algae do get their start at the bottom, but grow upward in a loose, slimy affiliation that looks like a mass of light green cotton candy or a cloud in the water. When you try to grab it, there is not much to grab, but your hand feels slimy. These algae produce a lot of mucilage, hence the slimy feel, and have enough structural strength to expand into these underwater, microscopic, “tinker-toy” conglomerations.

Sometimes a green mat will remain anchored to the sediment while part of it floats upward, creating a pillar in the water. Blue-green mats of Plectonema are brown to black and don’t rise in New England lakes until late summer, if at all, but from about Maryland south they can be a major impediment to lake use from early summer on. In New England, blue-green surface mats are most often chunks of Oscillatoria that break free of the bottom; these are very dark blue to black, often with brown sediment on the underside (still attached from the bottom) and they often have a distinct and unpleasant odor.

Algae mats are a clear indication that nutrients have accumulated in the sediment in water shallow enough for light to penetrate to the bottom. Once formed, algae mats are very hard to kill, as the outer filaments protect the inner filaments to a large degree. Removing the sediment is the most effective approach, but is very expensive and involves an often tedious permitting process. Treating the sediment with a phosphorus inactivator or algaecide before dense mats form is often effective, but results are not permanent.

Green algal mats   

Blue-green algal mats