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A New Weapon in the War Against Malaria

or

An Insidious1 Method for Larval Source Management

By Jeffrey S. Juel, PE

Abstract

This document describes a concept for an environment modification strategy which could be used for

larval source management to reduce mosquito populations. In some situations this method could

reduce the incidence of mosquito-borne disease.

This vector control strategy is functionally similar to the water level management scheme that has been

employed at the Tennessee Valley Authority’s reservoirs since the 1930’s. The TVA modulates the water

levels of the reservoirs to alternately strand and wash away mosquito larvae.2 Once each week during

the active mosquito season (May – October), the reservoirs are gradually raised by one foot and then

rapidly dropped by one foot. This disrupts mosquito breeding.

From the TVA’s website:

Mosquito breeding success is directly related to the extent and duration of shallow standing water, their breeding habitat. The degree to which a policy alternative would increase or decrease standing water throughout the Tennessee River watershed would directly affect the availability of breeding habitat, and indirectly affect the density and persistence of mosquito populations and the related potential transmission of disease3.

The drainage system for a low area protected from flooding by a levee can be designed such that it

seduces female mosquitoes to deposit their eggs in the shallow standing water of the lower reach of the

drainage system. If the drainage system discharges into a tidally-influenced body of water, a flow control

device known as a Variable Backflow Flap Gate, or VBFG, can be used to consistently fill and then drain

the lower reach with the flooding and ebbing tides. This filling and draining would be configured to

effectively dilute the mosquito larvae in the drainage system and discharge them into the receiving body

of water.

With a VBFG, during the flood tide the water level in the lower reach of the drainage system will rise to a

prescribed level and then the VBFG will automatically close. There will be standing water in the lower

reach for several hours during the remainder of the flood tide and for a portion of the following ebb

tide. To female adult mosquitoes4, this standing water will appear to be a viable stagnant pool on which

to deposit her eggs.

1 Definition of INSIDIOUS. 1. a: awaiting a chance to entrap : treacherous b: harmful but enticing : seductive.

2 The Fever, Sonia Shaw – page 190

3 http://www.tva.gov/environment/reports/ros_eis/4-12_vector_control.pdf

4 Note that there are thousands of species of mosquitoes with various breeding behaviors. Culex, Culiseta, and

Anopheles lay their eggs on water while Aedes lay their eggs on damp soil that will be flooded by water. Note that

some species are container-breeders and may not be easily seduced by this method of vector control.

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Ideally, the lower reach of the drainage system will be a broad channel with shallow side slopes and a

nearly flat gradient. The extent of the lower reach that will fill with each flood tide will be on the order

of several hundred yards up to a mile or more. When filled with water, this reach will effectively be a

canal with a depth of 1-2 feet.

The figure below shows the tidal water level downstream from the VBFG and the water level in the canal

upstream from the VBFG:

Figure 1- Tide curve & canal water level vs. time

If the water depth and other conditions are appropriate, egg-laden female pool-breeding mosquitoes

will deposit their eggs in the quiescent water of the filled canal. Later, when the flap gates open during

the ebb tide, the canal will drain. Some of the mosquito eggs and a fraction of the mosquito larvae will

pass downstream through the open flap gates into the tidally-influenced body of water.

After the canal has drained, shallow pools of standing water will be present at any low spots of the

canal. These pools of stagnant water may contain a few larvae and eggs. The shallow pools – which

briefly contain stagnant water - will seduce additional female mosquitoes to deposit their eggs during

the hours when the VBFG is closed. The eggs will eventually hatch and the larvae will inevitably be

flushed from the drainage system on the subsequent ebb tides.

Flood water breeding mosquitoes will deposit their eggs on the mud near the shallow pools during low

water periods. When the canal is re-filled on the following flood tide, these eggs will begin to hatch. The

eggs and larvae will then be flushed from the canal on the following ebb tide.

There are millions of acres of agricultural land and marshy lowland protected by levees where this

method of vector management could be used to reduce populations of mosquitoes to a tolerable level.

This scheme could be implemented as part of an integrated pest management program designed to

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reduce vector populations to a point below the disease transmission threshold level. This system could

also dilute the aquatic larvae of a number of other nuisance arthropods / vectors.

With this scheme, the dilution of mosquito larvae (as well as other aquatic parasites and pests) occurs

automatically without human intervention, with virtually no operational cost, and without the use of

insecticides or larvicides.

The canal could be connected to wetlands and marshes which would be filled and drained repeatedly

over time thereby reducing mosquito populations while preserving and enhancing important natural

wetland habitats.

By maintaining a proper slope5 along the fringe of the canal & the adjoining wetlands, the rising and

falling water level will strand larvae on the shoreline when the water level falls6. This will augment the

dilution effect; in fact the stranding of larvae could be a more relevant mechanism for killing mosquito

larvae than the dilution effect. This will be the case where a very large wetland is partially filled and

drained through a connection to the canal.

Introduction – Flood Plains, Flood Control, Interior Drainage & Flap Gates

Many towns and cities - as well as much of the world’s most productive agricultural land - is located

within riverine flood plains. Farms and developed areas within a river’s flood plain are commonly

protected from flooding by levees. The lands protected by levees require interior drainage systems and

a means of passing the collected water through the levee. The interior runoff is normally passed through

the levee using culverts with flap gates. If the drained area is below the river water level for extended

periods, pump stations may be necessary to manage runoff.

A traditional flap gate is a simple one-way valve that allows

water to pass downstream through a culvert while

preventing water from back-flowing through the culvert

and flooding the protected area during high tides and high

river stages. Since the flap gate has some weight and is

hinged at the top, some amount of differential head is

required to crack the flap gate open. Because of this

“cracking head”, traditional flap gates typically impound a

few inches of water during periods when there is no runoff.

If the flap gate does not leak, this impounded shallow

stagnant water will remain in the drainage system upstream

5 Controlling vegetation may be required in some locations.

6 The reservoirs of the Tennessee Valley are regulated such that the water level is raised by a foot and then

promptly lower by a foot once each week during mosquito breeding season. This is a very effective method for

controlling pool and floodwater-breeding mosquitoes.

Photo 1 - A traditional top-hinged flap gate on the

down-stream end of a culvert during a low tide.

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from the flap gate until there is a runoff-

producing event.

A typical drainage system for an area

protected by levees consists of a network of

ditches that drain to one or more traditional

flap-gated culverts which pass flow through

the levee. Since a riverine flood plain is

naturally flat, the inverts of the drainage

ditches will have very shallow gradients. Unless

they are precisely graded and meticulously

maintained, the ditches will not drain

completely.

When there is little or no runoff for an

extended period of time, vegetation will grow

in the ditches and the network of ditches will

not pass water efficiently. The ditches will

contain numerous pools of shallow stagnant

water. These pools could support mosquito

breeding when the right species of mosquitoes

are active in the area. The vegetation in the

ditches also impedes flow and degrades

drainage during runoff-producing rainfall

events.

If a VBFG fills and drains the ditches on a regular basis, many forms of vegetation will be unable to grow

in the ditch. In addition, the movement of the water will topple the few plants that can tolerate the

water.

What if the drainage system for a low-lying area protected by a levee was designed,

constructed, and maintained so that from a mosquito’s perspective, it appeared to be an ideal

place to deposit her eggs?

And what if the resulting mosquito larvae in this environment had virtually no chance of

maturing to become adult mosquitoes?

And what if female adult mosquitoes in the area had a very high probability of encountering the

drainage system while searching for a place to deposit their eggs?

This would be an ingenious and insidious method of larval source management.

Photo 2 – A drainage ditch with standing water.

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The Variable Backflow Flap Gate

I have designed and installed several working copies

of a revolutionary side-hinged flap gate that I call a

“variable backflow flap gate”, or VBFG. A VBFG

allows a finite amount of backflow to pass upstream

during the flood tide. For a given setting, the VBFG

will close during the flood tide when the water level

in the drainage system upstream from the VBFG

reaches a specific elevation.

The VBFG operates unattended for years and closes

consistently at a selected elevation on each flood

tide. This water control device is integral to the

hypothetical drainage system described herein.

The VBFG is a side-hinged flap gate that is open by

default. It only closes when the backflow through

the flap gate generates a “draft force” of sufficient

magnitude to draw the flap gate closed. The VBFG

can be set to close under a wide range of flows or

draft forces – hence the name “variable backflow flap gate”. A VBFG can close when it experiences a

very small draft force, or it can be set so that the draft force must be substantial before the flap gate will

close. It is convenient that the magnitude of the draft force grows at an increasing rate during the flood

tide; a given draft force correlates to a very specific upstream water level.7

The velocity of the flood tide’s flow passing by the open flap gate along with the “sail area” (the

submerged portion of the gate leaf) combine to produce the draft force drawing the VBFG closed. When

the draft force is sufficient to overpower the control mechanism, the VBFG will close. The VBFG will

remain closed as long as there is seating head acting against the flap gate. Since the closed VBFG is

spring-loaded, when there is little or no seating head8 the VBFG will literally “spring” open.

The VBFG will close and a few hours later it will spring open automatically with the rising and falling

water level. It does this automatically, over and over, without any human intervention for months or

years. By closing at a given tide level on each flood tide with uncanny accuracy, tidal exchange occurs

without sacrificing flood protection.

7 If there are higher than normal volumes of water entering the canal from upland runoff, the VBFG will close a few

minutes later than normal with an interior water level that is a few inches higher than normal. This is nearly always

inconsequential. 8 The seating head is zero when water levels are equal both upstream and downstream from the VBFG.

Photo 3 -A Variable Backflow Flap Gate (VBFG ) open and allowing

backflow on a flood tide.

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Photo 4 - The VBFG after closing.

If the invert of the culvert is at a reasonable elevation relative to the tidal range, the VBFG will open and

close at least once - if not twice - on most days. With each tide cycle, the lower reach of the drainage

system (the canal) will be partially filled and then drained. The repeated filling and draining will

effectively dilute the concentration of mosquito larvae in the canal and portions of the adjoining

drainage system to zero.

Photo 5 – A drainage channel “canal” upstream from a VBFG. During low tide (left); and with quiescent water after the canal

has filled and the VBFG has closed (right).

At some locations, multiple flap-gated culverts in parallel will be required to pass the maximum design

out-flow. It may be necessary for more than one of these culverts to be equipped with a VBFG in order

to pass a sufficient volume of backflow to reliably fill the canal and any connected wetlands to the

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desired depth during a typical flood tide. The diameter and number of culverts should be selected so

that the culverts are able to drain the bank-full canal during a single typical low tide.

Water Level Variations and the Tides

This vector control method relies on water level variations downstream from the VBFG to fill and drain

the targeted watercourse/drainage. Tide cycles are inherently reliable; however tidal heights will vary

depending on the location of the site in question as well as by the phase of the moon.

It should be obvious that rivers rise and fall with the tide and the tidal effect diminishes with the

distance from the river mouth. For example: the tide at the mouth of the Columbia River in Washington

State typically varies by about ten feet from low to high tide. Vancouver Washington is located at River

Mile 106. A 7.0 foot tide at the mouth of the river corresponds with a 1.65 foot tide at Vancouver, so the

extent of the levees along a large river that can use this insidious vector control method can be

considerable.

If the site is upstream from the tidally-influenced lower reaches of a river, the water level of the river

will only rise and fall as a result of changes in the river flows. Runoff, snowmelt, and/or reservoir

releases all increase the flow rate, and higher flow results in a higher river stage9. A continuous record of

historical river water levels over time should be used to select the invert elevation of the culverts, the

elevation of the U-shaped ditch, and the elevation that the VBFG closes at so that the canal will be filled

and drained at least once or twice each week under normal river level fluctuations. If a reservoir exists

upstream from the site, water releases can be planned to produce a flushing effect in the canals at

optimum times and with sufficient frequency for larval source management. The days following a full

moon would be good candidates for flushing since it is reported that mosquito biting increases 500

times during a full moon. Egg laying will occur within a few days of biting.

Tidal ranges on coastlines vary from place to place - from a few inches to tens of feet. Designing the

channel and culverts for a location that has a tidal range of less than two feet will be more challenging.

Achieving vector control with this scheme will be more challenging in locations having limited tidal

variations.

Traditional Flap Gates and Stagnant Water

Top-hinged flap gates are closed when there is no flow and they require some amount of unseating

head before they will crack open. In effect, a traditional flap gate is closed by default and only opens

when there is out flow. As the water level downstream fluctuates over time, the water level in the

drainage upstream from a traditional flap gate will ratchet down and become progressively lower.

During dry periods, any water upstream from the flap gate will be stagnant for extended periods.

9 River stage means the water surface elevation at a given location on the river.

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For contrast, a VBFG is wide open when there is outflow in addition to when there is no flow. It will also

be open when there is some amount of backflow. As long as there is little or no backflow, the water

level in the drainage system will rise and fall and effectively match the water level downstream from the

VBFG. This is very effective for reducing the amount and duration of stagnant water in the system. These

variations will reduce the amount of stagnant water available which will reduce mosquito breeding. The

level of effect will increase with the frequency and magnitude of the water level variations.

An Idealized Drainage System Using a VBFG for Larval Source Eradication

How this system works is best explained using an idealized drainage in a location which experiences

diurnal tides with a tidal variation of at least a few feet. The principles illustrated here can be applied to

more complicated configurations. The canal connecting the VBFG to the regular flap gate can take on

virtually any shape and can be several miles long.

Figure 2 - Idealized drainage system using a VBFG for larval source management.

The drainage system consists of a U-shaped canal surrounding an area adjacent to a flood control levee.

A VBFG that allows some backflow is installed on a culvert passing through the levee at one end of the

U-shaped canal. A side-hinged flap-gated culvert that does not allow backflow is installed on a culvert

passing through the levee at the other end of the canal. Culverts (or bridges) are used to provide

crossing points so that the canal can be crossed by vehicles or by foot traffic.

The existing ground in a typical flood plain is flat and at an elevation that is at most a few feet higher

than the normal high water level of the adjacent river, estuary or inlet. This terrain will allow the

construction of a shallow U-shaped (in plan) trapezoidal (in section) channel with an invert which slopes

very slightly towards the levee. If the project is in an area with a developing economy and the local labor

force is willing and able, the canal and drainage features could conceivably be hand-dug. This would

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provide useful and productive jobs for the local population in addition to improving drainage while

reducing mosquito populations and vector-borne disease. A win – win – win – win proposition. The

material excavated from the canal could be used to fill nearby depressions and/or raise the levee.

A number of ditches drain into the U-shaped canal. These ditches are graded and maintained so that

they drain completely into the canal whenever the canal is devoid of water during low tides. The land

surrounding the canal is graded to drain to the ditches or directly into the canal. Note that during

particularly wet periods, runoff and flow from the adjoining ditches will increase the dilution effect in

the canal and will sweep mosquito larvae from the surrounding drainage system into the canal and

ultimately to the receiving waters outboard of the levee.

Because the canal is graded to a very flat slope, it can be several miles long. The canal cross section and

the culverts passing through the levee should have sufficient capacity to drain 80-95% of the storage

volume of the canal on a typical ebb tide. A sinuous low-flow channel in the bottom of the canal will

form more or less spontaneously. The low flow channel will improve the flow efficiency and thereby

increase the volume of water that drains from the canal on each ebb tide.

Figure 3 - Drainage system during a low tide.

Since the canal has a very flat slope, it is likely that during the low tide there will be a number of water-

filled shallow depressions in the bottom of the canal. These depressions could be used by female

mosquitoes looking for water in which to deposit their eggs during low tides.

It is plausible that discarded or intentionally secured containers in the bottom of the canal could attract

container-breeding mosquitoes. This would be useful since the subsequent flood tides will submerge the

containers and carry the eggs and larvae toward the flap gates. In a subsequent ebb tide, the larvae

would be passed through the flap gates, into the river, and out to sea.

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Figure 4 - Drainage system being filled by the flood tide.

During the flood tide when the water level is higher than the invert elevation of the VBFG, water will fill

the canal10. The VBFG control mechanism should be set so that the canal fills to a depth that is ideal for

attracting egg-laden mosquitoes – presumably on the order of one foot or less. The canal will fill to this

depth within the time-span of an hour or two during a typical flood tide.

Figure 5 - Drainage system after VBFG closes.

After the VBFG closes, the partially-filled canal will contain quiescent shallow water that is presumably

attractive and, under normal circumstances, ideal for mosquito breeding. It will be in this state for

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And any containers secured in the canal.

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several hours while the tide cycle proceeds. If egg-laden female mosquitoes encounter the canal in this

condition and if other conditions are appropriate, they will likely deposit their eggs on or near the water

of the canal. The mosquitoes will not comprehend that the water will not be stagnant for long.11

The water depth in the canal, the canal side slopes, the vegetation on the side slopes, and the shading of

the canal should be maintained to make the canal attractive to egg-laden female mosquitoes. Adaptive

Management would be used to make the system as alluring as possible to the targeted species. Various

treatments could be tried on a limited basis. Adjustments that successfully entice more female

mosquitoes to deposit eggs would then be propagated to other insidious drainage systems.

The shoreline along the river outboard of the levee should be graded so that the spring tides do not

leave pools of stranded water between the levee and the river. This will be achieved if the levee

alignment is such that the outboard toe of the levee embankment is at an elevation that is slightly lower

than the typical spring tide’s high water level.

Figure 6 - Drainage system draining during the ebb tide.

Several hours later during the ebb tide, the water level outboard of the VBFG will be lower than the

water level in the canal and both flap gates will open. The water in the canal – including some of the

floating mosquito eggs and most of the larvae - will be flushed from the canal through the two flap

gates. If the low tide is lower than both of the culverts’ invert elevations for a sufficient length of time,

the canal and ditches will drain completely – other than a small amount of water trapped in a few

depressions.

11 By definition, they have the brains of a gnat.

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Since a series of tide cycles will produce a circulation in the canal rather than an in-and-out flow, the

flushing effect will be particularly effective at diluting the mosquito larvae population in the canal to

virtually zero.

The Dilution Effect

When disturbed, mosquito larvae swim or “wriggle” about randomly and they disburse into the water

column. They generally have no choice but to go with the flow. If 95% of the water is drained from the

system, it is plausible that roughly 95% of the larvae will either be carried to their fate through the flap

gates or stranded on the mud of the shoreline of the canal.12 If the receiving water is a river or a flowing

stream, the larvae will likely have a greatly diminished chance of surviving to become adults. Survival

will be particularly improbable if the travel time to the sea is less than a few days.

The sequence of dilutions of the mosquito larvae represents an inverse geometric progression. With

each dilution, the number of larvae that remain in the canal will become increasingly rare and, in time,

non-existent. The larvae will effectively be diluted to zero before they have sufficient time to mature

into adult mosquitoes.

Assume that the drainage is filled and then some percentage of the water is drained with each tide

cycle. If the canal experiences five or more cycles in the time required for the larvae to become adult

mosquitoes, and 85% of the mosquitoes are removed with each cycle, fewer than one in a million larvae

will survive to become adults. (1/1,000,000 = 0.0001%)

95% 90% 85%

tide

cycle

concentration

of remaining

larvae

concentration

of remaining

larvae

concentration

of remaining

larvae

1 5.0% 10.0% 15.0%

2 0.25% 0.50% 0.75%

3 0.0125% 0.0250% 0.0375%

4 0.00063% 0.00125% 0.00188%

5 0.00003% 0.00006% 0.00009%

In addition to the dilution effect, poor water quality tends to increase mosquito production rates. With

this control method the amount of organic material in the water will decrease due to dilution and the

dissolved oxygen will increase with the repeated filling and draining of the canal. Reduced organic

material in the water means fewer nutrients for the bacteria and algae that are food for mosquito

larvae. In addition, improved water quality (lower temperature; higher dissolved oxygen (DO); and lower

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To date this has not been tested and it is merely conjecture. Flow velocities, peripheral vegetation, and larvae

behavior could conceivably reduce the dilution effect.

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biological oxygen demand (BOD)) presumably will be beneficial to fish and other organisms that prey on

mosquito larvae.

Designing an Actual Drainage Systems Using VBFG’s for Larval Source Management

Levees commonly extend for dozens or even hundreds of miles and are frequently constructed on both

banks of a river. The U-shaped canal described herein would be repeated at some interval up and down

the river behind the levees on both banks of the river.

A proposed drainage system for a new levee (or a modified drainage system for an existing levee) should

be designed to accommodate the site features, topography, and existing drainage patterns. River water

level data, topographic maps, and plat maps showing property lines, will provide the basic information

necessary to design this type of drainage system.

Different species of mosquitoes can have very different breeding behaviors. Conditions that work well to

attract one species of mosquito may not work as well with others. An experienced entomologist should

be consulted to fine tune the variables of a particular Insidious Vector Control Canal in consideration of

the species of mosquitoes in the area. Consideration should also be given to the disease threat in the

area and the proportions of the local mosquito species that are vectors for the disease.

If several species of mosquitoes are targeted and their breeding preferences are dramatically different

(water depth, shading, vegetation, etc), it may be desirable to configure one drainage canal to attract

mosquito species “A” and the adjacent drainage system to attract species “B”.

If funds are limited and major modifications to the drainage system cannot be made, simply replacing

the existing flap gates with VBFG’s could significantly improving drainage and reduce mosquito breeding

habitat. Modifications to the drainage system upstream from the tide gate could be incrementally done

over time as funds become available.

It is estimated that there are one hundred thousand miles of levees in the U.S. – enough levees to circle

the globe four times. There are also hundreds of thousands of miles of levees outside of the U.S. Since

all of these levees have flap-gated culverts, this method of vector control could be applied to a

significant number of locations in the US and around the world.

Operational Requirements for the VBFG

A VBFG requires infrequent maintenance and will operate unattended for years at a time. Once set, the

VBFG will open and closed with high precision and with uncanny reliability. The design is very durable

with a design-life of 50 to 100 years. It is also fail-safe. If any part of the control mechanism fails, the flap

gate will operate like a regular flap gate (allowing no backflow). Flooding will not occur. One component

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in the control mechanism must be replaced on five year intervals. The material cost is less than $50 and

the replacement takes less than 30 minutes.

It is simple to adjust the control mechanism to increase or decrease the amount of tidal flushing.13

Adaptive management can be used over time to set the VBFG such that there is a balance between:

drainage needs; vector control; environmental benefits; the storage volume available for runoff in the

canal after the VBFG closes; and the need for groundwater recharge.

For example: When a vector-borne disease outbreak is underway, the VBFG should be adjusted to

maximize vector control benefits and potentially give up some of the storage available for runoff. During

rainy periods, the available storage volume for runoff should be increased by reducing the amount of

backflow allowed on the flood tides.

Mosquito Behavior

A female mosquito laden with a blood meal and later laden with eggs will typically not fly very far. In the

idealized drainage, source reduction will be implemented so that there will be no water-filled

depressions or containers in the vicinity of where she takes her blood meal. If the canals are laid out so

that a mosquito could not help but fly over one after flying for a few minutes, it will be next to

impossible for a female mosquito to resist the temptation to deposit her eggs or egg raft in one of the

canals or on the mud along the fringe of the canal.14

Salt Marsh Mosquitoes deposit their eggs on the mud flats or on the vegetation near water. They hatch

when the eggs are wetted during a high water level. In the natural system the higher tides – known as

“Spring Tides” reach more of the eggs, and these spring tides causes a large number of eggs to hatch.

Since all of the flood tides reach the same elevation upstream from the VBFG, any eggs deposited higher

than the level at which the VBFG closes will not hatch until there is a rainstorm and the eggs are washed

into the ditches and canal. When the Salt Marsh Mosquito eggs are flushed into the canal, their fate is

sealed. If the land is properly graded to drain to the canals and ditches, salt marsh mosquito breeding

could be effectively stymied.

Given the extent of the insidious drainage canals along the river and assuming that a very small number

of other potential breeding sites are available, the canals should receive the vast majority of the

mosquito eggs deposited in the area. The canals will typically fill with water in less than two hours and

will drain in a similar amount of time. The canals will contain quiescent/stagnant water at least 2/3 of

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This is done by selecting the water level at which the VBFG closes. A higher water level means more tidal

flushing. 14 Note that some species of mosquito are devout container-breeders. These vectors will not be controlled by this

drainage system. More hands-on control methods are required to combat container breeders. Mosquitoes

classified as “Floodwater Mosquitoes", "Swamp Breeding Mosquitoes”, and “Salt Marsh Mosquitoes” are the

targets of this vector control method.

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the time. If the quiescent water occurs when egg-laden mosquitoes are active, these temporarily

stagnant canals will be very effective at seducing female mosquitoes to deposit their eggs. If the canal is

filling with water (not quiescent) when an egg-laden mosquito is searching for stagnant water, the

mosquito will eventually give up looking for stagnant water and rest. In time the VBFG will close, the

water will be still, and the mosquito will foolishly deposit her eggs in the canal.

Integrated Pest Management

Integrated Pest Management (IPM) is an effective and environmentally sensitive approach to

pest management that relies on a combination of common-sense practices. IPM programs use

current, comprehensive information on the life cycles of pests and their interaction with the

environment. This information, in combination with available pest control methods, is used to

manage pest damage by the most economical means, and with the least possible hazard to

people, property, and the environment.

The IPM approach can be applied to both agricultural and non-agricultural settings, such as the

home, garden, and workplace. IPM takes advantage of all appropriate pest management options

including, but not limited to, the judicious use of pesticides. In contrast, organic food production

applies many of the same concepts as IPM but limits the use of pesticides to those that are

produced from natural sources, as opposed to synthetic chemicals.15

It should be obvious that this scheme is environmentally sensitive and in many locations it could be an

important component of Integrated Pest Management for mosquito control.

This scheme could significantly reduce the need for insecticides and larvicides. It will also save money by

reducing the need for larvicide/insecticide-spraying planes, helicopters, boats, trucks, and ATV’s

including the manpower required to operate and maintain this equipment.

The costs associated with the design and construction of this insidious drainage system could be

considerable, however once it is operational, it has virtually no operating expense. The life-cycle costs

could be very attractive and ultimately free-up resources for other vector control strategies.

If integrated pest management can be made more effective and this results in mosquito populations

below the disease transmission threshold level, substantial benefits will be accrued in the form of lives

saved, reduced medical expenses, and increased productivity.

Miscellaneous Details

This drainage system can be scaled-up to cover a larger area by increasing the size and number of flap-

gated culverts and by increasing the width of the canal. A larger cross sectional area will allow the canal

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U.S. EPA

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to fill and drain with a flatter slope and a greater length. Grade control structures may be required to

prevent scour in the canal, particularly as the length of the canal increases and the volume of water

required to fill the canal increases.

If the river is under high flow and is at an unusually-high stage for several days, the flap gate and VBFG

will remain closed for an extended period of time. As a result, the drainage system will not be flushed.

The drainage system will gradually fill with runoff and nuisance flooding may occur. A portable pump

with sufficient capacity could be used to de-water the drainage system periodically as needed to prevent

nuisance flooding during the high river stage event. A reduction in the concentration of mosquito larvae

in the canal would be another benefit derived from pumping. If pumping cannot be justified and

mosquito larvae are present, the application of larvicide may be sensible.

If the high river stage is anticipated, larvicide could be metered into the canal at the tide gate as the

canal fills during the rising leg of the hydrograph. This should be a very efficient and inexpensive way to

treat the drainage system.

If the adult mosquito population is effectively suppressed by the normal flushing of the drainage system

in advance of the high river stage event, two or three weeks of successful mosquito breeding in the

canal during a flood may not produce a large brood.

Some agricultural land protected by levees lies below sea level16 and relies primarily or entirely on

pumps for drainage. The drainage system bringing water to the pumps could be graded and the pumps

could be operated in a way that seduces mosquitoes to deposit their eggs. The pumps could be

operated on a schedule so as to pump out the larvae with the water before the larvae have time to

mature. A small siphon with a float control could add water to the ditches if the volume of seepage and

runoff are not sufficient to refill the ditches after they have been pumped-out.

The Basic Reduction Number

An important concept in the study of vector-borne disease is the Basic Reduction Number, or BRN. The

BRN of a disease describes the number of additional infections that an originally infected person will

generate under ideal conditions.17 Malaria can have a BRN which exceeds 100. In mosquito-borne

diseases, three factors exert a powerful influence on the equation that leads to the BRN. They are, from

least important to most important:

1. Vector abundance, or the population density of the mosquito that carries a disease.

2. Focused feeding, or the mosquitoes’ tendency to bite people and nothing else.

3. Vector Longevity. Will the mosquito live long enough to acquire a pathogen and then deliver it,

via bite, to people?

16

The Sacramento River is noticeably higher than the protected land to the SW of Sacramento. 17

Mosquito, Andrew Spielman, Sc.D., and Michael D’Antonio (Pages 96-97)

17

The control method described herein could significantly reduce mosquito populations in many locations.

By freeing up vector control equipment and manpower, this method will indirectly reduce the

abundance of container-breeding mosquitoes and mosquito populations in areas where this approach

cannot be applied.

Keeping livestock in fenced areas adjacent to the drainage canal could reduce the tendency of

mosquitoes to bite people. Many species of mosquitoes will feed on the most convenient source for a

blood meal. Screened windows and doors, personal use of insect repellents, and the use of insecticide

treated nets (ITN’s) will also reduce the number of opportunities for mosquitoes to feed on people. A

variety of control measures used in conjunction with this insidious vector control method could result in

less focused feeding, thereby reducing the BRN for malaria and other diseases in the area.

Vector longevity is not significantly decreased by this control method. Indoor residual spraying (IRS) with

DDT and the use of ITN’s are effective at reducing vector longevity for mosquitoes that prefer to bite

humans. Mosquitoes that feed exclusively on humans will not achieve longevity if a substantial portion

of the residences in an area are treated with IRS or if the mosquitoes land on ITNs. The use of IRS and/or

ITN’s – particularly during disease outbreaks - would complement the insidious vector control method

and would reduce the BRN.

When used in conjunction with other measures, this insidious new vector control method can reduce

the abundance of mosquito vectors, diminish focused feeding, reduce vector longevity, and thereby

significantly reduce the BRN for a given mosquito-borne disease.

Insecticide Resistant Mosquitoes

DDT-resistant mosquitoes are becoming more and more common in many locations – particularly in

Southeast Asia. Mosquitoes are also developing resistance to many other insecticides including those

used with ITN’s.

In consideration of the rise of insecticide-resistant mosquitoes, the use of insecticides should be used

with discretion and only if other alternatives are not viable. Overuse will hasten the day when the target

population of mosquitoes develops resistance to the insecticide being used.

Using VBFGs to produce tidal flushing in drainage systems will reduce mosquito populations and thereby

reduce the incidence of vector-borne disease. It does this without relying on pesticides such as DDT.

Malaria and the Resurgence of Vector-Borne Diseases

Malaria has reappeared in a number of countries in which it had been previously eradicated. This is

occurring primarily because effective vector and malaria control programs have deteriorated or have

18

been abandoned. Limited resources and socio-economic instability are the nearly always the key factors

when malaria & vector control efforts falter.

Plasmodium has developed resistance to anti-malaria drugs and mosquitoes have become resistant to

insecticides. These developments also contribute to malaria’s resurgence.

An increasing mosquito population is often the catalyst for a viscous cycle: More mosquitoes cause an

increase in the rate of mosquito-borne disease; this lowers productivity and increases health care

expenses - which results in a handicapped GDP. The consequence is a reduction in the revenue available

for various government functions including tasks such as maintenance of drainage infrastructure and

vector control activities - which leads to more mosquitoes, more mosquito-borne diseases, …

In many areas of the US, county and state vector control agencies have staff and equipment that require

a large annual expenditure. In 2009, county governments within the state of Florida spent an estimated

160 million dollars to provide mosquito control services18. Budget problems and brewing political

instability can subvert vector control activities, resulting in an increase in mosquito populations and,

ultimately, resurgence in vector-borne diseases.

One distinct advantage of the insidious vector control method is that VBFG’s will operate unattended for

years with perfect reliability and virtually no operation or maintenance costs. Vector control will

continue for years in spite of neglect, budget cuts, and socio-economic instability. Given the current

global economic crisis, this is an important attribute.

If mosquito density is significantly diminished to a level below the disease transmission threshold,

incidences of vector-borne disease outbreaks will be reduced and fewer anti-malaria drugs will be

dispensed. This will help to delay the rise of drug-resistant mosquito-borne viruses and parasites.

Why This Matters

Recent studies have shown that the overwhelming bulk of humanity is concentrated along or

near coasts on just 10% of the earth’s land surface. As of 1998, over half the population of the

planet — about 3.2 billion people — lives and works in a coastal strip just 200 kilometers wide

(120 miles).19

With the anticipated sea level rise attributed to Global Warming, drainage systems for populated land

near coasts and estuaries will inevitably require modifications and improvements. By using this insidious

new vector control method, these drainage modifications can be made in a way that will improve

drainage and thereby reduce nuisance flooding. This will enhance aquatic environments while

suppressing vector populations. This vector control scheme achieves this without contributing to the rise

18

STATUS OF FLORIDA STATE AID FOR MOSQUITO CONTROL (SAMC), Feb 2010 19

THE COASTAL POPULATION EXPLOSION Don Hinrichsen, United Nations consultant and author.

19

of insecticide-resistant mosquitoes or drug-resistant viruses and parasites. Given the distribution of

humanity near coasts, this will benefit a considerable fraction of the world’s population.

Mozambique, Malaria, and Tide Gates

Mozambique, a country in SE Africa, has a very high malaria rate and illustrates why this vector control

method is important. A recent nationwide survey confirmed that malaria - especially malaria caused by

plasmodium falciparum - remains endemic throughout the country and therefore represents a looming

public health problem. There are variations in the intensity of malaria transmission across different

geological and ecological settings throughout Mozambique.

In general, along the coastline and in flat terrains, malaria transmission can be categorized as

hyperendemic (high and continued incidence). The inland strata across central and southern

regions can be categorized as mesoendemic (some transmission).20

“Along the coastline and in flat terrains” is precisely where the insidious new vector control method can

be most easily implemented. Along the coastline and in flat terrain is coincidentally where malaria is

especially problematic in Mozambique – and presumably in other countries with high malaria infection

rates. If a large flood control project is ever constructed in Mozambique, the drainage infrastructure

should be designed to reduce mosquito breeding opportunities. VBFG’s could play an important role in

this.21

Vector Control is not simply a third-world problem. Hundreds of millions of dollars are spent every year

in the U.S. for vector control. Unfortunately, most Americans take for granted the benefits of the

remarkably successful American mosquito crusade from the past century.

No movement held greater consequence for Americans’ health and welfare than the “Mosquito

Crusade”. – Leland Howard, Chief Entomologist USDA (1894-1927)

Reducing mosquito populations obviously improves human health. It also improves the quality of life for

people who would otherwise be harassed by swarms of biting pests. Property values increase when

mosquito populations are reduced. A thriving tourism industry depends upon effective mosquito

control. Effective vector control is vital to the American way of life.

Vector control must be sustainable and economically viable - especially in light of the recent global

economic downturn. Implementing this insidious new vector control strategy may require a

considerable initial expenditure; however this scheme has a long design life and minimal operation and

maintenance costs. As such, over time it will free-up resources for other vector control measures such as

container source reduction, application of larvicides, biological control, education, and research.

20

The Epidemiology and Burden of Malaria in Mozambique, Samuel Jose’ Alves Mabunda, 2006 21

In 2011, USAID’s Malaria Coordinator for Mozambique was incapable of comprehending this notion.

20

Manpower, equipment and money will be available for traditional vector control in locations where this

strategy cannot be used.

Other Benefits Derived From Drainage Systems Using VBFG’s

In addition to vector control and the reduction in the transmission of mosquito-borne diseases, this

insidious vector control method provides several other benefits:

Improved drainage –VBFG’s and side-hinged flap gates open wide and have less head loss than

traditional flap gates. This allows more water to drain from the system during a given tide cycle. Not

having a trash rack also reduces system head loss and increases the volume of water that will pass on a

given tide cycle. A trash rack clogged with debris will act like a dam, creating a large body of stagnant

water. This could cause nuisance flooding in addition to mosquito breeding. Improved drainage in

agricultural areas will increase food production and lower pumping costs.

Increased Storage Volume for Runoff – The storage volume of the canal is available to hold runoff that

collects when the flap gates are closed during high river stages. If there is advance warning, the backflow

through the VBFG can be easily disabled until the event has passed. This maximizes the storage volume

available for runoff while the flap gates are closed. The water that accumulates during the high tide can

be efficiently discharged later during the brief low tide.

No trash racks – Since the VBFG’s and side-hinged flap gates open wide, debris that enters the upstream

end of the culvert will easily passes through the wide open flap gate during ebb tides. The VBFG is only

open and allowing backflow during the early stage of the flood tide. Debris could snag on the lead edge

of the open flap gate, however floating debris is not normally present during the early flood tide. If

debris somehow hangs up on the open flap gate, it will be released on the following ebb tide. Trash

racks are not needed. This reduces the operation and maintenance costs for the drainage system since

traditional top-hinged flap gates require trash racks that must be cleaned of debris at regular intervals.

Not having trash-racks also improves fish-passage which may increase biological control.

Groundwater Recharge – Filling the canal with fresh water on a regular basis helps recharge the

groundwater. A higher groundwater level reduces soil oxidation and land subsidence rates. It also makes

more ground water available for irrigation. If the water back-flowing through the VBFG is brackish,

saltwater intrusion should be monitored and backflow managed as needed. Since fresh water is less

dense than salt water, in many estuaries the fresh water flowing down-river rises and falls above a salt

wedge. The VBFG will draw the fresh water from above the salt water wedge and saltwater intrusion will

not contaminate the groundwater.

Dewatering Saturated Soil - If the soil of an agricultural area is excessively wet or waterlogged during a

critical point in the growing season, backflow could be reduced or completely curtailed for some

interval. To discharge the accumulating mosquito larvae in the canal while backflow is disabled, a

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nominal amount of backflow could be enabled and then disabled every two or three days until the soil

moisture level is suitable for the crop being grown.

Reduced Need for Irrigation – If the groundwater level is elevated such that the capillary fringe is within

the root zone of crops being grown, the plants will not require irrigation. A VBFG that allows the canal to

fill and drain with the tides will result in a higher groundwater level and a higher capillary fringe

compared to a traditional flap gate.

Environmental Benefits – Traditional flap gates effectively de-water wetlands. Replacing a traditional

flap gate with a VBFG will raise the average water level in a wetland. The regular tidal flushing afforded

by the VBFG will improve the water quality in the drainage system as well as in adjoining wetlands.

Because the VBFG opens wide, fish, waterfowl, and amphibians will have improved access to the

drainage system on the protected side of the levee. Wildlife will also benefit from the increased

biological activity in the canal and ditches. An increased in biological control of vectors is likely.

Reduced Maintenance of the Drainage System – It is not unusual for traditional flap gates to become

buried in sediment during dry periods due to sedimentation and the lack of flow through the flap gate.

Regular tidal flushing will reduce the deposition of sediment in the channel downstream from the VBFG.

The regular tidal pulse will result in an efficient ebb channel downstream from the VBFG. This will

increase overall system conveyance and improve drainage. The filling and draining of the canal on the

protected side of the levee will also suppress the growth of many types of vegetation in the canal. This

will increase the conveyance capacity of the canal and also reduce the labor associated with spraying

herbicides or mechanically removing the vegetation from the canal.

Schistosomiasis (Snail Fever) Reduction - The free-swimming miracidium parasite causes Schistosomiasis

and infects fresh-water snails. The parasites known as cercariae are the larvae capable of infecting

mammals - including humans. Both the parasites and their host snails could be effectively flushed from

the drainage system along with mosquito larvae. If the receiving body of water is not amenable to these

parasites and their snail hosts, the incidence of schistosomiasis will be reduced.

Public Relations and Environmentally Beneficial Vector Control

Vector control is frequently perceived as being environmentally unfriendly. This was true in the past

century when oil, Paris Green, and DDT were widely used to control mosquitoes. While modern control

methods are generally environmentally sound, vector control over the past century has resulted in

significant environmental impacts: filling and draining vast amounts of wetlands; pouring large

quantities of oil on swamps; the widespread use of Paris-Green (an arsenic and copper-based poison);

and the spraying of the much-maligned DDT.

This insidious vector control method will create and improve aquatic habitats. Strategically placed signs,

trails and overlooks, along with well-planned public outreach could be very useful for increasing public

awareness and support for vector control operations concurrent with environmental restoration work. A

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beautiful wetland and a thriving community of birds, fish and amphibians - courtesy of / supported by a

vector control agency - will be a wonderful backdrop for public outreach.

Vector control is vital to public health, but when there is an absence of a disease outbreak it is not

particularly glamorous and not especially trendy. Making vector control compatible with “saving the

planet” will help to preserve or increase funding in an era of falling revenues and budget cuts.

Cost Considerations

Replacing existing flap gates with VBFG’s and redesigning and significantly modifying an existing

drainage system could cost on the order of a few thousand of dollars per acre of land protected by the

levee. If a new flood control project is proposed, the incremental cost associated with including VBFG’s

and designing the drainage system to be effective for vector control would be a small fraction of the

overall project cost. When the design life of the VBFG and the operational costs are factored in, the life-

cycle cost associated with the interior drainage system will be less than the costs associated with

traditional flap gates and a basic drainage system.

The cost of a VBFG varies with the size of the culvert. A VBFG for a 48” culvert can be produced for

around $7,500. If mass-produced, the cost will be considerably less.

The culvert, headwalls, apron, and wing walls for a properly engineered, small gravity drain structure

using cast-in-place concrete will cost on the order of $100,000 to construct. I have a very novel

proprietary gravity drain design that uses corrugated high density polyethylene (HDPE) culverts, precast

concrete headwalls (which are cast around the end of HDPE culverts), and driven vinyl sheet pile wing

walls with stainless steel wales. My gravity drain is considerably less expensive than what is normally

constructed and has a longer design life. The cost savings associated with my gravity drain design will

easily pay for the higher cost of the VBFG compared to a top-hinged flap gate.

It is also important to recognize that a VBFG has lower life cycle costs than a traditional flap gate. Life-

cycle costs savings can be attributed to: not needing to clean trash racks; self-maintaining channels and

ditches (requiring less maintenance dredging); a longer useful life of the VBFG compared to a traditional

flap gate; and reduced frequency for removing vegetation from ditches.

Summary

Automatic tidal flushing using a VBFG may be a cost-effective and efficient vector control method. If the

river water level reliably rises and falls with the tides or with changing river flow at least every few days,

the flushing could be a very effective means of reducing larval vector populations. Multiple dilutions of

the water in the drainage system will decrease the larvae concentration as an inverse geometric

progression. The concentration of larvae becomes infinitesimal after a finite number of dilutions. This

vector control happens automatically without chemicals and without any human intervention.

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Vector control via water exchange using a VBFG has advantages over other means of vector control:

1. No insecticides or larvicides are required.

2. The larval kill rate via dilution could approach 100%.

3. It occurs automatically without human intervention and with no overhead.

4. Education / cooperation from the local populace is not required.

5. Improved water quality could increase biological vector control.

6. Mosquitoes are not likely to develop resistance to control using dilution.

Backflow and regular flushing of the drainage system have other benefits that are not realized with

traditional flap gates:

1. Improved water quality in the canal and drainage ditches.

2. Pollutants are diluted and dispersed.

3. Vegetative growth in the canal is suppressed.

4. Groundwater is recharged.

5. Wetland habitats on the protected side of the levee can be preserved and enhanced.

A VBFG has advantages over a conventional flap gate:

1. The side-hinged VBFG opens easily and conveys more flow.

2. A VBFG is heavy duty with a useful life of 50-100 years.

3. The design is fail-safe.

4. Since a trash rack is not needed there is no labor expended to clear debris from the trash rack.

Many populated areas and agricultural areas located on tidally-influenced rivers or near coastlines are

protected by levees. The drainage systems for these areas can be redesigned to improve drainage,

enhance the environment, and reduce mosquito populations using this insidious new vector control

method.

"To achieve elimination and eradication, we need to start with control, drive it up to very high

levels, and sustain it." – Bill Gates, October 2011

When used as part of Integrated Pest Management, this insidious scheme can be used to drive vector

control up to a very high level in an environmentally sensitive way without compromising drainage and

flood control.