Rotational Impoundment Management (RIM) is a technique that provides for source reduction mosquito control and the improvement of fish and wildlife habitat and water quality. It is a working compromise between totally open impoundments, where flooding is not possible, and totally closed impoundments, where many of the natural marsh functions are eliminated. RIM impoundments are minimally flooded from the late spring to early fall to prevent salt-marsh mosquito oviposition while protecting desirable vegetation. The impoundment is left open or partially open to the estuary for the remainder of the year to allow water, nutrient and organism exchange between the impoundment and estuary, and normal marsh drying. RIM water control structures may include: 1) flapgated culverts installed through the impoundment dike, and 2) a permanent or dedicated pump. General guidelines for RIM management have been developed by SOMM.

Water Control Structures.

CULVERTS. Research has shown that, to approach natural conditions, culverts should be made of a non-corroding material (most commonly aluminum), should be a minimum of 30 in. diameter, and be equipped with a structure that will allow trapping of tidal water when closed and passage of water when open. Culverts should be spaced at 500 m intervals or less and should be placed where natural tidal creeks, basins or sloughs intersect the dike. Invert elevations should be set at -1.0 ft. NGVD (National Geodetic Vertical Datum).

PUMPS. A permanent or dedicated pump is necessary for optimal water level manipulations. Pumps can be electric or diesel, with automatic or manual operation.

RIM Variations:

RIM is flexible, allowing for the incorporation of new techniques or management findings that prove beneficial. Some variations to a standard RIM protocol are discussed below.

BLEED DOWN OR BOTTOM WATER RELEASE STRUCTURES. During the summer, when culverts are closed, water quality in some impoundments can deteriorate, particularly in deep perimeter ditches. In such situations, water control structures that allow a slow release of bottom waters have usefulness in improving water quality within the impoundment.

PERIODIC DRAWDOWNS DURING THE CLOSED SUMMER PERIOD. Impoundments can be drained for a short duration during the normally flooded summer period to allow marsh soils to dry and oxidize. This technique may be used to reduce hydrogen sulfide build-up in marsh soils.

CONSTANT BLEED. This technique requires pumping of estuarine water into the impoundment in excess of what is necessary for mosquito control. Excess water is allowed to either spill over riser structures or escape through bottom water release structures thus creating a flushing effect. Flushing may affect the whole impoundment, or may only be effective throughout the perimeter ditch.

DITCHING. In some instances, shallow ditches constructed by hand or with a rotary ditcher can extend the open RIM period allowing the marsh to provide more natural functions. By connecting salt-marsh mosquito breeding areas to deeper ponds or ditches, mosquito production can sometimes be abated by improving drainage and/or allowing larvivorous fish improved high marsh access. However, ditching has the possibility to promote marsh drainage and may increase habitat for sandfly production (Culicoides spp.).

SPECIFIC SPECIES MANAGEMENT. Variations of the standard RIM technique may be incorporated to manage for an individual or group of animal species. An example is a winter flood period with gradual water level drawdown to concentrate fish in shallow waters, thus providing improved feeding possibilities for wading birds.

Advantages:

1) Many natural marsh functions are re-established (e.g., access to the marsh for transient organisms).

2) Allows for seasonal source reduction mosquito control.

3) By flooding to only minimal levels during the summer months with a winter dry-down, indigenous marsh vegetation has a better opportunity to survive and/or re-establish.

4) The technique is economical.

Disadvantages:

1) Because the marsh is not flooded year-round, some larvicide applications may be necessary.

2) Flooding the marsh during the summer months is not a natural occurrence in this area and thus may cause problems with water quality and problems to some indigenous flora and fauna (e.g., displacement of Uca populations from the marsh surface).

II.A.4 SEASONAL DRAWDOWN

Seasonal drawdowns take advantage of the water management capabilities of impounded marshes to enhance habitat utilization by groups of species, to promote growth of desirable food plants, and to maximize management opportunities by following seasonal management needs.

The exact schedule of culvert opening and closing, and the time periods during which impoundments are in a drawdown stage can vary considerably depending upon local conditions. During open portions of the cycles, the opening can be complete or partial, and may be continuous or alternating. Some likely drawdown scenarios are described below.

SUMMER DRAWDOWN

Summer drawdown management is a modified RIM technique that provides increased wading bird foraging habitat. The impoundment or cell of an impoundment is periodically drained during the summer months by opening the culverts and allowing water levels to fall over a period of 11 to 17 days to the level of the lagoon, which essentially drains the marsh. Later, a high tide can be trapped once the culverts are closed.

The summer months are particularly important for wading birds in coastal systems. During this time, water levels are naturally low, often nearly exposing seagrass beds, and coastal marshes are under low water levels and periodic natural drawdowns. Manipulated summer drawdowns may be equivalent to the historical conditions when the low water levels associated with the summer months may have provided an important foraging habitat for post-breeding and juvenile wading birds, classes that are often ignored in management plans due to their dispersed population structure.

Water Control Structures:

CULVERTS. Research has shown that culverts used for RIM management may be used to facilitate summer drawdowns.

PUMPS. A permanent or dedicated pump is desirable for optimal water level manipulation. Pumps can be electrical or diesel, with automatic or manual operation. However, studies have shown that in certain areas of the lagoon, the lunar tidal cycle is often sufficient to flood and drawdown impoundments.

Hydrological Regime:

Drawdown cycles begin with the flooding of the impoundment by entrapment of high tidal water associated with a new moon. The culverts are then closed and the impoundment remains flooded for approximately two weeks. Before the start of the cycle, impoundments can be pumped high for over one month. This high water period is referred to as the "pre-drawdown condition".

The exact schedule of culvert opening and closing, and the time periods during which the impoundment is in a drawdown stage can vary considerably depending upon local conditions. A typical scenario is described below.

The drawdown begins by partially opening the culverts at high tide during full moon, when estuarine water level is expected to be at the second highest point of the monthly lunar cycle. The timing is important to assure that the impoundment is drawn down gradually. After one week, the entire flapgate is opened, thus releasing the maximum amount of water from the impoundment. When the water in the impoundment reaches its lowest point, just after opening the entire flapgate, the impoundment is said to be in its maximum drawdown (max-drawdown) condition.

Drawdown impoundments usually remain open for two weeks (flapgate partially open for 1 week, entire flapgate open for 1 week) until the next new moon. The final stage involves catching the next high tide and closing the culverts. After this, the impoundment can be left flooded, or a new cycle can be started.

Faunal Management:

Experimental drawdown studies have shown that Great Egrets were more common during drawdowns while Snowy Egret numbers peaked at drawdown, but the species was rarely observed when the impoundment was closed and water level was high. Although quantitative data are limited, Wood Stork responses to the drawdowns paralleled those of the Great and Snowy Egrets.

Adult and juvenile White Ibis were present in high numbers in the impoundments under all water level conditions. The relationship between drawdown conditions and number of Roseate Spoonbills is far less clear. The ground survey data available do not show any increase in the number of Roseate Spoonbills in response to drawdowns; however, aereal census data appear to contradict this, with significant increases in numbers detected at the time of maximum drawdown. Other observations suggest that large number of Roseate Spoonbills and Great Egrets feed in the lagoon at low tide rather than in the impoundments.

Advantages:

Wading birds respond very favorably, at the regional scale, to drawdown conditions. When drawdowns are available, the average number of birds per acre for drawdown impoundments has been found to be up to twenty times higher than the average number of birds per acre in RIM impoundments, and thirty times higher than in open impoundments.

Disadvantages:

There may be individual species that are negatively impacted by this hydrological regime.

END OF SUMMER DRAWDOWN

RIM impoundments that have been flooded during the entire summer can follow the same drawdown pattern when they are opened to the lagoon in the fall. If the impoundments are opened before the fall high tides, drawdowns can occur and thus be beneficial to wading birds. If the impoundments are opened after the fall high tides, the drawdown effect will not be as significant.

Impoundment blocks may be opened rotationally before the high tides impact the system. For example, two impoundments may begin bottom water release on July 15, two on July 29 and two on August 12, etc., thus increasing the foraging habitat available to wading birds and shore birds. This approach may be more advantageous than opening all of the impoundments in a particular county during the same week.

FALL/WINTER DRAWDOWN

This strategy is used mainly in the Merritt Island National Wildlife Refuge to promote growth of food plants for migratory birds. It involves summer flooding, with a fall/winter drawdown and a spring opening. It is often implemented in a cyclic manner, with one to several years of fall/winter drawdown alternating with one to several years when the impoundment is left open all year. The exact opening, closing and drawdown schedule varies depending upon lagoon water levels, rainfall, and physical conditions within the impoundment.

Advantages:

Results in good food production and habitat for migratory birds and waterfowl.

Disadvantages:

Access to the marsh is restricted during most of the year.

WINTER FLOODING/SPRING DRAWDOWN

This technique was developed originally to improve feeding opportunities for wood storks during their nesting season. Under this protocol, the impoundment is flooded by pumping in January and holding the water up for a minimum of 60 days, followed by a gradual drawdown. The flooding period allows for the production of resident fish, which are then concentrated as the water levels recede, thus improving feeding conditions for wading birds. This technique is most appropriate in the vicinity of rookeries.

Advantages:

Provides food and suitable feeding conditions for birds during the nesting period, when forage requirements are greatest.

Disadvantages:

1) Marshes are flooded during a time of the year when they would normally be dry.

2) Increases cost of management.

3) If not timed correctly may interfere with natural bird feeding opportunities at start of winter.

PRE-SUMMER DRAWDOWN

This techniques involves drawing down a flooded impoundment for a minimum of two weeks in late spring or early summer. During draw-down, moist soil conditions promote re-seeding of Ruppia maritima, an important waterfowl food plant. The impoundment is then gradually re-flooded to encourage growth of the plants. To encourage waterfowl use, the impoundment remains flooded until late spring.

Advantages:

Good food production and habitat for waterfowl.

Disadvantages:

Access to the marsh is restricted during most of the year.