Lagoa dos Salgados
Key Proposals for Management
This document was written by Matt Self
This is an SPEA-RSPB joint project
For more contacts: Jose.firstname.lastname@example.org Ivan.email@example.com Or:http://www.spea.pt/MS2/ibas/35.html
(Notes from site visit 29-31 March 2005)
Lagoa dos Salgados, one of few wetlands on the western Algarve coast, is highly important for birds and has been recognised as an IBA by SPEA. Unfortunately, the site has not been designated as a SPA (under the Birds Directive) by the Portuguese government, and lobbying continues at a high level. SPA designation is unpopular, as it is seen as a hindrance to development. There are elections in 2005 and 2006, it is unlikely that this situation will change.
There has been an outcry in recent years over a proposed development on the north and west sides of the lagoon, consisting of a golf course, two hotels and many villas, a total of some 10,500 beds. SPA designation would prevent the project from going ahead. So far, the only obstacle to the developers has been an old planning law that prevents building within the historical flood limits of rivers and other low lying areas. The regional land planning agency have told the developers to work above this elevation. SPEA has also requested flexibility in the project boundaries to work with the ecological sensitivities of the site. Recent meetings have resulted in a sincere compromise from the developers to adapt their project.
Another threat to the lagoon comes from the proposed construction of a new sewage treatment works. The old works currently supplies most of the water to the lagoon, but there is no plan for the new works to include a transfer to the lagoon.
It is clear that the development will go ahead, and that SPEA and the RSPB have to achieve the best outcome within this. The Lagoa de Salgados faces an uncertain future, but there are considerable opportunities to work with the developers, Aguas de Algarve and others to get enhancement and management work carried out.
The role of this report is to consider possibilities for creative management of the site within the constraints of the impending development and potentially restricted water supply.
Summary of constraints:Potentially far less water entering the siteProximity of disturbance in the form of golf, hotels, roads etc.A reduction in the open aspect of the site on the north and west sides
Limitations on the water supply to a site are common in wetlands throughout the world, but in this case the loss of the ETAR supply would be unusually significant, as there is very infrequent flow in the Ribeira de Espiche to compensate. The normal response to water supply issues at wetlands is to maximise usage of the water available throughout the year (particularly if supply is highly seasonal) by storage of winter surplus, and to use compartmentalisation (installation of bunds or banks, and water control structures) to create concentrated areas of ideally managed land.
Key proposals for management
Ensure a supply of water from the new ETAR
This is the most fundamental of the proposals, and cannot be understated. Natural flows in the Ribeira de Espiche (and the Ribeira de Vale Rabelho) are highly intermittent and limited to winter flood events. They are unable to maintain a reasonable area of surface flooding in key periods such as the breeding season. Ideally, a flow could be routed to one of the proposed water storage areas (see below) so that it can be stored and distributed as necessary.
Presumably the new development would use considerable amounts of water, and their should be the potential to direct their discharged golf course water into the lagoon. Ideally this would go into a tertiary treatment reedbed first (see below).
Create a water storage area
A bunded area just to the north of the lagoon could be used to store water from winter rainfall and flood events. The bunds should be relatively low (eg.1.5m high) with gently shelving sides to avoid reducing sight lines, which can deter bird usage. Locating the bunded area (or indeed areas, as there could be more than one compartment) in the dry grassland area to the immediate north of the lagoon would make use of the gentle fall in land levels. Water could gravity feed into the lagoon in winter from the channels running north-south through the grassland, linked into the Ribeira de Espiche by a cross ditch at the north end (where the old farmhouse is). The storage area could also be filled or topped up by pumping from the lagoon. Soils are less sandy farther away from the dunes, and more able to retain surface water there.
Material for the bunds can come from excavation within the bunded area, to increase capacity, and also from excavation work in the core lagoon area (see below) as part of work to enhance features there.
It would not be possible for a water storage area to maintain a large area of surface water without supplementation from the new ETAR. However, the tens of thousands of cubic metres of water stored can be trickle fed to offset evaporational losses in spring, and because of the non-ETAR source, can improve water quality. The value of the water would be maximised if enhancement work was undertaken in the core lagoon area (see below).
The bunded water storage area has the potential to be a rich area for wildlife in itself. Some of the area could be planted as a reedbed, ideally if this was in a separate unit. This would be valuable marginal cover, as well as providing some screening from the new development. If suitably engineered, the reedbed would have the potential to improve water quality and be an attractive edge for the new golf course and hotel development. There could be opportunities here for the developers to create this as part of their work, to the benefit of both their project and wildlife. If the bunded area is a reasonable size (more than 2ha), it would be attractive to wildfowl when full, and to waders when the water level recedes leaving areas of newly exposed mud. The slope from the north down to the lagoon would be ideal for this, ensuring some open water and mud at a wide range of water levels. Ditch enhancement and some low ridges (c 0.5m high) would further increase the interest at a wide range of water levels.
In an ideal situation, a pipe from the new ETAR could be installed directly into the reedbed compartment for final treatment (‘polishing’), before feeding it into the main water storage area.
A valve or sluice would allow water to be gravity fed into the main lagoon, using ditches that are still present from the old rice fields. The sluice can be a very simple structure. Ideally the flows would be concentrated into a core area with enhanced features.
Enhance water features within the lagoon area
This would consist largely of enhancement of former ditches from the old rice fields, both by deepening and also reprofiling to ensure gently-shelving sides. Low bunding (less than 0.5m) would ensure that water is retained for longer, and would provide a place for the excavated spoil. Bunds of this size are almost imperceptible when viewed, and are a standard feature in many created and restored wetlands.
Water levels were high (4.00m) at the time of my visit, so it was not possible to examine features within the core lagoon area. However, the most suitable area is likely to be relatively low in the system and not adjacent to tracks and paths.
Install a pipe through the sand bar, linking lagoon and sea
At present the management of the outlet is ‘all or nothing’, based on sporadic opening of a channel using an excavator. This will still need to be done, to allow flushing of accumulated silt and debris, and to ensure an adequate discharge capacity for very large, infrequent flood events. The ingress of sea water also maintains brackish conditions in the lower lagoon, and increases overall water quality when the majority comes directly from the old, poorly-functioning ETAR.
However, a pipe will allow a more steady, small scale ingress and egress through the sand bar. This will maintain more stability of salinity, avoiding sudden changes from fresh to sea-water. It will allow outflow of small floods, a far more regular occurrence than large flood events, and a valve on the end will allow some degree of control. This is important in the current situation where the golf course people are not allowed to open the sand bar outside winter, and should be welcomed by them (and hopefully installed by them).
The water quality at the time of the visit was very poor, with a strong odour and noticeable algal growth.
Further work needed
A detailed topographical survey of the core lagoon area will be needed, to ensure suitable placement of the bunds and sluices. This will need to be done when the lagoon is drained down, and related to local datum.
Matt Self 5.5.05
Proposals for a water management system at Lagoa dos Salgados
SummaryThe area of the lagoon declines greatly when water levels fall from 4.0m to 3.75m - reinstatement of field ditches and some pool creation would maintain the interest of this area into spring and summerStorage of winter surplus rainfall would reduce dependence on water from the ETAR, improve water quality and permit larger areas to be kept wet through the springA shallow storage area with a low bund (<1m high) in the northern part of the area would increase wet habitat and allow gravity feed into lower areasA 2ha area in the northern half flooded to a maximum depth of 0.50m (mean depth 0.25m) would be able to retain lagoon levels at 3.75m into July in a typical year.
Currently, water levels in the Lagoa vary from around 4.2m in winter, down to 3.6m in mid summer. However, within these periods the levels can change rapidly due to opening of the sand bar at the lowest point, or storm events in the catchment. The effects of these water level changes are magnified by the very flat ground between the elevations of 3.75 to 4.0m. At present, when water levels drop from 4.0m to 3.75m, around 5.4ha of wetland dries out. The benefit of this area would be maintained if the current derelict ditches were reinstated (fig.1), to a bed depth of about 2.6m (above the existing gauge board), with gently shelving margins (a slope of around 30%). In addition, the junctions of ditches could be opened up into shallow pools. As water levels recede in spring, these features would retain approximately 1 metre depth of open water, and muddy edges.
There is very little water available in the Espiche catchment, and a high dependence on water from the water treatment plant nearby. This has led to some water quality problems. A solution to this is to make more use of surplus winter rainfall, which tends to be concentrated in relatively few days, leading to flash flooding of the catchment and lagoon area. Rapid rises in water levels tend to lead to the outlet channel being opened, and much of this water being lost to sea.
Storage of some winter surplus rainfall would reduce the severity of flood events and may result in less necessity to open the outlet channel. However, the primary benefit of the water would be to offset the evapotranspiration losses in and around the lagoon by a controlled gravity feed during the spring, and on into the summer if feasible. This would maintain more water in and around the lagoon in the breeding season than would otherwise be the case. The quality of the water will also be considerably better than that from the ETAR.
A water storage area would increase the wetness of the slightly higher northern part of the lagoon area during the spring, and create additional areas of wet habitat. Enhancement of ditches would not only improve water distribution (especially during low water periods), but also increase the availability of wet areas in the northern part of the lagoon area. The water storage area would not have the appearance of a reservoir (figure 1). A low bund would follow the approximate line of the 4.0m contour. The bund would only be 1.0m high, with a broad crest and gently sloping sides (less than 30%). The 400m length indicated would retain up to 50,000m3 if held to a depth of 0.5m at the bund, allowing a freeboard of 0.5m to avoid wave damage to the bund crest.
The additional features between the lagoon and the bund would create an additional 1.20ha of open water on top of the 0.93ha of the lagoon (at 3.75m). The area faces a severe water deficit for much of the year, starting in late February and increasing cululatively to 560mm by the end of June in a typical year. The 2.13ha of open water would lose 11,900m3 of water by the end of June. The bunded water storage body would lose a similar quantity over the same period. So given a full storage area in February, the water levels could be maintained at 3.75m in the lagoon and new features to the end of June using the stored water, whilst leaving 25,000m3 available for further supplementation into July and August, or as a contingency for an unusually dry year. Some of this will be lost to seepage, both to groundwater and to the sea.
The storage water calculations assume no contribution from the ETAR. Small quantities would be able to supplement the lagoon, and any quality problems would be diluted by the storage water. There is also the potential for sea water contribution through the outlet channel when it is opened in winter. ￼