Preparing for El Niño1 December 2002
Thanks to El Nino effects at the lower Piura scheme, estimates of the 100-year and 250-year design flood at the Chira-Piura project were far too low. César Alvarado Ancieta explains how the project is being upgraded
IN THE past thirty years El Niño has caused three severe floods in the Lower Piura area and Piura city. Urban areas and farmland were flooded and bridges collapsed, among other problems. The water level of the high floods in 1972, 1983 and 1998 have provided benchmarks for flood protection works in Piura as well as the river downstream.
The floods are caused by heavy rainfall in March, April and May and they cannot be managed by reservoirs in the Piura river or its tributaries. What is more, flooding seems to have increased during recent El Nino events and potential future flood damage is high, so existing flood protection works must be improved. The result will be flood forecasting, a warning system and preventive measures.
The Peruvian government carried out studies to improve,
refurbish and strengthen important hydraulic structures and flood protection schemes situated in the Piura Valley. One of the measures will be refurbishment of the most important hydraulic structure in the Piura river, the Los Ejidos diversion dam spillway.
The objectives of this dam are water diversion for irrigation and flood control management during large events like the El Niño phenomena.
It is likely to be severely tested in the coming months, since flooding has been forecast in northern Peru at the end of 2002 and the beginning of 2003, as a consequence of El Niño.
The Los Ejidos dam
The Los Ejidos diversion dam is part of the complex Chira-Piura project, which is in its final stages after more than 30 years of continuous engineering service - consulting and construction - from national and foreign companies. The Los Ejidos diversion dam spillway started operating in 1985 to divert water flows from the Piura river to the main irrigation canal for the Lower Piura area. This 40000ha valley traditionally produces cotton, rice and fruit and is characterised by subtropical weather with an average temperature of 25ºC.
The dam is 4km upstream of Piura, a city of around 0.75M inhabitants situated 1000km north of Lima. Its main purposes are water diversion for irrigation (up to 60m3/sec) and flood control management (up to 3200m3/sec) . It comprises a canal intake with three radial gates, a sluiceway with two radial gates, an overflow-type gated spillway with seven radial gates and a weir 120m long, ranged from left to right across the river.. Each structure has a stilling basin and the last has baffle blocks.
At the right abutment is located a training levee and at the left buttress an abutment wall.
The Piura river drains a part of the northern Coastal Andes Mountains and flows into the lagoon Ramon. The former main channel, now reduced, branches off 21km downstream of Piura city to flow into the Pacific Ocean at Sechura. The catchment area is around 7740km2.
From the main upper basin to Piura, the river stretches 180km. Long reaches are in a natural condition with large flood plains. The highest elevation of the watershed is close to 3600m asl, but in total only 34% of the catchment lies above 500m asl and less than 7% is higher than 2000m asl. The average slope ranges from 0.2% in Lower Piura river to 0.8% in the main upper basin. The bed material in the upper part is gravel and boulder, and in the lower sections is various sand fractions.
The initial design flow for the diversion dam was 2000m3/sec. Construction was completed in January 1983 without setting the gates but in May, four months after the concrete works were completed, they suffered a maximum event. As a result of El Niño, a flood abovethe initial flood design of 2000m3/sec was routed through the diversion dam and as consequence the weir was damaged and destroyed. Afterwards it was found that water levels downstream of the diversion dam were lower than expected because of the scouring of the river bed.
In 1984 the diversion dam was refurbished, considering a design flood of 3200m3/sec and assuming that water levels downstream of the dam were now 3.5m below the previous level.
The El Niño phenomenon appeared again in summer 1998.
It reached a maximum flood of between 3200m3/sec and 3500m3/sec, without damaging the diversion dam, but produced deep scouring downstream of the dam, scarring the rockfill bottom protection and removing it altogether between the sluiceway and gated spillway.
Between December 1999 and January 2000 further improvement works were completed to strengthen the rockfill.
Between 1985 and 1997 sedimentation upstream of the diversion dam occurred as had been conceived in the initial designs. After the 1997-1998 El Niño phenomenon, the sedimentation upstream of the dam is considered highest and as a result the capacity of the storage was reduced significantly.
A scouring at the rockfill bottom protection zone located downstream of the dam is considered disadvantageous and the floods recorded are nearly of the 100 year flood period - 3200m3/sec. In order to meet the new requirements established for dam safety according to the icold reports, a new flood design was needed.
An extensive study to refurbish the diversion dam and further physical models - 2D and 3D - were constructed to test hydraulic design, taking into consideration the need to strengthen the dam, and to avoid sedimentation process and scouring.
The diversion dam will be refurbished for a flood discharge of 3750m3/sec to 4700m3/sec. The main project objectives are:
• To reduce sedimentation upstream.
• To increase its flow routing capacity by setting of gate panels supported downstream by inflatable air bladders at the weir prior to a reduction from its crest level and therefore improving its specific routing discharge.
• To avoid scouring downstream at the rockfill bottom protection.
• To elaborate an adequate programme for gate operation management.
A 100-year design flood of 3200m3/sec was considered too low, considering that the last floods from the 1998 El Niño were around 3200m3/sec and 3500m3/sec. That means that a flood of 3200m3/sec would have a return period of 80 years. The new 100-year design flood will be 3750m3/sec.
A 4700m3/sec 250-year design flood was chosen as a maximum flood design routing by the diversion dam. It assumes a flow of 25% above the 100-year design flood of 3750m3/sec.
The Los Ejidos dam is located at the end of a curve of Piura river of around 500m. The river bed is right/rectangular downstream but with a curvature upstream. With the gates open, the dam has a single cross section with three steps, formed (from the left to the right river bank) by the sluiceway, the gated spillway and the weir. The downstream cross section at the river has two steps at elevations 19 and 22m asl.
This cross section would be adequate if the dam settlement was at the curve of the river, but at the expected water level for a design-flood of 2000m3/sec the dam's position is at the end of a radius of 500m. The flow routed by the diversion dam with full flow at this location allows a flow release similar to the expected flow distribution if the dam was placed inside the curve of the river. Under these conditions there is scouring at the left river bank and sedimentation at the right river bank.
With these assumptions it was determined that the diversion dam forces the flow at the curve of the river and as a consequence there is sedimentation at the interior of the right river bank and scouring at the external left river bank.
Therefore, a reduction of the crest of the weir from 30.7m asl to 29.7m asl gives an effective flow distribution. Gate operation management, particularly during low floods, will facilitate the flushing of sediments at the right bank and avoid scouring at the left bank of the river.
If the flow distribution allows sedimentation at the highest rate the freeboard under maximum floods conditions is inadequate and is low enough to affect dam safety.
Lowering the crest of the weir and installing flap gates supported downstream by inflatable air bladders makes it easier to flush the sediments, using appropriate gate management. As a result the water storage capacity is increased, and the water level is improved under the highest flood conditions, improving the available freeboard and dam safety. The diversion dam will then be able to route the maximum flood design.
Since storage at Los Ejidos is short, the velocities during maximum events are high and flushed sediments accumulate at the right river bank.
The sedimentation process increases the risk to the diversion dam in the case of a maximum flood because of the reduction in freeboard and storage. The initial designs from 1981 considered sedimentation, especially at the right river bank, but it was not expected that it would occur after less than 15 years.
Recent floods, both from the El Niño events and regular flooding (which can produce water flows between 2000m3/sec and 2500m3/sec) have been reducing storage volumes continuously and in a short time.
In 1985 the impounded volume at Los Ejidos reservoir for a normal water elevation of 30.5m asl was around 5Mm3. After 15 years of operation the impounded volume for the same water elevation has been reduced lower than 1Mm3 - an 80% reduction. This comparison alone is enough to show the critical situation of the reservoir.
Gate operation management
Tests on physical models and hydraulic computations both recommend that the objectives of the gate operation regime should be water diversion and flood management. A water management plan is required for river flows below and above 900m3/sec, along with an operation plan for maintenance.
For river flow below 900m3/sec, management for water diversion and flood control assumes a normal water level at the storage of 30.5m asl. River flow up to 350m3/sec can be managed by gate operation of the sluiceway. Higher flows require management of the spillway or flap gates at the weir, which facilitate the flushing of sediments accumulated upstream. River flow above 450m3/sec requires the operation of the gated spillway.
For flow above 900m3/sec management is for water diversion and flood control, assuming water levels in the reservoir above 30.5m asl. At this flow rate it is impossible to maintain normal water level in storage and accurate gate operation depends on control of the sluiceway and the gated spillway.
An operation plan for maintenance is required for river flow below 100m3/sec during dry and flood seasons. This involves occasional operation of the sluiceway and spillway gates. In the case of the flap gates located at the weir, this includes deflation of the air bladders that support the gate panels. If river flow is below 100m3/sec the water level in the reservoir is 29.7m asl.
The rockfill bottom is intended to protect the dam foot from scouring. Immediately downstream of the diversion dam there are stilling basins constructed before the 1983 El Niño event for a design flood of 2000m3/sec.
The stilling basins from the sluiceway and the gated spillway are appropriated for a Froude Number of 4.5. However, in the redesigned project the Froude Numbers are below 3 because the 100-year design flood has been increased to 3750m3/sec. The dissipaters are therefore not effective for the amount of water released by the sluiceway and gated spillway.
There are two phenomena that occur downstream of the dam. First, a general scour, which is due to the dam location and its behaviour. In this case the ground level of the river bed descends from too far downstream up to reach the foot of the dam. The other phenomenon is a local scour at the dam, which results from the flow behaviour at the dam foot, which happens immediately downstream of the stilling pools from the sluiceway and gated spillway and the baffled apron drop from the weir. This erosion produces a deep scouring.
To address these problems the rockfill must be modified. Its area, size and weight are increased as shown.
Testing the rockfill bottom protection suggests that for an appropriate strengthening of the rockfill dimensions must be increased by 50m above the original 1984 design.
Water surface profiles
Water surface profiles obtained upstream of the diversion dam by a hydraulic computational model were in agreement with those obtained from investigations of hydraulic model tests. Important factors for the backwater surface profiles were roughness coefficients considering the cross sections, type of soil, vegetation conditions, river morphology, river alignment, zones of erosion and sedimentation, river bed shape, water levels during flood and dry seasons and sediment material characteristics.
The water level immediately upstream of the diversion dam reaches 34.5m asl for a 250-year design flood. The influence of the diversion dam was expected to be felt up to 4km upstream when it was designed in 1984 but in fact its influence is only 2.5km.
Schedule of works
As mentioned earlier,Piura river has its season of heavy floods between the months of January and May, while the rest of the year is considered a dry season. For this reason the main works are scheduled for completion in a timeframe of seven months, from the middle of May. During this period the owner will complete work related to river training at the left river bank upstream of the weir, to facilitate the flushing of sediments, and the installation of the spillway gates at the crest of the weir. Supply and assembly of equipment will be within this timeframe.
The project is expected to cost some US$5M. The investigations and studies were carried out by a consortium of German and a Peruvian Consulting Engineering Companies: the Lima office of CES Consulting Engineers Salzgitter GmbH, and Lima-based Ortega & CLASS Consultores Latinoamericanos Asociados.