The Drain Down
Brent Reservoir - A case study

Seine netting contracts are commonly undertaken in conjunction with a full or partly drained water body. However, certain situations require further, more complex measures. Occasionally reservoirs are almost completely drained for one of three reasons. Designated filler reservoirs are commonly seen to have extremely low water levels come the end of the summer. Whether for drinking water purposes or feeding canals and rivers these water bodies lose water, however, the levels are controlled and observed daily which minimises the risk to fish stocks. More commonly the stock levels are calculated with water loss in mind thus problems are rare.

The second reason for dewatering directly relates to fisheries. The easiest way to crop or catch fish or a designated species is to remove their medium, water. Reservoirs are commonly used as stock ponds, therefore they are periodically drained. Again little problems are envisaged with this procedure as the water is usually well known by the contractors or owners and the knowledge and equipment is already in place. Conversely, islands or reed beds may need adding or altering thus water levels have to be dropped. The final, and most relevant, reason behind water removal regards large-scale engineering works. Dams leak, sluices block, silt deposits accrual and weed becomes out of hand. These all require plant and machinery which indicates access to difficult areas. Water needs to be removed to carry out these jobs - some times all of it!!

So why do fishery consultants become involved with large-scale drain downs? More emphasis is put onto the environmental issues than ever before and as one can imagine a large-scale fish loss is totally unacceptable. Fish that derive from the drained water body are usually dealt with in one of two ways. They can be stored in designated areas, ponds or lakes, or they can be removed from the site, sold and then replaced when the reservoir is re-filled. The reservoir owners or their designated fishery manager takes this decision. In December 2001 MEM Fisheries Ltd undertook one of the aforementioned drain downs which has offered me a case study to explain such procedures.

Site Description
Brent Reservoir (also known as the Welsh harp) is situated at the foot of the M1 motorway in the London Borough of Brent. This disused filler reservoir is approximately 110 acres in size and is used for many water activities - ironically angling is not undertaken on the reservoir. The reservoir is owned by British Waterways with John Ellis, Fisheries and Environmental manager, BW South East over seeing the fish removal. The image below shows the immense scale of the reservoir (left) and an early morning sunset (right). I would like to thank Mr Drake, Chief Executive, Brent Business for the Environment Ltd for allowing me to use the aerial photograph of the reservoir. Mr Drake has an excellent site outlining the history and uses of Brent Reservoir which should give all the readers of this page an excellent insight into the reservoir.


On the bottom of the aerial view (click for larger picture) the reservoir dam can be seen. Just to the right of centre is the sluice tower and this is where all of the following photographs have been taken from. The reservoir is gravity drained, a more efficient and cost effective method than pumping. It was drained very slowly as the reservoir has high silt levels thus increased water flow would cause this substrate to be sent down the river. The water is deposited into the River Brent via an advanced mechanically controlled sluice system. This sluice was the area of concern and the cause of the reservoir drain down. A section needed replacing and as it was situated at the bottom of the deepest area of the reservoir all of the water was needed to be removed.

The biggest concern with any drain down is knowing quite where the water is going to end up. Now, in theory the water will always drain to the deepest end, this is true for all sites. Unfortunately many lakes and reservoirs have uneven beds that tend to leave many pools of water scattered across the area. Deep silt deposits make retrieving fish from these hold ups very difficult and very time consuming. In general the contours of the lake bed should be assessed to allow the proper planning of large scale jobs; this is easiest done with an echo sounder. Fortunately, Brent Reservoir was drained 8 years ago for similar engineering reasons thus it was known exactly where the fish would be in the final stages. Commonly though, larger waters, when drained, are being done for the first time, therefore prior knowledge is unavailable. Bad planning will result is fish loss, a totally unacceptable and unnecessary scenario.

Job Preparation
It had been decided, in collaboration with English Nature and The Environment Agency, that the fish were to be stored in two secure canal pounds situated at Hanwell on the Grand Union Canal. These pounds would be closed to boat traffic use and were alongside British Waterway's offices for extra security. The stock densities of short canal pounds are commonly low, therefore a small-scale netting and electrofishing operation was undertaken to remove the original stock. These fish were allocated to the canal above and below the secure pounds; migration is common in canals thus fish would soon return to the holding pounds when the Brent Reservoir project was completed. After this small exercise was accomplished the drain down could commence.
The Netting

At low water level the access to the dam section was difficult. A small boat was the only option as it needed pulling across the silt to the remaining water. Seine netting was the only option available to catch the fish safely and quickly. Due to the silt levels and unknown quantities of fish a relatively small 180-metre seine net was used. Large quantities of juvenile fish combined with thick gill blocking silt would spell disaster. Although the activity of the larger fish in a landed seine net will clear large quantities of silt smaller fish tend to perish, therefore it was imperative not to over catch in any one single net.

The three pictures above outline the process of laying the seine net. The net was loaded onto the boat and then laid in a semi circular shape. The sluice tower is shown in the image on the right; the majority of the netting took place to the left of this tower. These images were taken about 6 days into the operation; there was approximately a further 7 acres of water at the start of netting. The net was reasonably heavy to land and required 4 men to achieve this task. The two main problems were silt levels and the deep sump hole in front of the tower. The silt in the net had a tendency to slide down the sump causing strain and pain to those left pulling the net.

With such a steep dam coupled with large quantities of silt the net was extremely heavy when it was time for the leads to be pulled in. On occasions three people were needed to hold up the corks to stop them from being towed underneath the water surface. A benefit was the solid concrete dam that allowed excellent grip under foot. It is common to be waist deep in silt at the net landing stage. The image on the left shows the leads being pulled in whilst the net and cork line is eased up the shelf. This is the most important stage of any netting as one false move would result in total loss of fish.

The Catch
As previously mentioned Brent Reservoir was drained 8 years prior to this fish rescue. The reservoir bed was exposed to the air for 3 months with the addition of many heavy frosts. This allows the break down of silt deposits and an overall increase of fertility with regards to promotion of fauna and flora. The benefits of this were directly shown in the current yield of fish. Even with high cormorant populations the total stock level had increased by 70% to that of 7 years before. Year classes of all fish were present compared to just roach the previous time. Only small numbers of carp and tench between 8oz and 1lb 8oz were caught. These were severely damaged by cormorants, therefore the levels of these species lower year classes could have been much higher. However, the carp and tench had spawned and achieved a mild success rate. All year classes of roach were present and healthy, bream and perch were also present alongside various gold fish and ornamentals, obviously stocked by the public. There were pike into double figures and carp to over twenty pounds. The three pictures below show the fish in all their glory. Left and right are the vast quantities of silver fish yielded by the reservoir. The centre image shows one of the ornamentals, in this case a white Koi carp.

The Fish Movement
Although catching the fish was a fairly straightforward process the removal and loading was far from easy. The small obstacle of a concrete dam wall stood in the way of easy loading. The only efficient way to get fish from the water to the tanks at the top of the dam was through sheer hard work. Slings of fish, approximately 20 lbs in weight, were loaded and then carried to the base of the wall. A rope was attached to the sling and then it was manually hauled to the top where the fish could be released into the tank. The period of the drain down saw over 1000 slings of fish pulled to the top of the dam. The relatively poor water quality, mainly caused by the silt, meant that the fish needed to be removed and loaded into clean, oxygenated water as quickly as possible.
As small fish tend to reside at the surface of the net, thus easily caught, a tank was designated for 0-2+ silver fish. This would remove the additional stress caused by larger fish charging around inside a tank. Carp and larger fish travelled separately as their lower dissolved oxygen requirements meant higher quantities can be moved together. The very low water temperatures allowed up to 900lbs of fish to be transported in one tank at any one time. Providing the water is clean, the temperature is low and the correct level of oxygen coupled with the correct diffusers are used there is no reason why 1200lbs of carp (850lbs of roach) cannot be successfully transported in one tank. The fish were loaded and taken to Hanwell where they were stocked into their temporary home.

With regards to water drainage Brent Reservoir is very efficient. Although silt levels are high and access is relatively poor the water does drain to one point on the dam. No pools of any note are left, therefore all of the fish can be successfully caught with nets. Once the water had been drained to a small pool approximately 7 square metres in size only one carp was left to rescue. The continued dragging of a seine net had resulted in the successful removal of all the fish resident in the reservoir. Overall a comparatively easy drain down to complete.

Re-capturing and returning Brent's stock
The re-capture of the fish required a very different process. Small nets, 50 metres in length, were dragged along the canal where they met strategically positioned stop nets. These stop nets are used to break up a length of canal into easily manageable sections. The net is then closed to the bank and landed. This process can be seen in the photographs below. The biggest problem encountered with netting urban canals is the multitude of items thrown in by the public. Bicycles, lawn mowers umbrellas etc all need removing before successful nets can be landed. The only way to find these items is by dragging the nets, therefore continued de-snagging was required.


The pictures below show some of the fish being returned to a brim full Brent Reservoir. On the left is one of the very large carp and the right is our friend, the white Koi, returning home. Our very own Professor Parr is holding both of the fish. The centre picture illustrates one of many nets of roach and silver fish being released. Although the natural stocks require a large return of small fish many resident fish eating birds also welcomed the re-stocking of such species.

The drain down, fish removal and engineering works on Brent reservoir in 2001/02 were a resounding success. Many thousands of fish were safely stored whilst large scale works were carried out. However, the picture on the right shows a frozen Brent Reservoir on Christmas Eve proving Mother Nature will always intervene if every thing is going to plan. This weather caused a cessation in work for over two weeks. It is highly unlikely Brent will be drained again in the foreseeable future, however, if the stock continues to thrive in a similar fashion to 8 years ago Brent Reservoir will once again become a successful haven for a diverse selection of fish species and fish eating birds.

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