Drainage
Most of the river is classified as a main river, and is therefore the responsibility of the Environment Agency. Below Narborough, the river acts as a highland carrier, as it is constrained by flood banks and the water level may be above the level of the surrounding land. Drainage from that land into the river is not, therefore, possible by gravity, and an area of 25.6 square miles (66 km2) is managed by the East of Ouse, Polver and Nar Internal Drainage Board (IDB), who maintain drainage channels and pumping stations to mitigate the risk of flooding. Their area of responsibility, of which 87 per cent is agricultural land, includes most of the Nar Valley. The catchment that supplies this area is much larger, at 46.89 square miles (121.4 km2), as water flows from the higher ground around its edges. Because of the low-lying nature of the area, a network of drainage ditches feeds into larger drains, and the IDB maintains seven pumping stations, to discharge water into the main rivers. Six of these are at Polver, Nar Valley, Mill Fen, Mow Fen, Chain Bridge and Saddlebow. The Puny Drain formerly ran to a gravity outfall sluice in King's Lynn, but has been re-routed to discharge into the Ouse Relief Channel.
The Polver Drain runs along the south side of the river, and continues to the Ouse Relief Channel, where there is a pumping station. The Puny Drain runs to the north of the river, and turns to the north near Setchey. It formerly ran to a syphon which carried it under the Nar just upstream of the 1884 sluice, and ended at a gravity outlet on the bank of the Ouse. As part of the redevelopment of the area to the south of King's Lynn, the drain was re-routed to a new outfall some 2.5 miles (4 km) further upstream. This involved the construction of a new channel from West Winch Common to near Saddlebow Bridge. The route had to cross the main railway line into King's Lynn and the River Nar. To accommodate this, syphons were constructed under both. For the railway, a 25-foot (7.6 m) shaft was constructed on each side of the track to a depth of 39 feet (12 m), and two pipes connected the shafts, running around 33 feet (10 m) below the level of the tracks. A culvert 7 feet (2.1 m) in diameter was constructed beneath the Nar, at a depth of around 20 feet (6 m), and 13 feet (4 m) below the bed of the river. A new pumping station, consisting of three pumps and located at the inland end of the syphons, is capable of pumping 76M gallons (345 Ml) per day. The design uses syphon breaking valves to reduce the size of the pumps required, and hence the power consumption.
The scheme cost £5.5 million, and was achieved over an 18-month period starting in September 2006, after two years of negotiation with Railtrack to get the design for the tunnel under the railway approved. Around 1 mile (1.6 km) of new drain were constructed and 1.2 miles (1.9 km) of the existing drain were regraded. The project was complicated by the fact that much of the site it within the River Nar Site of Special Scientific Interest, and had to be approved by Natural England. The work was done in small sections to mitigate the impact on badgers, water voles and other wildlife. The final section of the original route, from the syphon under the River Nar to the Great Ouse, has since been filled in as it ran through the Nar Ouse Regeneration Area, and the first building to be built on the reclaimed land was St Michael's Primary School.
The construction of the flood diversion channel in 2001, to discharge excess water into the Ouse Flood Relief Channel, was part of a larger scheme, which included the raising of flood banks further upstream, and the provision of a flood storage area. To allow for the drainage of surface water from the Nar Ouse Regeneration Area, the channel below the bridge on Wisbech Road has been enlarged. Part of Blubberhouse Creek has been re-excavated, to create a flood relief basin. Although it will not normally contain any water, in storm conditions where high water levels in the Ouse prevent the discharge of water from the Nar, the basin may be filled to a depth of up to 2.6 feet (0.79 m), and will drain back into the river once tide levels fall, normally after 5 to 6 hours. This work was completed in April 2011.
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