Levels

Levels and Flow

Introduction

The water level in rivers is measured in meters above Ordinance Datum Newlyn, in the same way as the contours on an Ordinance Survey Map, so a water level mentioned below at 5.0 mtr would be at the same height as a 5.0 mtr contour on a map and does not indicate the depth of water.

To the south of the Land Yeo is the much larger and deeper Blind Yeo this was  built to improve drainage of the farmland between Nailsea, Tickenham and Clevedon and also allowed the development of Clevedon between the two rivers.

When the motorway was built a channel was made to the east of the motorway (The Yearling Ditch), to connect the Land Yeo into the Blind Yeo. The Blind Yeo level is typically 1.5 mtr below the Land Yeo level, therefore,  so as to prevent all the water in the Land Yeo draining into the Blind Yeo, an adjustable weir (Cooks Clyse tilting weir), was installed to control the water levels in the Land Yeo.

As the level in the Land Yeo is close to the level of some of the farm land through which it flows it can be lowered in times of heavy rainfall to improve drainage and in dry periods it is increased to raise the water table.  The normal level for the summer (Apr 1st to Dec 1st) is normally 4.8 mtr and the rest of the year the winter level is 4.6 mtr.  During periods of exceptionally heavy rainfall, levels are lowered to increase river capacity and reduce the risk of flooding.

Another Tilting Weir is incorporated into the Marshalls Field Outfall where the Land Yeo flows into Clevedon Pill, this is used to hold water in the river when low levels are set at Cooks Clyse (4.5 mtr and below). Under these conditions any flow through the town would in effect stop. Without the weir at the Outfall most of the water between the Triangle and Marshalls Field would drain into the Pill. Under normal conditions the level at the Outfall is held at about 4.3 mtr, however during periods of heavy rainfall this level can also be lowered to increase river capacity and therefore reduce the risk of flooding.

A large Flap Valve is also built into the Outfall, this prevents the sea flowing back into the river when there are high tides. Predicted spring tides can be as high as 14.4 mtr above the Kingroad Datum, this would be the equivalent of 7.9 mtr above Ordinance Datum, so in effect the tide at that height would be approx 3.6 mtrs (7.9 – 4.3) above the level in the river. Hopefully this illustrates the need for increasing the capacity by lowering levels at times of heavy rainfall and when there is the likelihood of the Outfall becoming tide locked by spring tides.

During  2012 there have been periods of exceptionally heavy rain, as a result of efforts by the Environment Agency to reduce the risk of flooding and improve drainage, levels in the Land Yeo have been lower than normal for a large part of the summer and autumn.  The following is a record of the main events affecting levels and flow in the Land Yeo.

2015

January /February  

The level expected at this time of year has been mostly steady, though periods of heavy rain raised it for a few days. Following the removal of weed and the obstruction to flow that it causes, there have been indications of silt migration downstream. For the time of year there has been a reasonable flow through the town, despite the level on 4th February being below 4.7 m (the gauge was dirty below that value), a further indication of improved depths. The Outfall level was recently noted at 4.2 m.

March / April

The river has been mostly at the normal Towards the end of March the level through the town has increased by about 10 cm following the setting of the summer level (4.8 m) at Cook’s Clyse.

May / June

During the last 2 months we have been on summer level which has been fairly steady apart from sudden drop of 10 cm on 19 May. EA could not offer an explanation for this. SE reported that the level at Cooks Clyse on 25 May was about 4.75 m (it should be 4.8 m at this time of year). However, the top of the weir was about 10 cm above the water, so the entire flow from upstream was passing through the town. The level at the outfall was noted at 4.3 m in April. Apart from the week following the drop in level the flow has been mostly good. The owner of Watercress Farm has reported that the water from the Land Yeo is being diverted to fill a fishing lake. This is being investigated by the EA enforcement officer.

July / August

Over the last 2 months there has been no significant rainfall until the last week in July and the levels were mostly normal for the summer with a reasonable flow.  Following the rain the level increased for few days, however over the week end of the 25th/26th July the level suddenly dropped by about 6” in the town and has been low with little or no flow since.  On warm days fish have been spotted struggling near the surface probably as a result of low oxygen levels. A similar drop in water level occurred about this time last year as a result of the watercress choking the river upstream, this was reported to the EA  on the 4th August.  However further investigation has revealed a water level of just over 4.7 m (should be 4.8 m) at Cooks Clyse, with the weir higher than the water preventing any flow of water down the Yearling Ditch. The flow of water towards Clevedon appears to be lower than normal, this was confirmed by viewing the flow over the weir at Tickenham Mill, not far from the church.  Several springs feed the river and the lack of rainfall would go some way to towards explaining low water levels and flow, however it does not explain the sudden drop in level that occurred over the week end of the 25th/26th.  A photograph has been taken of the river next to the Conservative Club clearly showing the low level and along with an update has been e.mailed to the EA.

There has been no feed back from the EA regarding the complaint made by the owner of Watercress Farm that water was being diverted away from Land Yeo by a neighbour to fill a fishing lake!

Various causes of the poor flow in the river were discussed with no logical explanation apparent apart from a lack of supply from upstream. A check on flow in the Nailsea area and to the west of Nailsea to be undertaken.

September / October

The flow in the river was checked at Jacklands Bridge on 13 August where there appeared to be more water flowing there than downstream at Tickenham Mill. This could indicate either a possible leakage on the elevated section to Tickenham Church or that water was being extracted for crop irrigation. The EA had investigated complaint by owner of Watercress Farm that water was being diverted from river to fill a fishing lake, but nothing was found to support this claim. Dry conditions continued until 22nd August when there was a torrential downpour which caused flash flooding in Station Road, Old Church Road and Old Street. 17 mm rain fell in as many minutes and within an hour the river level had risen by 10 cm. Concerns have been raised by the LYF that such an event could result in flood water overflowing into the river. SE had e-mailed the EA and NSC with photographs to illustrate our concerns and to highlight the importance of keeping the town section of the river clear of obstructions. (Correspondence attached). The flow through the town is currently restricted by the spread of watercress. On 13 September the level at Cooks Clyse was 4.8 m with some water flowing over the weir. Level at outfall recently noted at 4.3 m. M Fearon reported that the spring by Towerhouse Wood is still flowing and that the Moorend Spout is a SSSI (Site of Special Scientific Interest). The Drainage Board keep that area wet.

2014

February  / March / April

The storms and wet weather continued until the middle of February, with river levels in the town at times up to 5” above the norm for the time of year, along with improved maintenance of the trash screen and cleared culverts this resulted in a significant improvement to the flow.  From mid February until mid March conditions were drier and the levels returned to that normally expected during the winter period.  On April 1st the tilting weir at Cooks Clyse was raised to the summer setting of 4.8 metre, the levels in the town are now steady and have risen by about 4”, the Outfall setting was recently noted at 4.3 metre.

May / June

The river has been mostly steady since the setting of the summer level of 4.8 metres on April 1st, although it has been noted that a few unusual fluctuations when the level dropped during the 2nd week in May.  At present there is a good flow however it is likely this might be affected by weed growth during the summer.

 July / August

The weather had been much warmer for this time of year without much rain. Concerns that the rapid growth of watercress would affect the flow through the town have proven correct, the level and flow dropped until some of the weed was removed. While the situation improved for a while it deteriorated again during July mainly due to heavy weed growth between the motorway and the Yearling Ditch. The summer level has been maintained at Cooks Clyse with water continuing to flow over the weir, unlike last year at this time when it all flowed through the town.  Following torrential rain on the 1st of August the river level rapidly increased by  8”returning to normal the following day.

September

The mostly dry and settled conditions have continued through September. Following on from the weed clearing on the 19th the level by Meadow road has risen about 3 – 4”, all the flow has been going through the town with nothing going over the weir at Cooks Clyse which is set at the summer level of 4.8 m.

October / November

During October and the first part of November the summer level was maintained at Cooks Clyse, with good levels and flow through the town.  Following a period of heavy rain in the middle of November the level was dropped to the same level as set last winter. Hopefully this level will be maintained until next April.

2013

January / February/March

Conditions were more settled for most of the month with the level steady at 4.6 mtr until melting snow and rain increased the level to a max of 4.8 mtr on the 30th.

Apart from  3 days of rain, for the most part this period has been relatively dry with the river level being controlled at 4.6 mtr by the Tilting Weir at Cooks Clyse. From April 1st or soon after the Weir should be raised to control the level at 4.8 mtr for the summer period, this will have the beneficial effect of increasing the level and flow through the town.

April / May / June

Adjustments were made at Cooks Clyse on April 5th, as a result the water level in the Land Yeo near the motorway has been raised from 4.6 to 4.8 mtr increasing the amount of water flowing through the town. The relatively dry weather has continued (compared to last year) as a result the river levels have been fairly steady.

The dry weather has continued, as a result water levels have remained steady having been controlled at 4.8 mtr by the weir at Cooks Clyse.  With the steady mobilisation of silt through the town section, levels have fallen slightly this year, at the same time there has been a beneficial increase in rate of flow. The removal of debris from the trash screen in the town and weed removal have all helped this process.

July/August

As a result of increased capacity through the town section and a reduction of flow from up-stream due to the continuing dry weather, the Land Yeo Level is now slightly below the top of the weir and there is no flow over it, in other words all the flow from upstream is now going through the town and nothing is being diverted down the Yearling Ditch.

Due to relatively dry conditions in June levels remained steady with the level maintained at 4.8m at Cooks Clyst. From first week in July water stopped flowing over the weir at the 4.8 setting. At the request of the IDB the weir was raised by 10 cm and also at the outfall to hold more water in the Land Yeo. There have been a few fluctuations due to recent rain although the weir has been raised the level at Cooks Clyst is about 4.75m and the outfall 4.3m. There has been no water flowing over Cooks Clyst and all flow has been passing through the town

September / October / November

Similar dry conditions have continued, water levels have been maintained at 4.8 m or just below at Cooks Clyse, with little or no water flowing over the weir and most of the time all of it going through the town. The Outfall level has been held at 4.3 m.  The upstream level did fall approx. 15 cm for a couple of days at the beginning of Aug due to a fault in the auto control of the weir at CC.

Dry conditions and steady level continued until mid Oct, with no water flowing over Cooks Clyse. There were several down pours towards the end of the month and as a result and also due to the forecast of heavy rain for the 27/28 the levels was reduced from 4.8 to 4.5 for about a week. November 3rd there was little if no flow through the town so the weir was raised to 4.7 this was maintained for the rest of the month. Since then levels have remained steady with some flow at this setting. Operations Delivery (EA) hope that they will be able to maintain this level for the rest of the winter* providing there is no prolonged periods of wet weather. * 1st April

 December / January 

We are all aware that over the last two months we have experienced some very wet weather, following on from the down pours there have been increases in the level by Meadow Road of about 3” to 4”, dropping back to normal after a couple of days. I understand the EA have allowed the tilting weir to maintain the winter level of 4.7 m most of the time, with the main part of the channel, culverts and trash screens clear, this has resulted in a much improved flow through the town. These improvements have been hard-won over the years, it is worth recalling that a couple of years ago the river bed in parts of the town would be without water for several weeks at this time of the year.  The level at the outfall was recently noted at 4.3 m.

2012

August / September

Following torrential rain at Nailsea (4/8) levels were lowered to 4.5 to increase the Land Yeo capacity, the Blind Yeo was also lowered to improve drainage further up-stream towards Nailsea. Unfortunately with  warm weather and shallow water some fish were becoming distressed. Following a call to Operations Delivery (EA) levels were increased to 4.6 (9/8) to maintain flow in the Land Yeo, the normal summer level of 4.8 was restored a week later. In the future EA will try to hold a minimum level of 4.6 in order to maintain the flow in the Land Yeo.

Torrential rain in Clevedon (23/24) saw 75 cm fall in the early hours of the 24th and having rained most of the day on the 23rd this saw the river level rise by 25 cm to a level of 5.05 mtr (approx). Although the weir at Cooks Clyse had been lowered, as a result of the exceptional high volumes of water it took 30 hours for the river to drop back to the summer level.

October / November / December

Due to continuing rain and waterlogged fields,  a lower than normal level for the time of the year was maintained at 4.6 mtr.

A level of 4.6 was maintained through much of Nov until another torrential downpour occurred on the 21st, the level increased to about 5.0mtr before falling to 4.5 mtr as the flood water was released through the Yearling ditch into the Blind Yeo, a 4.5 mtr level was then maintained until the end of the month.

At the beginning of Dec the level would normally be dropped from 4.8 to 4.6 mtr, this year it was raised from 4.5 to 4.6 mtr. This level was maintained for most of the month except for a few days before Christmas when further heavy rain caused levels to rise to a max of 4.85 mtr and after falling for a while increased again to 4.75 on the 30th.

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