wimbledon park lake seas... · (recommendation under section 12(3) for an inspection under section...

WIMBLEDON PARK LAKE

LONDON, SW19

Supervising Engineer's Annual Statement under Section 12(2) & Section 12(2A) of the Reservoirs Act 1975

Period up to 31st December 2018

Wimbledon Park Lake Supervising Engineer’s Annual Statement London SW19 Period up to 31st December 2018

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1 General Information

1.1 Reservoir name

Wimbledon Park Lake

1.2 Reservoir location

TQ 247 724

1.3 Name & address of Undertaker(s)

London Borough of Merton Merton Civic Centre London Road Morden Surrey SM4 5DX

Tom Sly 020 8545 3899 / 07739 341885

1.4 Name & address of Supervising Engineer

David Littlemore Stillwater Associates Limited Alma House – Suite 3 Alma Road Reigate Surrey RH2 0AX

Office: 01737 768237 Home: 01737 270301 Mobile: 07740 432292 Email: [email protected]

1.5 Alternative Supervising Engineer contact details

Alan Brown Stillwater Associates Limited

Office: 01737 768237 Home: 01189 467832 Mobile: 07769 176184 Email: [email protected]

If during an emergency the Supervising Engineer and alternative engineer named above cannot be contacted please call the Stillwater Associates main office telephone number (01737 768237), which will be answered 24 hours a day, 7 days a week.

1.6 Name of last Inspecting Engineer

Henry Hewlett


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2 Status & Findings

2.1 Inspection under Section 10(2)

Date of last Section 10 Inspection 5th November 2013

Date of last Section 10 Inspection Report 22nd April 2014

Date of last Section 10(5) certificate 22nd April 2014

Number of measures to be taken in the interests of safety Four

Date of Section 10(6) certificate confirming completion of measures in the interests of safety

Partial Section 10(6) Certificates were issued on:

25th November 2014

10th December 2015.

30th April 2018Recommendation ii) requires completion.

Date of next scheduled inspection under Section 10(2) of the Act By 5th November 2023

Does the Supervising Engineer consider that a Section 10 Inspection is required at this time?

(Recommendation under Section 12(3) for an inspection under Section 10(2))

No

If ‘Yes’ state reasons:

2.2 Recommendations as to measures to be taken in the interests of safety under Section 10(3)(c) of the Act

No. Recommendation by last Inspecting Engineer Status

i) Studies be carried out to enable categorisation of the dam in accordance with ‘Floods and Reservoir Safety – 4th Edition’ (due to be published late 2014). These should include a dam break analysis of the peak breach flow combined with the Probable Maximum Flood to assess the routing effect of the railway and its underbridge on peak flows and depths, the number of properties inundated and the extent of flooding of the railway embankment;

Completed. An initial assessment of the storage available behind the railway embankment has been made. This assessment indicates that the reservoir should be categorised as ‘High Risk’ (Category A).

Partial Section 10(6) Certificate was issued by Henry Hewlett on 9th December 2015.


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ii) A flood study be carried out in accordance with the latest methodology to determine the effect that the design rainfall event for the appropriate dam category could have on reservoir levels and options for improvements in flood capacity. The need for improvements in flood capacity and timescale for implementation shall be specified by an AR Panel Engineer; determined to reduce risk as low as reasonably practicable (ALARP).

Partially Complete.

A flood study has been completed and certified by Dr Andrew Hughes on 30th April 2018.

The flood study indicates that there is a shortfall in spillway capacity.

The QCE has recommended that the timescale for implementation of improvements should be within 3 years of the completion of the options report for the improvements i.e. by the 15th January 2022.

iii) The stoplog guides be modified to ensure that no additional stoplogs can be installed to raise the water level in the reservoir above 17.47m OD;

Completed. Partial Section 10(6) Certificate was issued by Henry Hewlett on 25th November 2014.

iv) The eroded area downstream of a gate in the timber fence at the left end of the embankment be reinstated.

Completed. Partial Section 10(6) Certificate was issued by Henry Hewlett on 25th November 2014.

2.3 Recommendations as to matters of maintenance under Section 10(3)(b)

No. Recommendation by last Inspecting Engineer Status*

No recommendations were made under this heading

2.4 Other recommendations made by the Inspecting Engineer

2.4.1 Other measures recommended to be taken but not requiring supervision by a Qualified Civil Engineer within the meaning of the Act


i) A capping beam be installed to the sheet piling at the right end of the embankment, with the level of the ground behind being raised to the level of the top of the sheet piles;

Not started.

ii) The embankment crest immediately to the left of the fishing club compound should be raised to at least the capping level of the sheet piles with a good grass cover or other erosion resistant surface being established;

Not started. Previously pond weed and spare turf have been added to build up the crest level but more work is needed in this area.

iii) The vegetation on the embankment in the fishing club compound be strimmed at least once a year to enable observations for movement and seepage to be carried out;

No vegetation clearance on the embankment has been undertaken in this period. Observations are very poor in this area.


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iv) A good grass cover be established and maintained on the central and right sections of the embankment, with the grass being maintained no longer than 100mm;

Some limited grass cutting has been undertaken on the central section of the embankment but no works undertaken on the right end of the embankment within the fishing club section.

v) Gaps made in the fence on the downstream side of the embankment crest in the vicinity of the ‘Waterfall Garden’ be repaired;

The gaps in the fence have been patched up and this has significantly reduced the amount of deterioration of the downstream face.

vi) The trees on the embankment be inspected by a tree specialist at least every three years to identify where management is required to minimise the risk of trees falling over and maintain them in a healthy condition. Any management recommended by the tree specialist should be implemented;

Routine inspections of the trees are reportedly made by the council on a 3 year cycle.

Furthermore, an independent tree survey was undertaken and this is published on the Wimbledon Park Lake project webpage.

vii) The area upstream of the overflow weir be maintained clear of any debris that could inhibit flows;

At the time of this site visit the Fishing Club had erected timber posts with netting in-between within the reservoir. This barrier was located approx. 10m to 15m upstream of the weir. This structure has since been removed.

viii) The overflow channel is maintained clear of any debris which could impede flows;

The channel was clear of debris at the time of this site visit.

ix) The screening arrangements at the upstream end of the stilling pool discharge culvert be modified, with a new screen being installed at the culvert inlet headwall and a new primary screen provided that can be easily cleared;

Not started.

x) The area upstream of the stilling pool culvert be maintained clear of debris;

Marginal vegetation has started to encroach into the pool and should be removed.

xi) A CCTV survey of the 18in diameter outlet pipe from the stilling pool be undertaken, with any problems identified being addressed;

Not started. The council have reported that these works will be undertaken in early Spring.

xii) The 100mm diameter valve on the outlet pipe be operated over its full range at least once a year and any blockages in the pipe are cleared;

An attempt to operate the valve was made during this visit, however, no flow was observed from the downstream end of the outlet pipe. Works required to clear blockage.

xiii) The valves on the pipe feeding the ‘Waterfall Garden’ be fully operated at least once a year;

Not operated during this visit The valve is regularly operated by the park staff. No problems have been reported.


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xiv) An on-site emergency plan be prepared by the Undertakers covering actions to be taken in the event of a structural problem with the dam.

Not started.

2.4.2 Measures recommended in the interests of maintenance and safety of personnel/public.


No recommendations were made under this heading

2.5 Matters to be watched by Supervising Engineer under Section 10(4)


i) The Supervising Engineer should visit the site at least once a year.

One visit made.

ii) Watch for any signs of settlement, leakage or movement; None observed

iii) Observe the condition of the embankment crest track and instruct any necessary repairs;

The condition of the embankment crest track is slowly deteriorating.

iv) Watch for any damage caused to the embankment by vehicles, humans or animals;

None observed.

v) The condition of screens upstream of the stilling pool and railway culverts;

Both screens were clear at the time of the visit. Regular checks and clearance are required to keep these screens clear.

vi) Check that there has been no change in top water level by measuring the depth of flow over the stoplogs and the distance to the water level measured from a fixed point on the pile capping: the sum of the two figures should always be the same;

No change in top water level has occurred and the stoplogs remain secure.

vii) Ensure the Prescribed Form of Record is being maintained up to date with water levels being recorded as specified in Part 1.

The Prescribed Form of Record is up to date, however, the PFR is in the old format


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2.6 Records, monitoring and supervision by the Undertaker under Section 11

2.6.1 Prescribed Form of Record (Section 11(1))

Item Comment

Location of the Prescribed Form of Record Held at Council offices

Was the Prescribed Form of Record made available? Yes

Was the Prescribed Form of Record up to date? The following minor changes to the current entries are required.

• Supervising Engineer contact details

• Details of recent certificates.

Are any changes required to the Prescribed Form of Record? The Prescribed Form of Record needs to be converted to the new format now that the reservoir has been designated ‘High Risk’.

2.6.2 Records prescribed by Inspecting Engineer (Section 11(2))

The following directions in respect of records under Section 11 of the Act were given by the last Inspecting Engineer in his Section 10 inspection report:

No. Direction given by the last Inspecting Engineer Status

i) A check be made to ensure that the top of the stoplog (top water level) and zero on the gauge board both correspond to the certified top water level of 17.47m OD;

The gauge board was in the correct position.

ii) Water levels continue to be entered in Part 1 of the Prescribed Form of Record on a monthly basis;

Monthly readings are being taken and recorded in Part 1.

iii) A monthly visit be made to the reservoir by a competent person who should complete a checklist of items that need to be kept under observation.

No formal checklist has been developed. Monthly walkovers are being made.

2.7 Directions by Supervising Engineer as to the visual inspection by the Undertaker at specified intervals under Section 12(6)

No directions have been given by the Supervising Engineer under this Section of the Act.

2.8 Directions and certification for Flood Plans under Section 12AA

The Undertaker has not been directed to prepare a flood plan under Section 12A but the Inspecting Engineer has recommended production of an on-site plan.


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2.9 Annual Statement Period & Visit reports

This Supervising Engineer’s annual statement covers the period up to 31st December 2018.

A site visit was made on Tuesday 25th September 2018.

Signed ……………………………………………. Date of Statement: 31st December 2018 David Littlemore Supervising Engineer

(Expiry date of current appointment to Supervising Panel: 08 February 2020)


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3 Site Visit Observations

3.1 General

Date of site visit: Tuesday 25th September 2018

Accompanied by: Mr Tom Sly – London Borough of Merton

Weather: Dry and bright. Blue skies with some clouds

Reservoir water level: 36mm over.

Photographs are included in Appendix A and a summary of key events that have occurred since the last Section 10 Inspection is included in Appendix B.

3.2 Summary of Reservoir Water Levels

The water level is being measured at monthly intervals on the gauge board adjacent to the spillway and recorded in Part 1 of the Form of Record.

The maximum and minimum reservoir water levels recorded during the past year were as follows:

Water Level Measurement Date

Maximum 32mm over 30/03/2018 – Higher reading during September visit.

Minimum 0mm 28/08/2018

Flows were observed passing over the weir during all monthly monitoring visits carried out in 2018.

3.3 Significant changes since last Annual Statement

No significant changes to the condition of the embankment or overflow structures were observed.

3.4 Main Dam

3.4.1 Upstream Face & Crest

The sheet piling within the Fishing Club section of the embankment remains in a poor condition with several large holes apparent at or slightly above top water level. The sheet piles continue to slowly deteriorate with the holes slowly increasing in size (Photo 3).

Approximately half of the timber fenders on the upstream face of the sheet piles in this section are now missing.

The Fishing Club have erected timber posts with netting in-between within the reservoir. This barrier is located approx. 10m to 15m upstream of the weir. The purpose of this structure is unknown [Note: the structure has been removed since my site visit].

The embankment crest within the Fishing Club section consists of a relatively narrow path with a reasonable grass cover (Photos 1 & 2). Previously the fishing club have used grass turf and pond weed to raise the level of the crest and this has reduced the amount of


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water that appears to be passing through the holes in the sheet piles. The condition of this section of the crest appeared to be reasonable during the site visit with no signs of ponded water or soft areas. This is likely to be due to the prolonged period of dry warm weather and the lower water levels during the summer.

The sheet piling in the section between the Fishing Club and the Water Garden access (adjacent to the spiked fence) also has a number of longstanding holes close to the water line that allow wave slop and elevated water levels to pass onto the crest of the embankment. The condition of the crest in this area continues to deteriorate slowly with the surface now consisting of a mixture of ponded water, bare earth and grass (Photo 4). The Inspecting Engineer has recommended that ‘The embankment crest immediately to the left of the fishing club compound should be raised to at least the capping level of the sheet piles with a good grass cover or other erosion resistant surface being established’. This work has not yet been started.

The galvanised steel capping installed over the top of the sheet piles along the rest of the embankment remains in satisfactory condition. The condition of the sheet piles below the capping cannot be assessed due to the very limited freeboard that exist at this reservoir. Some of the timber fenders in the central and left section of the embankment are missing (Photo 8).

The surface of the crest path within the central and left sections of the embankment remains in a reasonable condition (Photos 5 to 8). Gradual deterioration of the tarmac surface continues and appropriate repairs should be made where necessary. Some of this deterioration is caused by tree roots lifting the surface of the crest path (Photo 6).

The trees on the upstream edge of the embankment crest track appear to be largely unchanged. One of the trees continues to lean into the lake and its roots appear to have damaged the tarmac path (Photo 6). I suggest that the trees on the embankment crest be regularly inspected by the Council to ensure that they remain in a healthy balanced state.

3.4.2 Downstream Face

The condition of the vegetation and grass cover on the downstream face varies significantly along the length of the embankment.

Fishing Club The right end of the embankment within the fenced area used by the fishing club remains very heavily vegetated with mature shrubs, trees and ground cover. The extensive vegetation made access and observations in this area impossible. The Inspecting Engineer has recommended that ‘The vegetation on the embankment in the fishing club compound be strimmed at least once a year to enable observations for movement and seepage to be carried out’. No vegetation clearance has been undertaken in 2018 or since the last Section 10 Inspection.

Right of Water Garden The condition of the downstream face of the embankment to the right side of the Water Garden (looking downstream) was again largely unchanged from my previous site visit (Photos 11 & 12). The quality of the grass in this area varied considerably from a satisfactory grass cover (Photo 12 area to the far right) to areas dominated by weeds (Photo 11 area nearer Water Garden).

No signs of any instability, deformation or seepage were observed in this section of the embankment.

Left of Water Garden


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No significant changes in the downstream face of the embankment to the left of the Water Garden were observed. The condition of the grass cover was also highly variable; ranging from areas of reasonable grass cover (Photo 10 – nearer the boat store) to areas with no grass cover below tree canopies.

A number of saplings growing in this area have been removed since my 2017 site visit. This work is welcomed as this prevents the saplings becoming more established and creating issues on the downstream face of the embankment.

Embankments with poor grass coverage and numerous trees have poor resistance to overtopping flows. The best form of resistance is provided by well-maintained grass without obstructions (such as trees, rocks, park benches etc.) as these create significant turbulence. A good grass cover is considerably more effective in binding soil together than isolated trees that deflect flows and create additional turbulence and erosion of the downstream face. Allowing overtopping flows to pass over a protected surface in a uniform way will prolong the life of the structure during a flood event.

Trees located on the downstream face of embankments also create leakage problems in the medium and long term. Depending on the location of the tree on the downstream face their root structures can extend towards the impounded water and penetrate the core material. These roots (especially when the tree eventually dies) can create leakage paths through the embankment which will require remedial works before the leakage develops and internal erosion of the embankment commences.

I suggest measures are taken to improve the grass cover on the downstream face of the embankment in order to improve the embankment resistance to overtopping flows. Measures should include reseeding, regular cutting and the raising of tree canopies. No new trees should be planted on the downstream face and any young trees starting to grow on the downstream face should be completely removed before they become any further established and create leakage problems with their root systems. Existing mature trees should not be removed as rotting root systems generally create more leakage problems than if the tree is maintained in its current state. Existing trees should be regularly inspected and works undertaken to keep the trees in a healthy balanced state. Trees at risk of falling should either be removed or reduced in size as the damage caused by the rootball can often lead to significant leakage through the embankment.

Sailing Club Section Access to the sailing club compound was possible during this visit. No signs of any instability or seepage were observed in this area, however, observations were difficult in the compound due to the large amounts of sailing equipment present.

Left end (running track) The downstream face at the left end of the embankment (running track end) was covered by short well maintained grass. Some bare patches were observed in this section during this visit, however, generally the grass was in good condition. No signs of any instability, movement or seepage were observed in this area.

3.5 Spillways

3.5.1 Overflow Weir

The timber stoplogs forming the overflow weir generally appeared to be in a reasonable condition and were clear of debris and obstructions. No modifications to the level of the stoplogs were apparent during this site visit.


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3.5.2 Overflow Channel

The condition of the concrete overflow channel downstream of the weir appeared to be largely unchanged from my previous visits. The channel was clear of debris at the time of the visit.

3.5.3 Overflow Stilling Area & Downstream Channel

The stilling basin pool remains relatively clear of debris and was draining well. Some vegetation is starting to grow into the stilling area from the margins.

The makeshift trash screen at the upstream of the outlet pipe (steel reinforcing mesh) was partially blocked with debris at the time of the visit (Photo 14). The outlet pipe discharging flows from the stilling area appears to be operating satisfactorily.

The Inspecting Engineer has recommended that ‘the screening arrangements upstream of the stilling pool culvert be modified, with a new screen being installed at the culvert inlet headwall and a new primary screen provided that can be easily cleared’. This work has not been started. If this work is started then the details of the new screen be sent to the Supervising Engineer for comment prior to its fabrication and installation.

3.6 Outlet Control Structures

The outlet valve controlling the 100mm diameter outlet pipe adjacent to the overflow was operated during this visit. However, no flow was observed from the downstream end of the pipe following opening of the valve suggesting some form of upstream blockage. I suggest that all necessary measures are taken to restore the full discharge capacity of the lower outlet facility.

3.7 Other Pipework & Valves

The ‘Water Garden’ supply valve was not operated during this visit and the manhole covers located on the crest were not lifted.

No flows were observed from the pipe at the top of the ‘Water Garden’. The grounds staff at Wimbledon Park regularly operate this valve when there is water available. No problems with the valves were reported.

3.8 Monitoring & Instrumentation

3.8.1 Seepage & Drainage Flows

No signs of any seepage from the embankment or around the overflow structure were observed during this visit.

Previously a small wet area was observed adjacent to the cascade close to the toe of the embankment. In 2017 it was not possible to tell if this soft ground was as a result of leakage from the cascade structure or leakage through the embankment. However, this area was considerably drier during this site visit suggesting that the previous wet spot was probably caused by leakage or wave slop from the cascade rather than leakage from the embankment. This area will be monitored by the Supervising Engineer during future visits.

3.8.2 Settlement & Movement

No signs of any settlement or movement were observed during the site visit.


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3.8.3 Instrumentation

A gauge board located immediately upstream of the overflow is used to measure the reservoir water level on a monthly basis.

The depth of water passing over the top stoplog is also measured.

No other instrumentation is present at the reservoir.

3.9 Downstream Conditions

No changes in the conditions downstream of the dam were observed or reported by the Undertaker.

3.10 Flood Plan

The Undertaker has not yet prepared an On-site Flood Plan.


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3.11 Supervising Engineer Maintenance Suggestions

Following my site visit, I make the following maintenance suggestions which should be read in conjunction with the recommendations made by the last Inspecting Engineer (see Sections 2.2 to 2.6 above):

i) The embankment crest immediately to the left of the fishing club compound should be raised to at least the capping level of the sheet piles with a good grass cover or other erosion resistant surface being established.

ii) The trees on the embankment crest be regularly inspected by the Council to ensure that they remain in a healthy balanced state.

iii) Measures should be taken to improve the grass cover on the downstream face of the embankment in order to improve the embankment resistance to overtopping flows. Measures should include reseeding, regular cutting and the raising of tree canopies. No new trees should be planted on the downstream face and any young trees starting to grow on the downstream face should be completely removed before they become any further established and create leakage problems with their root systems. Existing mature trees should not be removed as rotting root systems generally create more leakage problems than if the tree is maintained in its current state. Existing trees should be regularly inspected and works undertaken to keep the trees in a healthy balanced state. Trees at risk of falling should either be removed or reduced in size as the damage caused by the rootball can often lead to significant leakage through the embankment.

iv) All necessary measures are taken to restore the full discharge capacity of the lower outlet facility.

This Supervising Engineer’s Annual Statement results from a visual inspection of the reservoir condition on the date of the visit(s). No liability can be accepted in respect of any defects not visually apparent on or that arise subsequent to the date of the visit(s). It is important that the Undertaker reports as soon as possible any change in the condition of the reservoir to the Supervising Engineer.


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Appendix A Photographs

Photo 1: Fishing club section with newly erected net screen [Now removed].

Photo 2: Fishing club section of embankment


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Photo 3: Fishing club section – Sheet piles with increasing deterioration

Photo 4: Continued waterlogging of embankment crest caused by defects in the upstream face sheet pile


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Photo 5: Embankment crest – Central section

Photo 6: Embankment crest – Central section disrupted by tree roots


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Photo 7: Embankment crest – boathouse section

Photo 8: Embankment crest – left end


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Photo 9: Downstream Face – Central Face

Photo 10: Downstream Face – Central Face


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Photo 11: Downstream Face – Left of Water Garden

Photo 12: Downstream Face – Right end


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Photo 13: Water Garden – Previous wet spot

Photo 14: Debris accumulating on the screen located at the upstream end of the stilling pool discharge pipe


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Appendix B Summary of events since last Section 10 Inspection

Year Date Physical works Other points of note

2013 5th Nov Last Section 10 Inspection

2014 22nd April Section 10 Inspection report issued

25th Nov Modification to overflow weir to stop further stoplogs being added to structure

Downstream face repaired to the left of the sailing club

Partial Section 10 (6) Certificate issued on 25th November 2014

2015 28th Sept Flooding of park downstream on the 28th September 2015

2016 20th Sept Clearance of stilling pool Consultation plans for park redevelopment available on side of tennis court. Desilting of the reservoir to be included within options.

No reference to potential new spillway

Some schemes include realignment of natural channel downstream of the embankment to prevent flooding within the park.

2017 12th Oct Re-adjustment of gauge board during site visit.

2018 25th Sept Fishing club have erected timber posts with nets in-between in the area upstream of the overflow weir

2019

2020

2021

2022

2023 Next Section 10 Inspection due by 5th November 2023

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