Shellfisheries Waters Compliance Assessments at Pegwell Bay in Kent. MIKE by DHI modelling (FM Series) formed a large part of the study, and Jonathan presents some interesting experiences in the use of the Software (Jonathan Short - URS). MIKE by DHI 15th UK User Group Meeting - Tuesday 19 March 2013

1. Shellfisheries Waters Compliance Assessments at
Pegwell Bay, Kent
DHI User Group Meeting 2013
Jonathan Short, BSc MSc
March 19th 2013

2. Overview
Objective:
“..to identify future investment which will make a contribution to
achievement of compliance with the Shellfish Waters Directive Guideline
bacteriological standards and support the client (Southern Water) in
delivering waste water improvement schemes around the Pegwell Bay,
Kent.”
• The investigations are a Southern Water Output of the Environment Agency’s
AMP 4/5 National Environment Programme under the S2 Shellfish Driver.
• The investigations will inform the Agency’s Pollution Reduction Plans drawn up
as part of the UK response to EC infraction proceedings concerning the quality
of UK Shellfish Waters.
• URS (formerly Scott Wilson) in partnership and Intertek (Metoc), investigated
the impact of various sources on water quality at the shellfish beds.
• MIKE modelling formed a large part of the study.
DHI UGM 2013

3. Pegwell Bay, Kent
DHI UGM 2013

4. Key Aspects of the Study
• The study included:
Extensive field survey programme (Titan Surveys
Calibration of a MIKE FM HD and AD models to stringent FWR standards.
Simulation of effluent discharges and plumes from WwTW and CSO.
Simulation of background loads e.g. rivers (Catchment and Network models)
Large scale production run modelling phase.
Compliance assessment in partnership with Intertek Metoc (STORM
IMPACT) including assessment of the relative impact of different point
sources on overall water quality.
Evaluation of pros and cons of implementing
UV treatment at key outfalls.
DHI UGM 2013

5. Extensive Field Survey Programme
• Hydrodynamics
Water levels - ADCP 30-day
period (and Deal Pier - CCO).
Currents – ADCP.
Meteorological data.
River flows - EA gauging.
• Water Quality
Dye Patch monitoring.
Drogues.
Water sampling and bacterial
load testing.
Salinity / Temperature– CTD
grid.
Tracer from WwTW in river.
Bathing water sampling.
DHI UGM 2013

6. Further Water Quality Data
• Microbiological survey sampling.
• Hourly sampling at the Shellfish Beds –
Spring and Neap tides, Winter and
Summer conditions.
• EA routine monitoring sites.
• Bathing water sampling records.
• Shellfish Bed monitoring data.
• Outfall monitoring.
DHI UGM 2013

7. Hydrodynamic Model – Calibration / Validation
Parameter Tolerance Applied
Absolute Relative
10% tidal range -spring
tide
Water Level +/- 0.1m
15% tidal range neap
tide
+/- 20% of observed
Current Speed +/- 0.1ms-1
speed
Current
+/- 30 degrees* N/A
Direction
Phase +/- 15mins N/A
DHI UGM 2013

8. Water Quality Modelling – Calibration / Validation
1.E-05
Simulation
Dye Measurement
1.E-06
Concentration (cc/cc) 1.E-07
1.E-08
1.E-09
05/06/10 04:00 05/06/10 05:30 05/06/10 07:00 05/06/10 08:30 05/06/10 10:00
DHI UGM 2013

9. Salinity Calibration / Validation
Comparison of Depth Averaged Salinities at CTD Site C
36
Measured
Modelled
35
34
Salinity (psu)
33
32
31
30
31/05/2010 12:00 31/05/2010 18:00 01/06/2010 00:00 01/06/2010 06:00 01/06/2010 12:00
DHI UGM 2013

10. Discharges Represented in the AD Model
3 WwTW
21 CSOs
1 River
14 Discharge groups
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11. Technical Challenges / Limitations
0mins
160mins
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12. New DHI Whale Module required?!...
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13. Production Runs
•Highly computationally
intensive one month phase of
production runs.
•Modelling outputs totalling
over 100 processor days.
•Various wind and seasonal
conditions.
•Hourly releases over a 12
hour tidal cycle for each
discharge location.
DHI UGM 2013

14. Simulating Decay Rates / Residence
Stour Estuary
E0001326
T90=30hrs
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15. Compliance Assessment Outputs
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16. Key Findings
• Water quality within the Pegwell Bay shellfish waters is considered good, and
compliant with current water quality standards.
• During periods of high rainfall local CSOs in the Sandwich, Ramsgate and
Margate catchments contribute the majority of the bacteria concentration in
Pegwell Bay. However these impacts are infrequent and are only significant at
high percentiles.
• Improvements to the treatment process at Weatherlees Hill A WwTW, i.e. UV
disinfection, would be expected to reduce its contribution the shellfish waters of
Pegwell Bay.
• However, given that shellfish waters meet current water quality standards and
the relatively high river load in storm conditions, the provision of UV disinfection
at Weatherlees Hill A would not provide significant additional benefit when
considered against cost and carbon footprint.
DHI UGM 2013

17. Summary
• Study provided additional knowledge and insight into the often complex
patterns of water movement in coastal and estuarine waters.
• MIKE modelling software provided a suitable predictive tool capable of
assessing water quality impacts arising from individual discharges.
• Formed part of the wider toolkit for for efficiently assessing the performance of
different mitigation options.
• Outputs will inform the Agency’s Pollution Reduction Plans drawn up as part of
the UK response to improvement of Shellfish Waters.
• Targeted investment to minimise storm water storage, treatment and pumping
requirements in a sustainable manner.
• Ensures minimum overall environmental impact, in terms of meeting receiving
water quality standards, balanced against reducing carbon emissions.
DHI UGM 2013

18. Questions?
Thank you!
Jonathan.Short@urs.com
DHI UGM 2013