Flood Risk Assessment for Proposed Quarry by the River

Flood Risk Assessment for

Proposed Quarry by the River Trent near Cromwell

September 2009

FINAL REPORT

JBA Consulting South Barn Broughton Hall SKIPTON North Yorkshire BD23 3AE UK t: +44 (0)1756 799 919 f: +44 (0)1756 799 449 www.jbaconsulting.co.uk

Cemex UK Operations Ltd Wolverhampton Road

OLDBURY West Midlands

B69 4RJ

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REVISION HISTORY

Revision Ref./ Date Issued

Amendments Issued to

Draft report July 2008

Chris Pointer, CEMEX One digital copy

Revised draft report August 2008

Modified model to include the raising of levels across the plant site.


Revised draft report August 2008

Modified model to include access road to quarrying site


Draft final report October 2008

Modified following comments by EA Chris Pointer, CEMEX One digital copy

Final report September 2009

Modified to add consideration of flood risk from IDB drains, and other clarifications, following further comments by EA


CONTRACT

This report describes work commissioned by CEMEX UK Operations Ltd under Order Number 1182668 of 9 May 2008. CEMEX’s representative for the contract was Chris Pointer. Duncan Faulkner, Colin Riggs and Chris Allman of JBA Consulting carried out the work. Prepared by: ................................................... Chris Allman BSc MSc Assistant Analyst Reviewed by: ................................................... Duncan Faulkner MA MSc DIC CSci FCIWEM Chief Hydrologist Approved by: ....................................…............ Richard Annable BSc, MSc, CEng, MICE, MCIWEM Managing Director

PURPOSE

This document has been prepared solely as a Flood Risk Assessment for CEMEX UK Operations Ltd. JBA Consulting accepts no responsibility or liability for any use that is made of this document other than by the Client for the purposes for which it was originally commissioned and prepared.

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ACKNOWLEDGMENTS

JBA would like to thank CEMEX for their cooperation in providing the data required for this study. The hydraulic model of the study area was acquired from the Environment Agency, LiDAR and hydraulic model output data was bought from the Environment Agency, and NextMap DTM data was purchased from The XYZ Digital Map Company (www.xyzmaps.com).

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EXECUTIVE SUMMARY

Purpose

This report considers the impact that proposed quarrying works at the Cromwell Quarry site might have on peak flows should a large fluvial flood event occur during the quarrying or restoration phase. As this site is entirely within the Environment Agency’s Flood Zone 3, a Flood Risk Assessment is required under PPS25 guidance. A conservative approach has been taken by modelling a range of worse case scenarios which might occur during this period.

Method

An existing HEC-RAS model supplied by the Environment Agency (“The Agency”) was imported into ISIS and linked to a hydrodynamic 2-dimensional (2D) TUFLOW model to simulate flow across the floodplain on the left bank of the River Trent. This approach facilitated an improved understanding of the complex flow paths across the study site.

Four versions of the model were developed; the first was designed to model the current ground levels across the site and to act as a base against which the effects of the quarrying could be measured. The other three models were developed to represent different phases of the proposed quarrying process. Three phases for the quarry development were selected on the basis that they represent the probable worst case periods in terms of flood risk. The elevation data for the ground was altered to recreate the conditions in the different phases - ground levels were increased to represent the dimensions and location of the storage bunds, but kept constant for the quarry pits. This method represents a conservative modelling approach and considers the fact that many topographic depressions, such as quarry pits, will fill from surface runoff prior to the arrival of the fluvial flood peak.

The restoration phase will result in a lowering of existing ground levels, and therefore an increase in flood storage in the quarry site when compared to existing conditions. Model results from the present day conditions were therefore assumed to be suitable to represent a worst-case scenario for the restoration phase. In this case, the depressions are assumed to have filled with surface runoff to a point where the topography is the same as the current. The restoration phase is not expected to result in any increase to flood risk.

The impact of the proposed works on peak water levels was considered for the 100-year return period flood. The impact of climate change was also simulated by including a 10% increase to the 100-year flood flow, in accordance with Table B2 of PPS 25.

The effects on the peak water levels were considered both within the study site and on surrounding floodplains. Alternative site arrangements were considered and modelled to minimise the impact of the works to the site and to third party landowners, however none of these resulted in improved conditions.

Conclusions

Of the three phases modelled, none caused a significant change to the flood risk to the quarry. The peak water levels and flood extent across the site remained the same throughout all stages of the development. Peak water levels in the vicinity of the quarry site increased by a maximum of 0.03m to the south west of the site during phase 3 (as defined in the Method of Working). The reason for this increase in water level is essentially because floodplain flow is being trapped behind the “L” shaped bund. Hence the effects are highly localised and do not result in a significant change to the flood outline. Peak water levels on the river and other sections of floodplain also remain essentially unchanged.

Recommendations

Owing to the fact the proposed quarrying works do not result in a significantly increased flood risk, this report cannot make any further recommendations as to how the proposed works could be altered to reduce flood risk. Results from this study have suggested, however, that the current positioning of the bunds on the site may result in localised high velocities and turbulence, which could lead to damage or erosion of the soil bunds in some locations. This effect is likely to be most pronounced where floodplain flow is deflected to the east around the most southern L shaped bund. Our recommendation is that this erosion potential is probably best mitigated by trying to strengthen the bund, particularly at its most southerly extent.

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CONTENTS

Page REVISION HISTORY i CONTRACT i PURPOSE i ACKNOWLEDGEMENTS i EXECUTIVE SUMMARY iii CONTENTS v

1� INTRODUCTION�'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''�(�

1.1� Site Location�///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////��1.2� Site Description�//////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////��/+� 1�'��2��3��&�3�3&��2�3��14 5 ��//////////////////////////////////////////////////////////////////�,��/�� 6 ��2��7��8)�%��//////////////////////////////////////////////////////////////////////////////////////////�,�1.5� Brief for hydraulic modelling work�//////////////////////////////////////////////////////////////////////////////////////////////////�9�1.6� Previous Studies�////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////�9�

2� HYDRAULIC MODELLING PROCESS�'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''�)�2.1� Introduction�/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////�:�2.2� Model adaptation�////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////�:�2.3� Greenfield Runoff�/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////��

3� MODEL RESULTS�'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''�((�3.1� Current conditions�////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////��3.2� During the quarrying process�///////////////////////////////////////////////////////////////////////////////////////////////////////��3.3� Mitigation measures�/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////��+�3.4� Climate Change////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////��

4� CONCLUSIONS AND RECOMMENDATIONS�''''''''''''''''''''''''''''''''''''''''''''''''''�(*�4.1� Conclusions�//////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////��,�4.2� Recommendations�///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////��,�

APPENDICES:

APPENDIX A: - METHOD OF WORKING (MAY 08)

APPENDIX B: - LAYOUT OF THE PROCESSING PLANT (JAN 08)

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ABBREVIATIONS 1D One-dimensional 2D Two-dimensional DTM Digital Terrain Model HEC-RAS Hydrologic Engineering Center – River Analysis System (developed by the US

Army) HX Model boundary linking ISIS and TUFLOW using the water level from ISIS ISIS Hydrology and hydraulic modelling software IDB Internal Drainage Board JBA Jeremy Benn Associates Ltd LiDAR Light Detection and Ranging PPS25 Planning Policy Statement 25: Development and Flood Risk RMC Ready-Mix Concrete Ltd. RMSE Root Mean Square Error SX Model boundary linking ISIS and TUFLOW using the water level from TUFLOW TUFLOW Two-dimensional hydraulic modelling software

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1 INTRODUCTION

1.1 Site Location

The Cromwell Quarry study site lies just over the left-bank of the River Trent, east of the village of Cromwell and the A1, roughly 7 km north of Newark (Figure 1.1). The centre point of the site is at SK 80500 61700. It lies just downstream of Cromwell Weir – a location which represents the normal tidal limit of the river.

Figure 1.1: Site Location

Digital map data @ Bartholomew

1.2 Site Description

CEMEX are proposing the establishment of a quarrying operation on the site. This 0.26km2 site currently consists of arable farmland between the A1 and a flood bund running along the left bank of the Trent. To the north of the site lie the remnants of a similar quarrying project, which is now a series of restored lakes. This was being worked until recently by another operator.

Under the current site conditions, floodwater which overtopped the flood bund would simply flow along the floodplain in a downstream direction before rejoining the river downstream. The proposed quarrying works would be likely to cause a more complicated response, where floodwater would be redirected around a series of bunds both surrounding and within the site.

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1.2.1 Site Proposal

Quarrying work on the Cromwell site will be undertaken in 12 phases (Figure 1.2). Initially, topsoil from the operational area in the North-West of the site will be moved to a soil bund on the western edge of the site (Bund B). Two temporary silt and clean water lagoons will be dug in the south-west of the site, and the topsoil stored in a series of bunds in the south-west (Bund A). Gravel extracted from the lagoons will be relocated onto the site to provide a 1.5m high solid platform in the north-west of the site for the Operational Area. The topsoil from the first section to be excavated will be both stored in Bund A and also used to improve the eastern bund adjacent to the Trent (Bund D). For the majority of the following phases, the topsoil from the section being excavated will be used to restore the previous section. During the final restoration phase (phase 11), the soil stored in Bund A will be transferred to fill up previous excavation sites at 9 and 10. The topsoil used to reinforce Bund D will be used in restoration of excavation site 7. There shall be no temporary stockpiling of soil other than in these bunds. The processing plant will be located within the Operational Area. This will contain the site office and a limited stockpiling of processed and as-dug materials, however the scale of these will be too small to have an effect on flood risk should an event occur. Details of the layout of the processing plant can be seen in Appendix B. Beyond the northern boundary of the quarry site is an access road linking the site to the Cromwell junction of the A1. This road will be elevated by 1.5m above the surrounding land during the quarrying process but will be removed during the restoration phase. The method of working diagrams supplied by Cemex have been included in Appendix A.

Following the completion of the excavation works, the Cromwell site will be restored to create a landscaped conservation lake. The lake margins will be created progressively using the available overburden and topsoil. As each phase of work is completed, lake margins will be created and dewatering will cease, allowing water to accumulate in the worked out area. Following the accumulation of water in two adjoining areas, the separating bund will be removed enabling the progressive creation of a single landscaped lake. The 1.5m gravel platform constructed in the north-west will be removed at the end of construction to fit in with the restoration of the rest of the site.

The site has approximately 2.4 million tonnes reserve to be worked at a rate of 200 000 tonnes per annum, giving a lifespan of 12 years for extraction, and an additional 2 years for the restoration phase.

1.2.2 Flood risk

A sequential approach to flood risk has been considered in the development of the site in accordance with PPS25 guidelines. Due to the nature of the process of gravel extraction from floodplains, the development site is wholly within Flood Zone 3 (Highest risk of flooding - 1% Annual Exceedance Probability). Although sand and gravel extractions are considered to be water compatible land uses by PPS25, other ancillary activities such as stockpiling of material or the establishment of screening bunds are not. A range of options for locating such activities were considered at the planning stage, however it was not possible to locate any of these activities outside of the development site. Consideration was taken to reduce the impact of development on the movement of flood water across the site, for example by orientating soil bunds in the direction of flow to reduce the obstruction to flow. The least-at-risk location has therefore been adopted.

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Figure 1.2: Overview of the Cromwell Site

Based on Ordnance Survey Land Line data with the permission of Her Majesty’s Stationary Office, © Crown Copyright Licence No 100018131

1.2.3 Hydrology and Hydrogeology

The local hydrological regime is dominated by the River Trent which forms the eastern boundary of the application site. Cromwell Weir, located on the Trent 0.5km south-east of the study site marks the divide between the non-tidal upper catchment and the tidal lower catchment. Consequently, the river is tidal where it adjoins the quarry site. Tidal fluctuations in the river level are less than 1m. Tidal fluctuations and the large catchment area combine to create large fluctuations in river level which create a threat of flooding following heavy and prolonged rainfall. In order to prevent frequent flooding the river is flanked by engineered flood bunds. These structures are designed to accommodate the maximum river level created by a 1 in 10 year event at high tide2. More severe floods result in the bunds being overtopped and the adjacent land, which includes the application site, being inundated.

Two small IDB land drains flow along the western boundary of the quarry site and meet, just downstream of the site, to form one single drain (Figure 1.3). Combined, these drain a catchment of approximately 12.5km2, including the site. They discharge into the River Trent at a location downstream and 1.5km north of the site at SK 80366 62932.

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Figure 1.3: IDB drains flowing past the western edge of the FRA site

Based on Ordnance Survey Land Line data with the permission of Her Majesty’s Stationary Office, © Crown Copyright Licence No 100018131

The most likely cause of flood-risk to the site from these drains is from flow coming out of bank and entering the site. Although local residents have observed occasions of flooding from these drains, the flows generated from the catchment are likely to be too small to cause significant out of bank flow (see below). As the watercourses are so close to the River Trent, flooding is more likely to be caused by water backing up from the main Trent channel than because of high flow volumes along these drains. The Newark IDB has also reported that flood risk from these drains is insignificant.

Although the main flood risk to the site is from the Trent, it is possible that a localised storm could cause a relatively large fluvial event for the drains, but not for the Trent. To quantify the potential for the site to be flooded from a localised event, a water-balance analysis was undertaken for the IDB catchment areas. This aimed to assess whether the existing network of drains in the catchment has

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enough available storage volume (freeboard above normal water level) to contain the runoff associated with the an extreme rainfall event.

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The losses model from the Revitalised Flood Hydrograph method1 was used to produce an estimate of total runoff volume during a 100-year return period rainfall event for each of the two IDB drains described above. The catchment area and length of drain network were estimated from OS maps. The average cross-sectional area for the portion of drains above the normal water level was assumed to be 7.5m2, based on personal experience of fieldwork on land drains and OS mapping. Rainfall and runoff volumes were estimated using catchment descriptors from the FEH CD-ROM v2.0 and the calculated catchment areas. A critical duration of 24 hours was used.

The volume of runoff produced in the catchment during the flood event was compared with the available storage in the drain network to assess the potential for overbank flow during a 100-year event. As there is no flow gauging or hydraulic model available, the outflow from the catchment was assumed to be zero. This simulates the worst-case scenario in which all runoff from the flood event remains in the catchment and provides a very conservative estimate. The results from this analysis are shown in Table 1.1.

Catchment West Drain South Drain West and South Drains

Combined Catchment Area (km2) 0.88 11.65 12.53

Drain length (km) 2.29 13.90 16.19

Runoff during 24-hour rainfall event with return period 100 years (m3)

11000 243000 254000

Storage volume accounted for within the freeboard of the drains (m3)

17000 105000 122000

Volume of runoff not stored (m3) 0 138000 132000

Table 1.1: Results of water-balance analysis for IDB drain catchments

Analysis of the combined drain network suggests that there is enough capacity to contain all the runoff from a localised 100-year storm in the Western IDB drain network, but not in the Southern network, which should be able to contain about half the runoff. Assuming that flow was able to drain out of this catchment throughout the 24 hour event, it is likely that the IDB drain network would be able to contain most of the flow, although it is possible that some may spill out, causing localised flooding of low-lying areas. This would not necessarily occur around the study site, however, and its magnitude would be minimal compared to flooding from the Trent.

For these reasons, and because the proposed work do not involve alterations to either of these drains, for the purpose of this study it is assumed that flood risk from the land drains is not affected by these works and is negligible.

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A hydrogeological assessment of the site2 found that groundwater flows in a north-easterly direction across the site. Although the capacity of the pumps used for dewatering are designed to comfortably accommodate volumes of water expected from groundwater flooding, the site would be inundated if the River Trent overtopped its banks.

All groundwater and surface water that is excess to the requirements of the washing plant will be discharged to the Trent via the settlement lagoon in the south-west of the site. Water will enter the Trent via the discharge point, which will be located near the old wharf in the south-east corner of the �� ;< � 6 �&&��23�/� ��9/� � !�'��&�� !��3�&&�!)�33� = ��7�/� � '��&�$&�� &��

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site (Figure 1.2). Discharge consent NPSWQD002441 is held for the discharge of up to 5,200m3/d (60 l/s) which is less than the calculated 2-year (5,400m3/d; 62 l/s) greenfield runoff rate (calculated within Section 2.3 of this report). The volume and peak flow rate of the quarry site should therefore be no greater than the pre-development rates, in accordance with PPS25 requirements. An application will shortly be submitted for a Land Drainage Consent for an outfall structure. The existing flood defence embankment will be left untouched.

1.5 Brief for hydraulic modelling work

This report considers the impact that the proposed works might have on peak flood levels should a large fluvial event occur during the quarrying phase. A conservative approach has been taken by modelling a range of worst case scenarios which might occur. The impact of the proposed works on peak water levels is considered for the 100-year return period flood. The impact of climate change is also simulated by including a 10% increase to the 100-year flood flow, in accordance with Table B2 of PPS 25.

1.6 Previous Studies

As the Trent is such a large catchment with a high population, a large amount of work has been completed previously and the Environment Agency holds hydraulic models for the entire length of the main channel. Separate hydraulic models have been created for the fluvial (upper catchment) and tidal (lower catchment) sections of the channel within ISIS using survey and flow data. A separate 1-dimensional (1D) model (within the HEC-RAS software) was developed internally by the Environment Agency Midlands Region for the lower catchment including the tidal boundary (from North Muskham to Blacktoft – where the Trent enters the Humber estuary) using geometry from these existing ISIS models. Due to the proximity of the proposed site to the tidal limit, it is vulnerable to both tidal and fluvial flooding and so data from this HEC-RAS model has been used for the present study.

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2 HYDRAULIC MODELLING PROCESS

2.1 Introduction

An existing 1D HEC-RAS model for the region around the tidal boundary on the Trent was supplied by the Environment Agency to be used as a basis for this study. The model was developed internally by the Environment Agency Midlands Region using survey data from the existing main ISIS models for the Trent. Results from the running of the original ISIS model were purchased from the Environment Agency and used to provide flow and stage boundary conditions for the model.

2.2 Model adaptation

In order to gain a better understanding of how the proposed works would affect the floodplain flow it was decided to take advantage of recent modelling advances and dynamically combine a 2D floodplain model with the existing 1D model of the river channel. The 2D model domain (left-bank floodplain) was developed in TUFLOW which can now be combined to run with ISIS such that water is free to flow to and from either model depending on the prevailing conditions. The supplied HEC-RAS model was therefore translated into ISIS to accommodate this method. Flow into the right-bank floodplain was modelled within ISIS using spill and reservoir units.

The model adaptations can be divided into alterations that were made to the existing 1D model and the subsequent development of the 2D model domain. The most significant of these alterations are summarised graphically in Figure 2.1.

No quality checks were carried out on the supplied model and it was assumed to be representative of the conditions of the river.

2.2.1 1D model alterations

The existing hydraulic model was developed to provide an overview of flood characteristics for the area around the tidal limit of the Trent and not for detailed hydraulic assessment at one site. For this reason, a number of alterations were required to improve the detailed representation of the study area.

Firstly, as the majority of the right-bank floodplain was not represented in the original model, lateral reservoir units were added to this section. This is necessary to account for the volumes of flow leaving the channel and entering the floodplain over the right-bank. The geometry of the spill units used to connect these reservoir units to the channel was taken simply by interpolating between the highest levels on the right bank of each of the hydraulic model cross sections.

As the left overbank area of the River Trent was to be modelled in TUFLOW very few alterations had to be made to the ISIS model. Analysis of the elevation data suggests that most of the floodplain flow will be directed back into the river by a ridge of high ground to the north of Carlton-on-Trent (Figure 2.1). This was therefore used as a northern boundary between the TUFLOW and ISIS model domains. The elevation of this ridge was extracted from the NextMap DTM and used as the geometry for a spill unit in ISIS. The upstream side was connected to the TUFLOW model, and the downstream to the ISIS model, providing a mechanism for any floodplain flow overtopping the ridge to return to the ISIS model. The ISIS cross-sections downstream of this were extended across the left-bank floodplain area using NextMap data to allow the simulation of flow over the floodplain downstream of the ridge, and permit it to re-enter the channel.

2.2.2 2D model development:

Topography and Extent:

The 2D model was built solely for this study. The topographic representation of the study site was based on filtered LiDAR data supplied by the Environment Agency. The available LiDAR data was not large enough to cover the whole study area and so lower-quality NextMap DTM data was merged to this to patch small holes and provide an adequate coverage. A comparison of elevation values at 12 locations on these two datasets showed an acceptable level of accuracy, with a mean difference of 0.12m, with a Root Mean Square Error (RMSE) of 0.315m. This topography was used

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to represent the Cromwell Site prior to any excavations. In order to represent the different phases in the excavation process, alterations to the ground level were stamped on to this DTM from within TUFLOW. Given that the study site is relatively uniform in terms of hydraulic roughness a single Manning’s n value of 0.04 was adopted throughout.

Hydraulic Boundaries:

Two types of boundary were used to represent flow between the 1D and 2D model domains. Both are dynamic boundaries allowing water to flow freely in either direction. Firstly, along the left bank of the River Trent an HX boundary was used to represent water spilling over the river banks and into the floodplain. In addition to this, a connection was made from the model domain to the floodplain area in a downstream direction. In this case, a different type of connection known as an SX boundary was used. The difference between these two types of boundary relate to the way the two modelling programs interact to determine the flow passing between them. In the HX boundary, used to represent the river bank, ISIS calculates the water level in the channel. This water level is then used by TUFLOW to calculate the flow passing into the 2D domain. However, these roles are reversed for the SX boundary, used where the transition between the 1D and 2D model domains occurs at a hydraulic structure. In this case, ISIS calculates the flow across the boundary using the water level from TUFLOW as the upstream boundary. Upon examination of the catchment, it was assumed that flood flow from the upstream floodplain unit would not have a large impact on the extent of inundation at the site. As a result of this, a connection between these two units was not included. A graphical summary of the connections between the ISIS and TUFLOW model domains is provided below in Figure 2.1.

Figure 2.1: Interactions between the ISIS and TUFLOW models

Based on Ordnance Survey LandRangerr data with the permission of Her Majesty’s Stationery Office, © Crown copyright Licence NO 100018131

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2.2.3 Modelling scenarios

The quarrying process is a continuous one and as such the topography within the Cromwell site is unlikely to remain constant for very long. It is therefore not feasible to model each phase of quarry development separately; instead a number of periods were chosen that were likely to have the greatest influence on the peak water levels both within and beyond the Cromwell site. In order to provide a conservative estimate of the impact on flood levels the assumption was made that the actual excavation pits would be full from surface water prior to the arrival of the fluvial flood peak3. Subsequently, this modelling approach does not provide additional floodplain attenuation in the form of the pits and is therefore conservative.

The bunds used to store overburden during the quarrying process will result in a slight decrease in available storage and perhaps more significantly affect the overland flow routes across the floodplain. Each of the proposed bund combinations were therefore modelled separately. Again, to ensure a conservative and simpler approach to modelling, the bunds were assumed to have vertical sides with elevations taken from the supplied quarrying plans.

Four quarrying phases were used to assess the maximum impact on peak water levels (Figure 2.2). Initially, the site was modelled with existing ground levels in order to provide a base against which to assess the effects of the other quarrying phases. The supplied Method of Working information was used to select which of the quarrying phases would be most likely to produce the largest increase in peak water levels. These phases chosen for modelling were Phase 1, Phase, 3 and Phase 10. The restoration phase was not modelled separately as the only significant changes in topography involve a lowering of existing ground levels. Realistically, this scenario can only increase the available floodplain storage. As a worst case scenario, it was assumed that the depression becomes filled with surface runoff prior to the arrival of the fluvial flood peak, and therefore the model of present conditions, using current ground levels, will provide a good representation of the restoration conditions.

Each of these modelling scenarios was tested for the 100-year return period flood flows and this flow with a 10% increase to account for the effect of climate change, as suggested in Table B2 of PPS 25.

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Figure 2.2: Location of bunds used in 2D modelling of different scenarios.

Bund location Operational area Access road

Based on Ordnance Survey Land Line data with the permission of Her Majesty’s Stationery Office, © Crown copyright Licence NO 100018131

2.3 Greenfield Runoff

The greenfield runoff for a site is the volume of surface runoff, calculated for a variety of magnitudes of rainfall event (as a return period), assuming no development. To maintain the natural equilibrium of a site during and after development, the surface discharge from a developed site should not exceed the natural greenfield runoff rate.

Greenfield runoff for the quarry site was calculated using a methodology described in the Institute of Hydrology Report 124 (IH124)4. This uses a rainfall-runoff approach based on the Flood Studies Report method to estimate the mean annual runoff from a catchment for a range of return periods. It is primarily designed for use on small-catchments (<0.5km2) and is therefore appropriate for this case.

The catchment area used for the analysis was 0.26km2, with no development, giving an URBAN value of 0. The SAAR4170 value was 569mm, estimated from the FEH CD-ROM v2.0. The analysis used the FSR Regional Growth Curve from Region 4.

The 2-year and 100-year greenfield runoff rates for the quarry site are 5,400m3/d (62 l/s) and 15 400m3/d (178 l/s) respectively.

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3 MODEL RESULTS

To fully understand the effect the proposed works will have on fluvial flood risk it is important to have an appreciation of the impact on peak water levels both within and beyond the Cromwell site.

3.1 Current conditions

The proposed quarrying site lies wholly within the existing Flood Zone 3 (100-year return period). This is confirmed by the findings of this study although the 100-year flood extent resulting from the TUFLOW model does not extend beyond the A1 in the vicinity of the quarry site (Figure 3.1); this represents a slight narrowing of the left bank floodplain compared to the existing Flood Zones data. The mean peak flood depth across the site is approximately 1.72m, and average velocity is 0.08m/s, with the largest velocities occurring to the west of the site.

Figure 3.1: Maximum depth across the site during present case scenario.

Key

Depth

Quarry Extent


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3.2 During the quarrying process

Peak water levels remain essentially unchanged throughout the quarrying process. All increases are small, however the largest change in peak water level occurs immediately upstream (to the south west) of the study site (Figure 3.2). At this location, floodplain flow is partially trapped behind the “L” shaped soil bund, resulting in locally increased water levels. This increase is very small, with the maximum change in water level less than 0.03m, and no significant change in the flood extent.

Construction of the bunds causes the west side of the site to be sheltered, reducing the flow velocities for this area. In contrast, velocities on the east of the site are marginally increased with peak velocities in this area becoming in the region of 0.15m/s. In the case of a real flood, the fastest flow velocities are likely to occur around the quarry depressions as flood water enters and fills them. Flow is likely to enter these at a rapid rate until they have reached capacity, when the velocities will be similar to the modelled results.

The model results show that construction of the access road will have a negligible impact (0.00 – 0.01m) on the flood level for the 100 year or climate change run.

Figure 3.2: Increase in maximum depth during phase three of the quarrying process

Key

Increase in depth

Quarry Extent

Storage bunds and operational area


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3.3 Mitigation measures

Upon consultation with the Environment Agency, further consideration was required to mitigate the impact of the proposed works on flooding to third party land immediately upstream of the quarry boundary.

A number of options were considered to reduce the impact of the bunds, including relocation of existing bunds to improve their streamlining, increasing the depth of soil deposited on the existing bunds and relocated removed material away from the floodplain. Due to space and logistic constraints, the majority of these were not feasible.

The most appropriate proposal was to create a 20m gap between the two main sections of Bund A to reduce its obstruction to flow. The bottom 2m of the end sections surrounding the gap would be reinforced to prevent erosion from high velocities during a flood event. The topsoil removed from this gap would be relocated to create a new bund on the west side of the site between the existing Bund A and the operational area Additonally, if the modelling results indicated that further access was required for the water, 0.7m diameter pipes could be placed at ground level before construction of the east-west bund over them. The new arrangement is shown in Figure 3.3.

The hydraulic model was re-run using the new bund arrangement, and the results suggested that the new arrangement would only have a small impact on the depth of flooding upstream of Bund A (a reduction in flow depth of 0 – 0.01m; Figure 3.3). A knock-on effect of this change was to reduce flow depths just outside the north-west of the site by around 0.02m due to a blockage in the flow path between Bunds A and B in the south-west of the site. Movement of material from Bund A to create this additional small bund obstructed one of the major flow paths by which flood water can exit the site and re-enter the IDB drain running along the west of the site. This had a knock-on effect of causing slightly increased depths (< 0.01m) across the rest of the site, particularly on the upstream side of the newly-created obstructing bund. It was deemed that the inclusion of pipes would not significantly affect the passage of flood water and so these were discarded.

After consideration of alternative bund arrangements, it was deemed that the original layout is the most appropriate to reduce the impact of flooding in and around the site.

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Figure 3.3: Change in maximum depth during Phase 10 of the quarrying process as a result of the revised bund layout

Key

Change in depth

Quarry Extent

Storage bunds, operational area and

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3.4 Climate Change

The model was re-run to simulate the effect of climate change for each of the phases modelled previously. Instabilities in the model restricted results for phases 1 and 3; however the rest of the runs gave good results. The increase in flows to account for climate change showed similar results to the runs for the 100-year return period, with a small, uniform increase of 0.05m in peak water levels. The maximum increase in flow velocity across the site is 0.03m/s.

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4 CONCLUSIONS AND RECOMMENDATIONS

4.1 Conclusions

This study focused on assessing the impacts of the proposed works on peak water levels associated with the 100-year flood event. The results indicate that the proposed quarrying works will have a negligible effect on the 100-year flood risk for either the quarry site or the surrounding floodplains. The maximum expected increase to flood levels is only 0.03m due to a build up of water behind an “L” shaped bund at the southern edge of the site and behind the access road on the northern edge.

The restoration phase will result in a lowering of existing ground levels, and therefore an increase in flood storage in the quarry site when compared to existing conditions. Model results from the present day conditions were therefore assumed to be suitable to represent a worst-case scenario for the restoration phase. In this case, the depressions are assumed to have filled with surface runoff to a point where the topography is the same as the current. Therefore the restoration phase should not result in an increase in flood risk.

The 2D model of the quarry site was able to provide a good understanding of the mechanisms by which the site is likely to flood. Inundation should first occur roughly two hours into the flood event, at the southern edge of the site due to the Trent overtopping upstream of Cromwell Weir. Under current conditions the flood wave will flow over the site in a northerly direction taking approximately three hours to cover the majority of the site, with the operational area being inundated approximately 30 hours later. After the commencement of the quarrying work the flood wave should be attenuated slightly by the presence of the southerly bund, causing it to enter from the south-east of the site and inundate the majority of the site within about three hours. Under these conditions the flow will negotiate the obstructions on the site but due to the speed of the flow it is unlikely that they will have a considerable affect on the flow paths. The fastest flow velocities on the site are likely to occur around the quarry depressions, where flood water enters and fills these. Flow is likely to enter these at a rapid rate until they have reached capacity, when the velocities will be similar to the modelled results.

Flooding initially occurs at the study site four days from the beginning of the ISIS model run, and therefore four days from when the flood wave enters the model reach. As the site is located at the downstream end of the Trent, the operators should expect to have warning of a flood event well before this.

This study has not examined the expected frequency of flooding to the site. Given the standard of protection of the flood bunds along the banks of the Trent, it can probably be expected to flood once every 5-10 years on average. The impact of climate change across the site is expected to show a small, roughly uniform increase in level across the site when compared to the simulation of the 100-year event. The increase in flow velocity is expected to be negligible.

4.2 Recommendations

The “L” shaped bund in the south west traps flood water behind it causing flow to be deflected around its eastern extent. This results in locally exaggerated velocities which may cause erosion of the bund. The location of the bunds also causes increased peak water levels to the south where water is essential trapped behind it. Re-orientation of this bund, so that it is not perpendicular to the dominant flow paths, would undoubtedly reduce these effects. Due to restrictions to the space available on site, this bund cannot be relocated and so a better solution may simply be to strengthen the southern extent of the bund in order to minimise erosion.

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APPENDICES

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Appendix A: - Method of Working (May 08)

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Appendix B: - Layout of the Processing Plant (Jan 08)

Documents

Flood Risk Assessment for Proposed Quarry by the River