Forth and Clyde Canal Surface Water Management Potential Hazel Smith, Hydrologist, AECOM [email protected] Introduction The Forth & Clyde Canal is a 39 mile (62.5km) long manmade inland waterway linking the Firth of Forth in the east to the River Clyde in the west. In and around Glasgow near the canal system there is the potential to develop land, however, with the existing sewer network at capacity and no nearby watercourses, many areas are essentially land locked. The Forth and Clyde Canal offers a significant opportunity for draining future development and reducing existing flood risk problems. AECOM UK Limited was commissioned in June 2010 by British Waterways Scotland (BWS) to undertake a review and update of the Forth and Clyde Canal hydrological and hydraulic model and subsequently to investigate the opportunities that the canal can offer, through changes to the operational management of the system. Figure 1 – Location Map Forth & Clyde Canal (West Branch) Maryhill Lock 21 to Bowling Forth & Clyde Lock 39 Canal Summit Pound Lock 20 Monkland Canal (piped and open) Forth & Clyde Canal Glasgow Branch The Initial Study A hydraulic model was constructed which allowed BWS to meet their an increased understanding of the operation of the canal system, but primarily to have a tool which can assess the future effects of surface water discharges from adjacent developments, in particularly associated with land locked development within the North Glasgow catchment. The study required the collation and review of available data, relevant to the modelled study reach, including hydrological information, structure dimensions and cross-sectional channel geometry information. Important information included outfalls/waste weirs and any contributing structures, see Figure 2 below for examples. Figure 2 – Structures An existing ISIS model of the Summit Pound and the Glasgow Branch was utilised and supplemented with additional hydrological and survey data to extend the model extents to include the West Branch from Maryhill to Bowling and the Monklands Canal. In addition, hydrological inputs and overflows have been substantially refined to now include sidelong catchments, incorporating overland runoff from uncontrolled catchments, which contribute to the canal flows, see section below for further details on controlled and uncontrolled catchments. Detailed hydrological assessments of the controlled and uncontrolled catchments were carried out. The purpose of the hydrological analysis was to enable accurate representation of all inflows to the canal system. Survey information and in some cases direct flow measurement data was available to help define the controlled feeder inflows. Controlled Catchments Flow to the Forth & Clyde Canal is provided by a number of controlled feeders. The Monkland Canal provides the main feed to the Forth & Clyde Canal. Secondary feeds to the Forth & Clyde Summit include Lenzie, and Craigmarloch feeders. Just upstream of a weir over the North Calder Water, the dual Woodhall sluices divert flow from the river to the upstream end of the Monkland Canal. The flow into the Monkland Canal is determined by the head difference between the North Calder Water and the Monkland Canal and the dimensions of the openings, and can be controlled by the sluice gates. The arrangement is fully represented in the hydraulic model. The Lenzie feeder flow to the canal is controlled by the water level in the Bothlin Burn, and the hydraulic characteristics of the sluices, feeder channels and culverts. The Craigmarloch feeder discharges into the summit pound on the north side of the canal, east of Craigmarloch Weir. Flow is discharged from Townhead Reservoir into the feeder channel, which splits some 880m downstream of the reservoir, where a single 1m wide sluice controls the flow into the remaining length of feeder channel. Sidelong Uncontrolled Catchments As a man-made structure, the canal is unlike a natural river that has a definable natural catchment that contributes runoff. In some reaches, the canal is embanked on one or both sides, preventing flow running off surrounding land entering the canal. However, the canal banks in some areas are built at the same level as natural ground or cut into the surrounding land. Here, runoff from surrounding land could discharge into the canal and augment the flow in the canal. Part of the scope of this project was to identify these sidelong catchments that would contribute flow to the canal and incorporate them in the hydraulic model. Figure 3 – Contributing Catchment Areas Figure 4 – Example of Contributing Catchment Locations Digital Terrain Model (DTM) along with OS Landform Panorama data was used to identify the contributing catchments. Having identified these catchment areas, TUFLOW 2D models were set up to determine the flows entering the canal system. Nimrod QC rainfall (Nov 2009) or FEH generated return period rainfall hyetographs were used as inputs to the model. Percentage runoff values to generate net rainfall used the value of SPRHOST (Standard Percentage Runoff generated using the HOST soil classification system) for the catchment if it was rural, or a nominal 75% runoff if urban. Modelling then allowed the hydrograph of flow entering the canal from each catchment to be determined. The November 2009 rainfall event was used as an initial calibration exercise to scale the inflow hydrographs such that modelled water levels in the canal matched recorded levels as closely as possible. These scaling factors were then applied to the return period modelling scenarios. Available historical data was used to carry out a verification of the model which, although limited, provided a good representation of recorded data at Maryhill Lock 21. Sensitivity analysis of the model was carried out to provide confidence in the representation that the model produces. Whilst it is accepted that the model cannot be considered fully calibrated, without substantial data collection this cannot be achieved. It is considered that the model provides a good representation of the canal system and can be used with confidence for the purposes of assessing the effect of permitting surface water discharges on both the canal and receiving watercourses. The model was simulted to provide peak discharge results for rainfall events with Annual Exceedance Probabilities (AEP) of 1, 30 and 200 year return periods and for a range of durations to ensure that both peak flow and total volume considerations have been included, the 24 hour rainfall event equated to the critical duration for the canal. The model provides detailed flows from the overflow structures to the surrounding water environment and we can consider the potential impact that flows can have and how modifications can be made to alter the flows that are discharged into receiving watercourses. The model now has good potential for considering options for altering the operation of the canal, in terms of controlled flows from feeders, to determine the effect on the canal system by using surface water discharges as a resource to feed the canal to allow existing resources to be retained for utilisation during dry periods. Optioneering – Stage 1 In January 2011, AECOM were commissioned to further develop the model to provide BWS with a way of assessing the impact on the flood risk of the canal of receiving runoff from development areas in Glasgow City centre. Updating of the existing model provided a tool to assess the performance of the canal system, under a variety of scenarios including extreme weather events and also to provide comparative assessments from incorporating runoff from potential new developments. Runoff from new developments were based on the site attenuating to greenfield runoff rates and the critical duration for the canal system was determined, which was the 24hour storm event. The full potential of the canal to take increased flows from developments, how the canal responded, where it impacted and any wider implications were investigated with the following options: • Option 1 - Add flows and vary levels of existing waste weirs, to increase flows within the canal system; • Option 2 - Add flows and raise existing waste weirs, vary levels of lock gate structures and increase capacity at one location to divert flows to the River Kelvin without any wider impacts; • Option 3 - Add flows and raise existing waste weirs and provide one ‘super weir’ at a single location, without the need to modify existing Lock gates. Results indicated that the Forth and Clyde Canal is capable of conveying significant quantities of surface water runoff from development sites. Modelling results indicate that the canal can accept up to twice the Greenfield runoff from the North Glasgow Development Area, with no overtopping of the canal during a 0.5% AEP rainfall event. A conclusion of the assessment described above identified that the full potential of the canal is yet to be realised as the options above were run under normal operating conditions with all canal feeders still contributing during the flood event. How much volume could be released in the system should the controlled feeders be switched off during an extreme event? Further scenarios were developed to fully understand the capacity of the canal to take surface waters with the feeders switched off. Optioneering – Stage 2 Moving on from the initial optioneering exercise, the ability of the canal to accept run off when all controlled inflows to the canal are reduced during flood conditions needs to be investigated. For this stage the inflows were increased until freeboards were encroached upon to determine the absolute maximum capacity of the canal under a range of conditions. The following scenarios were modelled: • Scenario 1 – all feeders were turned off and development discharge added until flows over waste weirs were equal to existing conditions. • Scenario 2 –same process as scenario 1 was carried out but in addition waste weirs were raised until the freeboard in canal was encroached upon in each branch. • Scenario 3 – existing waste weirs were shut off and the super weir concept was incorporated and scenario 2 was re-run.