Document Control
Revision:
| Version | Date | Author | Change Description |
| 1.0 | 27/08/20 | Initial Draft | |
| 2.0 | 07/03/2022 | K Northcott | Document Revised |
| 3.0 |
Introduction
The drainage infrastructure supports road safety, other health objectives and contributes to the City Vision of being “One of Europe’s most vibrant waterfront cities, where an outstanding quality of life is enjoyed by everyone”.
Historically there has been a high level of public dissatisfaction with the condition of drainage assets on the highway network as a result of low levels of investment in maintenance in the past.
This document outlines Plymouth Lifecycle Plan for Drainage.
Context
Lifecycle planning is key to asset management and acts as a means to link the priorities identified in the Asset Management Objectives (see Asset Management Strategy) with the works being carried out on the network. Through the analysis of asset management data this process forms a framework of identifying the required maintenance activities to meet these objectives, to select and prioritise these activities into a programme of works and to then assess the effectiveness of the process in meeting the defined objectives (see below).
Asset Management Objectives:
- To provide a safe, efficient, accessible and health-enabling highway network
- To reduce the environmental impact of the highway asset
- To deliver cost effect asset management
- To encourage the adoption of innovation in the highway service
Drainage Lifecycle Plan
This document sets out the lifecycle planning approach for drainage assets managed by Plymouth City Council. The drainage asset comprises of over 40,000 gullies, approximately 80 culverts and a number of manholes, soakaways, pumping stations and rain water conductors. The approximate replacement value of £17 million has been calculated for gullies and £1 million for highways culverts.
Maintenance of highway drainage not only prevents flooding but also has a positive impact on future highway maintenance as surface water will freeze, potentially forming potholes, degrade the condition of the carriageway surface and reduce the overall life span of the asset. The aim of this lifecycle plan is to drive efficient maintenance and long-term management of the drainage assets by adopting an asset management approach.
This lifecycle plan will inform Plymouth’s decision-making regarding the management of their drainage over the next 5-year period, with the following considerations:
- Cost-effectiveness: maximising the benefit of maintenance spending
- Investment planning: comparing investment profiles; forecasting budgetary requirements
- Public safety: ensuring that flooding risks are effectively managed and that claims are minimised or robustly defendable
- Acceptable condition levels: maintaining the drainage assets to an acceptable level of functionality
- Performance management: monitoring performance against targets
Drainage Asset Data Management
Inventory Data
The drainage asset is an integral part of the overall highway asset and comprises of a variety of different asset types. The current inventory of highway drainage assets across Plymouth includes:
Table 1 – Drainage Inventory (Gullies)
| Description | Number of gullies |
|---|---|
| “A” Classified Roads | 2,598 |
| “B” & “C” Classified Roads | 5,714 |
| Unclassified Roads | 31,213 |
| Service Lanes/Footpaths | 3,102 |
| TOTAL | 42,627 |
Table 2 – Drainage Inventory (Other Assets)
| Description | Number |
|---|---|
| Culverts | 57 |
| Pumping Stations | 2 |
| Balancing Ponds | 2 |
| Gully Condition Sensors | 53 |
Currently the inventory for drainage assets is held and maintained within the scope of Highway Infrastructure Management System. The maintenance crews who maintain the asset on a day to day basis also have the technology to update the inventory data if they find new gullies during their inspections and in response to works carried out. Note that culverts with a diameter over 900mm are maintained under the structures asset group.
As part of an invest-to-save initiative an inspection-led regime is being adopted in conjunction with the use of SmartWater, a risk-based prioritisation model, to focus gully cleansing activity on those gullies that need cleansing while utilising an Internet of Things (IoT) based approach. The prediction model will be verified through both re-inspection and real-time performance measurement through use of strategically placed sensors in gullies.
Condition Survey
Condition of the drainage assets is regularly monitored by highways staff in order to identify and prioritise future maintenance and renewal work. Our risk-based approach places emphasis on inspection and assembly of trusted data upon which to best prioritise routine and cyclic cleansing activity, detailed surveys e.g. CCTV and development of minor improvement schemes where the benefit cost ratio is high.
The condition of the gully assets is stored and analysed in real-time within the SmartWater system before being transferred on a regular basis into the HIMS system to enable works orders to be issued and for future analysis
Performance Management
Inspection of the drainage asset to manage performance is in its infancy. Performance of the drainage asset is measured through a number of performance indicators utilising data collected over the year. These include:
| Performance Monitor | Performance Monitor | 2020/21 Target | 2020/21 Results | Reporting Frequency |
|---|---|---|---|---|
| CE-DR-001 | Number of reactive Drainage works orders issued | <1292 | 1887 | Quarterly |
| CE-DR-002 | Number of Jobs in the Workbank for Drainage | <75 | 102 | Quarterly |
| SE-DR-001 | Number flooding incidents | <359 | 335 | Quarterly |
| SE-DR-002 | % of flooding incidents resulting in road closure | 0 | 4 | Quarterly |
| SE-DR-003 | Number of flooding hotspots in high risk category | <29 | 29 | Annually |
| SE-DR-004 | % of targeted highway gullies emptied within timescales | New Measure | New Measure | Quarterly |
| SE-DR-005 | % of reactive drainage works orders completed on-time | <=97% | 97.70% | Quarterly |
| SE-OA-011 | "Keeping Drains Clear and Working" satisfaction score from NHT survey (HMBI12) | 55% | 40% | Annually |
These performance measures are utilised to drive both reactive and planned maintenance programmes focusing on the hot spot areas around the city as well as a means to stimulate a cycle of continuous improvement.
Risk Management
Key Risks
Culverts, in particular are high risk assets. They are inherently dangerous places because of their position adjacent to moving and potentially deep water. There are also risks associated with culverts becoming blocked or overwhelmed by the volume of water passing through the watercourse, which could lead to significant flooding issues both to the highway and adjacent properties. There are therefore risks of death, serious injuries, claims, prosecutions and disruption to the network as a result of inadequate or inappropriate maintenance and care.
Whilst gullies could be considered less of a risk the potential for injuries, claims and prosecutions remain particularly in respect to cyclists and motorcyclists. Damaged or missing gratings or covers can create a serious safety hazard. Also, the presence of standing water adjacent to a flooded gully accelerates the deterioration of the carriageway which gives rise to formation of potholes and subsequent vehicle damage claims, loss of amenity value which impacts on the economic activity in the area and also the temporary and permanent repair costs in replacing the pavement asset. Blocked gullies and flooding remain an emotive topic for residents of the city and it is recognised that managing these risks is key to a well performing asset.
Risk Management
A programmed and targeted maintenance programme significantly reduces the risks associated with highway flooding and can improve safety and reduce claims against Plymouth City Council. Within the limited funding it is important to target repairs and maintenance works at those areas most in need.
The SmartWater gully asset management service adopts a proactive approach and relies on establishing trusted data which, when run through a sophisticated risk-based algorithm, allows the critical gullies to be targeted for cleansing.
Key locations, such as known flood hotspots and other identified potentially vulnerable areas also have sensors installed. The sensors provide flood alerts, record silt build up and enable the contractor to respond before problems impact the network, thus reducing complaints and expensive reactionary emergency call-outs. These locations are inspected on at least a weekly basis and more regularly if forecasted rainfall exceeds agreed intervention levels set out in the Drainage Protocol. When combined with external weather data we will also have a 5-day forecast available in order to be able to cleanse the gully ahead of weather conditions that could cause problems.
Known flooding hotspot sites are inspected at least weekly by a member of the Highways Maintenance team normally, but this could be divided out and spread amongst any available staff if a weather event is forecast or arrives at short notice. During these inspections, any gullies requiring cleansing will be raised for works on a risk based timescale depending on the location and whether any particularly heavy rain is forecast. A separate list of sites where the effects of heavy rain are less sensitive will be inspected on a lesser schedule or again, when heavy rain is forecast.
During these periods of heavy rainfall warning messages to the public, professional partners and the emergency services will be communicated. If a high risk of flooding is identified based on predicted rainfall levels, a central “Flood Hub” will be established to coordinate a proactive response to the event.
Reactive Maintenance and Defect Repairs
Routine and reactive maintenance for the drainage asset are identified through inspections or from reports from members of the public. Safety inspections of the highway network are carried out by driven inspections in accordance with the Highways Safety Inspection Manual and specific drainage inspections are undertaken to identify silt levels of gullies on a rolling programme.
The public can report flooding or blocked gullies through several different methods including Firmstep CRM system, or Telephone. Potential defects are inspected by Plymouth Highways staff and programmed for repair if appropriate.
Defects reported as a high priority, especially where there is a high level of risk or performance failure has occurred are remedied or made safe within 24 hours. Other defects are usually repaired within a week or 28 days, depending on the type and location of the defect as assessed using the risk based approach set out in the Safety Inspection Manual. Defects assessed to require an emergency response will be attended to within 2 hours of the report being received.
Gully sensors installed at hotspots across the city will also trigger reactive cleanses once defined thresholds of silt levels are met but these should be undertaken before the problem starts to impact the accessibility, amenity and/or the safety of the network.
Structural Maintenance Strategy and Investment
A programme of planned maintenance on the highway drainage assets is updated continually, with potential schemes being prioritised by an agreed risk matrix to ensure transparency and best use of the resources available. This programme is derived from the identified flooding hotspots as a means to address those sites with the highest level of risk.
The SmartWater system produces focussed work schedules to target only those gullies that need attention, namely gullies that are blocked or close to becoming blocked that need to be cleaned in order to provide confidence that the network meets a suitable level of functionality. In a first iteration of implementing the system we would expect the numbers of gullies identified to equate to 20-25% of the gully stock based on empirical evidence of a number of years of research and also implementation of this system in other local authorities. Subsequent iterations of the system could see the numbers of gullies that need to be targeted reduce toward 10% and maybe even lower.
Coupled with this, ongoing monitoring of hotspot sites will inherently reduce the risk of flooding by providing early intervention where gullies are found to be blocked. This also supports the development of a programme of sweeping routes in collaboration with the street cleansing teams to tackle areas where leaf fall of other debris is known to regularly block the surface inlets of gullies and reduces the risk of flooding at these sites as a result.
The need for major works is identified by assessments considering condition surveys and accident records, stakeholder needs, local engineering input, coordinator opportunities and engineering risks. The intention is that the application of asset management principles should continue to be applied to the drainage asset to remove the backlog and continue to maintain the condition of the network at a steady state whilst improving the condition of the drainage asset and wherever possible reducing the known levels of flooding hotspots.