COMHAIRLE CONTAE ÁTHA CLIATH THEAS
SOUTH DUBLIN COUNTY COUNCIL
MEETING OF TALLAGHT AREA COMMITTEE (1)
Monday, October 20, 2008
HEADED ITEM NO. 2
Flooding at Knockmore Park
Following a report which was noted at the meeting of this Committee on 15th September last and circulation of an updated report as was agreed, to all members of the Committee, the following are the main points of that report. The Senior Executive Engineer has reported that there is no further comment to be added on the matter at this stage. There are four main issues which contribute to the problem at Knockmore Park;
Issue No 1: Construction of the drain and high influx of Fats, Oils & Grease
Due to the unusual construction route of this drain attempts to excavate to carry out repairs proved unsuccessful. However, the Jetter Vac was employed for quite a considerable period of time in order to thoroughly clean the drain on 25th April 2008. On the 6th May the crew returned with a pipe tracking device to track the route of the meandering pipeline. The Jetter Vac was again used to clean the drain prior to use of the pipe tracking device. Following on from this on the 20th May the crew carried out a CCTV survey of the drain and the following was discovered. The pipe bends at a junction of earthenware to PVC pipe approx 1.5 metres downstream of the existing manhole. It is believed that the joint at this point may be allowing a constant ingress of small stones. One or more households are allowing a high influx of Fats Oils & Greases into the system. A medium sized plastic ball was removed from the line following continued cleaning by the Jetter Vac, and this has had a considerable effect on the flow of the line, which has been monitored regularly since.
Significant repairs / improvements were carried out on the local drainage system serving 29 – 35 Knockmore Park in June 2008 and the system appears to be working satisfactory since.
Issue Number 2: Surfacewater runoff from open space at Knockmore Avenue
Flooding occurred along Knockmore Avenue approximately 6/7 years ago. As a result, a land drain was constructed to alleviate further potential for flooding. This work was successful. In 2006 trial holes were dug in the open space to establish ground conditions as a part of the design process for an infill development adjacent to Knockmore Avenue. The trail holes created artesian conditions, releasing groundwater or perched water, which caused a build up of water on Knockmore Avenue. This was again dealt with and this flow of water was successfully diverted to the appropriate drainage system.
Issue Number 3: Blocking of Main Sewer, Knockmore
The main sewer between Donamore Green and Knockmore Park last blocked badly on 21/01/08 causing wastewater to flow across the park. This blockage was caused as a result of large quantities of rubbish being deposited in the manhole at the end of Donamore Green.
The blockage that occurred on the 05/09/08 was as a result of a similar incident. On this occasion some concrete blocks had been deposited in the main sewer, which proved very difficult to clear. The drainage inspector and jetter were on site for four hours before they managed to release the blockage. Over this time the level of flood water in Knockmore Park rose to a fairly high level and caused some property damage. The extremely high rainfall event also added to the problems, however surfacewater did not flow into Knockmore Park from the open space. In both instances the drainage maintenance crews carried out a full cleanup and disinfecting of the area.
Issue Number 4: Infill Development
The Architects Department of the Council is currently working through the design process for an infill development at Knockmore Avenue. The Drainage Department are aware of this and have been liaising on an ongoing basis with both the Architects Department and the Design Consultants.
As early as 30/11/2006, information was sought by the Drainage Department from Parks and Architects Departments regarding the trial hole tests, as it appeared that the site had a high water table and would require particular attention to be paid to the design of the surface water system.
In January 2008, the design consultants Downes Associates confirmed by email that to alleviate this problem (of the high water table and high surface water runoff), they have allowed for a land drain to be installed at the lower end of the site. In addition, detailed drawings provided to the drainage section by Downes Associates indicate that significant replacement and diversion of the foul system and surface water system are proposed. This should serve to improve the overall drainage system (foul and surface water) in the area.
The Architects Department have also confirmed as follows on 19/09/08:
“The drainage design for the proposed development is for the 1:100 year return storm event (i.e. the rainfall event statistically likely to occur every 100 years) and includes underground storm water storage tanks which cater for this level of rainfall. Outflow from the site is restricted to reduce potential flooding downstream of the site, hence the storage tanks. This outflow is restricted to the outflow which would normally be expected to discharge from a Greenfield site. We have also made provision to lay a new drainage pipe along the length of the site to cater for the drainage issues arising on the existing site. All of the proposed drainage is in accordance with the latest best practice requirements as set out in the Greater Dublin Strategic Drainage Study (GDSDS) document. We have issued our drainage proposals to the drainage department in SDCC as per a standard planning application requirement to allow comment and confirm compliance with the drainage design requirements. The new development will therefore address the local drainage issues arising on this site as the existing drainage solution always required to be addressed & the provisions in the proposed drainage arrangement are the best long term solution”
General
The Knockmore Area has been impacted on by a number of drainage related issues in recent years. The localised issues relating to 29-35 Knockmore Park and the flowing of surface water onto the footpath on Knockmore Avenue from the open space have been addressed by way of works carried out by the Drainage Maintenance Crews.
Two significant incidents of the sewer blocking between Donamore Green and Knockmore Park have occurred in 2008 and in both instances major sewer blockages were identified and cleared. These particular blockages occurred as a result of dumping of rubbish into the main sewer and in each case clearance of the blockages proved very difficult.
The capacity of the main sewer is satisfactory - the surcharge that occurred on 05/09/08 appeared to be directly as a result of the blockage rather than as a result of a lack of capacity in the main, despite the extreme rainfall event that day.
The proposal to divert and replace this section of sewer as part of an infill development will improve its operation as will the proposals for an improved surface water system.