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Appendix B - Engineering Letter
Attached to the e-mail REC17/0011 - Representations to achieve a Negotiated Decision Notice - email 1 of 2.
Callaghan & Toth Consulting Engineers Pty Ltd ABN No. 45140292324 8/41 Sunshine Beach Road Noosa Junction P.O Box 2305 Noosa Heads 4567 Callaghan & Toth www. »hanandtoth.com.au te eee ee ee Ph: 07 5474 908] Fax: 07 $474 Civil, Environmental & Hydraulic Engineers Ph: 07 5474 9081 Fax: 07 54749041 — es 10" May 2016 Mark Lewis Coordinator Engineering (By email) Dear Sir, RE: PROPOSED 17 LOT SUBDIVISION LOT 6 RP149498 EAGLE DRIVE, TEWANTIN STORMWATER DETENTION BASIN AND BIO BASIN LOCATION Mark on behalf of the Owner we would like to summarize the investigations to date to provide a list to Council of the benefits to local residents resulting from the placement of an external detention basin/bio basin with new park on the existing flood prone house lot at No.14 Eagle Drive. The Current Flooding Problems to Local Residents are listed below: 1. Currently there is no major flood dedicated overland flow path for the runoff from the 4.72ha catchment to the east of No.14 Eagle Drive (note: of the 4.72ha, our clients land represents 1.04ha). The problem is also compounded by the fact that existing gully pits and pipe drainage at the sag at No.14 Eagle Drive only has the capacity to take a one in one year storm runoff. Therefore runoff overtops the kerb regularly and floods No.14 Eagle Drive and in large storms the runoff also floods through No.16 Eagle Drive. Both of those occurrences have been verified on site by the owner of No.16 and Agent for No.14. The Owner of No.16 said that in large storms the runoff floods No.14 and has entered the house, it also backups and floods down his (No.16) driveway and floods his shed in the south west corner. The real estate agent handling the rental of No.14 said that carpet has been replaced as a result of flooding, sand bags have been used in other events to stop water entering the house, and that a mound has had to be built in the front yard to divert flooding from the road away from the front door and down the side of the house. A further problem is that this house is also built only 2.7m away from the boundary adjacent where it floods through the land. In this location there is not enough room for a Council 3m easement or to lay pipes without impact on the house footings. ; 2. Itcan be seen at the Eagle Driveway that Council has recently replaced and lowered the grates to the gully pits in this location in an attempt to alleviate a problem. 3. The existing undersized stormwater pipe from the sag gully pits at No.14 does not go down the side of the house in the lowest point in the street, instead it goes down the north side of No.12. This pipe has no easement and the house is built right next to the pipe approximately 0.8m to 1.1m off the side boundary. These facts mean this pipe cannot be replaced by a larger pipe to reduce flooding. 1403 I4eagledr
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4. Aseparate flooding problem exists for the properties 31, 29, 27 and 25 Tait Street. Our Clients 0.7ha of un-developed land slopes toward the rear of these properties and floods through their yards onto Tait Street. This problem was addressed by Council approximately 15 years ago by the construction of an extension of the underground drainage system along the frontages of these properties. We designed this system for Council. The system is in place even though no records exist. Its purpose was to allow residents to run a stormwater pipe from our Clients land boundary through to the piped extension so as to alleviate the flooding. Unfortunately from discussion with owners they were not aware of this option. Unfortunately there is no room between houses 31, 29, 27 and 25 Tait Street and their side boundaries to allow a 3m wide overland flow path and underground drainage. There is however enough for a 1.5m wide interlot drainage outlet which our Client has negotiated permission from the Owner to construct. This will allow properties in our Clients development on the low side of the new road to drain to Tait Street. Flooding Solutions Investigated to Date: 1. Our Client first investigated buying the property at No.14 Eagle Drive, removing the house and placing a bioretention basin and detention basin in the property with a new large underground drainage system through this block to the park. We did a costing for this work which our client the checked against the development budget and found the $450,000 cost of the house made this small development not viable. 2. We then suggested that Council be approached to allow a relaxation of their requirements to retain most of the house on a smaller 450sqm lot with a 3m wide overland flow path, larger underground drainage path and bio basin/detention basin in the remaining area between the park and the house which would be given to Council as park. This option was modelled and a design prepared and costed. It involved removing approximately 1m of the house. This was submitted to Council but rejected mainly as being difficult to maintain (refer to Plans 1403-10A & 02B). 3. We then investigated the alternative of placing a bioretention basin and detention basin inside the development and piping the overflow down the side of No.14 Eagle Drive. Two options were investigated: a) Underground detention storage with bio tree pit gardens behind the kerbs b) Using a development lot to place the detention basin and bio basin beside the entrance to the development. Option A proved not possible due to the large volume of storage required in the road reserve. Option B proved feasible but would only solve the problem of flooding the residents at 31,29,27 and 25 Tait Street and leave the flooding of No.14 Eagle Drive unresolved as the existing non subdivision contributing catchment is still of a similar size. Conclusions: That there are two possible solutions for the development: (a) Bio basin and detention basin inside the development (refer to Plan 1403-10B,1403- 11B,1403-12B,1403-14A) 1403 14eagledr
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(b) Bio basin and detention basin and overland flow path in No.14 Eagle Drive replacing the house (refer to Plan 1403-15A). The benefits of the local community are significant for Option B and are listed below: e Provide a Q100 overland flow path, piped drain path and easement which the existing catchment outside the development needs. e Provide 845m2 of additional park which will extend from the existing park to Eagle Drive e Allow for the existing piped drainage from the Eagle Drive sag to the existing park to be increased in size and allow an easement for maintenance. e Removal of the regular flooding issues associated with No.14 Eagle Drive that Council eventually have to address. Option A cannot provide the above benefits and so will only provide a minor reduction in flooding at No.14 Eagle Drive which may lead to the local community believing that the development has made the situation worse. Option A though would comply with the Planning Scheme requirements of providing non worsening of runoff quantity and quality. Option A would also not be a forced engineering design which would require no underground drainage for the new road up till the basin and also require shallow box culverts under the footpath to direct runoff into the basin. This is feasible and necessary due to the basin needing to be on the high side of the development road to drain to Eagle Drive. We therefore recommend to our Client and Council to attempt to find a way for Option B to be economically viable in the form of offsetting contributions or a portion of the work being payed for by Council or another Developer so that this option can be economically viable and so therefore the benefits to the local community can be realized. Yours faithfully, Callaghan & Toth A D Toth BE Environmental RPEQ (Env) (civil) MIEAust CPEng NPER CPESC Director 1403 14eagledr