1211 - Engineering Report Complete C · 132002.220527.1

2.0 STORMWATER MANAGEMENT PLAN

2.1 Existing Stormwater Runoff from Webb Road

 

One of the main reasons that the existing residence is often inundated with stormwater
runoff, is because stormwater from Webb Road spills over the kerb and channel and runs

down the private driveway to the house.

As detailed on our Stormwater Management Plan No. 1211/P02 and on our driveway
longsection Plan No. 1211/P03, this existing runoff from Webb Road will be redirected to
the Council Stormwater pit in the kerb and channel as legally intended. This will be
achieved by increasing the flood freeboard in the footpath by lifting the vertical curve crest

in the new driveway, thus providing a bund against stormwater runoff from the road.

2.2 Existing Drainage System

The ground floor level of the existing residence is RL15.85m. The driveway to the
residence drops from RL20.9 at Webb Road to RL15.81m adjacent to the existing house.

Attached Drawing No. 1211/P01 shows the layout of the existing driveway and residence.
Due to the steep drop along the sealed driveway and the sealed forecourt in front of the
house, stormwater runoff velocity is very significant by the time the stormwater reaches

the existing house.

Presently two grated drains, as shown on the attached Drawing No.1211/P01, have been
installed to intercept stormwater runoff from the driveway and forecourt, upstream of the
existing house. There is also a third open concrete drain on the north west boundary

which intercepts stormwater from the adjacent property.

Unfortunately the two grated drains in the driveway and the forecourt are often overtopped
by Stormwater Runoff from storms greater than a 2 year ARI storm. As can be seen the
floor level of the house at RL15.85m, is only 4cm higher than the grated drain which
catches stormwater runoff immediately upstream of the house.

The reason for this inundation is that the drains have not been designed for a 100 year
ARI storm, they are often clogged by debris, and the drain level is only 4cm below the

floor level of the house.

As a consequence the existing residence is often inundated by stormwater runoff during
short heavy storms (over a 2 year ARI storm). This is one of the main reasons the
alterations are proposed for this residence. The only safe drainage system is to have an

 

Engineering Report Page 2 of 11
1211-ENG-01_Version 1.0 (April 2015)