I need a engineer to provide a storm water flood report

Building a granny flat in our backyard, the structure has been approved, however the council want our access bridge and storm water flood survey approved by an engineer. The land is not in a flood zone but does have a small natural water course that in very heavy rain can get up to 500mm deep and 700mm wide (so anyone can step over it, however we have built a steel and timber bridge to ensure easy access regardless of weather). Even in the heaviest of rain there is minimal flow on the water due to it exiting to the property next door and the accumulated water is only sourced from neighboring houses, so there is no chance of any flow that could knock someone off their feet, much less sweep them away (of which they could at most only travel 5m until they get to the fence. I work in the automotive restoration and steel fabrication field and designed and built the bridge which I believe would safely support 5-10 ton across the 700mm span. It only needs to hold pedestrian weight however due to being 3m wide the council want to ensure it will support a regular car being driven across it. I can provide photos. I have a topographical survey map from when the site was surveyed for the building.

Text from Council letter:

Thank you for lodging the above Construction Certificate application. I have undertaken a
preliminary assessment and would like to request the following additional information:
Flooding Requirements (condition 4)
1. A site-specific flood report should be prepared to show the flood hazard and 1% AEP,
5% AEP and Probable Maximum Flood (PMF) flood event data;
2. Provide an emergency evacuation plan showing strategies for ingress and egress to
and from the proposed Secondary Dwelling in a major flood event;
3. Demonstrate that the existing concrete drain and bridge will be acceptable for use in a
major flood event with respect to structurability, velocities, height and accessibility.