R. Moses' Answer:
First, paving of any kind will impact drainage as it generally creates impervious surfaces, which direct a large amount of water and runoff in specific areas versus over wide areas. This costs money—a lot of money—to control. There are low impact designs that cost less but do require some ongoing maintenance and the general policy is unfortunately to spend a lot of money now and forget about the problem for 20 to 100 years with little chance of changing it, versus a smaller amount every year for the next 20 to 100 years with the option of tweaking the system as needed.
Streets and drainage lines are much the same in the sense that they have a certain capacity (supply) and varying rates of flow (demand). When we as a society find that demand sometimes exceeds supply, our gut instinct is to expand the supply by adding travel lanes or installing bigger pipes, which has large associated costs in terms of both construction and future maintenance.
Strong Towns invites us to strongly consider working to decrease the demand instead of increasing supply as a way to deal with these issues, or even to ask the question “Does this need to be dealt with at all?” Adding more pedestrian and bicycle facilities can, in the right context, reduce the number of vehicles on the roadways, potentially saving you from the expense of adding lanes. Installing stormwater BMPs to your city reduces the demand on the drainage pipes, potentially eliminating the need to expand the drainage system.
But it may not, either. As such, it all comes down to context, or the answer to the question “What does this place want to be?”
To more specifically address the objection you are facing, I would say that if we take as a fact that the drainage system is obsolete and that, even if the surface infrastructure was built to better manage stormwater, significant improvements would still be needed. And if we also take as fact that the place above these drainage lines is productive enough to be worth saving from its flooding predicament at all, then a Strong Towns approach could well be to improve the drainage system. After all, floods are really expensive and potentially life-threatening. Further, investing in surface improvements (even smart ones) that are likely to get washed away in a flood well before their design life is up is a poor way to use scarce public funds.
However, if it is shown that improving the drainage system is a smart investment, you are going to need to dig up the surface infrastructure to get it done. When the new pipes are all installed, the surface infrastructure will need to be rebuilt anyway. And when it is rebuilt, shouldn’t what is put back look like the answer to the question “What does this place want to be?” (In your case, that sounds like a place with bike and walk infrastructure.)
I suspect this is your best approach to deal with the objections you are facing. Good luck.
How would you respond to this question? Jump in with your answers in the comments.
Note: R. Moses is not meant to be professional engineering advice nor should be relied upon as such. Consult your own technical professional before proceeding with your own project.