You might try the state department of water resources, or the state enineer, or the division of dam safety. It might be stated as a fraction of the probable maximum percipitation (PMP).
At 80 acre-feet of storage and 20 ft. height, you are definately geeting into more than the typical farm pond. You need someone experience in dam design to help you out. You will probably also be getting into permitting issues, so the agency will probably require a P.E. certification.
Coal will degrade with water, and, will tend to burn when loose. Coal steam plants usually figure 30 - 90 days max. in a compacted stockpile before the coal ignites.
I don't know of any rule of thumb. You will have to convince your regulators that what you want to use is at least as good of a growing medium as the topsoil. You will probably need to get a biologist, agronomist, or such involved.
In the work I've done, Time of Concentration would be calculated for each subwatershed. In your example, it would be across the lawn to the gutter. Travel time would be calculated to the catch basin. Calculations for outlet works downstream of the catch basin would be calculated based on the...
Mabn is right. If I want to be conservative in fleet sizing, I will use 90% of the 2:1 capactiy, but I have often found the the equipment capacity is even more the the rated volume capacity. Be sure to check the material weights, and not overload the equipment based on weight capacity.
I agree that, if possible, the emergency should be located over native material. If the spillway is located over the embankment, any erosion of the spillway will erode your emankement. Failing that, it should be well armored.
It has also been my experience that basins do not get removed when...
You can determine what the flows whould have been under various rainfall conditions pre and post development of either lot. Ultimately though, you should be discussing this with the city attorney before having any discussions with the land owner about responsibility.
In many parts of the country, subsidence refers to collapse of the surface due to underground extraction, such as mining. In your instance, the geotehnical engineer may be refering to voids caused by some other activity. You should contact the preparer of the report to understand what he means.
You didn't say what the required storm is. If the requirement is for the 10 yr storm, I sould guess using the 25 yr storm would give you more than enough safety factor.