When dealing with large surfaces that are impervious to water, it is important to consider the effects of a large rainstorm in a relatively short time. If the parking lot is not set up to drain properly or has lots of irregularities in the surface, there will be large pools of water. These reservoirs of water can range from being a slight nuisance on the way to your car to rendering a lot useless. Drainage systems are usually setup to avoid problems. I have seen water pouring out the entry roadway to the lot several times while heading to class on particularly rainy days. Wide but shallow puddles do develop in the middle, but none the hinder the function of the parking or or impede Mercerians from getting to their cars. Infrastructure must be set up that can handle the flow rate from the large surface. I am guessing that this lot can dump out several hundred gallons per minute when absolutely necessary.