The developer of the 312 Glendale project recently issued a report of the Neighborhood meeting held on January 15, 2012 at the Cobblestone Farm. The report, in pdf file format, has been circulated and is posted here . The report seems comprehensive and the developer should be commended for their efforts to record and report the discussion of the meeting.
However the report does raise some questions, primarily about the proposed stormwater treatment. The devil is always in the details, so let's look at some of the details reported. First, both the City and the County assert that the developer must detain the “100 year storm”.
DEFINITION – 100 year storm
The storm that has a 1% probability of occurring in any year.
In Ann Arbor, usually a storm of 4.75 inches in a 24 hr period.
The developer states that the area to be developed is 115,011 square feet (sqft) The 100 year storm, 4.75 inches of rain over this area would result in 45,500 cubic feet (cuft) of water. The developer proposes an underground culvert detention system designed to hold 20,000 cuft. The developer also proposes surface swales, but did not provide any dimensions or estimates of the volume of water that would be detained.
The underground detention system proposed by the developer is really only sized for the additional impervious surface that will be added. I estimate the new impervious surface to be about 51,000 sqft. A rain of 4.75 inches over 51,000 sqft is about 20,200 cuft.
Is it the developers responsibility to detain all the water of a 100 year storm on the site, or just the water from the new impervious surfaces?
Another troubling aspect of the report is that the developers assert their right and their intent to discharge water from the site at the allowed County agricultural rate of 0.15 cubic feet per second (cfs) per acre. How can this be considered an agricultural site? How many agricultural acres are next to the site to receive the discharge?
The discharge rate for the entire site is about 0.4 cfs. This may not seem like much, but if it is constant, that is about 1,400 cuft/hr. There are several ways to consider the effect of this discharge rate. For any storm that is about 1/3 in/hr or less none of the water from the new impervious surfaces would be detained. This would be the case even if the total rain fall is the 100 year storm.
If the rain fall occurs over a shorter time period there will be partial detention. For example, if the 100 year storm occurs over 4 hrs about 26% of the water will be released during the storm Even the detained 72% will be released in the next 10 hours. This might help a small amount to decrease the magnitude of a flood, but only if there would be greater runoff from the present undeveloped lot.
Under light rain conditions there will be increased runoff after the development. Even in the case of a 100 year storm over several hours the entire amount from the new impervious surfaces will not be detained. Is this what the Planning Commission, the County Water Resources Commission and our City Council consider detaining the 100 year storm event? Is it the flood improvement they intended to get from the city stormwater ordinances? Is it the flood improvement the citizens expect from the increased storm water fees?
The real unfortunate aspect of this development is that the city is losing an opportunity to make a significant contribution to reducing the present flooding in the area. If the area were left undeveloped and converted to flood control it could detain several hundred thousand cubic feet of water. Water from Hillside Terrace and other developments farther west could be diverted and stored during heavy rains. The storage could be large enough that the detained water could be released, or used, over a longer time period.
Vet's Park is another example. It could be a huge water retention area. Instead, the city keeps developing and paving more like the proposed skate park, while adding marginal water retention and treatment in other areas like the proposal for 312 Glendale.