Rain and Runoff

Here’s a bit of local data on rainfall and runoff. The rainfall data comes from Durham’s NWS COOP Station No. 312515, and the Jordan Lake inflow data comes the Army Corps site for the reservoir.

Here’s the daily rainfall for the last 10-11 years:

So-called “best management practices” for detention ponds equate to a 1 inch rainfall. Anytime the rainfall exceeds one inch, runoff goes directly into the streams. Here’s the sum of two-day rainfalls (ponds can’t empty in just two days):

We get a lot of rain here.

This rain goes into two main reservoirs, Falls and Jordan Lakes. Inflows to these reservoirs are measured, and here’s a plot showing inflows and rain for a short period:

No surprise, we get high flows when we get lots of rain (more than and inch or so) over the previous 3 to 5 days. Impervious surfaces add to high flows in our streams, and those flows bring pollutants.

One more thing. Here’s a risk analysis of how likely detention ponds built to hold one inch of rain will over-fill:

What this plot tells us is that in any week there’s a 25 percent chance that the runoff ponds specified by Durham’s UDO will overflow with a two-day rainfall. In any given month, the probability is about 60 percent. This conclusion means that our runoff ponds are too small, hence impervious surfaces in Durham are pretty much directly connected to our streams.