Stormwater is in the news in Durham.

A new city ordinance has raised awareness of the issue, and the cost of meeting state requirements. Stormwater management in urban areas is not simple, but it is essential for healthy streams and lakes, and a safe water supply. To help people understand the situation, ECWA’s water quality committee has prepared an overview description of the underlying causes and impacts of stormwater in Durham, and a systematic approach to stormwater planning that would help clean up Ellerbe Creek and restoring its ability to provide valuable ecosystem services.

In brief, Durham is dealing with two water quality issues that are functionally related, but that differ significantly in their impacts and in the appropriate management strategies for dealing with them. The first issue is the need to maintain the quality of the water in Falls Lake and Jordan Lake. The second is degradation of Durham’s streams. Both issues need to be managed. Steps taken to improve the health of the streams would also improve conditions in the lakes; unfortunately, the reverse is not necessarily true.

Like most of Durham’s streams, Ellerbe Creek is in poor condition; in 1995, NC DENR identified it as one of the four lowest rated streams in the Neuse River basin. The main cause of impairment is stormwater runoff from impervious surfaces, particularly those in downtown Durham. This urban stormwater runoff damages streams in two different ways: by pollutants are added to the water, and by altering the rate at which water is added to the stream channel. Rather than soaking into the soil and becoming groundwater, rain flows quickly off of impervious surfaces into stormwater pipes that dump into streams immediately following a storm, producing extreme high flows and flash floods that scour the stream channels. The lack of groundwater recharge during storms results in extreme low flow conditions during the dry, hot summer months. The alternating periods of storm scour and stagnant water produced by unmanaged urban stormwater are devastating to the animals and plants that should be living in the stream.

Conserving undeveloped land in the western part of the Ellerbe Creek watershed will help keep conditions from getting worse. To actually improve conditions, it will be necessary to clean up and slow down stormwater runoff from Durham’s urban core. This will not be easy to do, but it is possible.

To learn more, see the full document: A Systematic Strategy for Stormwater Management in the Ellerbe Creek Watershed.

-- Kathi Beratan

What is meant by water quality?

The water in a stream does many things. It is a home for aquatic plants and animals; birds and other land animals rely on it; people drink it; people fish and swim in it. A stream with high water quality is able to do all these things well; as water quality deteriorates, so does the stream’s ability to fulfill these functions.

The North Carolina Division of Water Quality (NCDWQ) determines the water quality in a stream by counting aquatic macro-invertebrates – animals without backbones, such as insects and worms, that can be seen without a microscope. A healthy stream will have a lot of macroinvertebrates, of many different species living in it. If there aren’t many bugs, and/or if the ones that are there are from the few species that aren’t very sensitive to pollutants, then the stream is considered impaired. Ellerbe Creek has been classified as impaired.

Why does our creek have water quality problems?

Many different things can contribute to decreased water quality. There are two main categories: stuff in the water, and alterations in how fast water gets to the stream.

Our streams are a lot cleaner these days than they used to be because laws prohibit dumping of pollutants into them. Most of the unwanted stuff in the water comes from non-point sources – stuff washed off of roads and other surfaces by rainwater, and sediment eroded from stream banks and off of exposed soil at construction sites. NCDWQ considers sediment to be the leading cause of water quality impairment in North Carolina's streams and rivers, while nutrients (nitrogen and phosphorous) are the most significant problem in our lakes. Regulation of stream water quality is largely focused on reducing the amount of nutrients flowing into lakes. This is good for the lakes, but doesn’t necessarily help the streams

Often overlooked by regulators is the impact of changes in stream flow volume related to stormwater runoff from impervious surfaces - things that water can’t pass through, such as pavement, sidewalks, and roofs. Most of the rain falling within the watershed runs off into storm drains and ditches, and is dumped quickly into streams instead of soaking into the ground and traveling slowly to the streams, particularly in areas with a lot of impervious surfaces. Groundwater is the main source for water in streams between storms, and so urban streams often have very little water in them when it isn’t raining.

The Ellerbe Creek watershed is mostly urban, with a high percentage of impervious surfaces. During storms, too much water gets to the Creek too rapidly, causing scouring of the streambed and erosion of the stream banks, and may cause flash floods. The water flow drops nearly as rapidly after the storm passes. So much of the rain flows down the stream so quickly that the stream level is unnaturally low in between storms. This pattern of extreme low and high flows due to impervious surfaces and storm drains makes it very difficult for animals and plants to survive.

As any gardener knows, Durham’s soils are unusually clay-rich. Water has trouble sinking into such clay-rich soils unless those soils have been opened up by deeply-rooted plants like trees. Almost all of the rain that falls on a forest sinks into the ground or returns to the atmosphere as water vapor. Grass has very shallow roots, and doesn’t open up the soil very much. Therefore, although lawns hold more water than pavement, much more rainwater tends to run off of lawns than from forests.

Monitoring stream flow.

Stream flow, or discharge, is the volume of water that moves past a point in the stream over a given period of time. It is usually reported in units of cubic feet per second (ft³/sec). As in other urban streams, discharge varies enormously over time in Ellerbe Creek in response to storms.

The extreme high and low stream flows that are caused by impervious surface runoff do tremendous damage to Ellerbe Creek, and make it very difficult for animals and plants to live in it. To improve conditions, we need to learn more about stormwater flows in Durham, particularly about how different combinations of impervious surface, lawns, and forests affect that flow, given Durham’s clay-rich soils. This type of information is necessary in order to design effective stormwater management projects.

The U.S. Geological Survey monitors water flow in Ellerbe Creek near Gorman NC, just before the Creek reaches Falls Lake. Although extremely useful, this single monitoring station doesn’t provide quite enough information to let us figure out best management practices. The City of Durham doesn’t collect any additional stream flow data.

To help fill this information gap, ECWA has established two volunteer flow-monitoring stations. One is at the east end of the 17-Acre Wood trail, across from Westover Park; the second is near the corner of Washington St. and Club Blvd., at the west end of the new greenway trail that parallels I-85. Both are small tributaries that drain relatively small areas (sub-basins) within the watershed; the sub-basin draining to the 17-Acre Wood site has a lot of grass and forest, while the sub-basin draining to the Washington St. site is mostly urban residential. Comparing differences in how the two small tributaries respond to storms with the differences in land cover between the two sub-basins will give us the information we need for planning.

 

RELATED LINKS

• U.S. Geological Survey Real-Time Water Flow Data, Ellerbe Creek near Gorman, NC (close to Falls Lake) - waterdata.usgs.gov/usa/nwis/uv?site_no=02086849
• GoMap, Interactive GIS for Durham (City and County) - gisweb.durhamnc.gov/gomaps/map/index.cfm
• EPA Water Science homepage - www.epa.gov/waterscience/
• EPA Monitoring And Assessing Water Quality homepage - www.epa.gov/owow/monitoring/
• Durham’s Stormwater Services Division - www.durhamnc.gov/departments/works/stormwater.cfm