Streams in urbanizing landscapes: from syndrome diagnosis to watershed

prescriptionNSF Grant 1258017

Urban Streams:

Examples of How Secondary Mathematics is used in Scientific ResearchJULIE GRAVES

NCSSM TCM CONFERENCE

JANUARY 2015

The Water Cycle in an Undeveloped Watershed

The Water Cycle in a Developed Watershed

Moving Water

Water travels downhill from points of higher energy to point of lower energy, unless forced to do otherwise, until it reaches a point of equilibrium, such as a large lake or an ocean. This travel is facilitated by the presence of natural conveyance channels such as brooks, streams and rivers. The water’s journey may also be aided by man-made structures such as drainage swales, pipes, culverts and canals.

How do urban watersheds differ from undeveloped watersheds?

Impervious Surfaces As watersheds are urbanized, much of the vegetation is replaced by impervious surfaces, thus reducing the area where infiltration to ground water can occur.

The presence of more impervious surfaces means that more storm water runoff occurs. This runoff must be collected by man-made drainage systems that combine curbs, storm sewers, and ditches to carry storm water runoff directly to streams.

In a developed watershed, much more water arrives into a stream much more quickly. The result is an increased likelihood of more frequent and more severe flooding.

Urban Stream Syndrome Higher peak flows Channel erosion Increased pollutant

concentrations Loss of sensitive

organisms

Development Intensity and Hydrographs

Hydrographs for urban stream and natural streams are very different and present different sets of consequences for the environment.

It is well understood that the percent of impervious cover in a watershed is an important explanatory variable. More impervious cover is associated with “flashier” hydrographs, higher concentrations of downstream pollutants and more downstream erosion.

Time

Dis

char

ge

Pre-Development

Post-Development

Storm hydrograph

Research Goals

The status quo in watershed management has been to control the amount of impervious surface area. This approach does not take into account the fact that there are other aspects of development that could be the cause of adverse ecological impacts. It could be that the real drivers of the “urban stream syndrome” are other variables, including storm water infrastructure (density of pipes) , road densitythe configuration of impervious cover,

The key to reducing harmful ecological outcomes may be to control these other factors.

Research Goals Researchers hope to understand the relationship between the pattern or configuration of development and the ecological consequences (habitat loss, pollution). If they succeed, it will be possible to manage watersheds so that a given level of development intensity can have a smaller impact on the health of urban streams.

Configuration of Development Here are USGS maps of several watersheds in North Carolina that are being studied. The watersheds have roughly equal areas and have comparable impervious cover percentages. The differences are in other characteristics that are being investigated as explanatory variables.

Explanatory Variables that quantify “configuration of development”

% Developed in watershed

% impervious Surface Area

% forest

Pipe density Meters/square km

Inlet Density Meters/square km

Road Density Meters/square km

Stream Density Meters/square km

Number of Patches FRAGSTAT

Mean Patch Area FRAGSTAT

Clumpiness FRAGSTAT

Correlation Length FRAGSTAT

Response Variables Flooding: as represented by a hydrograph that shows discharge over time

Contaminant Loading: as measured by concentration of chemicals in streams

Secondary Mathematics at Work (1) Measuring Discharge (2) Using Stage to Predict Discharge (3) Regression and correlation for relationships between explanatory and response variables

Measuring Discharge

Calculating Discharge

Measuring Stage

Data Analysis at Work

Discharge, or flow, is measured in cubic meters per secondDischarge is expensive and time consuming to measure, though it can be doneStage (depth) is much easier and cheaper to measureStage and discharge are closely related; a graph that shows this relationship is called a

stage-discharge rating curveA measurement of stage, together with a rating curve, makes it possible to calculate discharge without having to directly measure it.

Stage Discharge Rating Curve

Stage-Discharge Rating Curve

Rating Curve A power function relationship is most frequently used to model the relationship between stage and discharge:

Q = discharge (cubic meters per second) h = measured water level (meters) a = water level corresponding to Q=0 c = coefficient that quantifies the particular characteristics of a stream channel

Rating Curve Calculations Based on measured data that provides a value for a, and that gives values of h and Q that occur together, we can do linear regression to ordered pairs in which one coordinate is log(h+a) and the other coordinate is log(Q).

The slope of the resulting regression line will be the value of b and the intercept of the line will be log(c).

If then

Real-time Data USGS provides an abundance of real time and historical data.

Go to http://waterdata.usgs.gov/nwis/rt for information about rivers anywhere in the country

Stage and Discharge Data for Eno River Hillsborough NC

January 2013

February 2013

March 2013 April 2013 May 2013 June 2013

Discharge (cubic feet/sec)

63.0 72.0 44.4 48.8 48.2 41.2

Stage (feet) 2.241 2.436 2.216 2.238 Missing data 2.026July 2013 August 2013 September

2013October 2013

November 2013

December 2013

Discharge (cubic feet/sec)

101.7 70.4 10.3 8.24 10.7 85.5

Stage (feet) 2.516 2.335 1.638 1.603 1.623 2.422

Preliminary Results

Conclusion To manage urban watersheds more effectively, we need to understand how to separate effects of development intensity from development pattern.

Ongoing urbanization will make us unable to actually reduce impervious surface cover. Therefore, we would like to learn if we can manage the configuration of development and its associated infrastructure. If we can, we may be able to mitigate the effects of developed surface area.

What are researchers interested in?

Paved parking lot photo Parking lot with vegetation

photo

Thank You