Storm Sewer Network (Figure 20)

Water cannot be understood as an isolated system. It is dynamic and moves from the oceans to the atmosphere, to the land and back to the oceans. During and after rainstorms, water moves across the land as stormwater runoff when the surface of the land is impermeable or becomes saturated. A portion of this rain or snowfall soaks into the ground and percolates slowly through the soil and weathered bedrock to a zone of saturation. Here water slows but does not stop, draining along the gradient of the water table through fractures, or pore spaces in the rock. It reappears weeks or months later at the surface of the land as springs or seeps. These sub-surface pathways reflect the underlying geology. Water that has infiltrated into the ground and emerges in the stream channel is called “base flow.” The springs and small tributaries known as “first order” streams combine to form larger creeks and stream such as Meeting of the Waters and Morgan Creeks. The shape and location of streams is also determined by the groundwater discharge or base flow combined with surface runoff of stormwater.

In the Piedmont, rain falls with sufficient intensity to produce surface runoff approximately thirty days in a given year. During the remaining eleven months, base flow keeps the streams flowing. In an undisturbed and forested landscape in the North Carolina Piedmont, the annual rainfall averages about forty-five inches a year. Of this total, only a relatively small fraction runs off the ground during storms — on the order of eight to ten inches a year. A much larger amount of thirty-four to thirty-six inches soaks into the soil mantle. Much of this water is returned to the air as evaporation and transpiration from the vegetation. Approximately 15% or about 6 to 8 inches finds its way to the aquifers in the deeper bedrock, to reappear as base flow in the streams.

The paved surfaces of our developed communities disrupt this natural hydrologic cycle. They unbalance the equation and turn rainfall into runoff. Every square foot of impervious surface — roofs, roads, parking lots, etc. — produces as much as three additional square feet of runoff per year. The immediate effect of this change in the hydrologic balance is the torrents of muddy water pouring off roofs and rushing down streets in a storm. Not so obvious is the critical loss of base flow to local streams. The net result is a stream which is either inundated by floods with unstable banks and a rapidly migrating channel or dry and without the water to support aquatic life.

The damaging effects of flood and drought are the unnecessary results of thoughtless and unsophisticated land development. This report will pinpoint areas of the University where the hydrologic cycle is out of balance and suggest methods and approaches to planning and design that will help to restore that balance and with it the health of the water system.

Small streams become inundated during floods and dry with a low base flow between storm events (Figure 21)
The report divides the University property into coherent small drainage units or sub-basins. While it often goes unnoticed that the University is part of two larger watersheds, it is even more difficult to identify the smaller drainage divides. Local ridges and high points, as well as drainage infrastructure, divide the Campus into 21 smaller watersheds or sub-basins (Figure 18). Within each sub-basin the land surface drains to a given surface stream or swale, although many of these drainage elements have been buried overtime and are not obvious (Figure 17). Most Campus residents are totally unaware of these drainage elements as they move across the campus and do not divide the landscape into watersheds, but this structure will serve to measure the basic land and water system for much of the analysis that follows.

Since these sub-basins form the basic unit of the analysis, both natural and man-made land cover characteristics are aggregated by sub-basin. Because the planning boundaries that exist within the Campus property rarely coincide with sub-basin boundaries, the analysis first examines sub-basin characteristics on a watershed basis in order to evaluate the overall health of each sub-basin (Table 1). The US EPA states that watersheds that experience urban growth resulting in impervious cover greater than 25% are highly impaired and present tremendous challenges to remedy. Many sub-basins within and surrounding the University property have already been impacted by development with impervious percentages exceeding 25%. Therefore, as we look toward the future and proposed expansion of the Campus, it is important that we are sensitive to the existing sub-basin conditions as we plan within the boundaries of the University.

In order to document the net impacts of development on stormwater runoff volume, the changing patterns and composition of the land cover are aggregated by sub-basin as well as the analysis planning area (Tables 2 and 3). For example, the total amount of stormwater runoff, as well as the pollutant load generated during a storm, can be estimated by measuring the amount of impervious surface that covers the sub-basin area within University property. This information lends itself to a comparison of existing sub-basin characteristics on Campus and the future characteristics proposed by the Campus Master Plan.

This report will use a “water balance model,” a form of bookkeeping based on the hydrologic cycle, to analyze the effects of the impervious and pervious surfaces at the University on the amounts and quality of runoff, as well as to highlight the net changes in the entire water system. The model Is based on the idea that the hydrologic cycle, as described above, is an input/output system. For example, if water is nor recharged within the sub-basin where it falls, this loss cannot be made up on another site in an unrelated sub-basin. W hen the natural hydrologic cycle is sufficiently changed by development, only solutions that replace water that previously infiltrated the ground (or evaporated and transpired by vegetation) will address the broad issues. These issues include increasing flood and drought peaks, non-point source pollution, the excessive nutrient loads that are carried to local reservoirs, and loss of soil and natural vegetation with their complex interrelationships to the health of both land and the water.

The various methods and materials that are proposed for sustaining the hydrologic balance on the developing Campus are intended to replicate the natural balance. Their efficiency can be tested using the Water Balance Model as detailed solutions evolve in different portions of the Campus. For example, the most highly impervious portions of the Campus offer very limited opportunities to return runoff to the soil mantle, since little open land remains. In these locations, structural systems that intercept rainfall on rooftops and return it to the atmosphere by vegetative systems attempt to replicate the lost vegetation from the land surface, and can reduce the runoff volume increase by 50%. Other systems that capture and re-use rainfall also serve to replace the natural functions of the soil.

Various types of impervious surfaces will generate greater or lesser concentrations of pollutants, and these differences can also be documented by the model and included in the analysis of mass transport of sediment conveyed by runoff. All of these analyses of the natural and man-made systems require a form and structure, which takes the form of the topologic boundaries of the sub-basins created by the drainage patterns.

Hydrologic Soils Group (Figure 22)

To understand if it possible to infiltrate water within the area Campus, it is necessary to identify the undeveloped degree of soil permeability. This physical property of soils largely governs the amount of stormwater runoff during precipitation. The USDA-SCS analysis of soil series by drainage characteristics has resulted in a classification system that ranks soil by its ability to infiltrate rainfall. The system classifies each soil in a hydrologic group ranked from A (dry) to D (wet), with the group providing the best soil drainage conditions. In Figure 22, soils with a greater infiltration capacity that are less likely to produce runoff are shown in lighter colors, while the wetter, less permeable soils are shown in darker colors.

Surface Permeability (Figure 23)

As can be seen in Figure 22, Main Campus, south of South Road is comprised mostly of B soils. These soils may be suitable for infiltration and recharge techniques. Mason Farm is comprised of both C and D soils, shown in Figure 22. These soil groups have a lower infiltration rate and consist of soils that are poorly drained.

Soils that have been significantly altered by filling or regrading are shown in gray, and may have variable drainage properties. The Historic Core of Main Campus consists largely of these altered soils, and any conclusions about drainage characteristics here must be determined by field tests.

Large relic trees on the Campus greens. (Figure 24)
Campus parking area with a wooded edge. (Figure 25)
In order to quantify the amounts of stormwater runoff currently generated and to estimate the increase in runoff after redevelopment of the Campus, land cover types were identified by permeability. Land Cover is generally considered as pervious or impervious. However, in order to model more accurately how a change in land cover will impact the runoff volume produced by a given rainfall, an additional classification has been created to describe the land surface that has been disturbed and probably re-compacted, and planted in turf or other landscaped areas. Therefore, three land cover types can be described by permeability; pervious, semi-pervious and impervious as described below:

  1. Pervious – Natural forest and woodland that are considered fully pervious for infiltration.
  2. Semi-Pervious – Lawns and landscaped areas where compaction and grading during construction have reduced soil permeability are considered semi-pervious.
  3. Impervious – Surfaces that include all the paved surfaces that are built to ensure that water does not penetrate.
Permeability Percentage (Figure 26)

Areas in forest retain spongy, absorbent soils that hold water and allow it to seep slowly back in to the groundwater. On the southern portion of Main Campus, some of the slopes have been left in forest and are the major areas of recharge returning stormwater to the aquifer. On the flat plateaus of the Historic Core, the land is covered in a mixture of impermeable paved areas and semi-permeable grassy open space — “the greens.”

Turf varies in the amount of runoff it produces. While well-maintained grass produces little runoff, turf in poor health, on compacted soils, or on steep slopes may produce volumes up to 25% (Watershed Protection Techniques, 2(1): 239-246). In these cases, the improvement of turf grass health, or its replacement with vegetation better suited to harsh growing conditions, may provide water quality benefits.

The relative timing of fertilizer applications and water inputs — from irrigation or heavy rainfall — will also influence the leaching and transport of nutrients to ground water. The careful monitoring and management of lawn care can, therefore, make a difference to groundwater quality within the current landscape.

At present, the soils of the “greens” are heavily compacted. Soil compaction occurs when surface weight, whether it is repeated foot traffic or machines, collapses the spaces where water, air, and soil organisms can move, creating a hard, solid, non-living mass. Soil compaction is an insidious problem because the effects are often understimated. Heavily compacted soils act as a barrier to root growth, inhibit the exchange of atmospheric gases and also restrict the infiltration of water. Significant recharge cannot occur in these areas unless the compacted surface layer is broken through and replaced with a porous substrate.

The high percentage of impermeable surface on the South Campus has exacerbated the amount and speed of stormwater runoff, as well as the amounts of non-point
source pollutants that reach Meeting of the Waters, Morgan Creek and other surface tributaries that lie south of the Campus. Consequences to these stream channels that flow in The North Carolina Botanical Garden, the Mason Farm Biological Reserve and adjacent community parks such as Jones Park are severe. In particular, stream channels have migrated and their banks have been destabilized in an effort to handle the increased water volume and velocities and the increased sediment load.

In the Water Balance Model, the description of land cover composition by sub-basin is used as the basis to estimate how much of a given rainfall amount will be transformed into direct and immediate runoff. Using a methodology called the Cover-Complex Method (USDA, SCS, 1967), the combination of vegetative cover and soil hydrologic group is classified by a term called Curve Number. This is a designation that refers to a set of curves derived from the basic equations for rainfall-runoff relationships. Based on these estimated Curve Numbers, each type of cover can be used to calculate the net runoff for a given frequency and amount of rainfall, as shown in Table 4.

The statistics developed in Table 5 can be considered preliminary based on the best available existing data. The initial comparison of 1998 impervious cover and projected Master Plan impervious cover indicated a net increase of only a few acres. The net increase of 13.5 acre-feet is still a relatively small increase in runoff volume from the Campus, but serves as the initial target for volume mitigation measures. A more meaningful statistic is the total runoff volume during a 2-year frequency rainfall of 3.6 inches, a figure that represents some 96% of the total rainfall in a normal year. This total volume of 1,234 acre-inches (161 acre-feet) from the Campus would fill the football stadium (4.3 acres at mid-height) to a depth of almost 25 feet. It is this volume of runoff that flows from the Campus downhill to the receiving surface streams every year.

Table 1. UNC-Chapel Hill: Existing (1998) Land Cover: Characteristics by Sub-Basins
October 2001
Sub-Basin Total Area (acres) Total Sub-Basin Area Within UNC Property Total Impervious Area* Total Pervious Area**
(acres) (%) (acres) (%) (acres) (%)
Battle Creek
BA-1 257 134 52% 52 20% 205 80%
Bolin Creek
BO-1 145 20 14% 67 46% 79 54%
Chapel Creek
CH-1 157 77 49% 20 13% 137 87%
CH-2 106 53 50% 17 16% 90 84%
CH-3 217 138 64% 37 17% 180 83%
Meeting of the Waters Creek
ME-1 89 86 97% 50 56% 39 44%
ME-2 48 48 100% 37 78% 10 22%
ME-3 88 78 89% 33 38% 54 62%
ME-4 54 54 100% 20 38% 33 62%
ME-5 137 132 96% 44 32% 93 68%
ME-6 32 18 55% 4 13% 28 87%
ME-7 49 24 49% 9 19% 40 81%
ME-8 244 22 9% 32 13% 213 87%
Morgan Creek
MO-1 608 51 8% 75 12% 533 88%
MO-2 157 16 10% 16 10% 142 90%
MO-3 261 33 12% 25 10% 236 90%
MO-4 179 83 46% 7 4% 172 96%
MO-5 407 227 56% 29 7% 378 93%
MO-6 197 85 43% 48 24% 149 76%
MO-7 674 438 65% 8 1% 667 99%
MO-8 979 133 14% 5 0% 974 100%

*Includes Building Rooftops, Roads and Walkways
**Includes both Pervious and Semi-pervious Surfaces

Table 2. UNC-Chapel Hill: Existing (1998) Land Cover
Land Cover Analysis by Campus Property
October 2001
Sub-Basin Total Sub-Basin Area Within UNC Property (acres) UNC Property Total Impervious Area Woodland (acres) Lawn/Maintained Grass & Shrubs (acres) UNC Property Total Pervious Area
(acres) (%) (acres) (%)
Battle Creek
BA-1 134 25 19% 76 33 109 80%
Bolin Creek
BO-1 20 7 37% 0 13 13 63%
Chapel Creek
CH-1 77 4 5% 55 18 73 95%
Meeting of the Waters Creek
ME-1 86 48 56% 3 35 37 44%
ME-2 48 37 78% 0 10 10 22%
ME-3 78 33 42% 7 38 45 58%
ME-4 54 19 36% 10 24 34 64%
ME-5 132 44 33% 39 49 88 67%
ME-6 18 2 11% 13 3 16 89%
ME-7 24 6 24% 13 5 18 76%
ME-8 22 1 6% 18 2 20 94%
Morgan Creek
MO-1 51 19 36% 12 20 33 64%
Totals 742 245 247 250 497
Table 3. UNC-Chapel Hill: Master Campus Development Plan
Land Cover: Land Cover Analysis by Campus Property
October 2001
Sub-Basin Total Sub-Basin Area Within UNC Property (acres) UNC Property Total Impervious Area Woodland (acres) Lawn/Maintained Grass & Shrubs (acres) UNC Property Total Pervious Area
(acres) (%) (acres) (%)
Battle Creek
BA-1 134 29 21% 76 29 105 79%
Bolin Creek
BO-1 20 9 46% 0 11 11 54%
Chapel Creek
CH-1 77 4 5% 55 18 73 95%
Meeting of the Waters Creek
ME-1 86 55 64% 2 29 31 36%
ME-2 48 36 76% 0 11 11 24%
ME-3 78 39 49% 6 33 39 50%
ME-4 54 23 42% 9 22 31 57%
ME-5 132 70 53% 17 43 60 45%
ME-6 18 3 18% 13 2 14 81%
ME-7 24 8 33% 9 6 15 64%
ME-8 22 8 35% 10 5 14 65%
Morgan Creek
MO-1 51 25 49% 9 17 26 51%
Totals 742 308 204 226 431

Note: Proposed water feature areas have been excluded from the analysis because they do not possess properties of either pervious or impervious surfaces for the runoff analysis.

Table 4. UNC Rainfall and Runoff Estimates based on the Cover-Complex Method
(Assumes all soils are hydrologic group B)
Rainfall (inches/24 hours) 1-yr 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr
3 3.6 4.7 5.4 6.4 7.2 8.0
Land Cover/Use Curve Number Runoff (inches/24 hours)
Roofs, pavement 98 2.8 3.4 4.5 5.1 6.2 7.0 7.8
Residential use 85 1.6 2.1 3.1 3.7 4.7 5.4 6.2
Lawn, fields 98 0.7 1.1 1.8 2.3 3.1 3.8 4.5
Meadow, shrubs 98 0.5 0.8 1.5 1.9 2.7 3.3 3.9
Woodland 98 0.3 0.6 1.1 1.5 2.2 2.7 3.3
Table 5. Comparison of Net Increase in Runoff Volume for 2-Year and 100-Year Storm Events
Sub-Basin Existing Volume
Master Plan Volume
Existing Volume
Master Plan Volume
2-Year 100-Year
BA-1 162 171 593 605
BO-1 39 43 115 121
CH-1 64 64 294 294
ME-1 202 216 540 561
ME-2 136 134 335 332
ME-3 155 169 448 468
ME-4 96 105 291 302
ME-5 221 291 687 790
ME-6 17 19 70 73
ME-7 32 39 111 119
ME-8 17 36 81 112
MO-1 91 108 275 301
Totals 1234 1395 3839 4079

Net Increase for Master Plan

161 ac-in (13 ac-ft)
239 ac-in (20 ac-ft)