Chapter 13: Land Use, Stream Flow, and Runoff Pollution

Questions
Exercises
Links

Summary
Water has a major effect on the landform, shaping the land through erosion and deposition. Likewise, land use has a significant influence on water balance, affecting infiltration and runoff, peak and baseflows. Although any land disturbance will change the water balance, land development and urbanization and their associated impervious surfaces inhibit infiltration and speed runoff. These effects combine to cause higher peak discharges and greater stormwater and flooding problems during major storm events and reduced baseflows and low flows between storms and during drought. While much attention has been given to land development impacts on peak flows because of potential economic damages, impacts on low flows have a significant effect on stream ecology. It is not uncommon for urban streams to dry up during drought because reduced infiltration limits subsurface baseflow for the streams. In addition, increased storm flows carry with them a range of water pollutants.

Several methods now exist to assess the effect of changing land use on water flows, including peak storm flows, runoff pollution, and stream integrity. This chapter and appendices describe a number of these techniques, including the Rational Method, TR 55, the Simple Method, stream surveys, and testing channel capacity. Managing stormwater and natural channels involves applying these and other analytical methods to inform land development and design as well as preservation and restoration decisions. The next chapter presents approaches to stormwater management and stream restoration, including a range of stormwater management practices.

Chapter 13 Discussion Questions

1. The graph below from Schueler (2000; Basic Concepts of Watershed Planning)) shows a relationship between stream health and watershed imperviousness.
a. Provide three basic reasons that may explain this relationship.
b. Give two measures can alter this relationship, i.e., reduce the negative effect of imperviousness on stream health?

Chapter 13 Exercises

1. The attached topographic map shows a point on Little Fox Creek that is being considered for a small park. The community wants to assess the effects of land use on the stream water quality at this point. The first step is to delineate the watershed, and they have asked you to do so. The watershed is defined by the watershed outlet point on the map. Clearly show and label the boundaries of the watershed on the map.

2. Using a topographic map of your area, find the point where a local stream flows into a larger one and delineate its watershed.

3. Use the Rational Method to determine the peak discharge from a 2-year storm from a 100 acre site in Miami (see Figure 13.4) consisting of 20 percent rooftops, 10 percent streets and driveways, 40 percent lawns at 2 percent slope on sandy soil, and 20 percent woodland. The height of the most remote point above the outlet is 20 feet and the maximum length of travel is 1200 feet.

4. The following hydrograph shows the baseflow of a stream draining a watershed as well as the discharge response to a 10 year storm. About one-half of the watershed is urbanized with conventional residential, commercial, and transportation uses, and about half is undeveloped. The baseflow is the flow during non-storm periods and is supported primarily by groundwater outflow. On the hydrographs, draw the changes in both the baseflow and storm discharge that would result from the three scenarios: (a) full conventional development, (b) existing development with retrofit of distributed detention ponds, and (c) existing development with retrofit of detention and infiltration measures. Briefly explain your results. (add graphics)
a. Overlay "full conventional development" hydrograph: Briefly explain:
b. Overlay "detention ponds" hydrograph: Briefly explain:
c. Overlay "detention and infiltration" hydrograph: Briefly explain:

-Two developers propose to build a planned community in the upper part of a 600-acre watershed. Consult chapters 13 and 14 and the TR-55 documentation posted on the Internet ftp://ftp.wcc.nrcs.usda.gov/downloads/hydrology_hydraulics/tr55/tr55.pdf.

The watershed has a 10-year, 24 hour storm of 5 inches and a 2 year, 24 hour storm of 3 inches. The area is currently about 120 ac. meadow, 110 ac. fair pasture, 120 ac. cultivated good straight row crop, and 250 acres fair woods. The soils are mostly silty clay and average drainage (Hydrologic Soil Class B). The watershed distance from the most remote drainage point to the outlet is 9000 feet. The drainage has three segments:

  Segment AB: Sheetflow, 200 feet long, 5% slope, 100% light underbrush
  Segment BC: Shallow concentrated flow, 1300 feet long, 1% slope, 100% unpaved
  Segment CD: Channel flow, 7500 feet long, Manning’s roughness coefficient 0.2, hydraulic radius 1.0, 0.5% slope.

The first development plan (A) would be equivalent to about 1 acre lots over 300 acres, 75 ac. in commercial and business use, 75 ac. in impervious streets and parking, 50 ac. in open space (80% grass cover), and 100 ac. remaining in woods. Segment AB same, except 50% light underbrush, 25% short grass, and 25% smooth surfaces; Segment BC same, except 50% unpaved, 50% paved; Segment CD same, except roughness coefficient 0.1.

T he second plan (B) provides for the same number of development units but in a different form: 72 acres would be in 1/4 acre lots, 48 ac. commercial and business, 30 ac. impervious roads and parking, but the rest is left in commonly held open land: 90 ac. in meadow, 60 ac. fair pasture, 30 ac. contoured row crops, 180 ac. in forest, and 90 ac. open space (80% grass cover). Segment AB same, except 70% light underbrush, 20% short grass, and 10% smooth surfaces; Segment BC same, except 75% unpaved, 25% paved; Segment CD same, except roughness coefficient 0.15.

  a. Download and print the TR-55 worksheets 2, 3, 4, and 6a from the TR-55 site ftp://ftp.wcc.nrcs.usda.gov/downloads/hydrology_hydraulics/tr55/tr55.pdf. Using the SCS TR 55 graphical methods determine the following.
    1. The composite runoff curve number and runoff in watershed-inches resulting from a 10-year, 5-inch, 24-hour storm for the existing conditions and for the two development plans (A & B). Assumptions of Table 13.5 (Table 2.2 in TR-55) hold.
    2. The channel segment travel time and total time of concentration for the three cases.
    3. The unit peak and peak discharge.
      b. A county ordinance requires that developments provide measures so that zero-net-increase in discharge results. What detention storage volume would be required by the two development options to comply with this ordinance? See chapter 14, pp.464-467.
Use TR 55 Worksheets 2, 3, 4 and 6a. Record values below. Submit set of worksheets for Plan B.
CN Runoff
inches. AB
Tt BC
Tt CD
Tt TC Ia Unit pk csm/in Peak
cfs qo /qi Vs /Vr Detent
ac-ft
Before
XXXXX
XXXXX
XXXXX
Plan A

Plan B

c. What means of mitigating peak discharge effects would you suggest for the development designs?
7. You have been asked to assess the capacity of a natural channel to carry increased flows caused by upstream development. A peak discharge analysis has shown that a ten-year, 24-hour storm is likely to create flows of 350 cfs. A stream survey has shown that the stream cross section is roughly parabolic with average top width of 28 feet average maximum depth of 3.5 feet. The average slope shown on the stream profile is 0.015 feet per foot. The channel banks are about 60% fine gravel, 25% shales, and 15% firm loam. The natural winding stream is in poor condition; it has moderate irregularities, frequent size changes, minor obstructions, and minor meandering; vegetative growth is characterized by moderately dense stemmy weeds about half the height of the average flow.
Determine the expected channel-full flow velocity and capacity and compare them to the permissible velocity and the expected 10-year peak stream flow. If the channel does not pass the tests, make some recommendations. Put your answers in table below, but show all work.

Vperm. Qpeak Area Hyd.Rad. Slope n1 n2 n3 n4 n5 n6 n Vfull Qfull Pass?

1. Using Schueler’s Simple Method, calculate the stormwater pollutant load of fecal coliform, total suspended solids (TSS), and lead (Pb) in runoff from 3-inch, 24 hour storm from a 4 acre urban site that is 40% impervious and has typical runoff pollutant concentrations.

2. You are part of a local watershed improvement group and one of your tasks is to assessing a stream reach with the following visual characteristics:
a. Altered channel
b. Flooding occurs once every four years
c. Natural vegetation extends half of the active channel
d. Stream banks are unstable
e. Water appearance is cloudy most of the time
f. Water is greenish and there is an abundance of algae
g. There are 3 fish cover types available
h. There is a 3 foot drop structure 2 miles from the reach
i. Pools are 3 or more feet deep, but not abundant
j. There appear to be 3 types of invertebrate habitat present.
A streamside biosurvey yielded the following data:
a. 3 waterpenny larvae
b. 5 stonefly numphs
c. 100 snowbugs
d. 10 crayfish
e. 8 Bettle larvae
f. 80 scuds
g. 10 fishfly larvae
h. 10 midge larvae
i. 100 snails
1. Using the USDA Stream Visual Assessment protocol (see Appendix 13.B), calculate the visual assessment score for the stream reach.
2. Using the EPA Stream Monitoring protocol (see Appendix 13.C), calculate the biosurvey assessment score for the stream reach.

Chapter 13 Weblinks:
TR-55 documentation: http://www.wcc.nrcs.usda.gov/water/quality/common/tr55/tr55.pdf
WQS for each state see http://www.epa.gov/ost/wqs/
Water quality data from STORET (1999 and after) and the Legacy Data System (before 1999): http://www.epa.gov/storet/about.html