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FLOOD HAZARD ASSESSMENT AND CHANNEL GEOMETRY:
WILLIAMSON CREEK




Prepared for:
 Dr. Earl
Geography 4430
     Southwest Texas State University

Prepared by:
Holly Mittel
And
Tonya Reese

April 18, 2002





 Table of Contents

            
I.    Background and Introduction                                                            2

II.    Approach                                                                                        3

III.    Description of Development Site                                                     3

IV.    Description of Drainage Basin                                                         4

V.    Calculation of Peak Discharge - USGS Regression Method              5

VI.    Calculation of Peak Discharge - Rational Method                            6

VII.    Time of Concentration                                                                   6

VIII.    Field Analysis and Verification                                                      7

IX.    Analysis                                                                                         8

X.    Conclusion                                                                                      8

XI.    References                                                                                     9






 Williamson Creek Flood Hazard Assessment
and Channel Geometry


Background
    South Austin is a heavily developed area and is continuing to grow at an ever-increasing rate.  Also, the occupancy rates of apartments in the Austin area are at an all time high, which has provided CNC Investments incentive to purchase and develop one of the last empty plots in South Austin.  CNC Investments intends on building a 496-unit apartment complex on 34 acres of land between Manchaca Road and Westgate Lane.  However, the residential area northwest and downstream from the property are already prone to flooding and now the Cherry Creek Neighborhood Association fears a threat of increased flooding due to such high-density development.  At the request of the neighborhood association, an environmental study is being conducted to assess the hazards for the proposed site.  Map 1 shows the current undeveloped site and Map 2 shows the proposed development of that site by CNC Investments.

Introduction
    An essential component of any environmental study is to determine if the flood hazard of the area will be increased due to increased impervious cover.  The purpose of this project was to determine if such a high-density development would, if in fact, increase the flood hazard of the surrounding neighborhood.  A tributary of Williamson Creek flows immediately adjacent to the proposed site, which, less than one-fourth of mile away, meets up with Williamson Creek.  Many residents in the area have recently begun to experience flooding problems, and the residents believe this is a result of high-density development upstream.   
    The proposed site is also situated in close proximity to the Balcones Escarpment.  “Many large storms and catastrophic floods have occurred along the Balcones Escarpment.  About a dozen storms with precipitation depths exceeding fifteen inches in a few days or less have been documented in this area in the past sixty years.  These large storms can cause some flood peaks which would exceed those that can be predicted accurately by analysis of available precipitation data or flood data” (Slade, Asquith, and Tasker 1995).


Approach
    This was a task-oriented project, consisting mainly of fieldwork and library research.
Fieldwork consisted of
 · Locating the flotsam
 · Measuring the stream channel width and depth
 · Measuring the elevation and determining the slope
Library research included
 
· Locating the stream basin on a topographic map
· Delineating the Williamson Creek watershed
· Recording climate and soil data
 The data was then analyzed and Soil Conservation (SCS) and U.S. Geological Survey (USGS) empirical methods were used in order to determine peak flood discharge and flood volume for Williamson Creek.
 

 Description of Development Site
     The area of study is located in South Austin at approximately 6000 Manchaca Road.  It consists of 48+/- acres, which are situated between Manchaca Road and Westgate Lane.  The property is currently undeveloped and is located in a primarily residential area. 
 
 
Description of Drainage Basin

    Williamson Creek rises just west of Oak Hill in southeast Travis County and runs southeast for eighteen miles, through a variety of business and residential areas, to its mouth on Onion Creek.  Map 3 is a topographic map of the drainage basin for reference.  It crosses flat to rolling prairie surfaced by clay and sandy loam that supports hardwoods, mesquite, cacti, and grasses (Williamson Creek 2002).

Elevations
 *    Elevation at outlet:              650 feet
 *    Highest elevation:             1,070 feet
 *    Relief:                                 420 feet

Area of Basin
Ÿ Area with dot planimeter:        10,720 acres          4,340 hectares

Determination of Storm Magnitude*
      Recurrence Interval        Storm Intensity       
       (years)                   (inches per hour)
            2                                  2
            5                                2.5
          10                                2.9
          25                                3.4
          50                                3.9
        100                                4.2

*(Hershfield 1961)
 

Calculation of Peak Discharge (USGS Regression Method*)

Length of Basin along Major Stream from Point of Measure to Drainage Divide: 

    42,400 feet


Determination of Basin Shape Factor (SF):

    SF = (stream length)2 / basin area
    SF = (42,400)2 / 10,720 = 167,600


Determination of Stream Slope, in feet per mile:
           
    Distance from point of measure to drainage divide: 7.0 miles

    Difference in elevation between point of measure to drainage divide:
        Divide Elevation:            1070 feet
        Point of Measure:             650 feet
        Difference in Elevation:    420 feet
                
    Slope in feet/mile:  Difference in Elevation / Distance in miles:
                420 ft. / 8 mi. =  52.5 feet/mile


Calculation of Peak Discharges:

    Formula                Estimated Discharge (cfs)

    Q2     = 252(17)0.721(167,600)-0.326                        40
    Q5     = 525(17)0.648                                             3,300
    Q10   = 732(17)0.667                                             4,800
    Q25   = 1034(17)0.685                                           7,200
    Q50   = 408(17)0.768(52.5)0.281                         11,000
    Q100 = 416(17)0.788(52.5)0.325                         14,000

*(Slade, Asquith, and Tasker 1995)


Calculation of Peak Discharge (Rational Method)

 Equation
 Q = CIA
 Q = Peak Discharge
 C = Rational Runoff Coefficient = 0.70; residential and business areas
  I = 1 hour storm intensity in inches
 A = Drainage basin area in acres
 
 Calculation of Peak Discharge
          Recurrence Interval                  Peak Discharge
                  (years)                                     (cfs)
                      2                                      15,000
                      5                                      18,800
                    10                                      21,800
                    25                                      25,500
                    50                                      29,000
                   100                                     31,500



Time of Concentration
            T = Time in concentration in hours
            L = Basin length in feet = 42,400 feet
            H = Basin relief in feet  = 420 feet
            Formula:    T = L1.15 / (7770 * H.38)
            Time =  2.7 hours or 162 minutes



Field Analysis and Verification

Stream Channel Cross Section


   Estimated Channel Roughness:
        n = 0.059
   
   Calculated bankfull discharge by the USGS slope area method:
        W = 112 feet
        D = 6.01 feet
        A = w*d = 670 feet2
        R =     A         =  670  =  5.4 feet
                2d + w        124

        S = 0.003
        Qpeak = A(1.49)R0.67S0.05
                                      N

        Qpeak =  (670)(1.49)(5.4) 0.67(0.003)0.5
                                             0.054

        Qpeak = 2900 cubic feet per second



Analysis

Calculated Discharge by Rational Method (cfs)
                Slope Area        2yr         5yr        10yr        25yr       50yr       100yr
Cfs              10,720        15,000   18,800    21,800    25,500    29,000    31,500
% of S.A.      xxx             140%     175%    200%      240%      270%     290%


Calculated Discharge by the USGS Regression Method (cfs)
                Slope Area        2yr         5yr        10yr        25yr       50yr       100yr
Cfs              10,720            40        3,300     4,800      7,200     11,000    14,000
% of S.A.      xxx            0.3%        31%       45%       67%      103%      131%
   



Conclusion

    Based upon these calculations, the bankfull discharge has a recurrence interval of about
1-2 years through the Rational Method and about 50 years through the Regression Method.  The neighborhoods surrounding this undeveloped site along with the neighborhoods in the Williamson Creek watershed are already experiencing flooding.  Development of this plot of land would increase the flooding in these neighborhoods due to the increase of impervious cover.  The small area set aside for recreation/park area on the western edge of the property is not an adequate amount of land to soak up the excess amount of water during a heavy rain.  The excess water will overflow the creek causing flooding on West Gate Street, which is a major thorough fare for many motorists.