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Home My WebLink AboutDrainage ReportDRAINAGE REPORT FOR THE K.W. SCHICK 9.627 ACRE TRACT NINE (VOLUME 542, PAGE 131) CRAWFORD BURNETT LEAGUE, A -7 HYDRAULIC STUDY FOR aw � BEE CREEK TRIBUTARY B MDG JOB NO. 000776-3596 COLLEGE STATION, BRAZOS COUNTY, TEXAS JULY 2003 2551 Texas Ave. South, Ste. A, College Station, TX 77840 Ofc: 979.693.5359 Fax: 979.693.4243 Email: mdgcstx @yahoo.com Purpose of study: MDG (Municipal Development Group) was engaged to conduct a floodplain analysis for tracts Four, Five, Six, Seven, Eight, and Nine of the K.W. Schick 36.182 Acre Tract. The objective of this study was to determine the extent of the floodway and floodplain through tracts Four, Five, Six, Seven, Eight and Nine. General Location and Description: The subject site is Tract Nine and is located along Wellborn Road between the city streets of Southwest Parkway and Christine Lane. Currently, the project site is undeveloped and exists in a relatively natural state that is covered with native grasses, underbrush and canopy tree cover. The combined 19.627 -acre tracts drain directly into Bee Creek Tributary B. The monument data for Tract Nine is College Station Monument # 119, 1994 from datum NGVD 1929. This monument is located on Welsh Avenue and is approximately in the southwest quadrant of the intersection of Welsh Avenue and F.M. 2818. Monument #119 is west 6.10 feet from the back of curb on Welsh Avenue and at the corner end of the sidewalk as shown in Exhibit 1. Scope of the Study: MDG reviewed existing FEMA (Federal Emergency Management Agency) FIRM'S (Flood Insurance Rate Maps) and determined that Bee Creek Tributary B was the subject stream, however, no data was available for the upper reaches of Bee Creek Tributary B. The first cross section available from FEMA data is cross section G as shown on the map in Exhibit 2. Cross section G is located downstream of Tract Nine, but lies in the watershed area. Due to the lack of available data, extensive ground topographic survey was conducted through out the upper reaches of this watershed. This survey provided the data required to conduct the analysis. This data was then incorporated into AutoDesk Land Desktop 3, where contours and cross - section profiles where generated. The watershed, as depicted in Table 1, consists of three areas totaling 591.14 acres. Area 1 consists of 351.53 acres and is the northern most area. The second area consists of 170.74 acres and is the western most area. The third area consists of 68.87 acres and is the eastern most area. The watershed flow into Tributary B, including the unnamed Tributary of Tributary B was calculated using the Rational Method. These calculations are shown in Table 1. The water surface elevations are computed using an iterative application of the Manning's Equation called the standard step. The accepted and well -used computer program for this type of computation is HEC -RAS 3. 1.1 (Hydraulic Engineering Center — River Analysis System), which was developed by USACE (United States Army Corp of Engineers). The calculated flow rates were entered into HEC -RAS at the beginning and end of each reach and are shown in Table 2. These discharges, as well as the generated cross - sections, were analyzed in HEC -RAS yielding a floodplain and floodway. The floodplain profile generated by HEC -RAS had elevation variances. In order to create a floodplain with a relatively constant elevation, higher elevations were chosen in order to create a conservative profile. Plan Data: A plan in HEC -RAS is the combination of geometric data and flow data. A total of forty - five (45) water surface profiles at each ground - surveyed cross - section location were developed using various geometric and flow data. Listed in Appendix A are the results of the hydraulic analysis of Bee Creek Tributary B performed with the HEC -RAS modeling program. Tributary B was analyzed from its beginning up to cross - section G, which is documented in FIRM map number 48041 CO182 C dated July 2, 1992. A map of the entire analyzed area is shown in Exhibit 3. Results: The HEC -RAS water surface elevation of 286.07 feet at cross - section 11.9 compares to the water surface elevation of 285.70 feet at cross - section G with acceptable standards, not exceeding ±0.50 foot. Knowing this, the floodplain was created using the water surface elevations at each cross - section. The floodplain generated by the flood study is considerably smaller than the floodplain produced by FEMA, resulting in more usable land than originally considered. A floodway map was also generated using encroachment method 1 in HEC -RAS. This method moves the encroachment stations, allowing the watershed elevation to raise no more than one foot. Both the floodway and floodplain are shown in Exhibit 4. The encroachment data is shown in Appendix B. Table 2 Station Number 2 -YR Flow Rate (cfs) 25 -YR Flow Rate (cfs) 100 -YR Flow Rate (cfs) 11 733.230 1189.292 1410.733 1 781.068 1266.884 1502.773 35 232.198 380.369 452.148 28 244.529 400.568 476.160 27 1025.597 1667.452 1978.933 11.6 1154.057 1811.748 2145.023 Area 1 GENERAL INFORMATION Description: Pre - Development Drainage Area 1 Total Drainage Area = 351.53 acres GIVEN Velocity of Runoff = 4.50 " /sec Coefficient of Runoff = 0.56 Maximum Travel Distance = 6600 ft Time of Concentration = 24.44 min "NOTE: Minimum T allowed = 10 min. 2 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.806 Coefficient (b) = 65 Coefficient (d) = 8 5 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.785 Coefficient (b) = 76 Coefficient (d) = 8.5 10 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.763 Coefficient (b) = 80 Coefficient (d) = 8.5 25 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.754 Coefficient (b) = 89 Coefficient (d) = 8.5 50 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.745 Coefficient (b) = 98 Coefficient (d) = 8.5 100 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.73 Coefficient (b) = 96 Coefficient (d) = 8 RESULT Rainfall Intensity (1 = 3.935 in /hr Q2 = 781.068 cfs Rainfall Intensity (1 = 4.891 /hr Q5 = 970.754 cfs Rainfall Intensity (I = 5.559 in /hr Quo = 1103.511 cfs Rainfall Intensity ( = 6.382 in /hr Q25 = 1266.884 cfs Rainfall Intensity (150) = 7.252 /hr F57_ 1439.570 cfs Rainfall Intensity (I,00) = 7.571 /hr Q�oo = 1502.773 cfs Area 2 GENERAL INFORMATION Description: Pre - Development Drainage Area 2 Total Drainage Area = 170.74 acres GIVEN Velocity of Runoff = 2.50 " /sec Coefficient of Runoff = 0.41 Maximum Travel Distance = 4500 ft Time of Concentration = 30.00 min *"NOTE: Minimum T allowed = 10 min. 2 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.806 Coefficient (b) = 65 Coefficient (d) = 8 5 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.785 Coefficient (b) = 76 Coefficient (d) = 8.5 10 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.763 Coefficient (b) = 80 Coefficient (d) = 8.5 25 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.754 Coefficient (b) = 89 Coefficient (d) = 8.5 50 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.754 Coefficient (b) = 98 Coefficient (d) = 8.5 100 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.73 Coefficient (b) = 96 Coefficient (d) = 8 RESULT Rainfall Intensity (I = 3.464 in /hr QZ = 244.529 cfs Rainfall Intensity (1 = 4.327 in /hr Q = 305.457 cfs Rainfall Intensity (I = 4.936 "' /hr Q, o = 348.423 cfs Rainfall Intensity ( = 5.675 in /hr Q25 = 400.568 cfs Rainfall Intensity ( = 6.249 /hr F7_ so = 441.074 cfs Rainfall Intensity (Iloo) = 6.746 in /h, Q,00 = 476.160 cfs Area 3 GENERAL INFORMATION Description: Pre - Development Drainage Area 3 Drainage Area = 68.87 acres GIVEN Velocity of Runoff = 1.50'7... Coefficient of Runoff = 0.51 Maximum Travel Distance = 4500 ft Time of Concentration = 50.00 min " *NOTE: Minimum T allowed= 10 min. 2 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.81 Coefficient (b) = 65 Coefficient (d) = 8 5 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.785 Coefficient (b) = 76 Coefficient (d) = 8.5 10 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.763 Coefficient (b) = 80 Coefficient (d) = 8.5 25 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.754 Coefficient (b) = 89 Coefficient (d) = 8.5 50 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.754 Coefficient (b) = 98 Coefficient (d) = 8.5 100 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.73 Coefficient (b) = 96 Coefficient (d) = 8 RESULT Rainfall Intensity (1 = 2.464 in1 hr Q7=- 87.256 cfs Rainfall Intensity (I5) = 3.116 'n /hr Q = 110.357 cfs Rainfall Intensity ( 1 10) = 3.587 in /hr Q, = 127.044 cfs Rainfall Intensity ( 1 25) = 4.140 in /hr Q2s = 146.608 cfs Rainfall Intensity ( 1 50) = 4.558 n th, Fs; = 161.434 cfs Rainfall Intensity ( 1 100) = 4.954 in /hr Foo = 175.458 cfs Total Area GENERAL INFORMATION Description: Pre - Development Drainage Total Total Drainage Area = 591.14 acres GIVEN Composite Velocity of Runoff = 3.57 " / 5eC Composite Coeffcient of Runoff = 0.51 Composite Travel Distance = 5748.80 ft Composite Time of Concentration = 29.03 min "NOTE: Minimum T allowed = 10 min. 2 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.800 Coefficient (b) = 68 Coefficient (d) = 7.9 5 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.771 Coefficient (b) = 73 Coefficient (d) = 7.7 10 YEAR FREQUENCY CONSTANTS RESULT Rainfall Intensity (I = 3.790 in /hr Q = 1154.057 cfs Rainfall Intensity (1 = 4.537 in /hr Q7=- 1381.378 cfs Coefficient (e) = 0.757 Coefficient (b) = 81 Coefficient (d) = 7.7 25 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.728 Coefficient (b) = 82 Coefficient (d) = 7.7 50 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.736 Coefficient (b) = 92 Coefficient (d) = 7.7 100 YEAR FREQUENCY CONSTANTS Coefficient (e) = 0.712 Coefficient (b) = 92 Coefficient (d) = 7.9 Rainfall Intensity ( 1 10) = 5.294 in /hr Q To = 1612.072 cfs Rainfall Intensity ( 1 25) = 5.950 in /h, Q25 = 1811.748 cfs Rainfall Intensity ( 1 50) = 6.486 in /hr Q57 1974.931 cfs Rainfall Intensity ( 1 100) = 7.044 in /hr F,oa = 2145.023 cfs Project Manager: Greg Taggert 4/29/2005 Job # 000776 -3596 Calculations: Ben, Mark, Preston