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Ehz report Mary Street
1. Page 1 of 3
EROSION HAZARD ZONE REPORT
for
3133 Mary Street
Del Valle, Texas, 78617
Prepared by:
Servant Engineering & Consulting, PLLC
F-16504
12000 Manchaca Rd. Suite C
Austin Texas, 78748
February 6, 2018
2. Page 2 of 3
SERVANT ENGINEERING & CONSULTING, PLLC F-16504
12000 Manchaca Rd Suite C Tel. 512-368-5251
Austin, Texas 78748
February 6, 2018
Development Services Department
City of Austin
PO Box 1088
Austin, Texas 78767
EROSION HAZARD ZONE REPORT
3133 Mary Street
Del Valle, Texas, 78617
The subject property is located on the north side of the intersection of Mary Street and Bessie
Avenue. The site is in the Colorado Creek Watershed, which is classified as a Suburban
Watershed.
According to FEMA Map No. 48453C0610K, no portion of this site is located within the 100-
year floodplain. However, a portion of the site is located in the erosion hazard zone review
buffer. For this site, a drainage study was performed to delineate the planimetric limits of the
erosion hazard zone using HEC-HMS and HEC-RAS.
A drainage study using HEC-HMS was performed to determine the runoff that drains to the
creek. There is a surrounding drainage area of 166.63 acres draining to the creek. This drainage
area was determined based on the topographic conditions.
The existing site impervious cover of the site for drainage purposes has been taken as 67.06%
which was the weight average of 50% for single family residential area and 80% for commercial
area. The residential area has been estimated at 967,768 S.F. and the commercial area has been
estimated at 6,290,517 S.F. The time of Concentration was determined using the Technical
Release 55 Program. The existing hydrologic condition of the soil is good.
The site runoff combined with the offsite runoff for 2-year flood event has been calculated at
the point of analysis, the results from the HEC-HMS have been summarized and are listed
below.
Return Existing
period flow
(yr) (cfs)
2 154.78
3. Page 3 of 3
HEC-HMS Output files are attached for your review. Time of concentration and HEC-HMS
Summary table are included on the “Drainage Map for Erosion Hazard Zone” sheet of the
attached plans.
A floodplain study using HEC-RAS was performed to determine the water surface elevation in
the existing creek based on a 2-year flood event. The floodplain model was created based on the
topography and cross-section of the existing creek. Floodplain elevations are calculated for
fully developed conditions upstream of the site. Output files are attached on Sheet 1.
The Manning’s coefficient was estimated to be 0.045 based on composite roughness coefficient
of its conditions. The existing creek has the following condition: material of mostly earth and
fine gravel (n0 = 0.022, avg.), moderate degree of irregularity (n1=0.01), alternating relative
effect of channel cross section (n2=0.005), negligible relative effect of obstructions (n3 = 0.000),
low vegetation (n4 = 0.008) and minor degree of meandering (m = 1.000).
After the 2-year waster surface elevations was determined, procedure described on CoA DCM
Appendix E was followed to determine the erosion hazard zone boundary. This procedure is
explained the attached “Erosion Hazard Zone” sheet.
Should you have any questions, please contact me at (512) 368-5251 or via email at
mauricio@servanteng.com.
Sincerely,
Mauricio Quintero-Rangel, P.E.
cc: file
encl.: Time of concentration, Drainage Summary Table, HEC-HMS Simulations, Floodplain output files.
