1. Guidance on Homework 5 5.1. Report Tc and R for each watershed
Input Information - Existing Conditions
Subbasin Number
Area (ac)
L (ft)
Lca (ft)
Slope (ft/mi)
Development %
Conveyance (%)
Impervious (%)
SCS CN 1
1280
7000
4000 20 30 80 78 2
1485
9000
5050 10 28 18 3
1517
8750
4975 75
100 52 85 4
2752
10000
6340 60 90
Computed Information - Existing Conditions
Area (mi2)
L (mi)
Lca (mi) C
TC+R C' TC R
1.33
0.76
6.183
2.62
2.004
0.31
2.32
TOTAL
2.0
Notes
Acres per square mile
ft/mi
3. C computed using 4295[%DEV]^ [%CONV]^-0.967
4. TC+R computed using C(L/sqrt(slope))^0.706
5. C' computed using 2.46-([%DEV]/100)*( ). All slopes are <=20 in this case. This interpolates between 2.46 for 0 %DEV and 0.94 for 100% Dev

2. 5.2 Basin Model

3. 5.3 Frequency Storms Area reduction Concept
Partial and annual duration
See HEC HMS Manual P 207

4. Intensity-Duration-Frequency
IDF design curves
All major cities
Based on NWS data
Various return periods & durations
Used for drainage design of pipes & roads
Used for floodplain designs - watersheds
From Bedient

5. Design Rainfalls Design Storm from HCFCD and NWS
Based on Statistical Analysis of Data
5, 10, 25, 50, 100 Year Events
Various Durations of 6 to 24 hours
Six Hour Rainfall
From Bedient

6. From NOAA Atlas 2 http://www. nws. noaa
CEE 3430 – Spring 2011

7. From NOAA Atlas 2 http://www. nws. noaa
CEE 3430 – Spring 2011

8. Realization of Frequency Storm in HEC-HMS

9. 5.4 Detention Pond

10. 5.6 Evaluating and Comparing multiple runs
10 year Storm
100 year storm
Existing
Det Pond
Dev
Dev+Det Pond
Peak Flow (cfs)
Time of Peak (from start in hours:min)