Results

Results:

HEC-HMS produces an output for the 2-year 24-hour storm that is summarized Table 10:

Table 10 - HEC-HMS Output for the 2-Year Event, Basin 1 Only

Hydrologic Element	Discharge Peak cfs	Time of Peak	Total Volume acre-ft	Drainage Area mi²
Sub-basin 5	90.0	17 Dec 02 00:15	10.26	0.215
Reach 1	89.3	17 Dec 02 00:15	10.24	0.215
Sub-basin 1	29.4	17 Dec 02 00:35	5.06	0.107
Sub-basin 7	188.9	16 Dec 02 23:55	11.14	0.232
Sub-basin 2	52.6	17 Dec 02 00:25	7.65	0.161
Sub-basin 6	156.3	16 Dec 02 23:55	9.22	0.192
Sub-basin 3	91.6	17 Dec 02 00:20	12.23	0.257
Sub-basin 4	36.6	16 Dec 02 24:00	2.43	0.051
Sink 1	449.3	16 Dec 02 23:55	57.98	1.22

Total volume of runoff contributed, time of peak, and peak flows are calculated via a simulation for the 2-year 24-hour storm. Peak flows occur around midnight on December 16 for all sub-basins – approximately 12 hours after the storm begins.

After simulation of 3 types of storms with 2 different basin models, the following results were obtained for total volume runoff to the lake. Peak discharge flows are calculated at the sink (ditch entrance into the lake):

Table 11 - HEC-HMS Output Summary

Scenario	Event Storm	Peak Discharge cfs	Peak Time	Total Volume Added acre-ft	Drainage Area mi²

	2 Year	439	12/16/02 23:55	58.0	1.22
	50 Year	1400	12/16/02 23:55	179	1.22
	100 Year	1810	12/16/02 23:55	229	1.22

	2 Year	463	12/16/02 24:00	81.9	1.72
	50 Year	1480	12/16/02 23:55	253	1.72
	100 Year	1910	12/16/02 23:55	323	1.72

In addition to calculating water contributions from land runoff, it is crucial to add the volume of water that falls directly on the lake surface. For a lake area of 134 acres (see Table 1), the volume of water added can be calculated for each event storm:

Table 12 – Volume of Water Added Through Direct Precipitation on the Lake

Event Storm	Precipitation in	Volume Added acre-ft
2-Year	2.7	30.2
50-Year	5.5	61.4
100-Year	6.5	72.6

Compiling this information with data from Table 11 gives us a grand total of volume added to the lake for each storm, with and without runoff from Basin 2. This data is shown in Table 13.

Table 13 – Final Results

Scenario	Event Storm	Total Volume acre-ft	Total Volume m²

Basin 1	2 Year	88.2	109,000
	50 Year	240	296,000
	100 Year	302	373,000

Basin 1 & 2 Combined	2 Year	112	138,000
	50 Year	314	387,000
	100 Year	396	489,000

Malcolm Pirnie created an elevation-volume relationship in their TMDL report. Based on this relationship, when a specified volume is added to the lake, elevation can be predicted. These can be seen in Tables 14 and 15 and Figures O and P, which predict lake elevation in response to 2-year, 50-year and 100-year 24-hour event storms.

It is important to note that peak runoff of sub-basins 8, 9 and 10 (those that discharge into the ditch) sometimes exceed the ditch’s capacity of 45 cfs. Cross sections will need to be repaired in order to accommodate peak flows:

Table 16 - Ditch Overflow Data

Hydrologic Element	Discharge Peak	Time of Peak	Total Volume	Drainage Area
2-Year
Subbasin-9	52.5	17 Dec 02 00:25	7.60	0.160
Reach-2	52.2	17 Dec 02 00:35	7.57	0.160
Subbasin-8	136	17 Dec 02 00:10	13.3	0.278
Ditch 1	151	17 Dec 02 00:20	20.8	0.438
Subbasin-10	24.6	17 Dec 02 00:20	3.18	0.0670
Ditch 2	173	17 Dec 02 00:20	240	0.505
50-Year
Subbasin-9	166	17 Dec 02 00:25	23.5	0.160
Reach-2	166	17 Dec 02 00:35	23.4	0.160
Subbasin-8	429	17 Dec 02 00:10	41.0	0.278
Ditch 1	479	17 Dec 02 00:20	64.2	0.438
Subbasin-10	78.1	17 Dec 02 00:15	9.80	0.067
Ditch 2	549	17 Dec 02 00:20	73.9	0.505
100-Year
Subbasin-9	213	17 Dec 02 00:25	30.0	0.160
Reach-2	213	17 Dec 02 00:35	29.9	0.160
Subbasin-8	550	17 Dec 02 00:10	52.4	0.278
Ditch 1	616	17 Dec 02 00:20	82.1	0.438
Subbasin-10	100.5	17 Dec 02 00:15	12.5	0.067
Ditch 2	707	17 Dec 02 00:20	94.6	0.505

Even though the ditch cannot currently accommodate peak runoff flows, I assumed full capture of runoff from the watershed in order to create a worst-case scenario in terms of possible flood damage (to homes on the lakefront). In addition, these values create estimates that may aid in upgrading the ditch in the future if the homeowners wish to capture all potential runoff from the adjacent basin.