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--- I I 1 1 I I 1 1 1 I 1 I I 1 1 1 I 1 I 111-2 Infiltration effects in the mountains and forested regions of the Ralston Creek Basin were comhined in Phase A of this study with the detention effects of forest cover by using SCS runoff curves rather than infiltration rates alone, In the lower areas of the basin, SCS curves were used also, but here they more directly represent infiltration rates, Since most soils in the lower part of the Ralston Creek Basin are similar to those tested for the Lena Gulch Basin in 1973, a coorelation was made to confirm the validity of the SCS curves. Based upon this comparison, it is our opinion that the SCS curves yield reasonable values of the runoff from areas in the lower Ralston Creek Basin, TABLEIII-l lOO-YEAR PEAK FLOW SUMMARY PRESENT CONDITIONS (All values in cfs) DETERMINATION OF DESIGN FLOWS - MITCAT Locat ion 10-Year 100-Year Ra I stan Creek above Ra 1 s ton Reservo i r 4000 7200 (48) Ra 1 ston Creek below Ralston Reservoir 2700 5000 (48) Ralston Creek above Leyden Creek 2800 4700 (91) Ra 1 stan Creek below Leyden Creek 3000 6000 (91) Ralston Creek above Van Bibber Creek 3000 6100 (91) Ra 1 ston Creek below Van Bibber Creek 3100 8700 (91) Ra 1 stan Creek at Mouth 3800 9300 (91) Leyden Creek above Leyden Lake 1300 3300 -k Leyden Creek below Leyden Lake 700 1900 * Leyden Creek at Mouth 900 2400 1. Van Bibber Creek at Mouth 1600 3400 * Due to the complex nature of the drainage basin tributary to Ralston/Leyden Creeks and the extensive hydrological evaluations necessary, the method used to compute runoff was a mathematical model known as MITCAT or the MIT Catchment model, The MITCAT model is more sensitive to urbanization, channel- ization and reservoir variables than other commonly accepted methods and was formulated to need only basic physical data and used a fairly simple input forma t. RALSTON CREEK FLOOD HYDROLOGY * No rainfall correction made Numbers in parenthesis represent area in square miles for areal rainfall corrections. A more detailed explanation of the hydrology was presented in the Hydrological Appendix of the Phase A Report which includes rainfall, infiltration, the mechanics of MITCAT; and its application to the Ralston Creek Basin. A summary of peak flows for various hydrological conditions will be presented here. A discussion of the differences between MITCAT computed values and those generated by the U.S, Army Corps of Engineers (USACE) may be found in the Phase A Report, Because of the myriad of combinations involved with many different areal corrections, return frequencies, and rainfall zones, peak flow summaries will be separated into present and future conditions, It has been assumed that there will be no additional channel encroachment under future conditions, TABLE III -2 100-YEAR PEAK FLOW SUMMARY - FUTURE BASIN CONDITIONS MASTER PLAN CONDITIONS (All Values in cfs) Location 10-Year 100-Year 4000 7200 (48) 2700 5000 (48) 3200 4800 (30) 4300 6200 (30) 4800 7100 (30) 5200 8000 (30) 6000 8800 (30 1500 3800 ~~ 800 2200 * 1000 2600 * 1100 Present Conditions Ralston Creek above Ralston Reservoir Ralston Creek below Ralston Reservoir Ralston Creek above Leyden Creek Ralston Creek below Leyden Creek Ralston Creek above Van Bibber Creek Ralston Creek below Van Bibber Creek Ralston Creek at Mouth Leyden Creek above Leyden Lake Leyden Creek below Leyden Lake Leyden Creek at Mouth Van Bibber Creek at Mouth Flow estimates for the lOO-year event under present conditions are presented in Table 111-1, Numbers in parentheses represent the area (in square miles) upon which the areal correction factors were based to obtain the proper values for the design flow at a given location, Future Conditions Flow estimates for the 100-year event under future conditions are presented in Table 111-2, These values are based upon future development with the existing channel, and include routing the flood through Ralston Reservoir, and Leyden Lake. The lOO-year floodplain was determined assuming that Blunn Res- ervoir was not in place; however. the recommended alternative assumes that Blunn Reservoir will be constructed and will be an integral part of the flood manage- ment program. It was found through hydrologic analysis and routing that Blunn Reservoir has a negligible effect on the peak flows of Ralston Creek below its confluence with Leyden Creek. It was assumed all reservoirs are full at the * No rainfall correction made, Numbers in parenthesis represent area in square miles for areal rainfall corrections. Different areal corrections result from taking the largest peak flow at each location.