Hydrologic Analysis

3.1 Overview

A new hydrologic model was prepared for the entire Leyden Creek watershed, updating the hydrology for the existing and future land use conditions, for the 2-, 5-, 10-, 25-, 50- and 100-year storm frequencies. The Colorado Urban Hydrograph Procedure (CUHP v1.4.3), was used to develop runoff hydrographs for each individual sub-watershed. The hydrographs from each subwatershed were then routed using the EPA Stormwater Management Model (SWMM), version 5.0.022 to determine discharges at each design point.

The Leyden Creek watershed was divided into a total of 44 subwatersheds encompassing 11.78 square miles. After correspondence with UDFCD it was determined that the flow design points would be prioritized over subwatershed size with sub-watersheds staying within the 3,000 acre limits of CUHP.

Hydrologic routing generally followed the connectivity diagram presented in the 2003 Hydrology Report by Boyle Engineering; however the basin upstream of Leyden Lake was subdivided into 20 smaller basins, averaging 294 acres in sizes; whereas the Boyle study only included two sub-watersheds for the same area.

The Blue Mountain Estates area contributed flow to the Leyden Creek watershed during the September 2013 event. This area was analyzed to determine whether to include it in the watershed for this study. Through a 2-dimentional analysis in FLO-2D, it was determined that the contributing flow was more of a result from erosion during the storm than surface topography. Therefore, the area was not included in the Leyden Creek watershed hydrology.

From field review, two sub-watersheds (L303, L321) in the 2003 Boyle study were found to be non-tributary to Leyden Creek. Therefore, the total watershed area of the current study is slightly less than that used for past work (11.78 square miles versus 11.89 square miles).

3.2 Design Rainfall

One-hour rainfall depths were derived from the USDCM Figures RA-3 through RA-12 for the project area and compared with those used for past studies. Generally there were minimal differences, and the past rainfall depths were carried forward. 2-, 5-, and 25-year rainfall depths, not provided in the previous studies, were determined from Figures RA-1, RA-2, and RA-4 (USDCM). Areal adjustments were not applied since only a minor portion of the watershed exceeded the 10 square mile threshold needed for adjustment. A summary of one-hour rainfall depths can be found below in Table 3-1.

Complete rainfall distributions are provided in APPENDIX B - Hydrologic Analysis.

Table 3-1 : 1-hr Rainfall Depths

Return Period	1 Hour Rainfall Depth (in)
2-yr	1.04
5-yr	1.45
10-yr	1.75
25-yr	2.05
50-yr	2.40
100-yr	2.65

3.3 Subwatershed Characteristics

Subwatershed characteristics are summarized in the CUHP input table and subwatershed figure located in APPENDIX B - Hydrologic Analysis.

3.3.1 Subwatershed Delineation

As discussed in Section 3.1, subwatersheds delineation focused on identifying flow change locations for Leyden Creek. All basins conform to a maximum of 3,000 acres in size, with smaller subwatersheds located in more urban areas. The previous 2003 Boyle study included only 21 subwatersheds ranging in size from 44 acres to 3187 acres. This study includes 44 subwatersheds averaging 171 acres, and ranging between 16 acres to 1413 acres in size. Downstream of Leyden Lake the average watershed size is only 69.1 acres. The Church Ditch, Farmers’ High line Canal and Croke Canal were assumed full for the subwatershed delineation.

3.3.2 Watershed Imperviousness

Imperviousness for each subwatershed was estimated from a GIS based interpretation of land use and zoning information described in Section 2.2 Land Use.

3.3.3 Length, Centroid Distance, Slope

CUHP parameters such as subwatershed length, distance to centroid, and slopes were derived for each subwatershed using topographic data. Slopes were computed using the length-weighted, corrected average slope from Equation RO-9 and Figure RO-10 (USDCM). Subwatersheds downstream of Leyden Lake average a slope of 2.59% while the average for the entire watershed is a 2.94% slope.

3.3.4 Depression Losses

Depression storage loss was determined based on Table RO-6 from the USDCM. Aerial imagery was used to examine each subwatershed. Generally the depression loss value of 0.35 for pervious areas and 0.1 for impervious areas was carried forward from previous studies. A pervious depression storage value of 0.4 was used upstream of Leyden Lake in open space areas.

3.3.5 Infiltration

Soil data was obtained from Natural Resources Conservation Service (NRCS). The majority of the basin contains Type C soil consistent with the 2003 Boyle Hydrology Report. Table RO-7 provided Horton’s infiltration parameters for Type C/D soil which consists of an initial infiltration rate of 3.0 inches per hour, a final infiltration rate of 0.5 inches per hour and a decay coefficient of 0.0018. There are several basins upstream of Leyden Lake that were found to contain Type B soil and parameters for these basins were adjusted accordingly.

3.4 Hydrograph Routing

3.4.1 Manning’s n Coefficient

Each hydraulic Manning’s n roughness coefficient value was selected using Table MD-1 from USDCM. Following UDFCD guidance the values were then increased by 25% to better represent modeling conditions.

3.4.2 Conveyance Elements

Each conveyance element was determined by examining the contour data and comparing to previous studies. Storm sewers sizes and alignments were modeled based on the City of Arvada GIS files. A SWMM routing map and a SWMM schematic figure are included in APPENDIX B - Hydrologic Analysis.

3.4.3 Detention Facilities

Detention facilities within the watershed were evaluated and those meeting size and public ownership requirements were included in the SWMM routing. The inclusion of detention areas was reviewed with project sponsors. Detention was modeled at Leyden Lake, the Leyden Ranch sump, Lamplighter Park, and at Ralston Valley High School. Storage volumes for each facility were determined from the project topographic mapping. Release rates were developed from orifice and weir calculations reflecting the existing outlet structure configurations. Storage and discharge curves for the four detention facilities are provided in APPENDIX B - Hydrologic Analysis.

Leyden Lake was examined in further detail due to past discrepancies in the spillway elevation. The City of Arvada conducted a survey to determine the elevation of the spillway in relation to the Leyden Creek Dam As-Built drawings and project mapping. A conversion value of +2.2 feet was provided by Arvada to convert from the City of Arvada datum to the project datum, NAVD 88. As-built elevations from the dam were adjusted to the project datum by adding 2.9 feet. Datum conversion information is included in APPENDIX B - Hydrologic Analysis.

3.5 Previous Studies

In 1977 Wright-McLaughlin Engineers utilized the Massachusetts Institute of Technology Catchment program (MITCAT) to develop a model for Ralston/Leyden Creeks. The results of this MITCAT model were used to develop the effective FEMA flood insurance study (FIS).

In 2003 Boyle Engineering Corporation updated the hydrology for Ralston Creek and Leyden Lake. The purpose of the Boyle study was to update the watershed model from MITCAT to CUHP/UDSWMM. The model reflected improvements made to Leyden Creek Dam. Boyle examined the 10-, 50-, 100-, and 500-year events. A FHAD downstream of Leyden Lake was also prepared which utilized the updated flow results.

3.6 Hydrograph Calibration

With input from the project sponsors, three basins (106, 115, 303) required calibration. Due to the topography of the project area, these basins were underestimating runoff due to excessive length to width ratios. The peak runoff rate coefficient, Cp, was increased for each basin to produce runoff comparable to basins with similar impervious characteristics. Increases in peak flows, especially upstream of Leyden Lake, can be attributed to the increases in imperviousness within the watershed and variations in sub-watershed routing.

3.7 Results of Analysis

The results of the analysis are presented in Table 3-2. As noted above, results compare well with past studies prepared for the UDFCD and adopted by FEMA. Increased discharges can be correlated with increases in watershed imperviousness and variations in sub-watershed routing.

It should also be noted that past hydrology for Leyden and Ralston Creeks performed rainfall adjustments for watershed areas in excess of 10 square miles. These adjustments have not been performed with the current update since at 11.8 square miles, the watershed is only moderately above the 10 square mile threshold for rainfall adjustment. Additionally, peak flows along Leyden Creek were already lower than the effective flows. If a similar adjustment was made, peak discharges east of Alkire Street would be reduced further.

The increase in flows from the previous study from below Leyden Lake Dam to Alkire Street is due to the increase in discharge from Leyden Lake. The discharge increased from 373 cfs from the 2003 Boyle Hydrology Report to 550 cfs with the updated information. The updated survey information combined with updates to land use projections lead to an increase of flows into and out of Leyden Lake.

The decreased flows in lower reaches are caused by updated land use and added detention facilities. The update in land use data lead to a lower impervious value within Reach 3. Ralston Valley High School and Lamplighter Park detention were included with this study reducing flows in basins that had higher impervious values.

Peak flow and runoff volume tables are included for both existing and future conditions for all design points, and all frequencies in APPENDIX B - Hydrologic Analysis.. Hydrographs and peak flow profiles are also provided in APPENDIX B - Hydrologic Analysis.

Table 3-2 : Previous Studies Reconciliation (100 yr. Flows)

		FIS (1977 MDP)		2004 Boyle FHAD		2014 Leyden Creek MDP
		Drainage Area	Discharge	Drainage Area	Discharge	Drainage Area	Ex. Conditions Discharge	Future Conditions Discharge
Design Point	SWMM Junction	(sq. mi)	(cfs)	(sq. mi)	(cfs)	(sq. mi)	(cfs)	(cfs)
Confluence with Ralston Creek	RalstonCreek	--	--	11.89	2465	11.78	2057	2171
Simms Street	101	11.80	2456	11.89	2456	11.72	2043	2157
W 72nd Avenue	102	--	--	11.74	2456	11.57	1973	2083
W 75th Place	109	--	--	10.72	1519	10.58	1330	1416
Alkire Street	202	--	--	10.21	1035	10.33	1175	1249
Indiana Street/Croke Canal	301	--	--	9.26	372	9.62	615	665
Below Leyden Lake	351	9.00	373	9.16	373	9.19	333	550
Above Leyden Lake	401	9.00	3750	9.16	4081	9.19	4975	5532
Leyden Crossing Church Ditch	403	--	--	--	--	8.69	4855	5380
Quaker Street	404	--	--	--	--	8.42	4713	5212
Leyden Crossing 82nd Ave	412	--	--	--	--	4.52	2371	2453
At Foothills Road	413	4.20	3300	4.18	3225	4.02	2068	2136

Table 3-3 : Summary of Peak Flows

Design Point	ICON Junction	Drainage Area (sq. mile)	Ex. 2-yr	Ex. 5-yr	Ex. 10-yr	Ex. 25-yr	Ex. 50-yr	Ex. 100-yr	Future 2-yr	Future 5-yr	Future 10-yr	Future 25-yr	Future 50-yr	Future 100-yr
Confluence with Ralston Creek	Ralston Creek	11.78	229	506	759	1203	1636	2057	229	538	818	1282	1733	2171
Simms Street	101	11.72	221	487	750	1195	1624	2043	221	533	810	1274	1721	2157
W 72nd Avenue	102	11.57	195	466	723	1163	1572	1973	195	516	781	1239	1666	2083
W 75th Place	109	10.58	119	306	466	802	1070	1330	167	347	520	867	1147	1416
Alkire Street	202	10.33	119	270	425	719	947	1175	166	312	478	778	1018	1249
Indiana Street / Croke Canal	301	9.62	112	200	266	392	508	615	155	236	305	439	555	665
Below Leyden Lake	351	9.19	111	174	186	201	212	333	153	178	190	204	215	550
Above Leyden Lake	401	9.19	339	1036	1759	2912	3939	4975	639	1418	2030	3312	4404	5532
Church Ditch	403	8.69	332	1008	1714	2844	3844	4855	637	1392	1971	3218	4278	5380
Quaker Street	404	8.42	314	976	1663	2763	3732	4713	613	1332	1908	3116	4141	5212
W. 82nd Ave	412	4.52	49	464	815	1390	1882	2371	81	512	866	1455	1955	2453
Foothills Road	413	4.02	35	404	703	1213	1640	2068	71	449	749	1271	1703	2136