A tool for assessing the status of drainage ditches and the need for remedial measures

Figures & data

Figure 1. Topographical map of the study area with the ditch evaluated shown as a blue line and the catchment area of the ditch marked with a black line.

Figure 2. The ditch evaluated (blue line), with the letters A, B, C, D, E and F showing the end of each ditch section. The arrow shows the flow direction.

Figure 3. The ditch evaluated (blue line), with the letters A, B, C, D, E and F showing the end of each ditch section. The selected figures show changes over time in the ditch cross section, with the blue lines representing the original cross section and the red lines the observed cross section. The scales are in cm.

Table 1. Results of the ditch assessment in six segments with total MADRAS scores.

	Start–A	A–B	B–C	C–D	D–E	E–F
Bank stability	0	5	5	10	10	–
Over-widening/undercut banks	3	3	10	10	10	–
Sediment deposition	0	0	3	10	10	–
Total score	3	8	18	30	30	–

Figure 4. Changes in land use in the ditch catchment area between 1952 and 2012, and the expected land use change by 2020. In the legend Settlements 1 are areas with houses with green areas in between them, Settlements 2 are building areas with sparse or no green areas in between them and New Settlements are planned settlements.

Figure 5. (a) Schematic presentation of the geographical location of flow accumulation points 1–5 and the calculated water flow in each point and (b) measured cross-section placements with the higher number upstream and the lowest number at the outlet.

Table 2. Calculated flows in relation to the land use of 1952, 2012 and 2020 at different accumulation points.

Accumulation points	Flow 1952 (m^3 s^−1)	Flow 2012 (m^3 s^−1)	Flow 2020 (m^3 s^−1)
1	0.30	0.30	0.60
2	0.44	0.44	1.20
3	0.66	1.14	2.05
4	1.04	3.49	3.80
5	1.10	4.78	5.00

Figure 6. Output of one selected HEC-RAS simulation showing the consequences of damming one culvert (light blue area indicates water). (a) The water level over the bank level in the ditch cross-section upstream from the culvert. (b) The overfill culvert on the upstream side. (c) The water level in the cross-section downstream from the culvert. (d) The water level in the culvert downstream side.

Table 3. Results of HEC-RAS simulations of the ditch sections evaluated, with precipitation data of 30-minute rain intensities with a return period of 10 years used as design flow under the land use of 2012.

Section number	Flow (m^3 s^−1)	Water level (m)	Critical level (m)	Channel slope (m m^−1)	Water velocity (m s^−1)	Shear bed (N m^−2)	Shear left bank (N m^−2)	Shear right bank (N m^−2)	Shear section (N m^−2)
25	0.3	26.72	26.43	0.0014	0.53	5.8	2.2	1.4	3.57
24.95^a	0.3	26.33	26.33	0.0313	1.41	55.38	14.24	4.01	47.35
24.4	0.3	25.3	24.82	0.0004	0.34	2.26	0.85	0.65	1.43
24.2^a	0.3	24.94		0.0076	0.85	17.98	5.57	1.59	15.32
24	0.3	23.55		0.0073	0.84	17.61	5.47	1.62	14.96
23	0.3	21.58	21.38	0.0083	0.82	17.64	4.26	2.23	14.12
22.9^a	0.3	21.59	21.28	0.0029	0.59	8.24	3.5	1.62	6.09
22	0.44	20.73	20.73	0.0324	1.37	53.42	15.68		48.23
21.9	0.44	19.93	19.23	0.0001	0.25	1.05	0.57	0.37	0.76
21.8^a	0.44	19.72		0.0004	0.33	2.11	1.15	0.75	1.58
21.7	0.44	19.71	18.93	0.0001	0.23	0.85	0.46	0.3	0.6
21.6^a	0.44	18.85	18.83	0.0263	1.28	45.76	14.75		41.21
21.5	0.44	18.14	17.86	0.0029	0.67	9.96	1.4	4.07	7.73
21.4^a	0.44	18.1		0.0019	0.59	7.34	1.33	3.31	5.6
21	1.14	17.82		0.0036	1.01	19.69	5.08	10.16	14.13
20.7	1.14	17.38	16.79	0.0011	0.72	8.64	2.5	4.21	5.54
20.6^a	1.14	16.69	16.69	0.0263	2.01	90.25		34.85	69.77
20.5	1.14	16.37	15.99	0.0027	0.96	16.92	3.56	7.81	10.94
20.4^a	1.14	16.16		0.0086	1.39	38.89	3.76	16.9	27.88
20	1.14	16.16	15.79	0.0028	0.97	17.32	3.57	7.97	11.19
19	1.14	15.3	15.3	0.0679	1.74	92.3	31.03	174.09	133.05
18	1.14	14.79	14.13	0.0004	0.45	3.38	1.3	1.25	2.48
17.6^a	1.14	14.03	14.03	0.0283	1.65	67.99	2.56		64.52
17.5	1.14	12.24	11.69	0.0033	0.91	16.38	6.34	4.54	12.59
17.4^a	1.14	12.05		0.0078	1.2	30.45	7.95	5.38	24.86
17	1.14	11.81	11.39	0.0059	1.1	24.91	7.2	4.83	19.34
16^a	3.49	10.35		0.0019	1.06	17.88	14.73	5.55	13.79
15	3.49	10.26	8.61	0.0003	0.68	5.88	1.19	1.62	3.02
14^a	3.49	7.95		0.0076	1.81	56.3	27.45	37.13	41.13
13	3.49	6.26		0.0091	1.9	63.17	40.06	19.29	45.3
12	3.49	5.99		0.0013	0.98	14.35	9.51	5.19	9.94
10	4.78	5.88	4.65	0.0007	0.91	11.19	5.14	5.87	7.09
9^a	4.78	5.11		0.0004	0.6	5.25	4	3.03	4.28
8	4.78	4.83	4.83	0.0275	2.7	142.18	79.61	100.51	110.14
7	4.78	4.6		0.0024	1.3	25.72	17.81	8.83	18.78
6	4.78	4.24		0.0033	1.49	34.36	27.81	7.91	24.99
5	4.78	4.21		0.0012	1.11	17.12	2.03	3.49	9.76
4	4.78	4.04		0.0118	1.88	66.71			66.71
3	4.78	3.96		0.0056	1.4	35.46			35.46
2	4.78	3.81		0.0072	1.56	44.37			44.37
1	4.78	3.03	3.03	0.0247	3.01	162.58	36.73	63.53	110.81

The cross sections are numbered from 25 to 1, moving from upstream (25) towards the outlet (1).

^a Cross sections situated after a culvert.